dnsmasq - A lightweight DHCP and caching DNS server.
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
The dnsmasq DHCP server supports static address assignments and multiple
networks. It automatically sends a sensible default set of DHCP options, and
can be configured to send any desired set of DHCP options, including
vendor-encapsulated options. It includes a secure, read-only, TFTP server to
allow net/PXE boot of DHCP hosts and also supports BOOTP. The PXE support is
full featured, and includes a proxy mode which supplies PXE information to
clients whilst DHCP address allocation is done by another server.
The dnsmasq DHCPv6 server provides the same set of features as the DHCPv4
server, and in addition, it includes router advertisements and a neat feature
which allows naming for clients which use DHCPv4 and stateless
autoconfiguration only for IPv6 configuration. There is support for doing
address allocation (both DHCPv6 and RA) from subnets which are dynamically
delegated via DHCPv6 prefix delegation.
Dnsmasq is coded with small embedded systems in mind. It aims for the smallest
possible memory footprint compatible with the supported functions, and allows
unneeded functions to be omitted from the compiled binary.
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.
- Read and syntax check configuration file(s). Exit with code 0 if all is
OK, or a non-zero code otherwise. Do not start up dnsmasq.
- -w, --help
- Display all command-line options. --help dhcp will display known
DHCPv4 configuration options, and --help dhcp6 will display DHCPv6
- -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
--no-hosts is given, read only the specified file. This option may
be repeated for more than one additional hosts file. If a directory is
given, then read all the files contained in that directory.
- Read all the hosts files contained in the directory. New or changed files
are read automatically. See --dhcp-hostsdir for details.
- -E, --expand-hosts
- Add the domain to simple names (without a period) in /etc/hosts in the
same way as for DHCP-derived names. Note that this does not apply to
domain names in cnames, PTR records, TXT records etc.
- -T, --local-ttl=<time>
- When replying with information from /etc/hosts or configuration or the
DHCP leases file dnsmasq by default sets the time-to-live field to zero,
meaning that the requester should not itself cache the information. This
is the correct thing to do in almost all situations. This option allows a
time-to-live (in seconds) to be given for these replies. This will reduce
the load on the server at the expense of clients using stale data under
- As for --local-ttl, but affects only replies with information from
DHCP leases. If both are given, --dhcp-ttl applies for DHCP
information, and --local-ttl for others. Setting this to zero
eliminates the effect of --local-ttl for DHCP.
- Negative replies from upstream servers normally contain time-to-live
information in SOA records which dnsmasq uses for caching. If the replies
from upstream servers omit this information, dnsmasq does not cache the
reply. This option gives a default value for time-to-live (in seconds)
which dnsmasq uses to cache negative replies even in the absence of an SOA
- 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.
- Set a maximum TTL value for entries in the cache.
- Extend short TTL values to the time given when caching them. Note that
artificially extending TTL values is in general a bad idea, do not do it
unless you have a good reason, and understand what you are doing. Dnsmasq
limits the value of this option to one hour, unless recompiled.
- Set the TTL value returned in answers from the authoritative server.
- -k, --keep-in-foreground
- Do not go into the background at startup but otherwise run as normal. This
is intended for use when dnsmasq is run under daemontools or launchd.
- -d, --no-daemon
- Debug mode: don't fork to the background, don't write a pid file, don't
change user id, generate a complete cache dump on receipt on SIGUSR1, log
to stderr as well as syslog, don't fork new processes to handle TCP
queries. Note that this option is for use in debugging only, to stop
dnsmasq daemonising in production, use --keep-in-foreground.
- -q, --log-queries
- Log the results of DNS queries handled by dnsmasq. Enable a full cache
dump on receipt of SIGUSR1. If the argument "extra" is supplied,
ie --log-queries=extra then the log has extra information at the
start of each line. This consists of a serial number which ties together
the log lines associated with an individual query, and the IP address of
- -8, --log-facility=<facility>
- Set the facility to which dnsmasq will send syslog entries, this defaults
to DAEMON, and to LOCAL0 when debug mode is in operation. If the facility
given contains at least one '/' character, it is taken to be a filename,
and dnsmasq logs to the given file, instead of syslog. If the facility is
'-' then dnsmasq logs to stderr. (Errors whilst reading configuration will
still go to syslog, but all output from a successful startup, and all
output whilst running, will go exclusively to the file.) When logging to a
file, dnsmasq will close and reopen the file when it receives SIGUSR2.
This allows the log file to be rotated without stopping dnsmasq.
- 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.
- -u, --user=<username>
- Specify the userid to which dnsmasq will change after startup. Dnsmasq
must normally be started as root, but it will drop root privileges after
startup by changing id to another user. Normally this user is
"nobody" but that can be over-ridden with this switch.
- -g, --group=<groupname>
- Specify the group which dnsmasq will run as. The default is
"dip", if available, to facilitate access to
/etc/ppp/resolv.conf which is not normally world readable.
- -v, --version
- Print the version number.
- -p, --port=<port>
- Listen on <port> instead of the standard DNS port (53). Setting this
to zero completely disables DNS function, leaving only DHCP and/or
- -P, --edns-packet-max=<size>
- Specify the largest EDNS.0 UDP packet which is supported by the DNS
forwarder. Defaults to 4096, which is the RFC5625-recommended size.
- -Q, --query-port=<query_port>
- Send outbound DNS queries from, and listen for their replies on, the
specific UDP port <query_port> instead of using random ports. NOTE
that using this option will make dnsmasq less secure against DNS spoofing
attacks but it may be faster and use less resources. Setting this option
to zero makes dnsmasq use a single port allocated to it by the OS: this
was the default behaviour in versions prior to 2.43.
- Do not use ports less than that given as source for outbound DNS queries.
Dnsmasq picks random ports as source for outbound queries: when this
option is given, the ports used will always to larger than that specified.
Useful for systems behind firewalls. If not specified, defaults to
- Use ports lower than that given as source for outbound DNS queries.
Dnsmasq picks random ports as source for outbound queries: when this
option is given, the ports used will always be lower than that specified.
Useful for systems behind firewalls.
- -i, --interface=<interface name>
- Listen only on the specified interface(s). Dnsmasq automatically adds the
loopback (local) interface to the list of interfaces to use when the
--interface option is used. If no --interface or
--listen-address options are given dnsmasq listens on all available
interfaces except any given in --except-interface options. On
Linux, when --bind-interfaces or --bind-dynamic are in
effect, IP alias interface labels (eg "eth1:0") are checked,
rather than interface names. In the degenerate case when an interface has
one address, this amounts to the same thing but when an interface has
multiple addresses it allows control over which of those addresses are
accepted. The same effect is achievable in default mode by using
--listen-address. A simple wildcard, consisting of a trailing '*',
can be used in --interface and --except-interface
- -I, --except-interface=<interface name>
- Do not listen on the specified interface. Note that the order of
--listen-address --interface and --except-interface
options does not matter and that --except-interface options always
override the others. The comments about interface labels for
--listen-address apply here.
- Enable DNS authoritative mode for queries arriving at an interface or
address. Note that the interface or address need not be mentioned in
--interface or --listen-address configuration, indeed
--auth-server will override these and provide a different DNS
service on the specified interface. The <domain> is the "glue
record". It should resolve in the global DNS to an A and/or AAAA
record which points to the address dnsmasq is listening on. When an
interface is specified, it may be qualified with "/4" or
"/6" to specify only the IPv4 or IPv6 addresses associated with
the interface. Since any defined authoritative zones are also available as
part of the normal recusive DNS service supplied by dnsmasq, it can make
sense to have an --auth-server declaration with no interfaces or address,
but simply specifying the glue record.
- Accept DNS queries only from hosts whose address is on a local subnet, ie
a subnet for which an interface exists on the server. This option only has
effect if there are no --interface, --except-interface,
--listen-address or --auth-server options. It is intended to
be set as a default on installation, to allow unconfigured installations
to be useful but also safe from being used for DNS amplification
- -2, --no-dhcp-interface=<interface name>
- Do not provide DHCP or TFTP on the specified interface, but do provide DNS
- -a, --listen-address=<ipaddr>
- Listen on the given IP address(es). Both --interface and
--listen-address options may be given, in which case the set of
both interfaces and addresses is used. Note that if no --interface
option is given, but --listen-address is, dnsmasq will not
automatically listen on the loopback interface. To achieve this, its IP
address, 127.0.0.1, must be explicitly given as a --listen-address
- -z, --bind-interfaces
- On systems which support it, dnsmasq binds the wildcard address, even when
it is listening on only some interfaces. It then discards requests that it
shouldn't reply to. This has the advantage of working even when interfaces
come and go and change address. This option forces dnsmasq to really bind
only the interfaces it is listening on. About the only time when this is
useful is when running another nameserver (or another instance of dnsmasq)
on the same machine. Setting this option also enables multiple instances
of dnsmasq which provide DHCP service to run in the same machine.
- Enable a network mode which is a hybrid between --bind-interfaces
and the default. Dnsmasq binds the address of individual interfaces,
allowing multiple dnsmasq instances, but if new interfaces or addresses
appear, it automatically listens on those (subject to any access-control
configuration). This makes dynamically created interfaces work in the same
way as the default. Implementing this option requires non-standard
networking APIs and it is only available under Linux. On other platforms
it falls-back to --bind-interfaces mode.
- -y, --localise-queries
- Return answers to DNS queries from /etc/hosts and --interface-name
which depend on the interface over which the query was received. If a name
has more than one address associated with it, and at least one of those
addresses is on the same subnet as the interface to which the query was
sent, then return only the address(es) on that subnet. This allows for a
server to have multiple addresses in /etc/hosts corresponding to each of
its interfaces, and hosts will get the correct address based on which
network they are attached to. Currently this facility is limited to
- -b, --bogus-priv
- Bogus private reverse lookups. All reverse lookups for private IP ranges
(ie 192.168.x.x, etc) which are not found in /etc/hosts or the DHCP leases
file are answered with "no such domain" rather than being
forwarded upstream. The set of prefixes affected is the list given in
RFC6303, for IPv4 and IPv6.
- 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=184.108.40.206,220.127.116.11,255.255.255.0 will map 18.104.22.168 to 22.214.171.124
and 126.96.36.199 to 188.8.131.52. 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
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 184.108.40.206
- Ignore replies to A-record queries which include the specified address. No
error is generated, dnsmasq simply continues to listen for another reply.
This is useful to defeat blocking strategies which rely on quickly
supplying a forged answer to a DNS request for certain domain, before the
correct answer can arrive.
- -f, --filterwin2k
- Later versions of windows make periodic DNS requests which don't get
sensible answers from the public DNS and can cause problems by triggering
dial-on-demand links. This flag turns on an option to filter such
requests. The requests blocked are for records of types SOA and SRV, and
type ANY where the requested name has underscores, to catch LDAP
- -r, --resolv-file=<file>
- Read the IP addresses of the upstream nameservers from <file>,
instead of /etc/resolv.conf. For the format of this file see
resolv.conf(5). The only lines relevant to dnsmasq are nameserver
ones. Dnsmasq can be told to poll more than one resolv.conf file, the
first file name specified overrides the default, subsequent ones add to
the list. This is only allowed when polling; the file with the currently
latest modification time is the one used.
- -R, --no-resolv
- Don't read /etc/resolv.conf. Get upstream servers only from the command
line or the dnsmasq configuration file.
- -1, --enable-dbus[=<service-name>]
- Allow dnsmasq configuration to be updated via DBus method calls. The
configuration which can be changed is upstream DNS servers (and
corresponding domains) and cache clear. Requires that dnsmasq has been
built with DBus support. If the service name is given, dnsmasq provides
service at that name, rather than the default which is
- Enable dnsmasq UBus interface. It sends notifications via UBus on DHCPACK
and DHCPRELEASE events. Furthermore it offers metrics. Requires that
dnsmasq has been built with UBus support.
- -o, --strict-order
- By default, dnsmasq will send queries to any of the upstream servers it
knows about and tries to favour servers that are known to be up. Setting
this flag forces dnsmasq to try each query with each server strictly in
the order they appear in /etc/resolv.conf
- 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.
- Enable code to detect DNS forwarding loops; ie the situation where a query
sent to one of the upstream server eventually returns as a new query to
the dnsmasq instance. The process works by generating TXT queries of the
form <hex>.test and sending them to each upstream server. The hex is
a UID which encodes the instance of dnsmasq sending the query and the
upstream server to which it was sent. If the query returns to the server
which sent it, then the upstream server through which it was sent is
disabled and this event is logged. Each time the set of upstream servers
changes, the test is re-run on all of them, including ones which were
- Reject (and log) addresses from upstream nameservers which are in the
private IP ranges. This blocks an attack where a browser behind a firewall
is used to probe machines on the local network.
- Exempt 127.0.0.0/8 from rebinding checks. This address range is returned
by realtime black hole servers, so blocking it may disable these
- 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.
- -n, --no-poll
- Don't poll /etc/resolv.conf for changes.
- 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
- -S, --local,
- Specify IP address of upstream servers directly. Setting this flag does
not suppress reading of /etc/resolv.conf, use --no-resolv to do
that. If one or more optional domains are given, that server is used only
for those domains and they are queried only using the specified server.
This is intended for private nameservers: if you have a nameserver on your
network which deals with names of the form xxx.internal.thekelleys.org.uk
at 192.168.1.1 then giving the flag
--server=/internal.thekelleys.org.uk/192.168.1.1 will send all
queries for internal machines to that nameserver, everything else will go
to the servers in /etc/resolv.conf. DNSSEC validation is turned off for
such private nameservers, UNLESS a --trust-anchor is specified for
the domain in question. An empty domain specification, // has the
special meaning of "unqualified names only" ie names without any
dots in them. A non-standard port may be specified as part of the IP
address using a # character. More than one --server flag is
allowed, with repeated domain or ipaddr parts as required.
More specific domains take precedence over less specific domains, so:
--server=/www.google.com/220.127.116.11 will send queries for *.google.com
to 18.104.22.168, except *www.google.com, which will go to 22.214.171.124
The special server address '#' means, "use the standard servers",
so --server=/google.com/126.96.36.199 --server=/www.google.com/#
will send queries for *.google.com to 188.8.131.52, except *www.google.com
which will be forwarded as usual.
Also permitted is a -S flag which gives a domain but no IP address; this
tells dnsmasq that a domain is local and it may answer queries from
/etc/hosts or DHCP but should never forward queries on that domain to any
upstream servers. --local is a synonym for --server to make
configuration files clearer in this case.
IPv6 addresses may include an %interface scope-id, eg
The optional string after the @ character tells dnsmasq how to set the
source of the queries to this nameserver. It can either be an ip-address,
an interface name or both. The ip-address should belong to the machine on
which dnsmasq is running, otherwise this server line will be logged and
then ignored. If an interface name is given, then queries to the server
will be forced via that interface; if an ip-address is given then the
source address of the queries will be set to that address; and if both are
given then a combination of ip-address and interface name will be used to
steer requests to the server. The query-port flag is ignored for any
servers which have a source address specified but the port may be
specified directly as part of the source address. Forcing queries to an
interface is not implemented on all platforms supported by dnsmasq.
- This is functionally the same as --server, but provides some
syntactic sugar to make specifying address-to-name queries easier. For
example --rev-server=184.108.40.206/24,192.168.0.1 is exactly equivalent
- 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 --address flags. To include
multiple IP addresses for a single query, use
--addn-hosts=<path> instead. Note that /etc/hosts and DHCP
leases override this for individual names. A common use of this is to
redirect the entire doubleclick.net domain to some friendly local web
server to avoid banner ads. The domain specification works in the same was
as for --server, with the additional facility that /#/
matches any domain. Thus --address=/#/220.127.116.11 will always return
18.104.22.168 for any query not answered from /etc/hosts or DHCP
and not sent to an upstream nameserver by a more specific --server
directive. As for --server, one or more domains with no address
returns a no-such-domain answer, so --address=/example.com/ is
equivalent to --server=/example.com/ and returns NXDOMAIN for
example.com and all its subdomains. An address specified as '#' translates
to the NULL address of 0.0.0.0 and its IPv6 equivalent of :: so
--address=/example.com/# will return NULL addresses for example.com
and its subdomains. This is partly syntactic sugar for
--address=/example.com/0.0.0.0 and --address=/example.com/::
but is also more efficient than including both as seperate configuration
lines. Note that NULL addresses normally work in the same way as
localhost, so beware that clients looking up these names are likely to end
up talking to themselves.
- Places the resolved IP addresses of queries for one or more domains in the
specified Netfilter IP set. If multiple setnames are given, then the
addresses are placed in each of them, subject to the limitations of an IP
set (IPv4 addresses cannot be stored in an IPv6 IP set and vice versa).
Domains and subdomains are matched in the same way as --address.
These IP sets must already exist. See ipset(8) for more
- -m, --mx-host=<mx
- Return an MX record named <mx name> pointing to the given hostname
(if given), or the host specified in the --mx-target switch or, if
that switch is not given, the host on which dnsmasq is running. The
default is useful for directing mail from systems on a LAN to a central
server. The preference value is optional, and defaults to 1 if not given.
More than one MX record may be given for a host.
- -t, --mx-target=<hostname>
- Specify the default target for the MX record returned by dnsmasq. See
--mx-host. If --mx-target is given, but not
--mx-host, then dnsmasq returns a MX record containing the MX
target for MX queries on the hostname of the machine on which dnsmasq is
- -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.
- Return a SRV DNS record. See RFC2782 for details. If not supplied, the
domain defaults to that given by --domain. The default for the
target domain is empty, and the default for port is one and the defaults
for weight and priority are zero. Be careful if transposing data from BIND
zone files: the port, weight and priority numbers are in a different
order. More than one SRV record for a given service/domain is allowed, all
that match are returned.
- 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.
If the time-to-live is given, it overrides the default, which is zero or the
value of --local-ttl. The value is a positive integer and gives the
time-to-live in seconds.
- -Y, --txt-record=<name>[[,<text>],<text>]
- Return a TXT DNS record. The value of TXT record is a set of strings, so
any number may be included, delimited by commas; use quotes to put commas
into a string. Note that the maximum length of a single string is 255
characters, longer strings are split into 255 character chunks.
- Return a PTR DNS record.
- Return an NAPTR DNS record, as specified in RFC3403.
- Return a CAA DNS record, as specified in RFC6844.
- Return a CNAME record which indicates that <cname> is really
<target>. There are significant limitations on the target; it must
be a DNS name which is known to dnsmasq from /etc/hosts (or additional
hosts files), from DHCP, from --interface-name or from another
--cname. If the target does not satisfy this criteria, the whole
cname is ignored. The cname must be unique, but it is permissible to have
more than one cname pointing to the same target. Indeed it's possible to
declare multiple cnames to a target in a single line, like so:
If the time-to-live is given, it overrides the default, which is zero or the
value of --local-ttl. The value is a positive integer and gives the
time-to-live in seconds.
- --dns-rr=<name>,<RR-number>,[<hex data>]
- Return an arbitrary DNS Resource Record. The number is the type of the
record (which is always in the C_IN class). The value of the record is
given by the hex data, which may be of the form 01:23:45 or 01 23 45 or
012345 or any mixture of these.
- Return DNS records associating the name with the address(es) of the given
interface. This flag specifies an A or AAAA record for the given name in
the same way as an /etc/hosts line, except that the address is not
constant, but taken from the given interface. The interface may be
followed by "/4" or "/6" to specify that only IPv4 or
IPv6 addresses of the interface should be used. If the interface is down,
not configured or non-existent, an empty record is returned. The matching
PTR record is also created, mapping the interface address to the name.
More than one name may be associated with an interface address by
repeating the flag; in that case the first instance is used for the
reverse address-to-name mapping. Note that a name used in
--interface-name may not appear in /etc/hosts.
- Create artificial A/AAAA and PTR records for an address range. The records
either seqential numbers or the address, with periods (or colons for IPv6)
replaced with dashes.
An examples should make this clearer. First sequential numbers.
results in the name internal-0.thekelleys.org.uk. returning 192.168.0.50,
internal-1.thekelleys.org.uk returning 192.168.0.51 and so on. (note the
*) The same principle applies to IPv6 addresses (where the numbers may be
very large). Reverse lookups from address to name behave as expected.
Second, --synth-domain=thekelleys.org.uk,192.168.0.0/24,internal- (no
*) will result in a query for internal-192-168-0-56.thekelleys.org.uk
returning 192.168.0.56 and a reverse query vice versa. The same applies to
IPv6, but IPv6 addresses may start with '::' but DNS labels may not start
with '-' so in this case if no prefix is configured a zero is added in
front of the label. ::1 becomes 0--1.
V4 mapped IPv6 addresses, which have a representation like ::ffff:22.214.171.124
are handled specially, and become like 0--ffff-1-2-3-4
The address range can be of the form <ip address>,<ip address>
or <ip address>/<netmask> in both forms of the option.
- Specify the location of a pcap-format file which dnsmasq uses to dump
copies of network packets for debugging purposes. If the file exists when
dnsmasq starts, it is not deleted; new packets are added to the end.
- Specify which types of packets should be added to the dumpfile. The
argument should be the OR of the bitmasks for each type of packet to be
dumped: it can be specified in hex by preceding the number with 0x in the
normal way. Each time a packet is written to the dumpfile, dnsmasq logs
the packet sequence and the mask representing its type. The current types
are: 0x0001 - DNS queries from clients 0x0002 DNS replies to clients
0x0004 - DNS queries to upstream 0x0008 - DNS replies from upstream 0x0010
- queries send upstream for DNSSEC validation 0x0020 - replies to queries
for DNSSEC validation 0x0040 - replies to client queries which fail DNSSEC
validation 0x0080 replies to queries for DNSSEC validation which fail
- Add the MAC address of the requestor to DNS queries which are forwarded
upstream. This may be used to DNS filtering by the upstream server. The
MAC address can only be added if the requestor is on the same subnet as
the dnsmasq server. Note that the mechanism used to achieve this (an EDNS0
option) is not yet standardised, so this should be considered
experimental. Also note that exposing MAC addresses in this way may have
security and privacy implications. The warning about caching given for
--add-subnet applies to --add-mac too. An alternative
encoding of the MAC, as base64, is enabled by adding the
"base64" parameter and a human-readable encoding of
hex-and-colons is enabled by added the "text" parameter.
- Add an arbitrary identifying string to o DNS queries which are forwarded
- --add-subnet[[=[<IPv4 address>/]<IPv4 prefix
length>][,[<IPv6 address>/]<IPv6 prefix length>]]
- Add a subnet address to the DNS queries which are forwarded upstream. If
an address is specified in the flag, it will be used, otherwise, the
address of the requestor will be used. The amount of the address forwarded
depends on the prefix length parameter: 32 (128 for IPv6) forwards the
whole address, zero forwards none of it but still marks the request so
that no upstream nameserver will add client address information either.
The default is zero for both IPv4 and IPv6. Note that upstream nameservers
may be configured to return different results based on this information,
but the dnsmasq cache does not take account. If a dnsmasq instance is
configured such that different results may be encountered, caching should
For example, --add-subnet=24,96 will add the /24 and /96 subnets of
the requestor for IPv4 and IPv6 requestors, respectively.
--add-subnet=126.96.36.199/24 will add 188.8.131.52/24 for IPv4 requestors and
::/0 for IPv6 requestors. --add-subnet=184.108.40.206/24,220.127.116.11/24 will
add 18.104.22.168/24 for both IPv4 and IPv6 requestors.
- -c, --cache-size=<cachesize>
- Set the size of dnsmasq's cache. The default is 150 names. Setting the
cache size to zero disables caching. Note: huge cache size impacts
- -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.
- Validate DNS replies and cache DNSSEC data. When forwarding DNS queries,
dnsmasq requests the DNSSEC records needed to validate the replies. The
replies are validated and the result returned as the Authenticated Data
bit in the DNS packet. In addition the DNSSEC records are stored in the
cache, making validation by clients more efficient. Note that validation
by clients is the most secure DNSSEC mode, but for clients unable to do
validation, use of the AD bit set by dnsmasq is useful, provided that the
network between the dnsmasq server and the client is trusted. Dnsmasq must
be compiled with HAVE_DNSSEC enabled, and DNSSEC trust anchors provided,
see --trust-anchor. Because the DNSSEC validation process uses the
cache, it is not permitted to reduce the cache size below the default when
DNSSEC is enabled. The nameservers upstream of dnsmasq must be
DNSSEC-capable, ie capable of returning DNSSEC records with data. If they
are not, then dnsmasq will not be able to determine the trusted status of
answers and this means that DNS service will be entirely broken.
- Provide DS records to act a trust anchors for DNSSEC validation. Typically
these will be the DS record(s) for Key Signing key(s) (KSK) of the root
zone, but trust anchors for limited domains are also possible. The current
root-zone trust anchors may be downloaded from
- As a default, dnsmasq checks that unsigned DNS replies are legitimate:
this entails possible extra queries even for the majority of DNS zones
which are not, at the moment, signed. If --dnssec-check-unsigned=no
appears in the configuration, then such replies they are assumed to be
valid and passed on (without the "authentic data" bit set, of
course). This does not protect against an attacker forging unsigned
replies for signed DNS zones, but it is fast.
Versions of dnsmasq prior to 2.80 defaulted to not checking unsigned
replies, and used --dnssec-check-unsigned to switch this on. Such
configurations will continue to work as before, but those which used the
default of no checking will need to be altered to explicitly select no
checking. The new default is because switching off checking for unsigned
replies is inherently dangerous. Not only does it open the possiblity of
forged replies, but it allows everything to appear to be working even when
the upstream namesevers do not support DNSSEC, and in this case no DNSSEC
validation at all is occuring.
- DNSSEC signatures are only valid for specified time windows, and should be
rejected outside those windows. This generates an interesting
chicken-and-egg problem for machines which don't have a hardware real time
clock. For these machines to determine the correct time typically requires
use of NTP and therefore DNS, but validating DNS requires that the correct
time is already known. Setting this flag removes the time-window checks
(but not other DNSSEC validation.) only until the dnsmasq process receives
SIGINT. The intention is that dnsmasq should be started with this flag
when the platform determines that reliable time is not currently
available. As soon as reliable time is established, a SIGINT should be
sent to dnsmasq, which enables time checking, and purges the cache of DNS
records which have not been thoroughly checked.
Earlier versions of dnsmasq overloaded SIGHUP (which re-reads much
configuration) to also enable time validation.
If dnsmasq is run in debug mode ( --no-daemon flag) then SIGINT
retains its usual meaning of terminating the dnsmasq process.
- Enables an alternative way of checking the validity of the system time for
DNSSEC (see --dnssec-no-timecheck). In this case, the system time
is considered to be valid once it becomes later than the timestamp on the
specified file. The file is created and its timestamp set automatically by
dnsmasq. The file must be stored on a persistent filesystem, so that it
and its mtime are carried over system restarts. The timestamp file is
created after dnsmasq has dropped root, so it must be in a location
writable by the unprivileged user that dnsmasq runs as.
- Copy the DNSSEC Authenticated Data bit from upstream servers to downstream
clients and cache it. This is an alternative to having dnsmasq validate
DNSSEC, but it depends on the security of the network between dnsmasq and
the upstream servers, and the trustworthiness of the upstream
- Set debugging mode for the DNSSEC validation, set the Checking Disabled
bit on upstream queries, and don't convert replies which do not validate
to responses with a return code of SERVFAIL. Note that setting this may
affect DNS behaviour in bad ways, it is not an extra-logging flag and
should not be set in production.
- Define a DNS zone for which dnsmasq acts as authoritative server. Locally
defined DNS records which are in the domain will be served. If subnet(s)
are given, A and AAAA records must be in one of the specified subnets.
As alternative to directly specifying the subnets, it's possible to give the
name of an interface, in which case the subnets implied by that
interface's configured addresses and netmask/prefix-length are used; this
is useful when using constructed DHCP ranges as the actual address is
dynamic and not known when configuring dnsmasq. The interface addresses
may be confined to only IPv6 addresses using <interface>/6 or to
only IPv4 using <interface>/4. This is useful when an interface has
dynamically determined global IPv6 addresses which should appear in the
zone, but RFC1918 IPv4 addresses which should not. Interface-name and
address-literal subnet specifications may be used freely in the same
It's possible to exclude certain IP addresses from responses. It can be
used, to make sure that answers contain only global routeable IP addresses
(by excluding loopback, RFC1918 and ULA addresses).
The subnet(s) are also used to define in-addr.arpa and ip6.arpa domains
which are served for reverse-DNS queries. If not specified, the prefix
length defaults to 24 for IPv4 and 64 for IPv6. For IPv4 subnets, the
prefix length should be have the value 8, 16 or 24 unless you are familiar
with RFC 2317 and have arranged the in-addr.arpa delegation accordingly.
Note that if no subnets are specified, then no reverse queries are
- Specify fields in the SOA record associated with authoritative zones. Note
that this is optional, all the values are set to sane defaults.
- 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.
- Specify the addresses of secondary servers which are allowed to initiate
zone transfer (AXFR) requests for zones for which dnsmasq is
authoritative. If this option is not given but --auth-sec-servers is, then
AXFR requests will be accepted from any secondary. Specifying
--auth-peer without --auth-sec-servers enables zone transfer
but does not advertise the secondary in NS records returned by
- Read the Linux connection track mark associated with incoming DNS queries
and set the same mark value on upstream traffic used to answer those
queries. This allows traffic generated by dnsmasq to be associated with
the queries which cause it, useful for bandwidth accounting and
firewalling. Dnsmasq must have conntrack support compiled in and the
kernel must have conntrack support included and configured. This option
cannot be combined with --query-port.
Enable the DHCP server. Addresses will be given out from the range
<start-addr> to <end-addr> and from statically defined
addresses given in --dhcp-host options. If the lease time is given,
then leases will be given for that length of time. The lease time is in
seconds, or minutes (eg 45m) or hours (eg 1h) or "infinite". If
not given, the default lease time is one hour. The minimum lease time is
two minutes. For IPv6 ranges, the lease time maybe "deprecated";
this sets the preferred lifetime sent in a DHCP lease or router
advertisement to zero, which causes clients to use other addresses, if
available, for new connections as a prelude to renumbering.
This option may be repeated, with different addresses, to enable DHCP
service to more than one network. For directly connected networks (ie,
networks on which the machine running dnsmasq has an interface) the
netmask is optional: dnsmasq will determine it from the interface
configuration. For networks which receive DHCP service via a relay agent,
dnsmasq cannot determine the netmask itself, so it should be specified,
otherwise dnsmasq will have to guess, based on the class (A, B or C) of
the network address. The broadcast address is always optional. It is
always allowed to have more than one --dhcp-range in a single
For IPv6, the parameters are slightly different: instead of netmask and
broadcast address, there is an optional prefix length which must be equal
to or larger then the prefix length on the local interface. If not given,
this defaults to 64. Unlike the IPv4 case, the prefix length is not
automatically derived from the interface configuration. The minimum size
of the prefix length is 64.
IPv6 (only) supports another type of range. In this, the start address and
optional end address contain only the network part (ie ::1) and they are
followed by constructor:<interface>. This forms a template
which describes how to create ranges, based on the addresses assigned to
the interface. For instance
will look for addresses on eth0 and then create a range from
<network>::1 to <network>::400. If the interface is assigned
more than one network, then the corresponding ranges will be automatically
created, and then deprecated and finally removed again as the address is
deprecated and then deleted. The interface name may have a final
"*" wildcard. Note that just any address on eth0 will not do: it
must not be an autoconfigured or privacy address, or be deprecated.
If a --dhcp-range is only being used for stateless DHCP and/or SLAAC,
then the address can be simply ::
The optional set:<tag> sets an alphanumeric label which marks
this network so that dhcp options may be specified on a per-network basis.
When it is prefixed with 'tag:' instead, then its meaning changes from
setting a tag to matching it. Only one tag may be set, but more than one
tag may be matched.
The optional <mode> keyword may be static which tells dnsmasq
to enable DHCP for the network specified, but not to dynamically allocate
IP addresses: only hosts which have static addresses given via
--dhcp-host or from /etc/ethers will be served. A static-only
subnet with address all zeros may be used as a "catch-all"
address to enable replies to all Information-request packets on a subnet
which is provided with stateless DHCPv6, ie --dhcp-range=::,static
For IPv4, the <mode> may be proxy in which case dnsmasq will
provide proxy-DHCP on the specified subnet. (See --pxe-prompt and
--pxe-service for details.)
For IPv6, the mode may be some combination of ra-only, slaac, ra-names,
ra-stateless, ra-advrouter, off-link.
ra-only tells dnsmasq to offer Router Advertisement only on this
subnet, and not DHCP.
slaac tells dnsmasq to offer Router Advertisement on this subnet and
to set the A bit in the router advertisement, so that the client will use
SLAAC addresses. When used with a DHCP range or static DHCP address this
results in the client having both a DHCP-assigned and a SLAAC address.
ra-stateless sends router advertisements with the O and A bits set,
and provides a stateless DHCP service. The client will use a SLAAC
address, and use DHCP for other configuration information.
ra-names enables a mode which gives DNS names to dual-stack hosts
which do SLAAC for IPv6. Dnsmasq uses the host's IPv4 lease to derive the
name, network segment and MAC address and assumes that the host will also
have an IPv6 address calculated using the SLAAC algorithm, on the same
network segment. The address is pinged, and if a reply is received, an
AAAA record is added to the DNS for this IPv6 address. Note that this is
only happens for directly-connected networks, (not one doing DHCP via a
relay) and it will not work if a host is using privacy extensions.
ra-names can be combined with ra-stateless and slaac.
ra-advrouter enables a mode where router address(es) rather than
prefix(es) are included in the advertisements. This is described in
RFC-3775 section 7.2 and is used in mobile IPv6. In this mode the interval
option is also included, as described in RFC-3775 section 7.3.
off-link tells dnsmasq to advertise the prefix without the on-link
(aka L) bit set.
- Specify per host parameters for the DHCP server. This allows a machine
with a particular hardware address to be always allocated the same
hostname, IP address and lease time. A hostname specified like this
overrides any supplied by the DHCP client on the machine. It is also
allowable to omit the hardware address and include the hostname, in which
case the IP address and lease times will apply to any machine claiming
that name. For example --dhcp-host=00:20:e0:3b:13:af,wap,infinite
tells dnsmasq to give the machine with hardware address 00:20:e0:3b:13:af
the name wap, and an infinite DHCP lease.
--dhcp-host=lap,192.168.0.199 tells dnsmasq to always allocate the
machine lap the IP address 192.168.0.199.
Addresses allocated like this are not constrained to be in the range given
by the --dhcp-range option, but they must be in the same subnet as
some valid dhcp-range. For subnets which don't need a pool of dynamically
allocated addresses, use the "static" keyword in the
It is allowed to use client identifiers (called client DUID in IPv6-land)
rather than hardware addresses to identify hosts by prefixing with 'id:'.
Thus: --dhcp-host=id:01:02:03:04,..... refers to the host with
client identifier 01:02:03:04. It is also allowed to specify the client ID
as text, like this: --dhcp-host=id:clientidastext,.....
A single --dhcp-host may contain an IPv4 address or an IPv6 address,
or both. IPv6 addresses must be bracketed by square brackets thus:
--dhcp-host=laptop,[1234::56] IPv6 addresses may contain only the
host-identifier part: --dhcp-host=laptop,[::56] in which case they
act as wildcards in constructed dhcp ranges, with the appropriate network
part inserted. Note that in IPv6 DHCP, the hardware address may not be
available, though it normally is for direct-connected clients, or clients
using DHCP relays which support RFC 6939.
For DHCPv4, the special option id:* means "ignore any client-id and use
MAC addresses only." This is useful when a client presents a
client-id sometimes but not others.
If a name appears in /etc/hosts, the associated address can be allocated to
a DHCP lease, but only if a --dhcp-host option specifying the name
also exists. Only one hostname can be given in a --dhcp-host
option, but aliases are possible by using CNAMEs. (See --cname ).
The special keyword "ignore" tells dnsmasq to never offer a DHCP
lease to a machine. The machine can be specified by hardware address,
client ID or hostname, for instance
--dhcp-host=00:20:e0:3b:13:af,ignore This is useful when there is
another DHCP server on the network which should be used by some machines.
The set:<tag> construct sets the tag whenever this --dhcp-host
directive is in use. This can be used to selectively send DHCP options
just for this host. More than one tag can be set in a --dhcp-host
directive (but not in other places where "set:<tag>" is
allowed). When a host matches any --dhcp-host directive (or one
implied by /etc/ethers) then the special tag "known" is set.
This allows dnsmasq to be configured to ignore requests from unknown
machines using --dhcp-ignore=tag:!known If the host matches only a
--dhcp-host directive which cannot be used because it specifies an
address on different subnet, the tag "known-othernet" is set.
Ethernet addresses (but not client-ids) may have wildcard bytes, so for
example --dhcp-host=00:20:e0:3b:13:*,ignore will cause dnsmasq to
ignore a range of hardware addresses. Note that the "*" will
need to be escaped or quoted on a command line, but not in the
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,22.214.171.124 will only match a
Token-Ring hardware address, since the ARP-address type for token ring is
As a special case, in DHCPv4, it is possible to include more than one
hardware address. eg:
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.
- Read DHCP host information from the specified file. If a directory is
given, then read all the files contained in that directory. The file
contains information about one host per line. The format of a line is the
same as text to the right of '=' in --dhcp-host. The advantage of
storing DHCP host information in this file is that it can be changed
without re-starting dnsmasq: the file will be re-read when dnsmasq
- Read DHCP option information from the specified file. If a directory is
given, then read all the files contained in that directory. The advantage
of using this option is the same as for --dhcp-hostsfile: the
--dhcp-optsfile will be re-read when dnsmasq receives SIGHUP. Note
that it is possible to encode the information in a --dhcp-boot flag
as DHCP options, using the options names bootfile-name, server-ip-address
and tftp-server. This allows these to be included in a
- This is equivalent to --dhcp-hostsfile, except for the following.
The path MUST be a directory, and not an individual file. Changed or new
files within the directory are read automatically, without the need to
send SIGHUP. If a file is deleted or changed after it has been read by
dnsmasq, then the host record it contained will remain until dnsmasq
receives a SIGHUP, or is restarted; ie host records are only added
- This is equivalent to --dhcp-optsfile, with the differences noted
- -Z, --read-ethers
- Read /etc/ethers for information about hosts for the DHCP server. The
format of /etc/ethers is a hardware address, followed by either a hostname
or dotted-quad IP address. When read by dnsmasq these lines have exactly
the same effect as --dhcp-host options containing the same
information. /etc/ethers is re-read when dnsmasq receives SIGHUP. IPv6
addresses are NOT read from /etc/ethers.
- Specify different or extra options to DHCP clients. By default, dnsmasq
sends some standard options to DHCP clients, the netmask and broadcast
address are set to the same as the host running dnsmasq, and the DNS
server and default route are set to the address of the machine running
dnsmasq. (Equivalent rules apply for IPv6.) If the domain name option has
been set, that is sent. This configuration allows these defaults to be
overridden, or other options specified. The option, to be sent may be
given as a decimal number or as "option:<option-name>" The
option numbers are specified in RFC2132 and subsequent RFCs. The set of
option-names known by dnsmasq can be discovered by running "dnsmasq
--help dhcp". For example, to set the default route option to
192.168.4.4, do --dhcp-option=3,192.168.4.4 or --dhcp-option =
option:router, 192.168.4.4 and to set the time-server address to
192.168.0.4, do --dhcp-option = 42,192.168.0.4 or --dhcp-option
= option:ntp-server, 192.168.0.4 The special address 0.0.0.0 is taken
to mean "the address of the machine running dnsmasq".
Data types allowed are comma separated dotted-quad IPv4 addresses,
-wrapped IPv6 addresses, a decimal number, colon-separated hex digits
and a text string. If the optional tags are given then this option is only
sent when all the tags are matched.
Special processing is done on a text argument for option 119, to conform
with RFC 3397. Text or dotted-quad IP addresses as arguments to option 120
are handled as per RFC 3361. Dotted-quad IP addresses which are followed
by a slash and then a netmask size are encoded as described in RFC 3442.
IPv6 options are specified using the option6: keyword, followed by
the option number or option name. The IPv6 option name space is disjoint
from the IPv4 option name space. IPv6 addresses in options must be
bracketed with square brackets, eg.
--dhcp-option=option6:ntp-server,[1234::56] For IPv6, [::] means
"the global address of the machine running dnsmasq", whilst
[fd00::] is replaced with the ULA, if it exists, and [fe80::] with the
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,"126.96.36.199"
Encapsulated Vendor-class options may also be specified (IPv4 only) using
--dhcp-option: for instance
--dhcp-option=vendor:PXEClient,1,0.0.0.0 sends the encapsulated
vendor class-specific option "mftp-address=0.0.0.0" to any
client whose vendor-class matches "PXEClient". The vendor-class
matching is substring based (see --dhcp-vendorclass for details).
If a vendor-class option (number 60) is sent by dnsmasq, then that is used
for selecting encapsulated options in preference to any sent by the
client. It is possible to omit the vendorclass completely;
--dhcp-option=vendor:,1,0.0.0.0 in which case the encapsulated
option is always sent.
Options may be encapsulated (IPv4 only) within other options: for instance
--dhcp-option=encap:175, 190, iscsi-client0 will send option 175,
within which is the option 190. If multiple options are given which are
encapsulated with the same option number then they will be correctly
combined into one encapsulated option. encap: and vendor: are may not both
be set in the same --dhcp-option.
The final variant on encapsulated options is "Vendor-Identifying Vendor
Options" as specified by RFC3925. These are denoted like this:
--dhcp-option=vi-encap:2, 10, text The number in the vi-encap:
section is the IANA enterprise number used to identify this option. This
form of encapsulation is supported in IPv6.
The address 0.0.0.0 is not treated specially in encapsulated options.
- 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.
- (IPv4 only) Disable re-use of the DHCP servername and filename fields as
extra option space. If it can, dnsmasq moves the boot server and filename
information (from --dhcp-boot) out of their dedicated fields into
DHCP options. This make extra space available in the DHCP packet for
options but can, rarely, confuse old or broken clients. This flag forces
"simple and safe" behaviour to avoid problems in such a
- --dhcp-relay=<local address>,<server
- Configure dnsmasq to do DHCP relay. The local address is an address
allocated to an interface on the host running dnsmasq. All DHCP requests
arriving on that interface will we relayed to a remote DHCP server at the
server address. It is possible to relay from a single local address to
multiple remote servers by using multiple --dhcp-relay configs with
the same local address and different server addresses. A server address
must be an IP literal address, not a domain name. In the case of DHCPv6,
the server address may be the ALL_SERVERS multicast address, ff05::1:3. In
this case the interface must be given, not be wildcard, and is used to
direct the multicast to the correct interface to reach the DHCP server.
Access control for DHCP clients has the same rules as for the DHCP server,
see --interface, --except-interface, etc. The optional
interface name in the --dhcp-relay config has a different function:
it controls on which interface DHCP replies from the server will be
accepted. This is intended for configurations which have three interfaces:
one being relayed from, a second connecting the DHCP server, and a third
untrusted network, typically the wider internet. It avoids the possibility
of spoof replies arriving via this third interface.
It is allowed to have dnsmasq act as a DHCP server on one set of interfaces
and relay from a disjoint set of interfaces. Note that whilst it is quite
possible to write configurations which appear to act as a server and a
relay on the same interface, this is not supported: the relay function
will take precedence.
Both DHCPv4 and DHCPv6 relay is supported. It's not possible to relay DHCPv4
to a DHCPv6 server or vice-versa.
- -U, --dhcp-vendorclass=set:<tag>,[enterprise:<IANA-enterprise
- Map from a vendor-class string to a tag. Most DHCP clients provide a
"vendor class" which represents, in some sense, the type of
host. This option maps vendor classes to tags, so that DHCP options may be
selectively delivered to different classes of hosts. For example
--dhcp-vendorclass=set:printers,Hewlett-Packard JetDirect will
allow options to be set only for HP printers like so:
--dhcp-option=tag:printers,3,192.168.4.4 The vendor-class string is
substring matched against the vendor-class supplied by the client, to
allow fuzzy matching. The set: prefix is optional but allowed for
Note that in IPv6 only, vendorclasses are namespaced with an IANA-allocated
enterprise number. This is given with enterprise: keyword and specifies
that only vendorclasses matching the specified number should be
- -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
- -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.
- Map from RFC3046 relay agent options to tags. This data may be provided by
DHCP relay agents. The circuit-id or remote-id is normally given as
colon-separated hex, but is also allowed to be a simple string. If an
exact match is achieved between the circuit or agent ID and one provided
by a relay agent, the tag is set.
--dhcp-remoteid (but not --dhcp-circuitid) is supported in
- (IPv4 and IPv6) Map from RFC3993 subscriber-id relay agent options to
- --dhcp-proxy[=<ip addr>]......
- (IPv4 only) A normal DHCP relay agent is only used to forward the initial
parts of a DHCP interaction to the DHCP server. Once a client is
configured, it communicates directly with the server. This is undesirable
if the relay agent is adding extra information to the DHCP packets, such
as that used by --dhcp-circuitid and --dhcp-remoteid. A full
relay implementation can use the RFC 5107 serverid-override option to
force the DHCP server to use the relay as a full proxy, with all packets
passing through it. This flag provides an alternative method of doing the
same thing, for relays which don't support RFC 5107. Given alone, it
manipulates the server-id for all interactions via relays. If a list of IP
addresses is given, only interactions via relays at those addresses are
- --dhcp-match=set:<tag>,<option number>|option:<option
- 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.
- Set the tag if the given name is supplied by a dhcp client. There may be a
single trailing wildcard *, which has the usual meaning. Combined with
dhcp-ignore or dhcp-ignore-names this gives the ability to ignore certain
clients by name, or disallow certain hostnames from being claimed by a
- Perform boolean operations on tags. Any tag appearing as set:<tag>
is set if all the tags which appear as tag:<tag> are set, (or unset
when tag:!<tag> is used) If no tag:<tag> appears
set:<tag> tags are set unconditionally. Any number of set: and tag:
forms may appear, in any order. --tag-if lines are executed in
order, so if the tag in tag:<tag> is a tag set by another
--tag-if, the line which sets the tag must precede the one which
- -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.
- When all the given tags appear in the tag set, ignore any hostname
provided by the host. Note that, unlike --dhcp-ignore, it is
permissible to supply no tags, in which case DHCP-client supplied
hostnames are always ignored, and DHCP hosts are added to the DNS using
only --dhcp-host configuration in dnsmasq and the contents of
/etc/hosts and /etc/ethers.
- (IPv4 only) Generate a name for DHCP clients which do not otherwise have
one, using the MAC address expressed in hex, separated by dashes. Note
that if a host provides a name, it will be used by preference to this,
unless --dhcp-ignore-names is set.
- (IPv4 only) When all the given tags appear in the tag set, always use
broadcast to communicate with the host when it is unconfigured. It is
permissible to supply no tags, in which case this is unconditional. Most
DHCP clients which need broadcast replies set a flag in their requests so
that this happens automatically, some old BOOTP clients do not.
- (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
- Dnsmasq is designed to choose IP addresses for DHCP clients using a hash
of the client's MAC address. This normally allows a client's address to
remain stable long-term, even if the client sometimes allows its DHCP
lease to expire. In this default mode IP addresses are distributed
pseudo-randomly over the entire available address range. There are
sometimes circumstances (typically server deployment) where it is more
convenient to have IP addresses allocated sequentially, starting from the
lowest available address, and setting this flag enables this mode. Note
that in the sequential mode, clients which allow a lease to expire are
much more likely to move IP address; for this reason it should not be
- Most uses of PXE boot-ROMS simply allow the PXE system to obtain an IP
address and then download the file specified by --dhcp-boot and
execute it. However the PXE system is capable of more complex functions
when supported by a suitable DHCP server.
This specifies a boot option which may appear in a PXE boot menu.
<CSA> is client system type, only services of the correct type will
appear in a menu. The known types are x86PC, PC98, IA64_EFI, Alpha,
Arc_x86, Intel_Lean_Client, IA32_EFI, X86-64_EFI, Xscale_EFI, BC_EFI,
ARM32_EFI and ARM64_EFI; an integer may be used for other types. The
parameter after the menu text may be a file name, in which case dnsmasq
acts as a boot server and directs the PXE client to download the file by
TFTP, either from itself ( --enable-tftp must be set for this to
work) or another TFTP server if the final server address/name is given.
Note that the "layer" suffix (normally ".0") is
supplied by PXE, and need not be added to the basename. Alternatively, the
basename may be a filename, complete with suffix, in which case no layer
suffix is added. If an integer boot service type, rather than a basename
is given, then the PXE client will search for a suitable boot service for
that type on the network. This search may be done by broadcast, or direct
to a server if its IP address/name is provided. If no boot service type or
filename is provided (or a boot service type of 0 is specified) then the
menu entry will abort the net boot procedure and continue booting from
local media. The server address can be given as a domain name which is
looked up in /etc/hosts. This name can be associated in /etc/hosts with
multiple IP addresses, which are used round-robin.
- Setting this provides a prompt to be displayed after PXE boot. If the
timeout is given then after the timeout has elapsed with no keyboard
input, the first available menu option will be automatically executed. If
the timeout is zero then the first available menu item will be executed
immediately. If --pxe-prompt is omitted the system will wait for
user input if there are multiple items in the menu, but boot immediately
if there is only one. See --pxe-service for details of menu items.
Dnsmasq supports PXE "proxy-DHCP", in this case another DHCP
server on the network is responsible for allocating IP addresses, and
dnsmasq simply provides the information given in --pxe-prompt and
--pxe-service to allow netbooting. This mode is enabled using the
proxy keyword in --dhcp-range.
- -X, --dhcp-lease-max=<number>
- Limits dnsmasq to the specified maximum number of DHCP leases. The default
is 1000. This limit is to prevent DoS attacks from hosts which create
thousands of leases and use lots of memory in the dnsmasq process.
- -K, --dhcp-authoritative
- Should be set when dnsmasq is definitely the only DHCP server on a
network. For DHCPv4, it changes the behaviour from strict RFC compliance
so that DHCP requests on unknown leases from unknown hosts are not
ignored. This allows new hosts to get a lease without a tedious timeout
under all circumstances. It also allows dnsmasq to rebuild its lease
database without each client needing to reacquire a lease, if the database
is lost. For DHCPv6 it sets the priority in replies to 255 (the maximum)
instead of 0 (the minimum).
- Enable DHCPv4 Rapid Commit Option specified in RFC 4039. When enabled,
dnsmasq will respond to a DHCPDISCOVER message including a Rapid Commit
option with a DHCPACK including a Rapid Commit option and fully committed
address and configuration information. Should only be enabled if either
the server is the only server for the subnet, or multiple servers are
present and they each commit a binding for all clients.
- --dhcp-alternate-port[=<server port>[,<client
- (IPv4 only) Change the ports used for DHCP from the default. If this
option is given alone, without arguments, it changes the ports used for
DHCP from 67 and 68 to 1067 and 1068. If a single argument is given, that
port number is used for the server and the port number plus one used for
the client. Finally, two port numbers allows arbitrary specification of
both server and client ports for DHCP.
- -3, --bootp-dynamic[=<network-id>[,<network-id>]]
- (IPv4 only) Enable dynamic allocation of IP addresses to BOOTP clients.
Use this with care, since each address allocated to a BOOTP client is
leased forever, and therefore becomes permanently unavailable for re-use
by other hosts. if this is given without tags, then it unconditionally
enables dynamic allocation. With tags, only when the tags are all set. It
may be repeated with different tag sets.
- -5, --no-ping
- (IPv4 only) By default, the DHCP server will attempt to ensure that an
address is not in use before allocating it to a host. It does this by
sending an ICMP echo request (aka "ping") to the address in
question. If it gets a reply, then the address must already be in use, and
another is tried. This flag disables this check. Use with caution.
- 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.
- (IPv6 only) Specify the server persistent UID which the DHCPv6 server will
use. This option is not normally required as dnsmasq creates a DUID
automatically when it is first needed. When given, this option provides
dnsmasq the data required to create a DUID-EN type DUID. Note that once
set, the DUID is stored in the lease database, so to change between
DUID-EN and automatically created DUIDs or vice-versa, the lease database
must be re-initialised. The enterprise-id is assigned by IANA, and the uid
is a string of hex octets unique to a particular device.
- -6 --dhcp-script=<path>
- Whenever a new DHCP lease is created, or an old one destroyed, or a TFTP
file transfer completes, the executable specified by this option is run.
<path> must be an absolute pathname, no PATH search occurs. The
arguments to the process are "add", "old" or
"del", the MAC address of the host (or DUID for IPv6) , the IP
address, and the hostname, if known. "add" means a lease has
been created, "del" means it has been destroyed, "old"
is a notification of an existing lease when dnsmasq starts or a change to
MAC address or hostname of an existing lease (also, lease length or expiry
and client-id, if --leasefile-ro is set). If the MAC address is
from a network type other than ethernet, it will have the network type
prepended, eg "06-01:23:45:67:89:ab" for token ring. The process
is run as root (assuming that dnsmasq was originally run as root) even if
dnsmasq is configured to change UID to an unprivileged user.
The environment is inherited from the invoker of dnsmasq, with some or all
of the following variables added
For both IPv4 and IPv6:
DNSMASQ_DOMAIN if the fully-qualified domain name of the host is known, this
is set to the domain part. (Note that the hostname passed to the script as
an argument is never fully-qualified.)
If the client provides a hostname, DNSMASQ_SUPPLIED_HOSTNAME
If the client provides user-classes,
If dnsmasq was compiled with HAVE_BROKEN_RTC, then the length of the lease
(in seconds) is stored in DNSMASQ_LEASE_LENGTH, otherwise the time of
lease expiry is stored in DNSMASQ_LEASE_EXPIRES. The number of seconds
until lease expiry is always stored in DNSMASQ_TIME_REMAINING.
If a lease used to have a hostname, which is removed, an "old"
event is generated with the new state of the lease, ie no name, and the
former name is provided in the environment variable DNSMASQ_OLD_HOSTNAME.
DNSMASQ_INTERFACE stores the name of the interface on which the request
arrived; this is not set for "old" actions when dnsmasq
DNSMASQ_RELAY_ADDRESS is set if the client used a DHCP relay to contact
dnsmasq and the IP address of the relay is known.
DNSMASQ_TAGS contains all the tags set during the DHCP transaction,
separated by spaces.
DNSMASQ_LOG_DHCP is set if --log-dhcp is in effect.
For IPv4 only:
DNSMASQ_CLIENT_ID if the host provided a client-id.
DNSMASQ_CIRCUIT_ID, DNSMASQ_SUBSCRIBER_ID, DNSMASQ_REMOTE_ID if a DHCP
relay-agent added any of these options.
If the client provides vendor-class, DNSMASQ_VENDOR_CLASS.
DNSMASQ_REQUESTED_OPTIONS a string containing the decimal values in the
Parameter Request List option, comma separated, if the parameter request
list option is provided by the client.
For IPv6 only:
If the client provides vendor-class, DNSMASQ_VENDOR_CLASS_ID, containing the
IANA enterprise id for the class, and
DNSMASQ_VENDOR_CLASS0..DNSMASQ_VENDOR_CLASSn for the data.
DNSMASQ_SERVER_DUID containing the DUID of the server: this is the same for
every call to the script.
DNSMASQ_IAID containing the IAID for the lease. If the lease is a temporary
allocation, this is prefixed to 'T'.
DNSMASQ_MAC containing the MAC address of the client, if known.
Note that the supplied hostname, vendorclass and userclass data is only
supplied for "add" actions or "old" actions when a
host resumes an existing lease, since these data are not held in dnsmasq's
All file descriptors are closed except stdin, which is open to /dev/null,
and stdout and stderr which capture output for logging by dnsmasq. (In
debug mode, stdio, stdout and stderr file are left as those inherited from
the invoker of dnsmasq).
The script is not invoked concurrently: at most one instance of the script
is ever running (dnsmasq waits for an instance of script to exit before
running the next). Changes to the lease database are which require the
script to be invoked are queued awaiting exit of a running instance. If
this queueing allows multiple state changes occur to a single lease before
the script can be run then earlier states are discarded and the current
state of that lease is reflected when the script finally runs.
At dnsmasq startup, the script will be invoked for all existing leases as
they are read from the lease file. Expired leases will be called with
"del" and others with "old". When dnsmasq receives a
HUP signal, the script will be invoked for existing leases with an
There are four further actions which may appear as the first argument to the
script, "init", "arp-add", "arp-del" and
"tftp". More may be added in the future, so scripts should be
written to ignore unknown actions. "init" is described below in
--leasefile-ro The "tftp" action is invoked when a TFTP
file transfer completes: the arguments are the file size in bytes, the
address to which the file was sent, and the complete pathname of the file.
The "arp-add" and "arp-del" actions are only called if
enabled with --script-arp They are are supplied with a MAC address
and IP address as arguments. "arp-add" indicates the arrival of
a new entry in the ARP or neighbour table, and "arp-del"
indicates the deletion of same.
- Specify a script written in Lua, to be run when leases are created,
destroyed or changed. To use this option, dnsmasq must be compiled with
the correct support. The Lua interpreter is initialised once, when dnsmasq
starts, so that global variables persist between lease events. The Lua
code must define a lease function, and may provide init and
shutdown functions, which are called, without arguments when
dnsmasq starts up and terminates. It may also provide a tftp
The lease function receives the information detailed in
--dhcp-script. It gets two arguments, firstly the action, which is
a string containing, "add", "old" or "del",
and secondly a table of tag value pairs. The tags mostly correspond to the
environment variables detailed above, for instance the tag
"domain" holds the same data as the environment variable
DNSMASQ_DOMAIN. There are a few extra tags which hold the data supplied as
arguments to --dhcp-script. These are mac_address,
ip_address and hostname for IPv4, and client_duid,
ip_address and hostname for IPv6.
The tftp function is called in the same way as the lease function,
and the table holds the tags destination_address, file_name
The arp and arp-old functions are called only when enabled
with --script-arp and have a table which holds the tags
mac_address and client_address.
- 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
- Enable the "arp" and "arp-old" functions in the
--dhcp-script and --dhcp-luascript.
- -9, --leasefile-ro
- Completely suppress use of the lease database file. The file will not be
created, read, or written. Change the way the lease-change script (if one
is provided) is called, so that the lease database may be maintained in
external storage by the script. In addition to the invocations given in
--dhcp-script the lease-change script is called once, at dnsmasq
startup, with the single argument "init". When called like this
the script should write the saved state of the lease database, in dnsmasq
leasefile format, to stdout and exit with zero exit code. Setting this
option also forces the leasechange script to be called on changes to the
client-id and lease length and expiry time.
- Treat DHCP (v4 and v6) requests and IPv6 Router Solicit packets arriving
at any of the <alias> interfaces as if they had arrived at
<interface>. This option allows dnsmasq to provide DHCP and RA
service over unaddressed and unbridged Ethernet interfaces, e.g. on an
OpenStack compute host where each such interface is a TAP interface to a
VM, or as in "old style bridging" on BSD platforms. A trailing
'*' wildcard can be used in each <alias>.
It is permissible to add more than one alias using more than one
--bridge-interface option since
--bridge-interface=int1,alias1,alias2 is exactly equivalent to
- -s, --domain=<domain>[,<address range>[,local]]
- Specifies DNS domains for the DHCP server. Domains may be be given
unconditionally (without the IP range) or for limited IP ranges. This has
two effects; firstly it causes the DHCP server to return the domain to any
hosts which request it, and secondly it sets the domain which it is legal
for DHCP-configured hosts to claim. The intention is to constrain
hostnames so that an untrusted host on the LAN cannot advertise its name
via dhcp as e.g. "microsoft.com" and capture traffic not meant
for it. If no domain suffix is specified, then any DHCP hostname with a
domain part (ie with a period) will be disallowed and logged. If suffix is
specified, then hostnames with a domain part are allowed, provided the
domain part matches the suffix. In addition, when a suffix is set then
hostnames without a domain part have the suffix added as an optional
domain part. Eg on my network I can set --domain=thekelleys.org.uk
and have a machine whose DHCP hostname is "laptop". The IP
address for that machine is available from dnsmasq both as
"laptop" and "laptop.thekelleys.org.uk". If the domain
is given as "#" then the domain is read from the first
"search" directive in /etc/resolv.conf (or equivalent).
The address range can be of the form <ip address>,<ip address>
or <ip address>/<netmask> or just a single <ip address>.
See --dhcp-fqdn which can change the behaviour of dnsmasq with
If the address range is given as ip-address/network-size, then a additional
flag "local" may be supplied which has the effect of adding
--local declarations for forward and reverse DNS queries. Eg.
--domain=thekelleys.org.uk,192.168.0.0/24,local is identical to
--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.
- In the default mode, dnsmasq inserts the unqualified names of DHCP clients
into the DNS. For this reason, the names must be unique, even if two
clients which have the same name are in different domains. If a second
DHCP client appears which has the same name as an existing client, the
name is transferred to the new client. If --dhcp-fqdn is set, this
behaviour changes: the unqualified name is no longer put in the DNS, only
the qualified name. Two DHCP clients with the same name may both keep the
name, provided that the domain part is different (ie the fully qualified
names differ.) To ensure that all names have a domain part, there must be
at least --domain without an address specified when
--dhcp-fqdn is set.
- Normally, when giving a DHCP lease, dnsmasq sets flags in the FQDN option
to tell the client not to attempt a DDNS update with its name and IP
address. This is because the name-IP pair is automatically added into
dnsmasq's DNS view. This flag suppresses that behaviour, this is useful,
for instance, to allow Windows clients to update Active Directory servers.
See RFC 4702 for details.
- Enable dnsmasq's IPv6 Router Advertisement feature. DHCPv6 doesn't handle
complete network configuration in the same way as DHCPv4. Router discovery
and (possibly) prefix discovery for autonomous address creation are
handled by a different protocol. When DHCP is in use, only a subset of
this is needed, and dnsmasq can handle it, using existing DHCP
configuration to provide most data. When RA is enabled, dnsmasq will
advertise a prefix for each --dhcp-range, with default router as
the relevant link-local address on the machine running dnsmasq. By
default, the "managed address" bits are set, and the "use
SLAAC" bit is reset. This can be changed for individual subnets with
the mode keywords described in --dhcp-range. RFC6106 DNS parameters
are included in the advertisements. By default, the relevant link-local
address of the machine running dnsmasq is sent as recursive DNS server. If
provided, the DHCPv6 options dns-server and domain-search are used for the
DNS server (RDNSS) and the domain search list (DNSSL).
- Set non-default values for router advertisements sent via an interface.
The priority field for the router may be altered from the default of
medium with eg --ra-param=eth0,high. The interval between router
advertisements may be set (in seconds) with --ra-param=eth0,60. The
lifetime of the route may be changed or set to zero, which allows a router
to advertise prefixes but not a route via itself.
--ra-parm=eth0,0,0 (A value of zero for the interval means the
default value.) All four parameters may be set at once.
The interface field may include a wildcard.
The mtu: parameter may be an arbitrary interface name, in which case the MTU
value for that interface is used. This is useful for (eg) advertising the
MTU of a WAN interface on the other interfaces of a router.
- Delays sending DHCPOFFER and proxydhcp replies for at least the specified
number of seconds. This can be used as workaround for bugs in PXE boot
firmware that does not function properly when receiving an instant reply.
This option takes into account the time already spent waiting (e.g.
performing ping check) if any.
- Enable the TFTP server function. This is deliberately limited to that
needed to net-boot a client. Only reading is allowed; the tsize and
blksize extensions are supported (tsize is only supported in octet mode).
Without an argument, the TFTP service is provided to the same set of
interfaces as DHCP service. If the list of interfaces is provided, that
defines which interfaces receive TFTP service.
- 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.
- Do not abort startup if specified tftp root directories are
- Add the IP or hardware address of the TFTP client as a path component on
the end of the TFTP-root. Only valid if a --tftp-root is set and
the directory exists. Defaults to adding IP address (in standard
dotted-quad format). For instance, if --tftp-root is
"/tftp" and client 188.8.131.52 requests file "myfile" then
the effective path will be "/tftp/184.108.40.206/myfile" if
/tftp/220.127.116.11 exists or /tftp/myfile otherwise. When "=mac" is
specified it will append the MAC address instead, using lowercase zero
padded digits separated by dashes, e.g.: 01-02-03-04-aa-bb Note that
resolving MAC addresses is only possible if the client is in the local
network or obtained a DHCP lease from us.
- Enable TFTP secure mode: without this, any file which is readable by the
dnsmasq process under normal unix access-control rules is available via
TFTP. When the --tftp-secure flag is given, only files owned by the
user running the dnsmasq process are accessible. If dnsmasq is being run
as root, different rules apply: --tftp-secure has no effect, but
only files which have the world-readable bit set are accessible. It is not
recommended to run dnsmasq as root with TFTP enabled, and certainly not
without specifying --tftp-root. Doing so can expose any
world-readable file on the server to any host on the net.
- Convert filenames in TFTP requests to all lowercase. This is useful for
requests from Windows machines, which have case-insensitive filesystems
and tend to play fast-and-loose with case in filenames. Note that
dnsmasq's tftp server always converts "\" to "/" in
- Set the maximum number of concurrent TFTP connections allowed. This
defaults to 50. When serving a large number of TFTP connections,
per-process file descriptor limits may be encountered. Dnsmasq needs one
file descriptor for each concurrent TFTP connection and one file
descriptor per unique file (plus a few others). So serving the same file
simultaneously to n clients will use require about n + 10 file
descriptors, serving different files simultaneously to n clients will
require about (2*n) + 10 descriptors. If --tftp-port-range is
given, that can affect the number of concurrent connections.
- --tftp-mtu=<mtu size>
- Use size as the ceiling of the MTU supported by the intervening network
when negotiating TFTP blocksize, overriding the MTU setting of the local
interface if it is larger.
- 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.
- A TFTP server listens on a well-known port (69) for connection initiation,
but it also uses a dynamically-allocated port for each connection.
Normally these are allocated by the OS, but this option specifies a range
of ports for use by TFTP transfers. This can be useful when TFTP has to
traverse a firewall. The start of the range cannot be lower than 1025
unless dnsmasq is running as root. The number of concurrent TFTP
connections is limited by the size of the port range.
- -C, --conf-file=<file>
- Specify a different configuration file. The --conf-file option is
also allowed in configuration files, to include multiple configuration
files. A filename of "-" causes dnsmasq to read configuration
- Read all the files in the given directory as configuration files. If
extension(s) are given, any files which end in those extensions are
skipped. Any files whose names end in ~ or start with . or start and end
with # are always skipped. If the extension starts with * then only files
which have that extension are loaded. So
--conf-dir=/path/to/dir,*.conf loads all files with the suffix
.conf in /path/to/dir. This flag may be given on the command line or in a
configuration file. If giving it on the command line, be sure to escape *
- A special case of --conf-file which differs in two respects.
Firstly, only --server and --rev-server are allowed in the
configuration file included. Secondly, the file is re-read and the
configuration therein is updated when dnsmasq receives SIGHUP.
At startup, dnsmasq reads /etc/dnsmasq.conf,
if it exists. (On FreeBSD,
the file is /usr/local/etc/dnsmasq.conf
) (but see the
options.) The format of this file
consists of one option per line, exactly as the long options detailed in the
OPTIONS section but without the leading "--". Lines starting with #
are comments and ignored. For options which may only be specified once, the
configuration file overrides the command line. Quoting is allowed in a config
file: between " quotes the special meanings of ,:. and # are removed and
the following escapes are allowed: \\ \" \t \e \b \r and \n. The later
corresponding to tab, escape, backspace, return and newline.
When it receives a SIGHUP, dnsmasq
clears its cache and then re-loads
and any file given by
. The dhcp
lease change script is called for all existing DHCP leases. If
is set SIGHUP also re-reads /etc/resolv.conf.
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 ( --log-queries
), 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
dig +short chaos txt cachesize.bind
When it receives SIGUSR2 and it is logging direct to a file (see
will close and reopen the log file.
Note that during this operation, dnsmasq will not be running as root. When it
first creates the logfile dnsmasq changes the ownership of the file to the
non-root user it will run as. Logrotate should be configured to create a new
log file with the ownership which matches the existing one before sending
SIGUSR2. If TCP DNS queries are in progress, the old logfile will remain open
in child processes which are handling TCP queries and may continue to be
written. There is a limit of 150 seconds, after which all existing TCP
processes will have expired: for this reason, it is not wise to configure
logfile compression for logfiles which have just been rotated. Using
logrotate, the required options are create
Dnsmasq is a DNS query forwarder: it is not capable of recursively answering
arbitrary queries starting from the root servers but forwards such queries to
a fully recursive upstream DNS server which is typically provided by an ISP.
By default, dnsmasq reads /etc/resolv.conf
to discover the IP addresses
of the upstream nameservers it should use, since the information is typically
stored there. Unless --no-poll
is used, dnsmasq
modification time of /etc/resolv.conf
(or equivalent if
is used) and re-reads it if it changes. This allows the
DNS servers to be set dynamically by PPP or DHCP since both protocols provide
the information. Absence of /etc/resolv.conf
is not an error since it
may not have been created before a PPP connection exists. Dnsmasq simply keeps
checking in case /etc/resolv.conf
is created at any time. Dnsmasq can
be told to parse more than one resolv.conf file. This is useful on a laptop,
where both PPP and DHCP may be used: dnsmasq can be set to poll both
and will use the
contents of whichever changed last, giving automatic switching between DNS
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
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
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
dnsmasq with the --resolv-file /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 18.104.22.168" in /etc/hosts
will ensure that queries for "mycompany.com" always return 22.214.171.124
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
The tag system works as follows: For each DHCP request, dnsmasq collects a set
of valid tags from active configuration lines which include set:<tag>,
including one from the --dhcp-range
used to allocate the address, one
from any matching --dhcp-host
(and "known" or
"known-othernet" if a --dhcp-host
matches) The tag
"bootp" is set for BOOTP requests, and a tag whose name is the name
of the interface on which the request arrived is also set.
Any configuration lines which include one or more tag:<tag> constructs
will only be valid if all that tags are matched in the set derived above.
Typically this is --dhcp-option
which has tags
will be used in preference to an untagged --dhcp-option,
_all_ the tags match somewhere in the set collected as described above. The
prefix '!' on a tag means 'not' so --dhcp-option=tag:!purple,3,126.96.36.199
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
class relative to other tags, to make it easy to override options for
individual hosts, so --dhcp-range=set:interface1,......
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
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,
"net:" may be used instead of "set:".) For the same
reason, '#' may be used instead of '!' to indicate NOT.
The DHCP server in dnsmasq will function as a BOOTP server also, provided that
the MAC address and IP address for clients are given, either using
configurations or in /etc/ethers
, and a
configuration option is present to activate the DHCP
server on a particular network. (Setting --bootp-dynamic
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.
Configuring dnsmasq to act as an authoritative DNS server is complicated by the
fact that it involves configuration of external DNS servers to provide
delegation. We will walk through three scenarios of increasing complexity.
Prerequisites for all of these scenarios are a globally accessible IP address,
an A or AAAA record pointing to that address, and an external DNS server
capable of doing delegation of the zone in question. For the first part of
this explanation, we will call the A (or AAAA) record for the globally
accessible address server.example.com, and the zone for which dnsmasq is
The simplest configuration consists of two lines of dnsmasq configuration;
and two records in the external DNS
server.example.com A 188.8.131.52
our.zone.com NS server.example.com
eth0 is the external network interface on which dnsmasq is listening, and has
(globally accessible) address 184.108.40.206.
Note that the external IP address may well be dynamic (ie assigned from an ISP
by DHCP or PPP) If so, the A record must be linked to this dynamic assignment
by one of the usual dynamic-DNS systems.
A more complex, but practically useful configuration has the address record for
the globally accessible IP address residing in the authoritative zone which
dnsmasq is serving, typically at the root. Now we have
our.zone.com A 220.127.116.11
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.
If the external address is dynamic, the address associated with our.zone.com
must be derived from the address of the relevant interface. This is done using
(The "eth0" argument in --auth-zone
adds the subnet containing
eth0's dynamic address to the zone, so that the --interface-name
returns the address in outside queries.)
Our final configuration builds on that above, but also adds a secondary DNS
server. This is another DNS server which learns the DNS data for the zone by
doing zones transfer, and acts as a backup should the primary server become
inaccessible. The configuration of the secondary is beyond the scope of this
man-page, but the extra configuration of dnsmasq is simple:
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
--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
reverse (address to name) lookups can be simply configured with a suitable NS
record, for instance in this example, where we allow 18.104.22.168/24 addresses.
3.2.1.in-addr.arpa NS our.zone.com
Note that at present, reverse (in-addr.arpa and ip6.arpa) zones are not
available in zone transfers, so there is no point arranging secondary servers
for reverse lookups.
When dnsmasq is configured to act as an authoritative server, the following data
is used to populate the authoritative zone.
--mx-host, --srv-host, --dns-rr, --txt-record, --naptr-record,
as long as the record names are in the authoritative domain.
as long as the record name is in the authoritative domain. If the
target of the CNAME is unqualified, then it is qualified with the
authoritative zone name. CNAME used in this way (only) may be wildcards, as in
IPv4 and IPv6 addresses from /etc/hosts (and --addn-hosts
provided the address falls
into one of the subnets specified in the --auth-zone.
Addresses of DHCP leases, provided the address falls into one of the subnets
specified in the --auth-zone.
(If constructed DHCP ranges are is use,
which depend on the address dynamically assigned to an interface, then the
form of --auth-zone
which defines subnets by the dynamic address of an
interface should be used to ensure this condition is met.)
In the default mode, where a DHCP lease has an unqualified name, and possibly a
qualified name constructed using --domain
then the name in the
authoritative zone is constructed from the unqualified name and the zone's
domain. This may or may not equal that specified by --domain.
is set, then the fully qualified names associated with DHCP
leases are used, and must match the zone's domain.
0 - Dnsmasq successfully forked into the background, or terminated normally if
backgrounding is not enabled.
1 - A problem with configuration was detected.
2 - A problem with network access occurred (address in use, attempt to use
privileged ports without permission).
3 - A problem occurred with a filesystem operation (missing file/directory,
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.
The default values for resource limits in dnsmasq are generally conservative,
and appropriate for embedded router type devices with slow processors and
limited memory. On more capable hardware, it is possible to increase the
limits, and handle many more clients. The following applies to dnsmasq-2.37:
earlier versions did not scale as well.
Dnsmasq is capable of handling DNS and DHCP for at least a thousand clients. The
DHCP lease times should not be very short (less than one hour). The value of
can be increased: start with it equal to the number
of clients and increase if DNS seems slow. Note that DNS performance depends
too on the performance of the upstream nameservers. The size of the DNS cache
may be increased: the hard limit is 10000 names and the default (150) is very
low. Sending SIGUSR1 to dnsmasq makes it log information which is useful for
tuning the cache size. See the NOTES
section for details.
The built-in TFTP server is capable of many simultaneous file transfers: the
absolute limit is related to the number of file-handles allowed to a process
and the ability of the select() system call to cope with large numbers of file
handles. If the limit is set too high using --tftp-max
it will be
scaled down and the actual limit logged at start-up. Note that more transfers
are possible when the same file is being sent than when each transfer sends a
It is possible to use dnsmasq to block Web advertising by using a list of known
banner-ad servers, all resolving to 127.0.0.1 or 0.0.0.0, in /etc/hosts
or an additional hosts file. The list can be very long, dnsmasq has been
tested successfully with one million names. That size file needs a 1GHz
processor and about 60Mb of RAM.
Dnsmasq can be compiled to support internationalisation. To do this, the make
targets "all-i18n" and "install-i18n" should be used
instead of the standard targets "all" and "install". When
internationalisation is compiled in, dnsmasq will produce log messages in the
local language and support internationalised domain names (IDN). Domain names
in /etc/hosts, /etc/ethers and /etc/dnsmasq.conf which contain non-ASCII
characters will be translated to the DNS-internal punycode representation.
Note that dnsmasq determines both the language for messages and the assumed
charset for configuration files from the LANG environment variable. This
should be set to the system default value by the script which is responsible
for starting dnsmasq. When editing the configuration files, be careful to do
so using only the system-default locale and not user-specific one, since
dnsmasq has no direct way of determining the charset in use, and must assume
that it is the system default.
This manual page was written by Simon Kelley <email@example.com>.