NAME
dnsmasq - A lightweight DHCP and caching DNS server.
SYNOPSIS
dnsmasq [OPTION]...
DESCRIPTION
dnsmasq is a lightweight DNS, TFTP and DHCP server. It is intended to
provide coupled DNS and DHCP service to a LAN.
Dnsmasq accepts DNS queries and either answers them from a small,
local, cache or forwards them to a real, recursive, DNS server. It
loads the contents of /etc/hosts so that local hostnames which do not
appear in the global DNS can be resolved and also answers DNS queries
for DHCP configured hosts.
The dnsmasq DHCP server supports static address assignments and
multiple networks. It automatically sends a sensible default set of
DHCP options, and can be configured to send any desired set of DHCP
options, including vendor-encapsulated options. It includes a secure,
read-only, TFTP server to allow net/PXE boot of DHCP hosts and also
supports BOOTP.
Dnsmasq supports IPv6 for DNS, but not DHCP.
OPTIONS
Note that in general missing parameters are allowed and switch off
functions, for instance "--pid-file" disables writing a PID file. On
BSD, unless the GNU getopt library is linked, the long form of the
options does not work on the command line; it is still recognised in
the configuration file.
--test Read and syntax check configuration file(s). Exit with code 0 if
all is OK, or a non-zero code otherwise. Do not start up
dnsmasq.
-h, --no-hosts
Don’t read the hostnames in /etc/hosts.
-H, --addn-hosts=<file>
Additional hosts file. Read the specified file as well as
/etc/hosts. If -h is given, read only the specified file. This
option may be repeated for more than one additional hosts file.
If a directory is given, then read all the files contained in
that directory.
-E, --expand-hosts
Add the domain to simple names (without a period) in /etc/hosts
in the same way as for DHCP-derived names. Note that this does
not apply to domain names in cnames, PTR records, TXT records
etc.
-T, --local-ttl=<time>
When replying with information from /etc/hosts or the DHCP
leases file dnsmasq by default sets the time-to-live field to
zero, meaning that the requestor should not itself cache the
information. This is the correct thing to do in almost all
situations. This option allows a time-to-live (in seconds) to be
given for these replies. This will reduce the load on the server
at the expense of clients using stale data under some
circumstances.
--neg-ttl=<time>
Negative replies from upstream servers normally contain time-to-
live information in SOA records which dnsmasq uses for caching.
If the replies from upstream servers omit this information,
dnsmasq does not cache the reply. This option gives a default
value for time-to-live (in seconds) which dnsmasq uses to cache
negative replies even in the absence of an SOA record.
--max-ttl=<time>
Set a maximum TTL value that will be handed out to clients. The
specified maximum TTL will be given to clients instead of the
true TTL value if it is lower. The true TTL value is however
kept in the cache to avoid flooding the upstream DNS servers.
-k, --keep-in-foreground
Do not go into the background at startup but otherwise run as
normal. This is intended for use when dnsmasq is run under
daemontools or launchd.
-d, --no-daemon
Debug mode: don’t fork to the background, don’t write a pid
file, don’t change user id, generate a complete cache dump on
receipt on SIGUSR1, log to stderr as well as syslog, don’t fork
new processes to handle TCP queries.
-q, --log-queries
Log the results of DNS queries handled by dnsmasq. Enable a full
cache dump on receipt of SIGUSR1.
-8, --log-facility=<facility>
Set the facility to which dnsmasq will send syslog entries, this
defaults to DAEMON, and to LOCAL0 when debug mode is in
operation. If the facility given contains at least one ’/’
character, it is taken to be a filename, and dnsmasq logs to the
given file, instead of syslog. If the facility is ’-’ then
dnsmasq logs to stderr. (Errors whilst reading configuration
will still go to syslog, but all output from a successful
startup, and all output whilst running, will go exclusively to
the file.) When logging to a file, dnsmasq will close and reopen
the file when it receives SIGUSR2. This allows the log file to
be rotated without stopping dnsmasq.
--log-async[=<lines>]
Enable asynchronous logging and optionally set the limit on the
number of lines which will be queued by dnsmasq when writing to
the syslog is slow. Dnsmasq can log asynchronously: this allows
it to continue functioning without being blocked by syslog, and
allows syslog to use dnsmasq for DNS queries without risking
deadlock. If the queue of log-lines becomes full, dnsmasq will
log the overflow, and the number of messages lost. The default
queue length is 5, a sane value would be 5-25, and a maximum
limit of 100 is imposed.
-x, --pid-file=<path>
Specify an alternate path for dnsmasq to record its process-id
in. Normally /var/run/dnsmasq.pid.
-u, --user=<username>
Specify the userid to which dnsmasq will change after startup.
Dnsmasq must normally be started as root, but it will drop root
privileges after startup by changing id to another user.
Normally this user is "nobody" but that can be over-ridden with
this switch.
-g, --group=<groupname>
Specify the group which dnsmasq will run as. The defaults to
"dip", if available, to facilitate access to
/etc/ppp/resolv.conf which is not normally world readable.
-v, --version
Print the version number.
-p, --port=<port>
Listen on <port> instead of the standard DNS port (53). Setting
this to zero completely disables DNS function, leaving only DHCP
and/or TFTP.
-P, --edns-packet-max=<size>
Specify the largest EDNS.0 UDP packet which is supported by the
DNS forwarder. Defaults to 4096, which is the
RFC5625-recommended size.
-Q, --query-port=<query_port>
Send outbound DNS queries from, and listen for their replies on,
the specific UDP port <query_port> instead of using random
ports. NOTE that using this option will make dnsmasq less secure
against DNS spoofing attacks but it may be faster and use less
resources. Setting this option to zero makes dnsmasq use a
single port allocated to it by the OS: this was the default
behaviour in versions prior to 2.43.
--min-port=<port>
Do not use ports less than that given as source for outbound DNS
queries. Dnsmasq picks random ports as source for outbound
queries: when this option is given, the ports used will always
to larger than that specified. Useful for systems behind
firewalls.
-i, --interface=<interface name>
Listen only on the specified interface(s). Dnsmasq automatically
adds the loopback (local) interface to the list of interfaces to
use when the --interface option is used. If no --interface or
--listen-address options are given dnsmasq listens on all
available interfaces except any given in --except-interface
options. IP alias interfaces (eg "eth1:0") cannot be used with
--interface or --except-interface options, use --listen-address
instead.
-I, --except-interface=<interface name>
Do not listen on the specified interface. Note that the order of
--listen-address --interface and --except-interface options does
not matter and that --except-interface options always override
the others.
-2, --no-dhcp-interface=<interface name>
Do not provide DHCP or TFTP on the specified interface, but do
provide DNS service.
-a, --listen-address=<ipaddr>
Listen on the given IP address(es). Both --interface and
--listen-address options may be given, in which case the set of
both interfaces and addresses is used. Note that if no
--interface option is given, but --listen-address is, dnsmasq
will not automatically listen on the loopback interface. To
achieve this, its IP address, 127.0.0.1, must be explicitly
given as a --listen-address option.
-z, --bind-interfaces
On systems which support it, dnsmasq binds the wildcard address,
even when it is listening on only some interfaces. It then
discards requests that it shouldn’t reply to. This has the
advantage of working even when interfaces come and go and change
address. This option forces dnsmasq to really bind only the
interfaces it is listening on. About the only time when this is
useful is when running another nameserver (or another instance
of dnsmasq) on the same machine. Setting this option also
enables multiple instances of dnsmasq which provide DHCP service
to run in the same machine.
-y, --localise-queries
Return answers to DNS queries from /etc/hosts which depend on
the interface over which the query was received. If a name in
/etc/hosts has more than one address associated with it, and at
least one of those addresses is on the same subnet as the
interface to which the query was sent, then return only the
address(es) on that subnet. This allows for a server to have
multiple addresses in /etc/hosts corresponding to each of its
interfaces, and hosts will get the correct address based on
which network they are attached to. Currently this facility is
limited to IPv4.
-b, --bogus-priv
Bogus private reverse lookups. All reverse lookups for private
IP ranges (ie 192.168.x.x, etc) which are not found in
/etc/hosts or the DHCP leases file are answered with "no such
domain" rather than being forwarded upstream.
-V, --alias=[<old-ip>]|[<start-ip>-<end-ip>],<new-ip>[,<mask>]
Modify IPv4 addresses returned from upstream nameservers; old-ip
is replaced by new-ip. If the optional mask is given then any
address which matches the masked old-ip will be re-written. So,
for instance --alias=1.2.3.0,6.7.8.0,255.255.255.0 will map
1.2.3.56 to 6.7.8.56 and 1.2.3.67 to 6.7.8.67. This is what
Cisco PIX routers call "DNS doctoring". If the old IP is given
as range, then only addresses in the range, rather than a whole
subnet, are re-written. So
--alias=192.168.0.10-192.168.0.40,10.0.0.0,255.255.255.0 maps
192.168.0.10->192.168.0.40 to 10.0.0.10->10.0.0.40
-B, --bogus-nxdomain=<ipaddr>
Transform replies which contain the IP address given into "No
such domain" replies. This is intended to counteract a devious
move made by Verisign in September 2003 when they started
returning the address of an advertising web page in response to
queries for unregistered names, instead of the correct NXDOMAIN
response. This option tells dnsmasq to fake the correct response
when it sees this behaviour. As at Sept 2003 the IP address
being returned by Verisign is 64.94.110.11
-f, --filterwin2k
Later versions of windows make periodic DNS requests which don’t
get sensible answers from the public DNS and can cause problems
by triggering dial-on-demand links. This flag turns on an option
to filter such requests. The requests blocked are for records of
types SOA and SRV, and type ANY where the requested name has
underscores, to catch LDAP requests.
-r, --resolv-file=<file>
Read the IP addresses of the upstream nameservers from <file>,
instead of /etc/resolv.conf. For the format of this file see
resolv.conf(5) the only lines relevant to dnsmasq are nameserver
ones. Dnsmasq can be told to poll more than one resolv.conf
file, the first file name specified overrides the default,
subsequent ones add to the list. This is only allowed when
polling; the file with the currently latest modification time is
the one used.
-R, --no-resolv
Don’t read /etc/resolv.conf. Get upstream servers only from the
command line or the dnsmasq configuration file.
-1, --enable-dbus
Allow dnsmasq configuration to be updated via DBus method calls.
The configuration which can be changed is upstream DNS servers
(and corresponding domains) and cache clear. Requires that
dnsmasq has been built with DBus support.
-o, --strict-order
By default, dnsmasq will send queries to any of the upstream
servers it knows about and tries to favour servers that are
known to be up. Setting this flag forces dnsmasq to try each
query with each server strictly in the order they appear in
/etc/resolv.conf
--all-servers
By default, when dnsmasq has more than one upstream server
available, it will send queries to just one server. Setting this
flag forces dnsmasq to send all queries to all available
servers. The reply from the server which answers first will be
returned to the original requestor.
--stop-dns-rebind
Reject (and log) addresses from upstream nameservers which are
in the private IP ranges. This blocks an attack where a browser
behind a firewall is used to probe machines on the local
network.
--rebind-localhost-ok
Exempt 127.0.0.0/8 from rebinding checks. This address range is
returned by realtime black hole servers, so blocking it may
disable these services.
--rebind-domain-ok=[<domain>]|[[/<domain>/[<domain>/]
Do not detect and block dns-rebind on queries to these domains.
The argument may be either a single domain, or multiple domains
surrounded by ’/’, like the --server syntax, eg. --rebind-
domain-ok=/domain1/domain2/domain3/
-n, --no-poll
Don’t poll /etc/resolv.conf for changes.
--clear-on-reload
Whenever /etc/resolv.conf is re-read, clear the DNS cache. This
is useful when new nameservers may have different data than that
held in cache.
-D, --domain-needed
Tells dnsmasq to never forward queries for plain names, without
dots or domain parts, to upstream nameservers. If the name is
not known from /etc/hosts or DHCP then a "not found" answer is
returned.
-S, --local,
--server=[/[<domain>]/[domain/]][<ipaddr>[#<port>][@<source-
ip>|<interface>[#<port>]]
Specify IP address of upstream servers directly. Setting this
flag does not suppress reading of /etc/resolv.conf, use -R to do
that. If one or more optional domains are given, that server is
used only for those domains and they are queried only using the
specified server. This is intended for private nameservers: if
you have a nameserver on your network which deals with names of
the form xxx.internal.thekelleys.org.uk at 192.168.1.1 then
giving the flag -S /internal.thekelleys.org.uk/192.168.1.1 will
send all queries for internal machines to that nameserver,
everything else will go to the servers in /etc/resolv.conf. An
empty domain specification, // has the special meaning of
"unqualified names only" ie names without any dots in them. A
non-standard port may be specified as part of the IP address
using a # character. More than one -S flag is allowed, with
repeated domain or ipaddr parts as required.
More specific domains take precendence over less specific
domains, so: --server=/google.com/1.2.3.4
--server=/www.google.com/2.3.4.5 will send queries for
*.google.com to 1.2.3.4, except *www.google.com, which will go
to 2.3.4.5
The special server address ’#’ means, "use the standard
servers", so --server=/google.com/1.2.3.4
--server=/www.google.com/# will send queries for *.google.com to
1.2.3.4, except *www.google.com which will be forwarded as
usual.
Also permitted is a -S flag which gives a domain but no IP
address; this tells dnsmasq that a domain is local and it may
answer queries from /etc/hosts or DHCP but should never forward
queries on that domain to any upstream servers. local is a
synonym for server to make configuration files clearer in this
case.
The optional string after the @ character tells dnsmasq how to
set the source of the queries to this nameserver. It should be
an ip-address, which should belong to the machine on which
dnsmasq is running otherwise this server line will be logged and
then ignored, or an interface name. If an interface name is
given, then queries to the server will be forced via that
interface; if an ip-address is given then the source address of
the queries will be set to that address. The query-port flag is
ignored for any servers which have a source address specified
but the port may be specified directly as part of the source
address. Forcing queries to an interface is not implemented on
all platforms supported by dnsmasq.
-A, --address=/<domain>/[domain/]<ipaddr>
Specify an IP address to return for any host in the given
domains. Queries in the domains are never forwarded and always
replied to with the specified IP address which may be IPv4 or
IPv6. To give both IPv4 and IPv6 addresses for a domain, use
repeated -A flags. Note that /etc/hosts and DHCP leases
override this for individual names. A common use of this is to
redirect the entire doubleclick.net domain to some friendly
local web server to avoid banner ads. The domain specification
works in the same was as for --server, with the additional
facility that /#/ matches any domain. Thus --address=/#/1.2.3.4
will always return 1.2.3.4 for any query not answered from
/etc/hosts or DHCP and not sent to an upstream nameserver by a
more specific --server directive.
-m, --mx-host=<mx name>[[,<hostname>],<preference>]
Return an MX record named <mx name> pointing to the given
hostname (if given), or the host specified in the --mx-target
switch or, if that switch is not given, the host on which
dnsmasq is running. The default is useful for directing mail
from systems on a LAN to a central server. The preference value
is optional, and defaults to 1 if not given. More than one MX
record may be given for a host.
-t, --mx-target=<hostname>
Specify the default target for the MX record returned by
dnsmasq. See --mx-host. If --mx-target is given, but not --mx-
host, then dnsmasq returns a MX record containing the MX target
for MX queries on the hostname of the machine on which dnsmasq
is running.
-e, --selfmx
Return an MX record pointing to itself for each local machine.
Local machines are those in /etc/hosts or with DHCP leases.
-L, --localmx
Return an MX record pointing to the host given by mx-target (or
the machine on which dnsmasq is running) for each local machine.
Local machines are those in /etc/hosts or with DHCP leases.
-W, --srv-
host=<_service>.<_prot>.[<domain>],[<target>[,<port>[,<priority>[,<weight>]]]]
Return a SRV DNS record. See RFC2782 for details. If not
supplied, the domain defaults to that given by --domain. The
default for the target domain is empty, and the default for port
is one and the defaults for weight and priority are zero. Be
careful if transposing data from BIND zone files: the port,
weight and priority numbers are in a different order. More than
one SRV record for a given service/domain is allowed, all that
match are returned.
-Y, --txt-record=<name>[[,<text>],<text>]
Return a TXT DNS record. The value of TXT record is a set of
strings, so any number may be included, split by commas.
--ptr-record=<name>[,<target>]
Return a PTR DNS record.
--naptr-
record=<name>,<order>,<preference>,<flags>,<service>,<regexp>[,<replacement>]
Return an NAPTR DNS record, as specified in RFC3403.
--cname=<cname>,<target>
Return a CNAME record which indicates that <cname> is really
<target>. There are significant limitations on the target; it
must be a DNS name which is known to dnsmasq from /etc/hosts (or
additional hosts files) or from DHCP. If the target does not
satisfy this criteria, the whole cname is ignored. The cname
must be unique, but it is permissable to have more than one
cname pointing to the same target.
--interface-name=<name>,<interface>
Return a DNS record associating the name with the primary
address on the given interface. This flag specifies an A record
for the given name in the same way as an /etc/hosts line, except
that the address is not constant, but taken from the given
interface. If the interface is down, not configured or non-
existent, an empty record is returned. The matching PTR record
is also created, mapping the interface address to the name. More
than one name may be associated with an interface address by
repeating the flag; in that case the first instance is used for
the reverse address-to-name mapping.
-c, --cache-size=<cachesize>
Set the size of dnsmasq’s cache. The default is 150 names.
Setting the cache size to zero disables caching.
-N, --no-negcache
Disable negative caching. Negative caching allows dnsmasq to
remember "no such domain" answers from upstream nameservers and
answer identical queries without forwarding them again.
-0, --dns-forward-max=<queries>
Set the maximum number of concurrent DNS queries. The default
value is 150, which should be fine for most setups. The only
known situation where this needs to be increased is when using
web-server log file resolvers, which can generate large numbers
of concurrent queries.
-F, --dhcp-
range=[interface:<interface>,][tag:<tag>[,tag:<tag>],][set:<tag],]<start-
addr>,<end-addr>[,<netmask>[,<broadcast>]][,<lease time>]
Enable the DHCP server. Addresses will be given out from the
range <start-addr> to <end-addr> and from statically defined
addresses given in dhcp-host options. If the lease time is
given, then leases will be given for that length of time. The
lease time is in seconds, or minutes (eg 45m) or hours (eg 1h)
or "infinite". If not given, the default lease time is one hour.
The minimum lease time is two minutes. This option may be
repeated, with different addresses, to enable DHCP service to
more than one network. For directly connected networks (ie,
networks on which the machine running dnsmasq has an interface)
the netmask is optional. It is, however, required for networks
which receive DHCP service via a relay agent. The broadcast
address is always optional. It is always allowed to have more
than one dhcp-range in a single subnet.
The optional set:<tag> sets an alphanumeric label which marks
this network so that dhcp options may be specified on a per-
network basis. When it is prefixed with ’tag:’ instead, then
its meaning changes from setting a tag to matching it. Only one
tag may be set, but more than one tag may be matched. The end
address may be replaced by the keyword static which tells
dnsmasq to enable DHCP for the network specified, but not to
dynamically allocate IP addresses: only hosts which have static
addresses given via dhcp-host or from /etc/ethers will be
served. The end address may be replaced by the keyword proxy in
which case dnsmasq will provide proxy-DHCP on the specified
subnet. (See pxe-prompt and pxe-service for details.)
The interface:<interface name> section is not normally used. See
the NOTES section for details of this.
-G, --dhcp-
host=[<hwaddr>][,id:<client_id>|*][,set:<tag>][,<ipaddr>][,<hostname>][,<lease_time>][,ignore]
Specify per host parameters for the DHCP server. This allows a
machine with a particular hardware address to be always
allocated the same hostname, IP address and lease time. A
hostname specified like this overrides any supplied by the DHCP
client on the machine. It is also allowable to ommit the
hardware address and include the hostname, in which case the IP
address and lease times will apply to any machine claiming that
name. For example --dhcp-host=00:20:e0:3b:13:af,wap,infinite
tells dnsmasq to give the machine with hardware address
00:20:e0:3b:13:af the name wap, and an infinite DHCP lease.
--dhcp-host=lap,192.168.0.199 tells dnsmasq to always allocate
the machine lap the IP address 192.168.0.199.
Addresses allocated like this are not constrained to be in the
range given by the --dhcp-range option, but they must be in the
same subnet as some valid dhcp-range. For subnets which don’t
need a pool of dynamically allocated addresses, use the "static"
keyword in the dhcp-range declaration.
It is allowed to use client identifiers rather than hardware
addresses to identify hosts by prefixing with ’id:’. Thus:
--dhcp-host=id:01:02:03:04,..... refers to the host with client
identifier 01:02:03:04. It is also allowed to specify the client
ID as text, like this: --dhcp-host=id:clientidastext,.....
The special option id:* means "ignore any client-id and use MAC
addresses only." This is useful when a client presents a client-
id sometimes but not others.
If a name appears in /etc/hosts, the associated address can be
allocated to a DHCP lease, but only if a --dhcp-host option
specifying the name also exists. Only one hostname can be given
in a dhcp-host option, but aliases are possible by using CNAMEs.
(See --cname ).
The special keyword "ignore" tells dnsmasq to never offer a DHCP
lease to a machine. The machine can be specified by hardware
address, client ID or hostname, for instance --dhcp-
host=00:20:e0:3b:13:af,ignore This is useful when there is
another DHCP server on the network which should be used by some
machines.
The set:<tag> contruct sets the tag whenever this dhcp-host
directive is in use. This can be used to selectively send DHCP
options just for this host. More than one tag can be set in a
dhcp-host directive (but not in other places where "set:<tag>"
is allowed). When a host matches any dhcp-host directive (or one
implied by /etc/ethers) then the special tag "known" is set.
This allows dnsmasq to be configured to ignore requests from
unknown machines using --dhcp-ignore=tag:!known Ethernet
addresses (but not client-ids) may have wildcard bytes, so for
example --dhcp-host=00:20:e0:3b:13:*,ignore will cause dnsmasq
to ignore a range of hardware addresses. Note that the "*" will
need to be escaped or quoted on a command line, but not in the
configuration file.
Hardware addresses normally match any network (ARP) type, but it
is possible to restrict them to a single ARP type by preceding
them with the ARP-type (in HEX) and "-". so --dhcp-
host=06-00:20:e0:3b:13:af,1.2.3.4 will only match a Token-Ring
hardware address, since the ARP-address type for token ring is
6.
As a special case, it is possible to include more than one
hardware address. eg: --dhcp-
host=11:22:33:44:55:66,12:34:56:78:90:12,192.168.0.2 This allows
an IP address to be associated with multiple hardware addresses,
and gives dnsmasq permission to abandon a DHCP lease to one of
the hardware addresses when another one asks for a lease. Beware
that this is a dangerous thing to do, it will only work reliably
if only one of the hardware addresses is active at any time and
there is no way for dnsmasq to enforce this. It is, for
instance, useful to allocate a stable IP address to a laptop
which has both wired and wireless interfaces.
--dhcp-hostsfile=<file>
Read DHCP host information from the specified file. The file
contains information about one host per line. The format of a
line is the same as text to the right of ’=’ in --dhcp-host. The
advantage of storing DHCP host information in this file is that
it can be changed without re-starting dnsmasq: the file will be
re-read when dnsmasq receives SIGHUP.
--dhcp-optsfile=<file>
Read DHCP option information from the specified file. The
advantage of using this option is the same as for --dhcp-
hostsfile: the dhcp-optsfile will be re-read when dnsmasq
receives SIGHUP. Note that it is possible to encode the
information in a --dhcp-boot flag as DHCP options, using the
options names bootfile-name, server-ip-address and tftp-server.
This allows these to be included in a dhcp-optsfile.
-Z, --read-ethers
Read /etc/ethers for information about hosts for the DHCP
server. The format of /etc/ethers is a hardware address,
followed by either a hostname or dotted-quad IP address. When
read by dnsmasq these lines have exactly the same effect as
--dhcp-host options containing the same information. /etc/ethers
is re-read when dnsmasq receives SIGHUP.
-O, --dhcp-option=[tag:<tag>,[tag:<tag>,]][encap:<opt>,][vi-
encap:<enterprise>,][vendor:[<vendor-class>],][<opt>|option:<opt-
name>],[<value>[,<value>]]
Specify different or extra options to DHCP clients. By default,
dnsmasq sends some standard options to DHCP clients, the netmask
and broadcast address are set to the same as the host running
dnsmasq, and the DNS server and default route are set to the
address of the machine running dnsmasq. If the domain name
option has been set, that is sent. This configuration allows
these defaults to be overridden, or other options specified. The
option, to be sent may be given as a decimal number or as
"option:<option-name>" The option numbers are specified in
RFC2132 and subsequent RFCs. The set of option-names known by
dnsmasq can be discovered by running "dnsmasq --help dhcp". For
example, to set the default route option to 192.168.4.4, do
--dhcp-option=3,192.168.4.4 or --dhcp-option = option:router,
192.168.4.4 and to set the time-server address to 192.168.0.4,
do --dhcp-option = 42,192.168.0.4 or --dhcp-option = option:ntp-
server, 192.168.0.4 The special address 0.0.0.0 is taken to mean
"the address of the machine running dnsmasq". Data types allowed
are comma separated dotted-quad IP addresses, a decimal number,
colon-separated hex digits and a text string. If the optional
tags are given then this option is only sent when all the tags
are matched.
Special processing is done on a text argument for option 119, to
conform with RFC 3397. Text or dotted-quad IP addresses as
arguments to option 120 are handled as per RFC 3361. Dotted-quad
IP addresses which are followed by a slash and then a netmask
size are encoded as described in RFC 3442.
Be careful: no checking is done that the correct type of data
for the option number is sent, it is quite possible to persuade
dnsmasq to generate illegal DHCP packets with injudicious use of
this flag. When the value is a decimal number, dnsmasq must
determine how large the data item is. It does this by examining
the option number and/or the value, but can be overridden by
appending a single letter flag as follows: b = one byte, s = two
bytes, i = four bytes. This is mainly useful with encapsulated
vendor class options (see below) where dnsmasq cannot determine
data size from the option number. Option data which consists
solely of periods and digits will be interpreted by dnsmasq as
an IP address, and inserted into an option as such. To force a
literal string, use quotes. For instance when using option 66 to
send a literal IP address as TFTP server name, it is necessary
to do --dhcp-option=66,"1.2.3.4"
Encapsulated Vendor-class options may also be specified using
--dhcp-option: for instance --dhcp-
option=vendor:PXEClient,1,0.0.0.0 sends the encapsulated vendor
class-specific option "mftp-address=0.0.0.0" to any client whose
vendor-class matches "PXEClient". The vendor-class matching is
substring based (see --dhcp-vendorclass for details). If a
vendor-class option (number 60) is sent by dnsmasq, then that is
used for selecting encapsulated options in preference to any
sent by the client. It is possible to omit the vendorclass
completely; --dhcp-option=vendor:,1,0.0.0.0 in which case the
encapsulated option is always sent.
Options may be encapsulated within other options: for instance
--dhcp-option=encap:175, 190, iscsi-client0 will send option
175, within which is the option 190. If multiple options are
given which are encapsulated with the same option number then
they will be correctly combined into one encapsulated option.
encap: and vendor: are may not both be set in the same dhcp-
option.
The final variant on encapsulated options is "Vendor-Identifying
Vendor Options" as specified by RFC3925. These are denoted like
this: --dhcp-option=vi-encap:2, 10, text The number in the vi-
encap: section is the IANA enterprise number used to identify
this option.
The address 0.0.0.0 is not treated specially in encapsulated
options.
--dhcp-option-force=[tag:<tag>,[tag:<tag>,]][encap:<opt>,][vi-
encap:<enterprise>,][vendor:[<vendor-class>],]<opt>,[<value>[,<value>]]
This works in exactly the same way as --dhcp-option except that
the option will always be sent, even if the client does not ask
for it in the parameter request list. This is sometimes needed,
for example when sending options to PXELinux.
--dhcp-no-override
Disable re-use of the DHCP servername and filename fields as
extra option space. If it can, dnsmasq moves the boot server and
filename information (from dhcp-boot) out of their dedicated
fields into DHCP options. This make extra space available in the
DHCP packet for options but can, rarely, confuse old or broken
clients. This flag forces "simple and safe" behaviour to avoid
problems in such a case.
-U, --dhcp-vendorclass=set:<tag>,<vendor-class>
Map from a vendor-class string to a tag. Most DHCP clients
provide a "vendor class" which represents, in some sense, the
type of host. This option maps vendor classes to tags, so that
DHCP options may be selectively delivered to different classes
of hosts. For example dhcp-vendorclass=set:printers,Hewlett-
Packard JetDirect will allow options to be set only for HP
printers like so: --dhcp-option=tag:printers,3,192.168.4.4 The
vendor-class string is substring matched against the vendor-
class supplied by the client, to allow fuzzy matching. The set:
prefix is optional but allowed for consistency.
-j, --dhcp-userclass=set:<tag>,<user-class>
Map from a user-class string to a tag (with substring matching,
like vendor classes). Most DHCP clients provide a "user class"
which is configurable. This option maps user classes to tags, so
that DHCP options may be selectively delivered to different
classes of hosts. It is possible, for instance to use this to
set a different printer server for hosts in the class "accounts"
than for hosts in the class "engineering".
-4, --dhcp-mac=set:<tag>,<MAC address>
Map from a MAC address to a tag. The MAC address may include
wildcards. For example --dhcp-mac=set:3com,01:34:23:*:*:* will
set the tag "3com" for any host whose MAC address matches the
pattern.
--dhcp-circuitid=set:<tag>,<circuit-id>, --dhcp-
remoteid=set:<tag>,<remote-id>
Map from RFC3046 relay agent options to tags. This data may be
provided by DHCP relay agents. The circuit-id or remote-id is
normally given as colon-separated hex, but is also allowed to be
a simple string. If an exact match is achieved between the
circuit or agent ID and one provided by a relay agent, the tag
is set.
--dhcp-subscrid=set:<tag>,<subscriber-id>
Map from RFC3993 subscriber-id relay agent options to tags.
--dhcp-proxy[=<ip addr>]......
A normal DHCP relay agent is only used to forward the initial
parts of a DHCP interaction to the DHCP server. Once a client is
configured, it communicates directly with the server. This is
undesirable if the relay agent is addding extra information to
the DHCP packets, such as that used by dhcp-circuitid and dhcp-
remoteid. A full relay implementation can use the RFC 5107
serverid-override option to force the DHCP server to use the
relay as a full proxy, with all packets passing through it. This
flag provides an alternative method of doing the same thing, for
relays which don’t support RFC 5107. Given alone, it manipulates
the server-id for all interactions via relays. If a list of IP
addresses is given, only interactions via relays at those
addresses are affected.
--dhcp-match=set:<tag>,<option number>|option:<option name>|vi-
encap:<enterprise>[,<value>]
Without a value, set the tag if the client sends a DHCP option
of the given number or name. When a value is given, set the tag
only if the option is sent and matches the value. The value may
be of the form "01:ff:*:02" in which case the value must match
(apart from widcards) but the option sent may have unmatched
data past the end of the value. The value may also be of the
same form as in dhcp-option in which case the option sent is
treated as an array, and one element must match, so
--dhcp-match=set:efi-ia32,option:client-arch,6
will set the tag "efi-ia32" if the the number 6 appears in the
list of architectures sent by the client in option 93. (See RFC
4578 for details.) If the value is a string, substring matching
is used.
The special form with vi-encap:<enterpise number> matches
against vendor-identifying vendor classes for the specified
enterprise. Please see RFC 3925 for more details of these rare
and interesting beasts.
--tag-if=set:<tag>[,set:<tag>[,tag:<tag>[,tag:<tag>]]]
Perform boolean operations on tags. Any tag appearing as
set:<tag> is set if all the tags which appear as tag:<tag> are
set, (or unset when tag:!<tag> is used) If no tag:<tag> appears
set:<tag> tags are set unconditionally. Any number of set: and
tag: forms may appear, in any order. Tag-if lines ares executed
in order, so if the tag in tag:<tag> is a tag set by another
tag-if, the line which sets the tag must precede the one which
tests it.
-J, --dhcp-ignore=tag:<tag>[,tag:<tag>]
When all the given tags appear in the tag set ignore the host
and do not allocate it a DHCP lease.
--dhcp-ignore-names[=tag:<tag>[,tag:<tag>]]
When all the given tags appear in the tag set, ignore any
hostname provided by the host. Note that, unlike dhcp-ignore, it
is permissible to supply no tags, in which case DHCP-client
supplied hostnames are always ignored, and DHCP hosts are added
to the DNS using only dhcp-host configuration in dnsmasq and the
contents of /etc/hosts and /etc/ethers.
--dhcp-generate-names=tag:<tag>[,tag:<tag>]
Generate a name for DHCP clients which do not otherwise have
one, using the MAC address expressed in hex, seperated by
dashes. Note that if a host provides a name, it will be used by
preference to this, unless --dhcp-ignore-names is set.
--dhcp-broadcast[=tag:<tag>[,tag:<tag>]]
When all the given tags appear in the tag set, always use
broadcast to communicate with the host when it is unconfigured.
It is permissible to supply no tags, in which case this is
unconditional. Most DHCP clients which need broadcast replies
set a flag in their requests so that this happens automatically,
some old BOOTP clients do not.
-M, --dhcp-boot=[tag:<tag>,]<filename>,[<servername>[,<server
address>]]
Set BOOTP options to be returned by the DHCP server. Server name
and address are optional: if not provided, the name is left
empty, and the address set to the address of the machine running
dnsmasq. If dnsmasq is providing a TFTP service (see --enable-
tftp ) then only the filename is required here to enable network
booting. If the optional tag(s) are given, they must match for
this configuration to be sent.
--pxe-service=[tag:<tag>,]<CSA>,<menu
text>[,<basename>|<bootservicetype>][,<server address>]
Most uses of PXE boot-ROMS simply allow the PXE system to obtain
an IP address and then download the file specified by dhcp-boot
and execute it. However the PXE system is capable of more
complex functions when supported by a suitable DHCP server.
This specifies a boot option which may appear in a PXE boot
menu. <CSA> is client system type, only services of the correct
type will appear in a menu. The known types are x86PC, PC98,
IA64_EFI, Alpha, Arc_x86, Intel_Lean_Client, IA32_EFI, BC_EFI,
Xscale_EFI and X86-64_EFI; an integer may be used for other
types. The parameter after the menu text may be a file name, in
which case dnsmasq acts as a boot server and directs the PXE
client to download the file by TFTP, either from itself (
enable-tftp must be set for this to work) or another TFTP server
if the final IP address is given. Note that the "layer" suffix
(normally ".0") is supplied by PXE, and should not be added to
the basename. If an integer boot service type, rather than a
basename is given, then the PXE client will search for a
suitable boot service for that type on the network. This search
may be done by broadcast, or direct to a server if its IP
address is provided. If no boot service type or filename is
provided (or a boot service type of 0 is specified) then the
menu entry will abort the net boot procedure and continue
booting from local media.
--pxe-prompt=[tag:<tag>,]<prompt>[,<timeout>]
Setting this provides a prompt to be displayed after PXE boot.
If the timeout is given then after the timeout has elapsed with
no keyboard input, the first available menu option will be
automatically executed. If the timeout is zero then the first
available menu item will be executed immediately. If pxe-prompt
is ommitted the system will wait for user input if there are
multiple items in the menu, but boot immediately if there is
only one. See pxe-service for details of menu items.
Dnsmasq supports PXE "proxy-DHCP", in this case another DHCP
server on the network is responsible for allocating IP
addresses, and dnsmasq simply provides the information given in
pxe-prompt and pxe-service to allow netbooting. This mode is
enabled using the proxy keyword in dhcp-range.
-X, --dhcp-lease-max=<number>
Limits dnsmasq to the specified maximum number of DHCP leases.
The default is 1000. This limit is to prevent DoS attacks from
hosts which create thousands of leases and use lots of memory in
the dnsmasq process.
-K, --dhcp-authoritative
Should be set when dnsmasq is definitely the only DHCP server on
a network. It changes the behaviour from strict RFC compliance
so that DHCP requests on unknown leases from unknown hosts are
not ignored. This allows new hosts to get a lease without a
tedious timeout under all circumstances. It also allows dnsmasq
to rebuild its lease database without each client needing to
reacquire a lease, if the database is lost.
--dhcp-alternate-port[=<server port>[,<client port>]]
Change the ports used for DHCP from the default. If this option
is given alone, without arguments, it changes the ports used for
DHCP from 67 and 68 to 1067 and 1068. If a single argument is
given, that port number is used for the server and the port
number plus one used for the client. Finally, two port numbers
allows arbitrary specification of both server and client ports
for DHCP.
-3, --bootp-dynamic[=<network-id>[,<network-id>]]
Enable dynamic allocation of IP addresses to BOOTP clients. Use
this with care, since each address allocated to a BOOTP client
is leased forever, and therefore becomes permanently unavailable
for re-use by other hosts. if this is given without tags, then
it unconditionally enables dynamic allocation. With tags, only
when the tags are all set. It may be repeated with different tag
sets.
-5, --no-ping
By default, the DHCP server will attempt to ensure that an
address in not in use before allocating it to a host. It does
this by sending an ICMP echo request (aka "ping") to the address
in question. If it gets a reply, then the address must already
be in use, and another is tried. This flag disables this check.
Use with caution.
--log-dhcp
Extra logging for DHCP: log all the options sent to DHCP clients
and the tags used to determine them.
-l, --dhcp-leasefile=<path>
Use the specified file to store DHCP lease information.
-6 --dhcp-script=<path>
Whenever a new DHCP lease is created, or an old one destroyed,
the executable specified by this option is run. <path> must be
an absolute pathname, no PATH search occurs. The arguments to
the process are "add", "old" or "del", the MAC address of the
host, the IP address, and the hostname, if known. "add" means a
lease has been created, "del" means it has been destroyed, "old"
is a notification of an existing lease when dnsmasq starts or a
change to MAC address or hostname of an existing lease (also,
lease length or expiry and client-id, if leasefile-ro is set).
If the MAC address is from a network type other than ethernet,
it will have the network type prepended, eg
"06-01:23:45:67:89:ab" for token ring. The process is run as
root (assuming that dnsmasq was originally run as root) even if
dnsmasq is configured to change UID to an unprivileged user.
The environment is inherited from the invoker of dnsmasq, with
some or all of the following variables added.
DNSMASQ_CLIENT_ID if the host provided a client-id.
DNSMASQ_DOMAIN if the fully-qualified domain name of the host is
known, this is set to the domain part.
If the client provides vendor-class, hostname or user-class,
these are provided in DNSMASQ_VENDOR_CLASS
DNSMASQ_SUPPLIED_HOSTNAME and
DNSMASQ_USER_CLASS0..DNSMASQ_USER_CLASSn variables, but only for
"add" actions or "old" actions when a host resumes an existing
lease, since these data are not held in dnsmasq’s lease
database.
If dnsmasq was compiled with HAVE_BROKEN_RTC, then the length of
the lease (in seconds) is stored in DNSMASQ_LEASE_LENGTH,
otherwise the time of lease expiry is stored in
DNSMASQ_LEASE_EXPIRES. The number of seconds until lease expiry
is always stored in DNSMASQ_TIME_REMAINING.
If a lease used to have a hostname, which is removed, an "old"
event is generated with the new state of the lease, ie no name,
and the former name is provided in the environment variable
DNSMASQ_OLD_HOSTNAME.
DNSMASQ_INTERFACE stores the name of the interface on which the
request arrived; this is not set for "old" actions when dnsmasq
restarts.
DNSMASQ_RELAY_ADDRESS is set if the client used a DHCP relay to
contact dnsmasq and the IP address of the relay is known.
DNSMASQ_TAGS contains all the tags set during the DHCP
transaction, separated by spaces.
All file descriptors are closed except stdin, stdout and stderr
which are open to /dev/null (except in debug mode).
The script is not invoked concurrently: at most one instance of
the script is ever running (dnsmasq waits for an instance of
script to exit before running the next). Changes to the lease
database are which require the script to be invoked are queued
awaiting exit of a running instance. If this queueing allows
multiple state changes occur to a single lease before the script
can be run then earlier states are discarded and the current
state of that lease is reflected when the script finally runs.
At dnsmasq startup, the script will be invoked for all existing
leases as they are read from the lease file. Expired leases will
be called with "del" and others with "old". When dnsmasq
receives a HUP signal, the script will be invoked for existing
leases with an "old " event.
--dhcp-scriptuser
Specify the user as which to run the lease-change script. This
defaults to root, but can be changed to another user using this
flag.
-9, --leasefile-ro
Completely suppress use of the lease database file. The file
will not be created, read, or written. Change the way the lease-
change script (if one is provided) is called, so that the lease
database may be maintained in external storage by the script. In
addition to the invocations given in --dhcp-script the lease-
change script is called once, at dnsmasq startup, with the
single argument "init". When called like this the script should
write the saved state of the lease database, in dnsmasq
leasefile format, to stdout and exit with zero exit code.
Setting this option also forces the leasechange script to be
called on changes to the client-id and lease length and expiry
time.
--bridge-interface=<interface>,<alias>[,<alias>]
Treat DHCP request packets arriving at any of the <alias>
interfaces as if they had arrived at <interface>. This option is
necessary when using "old style" bridging on BSD platforms,
since packets arrive at tap interfaces which don’t have an IP
address.
-s, --domain=<domain>[,<address range>]
Specifies DNS domains for the DHCP server. Domains may be be
given unconditionally (without the IP range) or for limited IP
ranges. This has two effects; firstly it causes the DHCP server
to return the domain to any hosts which request it, and secondly
it sets the domain which it is legal for DHCP-configured hosts
to claim. The intention is to constrain hostnames so that an
untrusted host on the LAN cannot advertise its name via dhcp as
e.g. "microsoft.com" and capture traffic not meant for it. If no
domain suffix is specified, then any DHCP hostname with a domain
part (ie with a period) will be disallowed and logged. If suffix
is specified, then hostnames with a domain part are allowed,
provided the domain part matches the suffix. In addition, when a
suffix is set then hostnames without a domain part have the
suffix added as an optional domain part. Eg on my network I can
set --domain=thekelleys.org.uk and have a machine whose DHCP
hostname is "laptop". The IP address for that machine is
available from dnsmasq both as "laptop" and
"laptop.thekelleys.org.uk". If the domain is given as "#" then
the domain is read from the first "search" directive in
/etc/resolv.conf (or equivalent). The address range can be of
the form <ip address>,<ip address> or <ip address>/<netmask> or
just a single <ip address>. See --dhcp-fqdn which can change the
behaviour of dnsmasq with domains.
--dhcp-fqdn
In the default mode, dnsmasq inserts the unqualified names of
DHCP clients into the DNS. For this reason, the names must be
unique, even if two clients which have the same name are in
different domains. If a second DHCP client appears which has the
same name as an existing client, the name is transfered to the
new client. If --dhcp-fqdn is set, this behaviour changes: the
unqualified name is no longer put in the DNS, only the qualified
name. Two DHCP clients with the same name may both keep the
name, provided that the domain part is different (ie the fully
qualified names differ.) To ensure that all names have a domain
part, there must be at least --domain without an address
specified when --dhcp-fqdn is set.
--enable-tftp[=<interface>]
Enable the TFTP server function. This is deliberately limited to
that needed to net-boot a client. Only reading is allowed; the
tsize and blksize extensions are supported (tsize is only
supported in octet mode). See NOTES section for use of the
interface argument.
--tftp-root=<directory>[,<interface>]
Look for files to transfer using TFTP relative to the given
directory. When this is set, TFTP paths which include ".." are
rejected, to stop clients getting outside the specified root.
Absolute paths (starting with /) are allowed, but they must be
within the tftp-root. If the optional interface argument is
given, the directory is only used for TFTP requests via that
interface.
--tftp-unique-root
Add the IP address of the TFTP client as a path component on the
end of the TFTP-root (in standard dotted-quad format). Only
valid if a tftp-root is set and the directory exists. For
instance, if tftp-root is "/tftp" and client 1.2.3.4 requests
file "myfile" then the effective path will be
"/tftp/1.2.3.4/myfile" if /tftp/1.2.3.4 exists or /tftp/myfile
otherwise.
--tftp-secure
Enable TFTP secure mode: without this, any file which is
readable by the dnsmasq process under normal unix access-control
rules is available via TFTP. When the --tftp-secure flag is
given, only files owned by the user running the dnsmasq process
are accessible. If dnsmasq is being run as root, different rules
apply: --tftp-secure has no effect, but only files which have
the world-readable bit set are accessible. It is not recommended
to run dnsmasq as root with TFTP enabled, and certainly not
without specifying --tftp-root. Doing so can expose any world-
readable file on the server to any host on the net.
--tftp-max=<connections>
Set the maximum number of concurrent TFTP connections allowed.
This defaults to 50. When serving a large number of TFTP
connections, per-process file descriptor limits may be
encountered. Dnsmasq needs one file descriptor for each
concurrent TFTP connection and one file descriptor per unique
file (plus a few others). So serving the same file
simultaneously to n clients will use require about n + 10 file
descriptors, serving different files simultaneously to n clients
will require about (2*n) + 10 descriptors. If --tftp-port-range
is given, that can affect the number of concurrent connections.
--tftp-no-blocksize
Stop the TFTP server from negotiating the "blocksize" option
with a client. Some buggy clients request this option but then
behave badly when it is granted.
--tftp-port-range=<start>,<end>
A TFTP server listens on a well-known port (69) for connection
initiation, but it also uses a dynamically-allocated port for
each connection. Normally these are allocated by the OS, but
this option specifies a range of ports for use by TFTP
transfers. This can be useful when TFTP has to traverse a
firewall. The start of the range cannot be lower than 1025
unless dnsmasq is running as root. The number of concurrent TFTP
connections is limited by the size of the port range.
-C, --conf-file=<file>
Specify a different configuration file. The conf-file option is
also allowed in configuration files, to include multiple
configuration files.
-7, --conf-dir=<directory>[,<file-extension>......]
Read all the files in the given directory as configuration
files. If extension(s) are given, any files which end in those
extensions are skipped. Any files whose names end in ~ or start
with . or start and end with # are always skipped. This flag may
be given on the command line or in a configuration file.
CONFIG FILE
At startup, dnsmasq reads /etc/dnsmasq.conf, if it exists. (On FreeBSD,
the file is /usr/local/etc/dnsmasq.conf ) (but see the -C and -7
options.) The format of this file consists of one option per line,
exactly as the long options detailed in the OPTIONS section but without
the leading "--". Lines starting with # are comments and ignored. For
options which may only be specified once, the configuration file
overrides the command line. Quoting is allowed in a config file:
between " quotes the special meanings of ,:. and # are removed and the
following escapes are allowed: \\ \" \t \e \b \r and \n. The later
corresponding to tab, escape, backspace, return and newline.
NOTES
When it receives a SIGHUP, dnsmasq clears its cache and then re-loads
/etc/hosts and /etc/ethers and any file given by --dhcp-hostsfile,
--dhcp-optsfile or --addn-hosts. The dhcp lease change script is
called for all existing DHCP leases. If --no-poll is set SIGHUP also
re-reads /etc/resolv.conf. SIGHUP does NOT re-read the configuration
file.
When it receives a SIGUSR1, dnsmasq writes statistics to the system
log. It writes the cache size, the number of names which have had to
removed from the cache before they expired in order to make room for
new names and the total number of names that have been inserted into
the cache. For each upstream server it gives the number of queries
sent, and the number which resulted in an error. In --no-daemon mode or
when full logging is enabled (-q), a complete dump of the contents of
the cache is made.
When it receives SIGUSR2 and it is logging direct to a file (see --log-
facility ) dnsmasq will close and reopen the log file. Note that during
this operation, dnsmasq will not be running as root. When it first
creates the logfile dnsmasq changes the ownership of the file to the
non-root user it will run as. Logrotate should be configured to create
a new log file with the ownership which matches the existing one before
sending SIGUSR2. If TCP DNS queries are in progress, the old logfile
will remain open in child processes which are handling TCP queries and
may continue to be written. There is a limit of 150 seconds, after
which all existing TCP processes will have expired: for this reason, it
is not wise to configure logfile compression for logfiles which have
just been rotated. Using logrotate, the required options are create and
delaycompress.
Dnsmasq is a DNS query forwarder: it it not capable of recursively
answering arbitrary queries starting from the root servers but forwards
such queries to a fully recursive upstream DNS server which is
typically provided by an ISP. By default, dnsmasq reads
/etc/resolv.conf to discover the IP addresses of the upstream
nameservers it should use, since the information is typically stored
there. Unless --no-poll is used, dnsmasq checks the modification time
of /etc/resolv.conf (or equivalent if --resolv-file is used) and re-
reads it if it changes. This allows the DNS servers to be set
dynamically by PPP or DHCP since both protocols provide the
information. Absence of /etc/resolv.conf is not an error since it may
not have been created before a PPP connection exists. Dnsmasq simply
keeps checking in case /etc/resolv.conf is created at any time. Dnsmasq
can be told to parse more than one resolv.conf file. This is useful on
a laptop, where both PPP and DHCP may be used: dnsmasq can be set to
poll both /etc/ppp/resolv.conf and /etc/dhcpc/resolv.conf and will use
the contents of whichever changed last, giving automatic switching
between DNS servers.
Upstream servers may also be specified on the command line or in the
configuration file. These server specifications optionally take a
domain name which tells dnsmasq to use that server only to find names
in that particular domain.
In order to configure dnsmasq to act as cache for the host on which it
is running, put "nameserver 127.0.0.1" in /etc/resolv.conf to force
local processes to send queries to dnsmasq. Then either specify the
upstream servers directly to dnsmasq using --server options or put
their addresses real in another file, say /etc/resolv.dnsmasq and run
dnsmasq with the -r /etc/resolv.dnsmasq option. This second technique
allows for dynamic update of the server addresses by PPP or DHCP.
Addresses in /etc/hosts will "shadow" different addresses for the same
names in the upstream DNS, so "mycompany.com 1.2.3.4" in /etc/hosts
will ensure that queries for "mycompany.com" always return 1.2.3.4 even
if queries in the upstream DNS would otherwise return a different
address. There is one exception to this: if the upstream DNS contains a
CNAME which points to a shadowed name, then looking up the CNAME
through dnsmasq will result in the unshadowed address associated with
the target of the CNAME. To work around this, add the CNAME to
/etc/hosts so that the CNAME is shadowed too.
The tag system works as follows: For each DHCP request, dnsmasq
collects a set of valid tags from active configuration lines which
include set:<tag>, including one from the dhcp-range used to allocate
the address, one from any matching dhcp-host (and "known" if a dhcp-
host matches) The tag "bootp" is set for BOOTP requests, and a tag
whose name is the name of the interface on which the request arrived is
also set.
Any configuration lines which includes one or more tag:<tag> contructs
will only be valid if all that tags are matched in the set derived
above. Typically this is dhcp-option. dhcp-option which has tags will
be used in preference to an untagged dhcp-option, provided that _all_
the tags match somewhere in the set collected as described above. The
prefix ’!’ on a tag means ’not’ so --dhcp=option=tag:!purple,3,1.2.3.4
sends the option when the tag purple is not in the set of valid tags.
(If using this in a command line rather than a configuration file, be
sure to escape !, which is a shell metacharacter)
Note that for dhcp-range both tag:<tag> and set:<tag> are allowed, to
both select the range in use based on (eg) dhcp-host, and to affect the
options sent, based on the range selected.
This system evolved from an earlier, more limited one and for backward
compatibility "net:" may be used instead of "tag:" and "set:" may be
omitted. (Except in dhcp-host, where "net:" may be used instead of
"set:".) For the same reason, ’#’ may be used instead of ’!’ to
indicate NOT.
The DHCP server in dnsmasq will function as a BOOTP server also,
provided that the MAC address and IP address for clients are given,
either using dhcp-host configurations or in /etc/ethers , and a dhcp-
range configuration option is present to activate the DHCP server on a
particular network. (Setting --bootp-dynamic removes the need for
static address mappings.) The filename parameter in a BOOTP request is
used as a tag, as is the tag "bootp", allowing some control over the
options returned to different classes of hosts.
dhcp-range may have an interface name supplied as
"interface:<interface-name>". The semantics if this are as follows: For
DHCP, if any other dhcp-range exists _without_ an interface name, then
the interface name is ignored and and dnsmasq behaves as if the
interface parts did not exist, otherwise DHCP is only provided to
interfaces mentioned in dhcp-range declarations. For DNS, if there are
no --interface or --listen-address flags, behaviour is unchanged by the
interface part. If either of these flags are present, the interfaces
mentioned in dhcp-ranges are added to the set which get DNS service.
Similarly, enable-tftp may take an interface name, which enables TFTP
only for a particular interface, ignoring --interface or --listen-
address flags. In addition --tftp-secure and --tftp-unique-root and
--tftp-no-blocksize are ignored for requests from such interfaces. (A
--tftp-root directive giving a root path and an interface should be
provided too.)
These rules may seem odd at first sight, but they allow a single line
of the form "dhcp-range=interface:virt0,192.168.0.4,192.168.0.200" to
be added to dnsmasq configuration which then supplies DHCP and DNS
services to that interface, without affecting what services are
supplied to other interfaces and irrespective of the existance or lack
of "interface=<interface>" lines elsewhere in the dnsmasq
configuration. "enable-tftp=virt0" and "tftp-root=<root>,virt0" do the
same job for TFTP.
The idea is that such a line can be added automatically by libvirt or
equivalent systems, without disturbing any manual configuration.
EXIT CODES
0 - Dnsmasq successfully forked into the background, or terminated
normally if backgrounding is not enabled.
1 - A problem with configuration was detected.
2 - A problem with network access occurred (address in use, attempt to
use privileged ports without permission).
3 - A problem occurred with a filesystem operation (missing
file/directory, permissions).
4 - Memory allocation failure.
5 - Other miscellaneous problem.
11 or greater - a non zero return code was received from the lease-
script process "init" call. The exit code from dnsmasq is the script’s
exit code with 10 added.
LIMITS
The default values for resource limits in dnsmasq are generally
conservative, and appropriate for embedded router type devices with
slow processors and limited memory. On more capable hardware, it is
possible to increase the limits, and handle many more clients. The
following applies to dnsmasq-2.37: earlier versions did not scale as
well.
Dnsmasq is capable of handling DNS and DHCP for at least a thousand
clients. The DHCP lease times should not be very short (less than one
hour). The value of --dns-forward-max can be increased: start with it
equal to the number of clients and increase if DNS seems slow. Note
that DNS performance depends too on the performance of the upstream
nameservers. The size of the DNS cache may be increased: the hard limit
is 10000 names and the default (150) is very low. Sending SIGUSR1 to
dnsmasq makes it log information which is useful for tuning the cache
size. See the NOTES section for details.
The built-in TFTP server is capable of many simultaneous file
transfers: the absolute limit is related to the number of file-handles
allowed to a process and the ability of the select() system call to
cope with large numbers of file handles. If the limit is set too high
using --tftp-max it will be scaled down and the actual limit logged at
start-up. Note that more transfers are possible when the same file is
being sent than when each transfer sends a different file.
It is possible to use dnsmasq to block Web advertising by using a list
of known banner-ad servers, all resolving to 127.0.0.1 or 0.0.0.0, in
/etc/hosts or an additional hosts file. The list can be very long,
dnsmasq has been tested successfully with one million names. That size
file needs a 1GHz processor and about 60Mb of RAM.
INTERNATIONALISATION
Dnsmasq can be compiled to support internationalisation. To do this,
the make targets "all-i18n" and "install-i18n" should be used instead
of the standard targets "all" and "install". When internationalisation
is compiled in, dnsmasq will produce log messages in the local language
and support internationalised domain names (IDN). Domain names in
/etc/hosts, /etc/ethers and /etc/dnsmasq.conf which contain non-ASCII
characters will be translated to the DNS-internal punycode
representation. Note that dnsmasq determines both the language for
messages and the assumed charset for configuration files from the LANG
environment variable. This should be set to the system default value by
the script which is responsible for starting dnsmasq. When editing the
configuration files, be careful to do so using only the system-default
locale and not user-specific one, since dnsmasq has no direct way of
determining the charset in use, and must assume that it is the system
default.
FILES
/etc/dnsmasq.conf
/usr/local/etc/dnsmasq.conf
/etc/resolv.conf
/etc/hosts
/etc/ethers
/var/lib/misc/dnsmasq.leases
/var/db/dnsmasq.leases
/var/run/dnsmasq.pid
SEE ALSO
hosts(5), resolver(5)
AUTHOR
This manual page was written by Simon Kelley <simon@thekelleys.org.uk>.