Provided by: dnsmasq-base_2.79-1ubuntu0.7_amd64 
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 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. 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 another 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 nameing for
clients which use DHCPv4 and stateless autoconfiguration only for IPv6 configuration.
There is support for doing address allocation (both DHCPv6 and RA) from subnets which are
dynamically delegated via DHCPv6 prefix delegation.
Dnsmasq is coded with small embedded systems in mind. It aims for the smallest possible
memory footprint 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 options, and --help dhcp6 will display DHCPv6 options.
-h, --no-hosts
Don't read the hostnames in /etc/hosts.
-H, --addn-hosts=<file>
Additional hosts file. Read the specified file as well as /etc/hosts. If -h is
given, read only the specified file. This option may be repeated for more than one
additional hosts file. If a directory is given, then read all the files contained
in that directory.
--hostsdir=<path>
Read all the hosts files contained in the directory. New or changed files are read
automatically. See --dhcp-hostsdir for details.
-E, --expand-hosts
Add the domain to simple names (without a period) in /etc/hosts in the same way as
for DHCP-derived names. Note that this does not apply to domain names in cnames,
PTR records, TXT records etc.
-T, --local-ttl=<time>
When replying with information from /etc/hosts or configuration or the DHCP leases
file dnsmasq by default sets the time-to-live field to zero, meaning that the
requester should not itself cache the information. This is the correct thing to do
in almost all situations. This option allows a time-to-live (in seconds) to be
given for these replies. This will reduce the load on the server at the expense of
clients using stale data under some circumstances.
--dhcp-ttl=<time>
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=<time>
Negative replies from upstream servers normally contain time-to-live information in
SOA records which dnsmasq uses for caching. If the replies from upstream servers
omit this information, dnsmasq does not cache the reply. This option gives a
default value for time-to-live (in seconds) which dnsmasq uses to cache negative
replies even in the absence of an SOA record.
--max-ttl=<time>
Set a maximum TTL value that will be handed out to clients. The specified maximum
TTL will be given to clients instead of the true TTL value if it is lower. The true
TTL value is however kept in the cache to avoid flooding the upstream DNS servers.
--max-cache-ttl=<time>
Set a maximum TTL value for entries in the cache.
--min-cache-ttl=<time>
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 understand what you are doing. Dnsmasq limits the value of this option
to one hour, unless recompiled.
--auth-ttl=<time>
Set the TTL value returned in answers from the authoritative server.
-k, --keep-in-foreground
Do not go into the background at startup but otherwise run as normal. This is
intended for use when dnsmasq is run under daemontools or launchd.
-d, --no-daemon
Debug mode: don't fork to the background, don't write a pid file, don't change user
id, generate a complete cache dump on receipt on SIGUSR1, log to stderr as well as
syslog, don't fork new processes to handle TCP queries. Note that this option is
for use in debugging only, to stop dnsmasq daemonising in production, use -k.
-q, --log-queries
Log the results of DNS queries handled by dnsmasq. Enable a full cache dump on
receipt of SIGUSR1. If the argument "extra" is supplied, ie --log-queries=extra
then the log has extra information 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.
-8, --log-facility=<facility>
Set the facility to which dnsmasq will send syslog entries, this defaults to
DAEMON, and to LOCAL0 when debug mode is in operation. If the facility given
contains at least one '/' character, it is taken to be a filename, and dnsmasq logs
to the given file, instead of syslog. If the facility is '-' then dnsmasq logs to
stderr. (Errors whilst reading configuration will still go to syslog, but all
output from a successful startup, and all output whilst running, will go
exclusively to the file.) When logging to a file, dnsmasq will close and reopen the
file when it receives SIGUSR2. This allows the log file to be rotated without
stopping dnsmasq.
--log-async[=<lines>]
Enable asynchronous logging and optionally set the limit on the number of lines
which will be queued by dnsmasq when writing to the syslog is slow. Dnsmasq can
log asynchronously: this allows it to continue functioning without being blocked by
syslog, and allows syslog to use dnsmasq for DNS queries without risking deadlock.
If the queue of log-lines becomes full, dnsmasq will log the overflow, and the
number of messages lost. The default queue length is 5, a sane value would be
5-25, and a maximum limit of 100 is imposed.
-x, --pid-file=<path>
Specify an alternate path for dnsmasq to record its process-id in. Normally
/var/run/dnsmasq.pid.
-u, --user=<username>
Specify the userid to which dnsmasq will change after startup. Dnsmasq must
normally be started as root, but it will drop root privileges after startup by
changing id to another user. Normally this user is "nobody" but that can be over-
ridden with this switch.
-g, --group=<groupname>
Specify the group which dnsmasq will run as. The defaults to "dip", if available,
to facilitate access to /etc/ppp/resolv.conf which is not normally world readable.
-v, --version
Print the version number.
-p, --port=<port>
Listen on <port> instead of the standard DNS port (53). Setting this to zero
completely disables DNS function, leaving only DHCP and/or TFTP.
-P, --edns-packet-max=<size>
Specify the largest EDNS.0 UDP packet which is supported by the DNS forwarder.
Defaults to 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=<query_port>
Send outbound DNS queries from, and listen for their replies on, the specific UDP
port <query_port> instead of using random ports. NOTE that using this option will
make dnsmasq less secure against DNS spoofing attacks but it may be faster and use
less resources. Setting this option to zero makes dnsmasq use a single port
allocated to it by the OS: this was the default behaviour in versions prior to
2.43.
--min-port=<port>
Do not use ports less than that given as source for outbound DNS queries. Dnsmasq
picks random ports as source for outbound queries: when this option is given, the
ports used will always to larger than that specified. Useful for systems behind
firewalls. If not specified, defaults to 1024.
--max-port=<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=<interface name>
Listen only on the specified interface(s). Dnsmasq automatically adds the loopback
(local) interface to the list of interfaces to use when the --interface option is
used. If no --interface or --listen-address options are given dnsmasq listens on
all available interfaces except any given in --except-interface options. 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 addresses are
accepted. The same effect is achievable in default mode by using --listen-address.
A simple wildcard, consisting of a trailing '*', can be used in --interface and
--except-interface options.
-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. The comments about interface labels
for --listen-address apply here.
--auth-server=<domain>,<interface>|<ip-address>
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 <domain> 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.
--local-service
Accept DNS queries only from hosts whose address is on a local subnet, ie a subnet
for which an interface exists on the server. This option only has effect if there
are no --interface --except-interface, --listen-address or --auth-server options.
It is intended to be set as a default on installation, to allow unconfigured
installations to be useful but also safe from being used for DNS amplification
attacks.
-2, --no-dhcp-interface=<interface name>
Do not provide DHCP or TFTP on the specified interface, but do provide DNS service.
-a, --listen-address=<ipaddr>
Listen on the given IP address(es). Both --interface and --listen-address options
may be given, in which case the set of both interfaces and addresses is used. Note
that if no --interface option is given, but --listen-address is, dnsmasq will not
automatically listen on the loopback interface. To achieve this, its IP address,
127.0.0.1, must be explicitly given as a --listen-address option.
-z, --bind-interfaces
On systems which support it, dnsmasq binds the wildcard address, even when it is
listening on only some interfaces. It then discards requests that it shouldn't
reply to. This has the advantage of working even when interfaces come and go and
change address. This option forces dnsmasq to really bind only the interfaces it is
listening on. About the only time when this is useful is when running another
nameserver (or another instance of dnsmasq) on the same machine. Setting this
option also enables multiple instances of dnsmasq which provide DHCP service to run
in the same machine.
--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-control configuration). This makes dynamically created
interfaces work in the same way as the default. 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 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. 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.
-V, --alias=[<old-ip>]|[<start-ip>-<end-ip>],<new-ip>[,<mask>]
Modify IPv4 addresses returned from upstream nameservers; old-ip is replaced by
new-ip. If the optional mask is given then any address which matches the masked
old-ip will be re-written. So, for instance --alias=1.2.3.0,6.7.8.0,255.255.255.0
will map 1.2.3.56 to 6.7.8.56 and 1.2.3.67 to 6.7.8.67. This is what Cisco PIX
routers call "DNS doctoring". If the old IP is given as range, then only addresses
in the range, rather than a whole subnet, are re-written. So
--alias=192.168.0.10-192.168.0.40,10.0.0.0,255.255.255.0 maps
192.168.0.10->192.168.0.40 to 10.0.0.10->10.0.0.40
-B, --bogus-nxdomain=<ipaddr>
Transform replies which contain the IP address given into "No such domain" replies.
This is intended to counteract a devious move made by Verisign in September 2003
when they started returning the address of an advertising web page in response to
queries for unregistered names, instead of the correct NXDOMAIN response. This
option tells dnsmasq to fake the correct response when it sees this behaviour. As
at Sept 2003 the IP address being returned by Verisign is 64.94.110.11
--ignore-address=<ipaddr>
Ignore replies to A-record queries which include the specified address. 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 certain 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 types SOA and SRV, and type ANY where the requested name has
underscores, to catch LDAP requests.
-r, --resolv-file=<file>
Read the IP addresses of the upstream nameservers from <file>, instead of
/etc/resolv.conf. For the format of this file see resolv.conf(5). The only lines
relevant to dnsmasq are nameserver ones. Dnsmasq can be told to poll more than one
resolv.conf file, the first file name specified overrides the default, subsequent
ones add to the list. This is only allowed when polling; the file with the
currently latest modification time is the one used.
-R, --no-resolv
Don't read /etc/resolv.conf. Get upstream servers only from the command line or the
dnsmasq configuration file.
-1, --enable-dbus[=<service-name>]
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 service at that name, rather than the default which is
uk.org.thekelleys.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 requester.
--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 <hex>.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
previously disabled.
--stop-dns-rebind
Reject (and log) addresses from upstream nameservers which are in the private IP
ranges. This blocks an attack where a browser behind a firewall is used to probe
machines on the local network.
--rebind-localhost-ok
Exempt 127.0.0.0/8 from rebinding checks. This address range is returned by
realtime black hole servers, so blocking it may disable these services.
--rebind-domain-ok=[<domain>]|[[/<domain>/[<domain>/]
Do not detect and block dns-rebind on queries to these domains. The argument may be
either a single domain, or multiple domains surrounded by '/', like the --server
syntax, eg. --rebind-domain-ok=/domain1/domain2/domain3/
-n, --no-poll
Don't poll /etc/resolv.conf for changes.
--clear-on-reload
Whenever /etc/resolv.conf is re-read 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>]/[domain/]][<ipaddr>[#<port>][@<interface>][@<source-
ip>[#<port>]]
Specify IP address of upstream servers directly. Setting this flag does not
suppress reading of /etc/resolv.conf, use -R to do that. If one or more optional
domains are given, that server is used only for those domains and they are queried
only using the specified server. This is intended for private nameservers: if you
have a nameserver on your network which deals with names of the form
xxx.internal.thekelleys.org.uk at 192.168.1.1 then giving the flag -S
/internal.thekelleys.org.uk/192.168.1.1 will send all queries for internal machines
to that nameserver, everything else will go to the servers in /etc/resolv.conf.
DNSSEC validation is turned off for such private nameservers, UNLESS a --trust-
anchor is specified for the domain in question. An empty domain specification, //
has the special meaning of "unqualified names only" ie names without any dots in
them. A non-standard port may be specified as part of the IP address using a #
character. More than one -S flag is allowed, with repeated domain or ipaddr parts
as required.
More specific domains take 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 to 1.2.3.4, except *www.google.com, which will go to 2.3.4.5
The special server address '#' means, "use the standard servers", so
--server=/google.com/1.2.3.4 --server=/www.google.com/# will send queries for
*.google.com to 1.2.3.4, except *www.google.com which will be forwarded as usual.
Also permitted is a -S flag which gives a domain but no IP address; this tells
dnsmasq that a domain is local and it may answer queries from /etc/hosts or DHCP
but should never forward queries on that domain to any upstream servers. local is
a synonym for server to make configuration files clearer in this case.
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 interface 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 platforms supported by dnsmasq.
--rev-server=<ip-address>/<prefix-len>,<ipaddr>[#<port>][@<interface>][@<source-
ip>[#<port>]]
This is functionally the same as --server, but provides some syntactic sugar to
make specifying 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
-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. To include multiple IP addresses for a single query, use
--addn-hosts=<path> instead. Note that /etc/hosts and DHCP leases override this
for individual names. A common use of this is to redirect the entire
doubleclick.net domain to some friendly local web server to avoid banner ads. The
domain specification works in the same was as for --server, with the additional
facility that /#/ matches any domain. Thus --address=/#/1.2.3.4 will always return
1.2.3.4 for any query not answered from /etc/hosts or DHCP and not sent to an
upstream nameserver by a more specific --server directive. As for --server, one or
more domains with no address returns a no-such-domain answer, so
--address=/example.com/ is equivalent to --server=/example.com/ and returns
NXDOMAIN for example.com and all its subdomains.
--ipset=/<domain>[/<domain>...]/<ipset>[,<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.
-m, --mx-host=<mx name>[[,<hostname>],<preference>]
Return an MX record named <mx name> pointing to the given hostname (if given), or
the host specified in the --mx-target switch or, if that switch is not given, the
host on which dnsmasq is running. The default is useful for directing mail from
systems on a LAN to a central server. The preference value is optional, and
defaults to 1 if not given. More than one MX record may be given for a host.
-t, --mx-target=<hostname>
Specify the default target for the MX record returned by dnsmasq. See --mx-host.
If --mx-target is given, but not --mx-host, then dnsmasq returns a MX record
containing the MX target for MX queries on the hostname of the machine on which
dnsmasq is running.
-e, --selfmx
Return an MX record pointing to itself for each local machine. Local machines are
those in /etc/hosts or with DHCP leases.
-L, --localmx
Return an MX record pointing to the host given by mx-target (or the machine on
which dnsmasq is running) for each local machine. Local machines are those in
/etc/hosts or with DHCP leases.
-W, --srv-host=<_service>.<_prot>.[<domain>],[<target>[,<port>[,<priority>[,<weight>]]]]
Return a SRV DNS record. See RFC2782 for details. If not supplied, the domain
defaults to that given by --domain. The default for the target domain is empty,
and the default for port is one and the defaults for weight and priority are zero.
Be careful if transposing data from BIND zone files: the port, weight and priority
numbers are in a different order. More than one SRV record for a given
service/domain is allowed, all that match are returned.
--host-record=<name>[,<name>....],[<IPv4-address>],[<IPv6-address>][,<TTL>]
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 therefore 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.
-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, 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=<name>[,<target>]
Return a PTR DNS record.
--naptr-record=<name>,<order>,<preference>,<flags>,<service>,<regexp>[,<replacement>]
Return an NAPTR DNS record, as specified in RFC3403.
--cname=<cname>,[<cname>,]<target>[,<TTL>]
Return a CNAME record which indicates that <cname> is really <target>. There are
significant limitations on the target; it must be a DNS name which is known to
dnsmasq from /etc/hosts (or additional hosts files), from DHCP, from --interface-
name or from another --cname. If the target does not satisfy this criteria, the
whole cname is ignored. 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=<name>,<RR-number>,[<hex data>]
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=<name>,<interface>[/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 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. Note that
a name used in --interface-name may not appear in /etc/hosts.
--synth-domain=<domain>,<address range>[,<prefix>[*]]
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=thekelleys.org.uk,192.168.0.50,192.168.0.70,internal-* results in the name
internal-0.thekelleys.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, but IPv6
addresses may start with '::' but DNS labels may not start with '-' so in this case
if no prefix is configured a zero is added in front of the label. ::1 becomes 0--1.
V4 mapped IPv6 addresses, which have a representation like ::ffff:1.2.3.4 are
handled specially, and become like 0--ffff-1-2-3-4
The address range can be of the form <ip address>,<ip address> or <ip
address>/<netmask> in both forms of the option.
--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 requestor 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 encoding 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.
--add-cpe-id=<string>
Add an arbitrary identifying string to o DNS queries which are forwarded upstream.
--add-subnet[[=[<IPv4 address>/]<IPv4 prefix length>][,[<IPv6 address>/]<IPv6 prefix
length>]]
Add a subnet address to the DNS queries which are forwarded upstream. If an address
is specified 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) forwards the whole address, zero forwards none of it
but still marks the request so that no upstream 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 requestors 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.
-c, --cache-size=<cachesize>
Set the size of dnsmasq's cache. The default is 150 names. Setting the cache size
to zero disables caching.
-N, --no-negcache
Disable negative caching. Negative caching allows dnsmasq to remember "no such
domain" answers from upstream nameservers and answer identical queries without
forwarding them again.
-0, --dns-forward-max=<queries>
Set the maximum number of concurrent DNS queries. The default value is 150, which
should be fine for most setups. The only known situation where this needs to be
increased is when using web-server log file resolvers, which can generate large
numbers of concurrent queries.
--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 returned 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. In the default mode, this
means that all replies will be marked as untrusted. If --dnssec-check-unsigned is
set and the upstream servers don't support DNSSEC, then DNS service will be
entirely broken.
--trust-anchor=[<class>],<domain>,<key-tag>,<algorithm>,<digest-type>,<digest>
Provide DS records to act a trust anchors for DNSSEC validation. 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. The current root-zone trust anchors
may be downloaded from https://data.iana.org/root-anchors/root-anchors.xml
--dnssec-check-unsigned
As a default, dnsmasq does not check that unsigned DNS replies are legitimate: they
are assumed to be valid and passed on (without the "authentic data" bit set, of
course). This does not protect against an attacker forging unsigned replies for
signed DNS zones, but it is fast. If this flag is set, dnsmasq will check the zones
of unsigned replies, to ensure that unsigned replies are allowed in those zones.
The cost of this is more upstream queries and slower performance. See also the
warning about upstream servers in the section on --dnssec
--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 validation.) 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
available. As soon as reliable time is established, a SIGINT should be sent to
dnsmasq, which enables 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 enable time validation.
If dnsmasq is run in debug mode (-d flag) then SIGINT retains its usual meaning of
terminating the dnsmasq process.
--dnssec-timestamp=<path>
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 becomes 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
and cache it. This is an alternative to having dnsmasq validate DNSSEC, but it
depends on the security of the network between dnsmasq and the upstream servers,
and the trustworthiness of the upstream servers.
--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-logging flag and should not be set in production.
--auth-zone=<domain>[,<subnet>[/<prefix length>][,<subnet>[/<prefix
length>].....][,exclude:<subnet>[/<prefix length>]].....]
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 interface, in which case the subnets implied by that interface's configured
addresses and netmask/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 confined to only IPv6 addresses using
<interface>/6 or to only IPv4 using <interface>/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=<serial>[,<hostmaster>[,<refresh>[,<retry>[,<expiry>]]]]
Specify fields in the SOA record associated with authoritative zones. Note that
this is optional, all the values are set to sane defaults.
--auth-sec-servers=<domain>[,<domain>[,<domain>...]]
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=<ip-address>[,<ip-address>[,<ip-address>...]]
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, then AXFR requests will be accepted from any secondary.
--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 conntrack support included and
configured. This option cannot be combined with --query-port.
-F, --dhcp-range=[tag:<tag>[,tag:<tag>],][set:<tag>,]<start-addr>[,<end-
addr>|<mode>][,<netmask>[,<broadcast>]][,<lease time>]
-F, --dhcp-range=[tag:<tag>[,tag:<tag>],][set:<tag>,]<start-IPv6addr>[,<end-
IPv6addr>|constructor:<interface>][,<mode>][,<prefix-len>][,<lease time>]
Enable the DHCP server. Addresses will be given out from the range <start-addr> to
<end-addr> and from statically defined addresses given in dhcp-host options. If the
lease time is given, then leases will be given for that length of time. The lease
time is in seconds, or minutes (eg 45m) or hours (eg 1h) or "infinite". If not
given, the default lease time is one hour. The minimum lease time is two minutes.
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 addresses, 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 configuration. For networks which receive DHCP
service via a relay agent, dnsmasq cannot determine 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 optional. 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 address contain only the network part (ie ::1) and they are followed by
constructor:<interface>. 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 <network>::1 to
<network>::400. If the interface is assigned more than one network, then the
corresponding ranges will be automatically created, and then deprecated and finally
removed again as the address is deprecated 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 deprecated.
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:<tag> sets an alphanumeric label which marks this network so that
dhcp options may be specified on a per-network basis. When it is prefixed with
'tag:' instead, then its meaning changes from setting a tag to matching it. Only
one tag may be set, but more than one tag may be matched.
The optional <mode> 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 addresses 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 <mode> may be proxy in which case dnsmasq will provide proxy-DHCP on
the specified 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-advrouter, 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 address.
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 information.
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 assumes 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 extensions. 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=[<hwaddr>][,id:<client_id>|*][,set:<tag>][,<ipaddr>][,<hostname>][,<lease_time>][,ignore]
Specify per host parameters for the DHCP server. This allows a machine with a
particular hardware address to be always allocated the same hostname, IP address
and lease time. A hostname specified like this overrides any supplied by the DHCP
client on the machine. It is also allowable to 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 an IPv6 address, or both. IPv6
addresses must be bracketed by square brackets thus: --dhcp-host=laptop,[1234::56]
IPv6 addresses 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. 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 ).
The special keyword "ignore" tells dnsmasq to never offer a DHCP lease to a
machine. The machine can be specified by hardware address, client ID or hostname,
for instance --dhcp-host=00:20:e0:3b:13:af,ignore This is useful when there is
another DHCP server on the network which should be used by some machines.
The set:<tag> 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:<tag>"
is allowed). When a host matches any dhcp-host directive (or one implied by
/etc/ethers) then the special tag "known" is set. This allows dnsmasq to be
configured to ignore requests from unknown machines using --dhcp-ignore=tag:!known
If the host matches only a dhcp-host directive which cannot be used because it
specifies an address on different subnet, the tag "known-othernet" is set.
Ethernet addresses (but not client-ids) may have wildcard bytes, so for example
--dhcp-host=00:20:e0:3b:13:*,ignore will cause dnsmasq to ignore a range of
hardware addresses. Note that the "*" will need to be escaped or quoted on a
command line, but not in the configuration file.
Hardware addresses normally match any network (ARP) type, but it is possible to
restrict them to a single ARP type by preceding them with the ARP-type (in HEX) and
"-". so --dhcp-host=06-00:20:e0:3b:13:af,1.2.3.4 will only match a Token-Ring
hardware address, since the ARP-address type for token ring is 6.
As a special case, 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, useful to
allocate a stable IP address to a laptop which has both wired and wireless
interfaces.
--dhcp-hostsfile=<path>
Read DHCP host information from the specified file. If a directory is given, then
read all the files contained in that directory. 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=<path>
Read DHCP option information from the specified file. If a directory is given,
then read all the files contained in that directory. The advantage of using this
option is the same as for --dhcp-hostsfile: the dhcp-optsfile will be re-read when
dnsmasq receives SIGHUP. Note that it is possible to encode the information in a
--dhcp-boot flag as DHCP options, using the options names bootfile-name, server-ip-
address and tftp-server. This allows these to be included in a dhcp-optsfile.
--dhcp-hostsdir=<path>
This is equivalent to dhcp-hostsfile, except for the following. The path MUST be a
directory, and not an individual file. Changed or new files within the directory
are read automatically, 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.
--dhcp-optsdir=<path>
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 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. IPv6 addresses are NOT read from /etc/ethers.
-O, --dhcp-option=[tag:<tag>,[tag:<tag>,]][encap:<opt>,][vi-
encap:<enterprise>,][vendor:[<vendor-class>],][<opt>|option:<opt-
name>|option6:<opt>|option6:<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. (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 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 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 described 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: 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 (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 vendor-class matching is substring
based (see --dhcp-vendorclass for details). If a vendor-class option (number 60) is
sent by dnsmasq, then that is used for selecting encapsulated options in preference
to any sent by the client. It is possible to omit the vendorclass completely;
--dhcp-option=vendor:,1,0.0.0.0 in which case the encapsulated option is always
sent.
Options may be encapsulated (IPv4 only) within other options: for instance --dhcp-
option=encap:175, 190, iscsi-client0 will send option 175, within which is the
option 190. If multiple options are given which are encapsulated with the same
option number then they will be correctly combined into one encapsulated option.
encap: and vendor: are may not both be set in the same dhcp-option.
The final variant on encapsulated options is "Vendor-Identifying Vendor Options" as
specified by RFC3925. These are denoted like this: --dhcp-option=vi-encap:2, 10,
text The number in the vi-encap: section is the IANA enterprise number used to
identify this option. This form of encapsulation is supported in IPv6.
The address 0.0.0.0 is not treated specially in encapsulated options.
--dhcp-option-force=[tag:<tag>,[tag:<tag>,]][encap:<opt>,][vi-
encap:<enterprise>,][vendor:[<vendor-class>],]<opt>,[<value>[,<value>]]
This works in exactly the same way as --dhcp-option except that the option will
always be sent, even if the client does not ask for it in the parameter request
list. This is sometimes needed, for example when sending options to PXELinux.
--dhcp-no-override
(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.
--dhcp-relay=<local address>,<server address>[,<interface]
Configure dnsmasq to do DHCP relay. The local address is an address allocated to an
interface on the host running dnsmasq. All DHCP requests arriving on that interface
will we relayed to a remote DHCP server at the server address. It is possible to
relay from a single local address to multiple remote servers by using multiple
dhcp-relay configs with the same local address and different server addresses. A
server address must be an IP literal address, not a domain name. In the case of
DHCPv6, the server address may be the ALL_SERVERS multicast address, ff05::1:3. In
this case the interface must be given, not be wildcard, and is used to direct the
multicast to the correct interface to reach the DHCP server.
Access control for DHCP clients has the same rules as for the DHCP server, see
--interface, --except-interface, etc. The optional interface name in the dhcp-relay
config has a different function: it controls on which interface DHCP replies from
the server will be accepted. This is intended for configurations which have three
interfaces: one being relayed from, a second connecting the DHCP server, and a
third untrusted network, typically the wider internet. It avoids the possibility of
spoof replies arriving via this third interface.
It is allowed to have dnsmasq act as a DHCP server on one set of interfaces and
relay from a disjoint set of interfaces. Note that whilst it is quite possible to
write configurations which appear to act as a server and a relay on the same
interface, this is not supported: the relay function will take precedence.
Both DHCPv4 and DHCPv6 relay is supported. It's not possible to relay DHCPv4 to a
DHCPv6 server or vice-versa.
-U, --dhcp-vendorclass=set:<tag>,[enterprise:<IANA-enterprise number>,]<vendor-class>
Map from a vendor-class string to a tag. Most DHCP clients provide a "vendor class"
which represents, in some sense, the type of host. This option maps vendor classes
to tags, so that DHCP options may be selectively delivered to different classes of
hosts. For example dhcp-vendorclass=set:printers,Hewlett-Packard JetDirect will
allow options to be set only for HP printers like so: --dhcp-
option=tag:printers,3,192.168.4.4 The vendor-class string is substring matched
against the vendor-class supplied by the client, to allow fuzzy matching. The set:
prefix is optional but allowed for consistency.
Note that in IPv6 only, vendorclasses are namespaced with an IANA-allocated
enterprise number. This is given with enterprise: keyword and specifies that only
vendorclasses matching the specified number should be searched.
-j, --dhcp-userclass=set:<tag>,<user-class>
Map from a user-class string to a tag (with substring matching, like vendor
classes). Most DHCP clients provide a "user class" which is configurable. This
option maps user classes to tags, so that DHCP options may be selectively delivered
to different classes of hosts. It is possible, for instance to use this to set a
different printer server for hosts in the class "accounts" than for hosts in the
class "engineering".
-4, --dhcp-mac=set:<tag>,<MAC address>
Map from a MAC address to a tag. The MAC address may include wildcards. For example
--dhcp-mac=set:3com,01:34:23:*:*:* will set the tag "3com" for any host whose MAC
address matches the pattern.
--dhcp-circuitid=set:<tag>,<circuit-id>, --dhcp-remoteid=set:<tag>,<remote-id>
Map from RFC3046 relay agent options to tags. This data may be provided by DHCP
relay agents. The circuit-id or remote-id is normally given as colon-separated hex,
but is also allowed to be a simple string. If an exact match is achieved between
the circuit or agent ID and one provided by a relay agent, the tag is set.
dhcp-remoteid (but not dhcp-circuitid) is supported in IPv6.
--dhcp-subscrid=set:<tag>,<subscriber-id>
(IPv4 and IPv6) Map from RFC3993 subscriber-id relay agent options to tags.
--dhcp-proxy[=<ip addr>]......
(IPv4 only) A normal DHCP relay agent is only used to forward the initial parts of
a DHCP interaction to the DHCP server. Once a client is configured, it communicates
directly with the server. This is undesirable if the relay agent is adding extra
information to the DHCP packets, such as that used by dhcp-circuitid and dhcp-
remoteid. A full relay implementation can use the RFC 5107 serverid-override
option to force the DHCP server to use the relay as a full proxy, with all packets
passing through it. This flag provides an alternative method of doing the same
thing, for relays which don't support RFC 5107. Given alone, it manipulates the
server-id for all interactions via relays. If a list of IP addresses is given, only
interactions via relays at those addresses are affected.
--dhcp-match=set:<tag>,<option number>|option:<option name>|vi-
encap:<enterprise>[,<value>]
Without a value, set the tag if the client sends a DHCP option of the given number
or name. When a value is given, set the tag only if the option is sent and matches
the value. The value may be of the form "01:ff:*:02" in which case the value must
match (apart from wildcards) but the option sent may have unmatched data past the
end of the value. The value may also be of the same form as in dhcp-option in which
case the option sent is treated as an array, and one element must match, so
--dhcp-match=set:efi-ia32,option:client-arch,6
will set the tag "efi-ia32" if the the number 6 appears in the list of
architectures sent by the client in option 93. (See RFC 4578 for details.) If the
value is a string, substring matching is used.
The special form with vi-encap:<enterprise number> matches against vendor-
identifying vendor classes for the specified enterprise. Please see RFC 3925 for
more details of these rare and interesting beasts.
--tag-if=set:<tag>[,set:<tag>[,tag:<tag>[,tag:<tag>]]]
Perform boolean operations on tags. Any tag appearing as set:<tag> is set if all
the tags which appear as tag:<tag> are set, (or unset when tag:!<tag> is used) If
no tag:<tag> appears set:<tag> tags are set unconditionally. Any number of set:
and tag: forms may appear, in any order. Tag-if lines are executed in order, so if
the tag in tag:<tag> is a tag set by another tag-if, the line which sets the tag
must precede the one which tests it.
-J, --dhcp-ignore=tag:<tag>[,tag:<tag>]
When all the given tags appear in the tag set ignore the host and do not allocate
it a DHCP lease.
--dhcp-ignore-names[=tag:<tag>[,tag:<tag>]]
When all the given tags appear in the tag set, ignore any hostname provided by the
host. Note that, unlike dhcp-ignore, it is permissible to supply no tags, in which
case DHCP-client supplied hostnames are always ignored, and DHCP hosts are added to
the DNS using only dhcp-host configuration in dnsmasq and the contents of
/etc/hosts and /etc/ethers.
--dhcp-generate-names=tag:<tag>[,tag:<tag>]
(IPv4 only) Generate a name for DHCP clients which do not otherwise have one, using
the MAC address expressed in hex, separated by dashes. Note that if a host provides
a name, it will be used by preference to this, unless --dhcp-ignore-names is set.
--dhcp-broadcast[=tag:<tag>[,tag:<tag>]]
(IPv4 only) When all the given tags appear in the tag set, always use broadcast to
communicate with the host when it is unconfigured. It is permissible to supply no
tags, in which case this is unconditional. Most DHCP clients which need broadcast
replies set a flag in their requests so that this happens automatically, some old
BOOTP clients do not.
-M, --dhcp-boot=[tag:<tag>,]<filename>,[<servername>[,<server address>|<tftp_servername>]]
(IPv4 only) Set BOOTP options to be returned by the DHCP server. Server name and
address are optional: if not provided, the name is left empty, and the address set
to the address of the machine running dnsmasq. If dnsmasq is providing a TFTP
service (see --enable-tftp ) then only the filename is required here to enable
network booting. If the optional tag(s) are given, they must match for this
configuration to be sent. Instead of an IP address, the TFTP server address can be
given as a domain name which is looked up in /etc/hosts. This name can be
associated in /etc/hosts with multiple IP addresses, which are used round-robin.
This facility can be used to load balance the tftp load among a set of servers.
--dhcp-sequential-ip
Dnsmasq is designed to choose IP addresses for DHCP clients using a hash of the
client's MAC address. This normally allows a client's address to remain stable
long-term, even if the client sometimes allows its DHCP lease to expire. In this
default mode IP addresses are distributed pseudo-randomly over the entire available
address range. There are sometimes circumstances (typically server deployment)
where it is more convenient to have IP addresses allocated sequentially, starting
from the lowest available address, and setting this flag enables this mode. Note
that in the sequential mode, clients which allow a lease to expire are much more
likely to move IP address; for this reason it should not be generally used.
--pxe-service=[tag:<tag>,]<CSA>,<menu text>[,<basename>|<bootservicetype>][,<server
address>|<server_name>]
Most uses of PXE boot-ROMS simply allow the PXE system to obtain an IP address and
then download the file specified by dhcp-boot and execute it. However the PXE
system is capable of more complex functions when supported by a suitable DHCP
server.
This specifies a boot option which may appear in a PXE boot menu. <CSA> is client
system type, only services of the correct type will appear in a menu. The known
types are x86PC, PC98, IA64_EFI, Alpha, Arc_x86, Intel_Lean_Client, IA32_EFI,
X86-64_EFI, Xscale_EFI, BC_EFI, ARM32_EFI and ARM64_EFI; an integer may be used for
other types. The parameter after the menu text may be a file name, in which case
dnsmasq acts as a boot server and directs the PXE client to download the file by
TFTP, either from itself ( enable-tftp must be set for this to work) or another
TFTP server if the final server address/name is given. Note that the "layer"
suffix (normally ".0") is supplied by PXE, and need not be added to the basename.
Alternatively, the basename may be a filename, complete with suffix, in which case
no layer suffix is added. If an integer boot service type, rather than a basename
is given, then the PXE client will search for a suitable boot service for that type
on the network. This search may be done by broadcast, or direct to a server if its
IP address/name is provided. If no boot service type or filename is provided (or a
boot service type of 0 is specified) then the menu entry will abort the net boot
procedure and continue booting from local media. The server address can be given as
a domain name which is looked up in /etc/hosts. This name can be associated in
/etc/hosts with multiple IP addresses, which are used round-robin.
--pxe-prompt=[tag:<tag>,]<prompt>[,<timeout>]
Setting this provides a prompt to be displayed after PXE boot. If the timeout is
given then after the timeout has elapsed with no keyboard input, the first
available menu option will be automatically executed. If the timeout is zero then
the first available menu item will be executed immediately. If pxe-prompt is
omitted the system will wait for user input if there are multiple items in the
menu, but boot immediately if there is only one. See pxe-service for details of
menu items.
Dnsmasq supports PXE "proxy-DHCP", in this case another DHCP server on the network
is responsible for allocating IP addresses, and dnsmasq simply provides the
information given in pxe-prompt and pxe-service to allow netbooting. This mode is
enabled using the proxy keyword in dhcp-range.
-X, --dhcp-lease-max=<number>
Limits dnsmasq to the specified maximum number of DHCP leases. The default is 1000.
This limit is to prevent DoS attacks from hosts which create thousands of leases
and use lots of memory in the dnsmasq process.
-K, --dhcp-authoritative
Should be set when dnsmasq is definitely the only DHCP server on a network. For
DHCPv4, 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. For DHCPv6 it sets the priority in replies to 255 (the
maximum) instead of 0 (the minimum).
--dhcp-alternate-port[=<server port>[,<client port>]]
(IPv4 only) Change the ports used for DHCP from the default. If this option is
given alone, without arguments, it changes the ports used for DHCP from 67 and 68
to 1067 and 1068. If a single argument is given, that port number is used for the
server and the port number plus one used for the client. Finally, two port numbers
allows arbitrary specification of both server and client ports for DHCP.
-3, --bootp-dynamic[=<network-id>[,<network-id>]]
(IPv4 only) Enable dynamic allocation of IP addresses to BOOTP clients. Use this
with care, since each address allocated to a BOOTP client is leased forever, and
therefore becomes permanently unavailable for re-use by other hosts. if this is
given without tags, then it unconditionally enables dynamic allocation. With tags,
only when the tags are all set. It may be repeated with different tag sets.
-5, --no-ping
(IPv4 only) By default, the DHCP server will attempt to ensure that an address is
not in use before allocating it to a host. It does this by sending an ICMP echo
request (aka "ping") to the address in question. If it gets a reply, then the
address must already be in use, and another is tried. This flag disables this
check. Use with caution.
--log-dhcp
Extra logging for DHCP: log all the options sent to DHCP clients and the tags used
to determine them.
--quiet-dhcp, --quiet-dhcp6, --quiet-ra
Suppress logging of the routine operation of these protocols. Errors and problems
will still be logged. --quiet-dhcp and quiet-dhcp6 are over-ridden by --log-dhcp.
-l, --dhcp-leasefile=<path>
Use the specified file to store DHCP lease information.
--dhcp-duid=<enterprise-id>,<uid>
(IPv6 only) Specify the server persistent UID which the DHCPv6 server will use.
This option is not normally required as dnsmasq creates a DUID automatically when
it is first needed. When given, this option provides dnsmasq the data required to
create a DUID-EN type DUID. Note that once set, the DUID is stored in the lease
database, so to change between DUID-EN and automatically created DUIDs or vice-
versa, the lease database must be re-initialised. The enterprise-id is assigned by
IANA, and the uid is a string of hex octets unique to a particular device.
-6 --dhcp-script=<path>
Whenever a new DHCP lease is created, or an old one destroyed, or a TFTP file
transfer completes, the executable specified by this option is run. <path> must be
an absolute pathname, no PATH search occurs. The arguments to the process are
"add", "old" or "del", the MAC address of the host (or DUID for IPv6) , the IP
address, and the hostname, if known. "add" means a lease has been created, "del"
means it has been destroyed, "old" is a notification of an existing lease when
dnsmasq starts or a change to MAC address or hostname of an existing lease (also,
lease length or expiry and client-id, if leasefile-ro is set). If the MAC address
is from a network type other than ethernet, it will have the network type
prepended, eg "06-01:23:45:67:89:ab" for token ring. The process is run as root
(assuming that dnsmasq was originally run as root) even if dnsmasq is configured to
change UID to an unprivileged user.
The environment is inherited from the invoker of dnsmasq, with some or all of the
following variables added
For both IPv4 and IPv6:
DNSMASQ_DOMAIN if the fully-qualified domain name of the host is known, this is set
to the domain part. (Note that the hostname passed to the script as an argument is
never fully-qualified.)
If the client provides a hostname, DNSMASQ_SUPPLIED_HOSTNAME
If the client provides user-classes, DNSMASQ_USER_CLASS0..DNSMASQ_USER_CLASSn
If dnsmasq was compiled with HAVE_BROKEN_RTC, then the length of the lease (in
seconds) is stored in DNSMASQ_LEASE_LENGTH, otherwise the time of lease expiry is
stored in DNSMASQ_LEASE_EXPIRES. The number of seconds until lease expiry is always
stored in DNSMASQ_TIME_REMAINING.
If a lease used to have a hostname, which is removed, an "old" event is generated
with the new state of the lease, ie no name, and the former name is provided in the
environment variable DNSMASQ_OLD_HOSTNAME.
DNSMASQ_INTERFACE stores the name of the interface on which the request arrived;
this is not set for "old" actions when dnsmasq restarts.
DNSMASQ_RELAY_ADDRESS is set if the client used a DHCP relay to contact dnsmasq and
the IP address of the relay is known.
DNSMASQ_TAGS contains all the tags set during the DHCP transaction, separated by
spaces.
DNSMASQ_LOG_DHCP is set if --log-dhcp is in effect.
For IPv4 only:
DNSMASQ_CLIENT_ID if the host provided a client-id.
DNSMASQ_CIRCUIT_ID, DNSMASQ_SUBSCRIBER_ID, DNSMASQ_REMOTE_ID if a DHCP relay-agent
added any of these options.
If the client provides vendor-class, DNSMASQ_VENDOR_CLASS.
DNSMASQ_REQUESTED_OPTIONS a string containing the decimal values in the Parameter
Request List option, comma separated, if the parameter request list option is
provided by the client.
For IPv6 only:
If the client provides vendor-class, DNSMASQ_VENDOR_CLASS_ID, containing the IANA
enterprise id for the class, and DNSMASQ_VENDOR_CLASS0..DNSMASQ_VENDOR_CLASSn for
the data.
DNSMASQ_SERVER_DUID containing the DUID of the server: this is the same for every
call to the script.
DNSMASQ_IAID containing the IAID for the lease. If the lease is a temporary
allocation, this is prefixed to 'T'.
DNSMASQ_MAC containing the MAC address of the client, if known.
Note that the supplied hostname, vendorclass and userclass data is only supplied
for "add" actions or "old" actions when a host resumes an existing lease, since
these data are not held in dnsmasq's lease database.
All file descriptors are closed except stdin, which is open to /dev/null, and
stdout and stderr which capture output for logging by dnsmasq. (In debug mode,
stdio, stdout and stderr file are left as those inherited from the invoker of
dnsmasq).
The script is not invoked concurrently: at most one instance of the script is ever
running (dnsmasq waits for an instance of script to exit before running the next).
Changes to the lease database are which require the script to be invoked are queued
awaiting exit of a running instance. If this queueing allows multiple state
changes occur to a single lease before the script can be run then earlier states
are discarded and the current state of that lease is reflected when the script
finally runs.
At dnsmasq startup, the script will be invoked for all existing leases as they are
read from the lease file. Expired leases will be called with "del" and others with
"old". When dnsmasq receives a HUP signal, the script will be invoked for existing
leases with an "old" event.
There are four further actions which may appear as the first argument to the
script, "init", "arp-add", "arp-del" and "tftp". More may be added in the future,
so scripts should be written to ignore unknown actions. "init" is described below
in --leasefile-ro The "tftp" action is invoked when a TFTP file transfer completes:
the arguments are the file size in bytes, the address to which the file was sent,
and the complete pathname of the file.
The "arp-add" and "arp-del" actions are only called if enabled with --script-arp
They are are supplied with a MAC address and IP address as arguments. "arp-add"
indicates the arrival of a new entry in the ARP or neighbour table, and "arp-del"
indicates the deletion of same.
--dhcp-luascript=<path>
Specify a script written in Lua, to be run when leases are created, destroyed or
changed. To use this option, dnsmasq must be compiled with the correct support. The
Lua interpreter is initialised once, when dnsmasq starts, so that global variables
persist between lease events. The Lua code must define a lease function, and may
provide init and shutdown functions, which are called, without arguments when
dnsmasq starts up and terminates. It may also provide a tftp function.
The lease function receives the information detailed in --dhcp-script. It gets two
arguments, firstly the action, which is a string containing, "add", "old" or "del",
and secondly a table of tag value pairs. The tags mostly correspond to the
environment variables detailed above, for instance the tag "domain" holds the same
data as the environment variable DNSMASQ_DOMAIN. There are a few extra tags which
hold the data supplied as arguments to --dhcp-script. These are mac_address,
ip_address and hostname for IPv4, and client_duid, ip_address and hostname for
IPv6.
The tftp function is called in the same way as the lease function, and the table
holds the tags destination_address, file_name and file_size.
The arp and arp-old functions are called only when enabled with --script-arp and
have a table which holds the tags mac_address and client_address.
--dhcp-scriptuser
Specify the user as which to run the lease-change script or Lua script. This
defaults to root, but can be changed to another user using this flag.
--script-arp
Enable the "arp" and "arp-old" functions in the dhcp-script and dhcp-luascript.
-9, --leasefile-ro
Completely suppress use of the lease database file. The file will not be created,
read, or written. Change the way the lease-change script (if one is provided) is
called, so that the lease database may be maintained in external storage by the
script. In addition to the invocations given in --dhcp-script the lease-change
script is called once, at dnsmasq startup, with the single argument "init". When
called like this the script should write the saved state of the lease database, in
dnsmasq leasefile format, to stdout and exit with zero exit code. Setting this
option also forces the leasechange script to be called on changes to the client-id
and lease length and expiry time.
--bridge-interface=<interface>,<alias>[,<alias>]
Treat DHCP (v4 and v6) requests and IPv6 Router Solicit packets arriving at any of
the <alias> interfaces as if they had arrived at <interface>. This option allows
dnsmasq to provide DHCP and RA service over unaddressed and unbridged Ethernet
interfaces, e.g. on an OpenStack compute host where each such interface is a TAP
interface to a VM, or as in "old style bridging" on BSD platforms. A trailing '*'
wildcard can be used in each <alias>.
It is permissible to add more than one alias using more than one --bridge-interface
option since --bridge-interface=int1,alias1,alias2 is exactly equivalent to
--bridge-interface=int1,alias1 --bridge-interface=int1,alias2
-s, --domain=<domain>[,<address range>[,local]]
Specifies DNS domains for the DHCP server. Domains may be be given unconditionally
(without the IP range) or for limited IP ranges. This has two effects; firstly it
causes the DHCP server to return the domain to any hosts which request it, and
secondly it sets the domain which it is legal for DHCP-configured hosts to claim.
The intention is to constrain hostnames so that an untrusted host on the LAN cannot
advertise its name via dhcp as e.g. "microsoft.com" and capture traffic not meant
for it. If no domain suffix is specified, then any DHCP hostname with a domain part
(ie with a period) will be disallowed and logged. If suffix is specified, then
hostnames with a domain part are allowed, provided the domain part matches the
suffix. In addition, when a suffix is set then hostnames without a domain part have
the suffix added as an optional domain part. Eg on my network I can set
--domain=thekelleys.org.uk and have a machine whose DHCP hostname is "laptop". The
IP address for that machine is available from dnsmasq both as "laptop" and
"laptop.thekelleys.org.uk". If the domain is given as "#" then the domain is read
from the first "search" directive in /etc/resolv.conf (or equivalent).
The address range can be of the form <ip address>,<ip address> or <ip
address>/<netmask> or just a single <ip address>. See --dhcp-fqdn which can change
the behaviour of dnsmasq with domains.
If the address range is given as ip-address/network-size, then a additional flag
"local" may be supplied which has the effect of adding --local declarations for
forward and reverse DNS queries. Eg.
--domain=thekelleys.org.uk,192.168.0.0/24,local is identical to
--domain=thekelleys.org.uk,192.168.0.0/24 --local=/thekelleys.org.uk/
--local=/0.168.192.in-addr.arpa/ The network size must be 8, 16 or 24 for this to
be legal.
--dhcp-fqdn
In the default mode, dnsmasq inserts the unqualified names of DHCP clients into the
DNS. For this reason, the names must be unique, even if two clients which have the
same name are in different domains. If a second DHCP client appears which has the
same name as an existing client, the name is transferred to the new client. If
--dhcp-fqdn is set, this behaviour changes: the unqualified name is no longer put
in the DNS, only the qualified name. Two DHCP clients with the same name may both
keep the name, provided that the domain part is different (ie the fully qualified
names differ.) To ensure that all names have a domain part, there must be at least
--domain without an address specified when --dhcp-fqdn is set.
--dhcp-client-update
Normally, when giving a DHCP lease, dnsmasq sets flags in the FQDN option to tell
the client not to attempt a DDNS update with its name and IP address. This is
because the name-IP pair is automatically added into dnsmasq's DNS view. This flag
suppresses that behaviour, this is useful, for instance, to allow Windows clients
to update Active Directory servers. See RFC 4702 for details.
--enable-ra
Enable dnsmasq's IPv6 Router Advertisement feature. DHCPv6 doesn't handle complete
network configuration in the same way as DHCPv4. Router discovery and (possibly)
prefix discovery for autonomous address creation are handled by a different
protocol. When DHCP is in use, only a subset of this is needed, and dnsmasq can
handle it, using existing DHCP configuration to provide most data. When RA is
enabled, dnsmasq will advertise a prefix for each dhcp-range, with default router
as the relevant link-local address on the machine running dnsmasq. By default, the
"managed address" bits are set, and the "use SLAAC" bit is reset. This can be
changed for individual subnets with the mode keywords described in --dhcp-range.
RFC6106 DNS parameters are included in the advertisements. By default, the relevant
link-local address of the machine running dnsmasq is sent as recursive DNS server.
If provided, the DHCPv6 options dns-server and domain-search are used for the DNS
server (RDNSS) and the domain search list (DNSSL).
--ra-param=<interface>,[mtu:<integer>|<interface>|off,][high,|low,]<ra-interval>[,<router
lifetime>]
Set non-default values for router advertisements sent via an interface. The
priority field for the router may be altered from the default of medium with eg
--ra-param=eth0,high. The interval between router advertisements may be set (in
seconds) with --ra-param=eth0,60. The lifetime of the route may be changed or set
to zero, which allows a router to advertise prefixes but not a route via itself.
--ra-parm=eth0,0,0 (A value of zero for the interval means the default value.) All
four parameters may be set at once. --ra-param=eth0,mtu:1280,low,60,1200
The interface field may include a wildcard.
The mtu: parameter may be an arbitrary interface name, in which case the MTU value
for that interface is used. This is useful for (eg) advertising the MTU of a WAN
interface on the other interfaces of a router.
--dhcp-reply-delay=[tag:<tag>,]<integer>
Delays sending DHCPOFFER and proxydhcp replies for at least the specified number of
seconds. This can be used as workaround for bugs in PXE boot firmware that does
not function properly when receiving an instant reply. This option takes into
account the time already spent waiting (e.g. performing ping check) if any.
--enable-tftp[=<interface>[,<interface>]]
Enable the TFTP server function. This is deliberately limited to that needed to
net-boot a client. Only reading is allowed; the tsize and blksize extensions are
supported (tsize is only supported in octet mode). Without an argument, the TFTP
service is provided to the same set of interfaces as DHCP service. If the list of
interfaces is provided, that defines which interfaces receive TFTP service.
--tftp-root=<directory>[,<interface>]
Look for files to transfer using TFTP relative to the given directory. When this is
set, TFTP paths which include ".." are rejected, to stop clients getting outside
the specified root. Absolute paths (starting with /) are allowed, but they must be
within the tftp-root. If the optional interface argument is given, the directory is
only used for TFTP requests via that interface.
--tftp-no-fail
Do not abort startup if specified tftp root directories are inaccessible.
--tftp-unique-root[=ip|mac]
Add the IP or hardware address of the TFTP client as a path component on the end of
the TFTP-root. Only valid if a tftp-root is set and the directory exists. Defaults
to adding IP address (in standard dotted-quad format). 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. When
"=mac" is specified it will append the MAC address instead, using lowercase zero
padded digits separated by dashes, e.g.: 01-02-03-04-aa-bb Note that resolving MAC
addresses is only possible if the client is in the local network or obtained a DHCP
lease from us.
--tftp-secure
Enable TFTP secure mode: without this, any file which is readable by the dnsmasq
process under normal unix access-control rules is available via TFTP. When the
--tftp-secure flag is given, only files owned by the user running the dnsmasq
process are accessible. If dnsmasq is being run as root, different rules apply:
--tftp-secure has no effect, but only files which have the world-readable bit set
are accessible. It is not recommended to run dnsmasq as root with TFTP enabled, and
certainly not without specifying --tftp-root. Doing so can expose any world-
readable file on the server to any host on the net.
--tftp-lowercase
Convert filenames in TFTP requests to all lowercase. This is useful for requests
from Windows machines, which have case-insensitive filesystems and tend to play
fast-and-loose with case in filenames. Note that dnsmasq's tftp server always
converts "\" to "/" in filenames.
--tftp-max=<connections>
Set the maximum number of concurrent TFTP connections allowed. This defaults to 50.
When serving a large number of TFTP connections, per-process file descriptor limits
may be encountered. Dnsmasq needs one file descriptor for each concurrent TFTP
connection and one file descriptor per unique file (plus a few others). So serving
the same file simultaneously to n clients will use require about n + 10 file
descriptors, serving different files simultaneously to n clients will require about
(2*n) + 10 descriptors. If --tftp-port-range is given, that can affect the number
of concurrent connections.
--tftp-mtu=<mtu size>
Use size as the ceiling of the MTU supported by the intervening network when
negotiating TFTP blocksize, overriding the MTU setting of the local interface if
it is larger.
--tftp-no-blocksize
Stop the TFTP server from negotiating the "blocksize" option with a client. Some
buggy clients request this option but then behave badly when it is granted.
--tftp-port-range=<start>,<end>
A TFTP server listens on a well-known port (69) for connection initiation, but it
also uses a dynamically-allocated port for each connection. Normally these are
allocated by the OS, but this option specifies a range of ports for use by TFTP
transfers. This can be useful when TFTP has to traverse a firewall. The start of
the range cannot be lower than 1025 unless dnsmasq is running as root. The number
of concurrent TFTP connections is limited by the size of the port range.
-C, --conf-file=<file>
Specify a different configuration file. The conf-file option is also allowed in
configuration files, to include multiple configuration files. A filename of "-"
causes dnsmasq to read configuration from stdin.
-7, --conf-dir=<directory>[,<file-extension>......],
Read all the files in the given directory as configuration files. If extension(s)
are given, any files which end in those extensions are skipped. Any files whose
names end in ~ or start with . or start and end with # are always skipped. If the
extension starts with * then only files which have that extension are loaded. So
--conf-dir=/path/to/dir,*.conf loads all files with the suffix .conf in
/path/to/dir. This flag may be given on the command line or in a configuration
file. If giving it on the command line, be sure to escape * characters.
--servers-file=<file>
A special case of --conf-file which differs in two respects. Firstly, only --server
and --rev-server are allowed in the configuration file included. Secondly, the file
is re-read and the configuration therein is updated when dnsmasq receives SIGHUP.
CONFIG FILE
At startup, dnsmasq reads /etc/dnsmasq.conf, if it exists. (On FreeBSD, the file is
/usr/local/etc/dnsmasq.conf ) (but see the -C and -7 options.) The format of this file
consists of one option per line, exactly as the long options detailed in the OPTIONS
section but without the leading "--". Lines starting with # are comments and ignored. For
options which may only be specified once, the configuration file overrides the command
line. Quoting is allowed in a config file: between " quotes the special meanings of ,:.
and # are removed and the following escapes are allowed: \\ \" \t \e \b \r and \n. The
later corresponding to tab, escape, backspace, return and newline.
NOTES
When it receives a SIGHUP, dnsmasq clears its cache and then re-loads /etc/hosts and
/etc/ethers and any file given by --dhcp-hostsfile, --dhcp-hostsdir, --dhcp-optsfile,
--dhcp-optsdir, --addn-hosts or --hostsdir. 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. The number of cache hits and misses and the number of
authoritative queries answered are also given. 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.
The cache statistics are also available in the DNS as answers to queries of class CHAOS
and type TXT in domain bind. The domain names are cachesize.bind, insertions.bind,
evictions.bind, misses.bind, hits.bind, auth.bind and servers.bind. An example command to
query this, using the dig utility would be
dig +short chaos txt cachesize.bind
When it receives SIGUSR2 and it is logging direct to a file (see --log-facility ) dnsmasq
will close and reopen the log file. Note that during this operation, dnsmasq will not be
running as root. When it first creates the logfile dnsmasq changes the ownership of the
file to the non-root user it will run as. Logrotate should be configured to create a new
log file with the ownership which matches the existing one before sending SIGUSR2. If TCP
DNS queries are in progress, the old logfile will remain open in child processes which are
handling TCP queries and may continue to be written. There is a limit of 150 seconds,
after which all existing TCP processes will have expired: for this reason, it is not wise
to configure logfile compression for logfiles which have just been rotated. Using
logrotate, the required options are create and delaycompress.
Dnsmasq is a DNS query forwarder: it is not capable of recursively answering arbitrary
queries starting from the root servers but forwards such queries to a fully recursive
upstream DNS server which is typically provided by an ISP. By default, dnsmasq reads
/etc/resolv.conf to discover the IP addresses of the upstream nameservers it should use,
since the information is typically stored there. Unless --no-poll is used, dnsmasq checks
the modification time of /etc/resolv.conf (or equivalent if --resolv-file is used) and re-
reads it if it changes. This allows the DNS servers to be set dynamically by PPP or DHCP
since both protocols provide the information. Absence of /etc/resolv.conf is not an error
since it may not have been created before a PPP connection exists. Dnsmasq simply keeps
checking in case /etc/resolv.conf is created at any time. Dnsmasq can be told to parse
more than one resolv.conf file. This is useful on a laptop, where both PPP and DHCP may be
used: dnsmasq can be set to poll both /etc/ppp/resolv.conf and /etc/dhcpc/resolv.conf and
will use the contents of whichever changed last, giving automatic switching between DNS
servers.
Upstream servers may also be specified on the command line or in the configuration file.
These server specifications optionally take a domain name which tells dnsmasq to use that
server only to find names in that particular domain.
In order to configure dnsmasq to act as cache for the host on which it is running, put
"nameserver 127.0.0.1" in /etc/resolv.conf to force local processes to send queries to
dnsmasq. Then either specify the upstream servers directly to dnsmasq using --server
options or put their addresses real in another file, say /etc/resolv.dnsmasq and run
dnsmasq with the -r /etc/resolv.dnsmasq option. This second technique allows for dynamic
update of the server addresses by PPP or DHCP.
Addresses in /etc/hosts will "shadow" different addresses for the same names in the
upstream DNS, so "mycompany.com 1.2.3.4" in /etc/hosts will ensure that queries for
"mycompany.com" always return 1.2.3.4 even if queries in the upstream DNS would otherwise
return a different address. There is one exception to this: if the upstream DNS contains a
CNAME which points to a shadowed name, then looking up the CNAME through dnsmasq will
result in the unshadowed address associated with the target of the CNAME. To work around
this, add the CNAME to /etc/hosts so that the CNAME is shadowed too.
The tag system works as follows: For each DHCP request, dnsmasq collects a set of valid
tags from active configuration lines which include set:<tag>, including one from the dhcp-
range used to allocate the address, one from any matching dhcp-host (and "known" or
"known-othernet" if a dhcp-host matches) The tag "bootp" is set for BOOTP requests, and a
tag whose name is the name of the interface on which the request arrived is also set.
Any configuration lines which include one or more tag:<tag> constructs will only be valid
if all that tags are matched in the set derived above. Typically this is dhcp-option.
dhcp-option which has tags will be used in preference to an untagged dhcp-option,
provided that _all_ the tags match somewhere in the set collected as described above. The
prefix '!' on a tag means 'not' so --dhcp-option=tag:!purple,3,1.2.3.4 sends the option
when the tag purple is not in the set of valid tags. (If using this in a command line
rather than a configuration file, be sure to escape !, which is a shell metacharacter)
When selecting dhcp-options, a tag from dhcp-range is second class relative to other tags,
to make it easy to override options for individual hosts, so dhcp-
range=set:interface1,...... dhcp-host=set:myhost,..... dhcp-
option=tag:interface1,option:nis-domain,"domain1" dhcp-option=tag:myhost,option:nis-
domain,"domain2" will set the NIS-domain to domain1 for hosts in the range, but override
that to domain2 for a particular host.
Note that for dhcp-range both tag:<tag> and set:<tag> are allowed, to both select the
range in use based on (eg) dhcp-host, and to affect the options sent, based on the range
selected.
This system evolved from an earlier, more limited one and for backward compatibility
"net:" may be used instead of "tag:" and "set:" may be omitted. (Except in dhcp-host,
where "net:" may be used instead of "set:".) For the same reason, '#' may be used instead
of '!' to indicate NOT.
The DHCP server in dnsmasq will function as a BOOTP server also, provided that the MAC
address and IP address for clients are given, either using dhcp-host configurations or in
/etc/ethers , and a dhcp-range configuration option is present to activate the DHCP server
on a particular network. (Setting --bootp-dynamic removes the need for static address
mappings.) The filename parameter in a BOOTP request is used as a tag, as is the tag
"bootp", allowing some control over the options returned to different classes of hosts.
AUTHORITATIVE CONFIGURATION
Configuring dnsmasq to act as an authoritative DNS server is complicated by the fact that
it involves configuration of external DNS servers to provide delegation. We will walk
through three scenarios of increasing complexity. Prerequisites for all of these scenarios
are a globally accessible IP address, an A or AAAA record pointing to that address, and an
external DNS server capable of doing delegation of the zone in question. For the first
part of this explanation, we will call the A (or AAAA) record for the globally accessible
address server.example.com, and the zone for which dnsmasq is authoritative our.zone.com.
The simplest configuration consists of two lines of dnsmasq configuration; something like
auth-server=server.example.com,eth0
auth-zone=our.zone.com,1.2.3.0/24
and two records in the external DNS
server.example.com A 192.0.43.10
our.zone.com NS server.example.com
eth0 is the external network interface on which dnsmasq is listening, and has (globally
accessible) address 192.0.43.10.
Note that the external IP address may well be dynamic (ie assigned from an ISP by DHCP or
PPP) If so, the A record must be linked to this dynamic assignment by one of the usual
dynamic-DNS systems.
A more complex, but practically useful configuration has the address record for the
globally accessible IP address residing in the authoritative zone which dnsmasq is
serving, typically at the root. Now we have
auth-server=our.zone.com,eth0
auth-zone=our.zone.com,1.2.3.0/24
our.zone.com A 1.2.3.4
our.zone.com NS our.zone.com
The A record for our.zone.com has now become a glue record, it solves the chicken-and-egg
problem of finding the IP address of the nameserver for our.zone.com when the A record is
within that zone. Note that this is the only role of this record: as dnsmasq is now
authoritative from our.zone.com it too must provide this record. If the external address
is static, this can be done with an /etc/hosts entry or --host-record.
auth-server=our.zone.com,eth0
host-record=our.zone.com,1.2.3.4
auth-zone=our.zone.com,1.2.3.0/24
If the external address is dynamic, the address associated with our.zone.com must be
derived from the address of the relevant interface. This is done using interface-name
Something like:
auth-server=our.zone.com,eth0
interface-name=our.zone.com,eth0
auth-zone=our.zone.com,1.2.3.0/24,eth0
(The "eth0" argument in auth-zone adds the subnet containing eth0's dynamic address to the
zone, so that the interface-name returns the address in outside queries.)
Our final configuration builds on that above, but also adds a secondary DNS server. This
is another DNS server which learns the DNS data for the zone by doing zones transfer, and
acts as a backup should the primary server become inaccessible. The configuration of the
secondary is beyond the scope of this man-page, but the extra configuration of dnsmasq is
simple:
auth-sec-servers=secondary.myisp.com
and
our.zone.com NS secondary.myisp.com
Adding auth-sec-servers enables zone transfer in dnsmasq, to allow the secondary to
collect the DNS data. If you wish to restrict this data to particular hosts then
auth-peer=<IP address of secondary>
will do so.
Dnsmasq acts as an authoritative server for in-addr.arpa and ip6.arpa domains associated
with the subnets given in auth-zone declarations, so reverse (address to name) lookups can
be simply configured with a suitable NS record, for instance in this example, where we
allow 1.2.3.0/24 addresses.
3.2.1.in-addr.arpa NS our.zone.com
Note that at present, reverse (in-addr.arpa and ip6.arpa) zones are not available in zone
transfers, so there is no point arranging secondary servers for reverse lookups.
When dnsmasq is configured to act as an authoritative server, the following data is used
to populate the authoritative zone.
--mx-host, --srv-host, --dns-rr, --txt-record, --naptr-record , as long as the record
names are in the authoritative domain.
--cname as long as the record name is in the authoritative domain. If the target of the
CNAME is unqualified, then it is qualified with the authoritative zone name. CNAME used
in this way (only) may be wildcards, as in
cname=*.example.com,default.example.com
IPv4 and IPv6 addresses from /etc/hosts (and --addn-hosts ) and --host-record and
--interface-name provided the address falls into one of the subnets specified in the
--auth-zone.
Addresses of DHCP leases, provided the address falls into one of the subnets specified in
the --auth-zone. (If constructed DHCP ranges are is use, which depend on the address
dynamically assigned to an interface, then the form of --auth-zone which defines subnets
by the dynamic address of an interface should be used to ensure this condition is met.)
In the default mode, where a DHCP lease has an unqualified name, and possibly a qualified
name constructed using --domain then the name in the authoritative zone is constructed
from the unqualified name and the zone's domain. This may or may not equal that specified
by --domain. If --dhcp-fqdn is set, then the fully qualified names associated with DHCP
leases are used, and must match the zone's domain.
EXIT CODES
0 - Dnsmasq successfully forked into the background, or terminated normally if
backgrounding is not enabled.
1 - A problem with configuration was detected.
2 - A problem with network access occurred (address in use, attempt to use privileged
ports without permission).
3 - A problem occurred with a filesystem operation (missing file/directory, permissions).
4 - Memory allocation failure.
5 - Other miscellaneous problem.
11 or greater - a non zero return code was received from the lease-script process "init"
call. The exit code from dnsmasq is the script's exit code with 10 added.
LIMITS
The default values for resource limits in dnsmasq are generally conservative, and
appropriate for embedded router type devices with slow processors and limited memory. On
more capable hardware, it is possible to increase the limits, and handle many more
clients. The following applies to dnsmasq-2.37: earlier versions did not scale as well.
Dnsmasq is capable of handling DNS and DHCP for at least a thousand clients. The DHCP
lease times should not be very short (less than one hour). The value of --dns-forward-max
can be increased: start with it equal to the number of clients and increase if DNS seems
slow. Note that DNS performance depends too on the performance of the upstream
nameservers. The size of the DNS cache may be increased: the hard limit is 10000 names and
the default (150) is very low. Sending SIGUSR1 to dnsmasq makes it log information which
is useful for tuning the cache size. See the NOTES section for details.
The built-in TFTP server is capable of many simultaneous file transfers: the absolute
limit is related to the number of file-handles allowed to a process and the ability of the
select() system call to cope with large numbers of file handles. If the limit is set too
high using --tftp-max it will be scaled down and the actual limit logged at start-up. Note
that more transfers are possible when the same file is being sent than when each transfer
sends a different file.
It is possible to use dnsmasq to block Web advertising by using a list of known banner-ad
servers, all resolving to 127.0.0.1 or 0.0.0.0, in /etc/hosts or an additional hosts file.
The list can be very long, dnsmasq has been tested successfully with one million names.
That size file needs a 1GHz processor and about 60Mb of RAM.
INTERNATIONALISATION
Dnsmasq can be compiled to support internationalisation. To do this, the make targets
"all-i18n" and "install-i18n" should be used instead of the standard targets "all" and
"install". When internationalisation is compiled in, dnsmasq will produce log messages in
the local language and support internationalised domain names (IDN). Domain names in
/etc/hosts, /etc/ethers and /etc/dnsmasq.conf which contain non-ASCII characters will be
translated to the DNS-internal punycode representation. Note that dnsmasq determines both
the language for messages and the assumed charset for configuration files from the LANG
environment variable. This should be set to the system default value by the script which
is responsible for starting dnsmasq. When editing the configuration files, be careful to
do so using only the system-default locale and not user-specific one, since dnsmasq has no
direct way of determining the charset in use, and must assume that it is the system
default.
FILES
/etc/dnsmasq.conf
/usr/local/etc/dnsmasq.conf
/etc/resolv.conf /var/run/dnsmasq/resolv.conf /etc/ppp/resolv.conf /etc/dhcpc/resolv.conf
/etc/hosts
/etc/ethers
/var/lib/misc/dnsmasq.leases
/var/db/dnsmasq.leases
/var/run/dnsmasq.pid
SEE ALSO
hosts(5), resolver(5)
AUTHOR
This manual page was written by Simon Kelley <simon@thekelleys.org.uk>.
DNSMASQ(8)