「dnsmasq」

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DNSMASQ(8) System Manager’s Manual DNSMASQ(8)

NAME

dnsmasq – A lightweight DHCP and caching DNS server.

SYNOPSIS

dnsmasq [OPTION]…

DESCRIPTION

dnsmasq is a lightweight DNS, TFTP, PXE, router advertisement and DHCP server. It is intended to provide cou‐

pled DNS and DHCP service to a LAN.

Dnsmasq accepts DNS queries and either answers them from a small, local, cache or forwards them to a real,

recursive, DNS server. It loads the contents of /etc/hosts so that local hostnames which do not appear in the

global DNS can be resolved and also answers DNS queries for DHCP configured hosts. It can also act as the

authoritative DNS server for one or more domains, allowing local names to appear in the global DNS. It can be

configured to do DNSSEC validation.

The dnsmasq DHCP server supports static address assignments and multiple networks. It automatically sends a

sensible default set of DHCP options, and can be configured to send any desired set of DHCP options, includ‐

ing vendor-encapsulated options. It includes a secure, read-only, TFTP server to allow net/PXE boot of DHCP

hosts and also supports BOOTP. The PXE support is full featured, and includes a proxy mode which supplies PXE

information to clients whilst DHCP address allocation is done by another server.

The dnsmasq DHCPv6 server provides the same set of features as the DHCPv4 server, and in addition, it

includes router advertisements and a neat feature which allows nameing for clients which use DHCPv4 and

stateless autoconfiguration only for IPv6 configuration. There is support for doing address allocation (both

DHCPv6 and RA) from subnets which are dynamically delegated via DHCPv6 prefix delegation.

Dnsmasq is coded with small embedded systems in mind. It aims for the smallest possible memory footprint com‐

patible with the supported functions, and allows uneeded functions to be omitted from the compiled binary.

OPTIONS

Note that in general missing parameters are allowed and switch off functions, for instance “–pid-file” dis‐

ables writing a PID file. On BSD, unless the GNU getopt library is linked, the long form of the options does

not work on the command line; it is still recognised in the configuration file.

–test Read and syntax check configuration file(s). Exit with code 0 if all is OK, or a non-zero code other‐
wise. Do not start up dnsmasq.

-h, –no-hosts
Don’t read the hostnames in /etc/hosts.

-H, –addn-hosts=<file>
Additional hosts file. Read the specified file as well as /etc/hosts. If -h is given, read only the

specified file. This option may be repeated for more than one additional hosts file. If a directory is

given, then read all the files contained in that directory.

–hostsdir=<path>
Read all the hosts files contained in the directory. New or changed files are read automatically. See

–dhcp-hostsdir for details.

-E, –expand-hosts
Add the domain to simple names (without a period) in /etc/hosts in the same way as for DHCP-derived

names. Note that this does not apply to domain names in cnames, PTR records, TXT records etc.

-T, –local-ttl=<time>
When replying with information from /etc/hosts or 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.

–neg-ttl=<time>
Negative replies from upstream servers normally contain time-to-live information in SOA records which

dnsmasq uses for caching. If the replies from upstream servers omit this information, dnsmasq does not

cache the reply. This option gives a default value for time-to-live (in seconds) which dnsmasq uses to

cache negative replies even in the absence of an SOA record.

–max-ttl=<time>
Set a maximum TTL value that will be handed out to clients. The specified maximum TTL will be given to

clients instead of the true TTL value if it is lower. The true TTL value is however kept in the cache

to avoid flooding the upstream DNS servers.

–max-cache-ttl=<time>
Set a maximum TTL value for entries in the cache.

–min-cache-ttl=<time>
Extend short TTL values to the time given when caching them. Note that artificially extending TTL val‐

ues is in general a bad idea, do not do it unless you have a good reason, and understand what you are

doing. Dnsmasq limits the value of this option to one hour, unless recompiled.

–auth-ttl=<time>
Set the TTL value returned in answers from the authoritative server.

-k, –keep-in-foreground
Do not go into the background at startup but otherwise run as normal. This is intended for use when

dnsmasq is run under daemontools or launchd.

-d, –no-daemon
Debug mode: don’t fork to the background, don’t write a pid file, don’t change user id, generate a

complete cache dump on receipt on SIGUSR1, log to stderr as well as syslog, don’t fork new processes

to handle TCP queries. Note that this option is for use in debugging only, to stop dnsmasq daemonising

in production, use -k.

-q, –log-queries
Log the results of DNS queries handled by dnsmasq. Enable a full cache dump on receipt of SIGUSR1. If

the argument “extra” is supplied, ie –log-queries=extra then the log has extra information at the

start of each line. This consists of a serial number which ties together the log lines associated

with an individual query, and the IP address of the requestor.

-8, –log-facility=<facility>
Set the facility to which dnsmasq will send syslog entries, this defaults to DAEMON, and to LOCAL0

when debug mode is in operation. If the facility given contains at least one ‘/’ character, it is

taken to be a filename, and dnsmasq logs to the given file, instead of syslog. If the facility is ‘-‘

then dnsmasq logs to stderr. (Errors whilst reading configuration will still go to syslog, but all

output from a successful startup, and all output whilst running, will go exclusively to the file.)

When logging to a file, dnsmasq will close and reopen the file when it receives SIGUSR2. This allows

the log file to be rotated without stopping dnsmasq.

–log-async[=<lines>]
Enable asynchronous logging and optionally set the limit on the number of lines which will be queued

by dnsmasq when writing to the syslog is slow. Dnsmasq can log asynchronously: this allows it to con‐

tinue 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 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 dns‐

masq use a single port allocated to it by the OS: this was the default behaviour in versions prior to

2.43.

–min-port=<port>
Do not use ports less than that given as source for outbound DNS queries. Dnsmasq picks random ports

as source for outbound queries: when this option is given, the ports used will always to larger than

that specified. Useful for systems behind firewalls.

-i, –interface=<interface name>
Listen only on the specified interface(s). Dnsmasq automatically adds the loopback (local) interface

to the list of interfaces to use when the –interface option is used. If no –interface or –listen-

address options are given dnsmasq listens on all available interfaces except any given in –except-

interface options. IP alias interfaces (eg “eth1:0”) cannot be used with –interface or –except-

interface options, use –listen-address instead. A simple wildcard, consisting of a trailing ‘*’, can

be used in –interface and –except-interface options.

-I, –except-interface=<interface name>
Do not listen on the specified interface. Note that the order of –listen-address –interface and

–except-interface options does not matter and that –except-interface options always override the

others.

–auth-server=<domain>,<interface>|<ip-address>
Enable DNS authoritative mode for queries arriving at an interface or address. Note that the interface

or address need not be mentioned in –interface or –listen-address configuration, indeed –auth-

server will overide these and provide a different DNS service on the specified interface. The <domain>

is the “glue record”. It should resolve in the global DNS to a A and/or AAAA record which points to

the address dnsmasq is listening on. When an interface is specified, it may be qualified with “/4” or

“/6” to specify only the IPv4 or IPv6 addresses associated with the interface.

–local-service
Accept DNS queries only from hosts whose address is on a local subnet, ie a subnet for which an inter‐

face exists on the server. This option only has effect is there are no –interface –except-interface,

–listen-address or –auth-server options. It is intended to be set as a default on installation, to

allow unconfigured installations to be useful but also safe from being used for DNS amplification

attacks.

-2, –no-dhcp-interface=<interface name>
Do not provide DHCP or TFTP on the specified interface, but do provide DNS service.

-a, –listen-address=<ipaddr>
Listen on the given IP address(es). Both –interface and –listen-address options may be given, in

which case the set of both interfaces and addresses is used. Note that if no –interface option is

given, but –listen-address is, dnsmasq will not automatically listen on the loopback interface. To

achieve this, its IP address, 127.0.0.1, must be explicitly given as a –listen-address option.

-z, –bind-interfaces
On systems which support it, dnsmasq binds the wildcard address, even when it is listening on only

some interfaces. It then discards requests that it shouldn’t reply to. This has the advantage of work‐

ing even when interfaces come and go and change address. This option forces dnsmasq to really bind

only the interfaces it is listening on. About the only time when this is useful is when running

another nameserver (or another instance of dnsmasq) on the same machine. Setting this option also

enables multiple instances of dnsmasq which provide DHCP service to run in the same machine.

–bind-dynamic
Enable a network mode which is a hybrid between –bind-interfaces and the default. Dnsmasq binds the

address of individual interfaces, allowing multiple dnsmasq instances, but if new interfaces or

addresses appear, it automatically listens on those (subject to any access-control configuration).

This makes dynamically created interfaces work in the same way as the default. Implementing this

option requires non-standard networking APIs and it is only available under Linux. On other platforms

it falls-back to –bind-interfaces mode.

-y, –localise-queries
Return answers to DNS queries from /etc/hosts 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>]|[<start-ip>-<end-ip>],<new-ip>[,<mask>]
Modify IPv4 addresses returned from upstream nameservers; old-ip is replaced by new-ip. If the

optional mask is given then any address which matches the masked old-ip will be re-written. So, for

instance –alias=1.2.3.0,6.7.8.0,255.255.255.0 will map 1.2.3.56 to 6.7.8.56 and 1.2.3.67 to 6.7.8.67.

This is what Cisco PIX routers call “DNS doctoring”. If the old IP is given as range, then only

addresses in the range, rather than a whole subnet, are re-written. So

–alias=192.168.0.10-192.168.0.40,10.0.0.0,255.255.255.0 maps 192.168.0.10->192.168.0.40 to

10.0.0.10->10.0.0.40

-B, –bogus-nxdomain=<ipaddr>
Transform replies which contain the IP address given into “No such domain” replies. This is intended

to counteract a devious move made by Verisign in September 2003 when they started returning the

address of an advertising web page in response to queries for unregistered names, instead of the cor‐

rect NXDOMAIN response. This option tells dnsmasq to fake the correct response when it sees this be‐

haviour. As at Sept 2003 the IP address being returned by Verisign is 64.94.110.11

-B, –ignore-address=<ipaddr>
Ignore replies to A-record queries which include the specified address. No error is generated, dns‐

masq simply continues to listen for another reply. This is useful to defeat blocking strategies which

rely on quickly supplying a forged answer to a DNS request for certain domain, before the correct

answer can arrive.

-f, –filterwin2k
Later versions of windows make periodic DNS requests which don’t get sensible answers from the public

DNS and can cause problems by triggering dial-on-demand links. This flag turns on an option to filter

such requests. The requests blocked are for records of types SOA and SRV, and type ANY where the

requested name has underscores, to catch LDAP requests.

-r, –resolv-file=<file>
Read the IP addresses of the upstream nameservers from <file>, instead of /etc/resolv.conf. For the

format of this file see resolv.conf(5). The only lines relevant to dnsmasq are nameserver ones. Dns‐

masq 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 cur‐

rently 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 configura‐

tion file.

-1, –enable-dbus[=<service-name>]
Allow dnsmasq configuration to be updated via DBus method calls. The configuration which can be

changed is upstream DNS servers (and corresponding domains) and cache clear. Requires that dnsmasq has

been built with DBus support. If the service name is given, dnsmasq provides service at that name,

rather than the default which is uk.org.thekelleys.dnsmasq

-o, –strict-order
By default, dnsmasq will send queries to any of the upstream servers it knows about and tries to

favour servers that are known to be up. Setting this flag forces dnsmasq to try each query with each

server strictly in the order they appear in /etc/resolv.conf

–all-servers
By default, when dnsmasq has more than one upstream server available, it will send queries to just one

server. Setting this flag forces dnsmasq to send all queries to all available servers. The reply from

the server which answers first will be returned to the original requester.

–dns-loop-detect
Enable code to detect DNS forwarding loops; ie the situation where a query sent to one of the upstream

server eventually returns as a new query to the dnsmasq instance. The process works by generating TXT

queries of the form <hex>.test and sending them to each upstream server. The hex is a UID which

encodes the instance of dnsmasq sending the query and the upstream server to which it was sent. If the

query returns to the server which sent it, then the upstream server through which it was sent is dis‐

abled and this event is logged. Each time the set of upstream servers changes, the test is re-run on

all of them, including ones which were previously disabled.

–stop-dns-rebind
Reject (and log) addresses from upstream nameservers which are in the private IP ranges. This blocks

an attack where a browser behind a firewall is used to probe machines on the local network.

–rebind-localhost-ok
Exempt 127.0.0.0/8 from rebinding checks. This address range is returned by realtime black hole

servers, so blocking it may disable these services.

–rebind-domain-ok=[<domain>]|[[/<domain>/[<domain>/]
Do not detect and block dns-rebind on queries to these domains. The argument may be either a single

domain, or multiple domains surrounded by ‘/’, like the –server syntax, eg. –rebind-domain-

ok=/domain1/domain2/domain3/

-n, –no-poll
Don’t poll /etc/resolv.conf for changes.

–clear-on-reload
Whenever /etc/resolv.conf is re-read or the upstream servers are set via DBus, clear the DNS cache.

This is useful when new nameservers may have different data than that held in cache.

-D, –domain-needed
Tells dnsmasq to never forward A or AAAA queries for plain names, without dots or domain parts, to

upstream nameservers. If the name is not known from /etc/hosts or DHCP then a “not found” answer is

returned.

-S, –local, –server=[/[<domain>]/[domain/]][<ipaddr>[#<port>][@<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 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 pri‐

vate 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.thekel‐

leys.org.uk/192.168.1.1 will send all queries for internal machines to that nameserver, everything

else will go to the servers in /etc/resolv.conf. An empty domain specification, // has the special

meaning of “unqualified names only” ie names without any dots in them. A non-standard port may be

specified as part of the IP address using a # character. More than one -S flag is allowed, with

repeated domain or ipaddr parts as required.

More specific domains take precendence over less specific domains, so: –server=/google.com/1.2.3.4

–server=/www.google.com/2.3.4.5 will send queries for *.google.com to 1.2.3.4, except

*www.google.com, which will go to 2.3.4.5

The special server address ‘#’ means, “use the standard servers”, so –server=/google.com/1.2.3.4

–server=/www.google.com/# will send queries for *.google.com to 1.2.3.4, except *www.google.com which

will be forwarded as usual.

Also permitted is a -S flag which gives a domain but no IP address; this tells dnsmasq that a domain

is local and it may answer queries from /etc/hosts or DHCP but should never forward queries on that

domain to any upstream servers. local is a synonym for server to make configuration files clearer in

this case.

IPv6 addresses may include a %interface scope-id, eg fe80::202:a412:4512:7bbf%eth0.

The optional string after the @ character tells dnsmasq how to set the source of the queries to this

nameserver. It should be an ip-address, which should belong to the machine on which 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 dns‐

masq.

–rev-server=<ip-address>/<prefix-len>,<ipaddr>[#<port>][@<source-ip>|<interface>[#<port>]]
This is functionally the same as –server, but provides some syntactic sugar to make specifying

address-to-name queries easier. For example –rev-server=1.2.3.0/24,192.168.0.1 is exactly equivalent

to –server=/3.2.1.in-addr.arpa/192.168.0.1

-A, –address=/<domain>/[domain/][<ipaddr>]
Specify an IP address to return for any host in the given domains. Queries in the domains are never

forwarded and always replied to with the specified IP address which may be IPv4 or IPv6. To give both

IPv4 and IPv6 addresses for a domain, use repeated -A flags. Note that /etc/hosts and DHCP leases

override this for individual names. A common use of this is to redirect the entire doubleclick.net

domain to some friendly local web server to avoid banner ads. The domain specification works in the

same was as for –server, with the additional facility that /#/ matches any domain. Thus

–address=/#/1.2.3.4 will always return 1.2.3.4 for any query not answered from /etc/hosts or DHCP and

not sent to an upstream nameserver by a more specific –server directive. As for –server, one or more

domains with no address returns a no-such-domain answer, so –address=/example.com/ is equivalent to

–server=/example.com/ and returns NXDOMAIN for example.com and all its subdomains.

–ipset=/<domain>/[domain/]<ipset>[,<ipset>]
Places the resolved IP addresses of queries for the specified domains in the specified netfilter ip

sets. Domains and subdomains are matched in the same way as –address. These ip sets must already

exist. See ipset(8) for more details.

-m, –mx-host=<mx name>[[,<hostname>],<preference>]
Return an MX record named <mx name> pointing to the given hostname (if given), or the host specified

in the –mx-target switch or, if that switch is not given, the host on which dnsmasq is running. The

default is useful for directing mail from systems on a LAN to a central server. The preference value

is optional, and defaults to 1 if not given. More than one MX record may be given for a host.

-t, –mx-target=<hostname>
Specify the default target for the MX record returned by dnsmasq. See –mx-host. If –mx-target is

given, but not –mx-host, then dnsmasq returns a MX record containing the MX target for MX queries on

the hostname of the machine on which dnsmasq is running.

-e, –selfmx
Return an MX record pointing to itself for each local machine. Local machines are those in /etc/hosts

or with DHCP leases.

-L, –localmx
Return an MX record pointing to the host given by mx-target (or the machine on which dnsmasq is run‐

ning) for each local machine. Local machines are those in /etc/hosts or with DHCP leases.

-W, –srv-host=<_service>.<_prot>.[<domain>],[<target>[,<port>[,<priority>[,<weight>]]]]
Return a SRV DNS record. See RFC2782 for details. If not supplied, the domain defaults to that given

by –domain. The default for the target domain is empty, and the default for port is one and the

defaults for weight and priority are zero. Be careful if transposing data from BIND zone files: the

port, weight and priority numbers are in a different order. More than one SRV record for a given ser‐

vice/domain is allowed, all that match are returned.

–host-record=<name>[,<name>….],[<IPv4-address>],[<IPv6-address>]
Add A, AAAA and PTR records to the DNS. This adds one or more names to the DNS with associated IPv4

(A) and IPv6 (AAAA) records. A name may appear in more than one host-record and therefore be assigned

more than one address. Only the first address creates a PTR record linking the address to the name.

This is the same rule as is used reading hosts-files. host-record options are considered to be read

before host-files, so a name appearing there inhibits PTR-record creation if it appears in hosts-file

also. Unlike hosts-files, names are not expanded, even when expand-hosts is in effect. Short and long

names may appear in the same host-record, eg. –host-record=laptop,laptop.thekel‐

leys.org,192.168.0.1,1234::100

-Y, –txt-record=<name>[[,<text>],<text>]
Return a TXT DNS record. The value of TXT record is a set of strings, so any number may be included,

delimited by commas; use quotes to put commas into a string. Note that the maximum length of a single

string is 255 characters, longer strings are split into 255 character chunks.

–ptr-record=<name>[,<target>]
Return a PTR DNS record.

–naptr-record=<name>,<order>,<preference>,<flags>,<service>,<regexp>[,<replacement>]
Return an NAPTR DNS record, as specified in RFC3403.

–cname=<cname>,<target>
Return a CNAME record which indicates that <cname> is really <target>. There are significant limita‐

tions on the target; it must be a DNS name which is known to dnsmasq from /etc/hosts (or additional

hosts files), from DHCP, from –interface-name or from another –cname. If the target does not sat‐

isfy this criteria, the whole cname is ignored. The cname must be unique, but it is permissable to

have more than one cname pointing to the same target.

–dns-rr=<name>,<RR-number>,[<hex data>]
Return an arbitrary DNS Resource Record. The number is the type of the record (which is always in the

C_IN class). The value of the record is given by the hex data, which may be of the form 01:23:45 or 01

23 45 or 012345 or any mixture of these.

–interface-name=<name>,<interface>[/4|/6]
Return a DNS record associating the name with the primary address on the given interface. This flag

specifies an A or AAAA record for the given name in the same way as an /etc/hosts line, except that

the address is not constant, but taken from the given interface. The interface may be followed by “/4”

or “/6” to specify that only IPv4 or IPv6 addresses of the interface should be used. If the interface

is down, not configured or non-existent, an empty record is returned. The matching PTR record is also

created, mapping the interface address to the name. More than one name may be associated with an

interface address by repeating the flag; in that case the first instance is used for the reverse

address-to-name mapping.

–synth-domain=<domain>,<address range>[,<prefix>]
Create artificial A/AAAA and PTR records for an address range. The records use the address, with peri‐

ods (or colons for IPv6) replaced with dashes.

An example should make this clearer. –synth-domain=thekelleys.org.uk,192.168.0.0/24,internal- will

result in a query for internal-192-168-0-56.thekelleys.org.uk returning 192.168.0.56 and a reverse

query vice versa. The same applies to IPv6, but IPv6 addresses may start with ‘::’ but DNS labels may

not start with ‘-‘ so in this case if no prefix is configured a zero is added in front of the label.

::1 becomes 0–1.

The address range can be of the form <ip address>,<ip address> or <ip address>/<netmask>

–add-mac
Add the MAC address of the requestor to DNS queries which are forwarded upstream. This may be used to

DNS filtering by the upstream server. The MAC address can only be added if the requestor is on the

same subnet as the dnsmasq server. Note that the mechanism used to achieve this (an EDNS0 option) is

not yet standardised, so this should be considered experimental. Also note that exposing MAC addresses

in this way may have security and privacy implications. The warning about caching given for –add-sub‐

net applies to –add-mac too.

–add-subnet[[=<IPv4 prefix length>],<IPv6 prefix length>]
Add the subnet address of the requestor to the DNS queries which are forwarded upstream. The amount of

the address forwarded depends on the prefix length parameter: 32 (128 for IPv6) forwards the whole

address, zero forwards none of it but still marks the request so that no upstream nameserver will add

client address information either. The default is zero for both IPv4 and IPv6. Note that upstream

nameservers may be configured to return different results based on this information, but the dnsmasq

cache does not take account. If a dnsmasq instance is configured such that different results may be

encountered, caching should be disabled.

-c, –cache-size=<cachesize>
Set the size of dnsmasq’s cache. The default is 150 names. Setting the cache size to zero disables

caching.

-N, –no-negcache
Disable negative caching. Negative caching allows dnsmasq to remember “no such domain” answers from

upstream nameservers and answer identical queries without forwarding them again.

-0, –dns-forward-max=<queries>
Set the maximum number of concurrent DNS queries. The default value is 150, which should be fine for

most setups. The only known situation where this needs to be increased is when using web-server log

file resolvers, which can generate large numbers of concurrent queries.

–dnssec
Validate DNS replies and cache DNSSEC data. When forwarding DNS queries, dnsmasq requests the DNSSEC

records needed to validate the replies. The replies are validated and the result returned as the

Authenticated Data bit in the DNS packet. In addition the DNSSEC records are stored in the cache, mak‐

ing validation by clients more efficient. Note that validation by clients is the most secure DNSSEC

mode, but for clients unable to do validation, use of the AD bit set by dnsmasq is useful, provided

that the network between the dnsmasq server and the client is trusted. Dnsmasq must be compiled with

HAVE_DNSSEC enabled, and DNSSEC trust anchors provided, see –trust-anchor. Because the DNSSEC vali‐

dation 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 sta‐

tus of answers. In the default mode, this menas that all replies will be marked as untrusted. If

–dnssec-check-unsigned is set and the upstream servers don’t support DNSSEC, then DNS service will be

entirely broken.

–trust-anchor=[<class>],<domain>,<key-tag>,<algorithm>,<digest-type>,<digest>
Provide DS records to act a trust anchors for DNSSEC validation. Typically these will be the DS

record(s) for Zone Signing key(s) of the root zone, but trust anchors for limited domains are also

possible. The current root-zone trust anchors may be downloaded from
https://data.iana.org/root

anchors/root-anchors.xml

–dnssec-check-unsigned
As a default, dnsmasq does not check that unsigned DNS replies are legitimate: they are assumed to be

valid and passed on (without the “authentic data” bit set, of course). This does not protect against

an attacker forging unsigned replies for signed DNS zones, but it is fast. If this flag is set, dns‐

masq will check the zones of unsigned replies, to ensure that unsigned replies are allowed in those

zones. The cost of this is more upstream queries and slower performance. See also the warning about

upstream servers in the section on –dnssec

–dnssec-no-timecheck
DNSSEC signatures are only valid for specified time windows, and should be rejected outside those win‐

dows. This generates an interesting chicken-and-egg problem for machines which don’t have a hardware

real time clock. For these machines to determine the correct time typically requires use of NTP and

therefore DNS, but validating DNS requires that the correct time is already known. Setting this flag

removes the time-window checks (but not other DNSSEC validation.) only until the dnsmasq process

receives SIGHUP. The intention is that dnsmasq should be started with this flag when the platform

determines that reliable time is not currently available. As soon as reliable time is established, a

SIGHUP should be sent to dnsmasq, which enables time checking, and purges the cache of DNS records

which have not been throughly checked.

–dnssec-timestamp=<path>
Enables an alternative way of checking the validity of the system time for DNSSEC (see –dnssec-no-

timecheck). In this case, the system time is considered to be valid once it becomes later than the

timestamp on the specified file. The file is created and its timestamp set automatically by dnsmasq.

The file must be stored on a persistent filesystem, so that it and its mtime are carried over system

restarts. The timestamp file is created after dnsmasq has dropped root, so it must be in a location

writable by the unprivileged user that dnsmasq runs as.

–proxy-dnssec
Copy the DNSSEC Authenticated Data bit from upstream servers to downstream clients and cache it. This

is an alternative to having dnsmasq validate DNSSEC, but it depends on the security of the network

between dnsmasq and the upstream servers, and the trustworthiness of the upstream servers.

–dnssec-debug
Set debugging mode for the DNSSEC validation, set the Checking Disabled bit on upstream queries, and

don’t convert replies which do not validate to responses with a return code of SERVFAIL. Note that

setting this may affect DNS behaviour in bad ways, it is not an extra-logging flag and should not be

set in production.

–auth-zone=<domain>[,<subnet>[/<prefix length>][,<subnet>[/<prefix length>]…..]]
Define a DNS zone for which dnsmasq acts as authoritative server. Locally defined DNS records which

are in the domain will be served. If subnet(s) are given, A and AAAA records must be in one of the

specified subnets.

As alternative to directly specifying the subnets, it’s possible to give the name of an interface, in

which case the subnets implied by that interface’s configured addresses and netmask/prefix-length are

used; this is useful when using constructed DHCP ranges as the actual address is dynamic and not known

when configuring dnsmasq. The interface addresses may be confined to only IPv6 addresses using <inter‐

face>/6 or to only IPv4 using <interface>/4. This is useful when an interface has dynamically deter‐

mined global IPv6 addresses which should appear in the zone, but RFC1918 IPv4 addresses which should

not. Interface-name and address-literal subnet specifications may be used freely in the same –auth-

zone declaration.

The subnet(s) are also used to define in-addr.arpa and ip6.arpa domains which are served for reverse-

DNS queries. If not specified, the prefix length defaults to 24 for IPv4 and 64 for IPv6. For IPv4

subnets, the prefix length should be have the value 8, 16 or 24 unless you are familiar with RFC 2317

and have arranged the in-addr.arpa delegation accordingly. Note that if no subnets are specified, then

no reverse queries are answered.

–auth-soa=<serial>[,<hostmaster>[,<refresh>[,<retry>[,<expiry>]]]]
Specify fields in the SOA record associated with authoritative zones. Note that this is optional, all

the values are set to sane defaults.

–auth-sec-servers=<domain>[,<domain>[,<domain>…]]
Specify any secondary servers for a zone for which dnsmasq is authoritative. These servers must be

configured to get zone data from dnsmasq by zone transfer, and answer queries for the same authorita‐

tive zones as dnsmasq.

–auth-peer=<ip-address>[,<ip-address>[,<ip-address>…]]
Specify the addresses of secondary servers which are allowed to initiate zone transfer (AXFR) requests

for zones for which dnsmasq is authoritative. If this option is not given, then AXFR requests will be

accepted from any secondary.

–conntrack
Read the Linux connection track mark associated with incoming DNS queries and set the same mark value

on upstream traffic used to answer those queries. This allows traffic generated by dnsmasq to be asso‐

ciated with the queries which cause it, useful for bandwidth accounting and firewalling. Dnsmasq must

have conntrack support compiled in and the kernel must have conntrack support included and configured.

This option cannot be combined with –query-port.

-F, –dhcp-range=[tag:<tag>[,tag:<tag>],][set:<tag>,]<start-addr>[,<end-addr>][,<mode>][,<netmask>[,<broad‐

cast>]][,<lease time>]

-F, –dhcp-range=[tag:<tag>[,tag:<tag>],][set:<tag>,]<start-IPv6addr>[,<end-IPv6addr>|constructor:<inter‐

face>][,<mode>][,<prefix-len>][,<lease time>]

Enable the DHCP server. Addresses will be given out from the range <start-addr> to <end-addr> and from

statically defined addresses given in dhcp-host options. If the lease time is given, then leases will

be given for that length of time. The lease time is in seconds, or minutes (eg 45m) or hours (eg 1h)

or “infinite”. If not given, the default lease time is one hour. The minimum lease time is two min‐

utes. 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 net‐

work. For directly connected networks (ie, networks on which the machine running dnsmasq has an inter‐

face) the netmask is optional: dnsmasq will determine it from the interface configuration. For net‐

works which receive DHCP service via a relay agent, dnsmasq cannot determine the netmask itself, so it

should be specified, otherwise dnsmasq will have to guess, based on the class (A, B or C) of the net‐

work address. The broadcast address is always optional. It is always allowed to have more than one

dhcp-range in a single 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 mimimum size of the prefix length is 64.

IPv6 (only) supports another type of range. In this, the start address and optional end address con‐

tain only the network part (ie ::1) and they are followed by constructor:<interface>. This forms a

template which describes how to create ranges, based on the addresses assigned to the interface. For

instance

–dhcp-range=::1,::400,constructor:eth0

will look for addresses on eth0 and then create a range from <network>::1 to <network>::400. If the

interface is assigned more than one network, then the corresponding ranges will be automatically cre‐

ated, and then deprecated and finally removed again as the address is deprecated and then deleted. The

interface name may have a final “*” wildcard. Note that just any address on eth0 will not do: it must

not be an autoconfigured or privacy address, or be deprecated.

If a dhcp-range is only being used for stateless DHCP and/or SLAAC, then the address can be simply ::

–dhcp-range=::,constructor:eth0

The optional set:<tag> sets an alphanumeric label which marks this network so that dhcp options may be

specified on a per-network basis. When it is prefixed with ‘tag:’ instead, then its meaning changes

from setting a tag to matching it. Only one tag may be set, but more than one tag may be matched.

The optional <mode> keyword may be static which tells dnsmasq to enable DHCP for the network speci‐

fied, but not to dynamically allocate IP addresses: only hosts which have static addresses given via

dhcp-host or from /etc/ethers will be served. A static-only subnet with address all zeros may be used

as a “catch-all” address to enable replies to all Information-request packets on a subnet which is

provided with stateless DHCPv6, ie –dhcp-range=::,static

For IPv4, the <mode> may be proxy in which case dnsmasq will provide proxy-DHCP on the specified sub‐

net. (See pxe-prompt and pxe-service for details.)

For IPv6, the mode may be some combination of ra-only, slaac, ra-names, ra-stateless, ra-advrouter,

off-link.

ra-only tells dnsmasq to offer Router Advertisement only on this subnet, and not DHCP.

slaac tells dnsmasq to offer Router Advertisement on this subnet and to set the A bit in the router

advertisement, so that the client will use SLAAC addresses. When used with a DHCP range or static DHCP

address this results in the client having both a DHCP-assigned and a SLAAC address.

ra-stateless sends router advertisements with the O and A bits set, and provides a stateless DHCP ser‐

vice. The client will use a SLAAC address, and use DHCP for other configuration information.

ra-names enables a mode which gives DNS names to dual-stack hosts which do SLAAC for IPv6. Dnsmasq

uses the host’s IPv4 lease to derive the name, network segment and MAC address and assumes that the

host will also have an IPv6 address calculated using the SLAAC algorithm, on the same network segment.

The address is pinged, and if a reply is received, an AAAA record is added to the DNS for this IPv6

address. Note that this is only happens for directly-connected networks, (not one doing DHCP via a

relay) and it will not work if a host is using privacy extensions. ra-names can be combined with ra-

stateless and slaac.

ra-advrouter enables a mode where router address(es) rather than prefix(es) are included in the adver‐

tisements. This is described in RFC-3775 section 7.2 and is used in mobile IPv6. In this mode the

interval option is also included, as described in RFC-3775 section 7.3.

off-link tells dnsmasq to advertise the prefix without the on-link (aka L) bit set.

-G, –dhcp-host=[<hwaddr>][,id:<client_id>|*][,set:<tag>][,<ipaddr>][,<hostname>][,<lease_time>][,ignore]
Specify per host parameters for the DHCP server. This allows a machine with a particular hardware

address to be always allocated the same hostname, IP address and lease time. A hostname specified like

this overrides any supplied by the DHCP client on the machine. It is also allowable to omit the hard‐

ware address and include the hostname, in which case the IP address and lease times will apply to any

machine claiming that name. For example –dhcp-host=00:20:e0:3b:13:af,wap,infinite tells dnsmasq to

give the machine with hardware address 00:20:e0:3b:13:af the name wap, and an infinite DHCP lease.

–dhcp-host=lap,192.168.0.199 tells dnsmasq to always allocate the machine lap the IP address

192.168.0.199.

Addresses allocated like this are not constrained to be in the range given by the –dhcp-range option,

but they must be in the same subnet as some valid dhcp-range. For subnets which don’t need a pool of

dynamically allocated addresses, use the “static” keyword in the dhcp-range declaration.

It is allowed to use client identifiers (called client DUID in IPv6-land rather than hardware

addresses to identify hosts by prefixing with ‘id:’. Thus: –dhcp-host=id:01:02:03:04,….. refers to

the host with client identifier 01:02:03:04. It is also allowed to specify the client ID as text, like

this: –dhcp-host=id:clientidastext,…..

A single dhcp-host may contain an IPv4 address or an IPv6 address, or both. IPv6 addresses must be

bracketed by square brackets thus: –dhcp-host=laptop,[1234::56] IPv6 addresses may contain only the

host-identifier part: –dhcp-host=laptop,[::56] in which case they act as wildcards in constructed

dhcp ranges, with the appropriate network part inserted. Note that in IPv6 DHCP, the hardware address

may not be available, though it normally is for direct-connected clients, or clients using DHCP relays

which support RFC 6939.

For DHCPv4, the special option id:* means “ignore any client-id and use MAC addresses only.” This is

useful when a client presents a client-id sometimes but not others.

If a name appears in /etc/hosts, the associated address can be allocated to a DHCP lease, but only if

a –dhcp-host option specifying the name also exists. Only one hostname can be given in a dhcp-host

option, but aliases are possible by using CNAMEs. (See –cname ).

The special keyword “ignore” tells dnsmasq to never offer a DHCP lease to a machine. The machine can

be specified by hardware address, client ID or hostname, for instance –dhcp-

host=00:20:e0:3b:13:af,ignore This is useful when there is another DHCP server on the network which

should be used by some machines.

The set:<tag> construct sets the tag whenever this dhcp-host directive is in use. This can be used to

selectively send DHCP options just for this host. More than one tag can be set in a dhcp-host direc‐

tive (but not in other places where “set:<tag>” is allowed). When a host matches any dhcp-host direc‐

tive (or one implied by /etc/ethers) then the special tag “known” is set. This allows dnsmasq to be

configured to ignore requests from unknown machines using –dhcp-ignore=tag:!known Ethernet addresses

(but not client-ids) may have wildcard bytes, so for example –dhcp-host=00:20:e0:3b:13:*,ignore will

cause dnsmasq to ignore a range of hardware addresses. Note that the “*” will need to be escaped or

quoted on a command line, but not in the configuration file.

Hardware addresses normally match any network (ARP) type, but it is possible to restrict them to a

single ARP type by preceding them with the ARP-type (in HEX) and “-“. so –dhcp-

host=06-00:20:e0:3b:13:af,1.2.3.4 will only match a Token-Ring hardware address, since the ARP-address

type for token ring is 6.

As a special case, in DHCPv4, it is possible to include more than one hardware address. eg: –dhcp-

host=11:22:33:44:55:66,12:34:56:78:90:12,192.168.0.2 This allows an IP address to be associated with

multiple hardware addresses, and gives dnsmasq permission to abandon a DHCP lease to one of the hard‐

ware addresses when another one asks for a lease. Beware that this is a dangerous thing to do, it will

only work reliably if only one of the hardware addresses is active at any time and there is no way for

dnsmasq to enforce this. It is, for instance, useful to allocate a stable IP address to a laptop which

has both wired and wireless interfaces.

–dhcp-hostsfile=<path>
Read DHCP host information from the specified file. If a directory is given, then read all the files

contained in that directory. The file contains information about one host per line. The format of a

line is the same as text to the right of ‘=’ in –dhcp-host. The advantage of storing DHCP host infor‐

mation in this file is that it can be changed without re-starting dnsmasq: the file will be re-read

when dnsmasq receives SIGHUP.

–dhcp-optsfile=<path>
Read DHCP option information from the specified file. If a directory is given, then read all the

files contained in that directory. The advantage of using this option is the same as for –dhcp-hosts‐

file: the dhcp-optsfile will be re-read when dnsmasq receives SIGHUP. Note that it is possible to

encode the information in a

–dhcp-hostsdir=<path>
This is equivalent to dhcp-hostsfile, except for the following. The path MUST be a directory, and not

an individual file. Changed or new files within the directory are read automatically, without the need

to send SIGHUP. If a file is deleted for changed after it has been read by dnsmasq, then the host

record it contained will remain until dnsmasq recieves a SIGHUP, or is restarted; ie host records are

only added dynamically.

–dhcp-optsdir=<path>
This is equivalent to dhcp-optsfile, with the differences noted for –dhcp-hostsdir.

–dhcp-boot
flag as DHCP options, using the options names bootfile-name, server-ip-address and tftp-server. This

allows these to be included in a dhcp-optsfile.

-Z, –read-ethers
Read /etc/ethers for information about hosts for the DHCP server. The format of /etc/ethers is a hard‐

ware address, followed by either a hostname or dotted-quad IP address. When read by dnsmasq these

lines have exactly the same effect as –dhcp-host options containing the same information. /etc/ethers

is re-read when dnsmasq receives SIGHUP. IPv6 addresses are NOT read from /etc/ethers.

-O, –dhcp-option=[tag:<tag>,[tag:<tag>,]][encap:<opt>,][vi-encap:<enterprise>,][vendor:[<vendor-

class>],][<opt>|option:<opt-name>|option6:<opt>|option6:<opt-name>],[<value>[,<value>]]
Specify different or extra options to DHCP clients. By default, dnsmasq sends some standard options to

DHCP clients, the netmask and broadcast address are set to the same as the host running dnsmasq, and

the DNS server and default route are set to the address of the machine running dnsmasq. (Equivalent

rules apply for IPv6.) If the domain name option has been set, that is sent. This configuration

allows these defaults to be overridden, or other options specified. The option, to be sent may be

given as a decimal number or as “option:<option-name>” The option numbers are specified in RFC2132 and

subsequent RFCs. The set of option-names known by dnsmasq can be discovered by running “dnsmasq –help

dhcp”. For example, to set the default route option to 192.168.4.4, do –dhcp-option=3,192.168.4.4 or

–dhcp-option = option:router, 192.168.4.4 and to set the time-server address to 192.168.0.4, do

–dhcp-option = 42,192.168.0.4 or –dhcp-option = option:ntp-server, 192.168.0.4 The special address

0.0.0.0 is taken to mean “the address of the machine running dnsmasq”.

Data types allowed are comma separated dotted-quad IPv4 addresses, []-wrapped IPv6 addresses, a deci‐

mal 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 dot‐

ted-quad IP addresses as arguments to option 120 are handled as per RFC 3361. Dotted-quad IP addresses

which are followed by a slash and then a netmask size are encoded as described in RFC 3442.

IPv6 options are specified using the option6: keyword, followed by the option number or option name.

The IPv6 option name space is disjoint from the IPv4 option name space. IPv6 addresses in options must

be bracketed with square brackets, eg. –dhcp-option=option6:ntp-server,[1234::56] For IPv6, [::]

means “the global address of the machine running dnsmasq”, whilst [fd00::] is replaced with the ULA,

if it exists, and [fe80::] with the link-local address.

Be careful: no checking is done that the correct type of data for the option number is sent, it is

quite possible to persuade dnsmasq to generate illegal DHCP packets with injudicious use of this flag.

When the value is a decimal number, dnsmasq must determine how large the data item is. It does this by

examining the option number and/or the value, but can be overridden by appending a single letter flag

as follows: b = one byte, s = two bytes, i = four bytes. This is mainly useful with encapsulated ven‐

dor class options (see below) where dnsmasq cannot determine data size from the option number. Option

data which consists solely of periods and digits will be interpreted by dnsmasq as an IP address, and

inserted into an option as such. To force a literal string, use quotes. For instance when using option

66 to send a literal IP address as TFTP server name, it is necessary to do –dhcp-option=66,”1.2.3.4″

Encapsulated Vendor-class options may also be specified (IPv4 only) using –dhcp-option: for instance

–dhcp-option=vendor:PXEClient,1,0.0.0.0 sends the encapsulated vendor class-specific option “mftp-

address=0.0.0.0″ to any client whose vendor-class matches “PXEClient”. The vendor-class matching is

substring based (see –dhcp-vendorclass for details). If a vendor-class option (number 60) is sent by

dnsmasq, then that is used for selecting encapsulated options in preference to any sent by the client.

It is possible to omit the vendorclass completely; –dhcp-option=vendor:,1,0.0.0.0 in which case the

encapsulated option is always sent.

Options may be encapsulated (IPv4 only) within other options: for instance –dhcp-option=encap:175,

190, iscsi-client0 will send option 175, within which is the option 190. If multiple options are given

which are encapsulated with the same option number then they will be correctly combined into one

encapsulated option. encap: and vendor: are may not both be set in the same dhcp-option.

The final variant on encapsulated options is “Vendor-Identifying Vendor Options” as specified by

RFC3925. These are denoted like this: –dhcp-option=vi-encap:2, 10, text The number in the vi-encap:

section is the IANA enterprise number used to identify this option. This form of encapsulation is sup‐

ported in IPv6.

The address 0.0.0.0 is not treated specially in encapsulated options.

–dhcp-option-force=[tag:<tag>,[tag:<tag>,]][encap:<opt>,][vi-encap:<enterprise>,][vendor:[<vendor-

class>],]<opt>,[<value>[,<value>]]
This works in exactly the same way as –dhcp-option except that the option will always be sent, even

if the client does not ask for it in the parameter request list. This is sometimes needed, for example

when sending options to PXELinux.

–dhcp-no-override
(IPv4 only) Disable re-use of the DHCP servername and filename fields as extra option space. If it

can, dnsmasq moves the boot server and filename information (from dhcp-boot) out of their dedicated

fields into DHCP options. This make extra space available in the DHCP packet for options but can,

rarely, confuse old or broken clients. This flag forces “simple and safe” behaviour to avoid problems

in such a case.

–dhcp-relay=<local address>,<server address>[,<interface]
Configure dnsmasq to do DHCP relay. The local address is an address allocated to an interface on the

host running dnsmasq. All DHCP requests arriving on that interface will we relayed to a remote DHCP

server at the server address. It is possible to relay from a single local address to multiple remote

servers by using multiple dhcp-relay configs with the same local address and different server

addresses. A server address must be an IP literal address, not a domain name. In the case of DHCPv6,

the server address may be the ALL_SERVERS multicast address, ff05::1:3. In this case the interface

must be given, not be wildcard, and is used to direct the multicast to the correct interface to reach

the DHCP server.

Access control for DHCP clients has the same rules as for the DHCP server, see –interface, –except-

interface, etc. The optional interface name in the dhcp-relay config has a different function: it con‐

trols on which interface DHCP replies from the server will be accepted. This is intended for configu‐

rations which have three interfaces: one being relayed from, a second connecting the DHCP server, and

a third untrusted network, typically the wider internet. It avoids the possibility of spoof replies

arriving via this third interface.

It is allowed to have dnsmasq act as a DHCP server on one set of interfaces and relay from a disjoint

set of interfaces. Note that whilst it is quite possible to write configurations which appear to act

as a server and a relay on the same interface, this is not supported: the relay function will take

precedence.

Both DHCPv4 and DHCPv6 relay is supported. It’s not possible to relay DHCPv4 to a DHCPv6 server or

vice-versa.

-U, –dhcp-vendorclass=set:<tag>,[enterprise:<IANA-enterprise number>,]<vendor-class>
Map from a vendor-class string to a tag. Most DHCP clients provide a “vendor class” which represents,

in some sense, the type of host. This option maps vendor classes to tags, so that DHCP options may be

selectively delivered to different classes of hosts. For example dhcp-vendorclass=set:print‐

ers,Hewlett-Packard JetDirect will allow options to be set only for HP printers like so: –dhcp-

option=tag:printers,3,192.168.4.4 The vendor-class string is substring matched against the vendor-

class supplied by the client, to allow fuzzy matching. The set: prefix is optional but allowed for

consistency.

Note that in IPv6 only, vendorclasses are namespaced with an IANA-allocated enterprise number. This is

given with enterprise: keyword and specifies that only vendorclasses matching the specified number

should be searched.

-j, –dhcp-userclass=set:<tag>,<user-class>
Map from a user-class string to a tag (with substring matching, like vendor classes). Most DHCP

clients provide a “user class” which is configurable. This option maps user classes to tags, so that

DHCP options may be selectively delivered to different classes of hosts. It is possible, for instance

to use this to set a different printer server for hosts in the class “accounts” than for hosts in the

class “engineering”.

-4, –dhcp-mac=set:<tag>,<MAC address>
Map from a MAC address to a tag. The MAC address may include wildcards. For example –dhcp-

mac=set:3com,01:34:23:*:*:* will set the tag “3com” for any host whose MAC address matches the pat‐

tern.

–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 cir‐

cuit-id or remote-id is normally given as colon-separated hex, but is also allowed to be a simple

string. If an exact match is achieved between the circuit or agent ID and one provided by a relay

agent, the tag is set.

dhcp-remoteid (but not dhcp-circuitid) is supported in IPv6.

–dhcp-subscrid=set:<tag>,<subscriber-id>
(IPv4 and IPv6) Map from RFC3993 subscriber-id relay agent options to tags.

–dhcp-proxy[=<ip addr>]……
(IPv4 only) A normal DHCP relay agent is only used to forward the initial parts of a DHCP interaction

to the DHCP server. Once a client is configured, it communicates directly with the server. This is

undesirable if the relay agent is adding extra information to the DHCP packets, such as that used by

dhcp-circuitid and dhcp-remoteid. A full relay implementation can use the RFC 5107 serverid-override

option to force the DHCP server to use the relay as a full proxy, with all packets passing through it.

This flag provides an alternative method of doing the same thing, for relays which don’t support RFC

5107. Given alone, it manipulates the server-id for all interactions via relays. If a list of IP

addresses is given, only interactions via relays at those addresses are affected.

–dhcp-match=set:<tag>,<option number>|option:<option name>|vi-encap:<enterprise>[,<value>]
Without a value, set the tag if the client sends a DHCP option of the given number or name. When a

value is given, set the tag only if the option is sent and matches the value. The value may be of the

form “01:ff:*:02” in which case the value must match (apart from wildcards) but the option sent may

have unmatched data past the end of the value. The value may also be of the same form as in dhcp-

option in which case the option sent is treated as an array, and one element must match, so

–dhcp-match=set:efi-ia32,option:client-arch,6

will set the tag “efi-ia32” if the the number 6 appears in the list of architectures sent by the

client in option 93. (See RFC 4578 for details.) If the value is a string, substring matching is

used.

The special form with vi-encap:<enterprise number> matches against vendor-identifying vendor classes

for the specified enterprise. Please see RFC 3925 for more details of these rare and interesting

beasts.

–tag-if=set:<tag>[,set:<tag>[,tag:<tag>[,tag:<tag>]]]
Perform boolean operations on tags. Any tag appearing as set:<tag> is set if all the tags which appear

as tag:<tag> are set, (or unset when tag:!<tag> is used) If no tag:<tag> appears set:<tag> tags are

set unconditionally. Any number of set: and tag: forms may appear, in any order. Tag-if lines ares

executed in order, so if the tag in tag:<tag> is a tag set by another tag-if, the line which sets the

tag must precede the one which tests it.

-J, –dhcp-ignore=tag:<tag>[,tag:<tag>]
When all the given tags appear in the tag set ignore the host and do not allocate it a DHCP lease.

–dhcp-ignore-names[=tag:<tag>[,tag:<tag>]]
When all the given tags appear in the tag set, ignore any hostname provided by the host. Note that,

unlike dhcp-ignore, it is permissible to supply no tags, in which case DHCP-client supplied hostnames

are always ignored, and DHCP hosts are added to the DNS using only dhcp-host configuration in dnsmasq

and the contents of /etc/hosts and /etc/ethers.

–dhcp-generate-names=tag:<tag>[,tag:<tag>]
(IPv4 only) Generate a name for DHCP clients which do not otherwise have one, using the MAC address

expressed in hex, separated by dashes. Note that if a host provides a name, it will be used by prefer‐

ence to this, unless –dhcp-ignore-names is set.

–dhcp-broadcast[=tag:<tag>[,tag:<tag>]]
(IPv4 only) When all the given tags appear in the tag set, always use broadcast to communicate with

the host when it is unconfigured. It is permissible to supply no tags, in which case this is uncondi‐

tional. Most DHCP clients which need broadcast replies set a flag in their requests so that this hap‐

pens automatically, some old BOOTP clients do not.

-M, –dhcp-boot=[tag:<tag>,]<filename>,[<servername>[,<server address>|<tftp_servername>]]
(IPv4 only) Set BOOTP options to be returned by the DHCP server. Server name and address are optional:

if not provided, the name is left empty, and the address set to the address of the machine running

dnsmasq. If dnsmasq is providing a TFTP service (see –enable-tftp ) then only the filename is

required here to enable network booting. If the optional tag(s) are given, they must match for this

configuration to be sent. Instead of an IP address, the TFTP server address can be given as a domain

name which is looked up in /etc/hosts. This name can be associated in /etc/hosts with multiple IP

addresses, which are used round-robin. This facility can be used to load balance the tftp load among

a set of servers.

–dhcp-sequential-ip
Dnsmasq is designed to choose IP addresses for DHCP clients using a hash of the client’s MAC address.

This normally allows a client’s address to remain stable long-term, even if the client sometimes

allows its DHCP lease to expire. In this default mode IP addresses are distributed pseudo-randomly

over the entire available address range. There are sometimes circumstances (typically server deploy‐

ment) where it is more convenient to have IP addresses allocated sequentially, starting from the low‐

est available address, and setting this flag enables this mode. Note that in the sequential mode,

clients which allow a lease to expire are much more likely to move IP address; for this reason it

should not be generally used.

–pxe-service=[tag:<tag>,]<CSA>,<menu text>[,<basename>|<bootservicetype>][,<server address>|<server_name>]
Most uses of PXE boot-ROMS simply allow the PXE system to obtain an IP address and then download the

file specified by dhcp-boot and execute it. However the PXE system is capable of more complex func‐

tions when supported by a suitable DHCP server.

This specifies a boot option which may appear in a PXE boot menu. <CSA> is client system type, only

services of the correct type will appear in a menu. The known types are x86PC, PC98, IA64_EFI, Alpha,

Arc_x86, Intel_Lean_Client, IA32_EFI, BC_EFI, Xscale_EFI and X86-64_EFI; an integer may be used for

other types. The parameter after the menu text may be a file name, in which case dnsmasq acts as a

boot server and directs the PXE client to download the file by TFTP, either from itself ( enable-tftp

must be set for this to work) or another TFTP server if the final server address/name is given. Note

that the “layer” suffix (normally “.0”) is supplied by PXE, and should not be added to the basename.

If an integer boot service type, rather than a basename is given, then the PXE client will search for

a suitable boot service for that type on the network. This search may be done by broadcast, or direct

to a server if its IP address/name is provided. If no boot service type or filename is provided (or a

boot service type of 0 is specified) then the menu entry will abort the net boot procedure and con‐

tinue booting from local media. The server address can be given as a domain name which is looked up in

/etc/hosts. This name can be associated in /etc/hosts with multiple IP addresses, which are used

round-robin.

–pxe-prompt=[tag:<tag>,]<prompt>[,<timeout>]
Setting this provides a prompt to be displayed after PXE boot. If the timeout is given then after the

timeout has elapsed with no keyboard input, the first available menu option will be automatically exe‐

cuted. If the timeout is zero then the first available menu item will be executed immediately. If pxe-

prompt is omitted the system will wait for user input if there are multiple items in the menu, but

boot immediately if there is only one. See pxe-service for details of menu items.

Dnsmasq supports PXE “proxy-DHCP”, in this case another DHCP server on the network is responsible for

allocating IP addresses, and dnsmasq simply provides the information given in pxe-prompt and pxe-ser‐

vice to allow netbooting. This mode is enabled using the proxy keyword in dhcp-range.

-X, –dhcp-lease-max=<number>
Limits dnsmasq to the specified maximum number of DHCP leases. The default is 1000. This limit is to

prevent DoS attacks from hosts which create thousands of leases and use lots of memory in the dnsmasq

process.

-K, –dhcp-authoritative
Should be set when dnsmasq is definitely the only DHCP server on a network. For DHCPv4, it changes

the behaviour from strict RFC compliance so that DHCP requests on unknown leases from unknown hosts

are not ignored. This allows new hosts to get a lease without a tedious timeout under all circum‐

stances. It also allows dnsmasq to rebuild its lease database without each client needing to reacquire

a lease, if the database is lost. For DHCPv6 it sets the priority in replies to 255 (the maximum)

instead of 0 (the minimum).

–dhcp-alternate-port[=<server port>[,<client port>]]
(IPv4 only) Change the ports used for DHCP from the default. If this option is given alone, without

arguments, it changes the ports used for DHCP from 67 and 68 to 1067 and 1068. If a single argument is

given, that port number is used for the server and the port number plus one used for the client.

Finally, two port numbers allows arbitrary specification of both server and client ports for DHCP.

-3, –bootp-dynamic[=<network-id>[,<network-id>]]
(IPv4 only) Enable dynamic allocation of IP addresses to BOOTP clients. Use this with care, since each

address allocated to a BOOTP client is leased forever, and therefore becomes permanently unavailable

for re-use by other hosts. if this is given without tags, then it unconditionally enables dynamic

allocation. With tags, only when the tags are all set. It may be repeated with different tag sets.

-5, –no-ping
(IPv4 only) By default, the DHCP server will attempt to ensure that an address is not in use before

allocating it to a host. It does this by sending an ICMP echo request (aka “ping”) to the address in

question. If it gets a reply, then the address must already be in use, and another is tried. This flag

disables this check. Use with caution.

–log-dhcp
Extra logging for DHCP: log all the options sent to DHCP clients and the tags used to determine them.

–quiet-dhcp, –quiet-dhcp6, –quiet-ra
Suppress logging of the routine operation of these protocols. Errors and problems will still be

logged. –quiet-dhcp and quiet-dhcp6 are over-ridden by –log-dhcp.

-l, –dhcp-leasefile=<path>
Use the specified file to store DHCP lease information.

–dhcp-duid=<enterprise-id>,<uid>
(IPv6 only) Specify the server persistent UID which the DHCPv6 server will use. This option is not

normally required as dnsmasq creates a DUID automatically when it is first needed. When given, this

option provides dnsmasq the data required to create a DUID-EN type DUID. Note that once set, the DUID

is stored in the lease database, so to change between DUID-EN and automatically created DUIDs or vice-

versa, the lease database must be re-intialised. The enterprise-id is assigned by IANA, and the uid is

a string of hex octets unique to a particular device.

-6 –dhcp-script=<path>
Whenever a new DHCP lease is created, or an old one destroyed, or a TFTP file transfer completes, the

executable specified by this option is run. <path> must be an absolute pathname, no PATH search

occurs. The arguments to the process are “add”, “old” or “del”, the MAC address of the host (or DUID

for IPv6) , the IP address, and the hostname, if known. “add” means a lease has been created, “del”

means it has been destroyed, “old” is a notification of an existing lease when dnsmasq starts or a

change to MAC address or hostname of an existing lease (also, lease length or expiry and client-id, if

leasefile-ro is set). If the MAC address is from a network type other than ethernet, it will have the

network type prepended, eg “06-01:23:45:67:89:ab” for token ring. The process is run as root (assuming

that dnsmasq was originally run as root) even if dnsmasq is configured to change UID to an unprivi‐

leged 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 num‐

ber of seconds until lease expiry is always stored in DNSMASQ_TIME_REMAINING.

If a lease used to have a hostname, which is removed, an “old” event is generated with the new state

of the lease, ie no name, and the former name is provided in the environment variable DNS‐

MASQ_OLD_HOSTNAME.

DNSMASQ_INTERFACE stores the name of the interface on which the request arrived; this is not set for

“old” actions when dnsmasq restarts.

DNSMASQ_RELAY_ADDRESS is set if the client used a DHCP relay to contact dnsmasq and the IP address of

the relay is known.

DNSMASQ_TAGS contains all the tags set during the DHCP transaction, separated by spaces.

DNSMASQ_LOG_DHCP is set if –log-dhcp is in effect.

For IPv4 only:

DNSMASQ_CLIENT_ID if the host provided a client-id.

DNSMASQ_CIRCUIT_ID, DNSMASQ_SUBSCRIBER_ID, DNSMASQ_REMOTE_ID if a DHCP relay-agent added any of these

options.

If the client provides vendor-class, DNSMASQ_VENDOR_CLASS.

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 pre‐

fixed to ‘T’.

DNSMASQ_MAC containing the MAC address of the client, if known.

Note that the supplied hostname, vendorclass and userclass data is only supplied for “add” actions or

“old” actions when a host resumes an existing lease, since these data are not held in dnsmasq’s lease

database.

All file descriptors are closed except stdin, stdout and stderr which are open to /dev/null (except in

debug mode).

The script is not invoked concurrently: at most one instance of the script is ever running (dnsmasq

waits for an instance of script to exit before running the next). Changes to the lease database are

which require the script to be invoked are queued awaiting exit of a running instance. If this queue‐

ing 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 sig‐

nal, the script will be invoked for existing leases with an “old ” event.

There are two further actions which may appear as the first argument to the script, “init” and “tftp”.

More may be added in the future, so scripts should be written to ignore unknown actions. “init” is

described below in –leasefile-ro The “tftp” action is invoked when a TFTP file transfer completes:

the arguments are the file size in bytes, the address to which the file was sent, and the complete

pathname of the file.

–dhcp-luascript=<path>
Specify a script written in Lua, to be run when leases are created, destroyed or changed. To use this

option, dnsmasq must be compiled with the correct support. The Lua interpreter is intialised once,

when dnsmasq starts, so that global variables persist between lease events. The Lua code must define a

lease function, and may provide init and shutdown functions, which are called, without arguments when

dnsmasq starts up and terminates. It may also provide a tftp function.

The lease function receives the information detailed in –dhcp-script. It gets two arguments, firstly

the action, which is a string containing, “add”, “old” or “del”, and secondly a table of tag value

pairs. The tags mostly correspond to the environment variables detailed above, for instance the tag

“domain” holds the same data as the environment variable DNSMASQ_DOMAIN. There are a few extra tags

which hold the data supplied as arguments to –dhcp-script. These are mac_address, ip_address and

hostname for IPv4, and client_duid, ip_address and hostname for IPv6.

The tftp function is called in the same way as the lease function, and the table holds the tags desti‐

nation_address, file_name and file_size.

–dhcp-scriptuser
Specify the user as which to run the lease-change script or Lua script. This defaults to root, but can

be changed to another user using this flag.

-9, –leasefile-ro
Completely suppress use of the lease database file. The file will not be created, read, or written.

Change the way the lease-change script (if one is provided) is called, so that the lease database may

be maintained in external storage by the script. In addition to the invocations given in –dhcp-

script the lease-change script is called once, at dnsmasq startup, with the single argument “init”.

When called like this the script should write the saved state of the lease database, in dnsmasq lease‐

file format, to stdout and exit with zero exit code. Setting this option also forces the leasechange

script to be called on changes to the client-id and lease length and expiry time.

–bridge-interface=<interface>,<alias>[,<alias>]
Treat DHCP (v4 and v6) request and IPv6 Router Solicit packets arriving at any of the <alias> inter‐

faces as if they had arrived at <interface>. This option allows dnsmasq to provide DHCP and RA ser‐

vice 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>.

-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-con‐

figured 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 dis‐

allowed 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=thekel‐

leys.org.uk and have a machine whose DHCP hostname is “laptop”. The IP address for that machine is

available from dnsmasq both as “laptop” and “laptop.thekelleys.org.uk”. If the domain is given as “#”

then the domain is read from the first “search” directive in /etc/resolv.conf (or equivalent).

The address range can be of the form <ip address>,<ip address> or <ip address>/<netmask> or just a

single <ip address>. See –dhcp-fqdn which can change the behaviour of dnsmasq with domains.

If the address range is given as ip-address/network-size, then a additional flag “local” may be sup‐

plied which has the effect of adding –local declarations for forward and reverse DNS queries. Eg.

–domain=thekelleys.org.uk,192.168.0.0/24,local is identical to –domain=thekel‐

leys.org.uk,192.168.0.0/24 –local=/thekelleys.org.uk/ –local=/0.168.192.in-addr.arpa/ The network

size must be 8, 16 or 24 for this to be legal.

–dhcp-fqdn
In the default mode, dnsmasq inserts the unqualified names of DHCP clients into the DNS. For this rea‐

son, the names must be unique, even if two clients which have the same name are in different domains.

If a second DHCP client appears which has the same name as an existing client, the name is transferred

to the new client. If –dhcp-fqdn is set, this behaviour changes: the unqualified name is no longer

put in the DNS, only the qualified name. Two DHCP clients with the same name may both keep the name,

provided that the domain part is different (ie the fully qualified names differ.) To ensure that all

names have a domain part, there must be at least –domain without an address specified when –dhcp-

fqdn is set.

–dhcp-client-update
Normally, when giving a DHCP lease, dnsmasq sets flags in the FQDN option to tell the client not to

attempt a DDNS update with its name and IP address. This is because the name-IP pair is automatically

added into dnsmasq’s DNS view. This flag suppresses that behaviour, this is useful, for instance, to

allow Windows clients to update Active Directory servers. See RFC 4702 for details.

–enable-ra
Enable dnsmasq’s IPv6 Router Advertisement feature. DHCPv6 doesn’t handle complete network configura‐

tion 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 and recursive DNS

server as the relevant link-local address on the machine running dnsmasq. By default, he “managed

address” bits are set, and the “use SLAAC” bit is reset. This can be changed for individual subnets

with the mode keywords described in –dhcp-range. RFC6106 DNS parameters are included in the adver‐

tisements. By default, the relevant link-local address of the machine running dnsmasq is sent as

recursive DNS server. If provided, the DHCPv6 options dns-server and domain-search are used for RDNSS

and DNSSL.

–ra-param=<interface>,[high|low],[[<ra-interval>],<router lifetime>]
Set non-default values for router advertisements sent via an interface. The priority field for the

router may be altered from the default of medium with eg –ra-param=eth0,high. The interval between

router advertisements may be set (in seconds) with –ra-param=eth0,60. The lifetime of the route may

be changed or set to zero, which allows a router to advertise prefixes but not a route via itself.

–ra-parm=eth0,0,0 (A value of zero for the interval means the default value.) All three parameters

may be set at once. –ra-param=low,60,1200 The interface field may include a wildcard.

–enable-tftp[=<interface>[,<interface>]]
Enable the TFTP server function. This is deliberately limited to that needed to net-boot a client.

Only reading is allowed; the tsize and blksize extensions are supported (tsize is only 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 recieve TFTP service.

–tftp-root=<directory>[,<interface>]
Look for files to transfer using TFTP relative to the given directory. When this is set, TFTP paths

which include “..” are rejected, to stop clients getting outside the specified root. Absolute paths

(starting with /) are allowed, but they must be within the tftp-root. If the optional interface argu‐

ment is given, the directory is only used for TFTP requests via that interface.

–tftp-no-fail
Do not abort startup if specified tftp root directories are inaccessible.

–tftp-unique-root
Add the IP address of the TFTP client as a path component on the end of the TFTP-root (in standard

dotted-quad format). Only valid if a tftp-root is set and the directory exists. For instance, if tftp-

root is “/tftp” and client 1.2.3.4 requests file “myfile” then the effective path will be

“/tftp/1.2.3.4/myfile” if /tftp/1.2.3.4 exists or /tftp/myfile otherwise.

–tftp-secure
Enable TFTP secure mode: without this, any file which is 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, differ‐

ent rules apply: –tftp-secure has no effect, but only files which have the world-readable bit set are

accessible. It is not recommended to run dnsmasq as root with TFTP enabled, and certainly not without

specifying –tftp-root. Doing so can expose any world-readable file on the server to any host on the

net.

–tftp-lowercase
Convert filenames in TFTP requests to all lowercase. This is useful for requests from Windows

machines, which have case-insensitive filesystems and tend to play fast-and-loose with case in file‐

names. Note that dnsmasq’s tftp server always converts “\” to “/” in filenames.

–tftp-max=<connections>
Set the maximum number of concurrent TFTP connections allowed. This defaults to 50. When serving a

large number of TFTP connections, per-process file descriptor limits may be encountered. Dnsmasq needs

one file descriptor for each concurrent TFTP connection and one file descriptor per unique file (plus

a few others). So serving the same file simultaneously to n clients will use require about n + 10 file

descriptors, serving different files simultaneously to n clients will require about (2*n) + 10

descriptors. If –tftp-port-range is given, that can affect the number of concurrent connections.

–tftp-no-blocksize
Stop the TFTP server from negotiating the “blocksize” option with a client. Some buggy clients request

this option but then behave badly when it is granted.

–tftp-port-range=<start>,<end>
A TFTP server listens on a well-known port (69) for connection initiation, but it also uses a dynami‐

cally-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 num‐

ber of concurrent TFTP connections is limited by the size of the port range.

-C, –conf-file=<file>
Specify a different configuration file. The conf-file option is also allowed in configuration files,

to include multiple configuration files. A filename of “-” causes dnsmasq to read configuration from

stdin.

-7, –conf-dir=<directory>[,<file-extension>……],
Read all the files in the given directory as configuration files. If extension(s) are given, any files

which end in those extensions are skipped. Any files whose names end in ~ or start with . or start and

end with # are always skipped. If the extension starts with * then only files which have that exten‐

sion are loaded. So –conf-dir=/path/to/dir,*.conf loads all files with the suffix .conf in

/path/to/dir. This flag may be given on the command line or in a configuration file. If giving it on

the command line, be sure to escape * characters.

–servers-file=<file>
A special case of –conf-file which differs in two respects. Firstly, only –server and –rev-server

are allowed in the configuration file included. Secondly, the file is re-read and the configuration

therein is updated when dnsmasq recieves SIGHUP.

CONFIG FILE

At startup, dnsmasq reads /etc/dnsmasq.conf, if it exists. (On FreeBSD, the file is /usr/local/etc/dns‐

masq.conf ) (but see the -C and -7 options.) The format of this file consists of one option per line, exactly

as the long options detailed in the OPTIONS section but without the leading “–“. Lines starting with # are

comments and ignored. For options which may only be specified once, the configuration file overrides the com‐

mand line. Quoting is allowed in a config file: between ” quotes the special meanings of ,:. and # are

removed and the following escapes are allowed: \\ \” \t \e \b \r and \n. The later corresponding to tab,

escape, backspace, return and newline.

NOTES

When it receives a SIGHUP, dnsmasq clears its cache and then re-loads /etc/hosts and /etc/ethers and any file

given by –dhcp-hostsfile, –dhcp-hostsdir, –dhcp-optsfile, –dhcp-optsdir, –addn-hosts or –hostsdir. The

dhcp lease change script is called for all existing DHCP leases. If –no-poll is set SIGHUP also re-reads

/etc/resolv.conf. SIGHUP does NOT re-read the configuration file.

When it receives a SIGUSR1, dnsmasq writes statistics to the system log. It writes the cache size, the number

of names which have had to removed from the cache before they expired in order to make room for new names and

the total number of names that have been inserted into the cache. The number of cache hits and misses and the

number of authoritative queries answered are also given. For each upstream server it gives the number of

queries sent, and the number which resulted in an error. In –no-daemon mode or when full logging is enabled

(-q), a complete dump of the contents of the cache is made.

The cache statistics are also available in the DNS as answers to queries of class CHAOS and type TXT in

domain bind. The domain names are cachesize.bind, insertions.bind, evictions.bind, misses.bind, hits.bind,

auth.bind and servers.bind. An example command to query this, using the dig utility would be

dig +short chaos txt cachesize.bind

When it receives SIGUSR2 and it is logging direct to a file (see –log-facility ) dnsmasq will close and

reopen the log file. Note that during this operation, dnsmasq will not be running as root. When it first cre‐

ates the logfile dnsmasq changes the ownership of the file to the non-root user it will run as. Logrotate

should be configured to create a new log file with the ownership which matches the existing one before send‐

ing SIGUSR2. If TCP DNS queries are in progress, the old logfile will remain open in child processes which

are handling TCP queries and may continue to be written. There is a limit of 150 seconds, after which all

existing TCP processes will have expired: for this reason, it is not wise to configure logfile compression

for logfiles which have just been rotated. Using logrotate, the required options are create and delay‐

compress.

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 pro‐

vided by an ISP. By default, dnsmasq reads /etc/resolv.conf to discover the IP addresses of the upstream

nameservers it should use, since the information is typically stored there. Unless –no-poll is used, dnsmasq

checks the modification time of /etc/resolv.conf (or equivalent if –resolv-file is used) and re-reads it if

it changes. This allows the DNS servers to be set dynamically by PPP or DHCP since both protocols provide the

information. Absence of /etc/resolv.conf is not an error since it may not have been created before a PPP

connection exists. Dnsmasq simply keeps checking in case /etc/resolv.conf is created at any time. Dnsmasq can

be told to parse more than one resolv.conf file. This is useful on a laptop, where both PPP and DHCP may be

used: dnsmasq can be set to poll both /etc/ppp/resolv.conf and /etc/dhcpc/resolv.conf and will use the con‐

tents 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 speci‐

fications optionally take a domain name which tells dnsmasq to use that server only to find names in that

particular domain.

In order to configure dnsmasq to act as cache for the host on which it is running, put “nameserver 127.0.0.1”

in /etc/resolv.conf to force local processes to send queries to dnsmasq. Then either specify the upstream

servers directly to dnsmasq using –server options or put their addresses real in another file, say

/etc/resolv.dnsmasq and run dnsmasq with the -r /etc/resolv.dnsmasq option. This second technique allows for

dynamic update of the server addresses by PPP or DHCP.

Addresses in /etc/hosts will “shadow” different addresses for the same names in the upstream DNS, so

“mycompany.com 1.2.3.4” in /etc/hosts will ensure that queries for “mycompany.com” always return 1.2.3.4 even

if queries in the upstream DNS would otherwise return a different address. There is one exception to this: if

the upstream DNS contains a CNAME which points to a shadowed name, then looking up the CNAME through dnsmasq

will result in the unshadowed address associated with the target of the CNAME. To work around this, add the

CNAME to /etc/hosts so that the CNAME is shadowed too.

The tag system works as follows: For each DHCP request, dnsmasq collects a set of valid tags from active con‐

figuration lines which include set:<tag>, including one from the dhcp-range used to allocate the address, one

from any matching dhcp-host (and “known” if a dhcp-host matches) The tag “bootp” is set for BOOTP requests,

and a tag whose name is the name of the interface on which the request arrived is also set.

Any configuration lines which include one or more tag:<tag> constructs will only be valid if all that tags

are matched in the set derived above. Typically this is dhcp-option. dhcp-option which has tags will be used

in preference to an untagged dhcp-option, provided that _all_ the tags match somewhere in the set collected

as described above. The prefix ‘!’ on a tag means ‘not’ so –dhcp-option=tag:!purple,3,1.2.3.4 sends the

option when the tag purple is not in the set of valid tags. (If using this in a command line rather than a

configuration file, be sure to escape !, which is a shell metacharacter)

When selecting dhcp-options, a tag from dhcp-range is second class relative to other tags, to make it easy to

override options for individual hosts, so dhcp-range=set:interface1,…… dhcp-host=set:myhost,….. dhcp-

option=tag:interface1,option:nis-domain,”domain1″ dhcp-option=tag:myhost,option:nis-domain,”domain2″ will set

the NIS-domain to domain1 for hosts in the range, but override that to domain2 for a particular host.

Note that for dhcp-range both tag:<tag> and set:<tag> are allowed, to both select the range in use based on

(eg) dhcp-host, and to affect the options sent, based on the range selected.

This system evolved from an earlier, more limited one and for backward compatibility “net:” may be used

instead of “tag:” and “set:” may be omitted. (Except in dhcp-host, where “net:” may be used instead of

“set:”.) For the same reason, ‘#’ may be used instead of ‘!’ to indicate NOT.

The DHCP server in dnsmasq will function as a BOOTP server also, provided that the MAC address and IP address

for clients are given, either using dhcp-host configurations or in /etc/ethers , and a dhcp-range configura‐

tion option is present to activate the DHCP server on a particular network. (Setting –bootp-dynamic removes

the need for static address mappings.) The filename parameter in a BOOTP request is used as a tag, as is the

tag “bootp”, allowing some control over the options returned to different classes of hosts.

AUTHORITATIVE CONFIGURATION

Configuring dnsmasq to act as an authoritative DNS server is complicated by the fact that it involves config‐

uration 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 ques‐

tion. For the first part of this explanation, we will call the A (or AAAA) record for the globally accessible

address server.example.com, and the zone for which dnsmasq is authoritative our.zone.com.

The simplest configuration consists of two lines of dnsmasq configuration; something like

auth-server=server.example.com,eth0

auth-zone=our.zone.com,1.2.3.0/24

and two records in the external DNS

server.example.com A 192.0.43.10

our.zone.com NS server.example.com

eth0 is the external network interface on which dnsmasq is listening, and has (globally accessible) address

192.0.43.10.

Note that the external IP address may well be dynamic (ie assigned from an ISP by DHCP or PPP) If so, the A

record must be linked to this dynamic assignment by one of the usual dynamic-DNS systems.

A more complex, but practically useful configuration has the address record for the globally accessible IP

address residing in the authoritative zone which dnsmasq is serving, typically at the root. Now we have

auth-server=our.zone.com,eth0

auth-zone=our.zone.com,1.2.3.0/24

our.zone.com A 1.2.3.4

our.zone.com NS our.zone.com

The A record for our.zone.com has now become a glue record, it solves the chicken-and-egg problem of finding

the IP address of the nameserver for our.zone.com when the A record is within that zone. Note that this is

the only role of this record: as dnsmasq is now authoritative from our.zone.com it too must provide this

record. If the external address is static, this can be done with an /etc/hosts entry or –host-record.

auth-server=our.zone.com,eth0

host-record=our.zone.com,1.2.3.4

auth-zone=our.zone.com,1.2.3.0/24

If the external address is dynamic, the address associated with our.zone.com must be derived from the address

of the relevant interface. This is done using interface-name Something like:

auth-server=our.zone.com,eth0

interface-name=our.zone.com,eth0

auth-zone=our.zone.com,1.2.3.0/24,eth0

(The “eth0” argument in auth-zone adds the subnet containing eth0’s dynamic address to the zone, so that the

interface-name returns the address in outside queries.)

Our final configuration builds on that above, but also adds a secondary DNS server. This is another DNS

server which learns the DNS data for the zone by doing zones transfer, and acts as a backup should the pri‐

mary server become inaccessible. The configuration of the secondary is beyond the scope of this man-page, but

the extra configuration of dnsmasq is simple:

auth-sec-servers=secondary.myisp.com

and

our.zone.com NS secondary.myisp.com

Adding auth-sec-servers enables zone transfer in dnsmasq, to allow the secondary to collect the DNS data. If

you wish to restrict this data to particular hosts then

auth-peer=<IP address of secondary>

will do so.

Dnsmasq acts as an authoritative server for in-addr.arpa and ip6.arpa domains associated with the subnets

given in auth-zone declarations, so reverse (address to name) lookups can be simply configured with a suit‐

able NS record, for instance in this example, where we allow 1.2.3.0/24 addresses.

3.2.1.in-addr.arpa NS our.zone.com

Note that at present, reverse (in-addr.arpa and ip6.arpa) zones are not available in zone transfers, so there

is no point arranging secondary servers for reverse lookups.

When dnsmasq is configured to act as an authoritative server, the following data is used to populate the

authoritative zone.

–mx-host, –srv-host, –dns-rr, –txt-record, –naptr-record , as long as the record names are in the

authoritative domain.

–cname as long as the record name is in the authoritative domain. If the target of the CNAME is unquali‐

fied, then it is qualified with the authoritative zone name.

IPv4 and IPv6 addresses from /etc/hosts (and –addn-hosts ) and –host-record and –interface-name provided

the address falls into one of the subnets specified in the –auth-zone.

Addresses of DHCP leases, provided the address falls into one of the subnets specified in the –auth-zone.

(If contructed DHCP ranges are is use, which depend on the address dynamically assigned to an interface, then

the form of –auth-zone which defines subnets by the dynamic address of an interface should be used to ensure

this condition is met.)

In the default mode, where a DHCP lease has an unqualified name, and possibly a qualified name constructed

using –domain then the name in the authoritative zone is constructed from the unqualified name and the

zone’s domain. This may or may not equal that specified by –domain. If –dhcp-fqdn is set, then the fully

qualified names associated with DHCP leases are used, and must match the zone’s domain.

EXIT CODES

0 – Dnsmasq successfully forked into the background, or terminated normally if backgrounding is not enabled.

1 – A problem with configuration was detected.

2 – A problem with network access occurred (address in use, attempt to use privileged ports without permis‐

sion).

3 – A problem occurred with a filesystem operation (missing file/directory, permissions).

4 – Memory allocation failure.

5 – Other miscellaneous problem.

11 or greater – a non zero return code was received from the lease-script process “init” call. The exit code

from dnsmasq is the script’s exit code with 10 added.

LIMITS

The default values for resource limits in dnsmasq are generally conservative, and appropriate for embedded

router type devices with slow processors and limited memory. On more capable hardware, it is possible to

increase the limits, and handle many more clients. The following applies to dnsmasq-2.37: earlier versions

did not scale as well.

Dnsmasq is capable of handling DNS and DHCP for at least a thousand clients. The DHCP lease times should not

be very short (less than one hour). The value of –dns-forward-max can be increased: start with it equal to

the number of clients and increase if DNS seems slow. Note that DNS performance depends too on the perfor‐

mance of the upstream nameservers. The size of the DNS cache may be increased: the hard limit is 10000 names

and the default (150) is very low. Sending SIGUSR1 to dnsmasq makes it log information which is useful for

tuning the cache size. See the NOTES section for details.

The built-in TFTP server is capable of many simultaneous file transfers: the absolute limit is related to the

number of file-handles allowed to a process and the ability of the select() system call to cope with large

numbers of file handles. If the limit is set too high using –tftp-max it will be scaled down and the actual

limit logged at start-up. Note that more transfers are possible when the same file is being sent than when

each transfer sends a different file.

It is possible to use dnsmasq to block Web advertising by using a list of known banner-ad servers, all

resolving to 127.0.0.1 or 0.0.0.0, in /etc/hosts or an additional hosts file. The list can be very long, dns‐

masq has been tested successfully with one million names. That size file needs a 1GHz processor and about

60Mb of RAM.

INTERNATIONALISATION

Dnsmasq can be compiled to support internationalisation. To do this, the make targets “all-i18n” and

“install-i18n” should be used instead of the standard targets “all” and “install”. When internationalisation

is compiled in, dnsmasq will produce log messages in the local language and support internationalised domain

names (IDN). Domain names in /etc/hosts, /etc/ethers and /etc/dnsmasq.conf which contain non-ASCII characters

will be translated to the DNS-internal punycode representation. Note that dnsmasq determines both the lan‐

guage 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 edit‐

ing the configuration files, be careful to do so using only the system-default locale and not user-specific

one, since dnsmasq has no direct way of determining the charset in use, and must assume that it is the system

default.

FILES

/etc/dnsmasq.conf

/usr/local/etc/dnsmasq.conf

/etc/resolv.conf /var/run/dnsmasq/resolv.conf /etc/ppp/resolv.conf /etc/dhcpc/resolv.conf

/etc/hosts

/etc/ethers

/var/lib/misc/dnsmasq.leases

/var/db/dnsmasq.leases

/var/run/dnsmasq.pid

SEE ALSO

hosts(5), resolver(5)

参考文献

  • man 8 dnsmasq, version Dnsmasq version 2.75