Section: Maintenance Commands (8)
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.
will display known DHCPv4 configuration options, and
will display DHCPv6 options.
- -h, --no-hosts
Don't read the hostnames in /etc/hosts.
- -H, --addn-hosts=<file>
Additional hosts file. Read the specified file as well as /etc/hosts. If --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 some circumstances.
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 record.
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
- -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
- -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
then the log has extra information at the start of each line.
This consists of a serial number which ties together the log lines associated with an individual query, and the IP address of the requestor.
- -8, --log-facility=<facility>
Set the facility to which dnsmasq will send syslog entries, this
defaults to DAEMON, and to LOCAL0 when debug mode is in operation. If
the facility given contains at least one '/' character, it is taken to
be a filename, and dnsmasq logs to the given file, instead of
syslog. If the facility is '-' then dnsmasq logs to stderr.
(Errors whilst reading configuration will still go to syslog,
but all output from a successful startup, and all output whilst
running, will go exclusively to the file.) When logging to a file,
dnsmasq will close and reopen the file when it receives SIGUSR2. This
allows the log file to be rotated without stopping dnsmasq.
Enable extra logging intended for debugging rather than information.
Enable asynchronous logging and optionally set the limit on the
number of lines
which will be queued by dnsmasq when writing to the syslog is slow.
Dnsmasq can log asynchronously: this
allows it to continue functioning without being blocked by syslog, and
allows syslog to use dnsmasq for DNS queries without risking deadlock.
If the queue of log-lines becomes full, dnsmasq will log the
overflow, and the number of messages lost. The default queue length is
5, a sane value would be 5-25, and a maximum limit of 100 is imposed.
- -x, --pid-file=<path>
Specify an alternate path for dnsmasq to record its process-id in. Normally /var/run/dnsmasq.pid.
- -u, --user=<username>
Specify the userid to which dnsmasq will change after startup. Dnsmasq must normally be started as root, but it will drop root
privileges after startup by changing id to another user. Normally this user is "nobody" but that
can be over-ridden with this switch.
- -g, --group=<groupname>
Specify the group which dnsmasq will run
as. The 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 TFTP.
- -P, --edns-packet-max=<size>
Specify the largest EDNS.0 UDP packet which is supported by the DNS
forwarder. Defaults to 4096, which is the RFC5625-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 be larger
than that specified. Useful for systems behind firewalls. If not specified,
defaults to 1024.
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
option is used. If no
options are given dnsmasq listens on all available interfaces except any
options. On Linux, when
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
A simple wildcard, consisting of a trailing '*',
can be used in
- -I, --except-interface=<interface name>
Do not listen on the specified interface. Note that the order of
options does not matter and that
options always override the others. The comments about interface labels for
Enable DNS authoritative mode for queries arriving at an interface or address. Note that the interface or address
need not be mentioned in
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 primary external nameserver.
Accept DNS queries only from hosts whose address is on a local subnet,
ie a subnet for which an interface exists on the server. This option
only has effect if there are no --interface, --except-interface,
--listen-address or --auth-server options. It is intended to be set as
a default on installation, to allow unconfigured installations to be
useful but also safe from being used for DNS amplification attacks.
- -2, --no-dhcp-interface=<interface name>
Do not provide DHCP or TFTP on the specified interface, but do provide DNS service.
- -a, --listen-address=<ipaddr>
Listen on the given IP address(es). Both
options may be given, in which case the set of both interfaces and
addresses is used. Note that if no
option is given, but
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
- -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
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 and --dynamic-host which depend on the interface over which the query was
received. If a name has more than one address associated with
it, and at least one of those addresses is on the same subnet as the
interface to which the query was sent, then return only the
address(es) on that subnet. This allows for a server to have multiple
addresses in /etc/hosts corresponding to each of its interfaces, and
hosts will get the correct address based on which network they are
attached to. Currently this facility is limited to IPv4.
- -b, --bogus-priv
Bogus private reverse lookups. All reverse lookups for private IP ranges (ie 192.168.x.x, etc)
which are not found in /etc/hosts or the DHCP leases file are answered
with "no such domain" rather than being forwarded upstream. The
set of prefixes affected is the list given in RFC6303, for IPv4 and IPv6.
- -V, --alias=[<old-ip>]|[<start-ip>-<end-ip>],<new-ip>[,<mask>]
Modify IPv4 addresses returned from upstream nameservers; old-ip is
replaced by new-ip. If the optional mask is given then any address
which matches the masked old-ip will be re-written. So, for instance
will map 188.8.131.52 to 184.108.40.206 and 220.127.116.11 to 18.104.22.168. 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
maps 192.168.0.10->192.168.0.40 to 10.0.0.10->10.0.0.40
- -B, --bogus-nxdomain=<ipaddr>[/prefix]
Transform replies which contain the IP specified address or subnet 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 22.214.171.124
Ignore replies to A-record queries which include the specified address or subnet.
No error is generated, dnsmasq simply continues to listen for another reply.
This is useful to defeat blocking strategies which rely on quickly supplying a
forged answer to a DNS request for certain domain, before the correct answer can arrive.
- -f, --filterwin2k
Later versions of windows make periodic DNS requests which don't get sensible answers from
the public DNS and can cause problems by triggering dial-on-demand links. This flag turns on an option
to filter such requests. The requests blocked are for records of types SOA and SRV, and type ANY where the
requested name has underscores, to catch LDAP requests.
- -r, --resolv-file=<file>
Read the IP addresses of the upstream nameservers from <file>, instead of
/etc/resolv.conf. For the format of this file see
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. If the service
name is given, dnsmasq provides service at that namespace, rather than
the default which is
- -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 previously disabled.
Reject (and log) addresses from upstream nameservers which are in the
private ranges. This blocks an attack where a browser behind a
firewall is used to probe machines on the local network. For IPv6, the
private range covers the IPv4-mapped addresses in private space plus
all link-local (LL) and site-local (ULA) addresses.
Exempt 127.0.0.0/8 and ::1 from rebinding checks. This address range is
returned by realtime black hole servers, so blocking it may disable
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 returned.
- -S, --local, --server=[/[<domain>]/[domain/]][<ipaddr>[#<port>]][@<interface>][@<source-ip>[#<port>]]
Specify IP address of upstream servers directly. Setting this flag does
not suppress reading of /etc/resolv.conf, use --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
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
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:
will send queries for *.google.com to 126.96.36.199, except *www.google.com,
which will go to 188.8.131.52
The special server address '#' means, "use the standard servers", so
will send queries for *.google.com to 184.108.40.206, 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
is a synonym for
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
but provides some syntactic sugar to make specifying address-to-name queries easier. For example
is exactly equivalent to
- -A, --address=/<domain>[/<domain>...]/[<ipaddr>]
Specify an IP address to return for any host in the given domains.
Queries in the domains are never forwarded and always replied to
with the specified IP address which may be IPv4 or IPv6. To give
both IPv4 and IPv6 addresses for a domain, use repeated --address flags.
To include multiple IP addresses for a single query, use
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 separate configuration lines. Note that NULL addresses normally work in the same way as localhost, so beware that clients looking up these names are likely to end up talking to themselves.
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
These IP sets must already exist. See
for more details.
- -m, --mx-host=<mx name>[[,<hostname>],<preference>]
Return an MX record named <mx name> pointing to the given hostname (if
the host specified in the --mx-target switch
or, if that switch is not given, the host on which dnsmasq
is running. The default is useful for directing mail from systems on a LAN
to a central server. The preference value is optional, and defaults to
1 if not given. More than one MX record may be given for a host.
- -t, --mx-target=<hostname>
Specify the default target for the MX record returned by dnsmasq. See
--mx-host. If --mx-target is given, but not --mx-host, then dnsmasq
returns a MX record containing the MX target for MX queries on the
hostname of the machine on which dnsmasq is running.
- -e, --selfmx
Return an MX record pointing to itself for each local
machine. Local machines are those in /etc/hosts or with DHCP leases.
- -L, --localmx
Return an MX record pointing to the host given by --mx-target (or the
machine on which dnsmasq is running) for each
local machine. Local machines are those in /etc/hosts or with DHCP
- -W, --srv-host=<_service>.<_prot>.[<domain>],[<target>[,<port>[,<priority>[,<weight>]]]]
Return a SRV DNS record. See RFC2782 for details. If not supplied, the
domain defaults to that given by
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
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.
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
is in effect. Short and long names may appear in the same
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.
Add A, AAAA and PTR records to the DNS in the same subnet as the specified interface. The address is derived from the network part of each address associated with the interface, and the host part from the specified address. For example
will, when eth0 has the address 192.168.78.x and netmask 255.255.255.0 give the
name example.com an A record for 192.168.78.8. The same principle applies to IPv6 addresses. Note that if an interface has more than one address, more than one A or AAAA record will be created. The TTL of the records is always zero, and any changes to interface addresses will be immediately reflected in them.
- -Y, --txt-record=<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 is a significant limitation on the target; it must be a
DNS record which is known to dnsmasq and NOT a DNS record which comes from
an upstream server. The cname must be unique, but it
is permissible to have more than one cname pointing to the same target. Indeed
it's possible to declare multiple cnames to a target in a single line, like so:
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.
- --synth-domain=<domain>,<address range>[,<prefix>[*]]
Create artificial A/AAAA and PTR records for an address range. The
records either seqential numbers or the address, with periods (or colons for IPv6) replaced with dashes.
An examples should make this clearer. First sequential numbers.
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.
--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 validation.
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 DNS queries which are
- --add-subnet[[=[<IPv4 address>/]<IPv4 prefix length>][,[<IPv6 address>/]<IPv6 prefix length>]]
Add a subnet address to the DNS queries which are forwarded
upstream. If an address is specified in the flag, it will be used,
otherwise, the address of the requestor will be used. The amount of
the address forwarded depends on the prefix length parameter: 32 (128
for IPv6) forwards the whole address, zero forwards none of it but
still marks the request so that no upstream nameserver will add client
address information either. The default is zero for both IPv4 and
IPv6. Note that upstream nameservers may be configured to return
different results based on this information, but the dnsmasq cache
does not take account. Caching is therefore disabled for such replies,
unless the subnet address being added is constant.
will add the /24 and /96 subnets of the requestor for IPv4 and IPv6 requestors, respectively.
will add 126.96.36.199/24 for IPv4 requestors and ::/0 for IPv6 requestors.
will add 188.8.131.52/24 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 performance.
- -N, --no-negcache
Disable negative caching. Negative caching allows dnsmasq to remember
"no such domain" answers from upstream nameservers and answer
identical queries without forwarding them again.
- -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
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 https://data.iana.org/root-anchors/root-anchors.xml
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
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
Versions of dnsmasq prior to 2.80 defaulted to not checking unsigned replies, and used
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 occurring.
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. This is an
alternative to having dnsmasq validate DNSSEC, but it depends on the security of the network between
dnsmasq and the upstream servers, and the trustworthiness of the upstream servers. Note that caching the
Authenticated Data bit correctly in all cases is not technically possible. If the AD bit is to be relied upon
when using this option, then the cache should be disabled using --cache-size=0. In most cases, enabling DNSSEC validation
within dnsmasq is a better option. See --dnssec for details.
Set debugging mode for the DNSSEC validation, set the Checking Disabled bit on upstream queries,
and don't convert replies which do not validate to responses with
a return code of SERVFAIL. Note that
setting this may affect DNS behaviour in bad ways, it is not an
extra-logging flag and should not be set in production.
- --auth-zone=<domain>[,<subnet>[/<prefix length>][,<subnet>[/<prefix length>].....][,exclude:<subnet>[/<prefix length>]].....]
Define a DNS zone for which dnsmasq acts as authoritative server. Locally defined DNS records which are in the domain
will be served. If subnet(s) are given, A and AAAA records must be in one of the
As alternative to directly specifying the subnets, it's possible to
give the name of an interface, in which case the subnets implied by
that interface's configured addresses and netmask/prefix-length are
used; this is useful when using constructed DHCP ranges as the actual
address is dynamic and not known when configuring dnsmasq. The
interface addresses may be confined to only IPv6 addresses using
<interface>/6 or to only IPv4 using <interface>/4. This is useful when
an interface has dynamically determined global IPv6 addresses which should
appear in the zone, but RFC1918 IPv4 addresses which should not.
Interface-name and address-literal subnet specifications may be used
freely in the same --auth-zone declaration.
It's possible to exclude certain IP addresses from responses. It can be
used, to make sure that answers contain only global routeable IP
addresses (by excluding loopback, RFC1918 and ULA addresses).
The subnet(s) are also used to define in-addr.arpa and
ip6.arpa domains which are served for reverse-DNS queries. If not
specified, the prefix length defaults to 24 for IPv4 and 64 for IPv6.
For IPv4 subnets, the prefix length should be have the value 8, 16 or 24
unless you are familiar with RFC 2317 and have arranged the
in-addr.arpa delegation accordingly. Note that if no subnets are
specified, then no reverse queries are answered.
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
enables zone transfer but does not advertise the secondary in NS records returned by dnsmasq.
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
- -F, --dhcp-range=[tag:<tag>[,tag:<tag>],][set:<tag>,]<start-addr>[,<end-addr>|<mode>][,<netmask>[,<broadcast>]][,<lease time>]
- -F, --dhcp-range=[tag:<tag>[,tag:<tag>],][set:<tag>,]<start-IPv6addr>[,<end-IPv6addr>|constructor:<interface>][,<mode>][,<prefix-len>][,<lease time>]
Enable the DHCP server. Addresses will be given out from the range
<start-addr> to <end-addr> and from statically defined addresses given
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 for IPv4 and one day for IPv6. The
minimum lease time is two minutes. For IPv6 ranges, the lease time
maybe "deprecated"; this sets the preferred lifetime sent in a DHCP
lease or router advertisement to zero, which causes clients to use
other addresses, if available, for new connections as a prelude to renumbering.
This option may be repeated, with different addresses, to enable DHCP
service to more than one network. For directly connected networks (ie,
networks on which the machine running dnsmasq has an interface) the
netmask is optional: dnsmasq will determine it from the interface
configuration. For networks which receive DHCP service via a relay
agent, dnsmasq cannot determine the netmask itself, so it should be
specified, otherwise dnsmasq will have to guess, based on the class (A, B or
C) of the network address. The broadcast address is
always optional. It is always
allowed to have more than one --dhcp-range in a single subnet.
For IPv6, the parameters are slightly different: instead of netmask
and broadcast address, there is an optional prefix length which must
be equal to or larger then the prefix length on the local interface. If not
given, this defaults to 64. Unlike the IPv4 case, the prefix length is not
automatically derived from the interface configuration. The minimum
size of the prefix length is 64.
IPv6 (only) supports another type of range. In this, the start address and optional end address contain only the network part (ie ::1) and they are followed by
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 ::
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
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
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
For IPv4, the <mode> may be
in which case dnsmasq will provide proxy-DHCP on the specified
For IPv6, the mode may be some combination of
ra-only, slaac, ra-names, ra-stateless, ra-advrouter, off-link.
tells dnsmasq to offer Router Advertisement only on this subnet,
and not DHCP.
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
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.
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.
can be combined with
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.
tells dnsmasq to advertise the prefix without the on-link (aka L) bit set.
- -G, --dhcp-host=[<hwaddr>][,id:<client_id>|*][,set:<tag>][tag:<tag>][,<ipaddr>][,<hostname>][,<lease_time>][,ignore]
Specify per host parameters for the DHCP server. This allows a machine
with a particular hardware address to be always allocated the same
hostname, IP address and lease time. A hostname specified like this
overrides any supplied by the DHCP client on the machine. It is also
allowable to omit the hardware address and include the hostname, in
which case the IP address and lease times will apply to any machine
claiming that name. For example
tells dnsmasq to give
the machine with hardware address 00:20:e0:3b:13:af the name wap, and
an infinite DHCP lease.
dnsmasq to always allocate the machine lap the IP address
Addresses allocated like this are not constrained to be
in the range given by the --dhcp-range option, but they must be in
the same subnet as some valid dhcp-range. For
subnets which don't need a pool of dynamically allocated addresses,
use the "static" keyword in the --dhcp-range declaration.
It is allowed to use client identifiers (called client
DUID in IPv6-land) rather than
hardware addresses to identify hosts by prefixing with 'id:'. Thus:
refers to the host with client identifier 01:02:03:04. It is also
allowed to specify the client ID as text, like this:
may contain an IPv4 address or one or more IPv6 addresses, or both. IPv6 addresses must be bracketed by square brackets thus:
IPv6 addresses may contain only the host-identifier part:
in which case they act as wildcards in constructed DHCP ranges, with
the appropriate network part inserted. For IPv6, an address may include a prefix length:
which (in this case) specifies four addresses, 1234::50 to 1234::53. This (an the ability
to specify multiple addresses) is useful
when a host presents either a consistent name or hardware-ID, but varying DUIDs, since it allows
dnsmasq to honour the static address allocation but assign a different adddress for each DUID. This
typically occurs when chain netbooting, as each stage of the chain gets in turn allocates an address.
Note that in IPv6 DHCP, the hardware address may not be
available, though it normally is for direct-connected clients, or
clients using DHCP relays which support RFC 6939.
For DHCPv4, the special option id:* means "ignore any client-id
and use MAC addresses only." This is useful when a client presents a client-id sometimes
but not others.
If a name appears in /etc/hosts, the associated address can be
allocated to a DHCP lease, but only if a
option specifying the name also exists. Only one hostname can be
given in a
option, but aliases are possible by using CNAMEs. (See
More than one
can be associated (by name, hardware address or UID) with a host. Which one is used
(and therefore which address is allocated by DHCP and appears in the DNS) depends
on the subnet on which the host last obtained a DHCP lease:
with an address within the subnet is used. If more than one address is within the subnet,
the result is undefined. A corollary to this is that the name associated with a host using
does not appear in the DNS until the host obtains a DHCP lease.
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
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
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.
The tag:<tag> construct filters which dhcp-host directives are used. Tagged directives are used in preference to untagged ones.
Ethernet addresses (but not client-ids) may have
wildcard bytes, so for example
will cause dnsmasq to ignore a range of hardware addresses. Note that
the "*" will need to be escaped or quoted on a command line, but not
in the configuration file.
Hardware addresses normally match any
network (ARP) type, but it is possible to restrict them to a single
ARP type by preceding them with the ARP-type (in HEX) and "-". so
will only match a
Token-Ring hardware address, since the ARP-address type for token ring
As a special case, in DHCPv4, it is possible to include more than one
hardware address. eg:
This allows an IP address to be associated with
multiple hardware addresses, and gives dnsmasq permission to abandon a
DHCP lease to one of the hardware addresses when another one asks for
a lease. Beware that this is a dangerous thing to do, it will only
work reliably if only one of the hardware addresses is active at any
time and there is no way for dnsmasq to enforce this. It is, for instance,
useful to allocate a stable IP address to a laptop which
has both wired and wireless interfaces.
Read DHCP host information from the specified file. If a directory
is given, then read all the files contained in that directory. The file contains
information about one host per line. The format of a line is the same
as text to the right of '=' in --dhcp-host. The advantage of storing DHCP host information
in this file is that it can be changed without re-starting dnsmasq:
the file will be re-read when dnsmasq receives SIGHUP.
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
flag as DHCP options, using the options names bootfile-name,
server-ip-address and tftp-server. This allows these to be included
in a --dhcp-optsfile.
This is equivalent to --dhcp-hostsfile, except for the following. The path MUST be a
directory, and not an individual file. Changed or new files within
the directory are read automatically, without the need to send SIGHUP.
If a file is deleted or changed after it has been read by dnsmasq, then the
host record it contained will remain until dnsmasq receives a SIGHUP, or
is restarted; ie host records are only added dynamically.
This is equivalent to --dhcp-optsfile, with the differences noted for --dhcp-hostsdir.
- -Z, --read-ethers
Read /etc/ethers for information about hosts for the DHCP server. The
format of /etc/ethers is a hardware address, followed by either a
hostname or dotted-quad IP address. When read by dnsmasq these lines
have exactly the same effect as
options containing the same information. /etc/ethers is re-read when
dnsmasq receives SIGHUP. IPv6 addresses are NOT read from /etc/ethers.
- -O, --dhcp-option=[tag:<tag>,[tag:<tag>,]][encap:<opt>,][vi-encap:<enterprise>,][vendor:[<vendor-class>],][<opt>|option:<opt-name>|option6:<opt>|option6:<opt-name>],[<value>[,<value>]]
Specify different or extra options to DHCP clients. By default,
dnsmasq sends some standard options to DHCP clients, the netmask and
broadcast address are set to the same as the host running dnsmasq, and
the DNS server and default route are set to the address of the machine
running dnsmasq. (Equivalent rules apply for IPv6.) If the domain name option has been set, that is sent.
This configuration allows these defaults to be overridden,
or other options specified. The option, to be sent may be given as a
decimal number or as "option:<option-name>" The option numbers are
specified in RFC2132 and subsequent RFCs. The set of option-names
known by dnsmasq can be discovered by running "dnsmasq --help dhcp".
For example, to set the default route option to
--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
--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
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.
For IPv6, [::] means "the global address of
the machine running dnsmasq", whilst [fd00::] is replaced with the
ULA, if it exists, and [fe80::] with the link-local address.
Be careful: no checking is done that the correct type of data for the
option number is sent, it is quite possible to
persuade dnsmasq to generate illegal DHCP packets with injudicious use
of this flag. When the value is a decimal number, dnsmasq must determine how
large the data item is. It does this by examining the option number and/or the
value, but can be overridden by appending a single letter flag as follows:
b = one byte, s = two bytes, i = four bytes. This is mainly useful with
encapsulated vendor class options (see below) where dnsmasq cannot
determine data size from the option number. Option data which
consists solely of periods and digits will be interpreted by dnsmasq
as an IP address, and inserted into an option as such. To force a
literal string, use quotes. For instance when using option 66 to send
a literal IP address as TFTP server name, it is necessary to do
Encapsulated Vendor-class options may also be specified (IPv4 only) using
--dhcp-option: for instance
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;
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
The address 0.0.0.0 is not treated specially in
This works in exactly the same way as
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 case.
- --dhcp-relay=<local address>,<server address>[,<interface]
Configure dnsmasq to do DHCP relay. The local address is an address
allocated to an interface on the host running dnsmasq. All DHCP
requests arriving on that interface will we relayed to a remote DHCP
server at the server address. It is possible to relay from a single local
address to multiple remote servers by using multiple --dhcp-relay
configs with the same local address and different server
addresses. A server address must be an IP literal address, not a
domain name. In the case of DHCPv6, the server address may be the
ALL_SERVERS multicast address, ff05::1:3. In this case the interface
must be given, not be wildcard, and is used to direct the multicast to the
correct interface to reach the DHCP server.
Access control for DHCP clients has the same rules as for the DHCP
server, see --interface, --except-interface, etc. The optional
interface name in the --dhcp-relay config has a different function: it
controls on which interface DHCP replies from the server will be
accepted. This is intended for configurations which have three
interfaces: one being relayed from, a second connecting the DHCP
server, and a third untrusted network, typically the wider
internet. It avoids the possibility of spoof replies arriving via this
It is allowed to have dnsmasq act as a DHCP server on one set of
interfaces and relay from a disjoint set of interfaces. Note that
whilst it is quite possible to write configurations which appear to
act as a server and a relay on the same interface, this is not
supported: the relay function will take precedence.
Both DHCPv4 and DHCPv6 relay is supported. It's not possible to relay
DHCPv4 to a DHCPv6 server or vice-versa.
- -U, --dhcp-vendorclass=set:<tag>,[enterprise:<IANA-enterprise number>,]<vendor-class>
Map from a vendor-class string to a tag. Most DHCP clients provide a
"vendor class" which represents, in some sense, the type of host. This option
maps vendor classes to tags, so that DHCP options may be selectively delivered
to different classes of hosts. For example
will allow options to be set only for HP printers like so:
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 searched.
- -j, --dhcp-userclass=set:<tag>,<user-class>
Map from a user-class string to a tag (with substring
matching, like vendor classes). Most DHCP clients provide a
"user class" which is configurable. This option
maps user classes to tags, so that DHCP options may be selectively delivered
to different classes of hosts. It is possible, for instance to use
this to set a different printer server for hosts in the class
"accounts" than for hosts in the class "engineering".
- -4, --dhcp-mac=set:<tag>,<MAC address>
Map from a MAC address to a tag. The MAC address may include
wildcards. For example
will set the tag "3com" for any host whose MAC address matches the pattern.
- --dhcp-circuitid=set:<tag>,<circuit-id>, --dhcp-remoteid=set:<tag>,<remote-id>
Map from RFC3046 relay agent options to tags. This data may
be provided by DHCP relay agents. The circuit-id or remote-id is
normally given as colon-separated hex, but is also allowed to be a
simple string. If an exact match is achieved between the circuit or
agent ID and one provided by a relay agent, the tag is set.
(but not --dhcp-circuitid) is supported in IPv6.
(IPv4 and IPv6) Map from RFC3993 subscriber-id relay agent options to tags.
- --dhcp-proxy[=<ip addr>]......
(IPv4 only) A normal DHCP relay agent is only used to forward the initial parts of
a DHCP interaction to the DHCP server. Once a client is configured, it
communicates directly with the server. This is undesirable if the
relay agent is adding extra information to the DHCP packets, such as
that used by
A full relay implementation can use the RFC 5107 serverid-override
option to force the DHCP server to use the relay as a full proxy, with all
packets passing through it. This flag provides an alternative method
of doing the same thing, for relays which don't support RFC
5107. Given alone, it manipulates the server-id for all interactions
via relays. If a list of IP addresses is given, only interactions via
relays at those addresses are affected.
- --dhcp-match=set:<tag>,<option number>|option:<option name>|vi-encap:<enterprise>[,<value>]
Without a value, set the tag if the client sends a DHCP
option of the given number or name. When a value is given, set the tag only if
the option is sent and matches the value. The value may be of the form
"01:ff:*:02" in which case the value must match (apart from wildcards)
but the option sent may have unmatched data past the end of the
value. The value may also be of the same form as in
in which case the option sent is treated as an array, and one element
must match, so
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 client.
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
the line which sets the tag must precede the one which tests it.
- -J, --dhcp-ignore=tag:<tag>[,tag:<tag>]
When all the given tags appear in the tag set ignore the host and do
not allocate it a DHCP lease.
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
(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,
(IPv4 only) When all the given tags appear in the tag set, always use broadcast to
communicate with the host when it is unconfigured. It is permissible
to supply no tags, in which case this is unconditional. Most DHCP clients which
need broadcast replies set a flag in their requests so that this
happens automatically, some old BOOTP clients do not.
- -M, --dhcp-boot=[tag:<tag>,]<filename>,[<servername>[,<server address>|<tftp_servername>]]
(IPv4 only) Set BOOTP options to be returned by the DHCP server. Server name and
address are optional: if not provided, the name is left empty, and the
address set to the address of the machine running dnsmasq. If dnsmasq
is providing a TFTP service (see
) then only the filename is required here to enable network booting.
If the optional tag(s) are given,
they must match for this configuration to be sent.
Instead of an IP address, the TFTP server address can be given as a domain
name which is looked up in /etc/hosts. This name can be associated in
/etc/hosts with multiple IP addresses, which are used round-robin.
This facility can be used to load balance the tftp load among a set of servers.
Dnsmasq is designed to choose IP addresses for DHCP clients using a
hash of the client's MAC address. This normally allows a client's
address to remain stable long-term, even if the client sometimes allows its DHCP
lease to expire. In this default mode IP addresses are distributed
pseudo-randomly over the entire available address range. There are
sometimes circumstances (typically server deployment) where it is more
convenient to have IP
addresses allocated sequentially, starting from the lowest available
address, and setting this flag enables this mode. Note that in the
sequential mode, clients which allow a lease to expire are much more
likely to move IP address; for this reason it should not be generally used.
Dnsmasq is reading 'client identifier' (RFC 2131) option sent by clients
(if available) to identify clients. This allow to serve same IP address
for a host using several interfaces. Use this option to disable 'client identifier'
reading, i.e. to always identify a host using the MAC address.
- --pxe-service=[tag:<tag>,]<CSA>,<menu text>[,<basename>|<bootservicetype>][,<server address>|<server_name>]
Most uses of PXE boot-ROMS simply allow the PXE
system to obtain an IP address and then download the file specified by
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 (
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
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
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
to allow netbooting. This mode is enabled using the
According to UEFI and PXE specifications, DHCP packets between PXE clients and
proxy PXE servers should have
in their vendor-class field. However, the firmware of computers from a few
vendors is customized to carry a different identifier in that field. This option
is used to consider such identifiers valid for identifying PXE clients. For
will enable dnsmasq to also provide proxy PXE service to those PXE clients with
in as their identifier.
>>>>>>> 907def3... pxe: support pxe clients with custom vendor-class
- -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
- -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 port>]]
(IPv4 only) Change the ports used for DHCP from the default. If this option is
given alone, without arguments, it changes the ports used for DHCP
from 67 and 68 to 1067 and 1068. If a single argument is given, that
port number is used for the server and the port number plus one used
for the client. Finally, two port numbers allows arbitrary
specification of both server and client ports for DHCP.
- -3, --bootp-dynamic[=<network-id>[,<network-id>]]
(IPv4 only) Enable dynamic allocation of IP addresses to BOOTP clients. Use this
with care, since each address allocated to a BOOTP client is leased
forever, and therefore becomes permanently unavailable for re-use by
other hosts. if this is given without tags, then it unconditionally
enables dynamic allocation. With tags, only when the tags are all
set. It may be repeated with different tag sets.
- -5, --no-ping
(IPv4 only) By default, the DHCP server will attempt to ensure that an address is
not in use before allocating it to a host. It does this by sending an
ICMP echo request (aka "ping") to the address in question. If it gets
a reply, then the address must already be in use, and another is
tried. This flag disables this check. Use with caution.
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
and lease expiry if --script-on-renewal is set).
If the MAC address is from a network type other than ethernet,
it will have the network type prepended, eg "06-01:23:45:67:89:ab" for
token ring. The process is run as root (assuming that dnsmasq was originally run as
root) even if dnsmasq is configured to change UID to an unprivileged user.
The environment is inherited from the invoker of dnsmasq, with some or
all of the following variables added
For both IPv4 and IPv6:
DNSMASQ_DOMAIN if the fully-qualified domain name of the host is
known, this is set to the domain part. (Note that the hostname passed
to the script as an argument is never fully-qualified.)
If the client provides a hostname, DNSMASQ_SUPPLIED_HOSTNAME
If the client provides user-classes, DNSMASQ_USER_CLASS0..DNSMASQ_USER_CLASSn
If dnsmasq was compiled with HAVE_BROKEN_RTC, then
the length of the lease (in seconds) is stored in
DNSMASQ_LEASE_LENGTH, otherwise the time of lease expiry is stored in
DNSMASQ_LEASE_EXPIRES. The number of seconds until lease expiry is
always stored in DNSMASQ_TIME_REMAINING.
If a lease used to have a hostname, which is
removed, an "old" event is generated with the new state of the lease,
ie no name, and the former name is provided in the environment
DNSMASQ_INTERFACE stores the name of
the interface on which the request arrived; this is not set for "old"
actions when dnsmasq restarts.
DNSMASQ_RELAY_ADDRESS is set if the client
used a DHCP relay to contact dnsmasq and the IP address of the relay
DNSMASQ_TAGS contains all the tags set during the
DHCP transaction, separated by spaces.
DNSMASQ_LOG_DHCP is set if
is in effect.
For IPv4 only:
DNSMASQ_CLIENT_ID if the host provided a client-id.
DNSMASQ_CIRCUIT_ID, DNSMASQ_SUBSCRIBER_ID, DNSMASQ_REMOTE_ID if a
DHCP relay-agent added any of these options.
If the client provides vendor-class, DNSMASQ_VENDOR_CLASS.
DNSMASQ_REQUESTED_OPTIONS a string containing the decimal values in the Parameter Request List option, comma separated, if the parameter request list option is provided by the client.
For IPv6 only:
If the client provides vendor-class, DNSMASQ_VENDOR_CLASS_ID,
containing the IANA enterprise id for the class, and
DNSMASQ_VENDOR_CLASS0..DNSMASQ_VENDOR_CLASSn for the data.
DNSMASQ_SERVER_DUID containing the DUID of the server: this is the same for
every call to the script.
DNSMASQ_IAID containing the IAID for the lease. If the lease is a
temporary allocation, this is prefixed to 'T'.
DNSMASQ_MAC containing the MAC address of the client, if known.
Note that the supplied hostname, vendorclass and userclass data is
only supplied for
"add" actions or "old" actions when a host resumes an existing lease,
since these data are not held in dnsmasq's lease
All file descriptors are
closed except stdin, which is open to /dev/null, and stdout and stderr which capture output for logging by dnsmasq.
(In debug mode, stdio, stdout and stderr file are left as those inherited from the invoker of dnsmasq).
The script is not invoked concurrently: at most one instance
of the script is ever running (dnsmasq waits for an instance of script to exit
before running the next). Changes to the lease database are which
require the script to be invoked are queued awaiting exit of a running instance.
If this queueing allows multiple state changes occur to a single
lease before the script can be run then
earlier states are discarded and the current state of that lease is
reflected when the script finally runs.
At dnsmasq startup, the script will be invoked for
all existing leases as they are read from the lease file. Expired
leases will be called with "del" and others with "old". When dnsmasq
receives a HUP signal, the script will be invoked for existing leases
with an "old" event.
There are four further actions which may appear as the first argument
to the script, "init", "arp-add", "arp-del" and "tftp". More may be added in the future, so
scripts should be written to ignore unknown actions. "init" is
described below in
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
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
function, and may provide
functions, which are called, without arguments when dnsmasq starts up
and terminates. It may also provide a
function receives the information detailed in
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
for IPv4, and
function is called in the same way as the lease function, and the
table holds the tags
functions are called only when enabled with
and have a table which holds the tags
Specify the user as which to run the lease-change script or Lua script. This defaults to root, but can be changed to another user using this flag.
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
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.
Call the DHCP script when the lease expiry time changes, for instance when the
lease is renewed.
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
The DHCP server determines which DHCP ranges are useable for allocating an
address to a DHCP client based on the network from which the DHCP request arrives,
and the IP configuration of the server's interface on that network. The shared-network
option extends the available subnets (and therefore DHCP ranges) beyond the
subnets configured on the arrival interface.
The first argument is either the
name of an interface, or an address that is configured on a local interface, and the
second argument is an address which defines another subnet on which addresses can be allocated.
To be useful, there must be a suitable dhcp-range which allows address allocation on this subnet
and this dhcp-range MUST include the netmask.
Using shared-network also needs extra
consideration of routing. Dnsmasq does not have the usual information that it uses to
determine the default route, so the default route option (or other routing) MUST be
configured manually. The client must have a route to the server: if the two-address form
of shared-network is used, this needs to be to the first specified address. If the interface,address
form is used, there must be a route to all of the addresses configured on the interface.
The two-address form of shared-network is also usable with a DHCP relay: the first address
is the address of the relay and the second, as before, specifies an extra subnet which
addresses may be allocated from.
- -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
and have a machine whose DHCP hostname is "laptop". The IP address for that machine is available from
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
which can change the behaviour of dnsmasq with domains.
If the address range is given as ip-address/network-size, then a
additional flag "local" may be supplied which has the effect of adding
--local declarations for forward and reverse DNS queries. Eg.
is identical to
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
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
without an address specified when
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
RFC6106 DNS parameters are included in the advertisements. By default,
the relevant link-local address of the machine running dnsmasq is sent
as recursive DNS server. If provided, the DHCPv6 options dns-server and
domain-search are used for the DNS server (RDNSS) and the domain search list (DNSSL).
- --ra-param=<interface>,[mtu:<integer>|<interface>|off,][high,|low,]<ra-interval>[,<router lifetime>]
Set non-default values for router advertisements sent via an
interface. The priority field for the router may be altered from the
default of medium with eg
The interval between router advertisements may be set (in seconds) with
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.
(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 inaccessible.
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 184.108.40.206 requests file "myfile"
then the effective path will be "/tftp/220.127.116.11/myfile" if /tftp/18.104.22.168 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 filenames.
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
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.
Run in a mode where the TFTP server uses ONLY the well-known port (69) for its end
of the TFTP transfer. This allows TFTP to work when there in NAT is the path between client and server. Note that
this is not strictly compliant with the RFCs specifying the TFTP protocol: use at your own risk.
- -C, --conf-file=<file>
Specify a configuration file. The presence of this option stops dnsmasq from reading the default configuration
file (normally /etc/dnsmasq.conf). Multiple files may be specified by repeating the option
either on the command line or in configuration files. A
filename of "-" causes dnsmasq to read configuration from stdin.
- -7, --conf-dir=<directory>[,<file-extension>......],
Read all the files in the given directory as configuration
files. If extension(s) are given, any files which end in those
extensions are skipped. Any files whose names end in ~ or start with . or start and end
with # are always skipped. If the extension starts with * then only files
which have that extension are loaded. So
loads all files with the suffix .conf in /path/to/dir. This flag may be given on the command
line or in a configuration file. If giving it on the command line, be sure to
escape * characters. Files are loaded in alphabetical order of filename.
A special case of
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
if it exists. (On
FreeBSD, the file is
) (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,
clears its cache and then re-loads
and any file given by --dhcp-hostsfile, --dhcp-hostsdir, --dhcp-optsfile,
--dhcp-optsdir, --addn-hosts or --hostsdir.
The DHCP lease change script is called for all
existing DHCP leases. If
is set SIGHUP also re-reads
does NOT re-read the configuration file.
When it receives a SIGUSR1,
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
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
utility would be
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
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
to discover the IP
addresses of the upstream nameservers it should use, since the
information is typically stored there. Unless
checks the modification time of
(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
is not an error
since it may not have been created before a PPP connection exists. Dnsmasq
simply keeps checking in case
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 servers.
Upstream servers may also be specified on the command line or in
the configuration file. These server specifications optionally take a
domain name which tells dnsmasq to use that server only to find names
in that particular domain.
In order to configure dnsmasq to act as cache for the host on which it is running, put "nameserver 127.0.0.1" in
to force local processes to send queries to
dnsmasq. Then either specify the upstream servers directly to dnsmasq
options or put their addresses real in another file, say
and run dnsmasq with the
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 22.214.171.124" in /etc/hosts will ensure that
queries for "mycompany.com" always return 126.96.36.199 even if queries in
the upstream DNS would otherwise return a different address. There is
one exception to this: if the upstream DNS contains a CNAME which
points to a shadowed name, then looking up the CNAME through dnsmasq
will result in the unshadowed address associated with the target of
the CNAME. To work around this, add the CNAME to /etc/hosts so that
the CNAME is shadowed too.
The tag system works as follows: For each DHCP request, dnsmasq
collects a set of valid tags from active configuration lines which
include set:<tag>, including one from the
used to allocate the address, one from any matching
(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
provided that _all_ the tags match somewhere in the
set collected as described above. The prefix '!' on a tag means 'not'
so --dhcp-option=tag:!purple,3,188.8.131.52 sends the option when the
tag purple is not in the set of valid tags. (If using this in a
command line rather than a configuration file, be sure to escape !,
which is a shell metacharacter)
When selecting --dhcp-options, a tag from --dhcp-range is second class
relative to other tags, to make it easy to override options for
individual hosts, so
will set the NIS-domain to domain1 for hosts in the range, but
override that to domain2 for a particular host.
Note that for
both tag:<tag> and set:<tag> are allowed, to both select the range in
use based on (eg) --dhcp-host, and to affect the options sent, based on
the range selected.
This system evolved from an earlier, more limited one and for backward
compatibility "net:" may be used instead of "tag:" and "set:" may be
omitted. (Except in
where "net:" may be used instead of "set:".) For the same reason, '#'
may be used instead of '!' to indicate NOT.
The DHCP server in dnsmasq will function as a BOOTP server also,
provided that the MAC address and IP address for clients are given,
configurations or in
, and a
configuration option is present to activate the DHCP server
on a particular network. (Setting --bootp-dynamic removes the need for
static address mappings.) The filename
parameter in a BOOTP request is used as a tag,
as is the tag "bootp", allowing some control over the options returned to
different classes of hosts.
Configuring dnsmasq to act as an authoritative DNS server is
complicated by the fact that it involves configuration of external DNS
servers to provide delegation. We will walk through three scenarios of
increasing complexity. Prerequisites for all of these scenarios
are a globally accessible IP address, an A or AAAA record pointing to that address,
and an external DNS server capable of doing delegation of the zone in
question. For the first part of this explanation, we will call the A (or AAAA) record
for the globally accessible address server.example.com, and the zone
for which dnsmasq is authoritative our.zone.com.
The simplest configuration consists of two lines of dnsmasq configuration; something like
and two records in the external DNS
server.example.com A 184.108.40.206
our.zone.com NS server.example.com
eth0 is the external network interface on which dnsmasq is listening,
and has (globally accessible) address 220.127.116.11.
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
our.zone.com A 18.104.22.168
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
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 then
--auth-peer=<IP address of secondary>
will do so.
Dnsmasq acts as an authoritative server for in-addr.arpa and
ip6.arpa domains associated with the subnets given in --auth-zone
declarations, so reverse (address to name) lookups can be simply
configured with a suitable NS record, for instance in this example,
where we allow 22.214.171.124/24 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, --caa-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
provided the address falls into one of the subnets specified in the
Addresses of DHCP leases, provided the address falls into one of the subnets specified in the
(If constructed DHCP ranges are is use, which depend on the address dynamically
assigned to an interface, then the form of
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
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
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
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
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
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
it will be scaled down and the actual limit logged at
start-up. Note that more transfers are possible when the same file is
being sent than when each transfer sends a different file.
It is possible to use dnsmasq to block Web advertising by using a list
of known banner-ad servers, all resolving to 127.0.0.1 or 0.0.0.0, in
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>.
- CONFIG FILE
- AUTHORITATIVE CONFIGURATION
- EXIT CODES
- SEE ALSO
This document was created by
using the manual pages.
Time: 20:41:31 GMT, April 07, 2021