DNSMASQ(8)DNSMASQ(8)NAMEdnsmasq - A lightweight DHCP and caching DNS server.
SYNOPSISdnsmasq [OPTION]...
DESCRIPTIONdnsmasq 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 multi‐
ple 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 alloca‐
tion 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 sup‐
port 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 func‐
tions, and allows uneeded 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 dns‐
masq.
-h, --no-hosts
Don't read the hostnames in /etc/hosts.
-H, --addn-hosts=<file>
Additional hosts file. Read the specified file as well as
/etc/hosts. If -h is given, read only the specified file. This
option may be repeated for more than one additional hosts file.
If a directory is given, then read all the files contained in
that directory.
-E, --expand-hosts
Add the domain to simple names (without a period) in /etc/hosts
in the same way as for DHCP-derived names. Note that this does
not apply to domain names in cnames, PTR records, TXT records
etc.
-T, --local-ttl=<time>
When replying with information from /etc/hosts or the DHCP
leases file dnsmasq by default sets the time-to-live field to
zero, meaning that the requester should not itself cache the
information. This is the correct thing to do in almost all situ‐
ations. This option allows a time-to-live (in seconds) to be
given for these replies. This will reduce the load on the server
at the expense of clients using stale data under some circum‐
stances.
--neg-ttl=<time>
Negative replies from upstream servers normally contain time-to-
live information in SOA records which dnsmasq uses for caching.
If the replies from upstream servers omit this information, dns‐
masq does not cache the reply. This option gives a default value
for time-to-live (in seconds) which dnsmasq uses to cache nega‐
tive replies even in the absence of an SOA record.
--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.
--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 dae‐
montools or launchd.
-d, --no-daemon
Debug mode: don't fork to the background, don't write a pid
file, don't change user id, generate a complete cache dump on
receipt on SIGUSR1, log to stderr as well as syslog, don't fork
new processes to handle TCP queries. Note that this option is
for use in debugging only, to stop dnsmasq daemonising in pro‐
duction, use -k.
-q, --log-queries
Log the results of DNS queries handled by dnsmasq. Enable a full
cache dump on receipt of SIGUSR1.
-8, --log-facility=<facility>
Set the facility to which dnsmasq will send syslog entries, this
defaults to DAEMON, and to LOCAL0 when debug mode is in opera‐
tion. If the facility given contains at least one '/' character,
it is taken to be a filename, and dnsmasq logs to the given
file, instead of syslog. 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. Nor‐
mally this user is "nobody" but that can be over-ridden with
this switch.
-g, --group=<groupname>
Specify the group which dnsmasq will run as. The defaults to
"dip", if available, to facilitate access to
/etc/ppp/resolv.conf which is not normally world readable.
-v, --version
Print the version number.
-p, --port=<port>
Listen on <port> instead of the standard DNS port (53). Setting
this to zero completely disables DNS function, leaving only DHCP
and/or TFTP.
-P, --edns-packet-max=<size>
Specify the largest EDNS.0 UDP packet which is supported by the
DNS forwarder. Defaults to 4096, which is the RFC5625-recom‐
mended size.
-Q, --query-port=<query_port>
Send outbound DNS queries from, and listen for their replies on,
the specific UDP port <query_port> instead of using random
ports. NOTE that using this option will make dnsmasq less secure
against DNS spoofing attacks but it may be faster and use less
resources. Setting this option to zero makes dnsmasq use a sin‐
gle port allocated to it by the OS: this was the default behav‐
iour in versions prior to 2.43.
--min-port=<port>
Do not use ports less than that given as source for outbound DNS
queries. Dnsmasq picks random ports as source for outbound
queries: when this option is given, the ports used will always
to larger than that specified. Useful for systems behind fire‐
walls.
-i, --interface=<interface name>
Listen only on the specified interface(s). Dnsmasq automatically
adds the loopback (local) interface to the list of interfaces to
use when the --interface option is used. If no --interface or
--listen-address options are given dnsmasq listens on all avail‐
able interfaces except any given in --except-interface options.
IP alias interfaces (eg "eth1:0") cannot be used with --inter‐
face or --except-interface options, use --listen-address
instead. 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.
--auth-server=<domain>,<interface>|<ip-address>
Enable DNS authoritative mode for queries arriving at an inter‐
face or address. Note that the interface or address need not be
mentioned in --interface or --listen-address configuration,
indeed --auth-server will overide 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 a 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 is 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 --lis‐
ten-address options may be given, in which case the set of both
interfaces and addresses is used. Note that if no --interface
option is given, but --listen-address is, dnsmasq will not auto‐
matically listen on the loopback interface. To achieve this, its
IP address, 127.0.0.1, must be explicitly given as a --listen-
address option.
-z, --bind-interfaces
On systems which support it, dnsmasq binds the wildcard address,
even when it is listening on only some interfaces. It then dis‐
cards requests that it shouldn't reply to. This has the advan‐
tage of working even when interfaces come and go and change
address. This option forces dnsmasq to really bind only the
interfaces it is listening on. About the only time when this is
useful is when running another nameserver (or another instance
of dnsmasq) on the same machine. Setting this option also
enables multiple instances of dnsmasq which provide DHCP service
to run in the same machine.
--bind-dynamic
Enable a network mode which is a hybrid between --bind-inter‐
faces 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 network‐
ing APIs and it is only available under Linux. On other plat‐
forms it falls-back to --bind-interfaces mode.
-y, --localise-queries
Return answers to DNS queries from /etc/hosts which depend on
the interface over which the query was received. If a name in
/etc/hosts has more than one address associated with it, and at
least one of those addresses is on the same subnet as the inter‐
face to which the query was sent, then return only the
address(es) on that subnet. This allows for a server to have
multiple addresses in /etc/hosts corresponding to each of its
interfaces, and hosts will get the correct address based on
which network they are attached to. Currently this facility is
limited to IPv4.
-b, --bogus-priv
Bogus private reverse lookups. All reverse lookups for private
IP ranges (ie 192.168.x.x, etc) which are not found in
/etc/hosts or the DHCP leases file are answered with "no such
domain" rather than being forwarded upstream.
-V, --alias=[<old-ip>]|[<start-ip>-<end-ip>],<new-ip>[,<mask>]
Modify IPv4 addresses returned from upstream nameservers; old-ip
is replaced by new-ip. If the optional mask is given then any
address which matches the masked old-ip will be re-written. So,
for instance --alias=1.2.3.0,6.7.8.0,255.255.255.0 will map
1.2.3.56 to 6.7.8.56 and 1.2.3.67 to 6.7.8.67. This is what
Cisco PIX routers call "DNS doctoring". If the old IP is given
as range, then only addresses in the range, rather than a whole
subnet, are re-written. So
--alias=192.168.0.10-192.168.0.40,10.0.0.0,255.255.255.0 maps
192.168.0.10->192.168.0.40 to 10.0.0.10->10.0.0.40
-B, --bogus-nxdomain=<ipaddr>
Transform replies which contain the IP address given into "No
such domain" replies. This is intended to counteract a devious
move made by Verisign in September 2003 when they started
returning the address of an advertising web page in response to
queries for unregistered names, instead of the correct NXDOMAIN
response. This option tells dnsmasq to fake the correct response
when it sees this behaviour. As at Sept 2003 the IP address
being returned by Verisign is 64.94.110.11
-f, --filterwin2k
Later versions of windows make periodic DNS requests which don't
get sensible answers from the public DNS and can cause problems
by triggering dial-on-demand links. This flag turns on an option
to filter such requests. The requests blocked are for records of
types SOA and SRV, and type ANY where the requested name has
underscores, to catch LDAP requests.
-r, --resolv-file=<file>
Read the IP addresses of the upstream nameservers from <file>,
instead of /etc/resolv.conf. For the format of this file see
resolv.conf(5). The only lines relevant to dnsmasq are name‐
server ones. Dnsmasq can be told to poll more than one
resolv.conf file, the first file name specified overrides the
default, subsequent ones add to the list. This is only allowed
when polling; the file with the currently latest modification
time is the one used.
-R, --no-resolv
Don't read /etc/resolv.conf. Get upstream servers only from the
command line or the dnsmasq configuration file.
-1, --enable-dbus[=<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 dns‐
masq 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.
--stop-dns-rebind
Reject (and log) addresses from upstream nameservers which are
in the private IP ranges. This blocks an attack where a browser
behind a firewall is used to probe machines on the local net‐
work.
--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 dis‐
able 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>][@<source-ip>|<inter‐
face>[#<port>]]
Specify IP address of upstream servers directly. Setting this
flag does not suppress reading of /etc/resolv.conf, use -R to do
that. If one or more optional domains are given, that server is
used only for those domains and they are queried only using the
specified server. This is intended for private nameservers: if
you have a nameserver on your network which deals with names of
the form xxx.internal.thekelleys.org.uk at 192.168.1.1 then giv‐
ing the flag -S /internal.thekelleys.org.uk/192.168.1.1 will
send all queries for internal machines to that nameserver,
everything else will go to the servers in /etc/resolv.conf. An
empty domain specification, // has the special meaning of
"unqualified names only" ie names without any dots in them. A
non-standard port may be specified as part of the IP address
using a # character. More than one -S flag is allowed, with
repeated domain or ipaddr parts as required.
More specific domains take precendence over less specific
domains, so: --server=/google.com/1.2.3.4
--server=/www.google.com/2.3.4.5 will send queries for
*.google.com to 1.2.3.4, except *www.google.com, which will go
to 2.3.4.5
The special server address '#' means, "use the standard
servers", so --server=/google.com/1.2.3.4
--server=/www.google.com/# will send queries for *.google.com to
1.2.3.4, except *www.google.com which will be forwarded as
usual.
Also permitted is a -S flag which gives a domain but no IP
address; this tells dnsmasq that a domain is local and it may
answer queries from /etc/hosts or DHCP but should never forward
queries on that domain to any upstream servers. local is a syn‐
onym for server to make configuration files clearer in this
case.
IPv6 addresses may include a %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 should be
an ip-address, which should belong to the machine on which dns‐
masq is running otherwise this server line will be logged and
then ignored, or an interface name. If an interface name is
given, then queries to the server will be forced via that inter‐
face; if an ip-address is given then the source address of the
queries will be set to that address. The query-port flag is
ignored for any servers which have a source address specified
but the port may be specified directly as part of the source
address. Forcing queries to an interface is not implemented on
all platforms supported by dnsmasq.
--rev-server=<ip-address>/<prefix-len>,<ipaddr>[#<port>][@<source-
ip>|<interface>[#<port>]]
This is functionally the same as --server, but provides some
syntactic sugar to make specifying address-to-name queries eas‐
ier. 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. Note that /etc/hosts and DHCP leases over‐
ride this for individual names. A common use of this is to redi‐
rect the entire doubleclick.net domain to some friendly local
web server to avoid banner ads. The domain specification works
in the same was as for --server, with the additional facility
that /#/ matches any domain. Thus --address=/#/1.2.3.4 will
always return 1.2.3.4 for any query not answered from /etc/hosts
or DHCP and not sent to an upstream nameserver by a more spe‐
cific --server directive.
--ipset=/<domain>/[domain/]<ipset>[,<ipset>]
Places the resolved IP addresses of queries for the specified
domains in the specified netfilter ip sets. Domains and subdo‐
mains 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 host‐
name (if given), or the host specified in the --mx-target switch
or, if that switch is not given, the host on which dnsmasq is
running. The default is useful for directing mail from systems
on a LAN to a central server. The preference value is optional,
and defaults to 1 if not given. More than one MX record may be
given for a host.
-t, --mx-target=<hostname>
Specify the default target for the MX record returned by dns‐
masq. See --mx-host. If --mx-target is given, but not --mx-
host, then dnsmasq returns a MX record containing the MX target
for MX queries on the hostname of the machine on which dnsmasq
is running.
-e, --selfmx
Return an MX record pointing to itself for each local machine.
Local machines are those in /etc/hosts or with DHCP leases.
-L, --localmx
Return an MX record pointing to the host given by mx-target (or
the machine on which dnsmasq is running) for each local machine.
Local machines are those in /etc/hosts or with DHCP leases.
-W, --srv-host=<_service>.<_prot>.[<domain>],[<target>[,<port>[,<prior‐
ity>[,<weight>]]]]
Return a SRV DNS record. See RFC2782 for details. If not sup‐
plied, the domain defaults to that given by --domain. The
default for the target domain is empty, and the default for port
is one and the defaults for weight and priority are zero. Be
careful if transposing data from BIND zone files: the port,
weight and priority numbers are in a different order. More than
one SRV record for a given service/domain is allowed, all that
match are returned.
--host-record=<name>[,<name>....][<IPv4-address>],[<IPv6-address>]
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,lap‐
top.thekelleys.org,192.168.0.1,1234::100
-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>,<reg‐
exp>[,<replacement>]
Return an NAPTR DNS record, as specified in RFC3403.
--cname=<cname>,<target>
Return a CNAME record which indicates that <cname> is really
<target>. There are significant limitations on the target; it
must be a DNS name which is known to dnsmasq from /etc/hosts (or
additional hosts files), from DHCP, from --interface-name or
from another --cname. If the target does not satisfy this cri‐
teria, the whole cname is ignored. The cname must be unique, but
it is permissable to have more than one cname pointing to the
same target.
--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 a DNS record associating the name with the primary
address on 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.
--synth-domain=<domain>,<address range>[,<prefix>]
Create artificial A/AAAA and PTR records for an address range.
The records use the address, with periods (or colons for IPv6)
replaced with dashes.
An example should make this clearer. --synth-domain=thekel‐
leys.org.uk,192.168.0.0/24,internal- 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.
The address range can be of the form <ip address>,<ip address>
or <ip address>/<netmask>
--add-mac
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.
--add-subnet[[=<IPv4 prefix length>],<IPv6 prefix length>]
Add the subnet address of the requestor to the DNS queries which
are forwarded upstream. The amount of the address forwarded
depends on the prefix length parameter: 32 (128 for IPv6) for‐
wards 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 dns‐
masq cache does not take account. If a dnsmasq instance is con‐
figured such that different results may be encountered, caching
should be disabled.
-c, --cache-size=<cachesize>
Set the size of dnsmasq's cache. The default is 150 names. Set‐
ting the cache size to zero disables caching.
-N, --no-negcache
Disable negative caching. Negative caching allows dnsmasq to
remember "no such domain" answers from upstream nameservers and
answer identical queries without forwarding them again.
-0, --dns-forward-max=<queries>
Set the maximum number of concurrent DNS queries. The default
value is 150, which should be fine for most setups. The only
known situation where this needs to be increased is when using
web-server log file resolvers, which can generate large numbers
of concurrent queries.
--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 valida‐
tion, 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 menas 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 Zone Signing key(s)
of the root zone, but trust anchors for limited domains are also
possible. The current root-zone trust anchors may be donwloaded
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 SIGHUP. The intention is that dnsmasq should be
started with this flag when the platform determines that reli‐
able time is not currently available. As soon as reliable time
is established, a SIGHUP should be sent to dnsmasq, which
enables time checking, and purges the cache of DNS records which
have not been throughly checked.
--proxy-dnssec
Copy the DNSSEC Authenticated Data bit from upstream servers to
downstream clients and cache it. This is an alternative to hav‐
ing 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 SERV‐
FAIL. 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>].....]]
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 net‐
mask/prefix-length are used; this is useful when using con‐
structed 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.
The subnet(s) are also used to define in-addr.arpa and ip6.arpa
domains which are served for reverse-DNS queries. If not speci‐
fied, 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 dns‐
masq 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 dns‐
masq to be associated with the queries which cause it, useful
for bandwidth accounting and firewalling. Dnsmasq must have con‐
ntrack 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 net‐
work address. The broadcast address is always optional. It is
always allowed to have more than one dhcp-range in a single sub‐
net.
For IPv6, the parameters are slightly different: instead of net‐
mask 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 mimimum 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 inter‐
face 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-net‐
work 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-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 informa‐
tion.
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 exten‐
sions. ra-names can be combined with ra-stateless and slaac.
-G, --dhcp-
host=[<hwaddr>][,id:<client_id>|*][,set:<tag>][,<ipaddr>][,<host‐
name>][,<lease_time>][,ignore]
Specify per host parameters for the DHCP server. This allows a
machine with a particular hardware address to be always allo‐
cated the same hostname, IP address and lease time. A hostname
specified like this overrides any supplied by the DHCP client on
the machine. It is also allowable to 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 nor‐
mally 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 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 direc‐
tory is given, then read all the files contained in that direc‐
tory. 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 informa‐
tion in a --dhcp-boot flag as DHCP options, using the options
names bootfile-name, server-ip-address and tftp-server. This
allows these to be included in a dhcp-optsfile.
-Z, --read-ethers
Read /etc/ethers for information about hosts for the DHCP
server. The format of /etc/ethers is a hardware address, fol‐
lowed by either a hostname or dotted-quad IP address. When read
by dnsmasq these lines have exactly the same effect as --dhcp-
host options containing the same information. /etc/ethers is re-
read when dnsmasq receives SIGHUP. 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 argu‐
ments to option 120 are handled as per RFC 3361. Dotted-quad IP
addresses which are followed by a slash and then a netmask size
are encoded as described in RFC 3442.
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=ven‐
dor:PXEClient,1,0.0.0.0 sends the encapsulated vendor class-spe‐
cific option "mftp-address=0.0.0.0" to any client whose vendor-
class matches "PXEClient". The vendor-class matching is sub‐
string 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 encapsu‐
lated 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 avail‐
able 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, typi‐
cally 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 num‐
ber>,]<vendor-class>
Map from a vendor-class string to a tag. Most DHCP clients pro‐
vide 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 spec‐
ified 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 cir‐
cuit 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-cir‐
cuitid 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 ares executed
in order, so if the tag in tag:<tag> is a tag set by another
tag-if, the line which sets the tag must precede the one which
tests it.
-J, --dhcp-ignore=tag:<tag>[,tag:<tag>]
When all the given tags appear in the tag set ignore the host
and do not allocate it a DHCP lease.
--dhcp-ignore-names[=tag:<tag>[,tag:<tag>]]
When all the given tags appear in the tag set, ignore any host‐
name provided by the host. Note that, unlike dhcp-ignore, it is
permissible to supply no tags, in which case DHCP-client sup‐
plied 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 other‐
wise 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 auto‐
matically, 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 (typ‐
ically 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>|<bootservice‐
type>][,<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 com‐
plex functions when supported by a suitable DHCP server.
This specifies a boot option which may appear in a PXE boot
menu. <CSA> is client system type, only services of the correct
type will appear in a menu. The known types are x86PC, PC98,
IA64_EFI, Alpha, Arc_x86, Intel_Lean_Client, IA32_EFI, BC_EFI,
Xscale_EFI and X86-64_EFI; an integer may be used for other
types. The parameter after the menu text may be a file name, in
which case dnsmasq acts as a boot server and directs the PXE
client to download the file by TFTP, either from itself (
enable-tftp must be set for this to work) or another TFTP server
if the final server address/name is given. Note that the
"layer" suffix (normally ".0") is supplied by PXE, and should
not be added to the basename. If an integer boot service type,
rather than a basename is given, then the PXE client will search
for a suitable boot service for that type on the network. This
search may be done by broadcast, or direct to a server if its IP
address/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 boot‐
ing 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 auto‐
matically executed. If the timeout is zero then the first avail‐
able menu item will be executed immediately. If pxe-prompt is
omitted the system will wait for user input if there are multi‐
ple 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 perma‐
nently unavailable for re-use by other hosts. if this is given
without tags, then it unconditionally enables dynamic alloca‐
tion. 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 in not in use before allocating it to a host. It
does this by sending an ICMP echo request (aka "ping") to the
address in question. If it gets a reply, then the address must
already be in use, and another is tried. This flag disables this
check. Use with caution.
--log-dhcp
Extra logging for DHCP: log all the options sent to DHCP clients
and the tags used to determine them.
--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 automati‐
cally created DUIDs or vice-versa, the lease database must be
re-intialised. 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..DNS‐
MASQ_USER_CLASSn
If dnsmasq was compiled with HAVE_BROKEN_RTC, then the length of
the lease (in seconds) is stored in DNSMASQ_LEASE_LENGTH, other‐
wise the time of lease expiry is stored in DNS‐
MASQ_LEASE_EXPIRES. The number of seconds until lease expiry is
always stored in DNSMASQ_TIME_REMAINING.
If a lease used to have a hostname, which is removed, an "old"
event is generated with the new state of the lease, ie no name,
and the former name is provided in the environment variable DNS‐
MASQ_OLD_HOSTNAME.
DNSMASQ_INTERFACE stores the name of the interface on which the
request arrived; this is not set for "old" actions when dnsmasq
restarts.
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 transac‐
tion, 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.
For IPv6 only:
If the client provides vendor-class, DNSMASQ_VENDOR_CLASS_ID,
containing the IANA enterprise id for the class, and DNS‐
MASQ_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 dns‐
masq's lease database.
All file descriptors are closed except stdin, stdout and stderr
which are open to /dev/null (except in debug mode).
The script is not invoked concurrently: at most one instance of
the script is ever running (dnsmasq waits for an instance of
script to exit before running the next). Changes to the lease
database are which require the script to be invoked are queued
awaiting exit of a running instance. If this queueing allows
multiple state changes occur to a single lease before the script
can be run then earlier states are discarded and the current
state of that lease is reflected when the script finally runs.
At dnsmasq startup, the script will be invoked for all existing
leases as they are read from the lease file. Expired leases will
be called with "del" and others with "old". When dnsmasq
receives a HUP signal, the script will be invoked for existing
leases with an "old " event.
There are two further actions which may appear as the first
argument to the script, "init" 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 argu‐
ments are the file size in bytes, the address to which the file
was sent, and the complete pathname of the file.
--dhcp-luascript=<path>
Specify a script written in Lua, to be run when leases are cre‐
ated, destroyed or changed. To use this option, dnsmasq must be
compiled with the correct support. The Lua interpreter is
intialised 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 environ‐
ment 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 argu‐
ments 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 func‐
tion, and the table holds the tags destination_address,
file_name and file_size.
--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.
-9, --leasefile-ro
Completely suppress use of the lease database file. The file
will not be created, read, or written. Change the way the lease-
change script (if one is provided) is called, so that the lease
database may be maintained in external storage by the script. In
addition to the invocations given in --dhcp-script the lease-
change script is called once, at dnsmasq startup, with the sin‐
gle argument "init". When called like this the script should
write the saved state of the lease database, in dnsmasq lease‐
file format, to stdout and exit with zero exit code. Setting
this option also forces the leasechange script to be called on
changes to the client-id and lease length and expiry time.
--bridge-interface=<interface>,<alias>[,<alias>]
Treat DHCP request packets arriving at any of the <alias> inter‐
faces as if they had arrived at <interface>. This option is nec‐
essary when using "old style" bridging on BSD platforms, since
packets arrive at tap interfaces which don't have an IP address.
-s, --domain=<domain>[,<address range>[,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 suf‐
fix added as an optional domain part. Eg on my network I can set
--domain=thekelleys.org.uk and have a machine whose DHCP host‐
name 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 identi‐
cal 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 dif‐
ferent 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 speci‐
fied 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 sup‐
presses 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 au‐
tonomous address creation are handled by a different protocol.
When DHCP is in use, only a subset of this is needed, and dns‐
masq 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 and recursive DNS
server as the relevant link-local address on the machine running
dnsmasq. By default, he "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 RDNSS and
DNSSL.
--ra-param=<interface>,[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 inter‐
val 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 three parameters may be
set at once. --ra-param=low,60,1200 The interface field may
include a wildcard.
--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 sup‐
ported 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
recieve 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-unique-root
Add the IP address of the TFTP client as a path component on the
end of the TFTP-root (in standard dotted-quad format). Only
valid if a tftp-root is set and the directory exists. For
instance, if tftp-root is "/tftp" and client 1.2.3.4 requests
file "myfile" then the effective path will be
"/tftp/1.2.3.4/myfile" if /tftp/1.2.3.4 exists or /tftp/myfile
otherwise.
--tftp-secure
Enable TFTP secure mode: without this, any file which is read‐
able by the dnsmasq process under normal unix access-control
rules is available via TFTP. When the --tftp-secure flag is
given, only files owned by the user running the dnsmasq process
are accessible. If dnsmasq is being run as root, different rules
apply: --tftp-secure has no effect, but only files which have
the world-readable bit set are accessible. It is not recommended
to run dnsmasq as root with TFTP enabled, and certainly not
without specifying --tftp-root. Doing so can expose any world-
readable file on the server to any host on the net.
--tftp-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 con‐
verts "\" 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 connec‐
tions, per-process file descriptor limits may be encountered.
Dnsmasq needs one file descriptor for each concurrent TFTP con‐
nection and one file descriptor per unique file (plus a few oth‐
ers). So serving the same file simultaneously to n clients will
use require about n + 10 file descriptors, serving different
files simultaneously to n clients will require about (2*n) + 10
descriptors. If --tftp-port-range is given, that can affect the
number of concurrent connections.
--tftp-no-blocksize
Stop the TFTP server from negotiating the "blocksize" option
with a client. Some buggy clients request this option but then
behave badly when it is granted.
--tftp-port-range=<start>,<end>
A TFTP server listens on a well-known port (69) for connection
initiation, but it also uses a dynamically-allocated port for
each connection. Normally these are allocated by the OS, but
this option specifies a range of ports for use by TFTP trans‐
fers. This can be useful when TFTP has to traverse a firewall.
The start of the range cannot be lower than 1025 unless dnsmasq
is running as root. The number of concurrent TFTP connections is
limited by the size of the port range.
-C, --conf-file=<file>
Specify a different configuration file. The conf-file option is
also allowed in configuration files, to include multiple config‐
uration files. A filename of "-" causes dnsmasq to read configu‐
ration 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. This flag may
be given on the command line or in a configuration file.
--servers-file=<file>
A special case of --conf-file which differs in two respects.
Firstly, only --server and --rev-server are allowed in the con‐
figuration file included. Secondly, the file is re-read and the
configuration therein is updated when dnsmasq recieves 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 over‐
rides the command line. Quoting is allowed in a config file: between "
quotes the special meanings of ,:. and # are removed and the following
escapes are allowed: \\ \" \t \e \b \r and \n. The later corresponding
to tab, escape, backspace, return and newline.
NOTES
When it receives a SIGHUP, dnsmasq clears its cache and then re-loads
/etc/hosts and /etc/ethers and any file given by --dhcp-hostsfile,
--dhcp-optsfile or --addn-hosts. The dhcp lease change script is
called for all existing DHCP leases. If --no-poll is set SIGHUP also
re-reads /etc/resolv.conf. SIGHUP does NOT re-read the configuration
file.
When it receives a SIGUSR1, dnsmasq writes statistics to the system
log. It writes the cache size, the number of names which have had to
removed from the cache before they expired in order to make room for
new names and the total number of names that have been inserted into
the cache. 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 cre‐
ates the logfile dnsmasq changes the ownership of the file to the non-
root user it will run as. Logrotate should be configured to create a
new log file with the ownership which matches the existing one before
sending SIGUSR2. If TCP DNS queries are in progress, the old logfile
will remain open in child processes which are handling TCP queries and
may continue to be written. There is a limit of 150 seconds, after
which all existing TCP processes will have expired: for this reason, it
is not wise to configure logfile compression for logfiles which have
just been rotated. Using logrotate, the required options are create and
delaycompress.
Dnsmasq is a DNS query forwarder: it it not capable of recursively
answering arbitrary queries starting from the root servers but forwards
such queries to a fully recursive upstream DNS server which is typi‐
cally provided by an ISP. By default, dnsmasq reads /etc/resolv.conf to
discover the IP addresses of the upstream nameservers it should use,
since the information is typically stored there. Unless --no-poll is
used, dnsmasq checks the modification time of /etc/resolv.conf (or
equivalent if --resolv-file is used) and re-reads it if it changes.
This allows the DNS servers to be set dynamically by PPP or DHCP since
both protocols provide the information. Absence of /etc/resolv.conf is
not an error since it may not have been created before a PPP connection
exists. Dnsmasq simply keeps checking in case /etc/resolv.conf is cre‐
ated at any time. Dnsmasq can be told to parse more than one
resolv.conf file. This is useful on a laptop, where both PPP and DHCP
may be used: dnsmasq can be set to poll both /etc/ppp/resolv.conf and
/etc/dhcpc/resolv.conf and will use the contents of whichever changed
last, giving automatic switching between DNS servers.
Upstream servers may also be specified on the command line or in the
configuration file. These server specifications optionally take a
domain name which tells dnsmasq to use that server only to find names
in that particular domain.
In order to configure dnsmasq to act as cache for the host on which it
is running, put "nameserver 127.0.0.1" in /etc/resolv.conf to force
local processes to send queries to dnsmasq. Then either specify the
upstream servers directly to dnsmasq using --server options or put
their addresses real in another file, say /etc/resolv.dnsmasq and run
dnsmasq with the -r /etc/resolv.dnsmasq option. This second technique
allows for dynamic update of the server addresses by PPP or DHCP.
Addresses in /etc/hosts will "shadow" different addresses for the same
names in the upstream DNS, so "mycompany.com 1.2.3.4" in /etc/hosts
will ensure that queries for "mycompany.com" always return 1.2.3.4 even
if queries in the upstream DNS would otherwise return a different
address. There is one exception to this: if the upstream DNS contains a
CNAME which points to a shadowed name, then looking up the CNAME
through dnsmasq will result in the unshadowed address associated with
the target of the CNAME. To work around this, add the CNAME to
/etc/hosts so that the CNAME is shadowed too.
The tag system works as follows: For each DHCP request, dnsmasq col‐
lects a set of valid tags from active configuration lines which include
set:<tag>, including one from the dhcp-range used to allocate the
address, one from any matching dhcp-host (and "known" if a dhcp-host
matches) The tag "bootp" is set for BOOTP requests, and a tag whose
name is the name of the interface on which the request arrived is also
set.
Any configuration lines which 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 rel‐
ative 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 indi‐
cate NOT.
The DHCP server in dnsmasq will function as a BOOTP server also, pro‐
vided that the MAC address and IP address for clients are given, either
using dhcp-host configurations or in /etc/ethers , and a dhcp-range
configuration option is present to activate the DHCP server on a par‐
ticular network. (Setting --bootp-dynamic removes the need for static
address mappings.) The filename parameter in a BOOTP request is 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 compli‐
cated 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.exam‐
ple.com, and the zone for which dnsmasq is authoritative our.zone.com.
The simplest configuration consists of two lines of dnsmasq configura‐
tion; 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 authori‐
tative 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 inter‐
face. 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 pri‐
mary server become inaccessible. The configuration of the secondary is
beyond the scope of this man-page, but the extra configuration of dns‐
masq 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 suit‐
able 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 fol‐
lowing 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.
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 contructed DHCP ranges are
is use, which depend on the address dynamically assigned to an inter‐
face, 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 nor‐
mally if backgrounding is not enabled.
1 - A problem with configuration was detected.
2 - A problem with network access occurred (address in use, attempt to
use privileged ports without permission).
3 - A problem occurred with a filesystem operation (missing file/direc‐
tory, permissions).
4 - Memory allocation failure.
5 - Other miscellaneous problem.
11 or greater - a non zero return code was received from the lease-
script process "init" call. The exit code from dnsmasq is the script's
exit code with 10 added.
LIMITS
The default values for resource limits in dnsmasq are generally conser‐
vative, and appropriate for embedded router type devices with slow pro‐
cessors and limited memory. On more capable hardware, it is possible to
increase the limits, and handle many more clients. The following
applies to dnsmasq-2.37: earlier versions did not scale as well.
Dnsmasq is capable of handling DNS and DHCP for at least a thousand
clients. 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 trans‐
fers: the absolute limit is related to the number of file-handles
allowed to a process and the ability of the select() system call to
cope with large numbers of file handles. If the limit is set too high
using --tftp-max it will be scaled down and the actual limit logged at
start-up. Note that more transfers are possible when the same file is
being sent than when each transfer sends a different file.
It is possible to use dnsmasq to block Web advertising by using a list
of known banner-ad servers, all resolving to 127.0.0.1 or 0.0.0.0, in
/etc/hosts or an additional hosts file. The list can be very long, dns‐
masq has been tested successfully with one million names. That size
file needs a 1GHz processor and about 60Mb of RAM.
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 representa‐
tion. 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 configura‐
tion files, be careful to do so using only the system-default locale
and not user-specific one, since dnsmasq has no direct way of determin‐
ing 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 ALSOhosts(5), resolver(5)AUTHOR
This manual page was written by Simon Kelley <simon@thekelleys.org.uk>.
DNSMASQ(8)
