NAME
traceroute - print the route packets trace to network host
SYNOPSIS
traceroute [-46dFITUnreAV] [-f first_ttl] [-g gate,...]
[-i device] [-m max_ttl] [-p port] [-s src_addr]
[-q nqueries] [-N squeries] [-t tos]
[-l flow_label] [-w waittime] [-z sendwait]
[-UL] [-P proto] [--sport=port] [-M method] [-O mod_options]
[--mtu] [--back]
host [packet_len]
traceroute6 [options]
lft [options]
DESCRIPTION
traceroute tracks the route packets taken from an IP network on their
way to a given host. It utilizes the IP protocol's time to live (TTL)
field and attempts to elicit an ICMP TIME_EXCEEDED response from each
gateway along the path to the host.
traceroute6 is equivalent to traceroute -6
lft is equivalent to traceroute -t
The only required parameter is the name or IP address of the
destination host . The optional packet_len`gth is the total size of
the probing packet (default 60 bytes for IPv4 and 80 for IPv6). The
specified size can be ignored in some situations or increased up to a
minimal value.
This program attempts to trace the route an IP packet would follow to
some internet host by launching probe packets with a small ttl (time to
live) then listening for an ICMP "time exceeded" reply from a gateway.
We start our probes with a ttl of one and increase by one until we get
an ICMP "port unreachable" (or TCP reset), which means we got to the
"host", or hit a max (which defaults to 30 hops). Three probes (by
default) are sent at each ttl setting and a line is printed showing the
ttl, address of the gateway and round trip time of each probe. The
address can be followed by additional information when requested. If
the probe answers come from different gateways, the address of each
responding system will be printed. If there is no response within a
5.0 seconds (default), an "*" (asterisk) is printed for that probe.
After the trip time, some additional annotation can be printed: !H, !N,
or !P (host, network or protocol unreachable), !S (source route
failed), !F (fragmentation needed), !X (communication administratively
prohibited), !V (host precedence violation), !C (precedence cutoff in
effect), or !<num> (ICMP unreachable code <num>). If almost all the
probes result in some kind of unreachable, traceroute will give up and
exit.
We don't want the destination host to process the UDP probe packets, so
the destination port is set to an unlikely value (you can change it
with the -p flag). There is no such a problem for ICMP or TCP
tracerouting (for TCP we use half-open technique, which prevents our
probes to be seen by applications on the destination host).
In the modern network environment the traditional traceroute methods
can not be always applicable, because of widespread use of firewalls.
Such firewalls filter the "unlikely" UDP ports, or even ICMP echoes.
To solve this, some additional tracerouting methods are implemented
(including tcp), see LIST OF AVAILABLE METHODS below. Such methods try
to use particular protocol and source/destination port, in order to
bypass firewalls (to be seen by firewalls just as a start of allowed
type of a network session).
OPTIONS
--help Print help info and exit.
-4, -6 Explicitly force IPv4 or IPv6 traceouting. By default, the
program will try to resolve the name given, and choose the
appropriate protocol automatically. If resolving a host name
returns both IPv4 and IPv6 addresses, traceroute will use IPv4.
-I Use ICMP ECHO for probes
-T Use TCP SYN for probes
-d Enable socket level debugging (when the Linux kernel supports
it)
-F Do not fragment probe packets. (For IPv4 it also sets DF bit,
which tells intermediate routers not to fragment remotely as
well).
Varying the size of the probing packet by the packet_len command
line parameter, you can manually obtain information about the
MTU of individual network hops. The --mtu option (see below)
tries to do this automatically.
Note, that non-fragmented features (like -F or --mtu) work
properly since the Linux kernel 2.6.22 only. Before that
version, IPv6 was always fragmented, IPv4 could use the once the
discovered final mtu only (from the route cache), which can be
less than the actual mtu of a device.
-f first_ttl
Specifies with what TTL to start. Defaults to 1.
-g gateway
Tells traceroute to add an IP source routing option to the
outgoing packet that tells the network to route the packet
through the specified gateway (most routers have disabled source
routing for security reasons). In general, several gateway's is
allowed (comma separated). For IPv6, the form of
num,addr,addr... is allowed, where num is a route header type
(default is type 2). Note the type 0 route header is now
deprecated (rfc5095).
-i interface
Specifies the interface through which traceroute should send
packets. By default, the interface is selected according to the
routing table.
-m max_ttl
Specifies the maximum number of hops (max time-to-live value)
traceroute will probe. The default is 30.
-N squeries
Specifies the number of probe packets sent out simultaneously.
Sending several probes concurrently can speed up traceroute
considerably. The default value is 16.
Note that some routers and hosts can use ICMP rate throttling.
In such a situation specifying too large number can lead to loss
of some responses.
-n Do not try to map IP addresses to host names when displaying
them.
-p port
For UDP tracing, specifies the destination port base traceroute
will use (the destination port number will be incremented by
each probe).
For ICMP tracing, specifies the initial icmp sequence value
(incremented by each probe too).
For TCP specifies just the (constant) destination port to
connect.
-t tos For IPv4, set the Type of Service (TOS) and Precedence value.
Useful values are 16 (low delay) and 8 (high throughput). Note
that in order to use some TOS precendence values, you have to be
super user.
For IPv6, set the Traffic Control value.
-w waittime
Set the time (in seconds) to wait for a response to a probe
(default 5.0 sec).
-q nqueries
Sets the number of probe packets per hop. The default is 3.
-r Bypass the normal routing tables and send directly to a host on
an attached network. If the host is not on a directly-attached
network, an error is returned. This option can be used to ping
a local host through an interface that has no route through it.
-s source_addr
Chooses an alternative source address. Note that you must select
the address of one of the interfaces. By default, the address
of the outgoing interface is used.
-z sendwait
Minimal time interval between probes (default 0). If the value
is more than 10, then it specifies a number in milliseconds,
else it is a number of seconds (float point values allowed too).
Useful when some routers use rate-limit for icmp messages.
-e Show ICMP extensions (rfc4884). The general form is CLASS/TYPE:
followed by a hexadecimal dump. The MPLS (rfc4950) is shown
parsed, in a form: MPLS:L=label,E=exp_use,S=stack_bottom,T=TTL
(more objects separated by / ).
-A Perform AS path lookups in routing registries and print results
directly after the corresponding addresses.
-V Print the version and exit.
There is a couple of additional options, intended for an advanced usage
(another trace methods etc.):
--sport=port
Chooses the source port to use. Implies -N 1. Normally source
ports (if applicable) are chosen by the system.
-M method
Use specified method for traceroute operations. Default
traditional udp method has name default, icmp (-I) and tcp (-T)
have names icmp and tcp respectively.
Method-specific options can be passed by -O . Most methods have
their simple shortcuts, (-I means -M icmp, etc).
-O option
Specifies some method-specific option. Several options are
separated by comma (or use several -O on cmdline). Each method
may have its own specific options, or many not have them at all.
To print information about available options, use -O help.
-U Use UDP to particular destination port for tracerouting (instead
of increasing the port per each probe). Default port is 53
(dns).
-UL Use UDPLITE for tracerouting (default port is 53).
-P protocol
Use raw packet of specified protocol for tracerouting. Default
protocol is 253 (rfc3692).
--mtu Discover MTU along the path being traced. Implies -F -N 1. New
mtu is printed once in a form of F=NUM at the first probe of a
hop which requires such mtu to be reached. (Actually, the
correspond "frag needed" icmp message normally is sent by the
previous hop).
Note, that some routers might cache once the seen information on
a fragmentation. Thus you can receive the final mtu from a
closer hop. Try to specify an unusual tos by -t , this can help
for one attempt (then it can be cached there as well).
See -F option for more info.
--back Print the number of backward hops when it seems different with
the forward direction. This number is guessed in assumption that
remote hops send reply packets with initial ttl set to either
64, or 128 or 255 (which seems a common practice). It is printed
as a negate value in a form of '-NUM' .
LIST OF AVAILABLE METHODS
In general, a particular traceroute method may have to be chosen by
-M name, but most of the methods have their simple cmdline switches
(you can see them after the method name, if present).
default
The traditional, ancient method of tracerouting. Used by default.
Probe packets are udp datagrams with so-called "unlikely" destination
ports. The "unlikely" port of the first probe is 33434, then for each
next probe it is incremented by one. Since the ports are expected to be
unused, the destination host normally returns "icmp unreach port" as a
final response. (Nobody knows what happens when some application
listens for such ports, though).
This method is allowed for unprivileged users.
icmp -I
Most usual method for now, which uses icmp echo packets for probes.
If you can ping(8) the destination host, icmp tracerouting is
applicable as well.
tcp -T
Well-known modern method, intended to bypass firewalls.
Uses the constant destination port (default is 80, http).
If some filters are present in the network path, then most probably any
"unlikely" udp ports (as for default method) or even icmp echoes (as
for icmp) are filtered, and whole tracerouting will just stop at such a
firewall. To bypass a network filter, we have to use only allowed
protocol/port combinations. If we trace for some, say, mailserver, then
more likely -T -p 25 can reach it, even when -I can not.
This method uses well-known "half-open technique", which prevents
applications on the destination host from seeing our probes at all.
Normally, a tcp syn is sent. For non-listened ports we receive tcp
reset, and all is done. For active listening ports we receive tcp
syn+ack, but answer by tcp reset (instead of expected tcp ack), this
way the remote tcp session is dropped even without the application ever
taking notice.
There is a couple of options for tcp method:
syn,ack,fin,rst,psh,urg,ece,cwr
Sets specified tcp flags for probe packet, in any combination.
flags=num
Sets the flags field in the tcp header exactly to num.
ecn Send syn packet with tcp flags ECE and CWR (for Explicit
Congestion Notification, rfc3168)
sack,timestamps,window_scaling
Use the corresponding tcp header option in the outgoing probe
packet.
sysctl Use current sysctl (/proc/sys/net/*) setting for the tcp header
options above and ecn. Always set by default, if nothing else
specified.
mss=num
Use value of num for maxseg tcp header option (when syn).
Default options is syn,sysctl.
tcpconn
An initial implementation of tcp method, simple using connect(2) call,
which does full tcp session opening. Not recommended for normal use,
because a destination application is always affected (and can be
confused).
udp -U
Use udp datagram with constant destination port (default 53, dns).
Intended to bypass firewall as well.
Note, that unlike in tcp method, the correspond application on the
destination host always receive our probes (with random data), and most
can easily be confused by them. Most cases it will not respond to our
packets though, so we will never see the final hop in the trace.
(Fortunately, it seems that at least dns servers replies with something
angry).
This method is allowed for unprivileged users.
udplite -UL
Use udplite datagram for probes (with constant destination port,
default 53).
This method is allowed for unprivileged users.
Options:
coverage=num
Set udplite send coverage to num.
raw -P proto
Send raw packet of protocol proto.
No protocol-specific headers are used, just IP header only.
Implies -N 1.
Options:
protocol=proto
Use IP protocol proto (default 253).
NOTES
To speed up work, normally several probes are sent simultaneously. On
the other hand, it creates a "storm of packages", especially in the
reply direction. Routers can throttle the rate of icmp responses, and
some of replies can be lost. To avoid this, decrease the number of
simultaneous probes, or even set it to 1 (like in initial traceroute
implementation), i.e. -N 1
The final (target) host can drop some of the simultaneous probes, and
might even answer only the latest ones. It can lead to extra "looks
like expired" hops near the final hop. We use a smart algorithm to
auto-detect such a situation, but if it cannot help in your case, just
use -N 1 too.
For even greater stability you can slow down the program's work by -z
option, for example use -z 0.5 for half-second pause between probes.
If some hops report nothing for every method, the last chance to obtain
something is to use ping -R command (IPv4, and for nearest 8 hops
only).
SEE ALSO
ping(8), ping6(8), tcpdump(8), netstat(8)