dnsmasq - Unix, Linux Command
dnsmasq - A lightweight DHCP and caching DNS server.
dnsmasq is a lightweight DNS, TFTP 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.
The dnsmasq DHCP server supports static address assignments, multiple
networks, DHCP-relay and RFC3011 subnet specifiers. 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.
supports IPv6 for DNS, but not DHCP.
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.
-h, --no-hosts |
Dont read the hostnames in /etc/hosts.
-H, --addn-hosts=<file> |
Additional hosts file. Read the specified file as well as /etc/hosts. If -h is given, read
only the specified file. This option may be repeated for more than one
additional hosts file.
-E, --expand-hosts |
Add the domain to simple names (without a period) in /etc/hosts
in the same way as for DHCP-derived names.
-T, --local-ttl=<time> |
When replying with information from /etc/hosts or the DHCP leases
file dnsmasq by default sets the time-to-live field to zero, meaning
that the requestor 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.
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.
-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: dont fork to the background, dont write a pid file,
dont change user id, generate a complete cache dump on receipt on
SIGUSR1, log to stderr as well as syslog, dont fork new processes
to handle TCP queries.
-q, --log-queries |
Log the results of DNS queries handled by dnsmasq. Enable a full cache dump on receipt of SIGUSR1.
-8, --log-facility=<facility> |
Set the facility to which dnsmasq will send syslog entries, this
defaults to DAEMON, and to LOCAL0 when debug mode is in 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. (Errors whilst reading configuration will still go to syslog,
but all output from a successful startup, and all output whilst
running, will go exclusively to the file.) When logging to a file,
dnsmasq will close and reopen the file when it receives SIGUSR2. This
allows the log file to be rotated without stopping dnsmasq.
Enable asynchronous logging and optionally set the limit on the
number of lines
which will be queued by dnsmasq when writing to the syslog is slow.
Dnsmasq can log asynchronously: this
allows it to continue functioning without being blocked by syslog, and
allows syslog to use dnsmasq for DNS queries without risking deadlock.
If the queue of log-lines becomes full, dnsmasq will log the
overflow, and the number of messages lost. The default queue length is
5, a sane value would be 5-25, and a maximum limit of 100 is imposed.
-x, --pid-file=<path> |
Specify an alternate path for dnsmasq to record its process-id in. Normally /var/run/dnsmasq.pid.
-u, --user=<username> |
Specify the userid to which dnsmasq will change after startup. Dnsmasq must normally be started as root, but it will drop root
privileges after startup by changing id to another user. Normally this user is "nobody" but that
can be over-ridden with this switch.
-g, --group=<groupname> |
Specify the group which dnsmasq will run
as. The defaults to "dip", if available, to facilitate access to
/etc/ppp/resolv.conf which is not normally world readable.
-v, --version |
Print the version number.
-p, --port=<port> |
Listen on <port> instead of the standard DNS port (53). Setting this
to zero completely disables DNS function, leaving only DHCP and/or TFTP.
-P, --edns-packet-max=<size> |
Specify the largest EDNS.0 UDP packet which is supported by the DNS
forwarder. Defaults to 1280, which is the RFC2671-recommended maximum
-Q, --query-port=<query_port> |
Send outbound DNS queries from, and listen for their replies on, the
specific UDP port <query_port> instead of using random ports. NOTE
that using this option will make dnsmasq less secure against DNS
spoofing attacks but it may be faster and use less resources. Setting this option
to zero makes dnsmasq use a single port allocated to it by the
OS: this was the default behaviour in versions prior to 2.43.
Do not use ports less than that given as source for outbound DNS
queries. Dnsmasq picks random ports as source for outbound queries:
when this option is given, the ports used will always to larger
than that specified. Useful for systems behind firewalls.
-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
--interface option is used. If no
--listen-address options are given dnsmasq listens on all available interfaces except any
--except-interface options. IP alias interfaces (eg "eth1:0") cannot be used with
--except-interface options, use --listen-address instead.
-I, --except-interface=<interface name> |
Do not listen on the specified interface. Note that the order of
--listen-address --interface and
--except-interface options does not matter and that
--except-interface options always override the others.
-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
--listen-address options may be given, in which case the set of both interfaces and
addresses is used. Note that if no
--interface option is given, but
--listen-address is, dnsmasq will not automatically listen on the loopback
interface. To achieve this, its IP address, 127.0.0.1, must be
explicitly given as a
-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 shouldnt 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.
-y, --localise-queries |
Return answers to DNS queries from /etc/hosts which depend on the interface over which the query was
received. If a name in /etc/hosts has more than one address associated with
it, and at least one of those addresses is on the same subnet as the
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.
-V, --alias=<old-ip>,<new-ip>[,<mask>] |
Modify IPv4 addresses returned from upstream nameservers; old-ip is
replaced by new-ip. If the optional mask is given then any address
which matches the masked old-ip will be re-written. So, for instance
--alias=18.104.22.168,22.214.171.124,255.255.255.0 will map 126.96.36.199 to 188.8.131.52 and 184.108.40.206 to 220.127.116.11. This is what
Cisco PIX routers call "DNS doctoring".
-B, --bogus-nxdomain=<ipaddr> |
Transform replies which contain the IP address given into "No such
domain" replies. This is intended to counteract a devious move made by
Verisign in September 2003 when they started returning the address of
an advertising web page in response to queries for unregistered names,
instead of the correct NXDOMAIN response. This option tells dnsmasq to
fake the correct response when it sees this behaviour. As at Sept 2003
the IP address being returned by Verisign is 18.104.22.168
-f, --filterwin2k |
Later versions of windows make periodic DNS requests which dont 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 |
Dont read /etc/resolv.conf. Get upstream servers only from the command
line or the dnsmasq configuration file.
-1, --enable-dbus |
Allow dnsmasq configuration to be updated via DBus method calls. The
configuration which can be changed is upstream DNS servers (and
corresponding domains) and cache clear. Requires that dnsmasq has
been built with DBus support.
-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 requestor.
Reject (and log) addresses from upstream nameservers which are in the
private IP ranges. This blocks an attack where a browser behind a
firewall is used to probe machines on the local network.
-n, --no-poll |
Dont poll /etc/resolv.conf for changes.
Whenever /etc/resolv.conf is re-read, 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 queries for plain names, without dots
or domain parts, to upstream nameservers. If the name is not known
from /etc/hosts or DHCP then a "not found" answer is returned.
-S, --local, --server=[/[<domain>]/[domain/]][<ipaddr>[#<port>][@<source-ip>|<interface>[#<port>]] |
Specify IP address of upstream servers directly. Setting this flag does
not suppress reading of /etc/resolv.conf, use -R to do that. If one or
optional domains are given, that server is used only for those domains
and they are queried only using the specified server. This is
intended for private nameservers: if you have a nameserver on your
network which deals with names of the form
xxx.internal.thekelleys.org.uk at 192.168.1.1 then giving the flag
-S /internal.thekelleys.org.uk/192.168.1.1 will send all queries for
internal machines to that nameserver, everything else will go to the
servers in /etc/resolv.conf. An empty domain specification,
// has the special meaning of "unqualified names only" ie names without any
dots in them. A non-standard port may be specified as
part of the IP
address using a # character.
More than one -S flag is allowed, with
repeated domain or ipaddr parts as required.
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
local is a synonym for
server to make configuration files clearer in this case.
The optional string after the @ character tells
dnsmasq how to set the source of the queries to this
nameserver. It should be an ip-address, which should belong to the machine on which
dnsmasq is running otherwise this server line will be logged and then
ignored, or an interface name. If an interface name is given, then
queries to the server will be forced via that interface; if an
ip-address is given then the source address of the queries will be set
to that address.
The query-port flag is ignored for any servers which have a
source address specified but the port may be specified directly as
part of the source address. Forcing queries to an interface is not
implemented on all platforms supported by dnsmasq.
-A, --address=/<domain>/[domain/]<ipaddr> |
Specify an IP address to return for any host in the given domains.
Queries in the domains are never forwarded and always replied to
with the specified IP address which may be IPv4 or IPv6. To give
both IPv4 and IPv6 addresses for a domain, use repeated -A flags.
Note that /etc/hosts and DHCP leases 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=/#/22.214.171.124 will always return 126.96.36.199 for any query not
answered from /etc/hosts or DHCP and not sent to an upstream
nameserver by a more specific --server directive.
-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
--domain. The default for the target domain is empty, and the default for port
is one and the defaults for
weight and priority are zero. Be careful if transposing data from BIND
zone files: the port, weight and priority numbers are in a different
order. More than one SRV record for a given service/domain is allowed,
all that match are returned.
-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, split by commas.
Return a PTR DNS record.
Return an NAPTR DNS record, as specified in RFC3403.
Return a DNS record associating the name with the primary address on
the given interface. This flag specifies an A 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. 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.
-c, --cache-size=<cachesize> |
Set the size of dnsmasqs cache. The default is 150 names. Setting the cache size to zero disables caching.
-N, --no-negcache |
Disable negative caching. Negative caching allows dnsmasq to remember
"no such domain" answers from upstream nameservers and answer
identical queries without forwarding them again.
-0, --dns-forward-max=<queries> |
Set the maximum number of concurrent DNS queries. The default value is
150, which should be fine for most setups. The only known situation
where this needs to be increased is when using web-server log file
resolvers, which can generate large numbers of concurrent queries.
-F, --dhcp-range=[[net:]network-id,]<start-addr>,<end-addr>[[,<netmask>],<broadcast>][,<default 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
dhcp-host options. If the lease time is given, then leases
will be given for that length of time. The lease time is in seconds,
or minutes (eg 45m) or hours (eg 1h) or the literal "infinite". The
minimum lease time is two minutes. 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. It is, however, required for networks which
receive DHCP service via a relay agent. The broadcast address is
always optional. On some broken systems, dnsmasq can listen on only
one interface when using DHCP, and the name of that interface must be
given using the
interface option. This limitation currently affects OpenBSD before version 4.0. It is always
allowed to have more than one dhcp-range in a single subnet. The optional
network-id is a alphanumeric label which marks this network so that
dhcp options may be specified on a per-network basis.
When it is prefixed with net: 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 end address may be replaced by the keyword
static which tells dnsmasq to enable DHCP for the network specified, but not
to dynamically allocate IP addresses. Only hosts which have static
addresses given via
dhcp-host or from /etc/ethers will be served.
-G, --dhcp-host=[<hwaddr>][,id:<client_id>|*][,net:<netid>][,<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 ommit the hardware address and include the hostname, in
which case the IP address and lease times will apply to any machine
claiming that name. For example
--dhcp-host=00:20:e0:3b:13:af,wap,infinite tells dnsmasq to give
the machine with hardware address 00:20:e0:3b:13:af the name wap, and
an infinite DHCP lease.
dnsmasq to always allocate the machine lap the IP address
192.168.0.199. Addresses allocated like this are not constrained to be
in the range given by the --dhcp-range option, but they must be on the
network being served by the DHCP server. It is allowed to use client identifiers rather than
hardware addresses to identify hosts by prefixing with id:. Thus:
--dhcp-host=id:01:02:03:04,..... refers to the host with client identifier 01:02:03:04. It is also
allowed to specify the client ID as text, like this:
--dhcp-host=id:clientidastext,..... The special option id:* means "ignore any client-id
and use MAC addresses only." This is useful when a client presents a client-id sometimes
but not others.
If a name appears in /etc/hosts, the associated address can be
allocated to a DHCP lease, but only if a
--dhcp-host option specifying the name also exists. 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
--dhcp-host=00:20:e0:3b:13:af,ignore This is
useful when there is another DHCP server on the network which should
be used by some machines. The net:<network-id> sets the network-id tag
whenever this dhcp-host directive is in use.This can be used to
selectively send DHCP options just for this host. When a host matches any
dhcp-host directive (or one implied by /etc/ethers) then the special
network-id tag "known" is set. This allows dnsmasq to be configured to
ignore requests from unknown machines using
--dhcp-ignore=#known Ethernet addresses (but not client-ids) may have
wildcard bytes, so for example
--dhcp-host=00:20:e0:3b:13:*,ignore will cause dnsmasq to ignore a range of hardware addresses. Note that
the "*" will need to be escaped or quoted on a command line, but not
in the configuration file. Hardware addresses normally match any
network (ARP) type, but it is possible to restrict them to a single
ARP type by preceding them with the ARP-type (in HEX) and "-". so
--dhcp-host=06-00:20:e0:3b:13:af,188.8.131.52 will only match a
Token-Ring hardware address, since the ARP-address type for token ring
Read DHCP host information from the specified file. 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. The advantage of
using this option is the same as for --dhcp-hostsfile: the
dhcp-optsfile will be re-read when dnsmasq receives SIGHUP.
-Z, --read-ethers |
Read /etc/ethers for information about hosts for the DHCP server. The
format of /etc/ethers is a hardware address, followed by either a
hostname or dotted-quad IP address. When read by dnsmasq these lines
have exactly the same effect as
--dhcp-host options containing the same information. /etc/ethers is re-read when
dnsmasq receives SIGHUP.
-O, --dhcp-option=[<network-id>,[<network-id>,]][vendor:[<vendor-class>],][<opt>|option:<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. 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 or
--dhcp-option = option:ntp-server, 192.168.0.4 The special address 0.0.0.0 is taken to mean "the address of the
machine running dnsmasq". Data types allowed are comma separated
dotted-quad IP addresses, a decimal number, colon-separated hex digits
and a text string. If the optional network-ids are given then
this option is only sent when all the network-ids 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.
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 using
--dhcp-option: for instance
--dhcp-option=vendor:PXEClient,1,0.0.0.0 sends the encapsulated vendor
class-specific option "mftp-address=0.0.0.0" to any client whose
vendor-class matches "PXEClient". The vendor-class matching is
substring based (see --dhcp-vendorclass for details). If a
vendor-class option (number 60) is sent by dnsmasq, then that is used
for selecting encapsulated options in preference to any sent by the
client. It is
possible to omit the vendorclass completely;
--dhcp-option=vendor:,1,0.0.0.0 in which case the encapsulated option is always sent.
The address 0.0.0.0 is not treated specially in
encapsulated vendor class options.
This works in exactly the same way as
--dhcp-option except that the option will always be sent, even if the client does
not ask for it in the parameter request list. This is sometimes
needed, for example when sending options to PXELinux.
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.
-U, --dhcp-vendorclass=<network-id>,<vendor-class> |
Map from a vendor-class string to a network id tag. Most DHCP clients provide a
"vendor class" which represents, in some sense, the type of host. This option
maps vendor classes to tags, so that DHCP options may be selectively delivered
to different classes of hosts. For example
dhcp-vendorclass=printers,Hewlett-Packard JetDirect will allow options to be set only for HP printers like so:
--dhcp-option=printers,3,192.168.4.4 The vendor-class string is
substring matched against the vendor-class supplied by the client, to
allow fuzzy matching.
-j, --dhcp-userclass=<network-id>,<user-class> |
Map from a user-class string to a network id 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=<network-id>,<MAC address> |
Map from a MAC address to a network-id tag. The MAC address may include
wildcards. For example
--dhcp-mac=3com,01:34:23:*:*:* will set the tag "3com" for any host whose MAC address matches the pattern.
--dhcp-circuitid=<network-id>,<circuit-id>, --dhcp-remoteid=<network-id>,<remote-id> |
Map from RFC3046 relay agent options to network-id 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 network-id tag is set.
Map from RFC3993 subscriber-id relay agent options to network-id tags.
--dhcp-match=<network-id>,<option number> |
Set the network-id tag if the client sends a DHCP option of the given
number. This can be used to identify particular clients which send
information using private option numbers.
-J, --dhcp-ignore=<network-id>[,<network-id>] |
When all the given network-ids match the set of network-ids derived
from the net, host, vendor and user classes, ignore the host and do
not allocate it a DHCP lease.
When all the given network-ids match the set of network-ids derived
from the net, host, vendor and user classes, ignore any hostname
provided by the host. Note that, unlike dhcp-ignore, it is permissible
to supply no netid 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
When all the given network-ids match the set of network-ids derived
from the net, host, vendor and user classes, always use broadcast to
communicate with the host when it is unconfigured. 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=[net:<network-id>,]<filename>,[<servername>[,<server address>]] |
Set BOOTP options to be returned by the DHCP server. Server name and
address are optional: if not provided, the name is left empty, and the
address set to the address of the machine running dnsmasq. If dnsmasq
is providing a TFTP service (see
--enable-tftp ) then only the filename is required here to enable network booting.
If the optional network-id(s) are given,
they must match for this configuration to be sent. Note that
network-ids are prefixed by "net:" to distinguish them.
-X, --dhcp-lease-max=<number> |
Limits dnsmasq to the specified maximum number of DHCP leases. The
default is 150. 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.
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.
--dhcp-alternate-port[=<server port>[,<client port>]] |
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 |
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
-5, --no-ping |
By default, the DHCP server will attempt to ensure that an address in
not in use before allocating it to a host. It does this by sending an
ICMP echo request (aka "ping") to the address in question. If it gets
a reply, then the address must already be in use, and another is
tried. This flag disables this check. Use with caution.
Extra logging for DHCP: log all the options sent to DHCP clients and
the netid tags used to determine them.
-l, --dhcp-leasefile=<path> |
Use the specified file to store DHCP lease information. If this option
is given but no dhcp-range option is given then dnsmasq version 1
behaviour is activated. The file given is assumed to be an ISC dhcpd
lease file and parsed for leases which are then added to the DNS
system if they have a hostname. This functionality may have been
excluded from dnsmasq at compile time, in which case an error will
occur. In any case note that ISC leasefile integration is a deprecated
feature. It should not be used in new installations, and will be
removed in a future release.
-6 --dhcp-script=<path> |
Whenever a new DHCP lease is created, or an old one destroyed, the
binary specified by this option is run. The arguments to the process
are "add", "old" or "del", the MAC
address of the host (or "<null>"), 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).
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, and if the
host provided a client-id, this is stored in the environment variable
DNSMASQ_CLIENT_ID. If the client provides vendor-class or user-class
information, these are provided in DNSMASQ_VENDOR_CLASS and
DNSMASQ_USER_CLASS0..DNSMASQ_USER_CLASSn variables, but only for
"add" actions or "old" actions when a host resumes an existing lease,
since these data are not held in dnsmasqs lease
database. If dnsmasq was compiled with HAVE_BROKEN_RTC, then
the length of the lease (in seconds) is stored in
DNSMASQ_LEASE_LENGTH, otherwise the time of lease expiry is stored in
DNSMASQ_LEASE_EXPIRES. The number of seconds until lease expiry is
always stored in DNSMASQ_TIME_REMAINING.
If a lease used to have a hostname, which is
removed, an "old" event is generated with the new state of the lease,
ie no name, and the former name is provided in the environment
variable DNSMASQ_OLD_HOSTNAME. DNSMASQ_INTERFACE stores the name of
the interface on which the request arrived; this is not set for "old"
actions when dnsmasq restarts.
All file descriptors are
closed except stdin, stdout and stderr which are open to /dev/null
(except in debug mode).
The script is not invoked concurrently: if subsequent lease
changes occur, the script is not invoked again until any existing
invocation exits. 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". <path>
must be an absolute pathname, no PATH search occurs. When dnsmasq
receives a HUP signal, the script will be invoked for existing leases
with an "old " event.
Specify the user as which to run the lease-change script. This defaults to root, but can be changed to another user using this flag.
-9, --leasefile-ro |
Completely suppress use of the lease database file. The file will not
be created, read, or written. Change the way the lease-change
script (if one is provided) is called, so that the lease database may
be maintained in external storage by the script. In addition to the
invocations given in
--dhcp-script the lease-change script is called once, at dnsmasq startup, with the
single argument "init". When called like this the script should write
the saved state of the lease database, in dnsmasq leasefile format, to
stdout and exit with zero exit code. Setting this
option also forces the leasechange script to be called on changes
to the client-id and lease length and expiry time.
Treat DHCP request packets arriving at any of the <alias> interfaces
as if they had arrived at <interface>. This option is only available
on BSD platforms, and is necessary when using "old style" bridging, since
packets arrive at tap interfaces which dont have an IP address.
-s, --domain=<domain> |
Specifies the domain for the DHCP server. This has two effects;
firstly it causes the DHCP server to return the domain to any hosts
which request it, and secondly it sets the domain which it is legal
for DHCP-configured hosts to claim. The intention is to constrain
hostnames so that an untrusted host on the LAN cannot advertise
its name via dhcp as e.g. "microsoft.com" and capture traffic not
meant for it. If no domain suffix is specified, then any DHCP
hostname with a domain part (ie with a period) will be disallowed
and logged. If suffix is specified, then hostnames with a domain
part are allowed, provided the domain part matches the suffix. In
addition, when a suffix is set then hostnames without a domain
part have the suffix added as an optional domain part. Eg on my network I can set
--domain=thekelleys.org.uk and have a machine whose DHCP hostname is "laptop". The IP address for that machine is available from
dnsmasq both as "laptop" and "laptop.thekelleys.org.uk". If the domain is
given as "#" then the domain is read from the first "search" directive
in /etc/resolv.conf (or equivalent).
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).
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
Add the IP address of the TFTP client as a path component on the end
of the TFTP-root (in standard dotted-quad format). Only valid if a
tftp-root is set and the directory exists. For instance, if tftp-root is "/tftp" and client
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.
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.
Set the maximum number of concurrent TFTP connections allowed. This
defaults to 50. When serving a large number of TFTP connections,
per-process file descriptor limits may be encountered. Dnsmasq needs
one file descriptor for each concurrent TFTP connection and one
file descriptor per unique file (plus a few others). So serving the
same file simultaneously to n clients will use require about n + 10 file
descriptors, serving different files simultaneously to n clients will
require about (2*n) + 10 descriptors. If
--tftp-port-range is given, that can affect the number of concurrent connections.
Stop the TFTP server from negotiating the "blocksize" option with a
client. Some buggy clients request this option but then behave badly
when it is granted.
A TFTP server listens on a well-known port (69) for connection initiation,
but it also uses a dynamically-allocated port for each
connection. Normally these are allocated by the OS, but this option
specifies a range of ports for use by TFTP transfers. This can be
useful when TFTP has to traverse a firewall. The start of the range
cannot be lower than 1025 unless dnsmasq is running as root. The number
of concurrent TFTP connections is limited by the size of the port range.
-C, --conf-file=<file> |
Specify a different configuration file. The conf-file option is also allowed in
configuration files, to include multiple configuration files.
-7, --conf-dir=<directory> |
Read all the files in the given directory as configuration
files. Files whose names end in ~ or start with . or start and end
with # are skipped. This flag may be given on the command
line or in a configuration file.
At startup, dnsmasq reads
/etc/dnsmasq.conf, if it exists. (On
FreeBSD, the file is
/usr/local/etc/dnsmasq.conf ) (but see the
-7 options.) The format of this
file consists of one option per line, exactly as the long options detailed
in the OPTIONS section but without the leading "--". Lines starting with # are comments and ignored. For
options which may only be specified once, the configuration file overrides
the command line. Quoting is allowed in a config file:
between " quotes the special meanings of ,:. and # are removed and the
following escapes are allowed: \\ \" \t \e \b \r and \n. The later
corresponding to tab, escape, backspace, return and newline.
When it receives a SIGHUP,
dnsmasq clears its cache and then re-loads
/etc/ethers and any file given by --dhcp-hostsfile, --dhcp-optsfile or --addn-hosts.
The dhcp lease change script is called for all
existing DHCP leases. If
is set SIGHUP also re-reads
does NOT re-read the configuration file.
When it receives a SIGUSR1,
dnsmasq writes statistics to the system log. It writes the cache size,
the number of names which have had to removed from the cache before
they expired in order to make room for new names and the total number
of names that have been inserted into the cache. For each upstream
server it gives the number of queries sent, and the number which
resulted in an error. In
--no-daemon mode or when full logging is enabled (-q), a complete dump of the
contents of the cache is made.
When it receives SIGUSR2 and it is logging direct to a file (see
dnsmasq will close and reopen the log file. Note that during this operation,
dnsmasq will not be running as root. When it first creates the logfile
dnsmasq changes the ownership of the file to the non-root user it will run
as. Logrotate should be configured to create a new log file with
the ownership which matches the existing one before sending SIGUSR2.
If TCP DNS queries are in progress, the old logfile will remain open in
child processes which are handling TCP queries and may continue to be
written. There is a limit of 150 seconds, after which all existing TCP
processes will have expired: for this reason, it is not wise to
configure logfile compression for logfiles which have just been
rotated. Using logrotate, the required options are
Dnsmasq is a DNS query forwarder: it it not capable of recursively
answering arbitrary queries starting from the root servers but
forwards such queries to a fully recursive upstream DNS server which is
typically provided by an ISP. By default, dnsmasq reads
/etc/resolv.conf to discover the IP
addresses of the upstream nameservers it should use, since the
information is typically stored there. Unless
--no-poll is used,
dnsmasq checks the modification time of
/etc/resolv.conf (or equivalent if
--resolv-file is used) and re-reads it if it changes. This allows the DNS servers to
be set dynamically by PPP or DHCP since both protocols provide the
/etc/resolv.conf is not an error
since it may not have been created before a PPP connection exists. Dnsmasq
simply keeps checking in case
/etc/resolv.conf is created at any
time. Dnsmasq can be told to parse more than one resolv.conf
file. This is useful on a laptop, where both PPP and DHCP may be used:
dnsmasq can be set to poll both
/etc/dhcpc/resolv.conf and will use the contents of whichever changed
last, giving automatic switching between DNS servers.
Upstream servers may also be specified on the command line or in
the configuration file. These server specifications optionally take a
domain name which tells dnsmasq to use that server only to find names
in that particular domain.
In order to configure dnsmasq to act as cache for the host on which it is running, put "nameserver 127.0.0.1" in
/etc/resolv.conf to force local processes to send queries to
dnsmasq. Then either specify the upstream servers directly to dnsmasq
--server options or put their addresses real in another file, say
/etc/resolv.dnsmasq and run dnsmasq with the
-r /etc/resolv.dnsmasq option. This second technique allows for dynamic update of the server
addresses by PPP or DHCP.
Addresses in /etc/hosts will "shadow" different addresses for the same
names in the upstream DNS, so "mycompany.com 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 network-id system works as follows: For each DHCP request, dnsmasq
collects a set of valid network-id tags, one from the
dhcp-range used to allocate the address, one from any matching
dhcp-host and possibly many from matching vendor classes and user
classes sent by the DHCP client. Any
dhcp-option which has network-id tags will be used in preference to an untagged
dhcp-option, provided that _all_ the tags match somewhere in the
set collected as described above. The prefix # on a tag means not
so --dhcp=option=#purple,3,188.8.131.52 sends the option when the
network-id tag purple is not in the set of valid tags.
If the network-id in a
dhcp-range is prefixed with net: then its meaning changes from setting a
tag to matching it. Thus if there is more than dhcp-range on a subnet,
and one is tagged with a network-id which is set (for instance
from a vendorclass option) then hosts which set the netid tag will be
allocated addresses in the tagged range.
The DHCP server in dnsmasq will function as a BOOTP server also,
provided that the MAC address and IP address for clients are given,
dhcp-host configurations or in
/etc/ethers , and a
dhcp-range configuration option is present to activate the DHCP server
on a particular network. (Setting --bootp-dynamic removes the need for
static address mappings.) The filename
parameter in a BOOTP request is matched against netids in
dhcp-option configurations, as is the tag "bootp", allowing some control over the options returned to
different classes of hosts.
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
scripts 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. Clearly to do this the value of
--dhcp-lease-max must be increased,
and lease times should not be very short (less than one hour). The
--dns-forward-max can be increased: start with it equal to
the number of clients and increase if DNS seems slow. Note that DNS
performance depends too on the performance of the upstream
nameservers. The size of the DNS cache may be increased: the hard
limit is 10000 names and the default (150) is very low. Sending
SIGUSR1 to dnsmasq makes it log information which is useful for tuning
the cache size. See the
NOTES section for details.
The built-in TFTP server is capable of many simultaneous file
transfers: the absolute limit is related to the number of file-handles
allowed to a process and the ability of the select() system call to
cope with large numbers of file handles. If the limit is set too high
--tftp-max it will be scaled down and the actual limit logged at
start-up. Note that more transfers are possible when the same file is
being sent than when each transfer sends a different file.
It is possible to use dnsmasq to block Web advertising by using a list
of known banner-ad servers, all resolving to 127.0.0.1 or 0.0.0.0, in
/etc/hosts or an additional hosts file. The list can be very long,
dnsmasq has been tested successfully with one million names. That size
file needs a 1GHz processor and about 60Mb of RAM.
This manual page was written by Simon Kelley <email@example.com>.