DNSMASQ(8)DNSMASQ(8)NAMEdnsmasq - A lightweight DHCP and caching DNS server.
SYNOPSISdnsmasq [OPTION]...
DESCRIPTIONdnsmasq 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, multiple
networks, DHCP-relay and RFC3011 subnet specifiers. 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.
-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.
-E, --expand-hosts
Add the domain to simple names (without a period) in /etc/hosts
in the same way as for DHCP-derived names.
-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 situ‐
ations. 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 circum‐
stances.
--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, dns‐
masq does not cache the reply. This option gives a default value
for time-to-live (in seconds) which dnsmasq uses to cache nega‐
tive replies even in the absence of an SOA record.
-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 dae‐
montools 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 opera‐
tion. 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. (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. Nor‐
mally 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 1280, which is the RFC2671-recom‐
mended maximum for ethernet.
-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 sin‐
gle port allocated to it by the OS: this was the default behav‐
iour 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 fire‐
walls.
-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 avail‐
able interfaces except any given in --except-interface options.
IP alias interfaces (eg "eth1:0") cannot be used with --inter‐
face 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 --lis‐
ten-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 auto‐
matically 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 dis‐
cards requests that it shouldn't reply to. This has the advan‐
tage 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 inter‐
face 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>,<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".
-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, sub‐
sequent 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 dns‐
masq 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 net‐
work.
-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>|<inter‐
face>[#<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 giv‐
ing 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.
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 syn‐
onym 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 dns‐
masq 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 inter‐
face; 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 over‐
ride this for individual names. A common use of this is to redi‐
rect 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 spe‐
cific --server directive.
-m, --mx-host=<mx name>[[,<hostname>],<preference>]
Return an MX record named <mx name> pointing to the given host‐
name (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 dns‐
masq. 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>[,<prior‐
ity>[,<weight>]]]]
Return a SRV DNS record. See RFC2782 for details. If not sup‐
plied, 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>,<reg‐
exp>[,<replacement>]
Return an NAPTR DNS record, as specified in RFC3403.
--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-exis‐
tent, 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. Set‐
ting 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=[[net:]network-id,]<start-addr>,<end-addr>[[,<net‐
mask>],<broadcast>][,<default 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 the literal "infinite". The minimum lease time is two min‐
utes. This option may be repeated, with different addresses, to
enable DHCP service to more than one network. For directly con‐
nected networks (ie, networks on which the machine running dns‐
masq 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. On some broken
systems, dnsmasq can listen on only one interface when using
DHCP, and the name of that interface must be given using the
interface option. This limitation currently affects OpenBSD
before version 4.0. It is always allowed to have more than one
dhcp-range in a single subnet. The optional network-id is a
alphanumeric label which marks this network so that dhcp options
may be specified on a per-network basis. When it is prefixed
with 'net:' 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 speci‐
fied, but not to dynamically allocate IP addresses. Only hosts
which have static addresses given via dhcp-host or from
/etc/ethers will be served.
-G, --dhcp-
host=[<hwaddr>][,id:<client_id>|*][,net:<netid>][,<ipaddr>][,<host‐
name>][,<lease_time>][,ignore]
Specify per host parameters for the DHCP server. This allows a
machine with a particular hardware address to be always allo‐
cated 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 on the network being served by the DHCP server. 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 iden‐
tifier 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.
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 net:<network-id> sets the network-id tag whenever
this dhcp-host directive is in use.This can be used to selec‐
tively send DHCP options just for this host. When a host matches
any dhcp-host directive (or one implied by /etc/ethers) then the
special network-id tag "known" is set. This allows dnsmasq to be
configured to ignore requests from unknown machines using
--dhcp-ignore=#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 configura‐
tion 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.
--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 advan‐
tage of using this option is the same as for --dhcp-hostsfile:
the dhcp-optsfile will be re-read when dnsmasq receives SIGHUP.
-Z, --read-ethers
Read /etc/ethers for information about hosts for the DHCP
server. The format of /etc/ethers is a hardware address, fol‐
lowed 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=[<network-id>,[<network-id>,]][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
network-ids are given then this option is only sent when all the
network-ids 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 argu‐
ments 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:PXE‐
Client,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. The address 0.0.0.0 is not treated spe‐
cially in encapsulated vendor class options.
--dhcp-option-force=[<network-id>,[<network-id>,]][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=<network-id>,<vendor-class>
Map from a vendor-class string to a network id 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 dif‐
ferent classes of hosts. For example dhcp-vendorclass=print‐
ers,Hewlett-Packard JetDirect will allow options to be set only
for HP printers like so: --dhcp-option=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.
-j, --dhcp-userclass=<network-id>,<user-class>
Map from a user-class string to a network id 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 deliv‐
ered 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=<network-id>,<MAC address>
Map from a MAC address to a network-id tag. The MAC address may
include wildcards. For example --dhcp-mac=3com,01:34:23:*:*:*
will set the tag "3com" for any host whose MAC address matches
the pattern.
--dhcp-circuitid=<network-id>,<circuit-id>, --dhcp-remoteid=<network-
id>,<remote-id>
Map from RFC3046 relay agent options to network-id 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 network-id tag is set.
--dhcp-subscrid=<network-id>,<subscriber-id>
Map from RFC3993 subscriber-id relay agent options to network-id
tags.
--dhcp-match=<network-id>,<option number>
Set the network-id tag if the client sends a DHCP option of the
given number. This can be used to identify particular clients
which send information using private option numbers.
-J, --dhcp-ignore=<network-id>[,<network-id>]
When all the given network-ids match the set of network-ids
derived from the net, host, vendor and user classes, ignore the
host and do not allocate it a DHCP lease.
--dhcp-ignore-names[=<network-id>[,<network-id>]]
When all the given network-ids match the set of network-ids
derived from the net, host, vendor and user classes, ignore any
hostname provided by the host. Note that, unlike dhcp-ignore, it
is permissible to supply no netid 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-broadcast=<network-id>[,<network-id>]
When all the given network-ids match the set of network-ids
derived from the net, host, vendor and user classes, always use
broadcast to communicate with the host when it is unconfigured.
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=[net:<network-id>,]<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 network-id(s) are given, they must
match for this configuration to be sent. Note that network-ids
are prefixed by "net:" to distinguish them.
-X, --dhcp-lease-max=<number>
Limits dnsmasq to the specified maximum number of DHCP leases.
The default is 150. 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 num‐
ber plus one used for the client. Finally, two port numbers
allows arbitrary specification of both server and client ports
for DHCP.
-3, --bootp-dynamic
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.
-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 netid tags used to determine them.
-l, --dhcp-leasefile=<path>
Use the specified file to store DHCP lease information. If this
option is given but no dhcp-range option is given then dnsmasq
version 1 behaviour is activated. The file given is assumed to
be an ISC dhcpd lease file and parsed for leases which are then
added to the DNS system if they have a hostname. This function‐
ality may have been excluded from dnsmasq at compile time, in
which case an error will occur. In any case note that ISC lease‐
file integration is a deprecated feature. It should not be used
in new installations, and will be removed in a future release.
-6 --dhcp-script=<path>
Whenever a new DHCP lease is created, or an old one destroyed,
the binary specified by this option is run. The arguments to the
process are "add", "old" or "del", the MAC address of the host
(or "<null>"), 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). The process is run as root (assuming that
dnsmasq was originally run as root) even if dnsmasq is config‐
ured to change UID to an unprivileged user. The environment is
inherited from the invoker of dnsmasq, and if the host provided
a client-id, this is stored in the environment variable DNS‐
MASQ_CLIENT_ID. If the client provides vendor-class or user-
class information, these are provided in DNSMASQ_VENDOR_CLASS
and DNSMASQ_USER_CLASS0..DNSMASQ_USER_CLASSn variables, but only
for "add" actions or "old" actions when a host resumes an exist‐
ing 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 DNS‐
MASQ_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 DNS‐
MASQ_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. 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: if subsequent lease changes occur, the script is
not invoked again until any existing invocation exits. At dns‐
masq 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". <path> must be an abso‐
lute pathname, no PATH search occurs. 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 sin‐
gle argument "init". When called like this the script should
write the saved state of the lease database, in dnsmasq lease‐
file 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> inter‐
faces as if they had arrived at <interface>. This option is only
available on BSD platforms, and is necessary when using "old
style" bridging, since packets arrive at tap interfaces which
don't have an IP address.
-s, --domain=<domain>
Specifies the domain for the DHCP server. 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=thekel‐
leys.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).
--enable-tftp
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 sup‐
ported in octet mode).
--tftp-root=<directory>
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.
--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 read‐
able 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 connec‐
tions, per-process file descriptor limits may be encountered.
Dnsmasq needs one file descriptor for each concurrent TFTP con‐
nection and one file descriptor per unique file (plus a few oth‐
ers). 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 trans‐
fers. 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 config‐
uration files.
-7, --conf-dir=<directory>
Read all the files in the given directory as configuration
files. Files whose names end in ~ or start with . or start and
end with # are 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 over‐
rides 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 cre‐
ates 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 typi‐
cally 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 cre‐
ated 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 network-id system works as follows: For each DHCP request, dnsmasq
collects a set of valid network-id tags, one from the dhcp-range used
to allocate the address, one from any matching dhcp-host and possibly
many from matching vendor classes and user classes sent by the DHCP
client. Any dhcp-option which has network-id tags will be used in pref‐
erence 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=#purple,3,1.2.3.4 sends the option
when the network-id tag purple is not in the set of valid tags.
If the network-id in a dhcp-range is prefixed with 'net:' then its
meaning changes from setting a tag to matching it. Thus if there is
more than dhcp-range on a subnet, and one is tagged with a network-id
which is set (for instance from a vendorclass option) then hosts which
set the netid tag will be allocated addresses in the tagged range.
The DHCP server in dnsmasq will function as a BOOTP server also, pro‐
vided 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 par‐
ticular network. (Setting --bootp-dynamic removes the need for static
address mappings.) The filename parameter in a BOOTP request is matched
against netids in dhcp-option configurations, as is the tag "bootp",
allowing some control over the options returned to different classes of
hosts.
EXIT CODES
0 - Dnsmasq successfully forked into the background, or terminated nor‐
mally 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/direc‐
tory, 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 conser‐
vative, and appropriate for embedded router type devices with slow pro‐
cessors 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. Clearly to do this the value of --dhcp-lease-max must be
increased, and 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 trans‐
fers: 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, dns‐
masq has been tested successfully with one million names. That size
file needs a 1GHz processor and about 60Mb of RAM.
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 ALSOhosts(5), resolver(5)AUTHOR
This manual page was written by Simon Kelley <simon@thekelleys.org.uk>.
DNSMASQ(8)
