NAME
nmap - Network exploration tool and security / port scanner
SYNOPSIS
nmap [Scan Type...] [Options] {target specification}
DESCRIPTION
Nmap ("Network Mapper") is an open source tool for network exploration
and security auditing. It was designed to rapidly scan large networks,
although it works fine against single hosts. Nmap uses raw IP packets
in novel ways to determine what hosts are available on the network,
what services (application name and version) those hosts are offering,
what operating systems (and OS versions) they are running, what type of
packet filters/firewalls are in use, and dozens of other
characteristics. While Nmap is commonly used for security audits, many
systems and network administrators find it useful for routine tasks
such as network inventory, managing service upgrade schedules, and
monitoring host or service uptime.
The output from Nmap is a list of scanned targets, with supplemental
information on each depending on the options used. Key among that
information is the "interesting ports table".. That table lists the
port number and protocol, service name, and state. The state is either
open, filtered, closed, or unfiltered. Open. means that an
application on the target machine is listening for connections/packets
on that port. Filtered. means that a firewall, filter, or other
network obstacle is blocking the port so that Nmap cannot tell whether
it is open or closed. Closed. ports have no application listening on
them, though they could open up at any time. Ports are classified as
unfiltered. when they are responsive to Nmap's probes, but Nmap cannot
determine whether they are open or closed. Nmap reports the state
combinations open|filtered. and closed|filtered. when it cannot
determine which of the two states describe a port. The port table may
also include software version details when version detection has been
requested. When an IP protocol scan is requested (-sO), Nmap provides
information on supported IP protocols rather than listening ports.
In addition to the interesting ports table, Nmap can provide further
information on targets, including reverse DNS names, operating system
guesses, device types, and MAC addresses.
A typical Nmap scan is shown in Example 1. The only Nmap arguments used
in this example are -A, to enable OS and version detection, script
scanning, and traceroute; -T4 for faster execution; and then the two
target hostnames.
Example 1. A representative Nmap scan
# nmap -A -T4 scanme.nmap.org
Starting Nmap ( http://nmap.org )
Interesting ports on scanme.nmap.org (64.13.134.52):
Not shown: 994 filtered ports
PORT STATE SERVICE VERSION
22/tcp open ssh OpenSSH 4.3 (protocol 2.0)
25/tcp closed smtp
53/tcp open domain ISC BIND 9.3.4
70/tcp closed gopher
80/tcp open http Apache httpd 2.2.2 ((Fedora))
|_ HTML title: Go ahead and ScanMe!
113/tcp closed auth
Device type: general purpose
Running: Linux 2.6.X
OS details: Linux 2.6.20-1 (Fedora Core 5)
TRACEROUTE (using port 80/tcp)
HOP RTT ADDRESS
[Cut first seven hops for brevity]
8 10.59 so-4-2-0.mpr3.pao1.us.above.net (64.125.28.142)
9 11.00 metro0.sv.svcolo.com (208.185.168.173)
10 9.93 scanme.nmap.org (64.13.134.52)
Nmap done: 1 IP address (1 host up) scanned in 17.00 seconds
The newest version of Nmap can be obtained from http://nmap.org. The
newest version of this man page is available at
http://nmap.org/book/man.html. It is also included as a chapter of
Nmap Network Scanning: The Official Nmap Project Guide to Network
Discovery and Security Scanning (see http://nmap.org/book/).
OPTIONS SUMMARY
This options summary is printed when Nmap is run with no arguments, and
the latest version is always available at
http://nmap.org/data/nmap.usage.txt. It helps people remember the most
common options, but is no substitute for the in-depth documentation in
the rest of this manual. Some obscure options aren't even included
here.
Nmap 5.21 ( http://nmap.org )
Usage: nmap [Scan Type(s)] [Options] {target specification}
TARGET SPECIFICATION:
Can pass hostnames, IP addresses, networks, etc.
Ex: scanme.nmap.org, microsoft.com/24, 192.168.0.1; 10.0.0-255.1-254
-iL <inputfilename>: Input from list of hosts/networks
-iR <num hosts>: Choose random targets
--exclude <host1[,host2][,host3],...>: Exclude hosts/networks
--excludefile <exclude_file>: Exclude list from file
HOST DISCOVERY:
-sL: List Scan - simply list targets to scan
-sP: Ping Scan - go no further than determining if host is online
-PN: Treat all hosts as online -- skip host discovery
-PS/PA/PU/PY[portlist]: TCP SYN/ACK, UDP or SCTP discovery to given ports
-PE/PP/PM: ICMP echo, timestamp, and netmask request discovery probes
-PO[protocol list]: IP Protocol Ping
-n/-R: Never do DNS resolution/Always resolve [default: sometimes]
--dns-servers <serv1[,serv2],...>: Specify custom DNS servers
--system-dns: Use OS's DNS resolver
--traceroute: Trace hop path to each host
SCAN TECHNIQUES:
-sS/sT/sA/sW/sM: TCP SYN/Connect()/ACK/Window/Maimon scans
-sU: UDP Scan
-sN/sF/sX: TCP Null, FIN, and Xmas scans
--scanflags <flags>: Customize TCP scan flags
-sI <zombie host[:probeport]>: Idle scan
-sY/sZ: SCTP INIT/COOKIE-ECHO scans
-sO: IP protocol scan
-b <FTP relay host>: FTP bounce scan
PORT SPECIFICATION AND SCAN ORDER:
-p <port ranges>: Only scan specified ports
Ex: -p22; -p1-65535; -p U:53,111,137,T:21-25,80,139,8080
-F: Fast mode - Scan fewer ports than the default scan
-r: Scan ports consecutively - don't randomize
--top-ports <number>: Scan <number> most common ports
--port-ratio <ratio>: Scan ports more common than <ratio>
SERVICE/VERSION DETECTION:
-sV: Probe open ports to determine service/version info
--version-intensity <level>: Set from 0 (light) to 9 (try all probes)
--version-light: Limit to most likely probes (intensity 2)
--version-all: Try every single probe (intensity 9)
--version-trace: Show detailed version scan activity (for debugging)
SCRIPT SCAN:
-sC: equivalent to --script=default
--script=<Lua scripts>: <Lua scripts> is a comma separated list of
directories, script-files or script-categories
--script-args=<n1=v1,[n2=v2,...]>: provide arguments to scripts
--script-trace: Show all data sent and received
--script-updatedb: Update the script database.
OS DETECTION:
-O: Enable OS detection
--osscan-limit: Limit OS detection to promising targets
--osscan-guess: Guess OS more aggressively
TIMING AND PERFORMANCE:
Options which take <time> are in milliseconds, unless you append 's'
(seconds), 'm' (minutes), or 'h' (hours) to the value (e.g. 30m).
-T<0-5>: Set timing template (higher is faster)
--min-hostgroup/max-hostgroup <size>: Parallel host scan group sizes
--min-parallelism/max-parallelism <time>: Probe parallelization
--min-rtt-timeout/max-rtt-timeout/initial-rtt-timeout <time>: Specifies
probe round trip time.
--max-retries <tries>: Caps number of port scan probe retransmissions.
--host-timeout <time>: Give up on target after this long
--scan-delay/--max-scan-delay <time>: Adjust delay between probes
--min-rate <number>: Send packets no slower than <number> per second
--max-rate <number>: Send packets no faster than <number> per second
FIREWALL/IDS EVASION AND SPOOFING:
-f; --mtu <val>: fragment packets (optionally w/given MTU)
-D <decoy1,decoy2[,ME],...>: Cloak a scan with decoys
-S <IP_Address>: Spoof source address
-e <iface>: Use specified interface
-g/--source-port <portnum>: Use given port number
--data-length <num>: Append random data to sent packets
--ip-options <options>: Send packets with specified ip options
--ttl <val>: Set IP time-to-live field
--spoof-mac <mac address/prefix/vendor name>: Spoof your MAC address
--badsum: Send packets with a bogus TCP/UDP/SCTP checksum
--adler32: Use deprecated Adler32 instead of CRC32C for SCTP checksums
OUTPUT:
-oN/-oX/-oS/-oG <file>: Output scan in normal, XML, s|<rIpt kIddi3,
and Grepable format, respectively, to the given filename.
-oA <basename>: Output in the three major formats at once
-v: Increase verbosity level (use twice or more for greater effect)
-d[level]: Set or increase debugging level (Up to 9 is meaningful)
--reason: Display the reason a port is in a particular state
--open: Only show open (or possibly open) ports
--packet-trace: Show all packets sent and received
--iflist: Print host interfaces and routes (for debugging)
--log-errors: Log errors/warnings to the normal-format output file
--append-output: Append to rather than clobber specified output files
--resume <filename>: Resume an aborted scan
--stylesheet <path/URL>: XSL stylesheet to transform XML output to HTML
--webxml: Reference stylesheet from Nmap.Org for more portable XML
--no-stylesheet: Prevent associating of XSL stylesheet w/XML output
MISC:
-6: Enable IPv6 scanning
-A: Enables OS detection and Version detection, Script scanning and Traceroute
--datadir <dirname>: Specify custom Nmap data file location
--send-eth/--send-ip: Send using raw ethernet frames or IP packets
--privileged: Assume that the user is fully privileged
--unprivileged: Assume the user lacks raw socket privileges
-V: Print version number
-h: Print this help summary page.
EXAMPLES:
nmap -v -A scanme.nmap.org
nmap -v -sP 192.168.0.0/16 10.0.0.0/8
nmap -v -iR 10000 -PN -p 80
SEE THE MAN PAGE (http://nmap.org/book/man.html) FOR MORE OPTIONS AND EXAMPLES
TARGET SPECIFICATION
Everything on the Nmap command-line that isn't an option (or option
argument) is treated as a target host specification. The simplest case
is to specify a target IP address or hostname for scanning.
Sometimes you wish to scan a whole network of adjacent hosts. For this,
Nmap supports CIDR-style. addressing. You can append /numbits to an
IPv4 address or hostname and Nmap will scan every IP address for which
the first numbits are the same as for the reference IP or hostname
given. For example, 192.168.10.0/24 would scan the 256 hosts between
192.168.10.0 (binary: 11000000 10101000 00001010 00000000) and
192.168.10.255 (binary: 11000000 10101000 00001010 11111111),
inclusive. 192.168.10.40/24 would scan exactly the same targets. Given
that the host scanme.nmap.org. is at the IP address 64.13.134.52, the
specification scanme.nmap.org/16 would scan the 65,536 IP addresses
between 64.13.0.0 and 64.13.255.255. The smallest allowed value is /0,
which scans the whole Internet. The largest value is /32, which scans
just the named host or IP address because all address bits are fixed.
CIDR notation is short but not always flexible enough. For example, you
might want to scan 192.168.0.0/16 but skip any IPs ending with .0 or
.255 because they may be used as subnet network and broadcast
addresses. Nmap supports this through octet range addressing. Rather
than specify a normal IP address, you can specify a comma-separated
list of numbers or ranges for each octet. For example,
192.168.0-255.1-254 will skip all addresses in the range that end in .0
or .255, and 192.168.3-5,7.1 will scan the four addresses 192.168.3.1,
192.168.4.1, 192.168.5.1, and 192.168.7.1. Either side of a range may
be omitted; the default values are 0 on the left and 255 on the right.
Using - by itself is the same as 0-255, but remember to use 0- in the
first octet so the target specification doesn't look like a
command-line option. Ranges need not be limited to the final octets:
the specifier 0-255.0-255.13.37 will perform an Internet-wide scan for
all IP addresses ending in 13.37. This sort of broad sampling can be
useful for Internet surveys and research.
IPv6 addresses can only be specified by their fully qualified IPv6
address or hostname. CIDR and octet ranges aren't supported for IPv6
because they are rarely useful.
Nmap accepts multiple host specifications on the command line, and they
don't need to be the same type. The command nmap scanme.nmap.org
192.168.0.0/8 10.0.0,1,3-7.- does what you would expect.
While targets are usually specified on the command lines, the following
options are also available to control target selection:
-iL inputfilename (Input from list) .
Reads target specifications from inputfilename. Passing a huge list
of hosts is often awkward on the command line, yet it is a common
desire. For example, your DHCP server might export a list of 10,000
current leases that you wish to scan. Or maybe you want to scan all
IP addresses except for those to locate hosts using unauthorized
static IP addresses. Simply generate the list of hosts to scan and
pass that filename to Nmap as an argument to the -iL option.
Entries can be in any of the formats accepted by Nmap on the
command line (IP address, hostname, CIDR, IPv6, or octet ranges).
Each entry must be separated by one or more spaces, tabs, or
newlines. You can specify a hyphen (-) as the filename if you want
Nmap to read hosts from standard input rather than an actual file.
The input file may contain comments that start with # and extend to
the end of the line.
-iR num hosts (Choose random targets) .
For Internet-wide surveys and other research, you may want to
choose targets at random. The num hosts argument tells Nmap how
many IPs to generate. Undesirable IPs such as those in certain
private, multicast, or unallocated address ranges are automatically
skipped. The argument 0 can be specified for a never-ending scan.
Keep in mind that some network administrators bristle at
unauthorized scans of their networks and may complain. Use this
option at your own risk! If you find yourself really bored one
rainy afternoon, try the command nmap -sS -PS80 -iR 0 -p 80 to
locate random web servers for browsing.
--exclude host1[,host2[,...]] (Exclude hosts/networks) .
Specifies a comma-separated list of targets to be excluded from the
scan even if they are part of the overall network range you
specify. The list you pass in uses normal Nmap syntax, so it can
include hostnames, CIDR netblocks, octet ranges, etc. This can be
useful when the network you wish to scan includes untouchable
mission-critical servers, systems that are known to react adversely
to port scans, or subnets administered by other people.
--excludefile exclude_file (Exclude list from file) .
This offers the same functionality as the --exclude option, except
that the excluded targets are provided in a newline, space, or tab
delimited exclude_file rather than on the command line.
The exclude file may contain comments that start with # and extend
to the end of the line.
HOST DISCOVERY
One of the very first steps in any network reconnaissance mission is to
reduce a (sometimes huge) set of IP ranges into a list of active or
interesting hosts. Scanning every port of every single IP address is
slow and usually unnecessary. Of course what makes a host interesting
depends greatly on the scan purposes. Network administrators may only
be interested in hosts running a certain service, while security
auditors may care about every single device with an IP address. An
administrator may be comfortable using just an ICMP ping to locate
hosts on his internal network, while an external penetration tester may
use a diverse set of dozens of probes in an attempt to evade firewall
restrictions.
Because host discovery needs are so diverse, Nmap offers a wide variety
of options for customizing the techniques used. Host discovery is
sometimes called ping scan, but it goes well beyond the simple ICMP
echo request packets associated with the ubiquitous ping tool. Users
can skip the ping step entirely with a list scan (-sL) or by disabling
ping (-PN), or engage the network with arbitrary combinations of
multi-port TCP SYN/ACK, UDP, SCTP INIT and ICMP probes. The goal of
these probes is to solicit responses which demonstrate that an IP
address is actually active (is being used by a host or network device).
On many networks, only a small percentage of IP addresses are active at
any given time. This is particularly common with private address space
such as 10.0.0.0/8. That network has 16 million IPs, but I have seen it
used by companies with less than a thousand machines. Host discovery
can find those machines in a sparsely allocated sea of IP addresses.
If no host discovery options are given, Nmap sends an ICMP echo
request, a TCP SYN packet to port 443, and TCP ACK packet to port 80,
and an ICMP timestamp request. These defaults are equivalent to the -PE
-PS443 -PA80 -PP options. An exception to this is that an ARP scan is
used for any targets which are on a local ethernet network. For
unprivileged Unix shell users, the default probes are a SYN packet to
ports 80 and 443 using the connect system call.. This host discovery
is often sufficient when scanning local networks, but a more
comprehensive set of discovery probes is recommended for security
auditing.
The -P* options (which select ping types) can be combined. You can
increase your odds of penetrating strict firewalls by sending many
probe types using different TCP ports/flags and ICMP codes. Also note
that ARP discovery (-PR). is done by default against targets on a
local ethernet network even if you specify other -P* options, because
it is almost always faster and more effective.
By default, Nmap does host discovery and then performs a port scan
against each host it determines is online. This is true even if you
specify non-default host discovery types such as UDP probes (-PU). Read
about the -sP option to learn how to perform only host discovery, or
use -PN to skip host discovery and port scan all target hosts. The
following options control host discovery:
-sL (List Scan) .
The list scan is a degenerate form of host discovery that simply
lists each host of the network(s) specified, without sending any
packets to the target hosts. By default, Nmap still does
reverse-DNS resolution on the hosts to learn their names. It is
often surprising how much useful information simple hostnames give
out. For example, fw.chi is the name of one company's Chicago
firewall. Nmap also reports the total number of IP addresses at
the end. The list scan is a good sanity check to ensure that you
have proper IP addresses for your targets. If the hosts sport
domain names you do not recognize, it is worth investigating
further to prevent scanning the wrong company's network.
Since the idea is to simply print a list of target hosts, options
for higher level functionality such as port scanning, OS detection,
or ping scanning cannot be combined with this. If you wish to
disable ping scanning while still performing such higher level
functionality, read up on the -PN (skip ping) option.
-sP (Skip port scan) .
This option tells Nmap not to do a port scan after host discovery,
and only print out the available hosts that responded to the scan.
This is often known as a "ping scan", but you can also request that
traceroute and NSE host scripts be run. This is by default one step
more intrusive than the list scan, and can often be used for the
same purposes. It allows light reconnaissance of a target network
without attracting much attention. Knowing how many hosts are up is
more valuable to attackers than the list provided by list scan of
every single IP and host name.
Systems administrators often find this option valuable as well. It
can easily be used to count available machines on a network or
monitor server availability. This is often called a ping sweep, and
is more reliable than pinging the broadcast address because many
hosts do not reply to broadcast queries.
The -sP option sends an ICMP echo request, TCP SYN to port 443, TCP
ACK to port 80, and an ICMP timestamp request by default. When
executed by an unprivileged user, only SYN packets are sent (using
a connect call) to ports 80 and 443 on the target. When a
privileged user tries to scan targets on a local ethernet network,
ARP requests are used unless --send-ip was specified. The -sP
option can be combined with any of the discovery probe types (the
-P* options, excluding -PN) for greater flexibility. If any of
those probe type and port number options are used, the default
probes are overridden. When strict firewalls are in place between
the source host running Nmap and the target network, using those
advanced techniques is recommended. Otherwise hosts could be missed
when the firewall drops probes or their responses.
-PN (No ping) .
This option skips the Nmap discovery stage altogether. Normally,
Nmap uses this stage to determine active machines for heavier
scanning. By default, Nmap only performs heavy probing such as port
scans, version detection, or OS detection against hosts that are
found to be up. Disabling host discovery with -PN causes Nmap to
attempt the requested scanning functions against every target IP
address specified. So if a class B sized target address space (/16)
is specified on the command line, all 65,536 IP addresses are
scanned. Proper host discovery is skipped as with the list scan,
but instead of stopping and printing the target list, Nmap
continues to perform requested functions as if each target IP is
active. To skip ping scan and port scan, while still allowing NSE
to run, use the two options -PN -sP together.
For machines on a local ethernet network, ARP scanning will still
be performed (unless --send-ip is specified) because Nmap needs MAC
addresses to further scan target hosts. This option flag used to be
P0 (uses zero), but was renamed to avoid confusion with protocol
ping's PO (uses the letter O) flag.
-PS port list (TCP SYN Ping) .
This option sends an empty TCP packet with the SYN flag set. The
default destination port is 80 (configurable at compile time by
changing DEFAULT_TCP_PROBE_PORT_SPEC in nmap.h). Alternate ports
can be specified as a parameter. The syntax is the same as for the
-p except that port type specifiers like T: are not allowed.
Examples are -PS22 and -PS22-25,80,113,1050,35000. Note that there
can be no space between -PS and the port list. If multiple probes
are specified they will be sent in parallel.
The SYN flag suggests to the remote system that you are attempting
to establish a connection. Normally the destination port will be
closed, and a RST (reset) packet sent back. If the port happens to
be open, the target will take the second step of a TCP
three-way-handshake. by responding with a SYN/ACK TCP packet. The
machine running Nmap then tears down the nascent connection by
responding with a RST rather than sending an ACK packet which would
complete the three-way-handshake and establish a full connection.
The RST packet is sent by the kernel of the machine running Nmap in
response to the unexpected SYN/ACK, not by Nmap itself.
Nmap does not care whether the port is open or closed. Either the
RST or SYN/ACK response discussed previously tell Nmap that the
host is available and responsive.
On Unix boxes, only the privileged user root. is generally able to
send and receive raw TCP packets.. For unprivileged users, a
workaround is automatically employed. whereby the connect system
call is initiated against each target port. This has the effect of
sending a SYN packet to the target host, in an attempt to establish
a connection. If connect returns with a quick success or an
ECONNREFUSED failure, the underlying TCP stack must have received a
SYN/ACK or RST and the host is marked available. If the connection
attempt is left hanging until a timeout is reached, the host is
marked as down. This workaround is also used for IPv6 connections,
as raw IPv6 packet building support is not yet available in Nmap..
-PA port list (TCP ACK Ping) .
The TCP ACK ping is quite similar to the just-discussed SYN ping.
The difference, as you could likely guess, is that the TCP ACK flag
is set instead of the SYN flag. Such an ACK packet purports to be
acknowledging data over an established TCP connection, but no such
connection exists. So remote hosts should always respond with a RST
packet, disclosing their existence in the process.
The -PA option uses the same default port as the SYN probe (80) and
can also take a list of destination ports in the same format. If an
unprivileged user tries this, or an IPv6 target is specified, the
connect workaround discussed previously is used. This workaround is
imperfect because connect is actually sending a SYN packet rather
than an ACK.
The reason for offering both SYN and ACK ping probes is to maximize
the chances of bypassing firewalls. Many administrators configure
routers and other simple firewalls to block incoming SYN packets
except for those destined for public services like the company web
site or mail server. This prevents other incoming connections to
the organization, while allowing users to make unobstructed
outgoing connections to the Internet. This non-stateful approach
takes up few resources on the firewall/router and is widely
supported by hardware and software filters. The Linux
Netfilter/iptables. firewall software offers the --syn convenience
option to implement this stateless approach. When stateless
firewall rules such as this are in place, SYN ping probes (-PS) are
likely to be blocked when sent to closed target ports. In such
cases, the ACK probe shines as it cuts right through these rules.
Another common type of firewall uses stateful rules that drop
unexpected packets. This feature was initially found mostly on
high-end firewalls, though it has become much more common over the
years. The Linux Netfilter/iptables system supports this through
the --state option, which categorizes packets based on connection
state. A SYN probe is more likely to work against such a system, as
unexpected ACK packets are generally recognized as bogus and
dropped. A solution to this quandary is to send both SYN and ACK
probes by specifying -PS and -PA.
-PU port list (UDP Ping) .
Another host discovery option is the UDP ping, which sends a UDP
packet to the given ports. For most ports, the packet will be
empty, though for a few a protocol-specific payload will be sent
that is more likely to get a response.. See the file payload.cc.
for exactly which ports have payloads. The --data-length. option
sends a fixed-length random payload for all ports.
The port list takes the same format as with the previously
discussed -PS and -PA options. If no ports are specified, the
default is 40125. This default can be configured at compile-time by
changing DEFAULT_UDP_PROBE_PORT_SPEC. in nmap.h.. A highly
uncommon port is used by default because sending to open ports is
often undesirable for this particular scan type.
Upon hitting a closed port on the target machine, the UDP probe
should elicit an ICMP port unreachable packet in return. This
signifies to Nmap that the machine is up and available. Many other
types of ICMP errors, such as host/network unreachables or TTL
exceeded are indicative of a down or unreachable host. A lack of
response is also interpreted this way. If an open port is reached,
most services simply ignore the empty packet and fail to return any
response. This is why the default probe port is 40125, which is
highly unlikely to be in use. A few services, such as the Character
Generator (chargen) protocol, will respond to an empty UDP packet,
and thus disclose to Nmap that the machine is available.
The primary advantage of this scan type is that it bypasses
firewalls and filters that only screen TCP. For example, I once
owned a Linksys BEFW11S4 wireless broadband router. The external
interface of this device filtered all TCP ports by default, but UDP
probes would still elicit port unreachable messages and thus give
away the device.
-PY port list (SCTP INIT Ping) .
This option sends an SCTP packet containing a minimal INIT chunk.
The default destination port is 80 (configurable at compile time by
changing DEFAULT_SCTP_PROBE_PORT_SPEC in nmap.h). Alternate ports
can be specified as a parameter. The syntax is the same as for the
-p except that port type specifiers like S: are not allowed.
Examples are -PY22 and -PY22,80,179,5060. Note that there can be no
space between -PY and the port list. If multiple probes are
specified they will be sent in parallel.
The INIT chunk suggests to the remote system that you are
attempting to establish an association. Normally the destination
port will be closed, and an ABORT chunk will be sent back. If the
port happens to be open, the target will take the second step of an
SCTP four-way-handshake. by responding with an INIT-ACK chunk. If
the machine running Nmap has a functional SCTP stack, then it tears
down the nascent association by responding with an ABORT chunk
rather than sending a COOKIE-ECHO chunk which would be the next
step in the four-way-handshake. The ABORT packet is sent by the
kernel of the machine running Nmap in response to the unexpected
INIT-ACK, not by Nmap itself.
Nmap does not care whether the port is open or closed. Either the
ABORT or INIT-ACK response discussed previously tell Nmap that the
host is available and responsive.
On Unix boxes, only the privileged user root. is generally able to
send and receive raw SCTP packets.. Using SCTP INIT Pings is
currently not possible for unprivileged users.. The same
limitation applies to IPv6, which is currently not supported for
SCTP INIT Ping..
-PE; -PP; -PM (ICMP Ping Types) .
In addition to the unusual TCP, UDP and SCTP host discovery types
discussed previously, Nmap can send the standard packets sent by
the ubiquitous ping program. Nmap sends an ICMP type 8 (echo
request) packet to the target IP addresses, expecting a type 0
(echo reply) in return from available hosts.. Unfortunately for
network explorers, many hosts and firewalls now block these
packets, rather than responding as required by RFC 1122[2]. For
this reason, ICMP-only scans are rarely reliable enough against
unknown targets over the Internet. But for system administrators
monitoring an internal network, they can be a practical and
efficient approach. Use the -PE option to enable this echo request
behavior.
While echo request is the standard ICMP ping query, Nmap does not
stop there. The ICMP standards (RFC 792[3]. and RFC 950[4]. "a
host SHOULD NOT implement these messages". Timestamp and address
mask queries can be sent with the -PP and -PM options,
respectively. A timestamp reply (ICMP code 14) or address mask
reply (code 18) discloses that the host is available. These two
queries can be valuable when administrators specifically block echo
request packets while forgetting that other ICMP queries can be
used for the same purpose.
-PO protocol list (IP Protocol Ping) .
The newest host discovery option is the IP protocol ping, which
sends IP packets with the specified protocol number set in their IP
header. The protocol list takes the same format as do port lists in
the previously discussed TCP, UDP and SCTP host discovery options.
If no protocols are specified, the default is to send multiple IP
packets for ICMP (protocol 1), IGMP (protocol 2), and IP-in-IP
(protocol 4). The default protocols can be configured at
compile-time by changing DEFAULT_PROTO_PROBE_PORT_SPEC. in nmap.h.
Note that for the ICMP, IGMP, TCP (protocol 6), UDP (protocol 17)
and SCTP (protocol 132), the packets are sent with the proper
protocol headers. while other protocols are sent with no
additional data beyond the IP header (unless the --data-length.
option is specified).
This host discovery method looks for either responses using the
same protocol as a probe, or ICMP protocol unreachable messages
which signify that the given protocol isn't supported on the
destination host. Either type of response signifies that the target
host is alive.
-PR (ARP Ping) .
One of the most common Nmap usage scenarios is to scan an ethernet
LAN. On most LANs, especially those using private address ranges
specified by RFC 1918[5], the vast majority of IP addresses are
unused at any given time. When Nmap tries to send a raw IP packet
such as an ICMP echo request, the operating system must determine
the destination hardware (ARP) address corresponding to the target
IP so that it can properly address the ethernet frame. This is
often slow and problematic, since operating systems weren't written
with the expectation that they would need to do millions of ARP
requests against unavailable hosts in a short time period.
ARP scan puts Nmap and its optimized algorithms in charge of ARP
requests. And if it gets a response back, Nmap doesn't even need to
worry about the IP-based ping packets since it already knows the
host is up. This makes ARP scan much faster and more reliable than
IP-based scans. So it is done by default when scanning ethernet
hosts that Nmap detects are on a local ethernet network. Even if
different ping types (such as -PE or -PS) are specified, Nmap uses
ARP instead for any of the targets which are on the same LAN. If
you absolutely don't want to do an ARP scan, specify --send-ip.
--traceroute (Trace path to host) .
Traceroutes are performed post-scan using information from the scan
results to determine the port and protocol most likely to reach the
target. It works with all scan types except connect scans (-sT) and
idle scans (-sI). All traces use Nmap's dynamic timing model and
are performed in parallel.
Traceroute works by sending packets with a low TTL (time-to-live)
in an attempt to elicit ICMP Time Exceeded messages from
intermediate hops between the scanner and the target host. Standard
traceroute implementations start with a TTL of 1 and increment the
TTL until the destination host is reached. Nmap's traceroute starts
with a high TTL and then decrements the TTL until it reaches zero.
Doing it backwards lets Nmap employ clever caching algorithms to
speed up traces over multiple hosts. On average Nmap sends 5-10
fewer packets per host, depending on network conditions. If a
single subnet is being scanned (i.e. 192.168.0.0/24) Nmap may only
have to send a single packet to most hosts.
-n (No DNS resolution) .
Tells Nmap to never do reverse DNS resolution on the active IP
addresses it finds. Since DNS can be slow even with Nmap's built-in
parallel stub resolver, this option can slash scanning times.
-R (DNS resolution for all targets) .
Tells Nmap to always do reverse DNS resolution on the target IP
addresses. Normally reverse DNS is only performed against
responsive (online) hosts.
--system-dns (Use system DNS resolver) .
By default, Nmap resolves IP addresses by sending queries directly
to the name servers configured on your host and then listening for
responses. Many requests (often dozens) are performed in parallel
to improve performance. Specify this option to use your system
resolver instead (one IP at a time via the getnameinfo call). This
is slower and rarely useful unless you find a bug in the Nmap
parallel resolver (please let us know if you do). The system
resolver is always used for IPv6 scans.
--dns-servers server1[,server2[,...]] (Servers to use for reverse DNS
queries) .
By default, Nmap determines your DNS servers (for rDNS resolution)
from your resolv.conf file (Unix) or the Registry (Win32).
Alternatively, you may use this option to specify alternate
servers. This option is not honored if you are using --system-dns
or an IPv6 scan. Using multiple DNS servers is often faster,
especially if you choose authoritative servers for your target IP
space. This option can also improve stealth, as your requests can
be bounced off just about any recursive DNS server on the Internet.
This option also comes in handy when scanning private networks.
Sometimes only a few name servers provide proper rDNS information,
and you may not even know where they are. You can scan the network
for port 53 (perhaps with version detection), then try Nmap list
scans (-sL) specifying each name server one at a time with
--dns-servers until you find one which works.
PORT SCANNING BASICS
While Nmap has grown in functionality over the years, it began as an
efficient port scanner, and that remains its core function. The simple
command nmap target scans more than 1660 TCP ports on the host target.
While many port scanners have traditionally lumped all ports into the
open or closed states, Nmap is much more granular. It divides ports
into six states: open, closed, filtered, unfiltered, open|filtered, or
closed|filtered.
These states are not intrinsic properties of the port itself, but
describe how Nmap sees them. For example, an Nmap scan from the same
network as the target may show port 135/tcp as open, while a scan at
the same time with the same options from across the Internet might show
that port as filtered.
The six port states recognized by Nmap
An application is actively accepting TCP connections, UDP datagrams
or SCTP associations on this port. Finding these is often the
primary goal of port scanning. Security-minded people know that
each open port is an avenue for attack. Attackers and pen-testers
want to exploit the open ports, while administrators try to close
or protect them with firewalls without thwarting legitimate users.
Open ports are also interesting for non-security scans because they
show services available for use on the network.
A closed port is accessible (it receives and responds to Nmap probe
packets), but there is no application listening on it. They can be
helpful in showing that a host is up on an IP address (host
discovery, or ping scanning), and as part of OS detection. Because
closed ports are reachable, it may be worth scanning later in case
some open up. Administrators may want to consider blocking such
ports with a firewall. Then they would appear in the filtered
state, discussed next.
Nmap cannot determine whether the port is open because packet
filtering prevents its probes from reaching the port. The filtering
could be from a dedicated firewall device, router rules, or
host-based firewall software. These ports frustrate attackers
because they provide so little information. Sometimes they respond
with ICMP error messages such as type 3 code 13 (destination
unreachable: communication administratively prohibited), but
filters that simply drop probes without responding are far more
common. This forces Nmap to retry several times just in case the
probe was dropped due to network congestion rather than filtering.
This slows down the scan dramatically.
The unfiltered state means that a port is accessible, but Nmap is
unable to determine whether it is open or closed. Only the ACK
scan, which is used to map firewall rulesets, classifies ports into
this state. Scanning unfiltered ports with other scan types such as
Window scan, SYN scan, or FIN scan, may help resolve whether the
port is open.
Nmap places ports in this state when it is unable to determine
whether a port is open or filtered. This occurs for scan types in
which open ports give no response. The lack of response could also
mean that a packet filter dropped the probe or any response it
elicited. So Nmap does not know for sure whether the port is open
or being filtered. The UDP, IP protocol, FIN, NULL, and Xmas scans
classify ports this way.
This state is used when Nmap is unable to determine whether a port
is closed or filtered. It is only used for the IP ID idle scan.
PORT SCANNING TECHNIQUES
As a novice performing automotive repair, I can struggle for hours
trying to fit my rudimentary tools (hammer, duct tape, wrench, etc.) to
the task at hand. When I fail miserably and tow my jalopy to a real
mechanic, he invariably fishes around in a huge tool chest until
pulling out the perfect gizmo which makes the job seem effortless. The
art of port scanning is similar. Experts understand the dozens of scan
techniques and choose the appropriate one (or combination) for a given
task. Inexperienced users and script kiddies,. on the other hand, try
to solve every problem with the default SYN scan. Since Nmap is free,
the only barrier to port scanning mastery is knowledge. That certainly
beats the automotive world, where it may take great skill to determine
that you need a strut spring compressor, then you still have to pay
thousands of dollars for it.
Most of the scan types are only available to privileged users.. This
is because they send and receive raw packets,. which requires root
access on Unix systems. Using an administrator account on Windows is
recommended, though Nmap sometimes works for unprivileged users on that
platform when WinPcap has already been loaded into the OS. Requiring
root privileges was a serious limitation when Nmap was released in
1997, as many users only had access to shared shell accounts. Now, the
world is different. Computers are cheaper, far more people have
always-on direct Internet access, and desktop Unix systems (including
Linux and Mac OS X) are prevalent. A Windows version of Nmap is now
available, allowing it to run on even more desktops. For all these
reasons, users have less need to run Nmap from limited shared shell
accounts. This is fortunate, as the privileged options make Nmap far
more powerful and flexible.
While Nmap attempts to produce accurate results, keep in mind that all
of its insights are based on packets returned by the target machines
(or firewalls in front of them). Such hosts may be untrustworthy and
send responses intended to confuse or mislead Nmap. Much more common
are non-RFC-compliant hosts that do not respond as they should to Nmap
probes. FIN, NULL, and Xmas scans are particularly susceptible to this
problem. Such issues are specific to certain scan types and so are
discussed in the individual scan type entries.
This section documents the dozen or so port scan techniques supported
by Nmap. Only one method may be used at a time, except that UDP scan
(-sU) and any one of the SCTP scan types (-sY, -sZ) may be combined
with any one of the TCP scan types. As a memory aid, port scan type
options are of the form -sC, where C is a prominent character in the
scan name, usually the first. The one exception to this is the
deprecated FTP bounce scan (-b). By default, Nmap performs a SYN Scan,
though it substitutes a connect scan if the user does not have proper
privileges to send raw packets (requires root access on Unix) or if
IPv6 targets were specified. Of the scans listed in this section,
unprivileged users can only execute connect and FTP bounce scans.
-sS (TCP SYN scan) .
SYN scan is the default and most popular scan option for good
reasons. It can be performed quickly, scanning thousands of ports
per second on a fast network not hampered by restrictive firewalls.
SYN scan is relatively unobtrusive and stealthy, since it never
completes TCP connections. It also works against any compliant TCP
stack rather than depending on idiosyncrasies of specific platforms
as Nmap's FIN/NULL/Xmas, Maimon and idle scans do. It also allows
clear, reliable differentiation between the open, closed, and
filtered states.
This technique is often referred to as half-open scanning, because
you don't open a full TCP connection. You send a SYN packet, as if
you are going to open a real connection and then wait for a
response. A SYN/ACK indicates the port is listening (open), while a
RST (reset) is indicative of a non-listener. If no response is
received after several retransmissions, the port is marked as
filtered. The port is also marked filtered if an ICMP unreachable
error (type 3, code 1, 2, 3, 9, 10, or 13) is received.
-sT (TCP connect scan) .
TCP connect scan is the default TCP scan type when SYN scan is not
an option. This is the case when a user does not have raw packet
privileges or is scanning IPv6 networks. Instead of writing raw
packets as most other scan types do, Nmap asks the underlying
operating system to establish a connection with the target machine
and port by issuing the connect system call. This is the same
high-level system call that web browsers, P2P clients, and most
other network-enabled applications use to establish a connection.
It is part of a programming interface known as the Berkeley Sockets
API. Rather than read raw packet responses off the wire, Nmap uses
this API to obtain status information on each connection attempt.
When SYN scan is available, it is usually a better choice. Nmap has
less control over the high level connect call than with raw
packets, making it less efficient. The system call completes
connections to open target ports rather than performing the
half-open reset that SYN scan does. Not only does this take longer
and require more packets to obtain the same information, but target
machines are more likely to log the connection. A decent IDS will
catch either, but most machines have no such alarm system. Many
services on your average Unix system will add a note to syslog, and
sometimes a cryptic error message, when Nmap connects and then
closes the connection without sending data. Truly pathetic services
crash when this happens, though that is uncommon. An administrator
who sees a bunch of connection attempts in her logs from a single
system should know that she has been connect scanned.
-sU (UDP scans) .
While most popular services on the Internet run over the TCP
protocol, UDP[6] services are widely deployed. DNS, SNMP, and DHCP
(registered ports 53, 161/162, and 67/68) are three of the most
common. Because UDP scanning is generally slower and more difficult
than TCP, some security auditors ignore these ports. This is a
mistake, as exploitable UDP services are quite common and attackers
certainly don't ignore the whole protocol. Fortunately, Nmap can
help inventory UDP ports.
UDP scan is activated with the -sU option. It can be combined with
a TCP scan type such as SYN scan (-sS) to check both protocols
during the same run.
UDP scan works by sending a UDP packet to every targeted port. For
some common ports such as 53 and 161, a protocol-specific payload
is sent, but for most ports the packet is empty.. The
--data-length option can be used to send a fixed-length random
payload to every port. If an ICMP port unreachable error (type 3,
code 3) is returned, the port is closed. Other ICMP unreachable
errors (type 3, codes 1, 2, 9, 10, or 13) mark the port as
filtered. Occasionally, a service will respond with a UDP packet,
proving that it is open. If no response is received after
retransmissions, the port is classified as open|filtered. This
means that the port could be open, or perhaps packet filters are
blocking the communication. Version detection (-sV) can be used to
help differentiate the truly open ports from the filtered ones.
A big challenge with UDP scanning is doing it quickly. Open and
filtered ports rarely send any response, leaving Nmap to time out
and then conduct retransmissions just in case the probe or response
were lost. Closed ports are often an even bigger problem. They
usually send back an ICMP port unreachable error. But unlike the
RST packets sent by closed TCP ports in response to a SYN or
connect scan, many hosts rate limit. ICMP port unreachable
messages by default. Linux and Solaris are particularly strict
about this. For example, the Linux 2.4.20 kernel limits destination
unreachable messages to one per second (in net/ipv4/icmp.c).
Nmap detects rate limiting and slows down accordingly to avoid
flooding the network with useless packets that the target machine
will drop. Unfortunately, a Linux-style limit of one packet per
second makes a 65,536-port scan take more than 18 hours. Ideas for
speeding your UDP scans up include scanning more hosts in parallel,
doing a quick scan of just the popular ports first, scanning from
behind the firewall, and using --host-timeout to skip slow hosts.
-sY (SCTP INIT scan) .
SCTP[7] is a relatively new alternative to the TCP and UDP
protocols, combining most characteristics of TCP and UDP, and also
adding new features like multi-homing and multi-streaming. It is
mostly being used for SS7/SIGTRAN related services but has the
potential to be used for other applications as well. SCTP INIT scan
is the SCTP equivalent of a TCP SYN scan. It can be performed
quickly, scanning thousands of ports per second on a fast network
not hampered by restrictive firewalls. Like SYN scan, INIT scan is
relatively unobtrusive and stealthy, since it never completes SCTP
associations. It also allows clear, reliable differentiation
between the open, closed, and filtered states.
This technique is often referred to as half-open scanning, because
you don't open a full SCTP association. You send an INIT chunk, as
if you are going to open a real association and then wait for a
response. An INIT-ACK chunk indicates the port is listening (open),
while an ABORT chunk is indicative of a non-listener. If no
response is received after several retransmissions, the port is
marked as filtered. The port is also marked filtered if an ICMP
unreachable error (type 3, code 1, 2, 3, 9, 10, or 13) is received.
-sN; -sF; -sX (TCP NULL, FIN, and Xmas scans) .
These three scan types (even more are possible with the --scanflags
option described in the next section) exploit a subtle loophole in
the TCP RFC[8] to differentiate between open and closed ports. Page
65 of RFC 793 says that "if the [destination] port state is CLOSED
.... an incoming segment not containing a RST causes a RST to be
sent in response." Then the next page discusses packets sent to
open ports without the SYN, RST, or ACK bits set, stating that:
"you are unlikely to get here, but if you do, drop the segment, and
return."
When scanning systems compliant with this RFC text, any packet not
containing SYN, RST, or ACK bits will result in a returned RST if
the port is closed and no response at all if the port is open. As
long as none of those three bits are included, any combination of
the other three (FIN, PSH, and URG) are OK. Nmap exploits this with
three scan types:
Null scan (-sN)
Does not set any bits (TCP flag header is 0)
FIN scan (-sF)
Sets just the TCP FIN bit.
Xmas scan (-sX)
Sets the FIN, PSH, and URG flags, lighting the packet up like a
Christmas tree.
These three scan types are exactly the same in behavior except for
the TCP flags set in probe packets. If a RST packet is received,
the port is considered closed, while no response means it is
open|filtered. The port is marked filtered if an ICMP unreachable
error (type 3, code 1, 2, 3, 9, 10, or 13) is received.
The key advantage to these scan types is that they can sneak
through certain non-stateful firewalls and packet filtering
routers. Another advantage is that these scan types are a little
more stealthy than even a SYN scan. Don't count on this though--
most modern IDS products can be configured to detect them. The big
downside is that not all systems follow RFC 793 to the letter. A
number of systems send RST responses to the probes regardless of
whether the port is open or not. This causes all of the ports to be
labeled closed. Major operating systems that do this are Microsoft
Windows, many Cisco devices, BSDI, and IBM OS/400. This scan does
work against most Unix-based systems though. Another downside of
these scans is that they can't distinguish open ports from certain
filtered ones, leaving you with the response open|filtered.
-sA (TCP ACK scan) .
This scan is different than the others discussed so far in that it
never determines open (or even open|filtered) ports. It is used to
map out firewall rulesets, determining whether they are stateful or
not and which ports are filtered.
The ACK scan probe packet has only the ACK flag set (unless you use
--scanflags). When scanning unfiltered systems, open and closed
ports will both return a RST packet. Nmap then labels them as
unfiltered, meaning that they are reachable by the ACK packet, but
whether they are open or closed is undetermined. Ports that don't
respond, or send certain ICMP error messages back (type 3, code 1,
2, 3, 9, 10, or 13), are labeled filtered.
-sW (TCP Window scan) .
Window scan is exactly the same as ACK scan except that it exploits
an implementation detail of certain systems to differentiate open
ports from closed ones, rather than always printing unfiltered when
a RST is returned. It does this by examining the TCP Window field
of the RST packets returned. On some systems, open ports use a
positive window size (even for RST packets) while closed ones have
a zero window. So instead of always listing a port as unfiltered
when it receives a RST back, Window scan lists the port as open or
closed if the TCP Window value in that reset is positive or zero,
respectively.
This scan relies on an implementation detail of a minority of
systems out on the Internet, so you can't always trust it. Systems
that don't support it will usually return all ports closed. Of
course, it is possible that the machine really has no open ports.
If most scanned ports are closed but a few common port numbers
(such as 22, 25, 53) are filtered, the system is most likely
susceptible. Occasionally, systems will even show the exact
opposite behavior. If your scan shows 1000 open ports and three
closed or filtered ports, then those three may very well be the
truly open ones.
-sM (TCP Maimon scan) .
The Maimon scan is named after its discoverer, Uriel Maimon.. He
described the technique in Phrack Magazine issue #49 (November
1996).. Nmap, which included this technique, was released two
issues later. This technique is exactly the same as NULL, FIN, and
Xmas scans, except that the probe is FIN/ACK. According to RFC
793[8] (TCP), a RST packet should be generated in response to such
a probe whether the port is open or closed. However, Uriel noticed
that many BSD-derived systems simply drop the packet if the port is
open.
--scanflags (Custom TCP scan) .
Truly advanced Nmap users need not limit themselves to the canned
scan types offered. The --scanflags option allows you to design
your own scan by specifying arbitrary TCP flags.. Let your
creative juices flow, while evading intrusion detection systems.
whose vendors simply paged through the Nmap man page adding
specific rules!
The --scanflags argument can be a numerical flag value such as 9
(PSH and FIN), but using symbolic names is easier. Just mash
together any combination of URG, ACK, PSH, RST, SYN, and FIN. For
example, --scanflags URGACKPSHRSTSYNFIN sets everything, though
it's not very useful for scanning. The order these are specified in
is irrelevant.
In addition to specifying the desired flags, you can specify a TCP
scan type (such as -sA or -sF). That base type tells Nmap how to
interpret responses. For example, a SYN scan considers no-response
to indicate a filtered port, while a FIN scan treats the same as
open|filtered. Nmap will behave the same way it does for the base
scan type, except that it will use the TCP flags you specify
instead. If you don't specify a base type, SYN scan is used.
-sZ (SCTP COOKIE ECHO scan) .
SCTP COOKIE ECHO scan is a more advanced SCTP scan. It takes
advantage of the fact that SCTP implementations should silently
drop packets containing COOKIE ECHO chunks on open ports, but send
an ABORT if the port is closed. The advantage of this scan type is
that it is not as obvious a port scan than an INIT scan. Also,
there may be non-stateful firewall rulesets blocking INIT chunks,
but not COOKIE ECHO chunks. Don't be fooled into thinking that this
will make a port scan invisible; a good IDS will be able to detect
SCTP COOKIE ECHO scans too. The downside is that SCTP COOKIE ECHO
scans cannot differentiate between open and filtered ports, leaving
you with the state open|filtered in both cases.
-sI zombie host[:probeport] (idle scan) .
This advanced scan method allows for a truly blind TCP port scan of
the target (meaning no packets are sent to the target from your
real IP address). Instead, a unique side-channel attack exploits
predictable IP fragmentation ID sequence generation on the zombie
host to glean information about the open ports on the target. IDS
systems will display the scan as coming from the zombie machine you
specify (which must be up and meet certain criteria). This
fascinating scan type is too complex to fully describe in this
reference guide, so I wrote and posted an informal paper with full
details at http://nmap.org/book/idlescan.html.
Besides being extraordinarily stealthy (due to its blind nature),
this scan type permits mapping out IP-based trust relationships
between machines. The port listing shows open ports from the
perspective of the zombie host. So you can try scanning a target
using various zombies that you think might be trusted. (via
router/packet filter rules).
You can add a colon followed by a port number to the zombie host if
you wish to probe a particular port on the zombie for IP ID
changes. Otherwise Nmap will use the port it uses by default for
TCP pings (80).
-sO (IP protocol scan) .
IP protocol scan allows you to determine which IP protocols (TCP,
ICMP, IGMP, etc.) are supported by target machines. This isn't
technically a port scan, since it cycles through IP protocol
numbers rather than TCP or UDP port numbers. Yet it still uses the
-p option to select scanned protocol numbers, reports its results
within the normal port table format, and even uses the same
underlying scan engine as the true port scanning methods. So it is
close enough to a port scan that it belongs here.
Besides being useful in its own right, protocol scan demonstrates
the power of open-source software. While the fundamental idea is
pretty simple, I had not thought to add it nor received any
requests for such functionality. Then in the summer of 2000,
Gerhard Rieger. conceived the idea, wrote an excellent patch
implementing it, and sent it to the nmap-hackers mailing list.. I
incorporated that patch into the Nmap tree and released a new
version the next day. Few pieces of commercial software have users
enthusiastic enough to design and contribute their own
improvements!
Protocol scan works in a similar fashion to UDP scan. Instead of
iterating through the port number field of a UDP packet, it sends
IP packet headers and iterates through the eight-bit IP protocol
field. The headers are usually empty, containing no data and not
even the proper header for the claimed protocol. The exceptions are
TCP, UDP, ICMP, SCTP, and IGMP. A proper protocol header for those
is included since some systems won't send them otherwise and
because Nmap already has functions to create them. Instead of
watching for ICMP port unreachable messages, protocol scan is on
the lookout for ICMP protocol unreachable messages. If Nmap
receives any response in any protocol from the target host, Nmap
marks that protocol as open. An ICMP protocol unreachable error
(type 3, code 2) causes the protocol to be marked as closed Other
ICMP unreachable errors (type 3, code 1, 3, 9, 10, or 13) cause the
protocol to be marked filtered (though they prove that ICMP is open
at the same time). If no response is received after
retransmissions, the protocol is marked open|filtered
-b FTP relay host (FTP bounce scan) .
An interesting feature of the FTP protocol (RFC 959[9]) is support
for so-called proxy FTP connections. This allows a user to connect
to one FTP server, then ask that files be sent to a third-party
server. Such a feature is ripe for abuse on many levels, so most
servers have ceased supporting it. One of the abuses this feature
allows is causing the FTP server to port scan other hosts. Simply
ask the FTP server to send a file to each interesting port of a
target host in turn. The error message will describe whether the
port is open or not. This is a good way to bypass firewalls because
organizational FTP servers are often placed where they have more
access to other internal hosts than any old Internet host would.
Nmap supports FTP bounce scan with the -b option. It takes an
argument of the form username:password@server:port. Server is the
name or IP address of a vulnerable FTP server. As with a normal
URL, you may omit username:password, in which case anonymous login
credentials (user: anonymous password:-wwwuser@) are used. The port
number (and preceding colon) may be omitted as well, in which case
the default FTP port (21) on server is used.
This vulnerability was widespread in 1997 when Nmap was released,
but has largely been fixed. Vulnerable servers are still around, so
it is worth trying when all else fails. If bypassing a firewall is
your goal, scan the target network for open port 21 (or even for
any FTP services if you scan all ports with version detection),
then try a bounce scan using each. Nmap will tell you whether the
host is vulnerable or not. If you are just trying to cover your
tracks, you don't need to (and, in fact, shouldn't) limit yourself
to hosts on the target network. Before you go scanning random
Internet addresses for vulnerable FTP servers, consider that
sysadmins may not appreciate you abusing their servers in this way.
PORT SPECIFICATION AND SCAN ORDER
In addition to all of the scan methods discussed previously, Nmap
offers options for specifying which ports are scanned and whether the
scan order is randomized or sequential. By default, Nmap scans the most
common 1,000 ports for each protocol.
-p port ranges (Only scan specified ports) .
This option specifies which ports you want to scan and overrides
the default. Individual port numbers are OK, as are ranges
separated by a hyphen (e.g. 1-1023). The beginning and/or end
values of a range may be omitted, causing Nmap to use 1 and 65535,
respectively. So you can specify -p- to scan ports from 1 through
65535. Scanning port zero. is allowed if you specify it
explicitly. For IP protocol scanning (-sO), this option specifies
the protocol numbers you wish to scan for (0-255).
When scanning both TCP and UDP ports, you can specify a particular
protocol by preceding the port numbers by T: or U:. The qualifier
lasts until you specify another qualifier. For example, the
argument -p U:53,111,137,T:21-25,80,139,8080 would scan UDP ports
53, 111,and 137, as well as the listed TCP ports. Note that to scan
both UDP and TCP, you have to specify -sU and at least one TCP scan
type (such as -sS, -sF, or -sT). If no protocol qualifier is given,
the port numbers are added to all protocol lists. Ports can also
be specified by name according to what the port is referred to in
the nmap-services. You can even use the wildcards * and ? with the
names. For example, to scan FTP and all ports whose names begin
with "http", use -p ftp,http*. Be careful about shell expansions
and quote the argument to -p if unsure.
Ranges of ports can be surrounded by square brackets to indicate
ports inside that range that appear in nmap-services. For example,
the following will scan all ports in nmap-services equal to or
below 1024: -p [-1024]. Be careful with shell expansions and quote
the argument to -p if unsure.
-F (Fast (limited port) scan) .
Specifies that you wish to scan fewer ports than the default.
Normally Nmap scans the most common 1,000 ports for each scanned
protocol. With -F, this is reduced to 100.
Nmap needs an nmap-services file with frequency information in
order to know which ports are the most common. If port frequency
information isn't available, perhaps because of the use of a custom
nmap-services file, -F means to scan only ports that are named in
the services file (normally Nmap scans all named ports plus ports
1-1024).
-r (Don't randomize ports) .
By default, Nmap randomizes the scanned port order (except that
certain commonly accessible ports are moved near the beginning for
efficiency reasons). This randomization is normally desirable, but
you can specify -r for sequential (sorted from lowest to highest)
port scanning instead.
--port-ratio <decimal number between 0 and 1>
Scans all ports in nmap-services file with a ratio greater than the
number specified as the argument.
--top-ports <integer of 1 or greater>
Scans the N highest-ratio ports found in nmap-services file.
SERVICE AND VERSION DETECTION
Point Nmap at a remote machine and it might tell you that ports 25/tcp,
80/tcp, and 53/udp are open. Using its nmap-services. database of
about 2,200 well-known services,. Nmap would report that those ports
probably correspond to a mail server (SMTP), web server (HTTP), and
name server (DNS) respectively. This lookup is usually accurate--the
vast majority of daemons listening on TCP port 25 are, in fact, mail
servers. However, you should not bet your security on this! People can
and do run services on strange ports..
Even if Nmap is right, and the hypothetical server above is running
SMTP, HTTP, and DNS servers, that is not a lot of information. When
doing vulnerability assessments (or even simple network inventories) of
your companies or clients, you really want to know which mail and DNS
servers and versions are running. Having an accurate version number
helps dramatically in determining which exploits a server is vulnerable
to. Version detection helps you obtain this information.
After TCP and/or UDP ports are discovered using one of the other scan
methods, version detection interrogates those ports to determine more
about what is actually running. The nmap-service-probes. database
contains probes for querying various services and match expressions to
recognize and parse responses. Nmap tries to determine the service
protocol (e.g. FTP, SSH, Telnet, HTTP), the application name (e.g. ISC
BIND, Apache httpd, Solaris telnetd), the version number, hostname,
device type (e.g. printer, router), the OS family (e.g. Windows, Linux)
and sometimes miscellaneous details like whether an X server is open to
connections, the SSH protocol version, or the KaZaA user name). Of
course, most services don't provide all of this information. If Nmap
was compiled with OpenSSL support, it will connect to SSL servers to
deduce the service listening behind that encryption layer.. When RPC
services are discovered, the Nmap RPC grinder. (-sR). is
automatically used to determine the RPC program and version numbers.
Some UDP ports are left in the open|filtered state after a UDP port
scan is unable to determine whether the port is open or filtered.
Version detection will try to elicit a response from these ports (just
as it does with open ports), and change the state to open if it
succeeds. open|filtered TCP ports are treated the same way. Note that
the Nmap -A option enables version detection among other things. A
paper documenting the workings, usage, and customization of version
detection is available at http://nmap.org/book/vscan.html.
When Nmap receives responses from a service but cannot match them to
its database, it prints out a special fingerprint and a URL for you to
submit if to if you know for sure what is running on the port. Please
take a couple minutes to make the submission so that your find can
benefit everyone. Thanks to these submissions, Nmap has about 3,000
pattern matches for more than 350 protocols such as SMTP, FTP, HTTP,
etc..
Version detection is enabled and controlled with the following options:
-sV (Version detection) .
Enables version detection, as discussed above. Alternatively, you
can use -A, which enables version detection among other things.
--allports (Don't exclude any ports from version detection) .
By default, Nmap version detection skips TCP port 9100 because some
printers simply print anything sent to that port, leading to dozens
of pages of HTTP GET requests, binary SSL session requests, etc.
This behavior can be changed by modifying or removing the Exclude
directive in nmap-service-probes, or you can specify --allports to
scan all ports regardless of any Exclude directive.
--version-intensity intensity (Set version scan intensity) .
When performing a version scan (-sV), Nmap sends a series of
probes, each of which is assigned a rarity value between one and
nine. The lower-numbered probes are effective against a wide
variety of common services, while the higher numbered ones are
rarely useful. The intensity level specifies which probes should be
applied. The higher the number, the more likely it is the service
will be correctly identified. However, high intensity scans take
longer. The intensity must be between 0 and 9. The default is 7.
When a probe is registered to the target port via the
nmap-service-probes ports directive, that probe is tried regardless
of intensity level. This ensures that the DNS probes will always be
attempted against any open port 53, the SSL probe will be done
against 443, etc.
--version-light (Enable light mode) .
This is a convenience alias for --version-intensity 2. This light
mode makes version scanning much faster, but it is slightly less
likely to identify services.
--version-all (Try every single probe) .
An alias for --version-intensity 9, ensuring that every single
probe is attempted against each port.
--version-trace (Trace version scan activity) .
This causes Nmap to print out extensive debugging info about what
version scanning is doing. It is a subset of what you get with
--packet-trace.
-sR (RPC scan) .
This method works in conjunction with the various port scan methods
of Nmap. It takes all the TCP/UDP ports found open and floods them
with SunRPC program NULL commands in an attempt to determine
whether they are RPC ports, and if so, what program and version
number they serve up. Thus you can effectively obtain the same info
as rpcinfo -p even if the target's portmapper is behind a firewall
(or protected by TCP wrappers). Decoys do not currently work with
RPC scan.. This is automatically enabled as part of version scan
(-sV) if you request that. As version detection includes this and
is much more comprehensive, -sR is rarely needed.
OS DETECTION
One of Nmap's best-known features is remote OS detection using TCP/IP
stack fingerprinting. Nmap sends a series of TCP and UDP packets to the
remote host and examines practically every bit in the responses. After
performing dozens of tests such as TCP ISN sampling, TCP options
support and ordering, IP ID sampling, and the initial window size
check, Nmap compares the results to its nmap-os-db. database of more
than a thousand known OS fingerprints and prints out the OS details if
there is a match. Each fingerprint includes a freeform textual
description of the OS, and a classification which provides the vendor
name (e.g. Sun), underlying OS (e.g. Solaris), OS generation (e.g. 10),
and device type (general purpose, router, switch, game console, etc).
If Nmap is unable to guess the OS of a machine, and conditions are good
(e.g. at least one open port and one closed port were found), Nmap will
provide a URL you can use to submit the fingerprint if you know (for
sure) the OS running on the machine. By doing this you contribute to
the pool of operating systems known to Nmap and thus it will be more
accurate for everyone.
OS detection enables some other tests which make use of information
that is gathered during the process anyway. One of these is TCP
Sequence Predictability Classification. This measures approximately how
hard it is to establish a forged TCP connection against the remote
host. It is useful for exploiting source-IP based trust relationships
(rlogin, firewall filters, etc) or for hiding the source of an attack.
This sort of spoofing is rarely performed any more, but many machines
are still vulnerable to it. The actual difficulty number is based on
statistical sampling and may fluctuate. It is generally better to use
the English classification such as "worthy challenge" or "trivial
joke". This is only reported in normal output in verbose (-v) mode.
When verbose mode is enabled along with -O, IP ID sequence generation
is also reported. Most machines are in the "incremental" class, which
means that they increment the ID field in the IP header for each packet
they send. This makes them vulnerable to several advanced information
gathering and spoofing attacks.
Another bit of extra information enabled by OS detection is a guess at
a target's uptime. This uses the TCP timestamp option (RFC 1323[10]) to
guess when a machine was last rebooted. The guess can be inaccurate due
to the timestamp counter not being initialized to zero or the counter
overflowing and wrapping around, so it is printed only in verbose mode.
A paper documenting the workings, usage, and customization of OS
detection is available at http://nmap.org/book/osdetect.html.
OS detection is enabled and controlled with the following options:
-O (Enable OS detection) .
Enables OS detection, as discussed above. Alternatively, you can
use -A to enable OS detection along with other things.
--osscan-limit (Limit OS detection to promising targets) .
OS detection is far more effective if at least one open and one
closed TCP port are found. Set this option and Nmap will not even
try OS detection against hosts that do not meet this criteria. This
can save substantial time, particularly on -PN scans against many
hosts. It only matters when OS detection is requested with -O or
-A.
--osscan-guess; --fuzzy (Guess OS detection results) .
When Nmap is unable to detect a perfect OS match, it sometimes
offers up near-matches as possibilities. The match has to be very
close for Nmap to do this by default. Either of these (equivalent)
options make Nmap guess more aggressively. Nmap will still tell you
when an imperfect match is printed and display its confidence level
(percentage) for each guess.
--max-os-tries (Set the maximum number of OS detection tries against a
target) .
When Nmap performs OS detection against a target and fails to find
a perfect match, it usually repeats the attempt. By default, Nmap
tries five times if conditions are favorable for OS fingerprint
submission, and twice when conditions aren't so good. Specifying a
lower --max-os-tries value (such as 1) speeds Nmap up, though you
miss out on retries which could potentially identify the OS.
Alternatively, a high value may be set to allow even more retries
when conditions are favorable. This is rarely done, except to
generate better fingerprints for submission and integration into
the Nmap OS database.
NMAP SCRIPTING ENGINE (NSE)
The Nmap Scripting Engine (NSE) is one of Nmap's most powerful and
flexible features. It allows users to write (and share) simple scripts
(using the Lua programming language[11],
Tasks we had in mind when creating the system include network
discovery, more sophisticated version detection, vulnerability
detection. NSE can even be used for vulnerability exploitation.
To reflect those different uses and to simplify the choice of which
scripts to run, each script contains a field associating it with one or
more categories. Currently defined categories are safe, intrusive,
malware, version, discovery, vuln, auth, and default. These are all
described at http://nmap.org/book/nse-usage.html#nse-categories.
Scripts are not run in a sandbox and thus could accidentally or
maliciously damage your system or invade your privacy. Never run
scripts from third parties unless you trust the authors or have
carefully audited the scripts yourself.
The Nmap Scripting Engine is described in detail at
http://nmap.org/book/nse.html
and is controlled by the following options:
-sC .
Performs a script scan using the default set of scripts. It is
equivalent to --script=default. Some of the scripts in this
category are considered intrusive and should not be run against a
target network without permission.
--script filename|category|directory|expression|all[,...] .
Runs a script scan using the comma-separated list of filenames,
script categories, and directories. Each element in the list may
also be a Boolean expression describing a more complex set of
scripts. Each element is interpreted first as an expression, then
as a category, and finally as a file or directory name. The special
argument all makes every script in Nmap's script database eligible
to run. The all argument should be used with caution as NSE may
contain dangerous scripts including exploits, brute force
authentication crackers, and denial of service attacks.
File and directory names may be relative or absolute. Absolute
names are used directly. Relative paths are looked for in the
following places until found:
--datadir
$NMAPDIR
~/.nmap (not searched on Windows)
NMAPDATADIR
the current directory
A scripts subdirectory is also tried in each of these.
When a directory name is given, Nmap loads every file in the
directory whose name ends with .nse. All other files are ignored
and directories are not searched recursively. When a filename is
given, it does not have to have the .nse extension; it will be
added automatically if necessary. Nmap scripts are stored in a
scripts subdirectory of the Nmap data directory by default (see
http://nmap.org/book/data-files.html).
For efficiency, scripts are indexed in a database stored in
scripts/script.db,. which lists the category or categories in
which each script belongs. When referring to scripts from
script.db by name, you can use a shell-style '*' wildcard.
nmap --script "http-*"
Loads all scripts whose name starts with http-, such as
http-auth.nse and http-open-proxy.nse. The argument to --script
had to be in quotes to protect the wildcard from the shell.
More complicated script selection can be done using the and, or,
and not operators to build Boolean expressions. The operators have
the same precedence[12] as in Lua: not is the highest, followed by
and and then or. You can alter precedence by using parentheses.
Because expressions contain space characters it is necessary to
quote them.
nmap --script "not intrusive"
Loads every script except for those in the intrusive category.
nmap --script "default or safe"
This is functionally equivalent to nmap --script
"default,safe". It loads all scripts that are in the default
category or the safe category or both.
nmap --script "default and safe"
Loads those scripts that are in both the default and safe
categories.
nmap --script "(default or safe or intrusive) and not http-*"
Loads scripts in the default, safe, or intrusive categories,
except for those whose names start with http-.
--script-args name1=value1,name2={name3=value3},name4={value4,value5} .
Lets you provide arguments to NSE scripts. Arguments are a
comma-separated list of name=value pairs. Names and values may be
strings not containing whitespace or the characters '{', '}', '=',
or ','. To include one of these characters in a string, enclose the
string in single or double quotes. Within a quoted string, '\'
escapes a quote. A backslash is only used to escape quotation marks
in this special case; in all other cases a backslash is interpreted
literally. Values may also be tables enclosed in {}, just as in
Lua. A table may contain simple string values or more name-value
pairs, including nested tables. An example of script arguments:
--script-args
auth={user=foo,pass=',{}=bar'},userdb=C:\Path\To\File. The online
NSE Documentation Portal at http://nmap.org/nsedoc/ lists the
arguments that each script accepts.
--script-trace .
This option does what --packet-trace does, just one ISO layer
higher. If this option is specified all incoming and outgoing
communication performed by a script is printed. The displayed
information includes the communication protocol, the source, the
target and the transmitted data. If more than 5% of all transmitted
data is not printable, then the trace output is in a hex dump
format. Specifying --packet-trace enables script tracing too.
--script-updatedb .
This option updates the script database found in scripts/script.db
which is used by Nmap to determine the available default scripts
and categories. It is only necessary to update the database if you
have added or removed NSE scripts from the default scripts
directory or if you have changed the categories of any script. This
option is generally used by itself: nmap --script-updatedb.
TIMING AND PERFORMANCE
One of my highest Nmap development priorities has always been
performance. A default scan (nmap hostname) of a host on my local
network takes a fifth of a second. That is barely enough time to blink,
but adds up when you are scanning hundreds or thousands of hosts.
Moreover, certain scan options such as UDP scanning and version
detection can increase scan times substantially. So can certain
firewall configurations, particularly response rate limiting. While
Nmap utilizes parallelism and many advanced algorithms to accelerate
these scans, the user has ultimate control over how Nmap runs. Expert
users carefully craft Nmap commands to obtain only the information they
care about while meeting their time constraints.
Techniques for improving scan times include omitting non-critical
tests, and upgrading to the latest version of Nmap (performance
enhancements are made frequently). Optimizing timing parameters can
also make a substantial difference. Those options are listed below.
Some options accept a time parameter. This is specified in milliseconds
by default, though you can append 's', 'm', or 'h' to the value to
specify seconds, minutes, or hours. So the --host-timeout arguments
900000, 900s, and 15m all do the same thing.
--min-hostgroup numhosts; --max-hostgroup numhosts (Adjust parallel
scan group sizes) .
Nmap has the ability to port scan or version scan multiple hosts in
parallel. Nmap does this by dividing the target IP space into
groups and then scanning one group at a time. In general, larger
groups are more efficient. The downside is that host results can't
be provided until the whole group is finished. So if Nmap started
out with a group size of 50, the user would not receive any reports
(except for the updates offered in verbose mode) until the first 50
hosts are completed.
By default, Nmap takes a compromise approach to this conflict. It
starts out with a group size as low as five so the first results
come quickly and then increases the groupsize to as high as 1024.
The exact default numbers depend on the options given. For
efficiency reasons, Nmap uses larger group sizes for UDP or
few-port TCP scans.
When a maximum group size is specified with --max-hostgroup, Nmap
will never exceed that size. Specify a minimum size with
--min-hostgroup and Nmap will try to keep group sizes above that
level. Nmap may have to use smaller groups than you specify if
there are not enough target hosts left on a given interface to
fulfill the specified minimum. Both may be set to keep the group
size within a specific range, though this is rarely desired.
These options do not have an effect during the host discovery phase
of a scan. This includes plain ping scans (-sP). Host discovery
always works in large groups of hosts to improve speed and
accuracy.
The primary use of these options is to specify a large minimum
group size so that the full scan runs more quickly. A common choice
is 256 to scan a network in Class C sized chunks. For a scan with
many ports, exceeding that number is unlikely to help much. For
scans of just a few port numbers, host group sizes of 2048 or more
may be helpful.
--min-parallelism numprobes; --max-parallelism numprobes (Adjust probe
parallelization) .
These options control the total number of probes that may be
outstanding for a host group. They are used for port scanning and
host discovery. By default, Nmap calculates an ever-changing ideal
parallelism based on network performance. If packets are being
dropped, Nmap slows down and allows fewer outstanding probes. The
ideal probe number slowly rises as the network proves itself
worthy. These options place minimum or maximum bounds on that
variable. By default, the ideal parallelism can drop to one if the
network proves unreliable and rise to several hundred in perfect
conditions.
The most common usage is to set --min-parallelism to a number
higher than one to speed up scans of poorly performing hosts or
networks. This is a risky option to play with, as setting it too
high may affect accuracy. Setting this also reduces Nmap's ability
to control parallelism dynamically based on network conditions. A
value of ten might be reasonable, though I only adjust this value
as a last resort.
The --max-parallelism option is sometimes set to one to prevent
Nmap from sending more than one probe at a time to hosts. The
--scan-delay option, discussed later, is another way to do this.
--min-rtt-timeout time, --max-rtt-timeout time, --initial-rtt-timeout
time (Adjust probe timeouts) .
Nmap maintains a running timeout value for determining how long it
will wait for a probe response before giving up or retransmitting
the probe. This is calculated based on the response times of
previous probes.
If the network latency shows itself to be significant and variable,
this timeout can grow to several seconds. It also starts at a
conservative (high) level and may stay that way for a while when
Nmap scans unresponsive hosts.
Specifying a lower --max-rtt-timeout and --initial-rtt-timeout than
the defaults can cut scan times significantly. This is particularly
true for pingless (-PN) scans, and those against heavily filtered
networks. Don't get too aggressive though. The scan can end up
taking longer if you specify such a low value that many probes are
timing out and retransmitting while the response is in transit.
If all the hosts are on a local network, 100 milliseconds is a
reasonable aggressive --max-rtt-timeout value. If routing is
involved, ping a host on the network first with the ICMP ping
utility, or with a custom packet crafter such as hping2. that is
more likely to get through a firewall. Look at the maximum round
trip time out of ten packets or so. You might want to double that
for the --initial-rtt-timeout and triple or quadruple it for the
--max-rtt-timeout. I generally do not set the maximum RTT below
100 ms, no matter what the ping times are. Nor do I exceed 1000 ms.
--min-rtt-timeout is a rarely used option that could be useful when
a network is so unreliable that even Nmap's default is too
aggressive. Since Nmap only reduces the timeout down to the minimum
when the network seems to be reliable, this need is unusual and
should be reported as a bug to the nmap-dev mailing list..
--max-retries numtries (Specify the maximum number of port scan probe
retransmissions) .
When Nmap receives no response to a port scan probe, it could mean
the port is filtered. Or maybe the probe or response was simply
lost on the network. It is also possible that the target host has
rate limiting enabled that temporarily blocked the response. So
Nmap tries again by retransmitting the initial probe. If Nmap
detects poor network reliability, it may try many more times before
giving up on a port. While this benefits accuracy, it also lengthen
scan times. When performance is critical, scans may be sped up by
limiting the number of retransmissions allowed. You can even
specify --max-retries 0 to prevent any retransmissions, though that
is only recommended for situations such as informal surveys where
occasional missed ports and hosts are acceptable.
The default (with no -T template) is to allow ten retransmissions.
If a network seems reliable and the target hosts aren't rate
limiting, Nmap usually only does one retransmission. So most target
scans aren't even affected by dropping --max-retries to a low value
such as three. Such values can substantially speed scans of slow
(rate limited) hosts. You usually lose some information when Nmap
gives up on ports early, though that may be preferable to letting
the --host-timeout expire and losing all information about the
target.
--host-timeout time (Give up on slow target hosts) .
Some hosts simply take a long time to scan. This may be due to
poorly performing or unreliable networking hardware or software,
packet rate limiting, or a restrictive firewall. The slowest few
percent of the scanned hosts can eat up a majority of the scan
time. Sometimes it is best to cut your losses and skip those hosts
initially. Specify --host-timeout with the maximum amount of time
you are willing to wait. For example, specify 30m to ensure that
Nmap doesn't waste more than half an hour on a single host. Note
that Nmap may be scanning other hosts at the same time during that
half an hour, so it isn't a complete loss. A host that times out is
skipped. No port table, OS detection, or version detection results
are printed for that host.
--scan-delay time; --max-scan-delay time (Adjust delay between probes)
.
This option causes Nmap to wait at least the given amount of time
between each probe it sends to a given host. This is particularly
useful in the case of rate limiting.. Solaris machines (among many
others) will usually respond to UDP scan probe packets with only
one ICMP message per second. Any more than that sent by Nmap will
be wasteful. A --scan-delay of 1s will keep Nmap at that slow rate.
Nmap tries to detect rate limiting and adjust the scan delay
accordingly, but it doesn't hurt to specify it explicitly if you
already know what rate works best.
When Nmap adjusts the scan delay upward to cope with rate limiting,
the scan slows down dramatically. The --max-scan-delay option
specifies the largest delay that Nmap will allow. A low
--max-scan-delay can speed up Nmap, but it is risky. Setting this
value too low can lead to wasteful packet retransmissions and
possible missed ports when the target implements strict rate
limiting.
Another use of --scan-delay is to evade threshold based intrusion
detection and prevention systems (IDS/IPS)..
--min-rate number; --max-rate number (Directly control the scanning
rate) .
Nmap's dynamic timing does a good job of finding an appropriate
speed at which to scan. Sometimes, however, you may happen to know
an appropriate scanning rate for a network, or you may have to
guarantee that a scan will be finished by a certain time. Or
perhaps you must keep Nmap from scanning too quickly. The
--min-rate and --max-rate options are designed for these
situations.
When the --min-rate option is given Nmap will do its best to send
packets as fast as or faster than the given rate. The argument is a
positive real number representing a packet rate in packets per
second. For example, specifying --min-rate 300 means that Nmap will
try to keep the sending rate at or above 300 packets per second.
Specifying a minimum rate does not keep Nmap from going faster if
conditions warrant.
Likewise, --max-rate limits a scan's sending rate to a given
maximum. Use --max-rate 100, for example, to limit sending to 100
packets per second on a fast network. Use --max-rate 0.1 for a slow
scan of one packet every ten seconds. Use --min-rate and --max-rate
together to keep the rate inside a certain range.
These two options are global, affecting an entire scan, not
individual hosts. They only affect port scans and host discovery
scans. Other features like OS detection implement their own timing.
There are two conditions when the actual scanning rate may fall
below the requested minimum. The first is if the minimum is faster
than the fastest rate at which Nmap can send, which is dependent on
hardware. In this case Nmap will simply send packets as fast as
possible, but be aware that such high rates are likely to cause a
loss of accuracy. The second case is when Nmap has nothing to send,
for example at the end of a scan when the last probes have been
sent and Nmap is waiting for them to time out or be responded to.
It's normal to see the scanning rate drop at the end of a scan or
in between hostgroups. The sending rate may temporarily exceed the
maximum to make up for unpredictable delays, but on average the
rate will stay at or below the maximum.
Specifying a minimum rate should be done with care. Scanning faster
than a network can support may lead to a loss of accuracy. In some
cases, using a faster rate can make a scan take longer than it
would with a slower rate. This is because Nmap's
adaptive retransmission algorithms will detect the network
congestion caused by an excessive scanning rate and increase the
number of retransmissions in order to improve accuracy. So even
though packets are sent at a higher rate, more packets are sent
overall. Cap the number of retransmissions with the --max-retries
option if you need to set an upper limit on total scan time.
--defeat-rst-ratelimit .
Many hosts have long used rate limiting. to reduce the number of
ICMP error messages (such as port-unreachable errors) they send.
Some systems now apply similar rate limits to the RST (reset)
packets they generate. This can slow Nmap down dramatically as it
adjusts its timing to reflect those rate limits. You can tell Nmap
to ignore those rate limits (for port scans such as SYN scan which
don't treat non-responsive ports as open) by specifying
--defeat-rst-ratelimit.
Using this option can reduce accuracy, as some ports will appear
non-responsive because Nmap didn't wait long enough for a
rate-limited RST response. With a SYN scan, the non-response
results in the port being labeled filtered rather than the closed
state we see when RST packets are received. This option is useful
when you only care about open ports, and distinguishing between
closed and filtered ports isn't worth the extra time.
-T paranoid|sneaky|polite|normal|aggressive|insane (Set a timing
template) .
While the fine-grained timing controls discussed in the previous
section are powerful and effective, some people find them
confusing. Moreover, choosing the appropriate values can sometimes
take more time than the scan you are trying to optimize. So Nmap
offers a simpler approach, with six timing templates. You can
specify them with the -T option and their number (0-5) or their
name. The template names are paranoid (0), sneaky (1), polite (2),
normal (3), aggressive (4), and insane (5). The first two are for
IDS evasion. Polite mode slows down the scan to use less bandwidth
and target machine resources. Normal mode is the default and so -T3
does nothing. Aggressive mode speeds scans up by making the
assumption that you are on a reasonably fast and reliable network.
Finally insane mode. assumes that you are on an extraordinarily
fast network or are willing to sacrifice some accuracy for speed.
These templates allow the user to specify how aggressive they wish
to be, while leaving Nmap to pick the exact timing values. The
templates also make some minor speed adjustments for which
fine-grained control options do not currently exist. For example,
-T4. prohibits the dynamic scan delay from exceeding 10 ms for TCP
ports and -T5 caps that value at 5 ms. Templates can be used in
combination with fine-grained controls, and the fine-grained
controls will you specify will take precedence over the timing
template default for that parameter. I recommend using -T4 when
scanning reasonably modern and reliable networks. Keep that option
even when you add fine-grained controls so that you benefit from
those extra minor optimizations that it enables.
If you are on a decent broadband or ethernet connection, I would
recommend always using -T4. Some people love -T5 though it is too
aggressive for my taste. People sometimes specify -T2 because they
think it is less likely to crash hosts or because they consider
themselves to be polite in general. They often don't realize just
how slow -T polite. really is. Their scan may take ten times
longer than a default scan. Machine crashes and bandwidth problems
are rare with the default timing options (-T3) and so I normally
recommend that for cautious scanners. Omitting version detection is
far more effective than playing with timing values at reducing
these problems.
While -T0. and -T1. may be useful for avoiding IDS alerts, they
will take an extraordinarily long time to scan thousands of
machines or ports. For such a long scan, you may prefer to set the
exact timing values you need rather than rely on the canned -T0 and
-T1 values.
The main effects of T0 are serializing the scan so only one port is
scanned at a time, and waiting five minutes between sending each
probe. T1 and T2 are similar but they only wait 15 seconds and 0.4
seconds, respectively, between probes. T3 is Nmap's default
behavior, which includes parallelization.. -T4 does the equivalent
of --max-rtt-timeout 1250 --initial-rtt-timeout 500 --max-retries 6
and sets the maximum TCP scan delay to 10 milliseconds. T5 does
the equivalent of --max-rtt-timeout 300 --min-rtt-timeout 50
--initial-rtt-timeout 250 --max-retries 2 --host-timeout 15m as
well as setting the maximum TCP scan delay to 5 ms.
FIREWALL/IDS EVASION AND SPOOFING
Many Internet pioneers envisioned a global open network with a
universal IP address space allowing virtual connections between any two
nodes. This allows hosts to act as true peers, serving and retrieving
information from each other. People could access all of their home
systems from work, changing the climate control settings or unlocking
the doors for early guests. This vision of universal connectivity has
been stifled by address space shortages and security concerns. In the
early 1990s, organizations began deploying firewalls for the express
purpose of reducing connectivity. Huge networks were cordoned off from
the unfiltered Internet by application proxies, network address
translation, and packet filters. The unrestricted flow of information
gave way to tight regulation of approved communication channels and the
content that passes over them.
Network obstructions such as firewalls can make mapping a network
exceedingly difficult. It will not get any easier, as stifling casual
reconnaissance is often a key goal of implementing the devices.
Nevertheless, Nmap offers many features to help understand these
complex networks, and to verify that filters are working as intended.
It even supports mechanisms for bypassing poorly implemented defenses.
One of the best methods of understanding your network security posture
is to try to defeat it. Place yourself in the mind-set of an attacker,
and deploy techniques from this section against your networks. Launch
an FTP bounce scan, idle scan, fragmentation attack, or try to tunnel
through one of your own proxies.
In addition to restricting network activity, companies are increasingly
monitoring traffic with intrusion detection systems (IDS). All of the
major IDSs ship with rules designed to detect Nmap scans because scans
are sometimes a precursor to attacks. Many of these products have
recently morphed into intrusion prevention systems (IPS). that
actively block traffic deemed malicious. Unfortunately for network
administrators and IDS vendors, reliably detecting bad intentions by
analyzing packet data is a tough problem. Attackers with patience,
skill, and the help of certain Nmap options can usually pass by IDSs
undetected. Meanwhile, administrators must cope with large numbers of
false positive results where innocent activity is misdiagnosed and
alerted on or blocked.
Occasionally people suggest that Nmap should not offer features for
evading firewall rules or sneaking past IDSs. They argue that these
features are just as likely to be misused by attackers as used by
administrators to enhance security. The problem with this logic is that
these methods would still be used by attackers, who would just find
other tools or patch the functionality into Nmap. Meanwhile,
administrators would find it that much harder to do their jobs.
Deploying only modern, patched FTP servers is a far more powerful
defense than trying to prevent the distribution of tools implementing
the FTP bounce attack.
There is no magic bullet (or Nmap option) for detecting and subverting
firewalls and IDS systems. It takes skill and experience. A tutorial is
beyond the scope of this reference guide, which only lists the relevant
options and describes what they do.
-f (fragment packets); --mtu (using the specified MTU) .
The -f option causes the requested scan (including ping scans) to
use tiny fragmented IP packets. The idea is to split up the TCP
header over several packets to make it harder for packet filters,
intrusion detection systems, and other annoyances to detect what
you are doing. Be careful with this! Some programs have trouble
handling these tiny packets. The old-school sniffer named Sniffit
segmentation faulted immediately upon receiving the first fragment.
Specify this option once, and Nmap splits the packets into eight
bytes or less after the IP header. So a 20-byte TCP header would be
split into three packets. Two with eight bytes of the TCP header,
and one with the final four. Of course each fragment also has an IP
header. Specify -f again to use 16 bytes per fragment (reducing the
number of fragments).. Or you can specify your own offset size
with the --mtu option. Don't also specify -f if you use --mtu. The
offset must be a multiple of eight. While fragmented packets won't
get by packet filters and firewalls that queue all IP fragments,
such as the CONFIG_IP_ALWAYS_DEFRAG option in the Linux kernel,
some networks can't afford the performance hit this causes and thus
leave it disabled. Others can't enable this because fragments may
take different routes into their networks. Some source systems
defragment outgoing packets in the kernel. Linux with the iptables.
connection tracking module is one such example. Do a scan while a
sniffer such as Wireshark. is running to ensure that sent packets
are fragmented. If your host OS is causing problems, try the
--send-eth. option to bypass the IP layer and send raw ethernet
frames.
Fragmentation is only supported for Nmap's raw packet features,
which includes TCP and UDP port scans (except connect scan and FTP
bounce scan) and OS detection. Features such as version detection
and the Nmap Scripting Engine generally don't support fragmentation
because they rely on your host's TCP stack to communicate with
target services.
-D decoy1[,decoy2][,ME][,...] (Cloak a scan with decoys) .
Causes a decoy scan to be performed, which makes it appear to the
remote host that the host(s) you specify as decoys are scanning the
target network too. Thus their IDS might report 5-10 port scans
from unique IP addresses, but they won't know which IP was scanning
them and which were innocent decoys. While this can be defeated
through router path tracing, response-dropping, and other active
mechanisms, it is generally an effective technique for hiding your
IP address.
Separate each decoy host with commas, and you can optionally use
ME. as one of the decoys to represent the position for your real
IP address. If you put ME in the sixth position or later, some
common port scan detectors (such as Solar Designer's. excellent
Scanlogd). are unlikely to show your IP address at all. If you
don't use ME, Nmap will put you in a random position. You can also
use RND. to generate a random, non-reserved IP address, or
RND:number to generate number addresses.
Note that the hosts you use as decoys should be up or you might
accidentally SYN flood your targets. Also it will be pretty easy to
determine which host is scanning if only one is actually up on the
network. You might want to use IP addresses instead of names (so
the decoy networks don't see you in their nameserver logs).
Decoys are used both in the initial ping scan (using ICMP, SYN,
ACK, or whatever) and during the actual port scanning phase. Decoys
are also used during remote OS detection (-O). Decoys do not work
with version detection or TCP connect scan. When a scan delay is in
effect, the delay is enforced between each batch of spoofed probes,
not between each individual probe. Because decoys are sent as a
batch all at once, they may temporarily violate congestion control
limits.
It is worth noting that using too many decoys may slow your scan
and potentially even make it less accurate. Also, some ISPs will
filter out your spoofed packets, but many do not restrict spoofed
IP packets at all.
-S IP_Address (Spoof source address) .
In some circumstances, Nmap may not be able to determine your
source address (Nmap will tell you if this is the case). In this
situation, use -S with the IP address of the interface you wish to
send packets through.
Another possible use of this flag is to spoof the scan to make the
targets think that someone else is scanning them. Imagine a company
being repeatedly port scanned by a competitor! The -e option and
-PN are generally required for this sort of usage. Note that you
usually won't receive reply packets back (they will be addressed to
the IP you are spoofing), so Nmap won't produce useful reports.
-e interface (Use specified interface) .
Tells Nmap what interface to send and receive packets on. Nmap
should be able to detect this automatically, but it will tell you
if it cannot.
--source-port portnumber; -g portnumber (Spoof source port number) .
One surprisingly common misconfiguration is to trust traffic based
only on the source port number. It is easy to understand how this
comes about. An administrator will set up a shiny new firewall,
only to be flooded with complains from ungrateful users whose
applications stopped working. In particular, DNS may be broken
because the UDP DNS replies from external servers can no longer
enter the network. FTP is another common example. In active FTP
transfers, the remote server tries to establish a connection back
to the client to transfer the requested file.
Secure solutions to these problems exist, often in the form of
application-level proxies or protocol-parsing firewall modules.
Unfortunately there are also easier, insecure solutions. Noting
that DNS replies come from port 53 and active FTP from port 20,
many administrators have fallen into the trap of simply allowing
incoming traffic from those ports. They often assume that no
attacker would notice and exploit such firewall holes. In other
cases, administrators consider this a short-term stop-gap measure
until they can implement a more secure solution. Then they forget
the security upgrade.
Overworked network administrators are not the only ones to fall
into this trap. Numerous products have shipped with these insecure
rules. Even Microsoft has been guilty. The IPsec filters that
shipped with Windows 2000 and Windows XP contain an implicit rule
that allows all TCP or UDP traffic from port 88 (Kerberos). In
another well-known case, versions of the Zone Alarm personal
firewall up to 2.1.25 allowed any incoming UDP packets with the
source port 53 (DNS) or 67 (DHCP).
Nmap offers the -g and --source-port options (they are equivalent)
to exploit these weaknesses. Simply provide a port number and Nmap
will send packets from that port where possible. Nmap must use
different port numbers for certain OS detection tests to work
properly, and DNS requests ignore the --source-port flag because
Nmap relies on system libraries to handle those. Most TCP scans,
including SYN scan, support the option completely, as does UDP
scan.
--data-length number (Append random data to sent packets) .
Normally Nmap sends minimalist packets containing only a header. So
its TCP packets are generally 40 bytes and ICMP echo requests are
just 28. Some UDP ports. and IP protocols. get a custom payload
by default. This option tells Nmap to append the given number of
random bytes to most of the packets it sends, and not to use any
protocol-specific payloads. (Use --data-length 0 for no random or
protocol-specific payloads.. OS detection (-O) packets are not
affected. because accuracy there requires probe consistency, but
most pinging and portscan packets support this. It slows things
down a little, but can make a scan slightly less conspicuous.
--ip-options S|R [route]|L [route]|T|U ... ; --ip-options hex string
(Send packets with specified ip options) .
The IP protocol[13] offers several options which may be placed in
packet headers. Unlike the ubiquitous TCP options, IP options are
rarely seen due to practicality and security concerns. In fact,
many Internet routers block the most dangerous options such as
source routing. Yet options can still be useful in some cases for
determining and manipulating the network route to target machines.
For example, you may be able to use the record route option to
determine a path to a target even when more traditional
traceroute-style approaches fail. Or if your packets are being
dropped by a certain firewall, you may be able to specify a
different route with the strict or loose source routing options.
The most powerful way to specify IP options is to simply pass in
values as the argument to --ip-options. Precede each hex number
with \x then the two digits. You may repeat certain characters by
following them with an asterisk and then the number of times you
wish them to repeat. For example, \x01\x07\x04\x00*36\x01 is a hex
string containing 36 NUL bytes.
Nmap also offers a shortcut mechanism for specifying options.
Simply pass the letter R, T, or U to request record-route,.
record-timestamp,. or both options together, respectively. Loose
or strict source routing. may be specified with an L or S followed
by a space and then a space-separated list of IP addresses.
If you wish to see the options in packets sent and received,
specify --packet-trace. For more information and examples of using
IP options with Nmap, see
http://seclists.org/nmap-dev/2006/q3/00.html.
--ttl value (Set IP time-to-live field) .
Sets the IPv4 time-to-live field in sent packets to the given
value.
--randomize-hosts (Randomize target host order) .
Tells Nmap to shuffle each group of up to 16384 hosts before it
scans them. This can make the scans less obvious to various network
monitoring systems, especially when you combine it with slow timing
options. If you want to randomize over larger group sizes, increase
PING_GROUP_SZ. in nmap.h. and recompile. An alternative solution
is to generate the target IP list with a list scan (-sL -n -oN
filename), randomize it with a Perl script, then provide the whole
list to Nmap with -iL..
--spoof-mac MAC address, prefix, or vendor name (Spoof MAC address) .
Asks Nmap to use the given MAC address for all of the raw ethernet
frames it sends. This option implies --send-eth. to ensure that
Nmap actually sends ethernet-level packets. The MAC given can take
several formats. If it is simply the number 0, Nmap chooses a
completely random MAC address for the session. If the given string
is an even number of hex digits (with the pairs optionally
separated by a colon), Nmap will use those as the MAC. If fewer
than 12 hex digits are provided, Nmap fills in the remainder of the
six bytes with random values. If the argument isn't a zero or hex
string, Nmap looks through nmap-mac-prefixes to find a vendor name
containing the given string (it is case insensitive). If a match is
found, Nmap uses the vendor's OUI (three-byte prefix). and fills
out the remaining three bytes randomly. Valid --spoof-mac argument
examples are Apple, 0, 01:02:03:04:05:06, deadbeefcafe, 0020F2, and
Cisco. This option only affects raw packet scans such as SYN scan
or OS detection, not connection-oriented features such as version
detection or the Nmap Scripting Engine.
--badsum (Send packets with bogus TCP/UDP checksums) .
Asks Nmap to use an invalid TCP, UDP or SCTP checksum for packets
sent to target hosts. Since virtually all host IP stacks properly
drop these packets, any responses received are likely coming from a
firewall or IDS that didn't bother to verify the checksum. For more
details on this technique, see http://nmap.org/p60-.html
--adler32 (Use deprecated Adler32 instead of CRC32C for SCTP checksums)
.
Asks Nmap to use the deprecated Adler32 algorithm for calculating
the SCTP checksum. If --adler32 is not given, CRC-32C (Castagnoli)
is used. RFC 2960[14] originally defined Adler32 as checksum
algorithm for SCTP; RFC 4960[7] later redefined the SCTP checksums
to use CRC-32C. Current SCTP implementations should be using
CRC-32C, but in order to elicit responses from old, legacy SCTP
implementations, it may be preferrable to use Adler32.
OUTPUT
Any security tool is only as useful as the output it generates. Complex
tests and algorithms are of little value if they aren't presented in an
organized and comprehensible fashion. Given the number of ways Nmap is
used by people and other software, no single format can please
everyone. So Nmap offers several formats, including the interactive
mode for humans to read directly and XML for easy parsing by software.
In addition to offering different output formats, Nmap provides options
for controlling the verbosity of output as well as debugging messages.
Output types may be sent to standard output or to named files, which
Nmap can append to or clobber. Output files may also be used to resume
aborted scans.
Nmap makes output available in five different formats. The default is
called interactive output,. and it is sent to standard output
(stdout).. There is also normal output,. which is similar to
interactive except that it displays less runtime information and
warnings since it is expected to be analyzed after the scan completes
rather than interactively.
XML output. is one of the most important output types, as it can be
converted to HTML, easily parsed by programs such as Nmap graphical
user interfaces, or imported into databases.
The two remaining output types are the simple grepable output. which
includes most information for a target host on a single line, and
sCRiPt KiDDi3 0utPUt. for users who consider themselves |<-r4d.
While interactive output is the default and has no associated
command-line options, the other four format options use the same
syntax. They take one argument, which is the filename that results
should be stored in. Multiple formats may be specified, but each format
may only be specified once. For example, you may wish to save normal
output for your own review while saving XML of the same scan for
programmatic analysis. You might do this with the options -oX
myscan.xml -oN myscan.nmap. While this chapter uses the simple names
like myscan.xml for brevity, more descriptive names are generally
recommended. The names chosen are a matter of personal preference,
though I use long ones that incorporate the scan date and a word or two
describing the scan, placed in a directory named after the company I'm
scanning.
While these options save results to files, Nmap still prints
interactive output to stdout as usual. For example, the command nmap
-oX myscan.xml target prints XML to myscan.xml and fills standard
output with the same interactive results it would have printed if -oX
wasn't specified at all. You can change this by passing a hyphen
character as the argument to one of the format types. This causes Nmap
to deactivate interactive output, and instead print results in the
format you specified to the standard output stream. So the command nmap
-oX - target will send only XML output to stdout.. Serious errors may
still be printed to the normal error stream, stderr..
Unlike some Nmap arguments, the space between the logfile option flag
(such as -oX) and the filename or hyphen is mandatory. If you omit the
flags and give arguments such as -oG- or -oXscan.xml, a backwards
compatibility feature of Nmap will cause the creation of normal format
output files named G- and Xscan.xml respectively.
All of these arguments support strftime-like. conversions in the
filename. %H, %M, %S, %m, %d, %y, and %Y are all exactly the same as
in strftime. %T is the same as %H%M%S, %R is the same as %H%M, and %D
is the same as %m%d%y. A % followed by any other character just yields
that character (%% gives you a percent symbol). So -oX 'scan-%T-%D.xml'
will use an XML file in the form of scan-144840-121307.xml.
Nmap also offers options to control scan verbosity and to append to
output files rather than clobbering them. All of these options are
described below.
Nmap Output Formats
-oN filespec (normal output) .
Requests that normal output be directed to the given filename. As
discussed above, this differs slightly from interactive output.
-oX filespec (XML output) .
Requests that XML output be directed to the given filename. Nmap
includes a document type definition (DTD) which allows XML parsers
to validate Nmap XML output. While it is primarily intended for
programmatic use, it can also help humans interpret Nmap XML
output. The DTD defines the legal elements of the format, and often
enumerates the attributes and values they can take on. The latest
version is always available from http://nmap.org/data/nmap.dtd.
XML offers a stable format that is easily parsed by software. Free
XML parsers are available for all major computer languages,
including C/C++, Perl, Python, and Java. People have even written
bindings for most of these languages to handle Nmap output and
execution specifically. Examples are Nmap::Scanner[15] and
Nmap::Parser[16] in Perl CPAN. In almost all cases that a
non-trivial application interfaces with Nmap, XML is the preferred
format.
The XML output references an XSL stylesheet which can be used to
format the results as HTML. The easiest way to use this is simply
to load the XML output in a web browser such as Firefox or IE. By
default, this will only work on the machine you ran Nmap on (or a
similarly configured one) due to the hard-coded nmap.xsl filesystem
path. Use the --webxml or --stylesheet options to create portable
XML files that render as HTML on any web-connected machine.
-oS filespec (ScRipT KIdd|3 oUTpuT) .
Script kiddie output is like interactive output, except that it is
post-processed to better suit the l33t HaXXorZ who previously
looked down on Nmap due to its consistent capitalization and
spelling. Humor impaired people should note that this option is
making fun of the script kiddies before flaming me for supposedly
"helping them".
-oG filespec (grepable output) .
This output format is covered last because it is deprecated. The
XML output format is far more powerful, and is nearly as convenient
for experienced users. XML is a standard for which dozens of
excellent parsers are available, while grepable output is my own
simple hack. XML is extensible to support new Nmap features as they
are released, while I often must omit those features from grepable
output for lack of a place to put them.
Nevertheless, grepable output is still quite popular. It is a
simple format that lists each host on one line and can be trivially
searched and parsed with standard Unix tools such as grep, awk,
cut, sed, diff, and Perl. Even I usually use it for one-off tests
done at the command line. Finding all the hosts with the SSH port
open or that are running Solaris takes only a simple grep to
identify the hosts, piped to an awk or cut command to print the
desired fields.
Grepable output consists of comments (lines starting with a pound
(#)). and target lines. A target line includes a combination of
six labeled fields, separated by tabs and followed with a colon.
The fields are Host, Ports, Protocols, Ignored State, OS, Seq
Index, IP ID, and Status.
The most important of these fields is generally Ports, which gives
details on each interesting port. It is a comma separated list of
port entries. Each port entry represents one interesting port, and
takes the form of seven slash (/) separated subfields. Those
subfields are: Port number, State, Protocol, Owner, Service, SunRPC
info, and Version info.
As with XML output, this man page does not allow for documenting
the entire format. A more detailed look at the Nmap grepable output
format is available from
http://nmap.org/book/output-formats-grepable-output.html.
-oA basename (Output to all formats) .
As a convenience, you may specify -oA basename to store scan
results in normal, XML, and grepable formats at once. They are
stored in basename.nmap, basename.xml, and basename.gnmap,
respectively. As with most programs, you can prefix the filenames
with a directory path, such as ~/nmaplogs/foocorp/ on Unix or
c:\hacking\sco on Windows.
Verbosity and debugging options
-v (Increase verbosity level) .
Increases the verbosity level, causing Nmap to print more
information about the scan in progress. Open ports are shown as
they are found and completion time estimates are provided when Nmap
thinks a scan will take more than a few minutes. Use it twice or
more for even greater verbosity.
Most changes only affect interactive output, and some also affect
normal and script kiddie output. The other output types are meant
to be processed by machines, so Nmap can give substantial detail by
default in those formats without fatiguing a human user. However,
there are a few changes in other modes where output size can be
reduced substantially by omitting some detail. For example, a
comment line in the grepable output that provides a list of all
ports scanned is only printed in verbose mode because it can be
quite long.
-d [level] (Increase or set debugging level) .
When even verbose mode doesn't provide sufficient data for you,
debugging is available to flood you with much more! As with the
verbosity option (-v), debugging is enabled with a command-line
flag (-d) and the debug level can be increased by specifying it
multiple times.. Alternatively, you can set a debug level by
giving an argument to -d. For example, -d9 sets level nine. That is
the highest effective level and will produce thousands of lines
unless you run a very simple scan with very few ports and targets.
Debugging output is useful when a bug is suspected in Nmap, or if
you are simply confused as to what Nmap is doing and why. As this
feature is mostly intended for developers, debug lines aren't
always self-explanatory. You may get something like: Timeout vals:
srtt: -1 rttvar: -1 to: 1000000 delta 14987 ==> srtt: 14987 rttvar:
14987 to: 100000. If you don't understand a line, your only
recourses are to ignore it, look it up in the source code, or
request help from the development list (nmap-dev).. Some lines are
self explanatory, but the messages become more obscure as the debug
level is increased.
--reason (Host and port state reasons) .
Shows the reason each port is set to a specific state and the
reason each host is up or down. This option displays the type of
the packet that determined a port or hosts state. For example, A
RST packet from a closed port or an echo reply from an alive host.
The information Nmap can provide is determined by the type of scan
or ping. The SYN scan and SYN ping (-sS and -PS) are very detailed,
but the TCP connect scan (-sT) is limited by the implementation of
the connect system call. This feature is automatically enabled by
the debug option (-d). and the results are stored in XML log files
even if this option is not specified.
--stats-every time (Print periodic timing stats) .
Periodically prints a timing status message after each interval of
time. The time is a specification of the kind described in the
section called "TIMING AND PERFORMANCE"; so for example, use
--stats-every 10s to get a status update every 10 seconds. Updates
are printed to interactive output (the screen) and XML output.
--packet-trace (Trace packets and data sent and received) .
Causes Nmap to print a summary of every packet sent or received.
This is often used for debugging, but is also a valuable way for
new users to understand exactly what Nmap is doing under the
covers. To avoid printing thousands of lines, you may want to
specify a limited number of ports to scan, such as -p20-30. If you
only care about the goings on of the version detection subsystem,
use --version-trace instead. If you only care about script tracing,
specify --script-trace. With --packet-trace, you get all of the
above.
--open (Show only open (or possibly open) ports) .
Sometimes you only care about ports you can actually connect to
(open ones), and don't want results cluttered with closed,
filtered, and closed|filtered ports. Output customization is
normally done after the scan using tools such as grep, awk, and
Perl, but this feature was added due to overwhelming requests.
Specify --open to only see open, open|filtered, and unfiltered
ports. These three ports are treated just as they normally are,
which means that open|filtered and unfiltered may be condensed into
counts if there are an overwhelming number of them.
--iflist (List interfaces and routes) .
Prints the interface list and system routes as detected by Nmap.
This is useful for debugging routing problems or device
mischaracterization (such as Nmap treating a PPP connection as
ethernet).
--log-errors (Log errors/warnings to normal mode output file) .
Warnings and errors printed by Nmap usually go only to the screen
(interactive output), leaving any normal-format output files
(usually specified with -oN) uncluttered. When you do want to see
those messages in the normal output file you specified, add this
option. It is useful when you aren't watching the interactive
output or when you want to record errors while debugging a problem.
The error and warning messages will still appear in interactive
mode too. This won't work for most errors related to bad
command-line arguments because Nmap may not have initialized its
output files yet. In addition, some Nmap error and warning messages
use a different system which does not yet support this option.
An alternative to --log-errors is redirecting interactive output
(including the standard error stream) to a file. Most Unix shells
make this approach easy, though it can be difficult on Windows.
Miscellaneous output options
--append-output (Append to rather than clobber output files) .
When you specify a filename to an output format flag such as -oX or
-oN, that file is overwritten by default. If you prefer to keep the
existing content of the file and append the new results, specify
the --append-output option. All output filenames specified in that
Nmap execution will then be appended to rather than clobbered. This
doesn't work well for XML (-oX) scan data as the resultant file
generally won't parse properly until you fix it up by hand.
--resume filename (Resume aborted scan) .
Some extensive Nmap runs take a very long time--on the order of
days. Such scans don't always run to completion. Restrictions may
prevent Nmap from being run during working hours, the network could
go down, the machine Nmap is running on might suffer a planned or
unplanned reboot, or Nmap itself could crash. The administrator
running Nmap could cancel it for any other reason as well, by
pressing ctrl-C. Restarting the whole scan from the beginning may
be undesirable. Fortunately, if normal (-oN) or grepable (-oG) logs
were kept, the user can ask Nmap to resume scanning with the target
it was working on when execution ceased. Simply specify the
--resume option and pass the normal/grepable output file as its
argument. No other arguments are permitted, as Nmap parses the
output file to use the same ones specified previously. Simply call
Nmap as nmap --resume logfilename. Nmap will append new results to
the data files specified in the previous execution. Resumption does
not support the XML output format because combining the two runs
into one valid XML file would be difficult.
--stylesheet path or URL (Set XSL stylesheet to transform XML output) .
Nmap ships with an XSL stylesheet named nmap.xsl for viewing or
translating XML output to HTML. The XML output includes an
xml-stylesheet directive which points to nmap.xml where it was
initially installed by Nmap (or in the current working directory on
Windows). Simply load Nmap's XML output in a modern web browser and
it should retrieve nmap.xsl from the filesystem and use it to
render results. If you wish to use a different stylesheet, specify
it as the argument to --stylesheet. You must pass the full pathname
or URL. One common invocation is --stylesheet
http://nmap.org/data/nmap.xsl. This tells a browser to load the
latest version of the stylesheet from Nmap.Org. The --webxml option
does the same thing with less typing and memorization. Loading the
XSL from Nmap.Org makes it easier to view results on a machine that
doesn't have Nmap (and thus nmap.xsl) installed. So the URL is
often more useful, but the local filesystem location of nmap.xsl is
used by default for privacy reasons.
--webxml (Load stylesheet from Nmap.Org) .
This convenience option is simply an alias for --stylesheet
http://nmap.org/data/nmap.xsl.
--no-stylesheet (Omit XSL stylesheet declaration from XML) .
Specify this option to prevent Nmap from associating any XSL
stylesheet with its XML output. The xml-stylesheet directive is
omitted.
MISCELLANEOUS OPTIONS
This section describes some important (and not-so-important) options
that don't really fit anywhere else.
-6 (Enable IPv6 scanning) .
Since 2002, Nmap has offered IPv6 support for its most popular
features. In particular, ping scanning (TCP-only), connect
scanning, and version detection all support IPv6. The command
syntax is the same as usual except that you also add the -6 option.
Of course, you must use IPv6 syntax if you specify an address
rather than a hostname. An address might look like
3ffe:7501:4819:2000:210:f3ff:fe03:14d0, so hostnames are
recommended. The output looks the same as usual, with the IPv6
address on the "interesting ports" line being the only IPv6 give
away.
While IPv6 hasn't exactly taken the world by storm, it gets
significant use in some (usually Asian) countries and most modern
operating systems support it. To use Nmap with IPv6, both the
source and target of your scan must be configured for IPv6. If your
ISP (like most of them) does not allocate IPv6 addresses to you,
free tunnel brokers are widely available and work fine with Nmap. I
use the free IPv6 tunnel broker. service at
http://www.tunnelbroker.net. Other tunnel brokers are listed at
Wikipedia[17]. 6to4 tunnels are another popular, free approach.
-A (Aggressive scan options) .
This option enables additional advanced and aggressive options. I
haven't decided exactly which it stands for yet. Presently this
enables OS detection (-O), version scanning (-sV), script scanning
(-sC) and traceroute (--traceroute). More features may be added in
the future. The point is to enable a comprehensive set of scan
options without people having to remember a large set of flags.
However, because script scanning with the default set is considered
intrusive, you should not use -A against target networks without
permission. This option only enables features, and not timing
options (such as -T4) or verbosity options (-v) that you might want
as well.
--datadir directoryname (Specify custom Nmap data file location) .
Nmap obtains some special data at runtime in files named
nmap-service-probes, nmap-services, nmap-protocols, nmap-rpc,
nmap-mac-prefixes, and nmap-os-db. If the location of any of these
files has been specified (using the --servicedb or --versiondb
options), that location is used for that file. After that, Nmap
searches these files in the directory specified with the --datadir
option (if any). Any files not found there, are searched for in the
directory specified by the NMAPDIR environmental variable.
~/.nmap. for real and effective UIDs (POSIX systems only) or
location of the Nmap executable (Win32 only), and then a
compiled-in location such as /usr/local/share/nmap or
/usr/share/nmap . As a last resort, Nmap will look in the current
directory.
--servicedb services file (Specify custom services file) .
Asks Nmap to use the specified services file rather than the
nmap-services data file that comes with Nmap. Using this option
also causes a fast scan (-F) to be used. See the description for
--datadir for more information on Nmap's data files.
--versiondb service probes file (Specify custom service probes file) .
Asks Nmap to use the specified service probes file rather than the
nmap-service-probes data file that comes with Nmap. See the
description for --datadir for more information on Nmap's data
files.
--send-eth (Use raw ethernet sending) .
Asks Nmap to send packets at the raw ethernet (data link) layer
rather than the higher IP (network) layer. By default, Nmap chooses
the one which is generally best for the platform it is running on.
Raw sockets (IP layer). are generally most efficient for Unix
machines, while ethernet frames are required for Windows operation
since Microsoft disabled raw socket support. Nmap still uses raw IP
packets on Unix despite this option when there is no other choice
(such as non-ethernet connections).
--send-ip (Send at raw IP level) .
Asks Nmap to send packets via raw IP sockets rather than sending
lower level ethernet frames. It is the complement to the --send-eth
option discussed previously.
--privileged (Assume that the user is fully privileged) .
Tells Nmap to simply assume that it is privileged enough to perform
raw socket sends, packet sniffing, and similar operations that
usually require root privileges. on Unix systems. By default Nmap
quits if such operations are requested but geteuid is not zero.
--privileged is useful with Linux kernel capabilities and similar
systems that may be configured to allow unprivileged users to
perform raw-packet scans. Be sure to provide this option flag
before any flags for options that require privileges (SYN scan, OS
detection, etc.). The NMAP_PRIVILEGED. environmental variable may
be set as an equivalent alternative to --privileged.
--unprivileged (Assume that the user lacks raw socket privileges) .
This option is the opposite of --privileged. It tells Nmap to treat
the user as lacking network raw socket and sniffing privileges.
This is useful for testing, debugging, or when the raw network
functionality of your operating system is somehow broken. The
NMAP_UNPRIVILEGED. environmental variable may be set as an
equivalent alternative to --unprivileged.
--release-memory (Release memory before quitting) .
This option is only useful for memory-leak debugging. It causes
Nmap to release allocated memory just before it quits so that
actual memory leaks are easier to spot. Normally Nmap skips this as
the OS does this anyway upon process termination.
--interactive (Start in interactive mode) .
Starts Nmap in interactive mode, which offers an interactive Nmap
prompt allowing easy launching of multiple scans (either
synchronously or in the background). This is useful for people who
scan from multi-user systems as they often want to test their
security without letting everyone else on the system know exactly
which systems they are scanning. Use --interactive to activate this
mode and then type h for help. This option is rarely used because
proper shells are usually more familiar and feature-complete. This
option includes a bang (!) operator for executing shell commands,
which is one of many reasons not to install Nmap setuid root..
-V; --version (Print version number) .
Prints the Nmap version number and exits.
-h; --help (Print help summary page) .
Prints a short help screen with the most common command flags.
Running Nmap without any arguments does the same thing.
RUNTIME INTERACTION
During the execution of Nmap, all key presses are captured. This allows
you to interact with the program without aborting and restarting it.
Certain special keys will change options, while any other keys will
print out a status message telling you about the scan. The convention
is that lowercase letters increase the amount of printing, and
uppercase letters decrease the printing. You may also press '?' for
help.
v / V
Increase / decrease the verbosity level
d / D
Increase / decrease the debugging Level
p / P
Turn on / off packet tracing
?
Print a runtime interaction help screen
Anything else
Print out a status message like this:
Stats: 0:00:08 elapsed; 111 hosts completed (5 up), 5 undergoing
Service Scan
Service scan Timing: About 28.00% done; ETC: 16:18 (0:00:15
remaining)
EXAMPLES
Here are some Nmap usage examples, from the simple and routine to a
little more complex and esoteric. Some actual IP addresses and domain
names are used to make things more concrete. In their place you should
substitute addresses/names from your own network.. While I don't think
port scanning other networks is or should be illegal, some network
administrators don't appreciate unsolicited scanning of their networks
and may complain. Getting permission first is the best approach.
For testing purposes, you have permission to scan the host
scanme.nmap.org. This permission only includes scanning via Nmap and
not testing exploits or denial of service attacks. To conserve
bandwidth, please do not initiate more than a dozen scans against that
host per day. If this free scanning target service is abused, it will
be taken down and Nmap will report Failed to resolve given hostname/IP:
scanme.nmap.org. These permissions also apply to the hosts
scanme2.nmap.org, scanme3.nmap.org, and so on, though those hosts do
not currently exist.
nmap -v scanme.nmap.org
This option scans all reserved TCP ports on the machine scanme.nmap.org
. The -v option enables verbose mode.
nmap -sS -O scanme.nmap.org/24
Launches a stealth SYN scan against each machine that is up out of the
256 IPs on "class C" sized network where Scanme resides. It also tries
to determine what operating system is running on each host that is up
and running. This requires root privileges because of the SYN scan and
OS detection.
nmap -sV -p 22,53,110,143,4564 198.116.0-255.1-127
Launches host enumeration and a TCP scan at the first half of each of
the 255 possible eight-bit subnets in the 198.116 class B address
space. This tests whether the systems run SSH, DNS, POP3, or IMAP on
their standard ports, or anything on port 4564. For any of these ports
found open, version detection is used to determine what application is
running.
nmap -v -iR 100000 -PN -p 80
Asks Nmap to choose 100,000 hosts at random and scan them for web
servers (port 80). Host enumeration is disabled with -PN since first
sending a couple probes to determine whether a host is up is wasteful
when you are only probing one port on each target host anyway.
nmap -PN -p80 -oX logs/pb-port80scan.xml -oG logs/pb-port80scan.gnmap
216.163.128.20/20
This scans 4096 IPs for any web servers (without pinging them) and
saves the output in grepable and XML formats.
NMAP BOOK
While this reference guide details all material Nmap options, it can't
fully demonstrate how to apply those features to quickly solve
real-world tasks. For that, we released Nmap Network Scanning: The
Official Nmap Project Guide to Network Discovery and Security Scanning.
Topics include subverting firewalls and intrusion detection systems,
optimizing Nmap performance, and automating common networking tasks
with the Nmap Scripting Engine. Hints and instructions are provided for
common Nmap tasks such as taking network inventory, penetration
testing, detecting rogue wireless access points, and quashing network
worm outbreaks. Examples and diagrams show actual communication on the
wire. More than half of the book is available free online. See
http://nmap.org/book for more information.
BUGS
Like its author, Nmap isn't perfect. But you can help make it better by
sending bug reports or even writing patches. If Nmap doesn't behave the
way you expect, first upgrade to the latest version available from
http://nmap.org. If the problem persists, do some research to determine
whether it has already been discovered and addressed. Try searching for
the error message on our search page at http://insecure.org/search.html
or at Google. Also try browsing the nmap-dev archives at
http://seclists.org/.. Read this full manual page as well. If nothing
comes of this, mail a bug report to nmap-dev@insecure.org. Please
include everything you have learned about the problem, as well as what
version of Nmap you are running and what operating system version it is
running on. Problem reports and Nmap usage questions sent to
nmap-dev@insecure.org are far more likely to be answered than those
sent to Fyodor directly. If you subscribe to the nmap-dev list before
posting, your message will bypass moderation and get through more
quickly. Subscribe at
http://cgi.insecure.org/mailman/listinfo/nmap-dev.
Code patches to fix bugs are even better than bug reports. Basic
instructions for creating patch files with your changes are available
at http://nmap.org/data/HACKING. Patches may be sent to nmap-dev
(recommended) or to Fyodor directly.
AUTHOR
Gordon "Fyodor" Lyon fyodor@insecure.org (http://insecure.org)
Hundreds of people have made valuable contributions to Nmap over the
years. These are detailed in the CHANGELOG. file which is distributed
with Nmap and also available from http://nmap.org/changelog.html.
LEGAL NOTICES
Nmap Copyright and Licensing
The Nmap Security Scanner is (C) 1996-2009 Insecure.Com LLC. Nmap is
also a registered trademark of Insecure.Com LLC. This program is free
software; you may redistribute and/or modify it under the terms of the
GNU General Public License as published by the Free Software
Foundation; Version 2 with the clarifications and exceptions described
below. This guarantees your right to use, modify, and redistribute this
software under certain conditions. If you wish to embed Nmap technology
into proprietary software, we sell alternative licenses (contact
sales@insecure.com). Dozens of software vendors already license Nmap
technology such as host discovery, port scanning, OS detection, and
version detection.
Note that the GPL places important restrictions on "derived works", yet
it does not provide a detailed definition of that term. To avoid
misunderstandings, we consider an application to constitute a
"derivative work" for the purpose of this license if it does any of the
following:
o Integrates source code from Nmap
o Reads or includes Nmap copyrighted data files, such as nmap-os-db
or nmap-service-probes.
o Executes Nmap and parses the results (as opposed to typical shell
or execution-menu apps, which simply display raw Nmap output and so
are not derivative works.)
o Integrates/includes/aggregates Nmap into a proprietary executable
installer, such as those produced by InstallShield.
o Links to a library or executes a program that does any of the
above.
The term "Nmap" should be taken to also include any portions or derived
works of Nmap. This list is not exclusive, but is meant to clarify our
interpretation of derived works with some common examples. Our
interpretation applies only to Nmap--we don't speak for other people's
GPL works.
If you have any questions about the GPL licensing restrictions on using
Nmap in non-GPL works, we would be happy to help. As mentioned above,
we also offer alternative license to integrate Nmap into proprietary
applications and appliances. These contracts have been sold to many
security vendors, and generally include a perpetual license as well as
providing for priority support and updates as well as helping to fund
the continued development of Nmap technology. Please email
sales@insecure.com for further information.
As a special exception to the GPL terms, Insecure.Com LLC grants
permission to link the code of this program with any version of the
OpenSSL library which is distributed under a license identical to that
listed in the included COPYING.OpenSSL file, and distribute linked
combinations including the two.. You must obey the GNU GPL in all
respects for all of the code used other than OpenSSL. If you modify
this file, you may extend this exception to your version of the file,
but you are not obligated to do so.
If you received these files with a written license agreement or
contract stating terms other than the terms above, then that
alternative license agreement takes precedence over these comments.
Creative Commons License for this Nmap Guide
This Nmap Reference Guide is (C) 2005-2009 Insecure.Com LLC. It is
hereby placed under version 3.0 of the Creative Commons Attribution
License[18]. This allows you redistribute and modify the work as you
desire, as long as you credit the original source. Alternatively, you
may choose to treat this document as falling under the same license as
Nmap itself (discussed previously).
Source Code Availability and Community Contributions
Source is provided to this software because we believe users have a
right to know exactly what a program is going to do before they run it.
This also allows you to audit the software for security holes (none
have been found so far).
Source code also allows you to port Nmap to new platforms, fix bugs,
and add new features. You are highly encouraged to send your changes to
nmap-dev@insecure.org for possible incorporation into the main
distribution. By sending these changes to Fyodor or one of the
Insecure.Org development mailing lists, it is assumed that you are
offering the Nmap Project (Insecure.Com LLC) the unlimited,
non-exclusive right to reuse, modify, and relicense the code. Nmap will
always be available Open Source,. but this is important because the
inability to relicense code has caused devastating problems for other
Free Software projects (such as KDE and NASM). We also occasionally
relicense the code to third parties as discussed above. If you wish to
specify special license conditions of your contributions, just say so
when you send them.
No Warranty.
This program is distributed in the hope that it will be useful, but
WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
General Public License v2.0 for more details at
http://www.gnu.org/licenses/gpl-2.0.html, or in the COPYING file
included with Nmap.
It should also be noted that Nmap has occasionally been known to crash
poorly written applications, TCP/IP stacks, and even operating
systems.. While this is extremely rare, it is important to keep in
mind. Nmap should never be run against mission critical systems unless
you are prepared to suffer downtime. We acknowledge here that Nmap may
crash your systems or networks and we disclaim all liability for any
damage or problems Nmap could cause.
Inappropriate Usage
Because of the slight risk of crashes and because a few black hats like
to use Nmap for reconnaissance prior to attacking systems, there are
administrators who become upset and may complain when their system is
scanned. Thus, it is often advisable to request permission before doing
even a light scan of a network.
Nmap should never be installed with special privileges (e.g. suid root)
for security reasons..
Third-Party Software
This product includes software developed by the Apache Software
Foundation[19]. A modified version of the Libpcap portable packet
capture library[20]. is distributed along with Nmap. The Windows
version of Nmap utilized the Libpcap-derived WinPcap library[21].
instead. Regular expression support is provided by the PCRE
library[22],. which is open-source software, written by Philip Hazel..
Certain raw networking functions use the Libdnet[23]. networking
library, which was written by Dug Song.. A modified version is
distributed with Nmap. Nmap can optionally link with the OpenSSL
cryptography toolkit[24]. for SSL version detection support. The Nmap
Scripting Engine uses an embedded version of the Lua programming
language[25].. All of the third-party software described in this
paragraph is freely redistributable under BSD-style software licenses.
United States Export Control.
Nmap only uses encryption when compiled with the optional OpenSSL
support and linked with OpenSSL. When compiled without OpenSSL support,
Insecure.Com LLC believes that Nmap is not subject to U.S. Export
Administration Regulations (EAR)[26] export control. As such, there is
no applicable ECCN (explort control classification number) and
exportation does not require any special license, permit, or other
governmental authorization.
When compiled with OpenSSL support or distributed as source code,
Insecure.Com LLC believes that Nmap falls under U.S. ECCN 5D002[27]
("Information Security Software"). We distribute Nmap under the TSU
exception for publicly available encryption software defined in EAR
740.13(e)[28].
NOTES
1. Nmap Network Scanning: The Official Nmap Project Guide to Network
Discovery and Security Scanning
http://nmap.org/book/
2. RFC 1122
http://www.rfc-editor.org/rfc/rfc1122.txt
3. RFC 792
http://www.rfc-editor.org/rfc/rfc792.txt
4. RFC 950
http://www.rfc-editor.org/rfc/rfc950.txt
5. RFC 1918
http://www.rfc-editor.org/rfc/rfc1918.txt
6. UDP
http://www.rfc-editor.org/rfc/rfc768.txt
7. SCTP
http://www.rfc-editor.org/rfc/rfc4960.txt
8. TCP RFC
http://www.rfc-editor.org/rfc/rfc793.txt
9. RFC 959
http://www.rfc-editor.org/rfc/rfc959.txt
10. RFC 1323
http://www.rfc-editor.org/rfc/rfc1323.txt
11. Lua programming language
http://lua.org
12. precedence
http://www.lua.org/manual/5.1/manual.html#2.5.3
13. IP protocol
http://www.rfc-editor.org/rfc/rfc791.txt
14. RFC 2960
http://www.rfc-editor.org/rfc/rfc2960.txt
15. Nmap::Scanner
http://sourceforge.net/projects/nmap-scanner/
16. Nmap::Parser
http://nmapparser.wordpress.com/
17. listed at Wikipedia
http://en.wikipedia.org/wiki/List_of_IPv6_tunnel_brokers
18. Creative Commons Attribution License
http://creativecommons.org/licenses/by/3.0/
19. Apache Software Foundation
http://www.apache.org
20. Libpcap portable packet capture library
http://www.tcpdump.org
21. WinPcap library
http://www.winpcap.org
22. PCRE library
http://www.pcre.org
23. Libdnet
http://libdnet.sourceforge.net
24. OpenSSL cryptography toolkit
http://www.openssl.org
25. Lua programming language
http://www.lua.org
26. Export Administration Regulations (EAR)
http://www.access.gpo.gov/bis/ear/ear_data.html
27. 5D002
http://www.access.gpo.gov/bis/ear/pdf/ccl5-pt2.pdf
28. EAR 740.13(e)
http://www.access.gpo.gov/bis/ear/pdf/740.pdf