Manual: dnsmasq
DNSMASQ(8) System Manager's Manual DNSMASQ(8)
NAME
dnsmasq - A lightweight DHCP and caching DNS server.
SYNOPSIS
dnsmasq [OPTION]...
DESCRIPTION
dnsmasq is a lightweight DNS, TFTP, PXE, router advertisement and DHCP server. It is intended to proā
vide 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. It
can also act as the authoritative DNS server for one or more domains, allowing local names to appear
in the global DNS. It can be configured to do DNSSEC validation.
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. The PXE support is full featured, and includes a
proxy mode which supplies PXE information to clients whilst DHCP address allocation is done by anā
other server.
The dnsmasq DHCPv6 server provides the same set of features as the DHCPv4 server, and in addition, it
includes router advertisements and a neat feature which allows naming for clients which use DHCPv4
and stateless autoconfiguration only for IPv6 configuration. There is support for doing address alloā
cation (both DHCPv6 and RA) from subnets which are dynamically delegated via DHCPv6 prefix delegaā
tion.
Dnsmasq is coded with small embedded systems in mind. It aims for the smallest possible memory footā
print compatible with the supported functions, and allows unneeded functions to be omitted from the
compiled binary.
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.
-w, --help
Display all command-line options. --help dhcp will display known DHCPv4 configuration opā
tions, and --help dhcp6 will display DHCPv6 options.
-h, --no-hosts
Don't read the hostnames in /etc/hosts.
-H, --addn-hosts=
Additional hosts file. Read the specified file as well as /etc/hosts. If --no-hosts 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 in alphaā
betical order.
--hostsdir=
Read all the hosts files contained in the directory. New or changed files are read automatiā
cally and modified and deleted files have removed records automatically deleted.
-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=
When replying with information from /etc/hosts or configuration or the DHCP leases file dnsā
masq by default sets the time-to-live field to zero, meaning that the requester should not itā
self cache the information. This is the correct thing to do in almost all situations. This opā
tion 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.
--dhcp-ttl=
As for --local-ttl, but affects only replies with information from DHCP leases. If both are
given, --dhcp-ttl applies for DHCP information, and --local-ttl for others. Setting this to
zero eliminates the effect of --local-ttl for DHCP.
--neg-ttl=
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 inforā
mation, 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=
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.
--max-cache-ttl=
Set a maximum TTL value for entries in the cache.
--min-cache-ttl=
Extend short TTL values to the time given when caching them. Note that artificially extending
TTL values is in general a bad idea, do not do it unless you have a good reason, and underā
stand what you are doing. Dnsmasq limits the value of this option to one hour, unless recomā
piled.
--auth-ttl=
Set the TTL value returned in answers from the authoritative server.
--fast-dns-retry=[[,]]
Under normal circumstances, dnsmasq relies on DNS clients to do retries; it does not generate
timeouts itself. Setting this option instructs dnsmasq to generate its own retries starting
after a delay which defaults to 1000ms. If the second parameter is given this controls how
long the retries will continue for otherwise this defaults to 10000ms. Retries are repeated
with exponential backoff. Using this option increases memory usage and network bandwidth. If
not otherwise configured, this option is activated with the default parameters when --dnssec
is set.
-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, generā
ate a complete cache dump on receipt on SIGUSR1, log to stderr as well as syslog, don't fork
new processes to handle TCP queries. Note that this option is for use in debugging only, to
stop dnsmasq daemonising in production, use --keep-in-foreground.
-q, --log-queries
Log the results of DNS queries handled by dnsmasq. Enable a full cache dump on receipt of SIā
GUSR1. If the argument "extra" is supplied, ie --log-queries=extra then the log has extra inā
formation at the start of each line. This consists of a serial number which ties together the
log lines associated with an individual query, and the IP address of the requestor. If the arā
gument "proto" is supplied, this shows everything that "extra" does and also the network proā
tocol used to communicate the queries. Logging of only queries to the authoritative server can
be configured with --log-queries=auth
-8, --log-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 '/' charā
acter, 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 dnsā
masq.
--log-debug
Enable extra logging intended for debugging rather than information.
--log-async[=]
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=
Specify an alternate path for dnsmasq to record its process-id in. Normally /var/run/dnsā
masq.pid.
-u, --user=
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=
Specify the group which dnsmasq will run as. The default is "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=
Listen on instead of the standard DNS port (53). Setting this to zero completely disā
ables DNS function, leaving only DHCP and/or TFTP.
-P, --edns-packet-max=
Specify the largest EDNS.0 UDP packet which is supported by the DNS forwarder. Defaults to
1232, which is the recommended size following the DNS flag day in 2020. Only increase if you
know what you are doing.
-Q, --query-port=
Send outbound DNS queries from, and listen for their replies on, the specific UDP 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.
--port-limit=<#ports>
By default, when sending a query via random ports to multiple upstream servers or retrying a
query dnsmasq will use a single random port for all the tries/retries. This option allows a
larger number of ports to be used, which can increase robustness in certain network configuraā
tions. Note that increasing this to more than two or three can have security and resource imā
plications and should only be done with understanding of those.
--min-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 be
larger than that specified. Useful for systems behind firewalls. If not specified, defaults to
1024.
--max-port=
Use ports lower 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 be
lower than that specified. Useful for systems behind firewalls.
-i, --interface=
Listen only on the specified interface(s). Dnsmasq automatically adds the loopback (local) inā
terface to the list of interfaces to use when the --interface option is used. If no --interā
face or --listen-address options are given dnsmasq listens on all available interfaces except
any given in --except-interface options. On Linux, when --bind-interfaces or --bind-dynamic
are in effect, IP alias interface labels (eg "eth1:0") are checked, rather than interface
names. In the degenerate case when an interface has one address, this amounts to the same
thing but when an interface has multiple addresses it allows control over which of those adā
dresses are accepted. The same effect is achievable in default mode by using --listen-adā
dress. A simple wildcard, consisting of a trailing '*', can be used in --interface and --exā
cept-interface options.
-I, --except-interface=
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. The comments about interface labels for --listen-address apply here.
--auth-server=,[|...]
Enable DNS authoritative mode for queries arriving at an interface or address. Note that the
interface or address need not be mentioned in --interface or --listen-address configuration,
indeed --auth-server will override these and provide a different DNS service on the specified
interface. The is the "glue record". It should resolve in the global DNS to an A
and/or AAAA record which points to the address dnsmasq is listening on. When an interface is
specified, it may be qualified with "/4" or "/6" to specify only the IPv4 or IPv6 addresses
associated with the interface. Since any defined authoritative zones are also available as
part of the normal recusive DNS service supplied by dnsmasq, it can make sense to have an
--auth-server declaration with no interfaces or address, but simply specifying the primary exā
ternal nameserver.
--local-service[=net|host]
Without parameter or with net parameter, restricts service to connected network. Accept DNS
queries only from hosts whose address is on a local subnet, ie a subnet for which an interface
exists on the server. With host parameter, listens only on lo interface and accepts queries
from localhost only. This option only has effect if there are no --interface, --except-interā
face, --listen-address or --auth-server options. It is intended to be set as a default on inā
stallation, to allow unconfigured installations to be useful but also safe from being used for
DNS amplification attacks.
-2, --no-dhcp-interface=
Do not provide DHCP, TFTP or router advertisement on the specified interface, but do provide
DNS service.
--no-dhcpv4-interface=
Disable only IPv4 DHCP on the specified interface.
--no-dhcpv6-interface=
Disable IPv6 DHCP and router advertisement on the specified interface.
-a, --listen-address=
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 --inā
terface 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 adā
vantage 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 maā
chine. Setting this option also enables multiple instances of dnsmasq which provide DHCP serā
vice to run in the same machine.
--bind-dynamic
Enable a network mode which is a hybrid between --bind-interfaces and the default. Dnsmasq
binds the address of individual interfaces, allowing multiple dnsmasq instances, but if new
interfaces or addresses appear, it automatically listens on those (subject to any access-conā
trol configuration). This makes dynamically created interfaces work in the same way as the deā
fault. Implementing this option requires non-standard networking APIs and it is only available
under Linux. On other platforms it falls-back to --bind-interfaces mode.
-y, --localise-queries
Return answers to DNS queries from /etc/hosts and --interface-name and --dynamic-host which
depend on the interface over which the query was received. If a name 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 and return all
the available addresses otherwise. 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. The set of prefixes affected is the list given in
RFC6303, for IPv4 and IPv6. Enabling this also subtly alters DNSSEC validation for reverse
lookups in the private ranges such that a non-secure DS record is accepted as proof that the
range is not signed. This works around behaviour by the public DNS services which seem not to
return validated proof-of-non-existence for DS records in these domains.
-V, --alias=[]|[-],[,]
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=[/prefix]
Transform replies which contain the specified address or subnet into "No such domain" replies.
IPv4 and IPv6 are supported. 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 adā
dress being returned by Verisign is 64.94.110.11
--ignore-address=[/prefix]
Ignore replies to A or AAAA queries which include the specified address or subnet. No error
is generated, dnsmasq simply continues to listen for another reply. This is useful to defeat
blocking strategies which rely on quickly supplying a forged answer to a DNS request for cerā
tain domain, before the correct answer can arrive.
-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 type ANY where the reā
quested name has underscores, to catch LDAP requests, and for all records of types SOA and
SRV.
--filter-A
Remove A records from answers. No IPv4 addresses will be returned.
--filter-AAAA
Remove AAAA records from answers. No IPv6 addresses will be returned.
--filter-rr=[,...]
Remove records of the specified type(s) from answers. The otherwise-nonsensical --filter-
rr=ANY has a special meaning: it filters replies to queries for type ANY. Everything other
than A, AAAA, MX and CNAME records are removed. Since ANY queries with forged source addresses
can be used in DNS amplification attacks (replies to ANY queries can be large) this defangs
such attacks, whilst still supporting the one remaining possible use of ANY queries. See RFC
8482 para 4.3 for details.
--cache-rr=[,...]
By default, dnsmasq caches A, AAAA, CNAME and SRV DNS record types. This option adds other
record types to the cache. The RR-type can be given as a name such as TXT or MX or a decimal
number. A single --cache-rr option can take a comma-separated list of RR-types and more than
one --cache-rr option is allowed. Use --cache-rr=ANY to enable caching for all RR-types.
-r, --resolv-file=
Read the IP addresses of the upstream nameservers from , instead of /etc/resolv.conf.
For the format of this file see resolv.conf(5) . The only lines relevant to dnsmasq are nameā
server 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. If the service name is given, dnsmasq provides serā
vice at that name, rather than the default which is uk.org.thekelleys.dnsmasq
--enable-ubus[=]
Enable dnsmasq UBus interface. It sends notifications via UBus on DHCPACK and DHCPRELEASE
events. Furthermore it offers metrics and allows configuration of Linux connection track mark
based filtering. When DNS query filtering based on Linux connection track marks is enabled
UBus notifications are generated for each resolved or filtered DNS query. Requires that dnsā
masq has been built with UBus support. If the service name is given, dnsmasq provides service
at that namespace, rather than the default which is dnsmasq
-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 reā
quester.
--dns-loop-detect
Enable code to detect DNS forwarding loops; ie the situation where a query sent to one of the
upstream server eventually returns as a new query to the dnsmasq instance. The process works
by generating TXT queries of the form .test and sending them to each upstream server. The
hex is a UID which encodes the instance of dnsmasq sending the query and the upstream server
to which it was sent. If the query returns to the server which sent it, then the upstream
server through which it was sent is disabled and this event is logged. Each time the set of
upstream servers changes, the test is re-run on all of them, including ones which were previā
ously disabled.
--stop-dns-rebind
Reject (and log) addresses from upstream nameservers which are in the private ranges. This
blocks an attack where a browser behind a firewall is used to probe machines on the local netā
work. For IPv6, the private range covers the IPv4-mapped addresses in private space plus all
link-local (LL) and site-local (ULA) addresses.
--rebind-localhost-ok
Exempt 127.0.0.0/8 and ::1 from rebinding checks. This address range is returned by realtime
black hole servers, so blocking it may disable these services.
--rebind-domain-ok=[]|[[//[/]
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 or the upstream servers are set via DBus, 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 A or AAAA 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/]][[#]][@][@[#]]
Specify upstream servers directly. Setting this flag does not suppress reading of /etc/reā
solv.conf, use --no-resolv 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
--server=/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 doā
main 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 --server flag is allowed, with repeated domain or ipaddr parts as
required.
More specific domains take precedence 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
and gmail.google.com to 1.2.3.4, but www.google.com will go to 2.3.4.5
Matching of domains is normally done on complete labels, so /google.com/ matches google.com
and www.google.com but NOT supergoogle.com. This can be overridden with a * at the start of a
pattern only: /*google.com/ will match google.com and www.google.com AND supergoogle.com. The
non-wildcard form has priority, so if /google.com/ and /*google.com/ are both specified then
google.com and www.google.com will match /google.com/ and /*google.com/ will only match superā
google.com.
For historical reasons, the pattern /.google.com/ is equivalent to /google.com/ if you wish to
match any subdomain of google.com but NOT google.com itself, use /*.google.com/
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 and
its subdomains to 1.2.3.4, except www.google.com (and its subdomains) 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.
IPv6 addresses may include an %interface scope-id, eg fe80::202:a412:4512:7bbf%eth0.
The optional string after the @ character tells dnsmasq how to set the source of the queries
to this nameserver. It can either be an ip-address, an interface name or both. The ip-address
should belong to the machine on which dnsmasq is running, otherwise this server line will be
logged and then ignored. 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; and if both are given then a combination of ip-address and interā
face name will be used to steer requests to the server. 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 platā
forms supported by dnsmasq.
Upstream servers may be specified with a hostname rather than an IP address. In this case,
dnsmasq will try to use the system resolver to get the IP address of a server during startup.
If name resolution fails, starting dnsmasq fails, too. If the system's configuration is such
that the system resolver sends DNS queries through the dnsmasq instance which is starting up
then this will time-out and fail.
--rev-server=[/][,][#][@][@[#]]
This is functionally the same as --server, but provides some syntactic sugar to make specifyā
ing address-to-name queries easier. For example --rev-server=1.2.3.0/24,192.168.0.1 is exactly
equivalent to --server=/3.2.1.in-addr.arpa/192.168.0.1 Allowed prefix lengths are 1-32 (IPv4)
and 1-128 (IPv6). If the prefix length is omitted, dnsmasq substitutes either 32 (IPv4) or 128
(IPv6).
-A, --address=/[/...]/[]
Specify an IP address to return for any host in the given domains. A (or AAAA) queries in the
domains are never forwarded and always replied to with the specified IP address which may be
IPv4 or IPv6. To give multiple addresses or both IPv4 and IPv6 addresses for a domain, use reā
peated --address 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 way 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. As for --server, one or more doā
mains with no address returns a no-such-domain answer, so --address=/example.com/ is equivaā
lent to --server=/example.com/ and returns NXDOMAIN for example.com and all its subdomains. An
address specified as '#' translates to the NULL address of 0.0.0.0 and its IPv6 equivalent of
:: so --address=/example.com/# will return NULL addresses for example.com and its subdomains.
This is partly syntactic sugar for --address=/example.com/0.0.0.0 and --address=/examā
ple.com/:: but is also more efficient than including both as separate configuration lines.
Note that NULL addresses normally work in the same way as localhost, so beware that clients
looking up these names are likely to end up talking to themselves.
Note that the behaviour for queries which don't match the specified address literal changed in
version 2.86. Previous versions, configured with (eg) --address=/example.com/1.2.3.4 and then
queried for a RR type other than A would return a NoData answer. From 2.86, the query is sent
upstream. To restore the pre-2.86 behaviour, use the configuration --address=/examā
ple.com/1.2.3.4 --local=/example.com/
--ipset=/[/...]/[,...]
Places the resolved IP addresses of queries for one or more domains in the specified Netfilter
IP set. If multiple setnames are given, then the addresses are placed in each of them, subject
to the limitations of an IP set (IPv4 addresses cannot be stored in an IPv6 IP set and vice
versa). Domains and subdomains are matched in the same way as --address. These IP sets must
already exist. See ipset(8) for more details.
--nftset=/[/...]/[(6|4)#[#]#[,[(6|4)#[#]#]...]
Similar to the --ipset option, but accepts one or more nftables sets to add IP addresses into.
These sets must already exist. See nft(8) for more details. The family, table and set are
passed directly to the nft. If the spec starts with 4# or 6# then only A or AAAA records reā
spectively are added to the set. Since an nftset can hold only IPv4 or IPv6 addresses, this
avoids errors being logged for addresses of the wrong type.
--connmark-allowlist-enable[=]
Enables filtering of incoming DNS queries with associated Linux connection track marks accordā
ing to individual allowlists configured via a series of --connmark-allowlist options. Disalā
lowed queries are not forwarded; they are rejected with a REFUSED error code. DNS queries are
only allowed if they do not have an associated Linux connection track mark, or if the queried
domains match the configured DNS patterns for the associated Linux connection track mark. If
no allowlist is configured for a Linux connection track mark, all DNS queries associated with
that mark are rejected. If a mask is specified, Linux connection track marks are first bitā
wise ANDed with the given mask before being processed.
--connmark-allowlist=[/][,[/...]]
Configures the DNS patterns that are allowed in DNS queries associated with the given Linux
connection track mark. If a mask is specified, Linux connection track marks are first bitwise
ANDed with the given mask before they are compared to the given connection track mark. Patā
terns follow the syntax of DNS names, but additionally allow the wildcard character "*" to be
used up to twice per label to match 0 or more characters within that label. Note that the
wildcard never matches a dot (e.g., "*.example.com" matches "api.example.com" but not
"api.us.example.com"). Patterns must be fully qualified, i.e., consist of at least two labels.
The final label must not be fully numeric, and must not be the "local" pseudo-TLD. A pattern
must end with at least two literal (non-wildcard) labels. Instead of a pattern, "*" can be
specified to disable allowlist filtering for a given Linux connection track mark entirely.
-m, --mx-host=[[,],]
Return an MX record named 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=
Specify the default target for the MX record returned by dnsmasq. See --mx-host. If --mx-tarā
get 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>[.],[[,[,[,]]]]
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.
--host-record=[,....],[],[][,]
Add A, AAAA and PTR records to the DNS. This adds one or more names to the DNS with associated
IPv4 (A) and IPv6 (AAAA) records. A name may appear in more than one --host-record and thereā
fore be assigned more than one address. Only the first address creates a PTR record linking
the address to the name. This is the same rule as is used reading hosts-files. --host-record
options are considered to be read before host-files, so a name appearing there inhibits PTR-
record creation if it appears in hosts-file also. Unlike hosts-files, names are not expanded,
even when --expand-hosts is in effect. Short and long names may appear in the same --host-
record, eg. --host-record=laptop,laptop.thekelleys.org,192.168.0.1,1234::100
If the time-to-live is given, it overrides the default, which is zero or the value of --local-
ttl. The value is a positive integer and gives the time-to-live in seconds.
--dynamic-host=,[IPv4-address],[IPv6-address],
Add A, AAAA and PTR records to the DNS in the same subnet as the specified interface. The adā
dress is derived from the network part of each address associated with the interface, and the
host part from the specified address. For example --dynamic-host=example.com,0.0.0.8,eth0
will, when eth0 has the address 192.168.78.x and netmask 255.255.255.0 give the name examā
ple.com an A record for 192.168.78.8. The same principle applies to IPv6 addresses. Note that
if an interface has more than one address, more than one A or AAAA record will be created. The
TTL of the records is always zero, and any changes to interface addresses will be immediately
reflected in them.
-Y, --txt-record=[[,],]
Return a TXT DNS record. The value of TXT record is a set of strings, so any number may be
included, delimited by commas; use quotes to put commas into a string. Note that the maximum
length of a single string is 255 characters, longer strings are split into 255 character
chunks.
--ptr-record=[,]
Return a PTR DNS record.
--naptr-record=,,,,,[,]
Return an NAPTR DNS record, as specified in RFC3403.
--caa-record=,,,
Return a CAA DNS record, as specified in RFC6844.
--cname=,[,][,]
Return a CNAME record which indicates that is really . There is a significant
limitation on the target; it must be a DNS record which is known to dnsmasq and NOT a DNS
record which comes from an upstream server. The cname must be unique, but it is permissible to
have more than one cname pointing to the same target. Indeed it's possible to declare multiple
cnames to a target in a single line, like so: --cname=cname1,cname2,target
If the time-to-live is given, it overrides the default, which is zero or the value of --local-
ttl. The value is a positive integer and gives the time-to-live in seconds.
--dns-rr=,,[]
Return an arbitrary DNS Resource Record. The number is the type of the record (which is always
in the C_IN class). The value of the record is given by the hex data, which may be of the form
01:23:45 or 01 23 45 or 012345 or any mixture of these.
--interface-name=,[/4|/6]
Return DNS records associating the name with the address(es) of the given interface. This flag
specifies an A or AAAA 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. The interface may be
followed by "/4" or "/6" to specify that only IPv4 or IPv6 addresses of the interface should
be used. If the interface is down, not configured or non-existent, an empty record is reā
turned. 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. Note that a name used
in --interface-name may not appear in /etc/hosts.
--synth-domain=,[,[*]]
Create artificial A/AAAA and PTR records for an address range. The records either seqential
numbers or the address, with periods (or colons for IPv6) replaced with dashes.
An examples should make this clearer. First sequential numbers. --synth-domain=thekelā
leys.org.uk,192.168.0.50,192.168.0.70,internal-* results in the name internal-0.thekelā
leys.org.uk. returning 192.168.0.50, internal-1.thekelleys.org.uk returning 192.168.0.51 and
so on. (note the *) The same principle applies to IPv6 addresses (where the numbers may be
very large). Reverse lookups from address to name behave as expected.
Second, --synth-domain=thekelleys.org.uk,192.168.0.0/24,internal- (no *) will result in a
query for internal-192-168-0-56.thekelleys.org.uk returning 192.168.0.56 and a reverse query
vice versa. The same applies to IPv6; the representation doesn't use the :: compression feaā
ture or the special representation of V4 mapped IPv6 addresses as these can generate illegal
domain names, so all domains are of the form interā
nal-1000-0000-0000-0000-0000-0000-0000-0008.example.com
The address range can be of the form , or / in both forms of the option. For IPv6 the start and end addresses must fall in the
same /64 network, or prefix-length must be greater than or equal to 64 except that shorter
prefix lengths than 64 are allowed only if non-sequential names are in use.
--dumpfile=
Specify the location of a pcap-format file which dnsmasq uses to dump copies of network packā
ets for debugging purposes. If the file exists when dnsmasq starts, it is not deleted; new
packets are added to the end. The file may be a named-pipe which Wireshark is listening to.
--dumpmask=
Specify which types of packets should be added to the dumpfile. The argument should be the OR
of the bitmasks for each type of packet to be dumped: it can be specified in hex by preceding
the number with 0x in the normal way. Each time a packet is written to the dumpfile, dnsmasq
logs the packet sequence and the mask representing its type. The current types are: 0x0001 -
DNS queries from clients, 0x0002 DNS replies to clients, 0x0004 - DNS queries to upstream,
0x0008 - DNS replies from upstream, 0x0010 - queries send upstream for DNSSEC validation,
0x0020 - replies to queries for DNSSEC validation, 0x0040 - replies to client queries which
fail DNSSEC validation, 0x0080 replies to queries for DNSSEC validation which fail validation,
0x1000 - DHCPv4, 0x2000 - DHCPv6, 0x4000 - Router advertisement, 0x8000 - TFTP.
--add-mac[=base64|text]
Add the MAC address of the requestor to DNS queries which are forwarded upstream. This may be
used to DNS filtering by the upstream server. The MAC address can only be added if the reā
questor is on the same subnet as the dnsmasq server. Note that the mechanism used to achieve
this (an EDNS0 option) is not yet standardised, so this should be considered experimental.
Also note that exposing MAC addresses in this way may have security and privacy implications.
The warning about caching given for --add-subnet applies to --add-mac too. An alternative enā
coding of the MAC, as base64, is enabled by adding the "base64" parameter and a human-readable
encoding of hex-and-colons is enabled by added the "text" parameter.
--strip-mac
Remove any MAC address information already in downstream queries before forwarding upstream.
--add-cpe-id=
Add an arbitrary identifying string to DNS queries which are forwarded upstream.
--add-subnet[[=[/]][,[/]]]
Add a subnet address to the DNS queries which are forwarded upstream. If an address is speciā
fied in the flag, it will be used, otherwise, the address of the requestor will be used. The
amount of the address forwarded depends on the prefix length parameter: 32 (128 for IPv6) forā
wards the whole address, zero forwards none of it but still marks the request so that no upā
stream nameserver will add client address information either. The default is zero for both
IPv4 and IPv6. Note that upstream nameservers may be configured to return different results
based on this information, but the dnsmasq cache does not take account. Caching is therefore
disabled for such replies, unless the subnet address being added is constant.
For example, --add-subnet=24,96 will add the /24 and /96 subnets of the requestor for IPv4 and
IPv6 requestors, respectively. --add-subnet=1.2.3.4/24 will add 1.2.3.0/24 for IPv4 reā
questors and ::/0 for IPv6 requestors. --add-subnet=1.2.3.4/24,1.2.3.4/24 will add 1.2.3.0/24
for both IPv4 and IPv6 requestors.
--strip-subnet
Remove any subnet address already present in a downstream query before forwarding it upstream.
If --add-subnet is set this also ensures that any downstream-provided subnet is replaced by
the one added by dnsmasq. Otherwise, dnsmasq will NOT replace an existing subnet in the query.
--umbrella[=[deviceid:][,orgid:][,assetid:]]
Embeds the requestor's IP address in DNS queries forwarded upstream. If device id or, asset
id or organization id are specified, the information is included in the forwarded queries and
may be able to be used in filtering policies and reporting. The order of the id attributes is
irrelevant, but they must be separated by a comma. Deviceid is a sixteen digit hexadecimal
number, org and asset ids are decimal numbers.
-c, --cache-size=
Set the size of dnsmasq's cache. The default is 150 names. Setting the cache size to zero disā
ables caching. Note: huge cache size impacts performance.
-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.
--no-round-robin
Dnsmasq normally permutes the order of A or AAAA records for the same name on successive
queries, for load-balancing. This turns off that behaviour, so that the records are always reā
turned in the order that they are received from upstream.
--do-0x20-encode, --no-0x20-encode
Dnsmasq can scramble the case of letters in DNS queries it sends upstream as a security feaā
ture. This technique can interact badly with rare broken DNS servers which don't preserve the
case of the query in their reply. The first time a reply is returned which matches the query
in all respects except case, a warning will be logged. If this coincides with DNS not funcā
tioning, it is necessary to disable the feature. As at version 2.91, 0x20 encoding is disabled
by default, and must be enabled with --do-0x20-encode. The default may change in the future,
so to be sure of its status after an upgrade, set --do-0x20-encode or --no-0x20-encode in your
config. --no-0x20-encode overrides --do-x20-encode or a future default 0x20-encode enable.
--use-stale-cache[=]
When set, if a DNS name exists in the cache, but its time-to-live has expired, dnsmasq will
return the data anyway. (It attempts to refresh the data with an upstream query after returnā
ing the stale data.) This can improve speed and reliability. It comes at the expense of someā
times returning out-of-date data and less efficient cache utilisation, since old data cannot
be flushed when its TTL expires, so the cache becomes mostly least-recently-used. To mitigate
issues caused by massively outdated DNS replies, the maximum overaging of cached records can
be specified in seconds (defaulting to not serve anything older than one day). Setting the TTL
excess time to zero will serve stale cache data regardless how long it has expired.
-0, --dns-forward-max=
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. This
parameter actually controls the number of concurrent queries per server group, where a server
group is the set of server(s) associated with a single domain. So if a domain has its own
server via --server=/example.com/1.2.3.4 and 1.2.3.4 is not responding, but queries for *.exā
ample.com cannot go elsewhere, then other queries will not be affected. On configurations with
many such server groups and tight resources, this value may need to be reduced.
--dnssec
Validate DNS replies and cache DNSSEC data. When forwarding DNS queries, dnsmasq requests the
DNSSEC records needed to validate the replies. The replies are validated and the result reā
turned as the Authenticated Data bit in the DNS packet. In addition the DNSSEC records are
stored in the cache, making validation by clients more efficient. Note that validation by
clients is the most secure DNSSEC mode, but for clients unable to do validation, use of the AD
bit set by dnsmasq is useful, provided that the network between the dnsmasq server and the
client is trusted. Dnsmasq must be compiled with HAVE_DNSSEC enabled, and DNSSEC trust anchors
provided, see --trust-anchor. Because the DNSSEC validation process uses the cache, it is not
permitted to reduce the cache size below the default when DNSSEC is enabled. The nameservers
upstream of dnsmasq must be DNSSEC-capable, ie capable of returning DNSSEC records with data.
If they are not, then dnsmasq will not be able to determine the trusted status of answers and
this means that DNS service will be entirely broken.
--trust-anchor=,[,][,,,]
Provide DS records to act a trust anchors for DNSSEC validation. The class defaults to IN.
Typically these will be the DS record(s) for Key Signing key(s) (KSK) of the root zone, but
trust anchors for limited domains are also possible. A negative trust anchor (ie. proof that
a DS record doesn't exist) may be configured be specifying only the name or only the name and
class. This can be useful for forcing dnsmasq to treat zones delegated using --server=// as unsigned. The current root-zone trust anchors may be downloaded from
https://data.iana.org/root-anchors/root-anchors.xml
--dnssec-check-unsigned[=no]
As a default, dnsmasq checks that unsigned DNS replies are legitimate: this entails possible
extra queries even for the majority of DNS zones which are not, at the moment, signed. If
--dnssec-check-unsigned=no appears in the configuration, then such replies they are assumed to
be valid and passed on (without the "authentic data" bit set, of course). This does not proā
tect against an attacker forging unsigned replies for signed DNS zones, but it is fast.
Versions of dnsmasq prior to 2.80 defaulted to not checking unsigned replies, and used
--dnssec-check-unsigned to switch this on. Such configurations will continue to work as beā
fore, but those which used the default of no checking will need to be altered to explicitly
select no checking. The new default is because switching off checking for unsigned replies is
inherently dangerous. Not only does it open the possibility of forged replies, but it allows
everything to appear to be working even when the upstream namesevers do not support DNSSEC,
and in this case no DNSSEC validation at all is occurring.
--dnssec-no-timecheck
DNSSEC signatures are only valid for specified time windows, and should be rejected outside
those windows. This generates an interesting chicken-and-egg problem for machines which don't
have a hardware real time clock. For these machines to determine the correct time typically
requires use of NTP and therefore DNS, but validating DNS requires that the correct time is
already known. Setting this flag removes the time-window checks (but not other DNSSEC validaā
tion.) only until the dnsmasq process receives SIGINT. The intention is that dnsmasq should be
started with this flag when the platform determines that reliable time is not currently availā
able. As soon as reliable time is established, a SIGINT should be sent to dnsmasq, which enā
ables time checking, and purges the cache of DNS records which have not been thoroughly
checked.
Earlier versions of dnsmasq overloaded SIGHUP (which re-reads much configuration) to also enā
able time validation.
If dnsmasq is run in debug mode (--no-daemon flag) then SIGINT retains its usual meaning of
terminating the dnsmasq process.
--dnssec-timestamp=
Enables an alternative way of checking the validity of the system time for DNSSEC (see
--dnssec-no-timecheck). In this case, the system time is considered to be valid once it beā
comes later than the timestamp on the specified file. The file is created and its timestamp
set automatically by dnsmasq. The file must be stored on a persistent filesystem, so that it
and its mtime are carried over system restarts. The timestamp file is created after dnsmasq
has dropped root, so it must be in a location writable by the unprivileged user that dnsmasq
runs as.
--proxy-dnssec
Copy the DNSSEC Authenticated Data bit from upstream servers to downstream clients. This is
an alternative to having dnsmasq validate DNSSEC, but it depends on the security of the netā
work between dnsmasq and the upstream servers, and the trustworthiness of the upstream
servers. Note that caching the Authenticated Data bit correctly in all cases is not techniā
cally possible. If the AD bit is to be relied upon when using this option, then the cache
should be disabled using --cache-size=0. In most cases, enabling DNSSEC validation within dnsā
masq is a better option. See --dnssec for details.
--dnssec-limits=[,.......]
Override the default resource limits applied to DNSSEC validation. Cryptographic operations
are expensive and crafted domains can DoS a DNSSEC validator by forcing it to do hundreds of
thousands of such operations. To avoid this, the dnsmasq validation code applies limits on how
much work will be expended in validation. If any of the limits are exceeded, the validation
will fail and the domain treated as BOGUS. There are four limits, in order(default values in
parens): number a signature validation fails per RRset(20), number of signature validations
and hash computations per query(200), number of sub-queries to fetch DS and DNSKEY RRsets per
query(40), and the number of iterations in a NSEC3 record(150). The maximum values reached
during validation are stored, and dumped as part of the stats generated by SIGUSR1. Supplying
a limit value of 0 leaves the default in place, so --dnssec-limits=0,0,20 sets the number of
sub-queries to 20 whilst leaving the other limits at default values.
--dnssec-debug
Set debugging mode for the DNSSEC validation, set the Checking Disabled bit on upstream
queries, and don't convert replies which do not validate to responses with a return code of
SERVFAIL. Note that setting this may affect DNS behaviour in bad ways, it is not an extra-logā
ging flag and should not be set in production.
--auth-zone=[,[/][,[/].....][,exclude:[/]].....]
Define a DNS zone for which dnsmasq acts as authoritative server. Locally defined DNS records
which are in the domain will be served. If subnet(s) are given, A and AAAA records must be in
one of the specified subnets.
As alternative to directly specifying the subnets, it's possible to give the name of an interā
face, in which case the subnets implied by that interface's configured addresses and netā
mask/prefix-length are used; this is useful when using constructed DHCP ranges as the actual
address is dynamic and not known when configuring dnsmasq. The interface addresses may be conā
fined to only IPv6 addresses using /6 or to only IPv4 using /4. This is
useful when an interface has dynamically determined global IPv6 addresses which should appear
in the zone, but RFC1918 IPv4 addresses which should not. Interface-name and address-literal
subnet specifications may be used freely in the same --auth-zone declaration.
It's possible to exclude certain IP addresses from responses. It can be used, to make sure
that answers contain only global routeable IP addresses (by excluding loopback, RFC1918 and
ULA addresses).
The subnet(s) are also used to define in-addr.arpa and ip6.arpa domains which are served for
reverse-DNS queries. If not specified, the prefix length defaults to 24 for IPv4 and 64 for
IPv6. For IPv4 subnets, the prefix length should be have the value 8, 16 or 24 unless you are
familiar with RFC 2317 and have arranged the in-addr.arpa delegation accordingly. Note that if
no subnets are specified, then no reverse queries are answered.
--auth-soa=[,[,[,[,]]]]
Specify fields in the SOA record associated with authoritative zones. Note that this is opā
tional, all the values are set to sane defaults.
--auth-sec-servers=[,[,...]]
Specify any secondary servers for a zone for which dnsmasq is authoritative. These servers
must be configured to get zone data from dnsmasq by zone transfer, and answer queries for the
same authoritative zones as dnsmasq.
--auth-peer=[,[,...]]
Specify the addresses of secondary servers which are allowed to initiate zone transfer (AXFR)
requests for zones for which dnsmasq is authoritative. If this option is not given but --auth-
sec-servers is, then AXFR requests will be accepted from any secondary. Specifying --auth-peer
without --auth-sec-servers enables zone transfer but does not advertise the secondary in NS
records returned by dnsmasq.
--conntrack
Read the Linux connection track mark associated with incoming DNS queries and set the same
mark value on upstream traffic used to answer those queries. This allows traffic generated by
dnsmasq to be associated with the queries which cause it, useful for bandwidth accounting and
firewalling. Dnsmasq must have conntrack support compiled in and the kernel must have conā
ntrack support included and configured. This option cannot be combined with --query-port.
-F, --dhcp-range=[tag:[,tag:],][set:,][,|[,[,]]][,]
-F, --dhcp-range=[tag:[,tag:],][set:,][,|construcā
tor:][,][,][,]
Enable the DHCP server. Addresses will be given out from the range to
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 days (2d) or weeks (1w) or "infinite". If not given, the
default lease time is one hour for IPv4 and one day for IPv6. The minimum lease time is two
minutes. For IPv6 ranges, the lease time maybe "deprecated"; this sets the preferred lifetime
sent in a DHCP lease or router advertisement to zero, which causes clients to use other adā
dresses, if available, for new connections as a prelude to renumbering.
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: dnsmasq will determine it from the interface conā
figuration. For networks which receive DHCP service via a relay agent, dnsmasq cannot deterā
mine the netmask itself, so it should be specified, otherwise dnsmasq will have to guess,
based on the class (A, B or C) of the network address. The broadcast address is always opā
tional. It is always allowed to have more than one --dhcp-range in a single subnet.
For IPv6, the parameters are slightly different: instead of netmask and broadcast address,
there is an optional prefix length which must be equal to or larger then the prefix length on
the local interface. If not given, this defaults to 64. Unlike the IPv4 case, the prefix
length is not automatically derived from the interface configuration. The minimum size of the
prefix length is 64.
IPv6 (only) supports another type of range. In this, the start address and optional end adā
dress contain only the network part (ie ::1) and they are followed by constructor:.
This forms a template which describes how to create ranges, based on the addresses assigned to
the interface. For instance
--dhcp-range=::1,::400,constructor:eth0
will look for addresses on eth0 and then create a range from ::1 to ::400.
If the interface is assigned more than one network, then the corresponding ranges will be auā
tomatically created, and then deprecated and finally removed again as the address is depreā
cated and then deleted. The interface name may have a final "*" wildcard. Note that just any
address on eth0 will not do: it must not be an autoconfigured or privacy address, or be depreā
cated.
If a --dhcp-range is only being used for stateless DHCP and/or SLAAC, then the address can be
simply ::
--dhcp-range=::,constructor:eth0
The optional set: sets an alphanumeric label which marks this network so that DHCP opā
tions 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 optional keyword may be static which tells dnsmasq to enable DHCP for the network
specified, but not to dynamically allocate IP addresses: only hosts which have static adā
dresses given via --dhcp-host or from /etc/ethers will be served. A static-only subnet with
address all zeros may be used as a "catch-all" address to enable replies to all Information-
request packets on a subnet which is provided with stateless DHCPv6, ie --dhcp-range=::,static
For IPv4, the may be proxy in which case dnsmasq will provide proxy-DHCP on the speciā
fied subnet. (See --pxe-prompt and --pxe-service for details.)
For IPv6, the mode may be some combination of ra-only, slaac, ra-names, ra-stateless, ra-adā
vrouter, off-link.
ra-only tells dnsmasq to offer Router Advertisement only on this subnet, and not DHCP.
slaac tells dnsmasq to offer Router Advertisement on this subnet and to set the A bit in the
router advertisement, so that the client will use SLAAC addresses. When used with a DHCP range
or static DHCP address this results in the client having both a DHCP-assigned and a SLAAC adā
dress.
ra-stateless sends router advertisements with the O and A bits set, and provides a stateless
DHCP service. The client will use a SLAAC address, and use DHCP for other configuration inforā
mation.
ra-names enables a mode which gives DNS names to dual-stack hosts which do SLAAC for IPv6.
Dnsmasq uses the host's IPv4 lease to derive the name, network segment and MAC address and asā
sumes that the host will also have an IPv6 address calculated using the SLAAC algorithm, on
the same network segment. The address is pinged, and if a reply is received, an AAAA record is
added to the DNS for this IPv6 address. Note that this is only happens for directly-connected
networks, (not one doing DHCP via a relay) and it will not work if a host is using privacy exā
tensions. ra-names can be combined with ra-stateless and slaac.
ra-advrouter enables a mode where router address(es) rather than prefix(es) are included in
the advertisements. This is described in RFC-3775 section 7.2 and is used in mobile IPv6. In
this mode the interval option is also included, as described in RFC-3775 section 7.3.
off-link tells dnsmasq to advertise the prefix without the on-link (aka L) bit set.
-G, --dhcp-host=[][,id:|*][,set:][,tag:][,][,][,][,ignore]
Specify per host parameters for the DHCP server. This allows a machine with a particular hardā
ware 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 alā
lowable to omit 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 (called client DUID in IPv6-land) 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,.....
A single --dhcp-host may contain an IPv4 address or one or more IPv6 addresses, or both. IPv6
addresses must be bracketed by square brackets thus: --dhcp-host=laptop,[1234::56] IPv6 adā
dresses may contain only the host-identifier part: --dhcp-host=laptop,[::56] in which case
they act as wildcards in constructed DHCP ranges, with the appropriate network part inserted.
For IPv6, an address may include a prefix length: --dhcp-host=laptop,[1234:50/126] which (in
this case) specifies four addresses, 1234::50 to 1234::53. This (an the ability to specify
multiple addresses) is useful when a host presents either a consistent name or hardware-ID,
but varying DUIDs, since it allows dnsmasq to honour the static address allocation but assign
a different address for each DUID. This typically occurs when chain netbooting, as each stage
of the chain gets in turn allocates an address.
Note that in IPv6 DHCP, the hardware address may not be available, though it normally is for
direct-connected clients, or clients using DHCP relays which support RFC 6939.
For DHCPv4, 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 ). Note that
/etc/hosts is NOT used when the DNS server side of dnsmasq is disabled by setting the DNS
server port to zero.
More than one --dhcp-host can be associated (by name, hardware address or UID) with a host.
Which one is used (and therefore which address is allocated by DHCP and appears in the DNS)
depends on the subnet on which the host last obtained a DHCP lease: the --dhcp-host with an
address within the subnet is used. If more than one address is within the subnet, the result
is undefined. A corollary to this is that the name associated with a host using --dhcp-host
does not appear in the DNS until the host obtains a DHCP lease.
The special keyword ignore tells dnsmasq never to offer a DHCP lease to a machine. The machine
can be specified by hardware address, client ID or hostname. For example: --dhcp-
host=00:20:e0:3b:13:af,ignore. This can be useful when there is another DHCP server on the
network which should be used by some machines.
The set: construct 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: 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. If the host matches only a --dhcp-host directive which cannot be
used because it specifies an address on a different subnet, the tag known-othernet is set.
The tag: construct filters which dhcp-host directives are used. More than one tag can be
provided. In this case, the request must match all tags. Tagged directives are used in preferā
ence to untagged ones. Note that one of , or still must be
specified (can be a wildcard).
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 the given range of hardware adā
dresses. 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, in DHCPv4, 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, useā
ful to allocate a stable IP address to a laptop which has both wired and wireless interfaces.
--dhcp-hostsfile=
Read DHCP host information from the specified file. If a directory is given, then read all the
files contained in that directory in alphabetical order. 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=
Read DHCP option information from the specified file. If a directory is given, then read all
the files contained in that directory in alphabetical order. The advantage of using this opā
tion is the same as for --dhcp-hostsfile: the --dhcp-optsfile will be re-read when dnsmasq reā
ceives 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.
--dhcp-hostsdir=
This is equivalent to --dhcp-hostsfile, except for the following. The path MUST be a direcā
tory, and not an individual file. Changed or new files within the directory are read automatiā
cally, without the need to send SIGHUP. If a file is deleted or changed after it has been
read by dnsmasq, then the host record it contained will remain until dnsmasq receives a
SIGHUP, or is restarted; ie host records are only added dynamically. The order in which the
files in a directory are read is not defined.
--dhcp-optsdir=
This is equivalent to --dhcp-optsfile, with the differences noted for --dhcp-hostsdir.
-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 dnsā
masq these lines have exactly the same effect as --dhcp-host options containing the same inā
formation. /etc/ethers is re-read when dnsmasq receives SIGHUP. IPv6 addresses are NOT read
from /etc/ethers.
-O, --dhcp-option=[tag:,[tag:,]][encap:,][vi-encap:,][vendor:[],][|option:|option6:|option6:],[[,]]
Send various 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 dnsā
masq, and the DNS server and default route are set to the address of the machine running dnsā
masq. (Equivalent rules apply for IPv6.) If the domain name option has been set, that is sent.
This configuration allows these defaults to be overridden, or other options specified. The opā
tion to be sent may be given as a decimal number or as option:.
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, use --dhcp-option=3,192.168.4.4 or --dhcp-option=opā
tion:router,192.168.4.4 and to set the time-server address to 192.168.0.4, use --dhcp-opā
tion=42,192.168.0.4 or --dhcp-option=option:ntp-server,192.168.0.4. The special address
0.0.0.0 means "the address of the system running dnsmasq".
An option without data is valid, and includes just the option without data. (There is only
one option with a zero length data field currently defined for DHCPv4, 80:rapid commit, so
this feature is not very useful in practice). Options for which dnsmasq normally provides deā
fault values can be omitted by defining the option with no data. These are netmask, broadcast,
router, DNS server, domainname and hostname. Thus, for DHCPv4 --dhcp-option = option:router
will result in no router option being sent, rather than the default of the host on which dnsā
masq is running. For DHCPv6, the same is true of the options DNS server and refresh time.
Data types allowed are comma separated dotted-quad IPv4 addresses, []-wrapped IPv6 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 deā
scribed in RFC 3442.
IPv6 options are specified using the option6: keyword, followed by the option number or option
name. The IPv6 option name space is disjoint from the IPv4 option name space. IPv6 addresses
in options must be bracketed with square brackets, eg.
--dhcp-option=option6:ntp-server,[1234::56]. For IPv6, [::] means "the global address of the
machine running dnsmasq", whilst [fd00::] is replaced with the ULA, if it exists, and [fe80::]
with the link-local address.
Be careful: data-type suitability for the option number sent is not checked. It is quite posā
sible 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 litā
eral 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 (IPv4 only) 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 venā
dor-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 (IPv4 only) within other options: for instance --dhcp-option=enā
cap: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 corā
rectly combined into one encapsulated option. encap: and vendor: 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. This form of enā
capsulation is supported in IPv6.
The address 0.0.0.0 is not treated specially in encapsulated options.
--dhcp-option-force=[tag:,[tag:,]][encap:,][vi-encap:,][vendor:[],],[[,]]
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-option-pxe=[tag:,[tag:,]][encap:,][vi-encap:,],[[,]]
A variant of --dhcp-option which defines options only sent in reply to PXE clients. In addiā
tion, such options are sent in reply to PXE clients when dnsmasq is acting as a PXE proxy, unā
like other options. A typical use-case is option 175, sent to iPXE.
--dhcp-no-override
(IPv4 only) 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.
--leasequery[=[/]]
Enable RFC 4388 leasequery. The dnsmasq DHCP server will answer LEASEQUERY messages from DHCP
relays when this option is given. If an address or subnet is given, only queries from a relay
at that source are allowed. Repeat the --leasequery option to specify multiple allowed
sources. To correctly answer lease queries it is necessary to store extra data in the lease
database, and this is also enabled by the --leasequery option. The extra fields (agent-info
and vendorclass) are stored in the leases file in a somewhat backwards compatible manner. Enā
abling and then disabling leasequery will not cause problems; the extra information will be
aged out of the database. However, enabling leasequery in release 2.92 or later, storing
leases which come via DHCP relays which add option-82 agent-info data, and then moving back to
a pre-2.92 dnsmasq binary may cause problems reading the leases file. As of release 2.92,
leasequery is only supported for DHCPv4.
--dhcp-relay=