xref: /external/dnsmasq/FAQ

Q: Why does dnsmasq open UDP ports >1024 as well as port 53.
   Is this a security problem/trojan/backdoor?

A: The high ports that dnsmasq opens are for replies from the upstream
   nameserver(s). Queries from dnsmasq to upstream nameservers are sent
   from these ports and replies received to them. The reason for doing this is
   that most firewall setups block incoming packets _to_ port 53, in order
   to stop DNS queries from the outside world. If dnsmasq sent its queries
   from port 53 the replies would be _to_ port 53 and get blocked.

   This is not a security hole since dnsmasq will only accept replies to that
   port: queries are  dropped. The replies must be to oustanding queries
   which dnsmasq has forwarded, otherwise they are dropped too.

   Addendum: dnsmasq now has the option "query-port" (-Q), which allows
   you to specify the UDP port to be used for this purpose.  If not
   specified, the operating system will select an available port number
   just as it did before.

   Second addendum: following the discovery of a security flaw in the
   DNS protocol, dnsmasq from version 2.43 has changed behavior. It
   now uses a new, randomly selected, port for each query. The old
   default behaviour (use one port allocated by the OS) is available by
   setting --query-port=0, and setting the query port to a positive
   value is still works. You should think hard and know what you are
   doing before using either of these options.

Q: Why doesn't dnsmasq support DNS queries over TCP? Don't the RFC's specify
   that?

A: Update: from version 2.10, it does. There are a few limitations:
   data obtained via TCP is not cached, and source-address
   or query-port specifications are ignored for TCP.

Q: When I send SIGUSR1 to dump the contents of the cache, some entries have
   no IP address and are for names like mymachine.mydomain.com.mydomain.com.
   What are these?

A: They are negative entries: that's what the N flag means. Dnsmasq asked
   an upstream nameserver to resolve that address and it replied "doesn't
   exist, and won't exist for <n> hours" so dnsmasq saved that information so
   that if _it_ gets asked the same question it can answer directly without
   having to go back to the upstream server again. The strange repeated domains
   result from the way resolvers search short names. See "man resolv.conf" for
   details.


Q: Will dnsmasq compile/run on non-Linux systems?

A: Yes, there is explicit support for *BSD and MacOS X and Solaris.
   There are start-up scripts for MacOS X Tiger and Panther
   in /contrib. Dnsmasq will link with uclibc to provide small
   binaries suitable for use in embedded systems such as
   routers. (There's special code to support machines with flash
   filesystems and no battery-backed RTC.)
   If you encounter make errors with *BSD, try installing gmake from
   ports and building dnsmasq with "make MAKE=gmake"
   For other systems, try altering the settings in config.h.

Q: My company's nameserver knows about some names which aren't in the
   public DNS. Even though I put it first in /etc/resolv.conf, it
   dosen't work: dnsmasq seems not to use the nameservers in the order
   given. What am I doing wrong?

A: By default, dnsmasq treats all the nameservers it knows about as
   equal: it picks the one to use using an algorithm designed to avoid
   nameservers which aren't responding. To make dnsmasq use the
   servers in order, give it the -o flag. If you want some queries
   sent to a special server, think about using the -S flag to give the
   IP address of that server, and telling dnsmasq exactly which
   domains to use the server for.

Q: OK, I've got queries to a private nameserver working, now how about
   reverse queries for a range of IP addresses?

A: Use the standard DNS convention of <reversed address>.in-addr.arpa.
   For instance to send reverse queries on the range 192.168.0.0 to
   192.168.0.255 to a nameserver at 10.0.0.1 do
   server=/0.168.192.in-addr.arpa/10.0.0.1
   Note that the "bogus-priv" option take priority over this option,
   so the above will not work when the bogus-priv option is set.

Q: Dnsmasq fails to start with an error like this: "dnsmasq: bind
   failed: Cannot assign requested address". What's the problem?

A: This has been seen when a system is bringing up a PPP interface at
   boot time: by the time dnsmasq start the interface has been
   created, but not brought up and assigned an address. The easiest
   solution is to use --interface flags to specify which interfaces
   dnsmasq should listen on. Since you are unlikely to want dnsmasq to
   listen on a PPP interface and offer DNS service to the world, the
   problem is solved.

Q: I'm running on BSD and dnsmasq won't accept long options on the
   command line.

A: Dnsmasq when built on some BSD systems doesn't use GNU getopt by
   default. You can either just use the single-letter options or
   change config.h and the Makefile to use getopt-long. Note that
   options in /etc/dnsmasq.conf must always be the long form,
   on all platforms.

Q: Names on the internet are working fine, but looking up local names
   from /etc/hosts or DHCP doesn't seem to work.

A: Resolver code sometime does strange things when given names without
   any dots in. Win2k and WinXP may not use the DNS at all and just
   try and look up the name using WINS. On unix look at "options ndots:"
   in "man resolv.conf" for details on this topic. Testing lookups
   using "nslookup" or "dig" will work, but then attempting to run
   "ping" will get a lookup failure, appending a dot to the end of the
   hostname  will fix things. (ie "ping myhost" fails, but "ping
   myhost." works. The solution is to make sure that all your hosts
   have a domain set ("domain" in resolv.conf, or set a domain in
   your DHCP server, see below fr Windows XP and Mac OS X).
   Any domain  will do, but "localnet" is traditional. Now when you
   resolve "myhost" the resolver will attempt to look up
   "myhost.localnet" so you need to have dnsmasq reply to that name.
   The way to do that is to include the domain in each name on
   /etc/hosts  and/or to use the --expand-hosts and --domain options.

Q: How do I set the DNS domain in Windows XP or MacOS X (ref: previous
   question)?

A: for XP, Control Panel > Network Connections > { Connection to gateway /
   DNS } > Properties > { Highlight TCP/IP } > Properties > Advanced >
   DNS Tab > DNS suffix for this connection:

A: for OS X, System Preferences > Network > {Connection to gateway / DNS } >
   Search domains:

Q: Can I get dnsmasq to save the contents of its cache to disk when
   I shut my machine down and re-load when it starts again?

A: No, that facility is not provided. Very few names in the DNS have
   their time-to-live set for longer than a few hours so most of the
   cache entries would have expired after a shutdown. For longer-lived
   names it's much cheaper to just reload them from the upstream
   server. Note that dnsmasq is not shut down between PPP sessions so
   go off-line and then on-line again will not lose the contents of
   the cache.

Q: Who are Verisign, what do they have to do with the bogus-nxdomain
   option in dnsmasq and why should I wory about it?

A: [note: this was written in September 2003, things may well change.]
   Versign run the .com and .net top-level-domains. They have just
   changed the configuration of their servers so that unknown .com and
   .net domains, instead of returning an error code NXDOMAIN, (no such
   domain) return the address of a host at Versign which runs a web
   server showing a search page. Most right-thinking people regard
   this new behaviour as broken :-).  You can test to see if you are
   suffering Versign brokeness by run a command like

   host jlsdajkdalld.com

   If you get "jlsdajkdalld.com" does not exist, then all is fine, if
   host returns an IP address, then the DNS is broken. (Try a few
   different unlikely domains, just in case you picked a wierd one
   which really _is_ registered.)

   Assuming that your DNS is broken, and you want to fix it, simply
   note the IP address being returned and pass it to dnsmasq using the
   --bogus-nxdomain flag. Dnsmasq will check for results returning
   that address and substitute an NXDOMAIN instead.

   As of writing, the IP address in question for the .com and .net
   domains is is 64.94.110.11. Various other, less prominent,
   registries pull the same stunt; there is a list of them all, and
   the addresses to block, at http://winware.org/bogus-domains.txt

Q: This new DHCP server is well and good, but it doesn't work for me.
   What's the problem?

A: There are a couple of configuration gotchas which have been
   encountered by people moving from the ISC dhcpd to the dnsmasq
   integrated DHCP daemon. Both are related to differences in
   in the way the two daemons bypass the IP stack to do "ground up"
   IP configuration and can lead to the dnsmasq daemon failing
   whilst the ISC one works.

   The first thing to check is the broadcast address set for the
   ethernet interface. This is normally the adddress on the connected
   network with all ones in the host part. For instance if the
   address of the ethernet interface is 192.168.55.7 and the netmask
   is 255.255.255.0 then the broadcast address should be
   192.168.55.255. Having a broadcast address which is not on the
   network to which the interface is connected kills things stone
   dead.

   The second potential problem relates to firewall rules: since the ISC
   daemon in some configurations bypasses the kernel firewall rules
   entirely, the ability to run the ISC daemon does not indicate
   that the current configuration is OK for the dnsmasq daemon.
   For the dnsmasq daemon to operate it's vital that UDP packets to
   and from ports 67 and 68 and broadcast packets with source
   address 0.0.0.0 and destination address 255.255.255.255 are not
   dropped by iptables/ipchains.

Q: I'm running Debian, and my machines get an address fine with DHCP,
   but their names are not appearing in the DNS.

A: By default, none of the DHCP clients send the host-name when asking
   for a lease. For most of the clients, you can set the host-name to
   send with the "hostname" keyword in /etc/network/interfaces. (See
   "man interfaces" for details.) That doesn't work for dhclient, were
   you have to add something like "send host-name daisy" to
   /etc/dhclient.conf [Update: the lastest dhcpcd packages _do_ send
   the hostname by default.

Q: I'm network booting my machines, and trying to give them static
   DHCP-assigned addresses. The machine gets its correct address
   whilst booting, but then the OS starts and it seems to get
   allocated a different address.

A: What is happening is this: The boot process sends a DHCP
   request and gets allocated the static address corresponding to its
   MAC address. The boot loader does not send a client-id. Then the OS
   starts and repeats the DHCP process, but it it does send a
   client-id. Dnsmasq cannot assume that the two requests are from the
   same machine (since the client ID's don't match) and even though
   the MAC address has a static allocation, that address is still in
   use by the first incarnation of the machine (the one from the boot,
   without a client ID.) dnsmasq therefore has to give the machine a
   dynamic address from its pool. There are three ways to solve this:
   (1) persuade your DHCP client not to send a client ID, or (2) set up
   the static assignment to the client ID, not the MAC address. The
   default client-id will be 01:<MAC address>, so change the dhcp-host
   line from "dhcp-host=11:22:33:44:55:66,1.2.3.4" to
   "dhcp-host=id:01:11:22:33:44:55:66,1.2.3.4" or (3) tell dnsmasq to
   ignore client IDs for a particular MAC address, like this:
   dhcp-host=11:22:33:44:55:66,id:*

Q: What network types are supported by the DHCP server?

A: Ethernet (and 802.11 wireless) are supported on all platforms. On
   Linux all network types (including FireWire) are supported.

Q: What is this strange "bind-interface" option?

A: The DNS spec says that the reply to a DNS query must come from the
   same address it was sent to. The traditional way to write an UDP
   server to do this is to find all of the addresses belonging to the
   machine (ie all the interfaces on the machine) and then create a
   socket for each interface which is bound to the address of the
   interface. Then when a packet is sent to address A, it is received
   on the socket bound to address A and when the reply is also sent
   via that socket, the source address is set to A by the kernel and
   everything works. This is the how dnsmasq works when
   "bind-interfaces" is set, with the obvious extension that is misses
   out creating sockets for some interfaces depending on the
   --interface, --address and --except-interface flags.  The
   disadvantage of this approach is that it breaks if interfaces don't
   exist or are not configured when the daemon starts and does the
   socket creation step. In a hotplug-aware world this is a real
   problem.

   The alternative approach is to have only one socket, which is bound
   to the correct port and the wildcard IP address (0.0.0.0). That
   socket will receive _all_ packets sent to port 53, no matter what
   destination address they have. This solves the problem of
   interfaces which are created or reconfigured after daemon
   start-up. To make this work is more complicated because of the
   "reply source address" problem. When a UDP packet is sent by a
   socket bound to 0.0.0.0 its source address will be set to the
   address of one of the machine's interfaces, but which one is not
   determined and can vary depending on the OS being run. To get round
   this it is neccessary to use a scary advanced API to determine the
   address to which a query was sent, and force that to be the source
   address in the reply. For IPv4 this stuff in non-portable and quite
   often not even available (It's different between FreeBSD 5.x and
   Linux, for instance, and FreeBSD 4.x, Linux 2.0.x and OpenBSD don't
   have it at all.) Hence "bind-interfaces" has to always be available
   as a fall back. For IPv6 the API is standard and universally
   available.

   It could be argued that if the --interface or --address flags are
   used then binding interfaces is more appropriate, but using
   wildcard binding means that dnsmasq will quite happily start up
   after being told to use interfaces which don't exist, but which are
   created later. Wildcard binding breaks the scenario when dnsmasq is
   listening on one interface and another server (most probably BIND)
   is listening on another. It's not possible for BIND to bind to an
   (address,port) pair when dnsmasq has bound (wildcard,port), hence
   the ability to explicitly turn off wildcard binding.

Q: Why doesn't Kerberos work/why can't I get sensible answers to
   queries for SRV records.

A: Probably because you have the "filterwin2k" option set. Note that
   it was on by default in example configuration files included in
   versions before 2.12, so you might have it set on without
   realising.

Q: Can I get email notification when a new version of dnsmasq is
   released?

A: Yes, new releases of dnsmasq are always announced through
   freshmeat.net, and they allow you to subcribe to email alerts when
   new versions of particular projects are released. New releases are
   also announced in the dnsmasq-discuss mailing list, subscribe at
   http://lists.thekelleys.org.uk/mailman/listinfo/dnsmasq-discuss

Q: What does the dhcp-authoritative option do?

A: See http://www.isc.org/index.pl?/sw/dhcp/authoritative.php - that's
   for the ISC daemon, but the same applies to dnsmasq.

Q: Why does my Gentoo box pause for a minute before getting a new
   lease?

A: Because when a Gentoo box shuts down, it releases its lease with
   the server but remembers it on the client; this seems to be a
   Gentoo-specific patch to dhcpcd. On restart it tries to renew
   a lease which is long gone, as far as dnsmasq is concerned, and
   dnsmasq ignores it until is times out and restarts the process.
   To fix this, set the dhcp-authoritative flag in dnsmasq.

Q: My laptop has two network interfaces, a wired one and a wireless
   one. I never use both interfaces at the same time, and I'd like the
   same IP and configuration to be used irrespective of which
   interface is in use. How can I do that?

A: By default, the identity of a machine is determined by using the
   MAC address, which is associated with interface hardware. Once an
   IP is bound to the MAC address of one interface, it cannot be
   associated with another MAC address until after the DHCP lease
   expires. The solution to this is to use a client-id as the machine
   identity rather than the MAC address. If you arrange for the same
   client-id to sent when either interface is in use, the DHCP server
   will recognise the same machine, and use the same address. The
   method for setting the client-id varies with DHCP client software,
   dhcpcd uses the "-I" flag. Windows uses a registry setting,
   see http://www.jsiinc.com/SUBF/TIP2800/rh2845.htm
Addendum:
   From version 2.46, dnsmasq has a solution to this which doesn't
   involve setting client-IDs. It's possible to put more than one MAC
   address in a --dhcp-host configuration. This tells dnsmasq that it
   should use the specified IP for any of the specified MAC addresses,
   and furthermore it gives dnsmasq permission to sumarily abandon a
   lease to one of the MAC addresses if another one comes along. Note
   that this will work fine only as longer as only one interface is
   up at any time. There is no way for dnsmasq to enforce this
   constraint: if you configure multiple MAC addresses and violate
   this rule, bad things will happen.

Q: Can dnsmasq do DHCP on IP-alias interfaces?

A: Yes, from version-2.21. The support is only available running under
   Linux, on a kernel which provides the RT-netlink facility. All 2.4
   and 2.6 kernels provide RT-netlink and it's an option in 2.2
   kernels.

   If a physical interface has more than one IP address or aliases
   with extra IP addresses, then any dhcp-ranges corresponding to
   these addresses can be used for address allocation. So if an
   interface has addresses 192.168.1.0/24 and 192.68.2.0/24 and there
   are DHCP ranges 192.168.1.100-192.168.1.200 and
   192.168.2.100-192.168.2.200 then both ranges would be used for host
   connected to the physical interface. A more typical use might be to
   have one of the address-ranges as static-only, and have known
   hosts allocated addresses on that subnet using dhcp-host options,
   while  anonymous hosts go on the other.


Q: Dnsmasq sometimes logs "nameserver xxx.xxx.xxx.xxx refused
   to do a recursive query" and DNS stops working. What's going on?

A: Probably the nameserver is an authoritative nameserver for a
   particular domain, but is not configured to answer general DNS
   queries for an arbitrary domain. It is not suitable for use by
   dnsmasq as an upstream server and should be removed from the
   configuration. Note that if you have more than one upstream
   nameserver configured dnsmasq will load-balance across them and
   it may be some time before dnsmasq gets around to using a
   particular nameserver. This means that a particular configuration
   may work for sometime with a broken upstream nameserver
   configuration.


Q: Does the dnsmasq DHCP server probe addresses before allocating
   them, as recommended in RFC2131?

A: Yes, dynmaically allocated IP addresses are checked by sending an
   ICMP echo request (ping). If a reply is received, then dnsmasq
   assumes that the address is in use, and attempts to allocate an
   different address. The wait for a reply is between two and three
   seconds. Because the DHCP server is not re-entrant, it cannot serve
   other DHCP requests during this time. To avoid dropping requests,
   the address probe may be skipped when dnsmasq is under heavy load.


Q: I'm using dnsmasq on a machine with the Firestarter firewall, and
   DHCP doesn't work. What's the problem?

A: This a variant on the iptables problem. Explicit details on how to
   proceed can be found at
   http://lists.thekelleys.org.uk/pipermail/dnsmasq-discuss/2005q3/000431.html


Q: I'm using dnsmasq on a machine with the shorewall firewall, and
   DHCP doesn't work. What's the problem?

A: This a variant on the iptables problem. Explicit details on how to
   proceed can be found at
   http://lists.thekelleys.org.uk/pipermail/dnsmasq-discuss/2007q4/001764.html


Q: Dnsmasq fails to start up with a message about capabilities.
   Why did that happen and what can do to fix it?

A: Change your kernel configuration: either deselect CONFIG_SECURITY
   _or_ select CONFIG_SECURITY_CAPABILITIES. Alternatively, you can
   remove the need to set capabilities by running dnsmasq as root.

Q: Where can I get .rpms Suitable for Suse?

A: Dnsmasq is in Suse itself, and the latest releases are also
   available at ftp://ftp.suse.com/pub/people/ug/


Q: Can I run dnsmasq in a Linux vserver?

A: Yes, as a DNS server, dnsmasq will just work in a vserver.
   To use dnsmasq's DHCP function you need to give the vserver
   extra system capabilities. Please note that doing so will lesser
   the overall security of your system. The capabilities
   required are NET_ADMIN and NET_RAW. NET_ADMIN is essential, NET_RAW
   is required to do an ICMP "ping" check on newly allocated
   addresses. If you don't need this check, you can disable it with
   --no-ping and omit the NET_RAW capability.
   Adding the capabilities is done by adding them, one per line, to
   either /etc/vservers/<vservername>/ccapabilities for a 2.4 kernel or
   /etc/vservers/<vservername>/bcapabilities for a 2.6 kernel (please
   refer to the vserver documentation for more information).


Q: What's the problem with syslog and dnsmasq?

A: In almost all cases: none. If you have the normal arrangement with
   local daemons logging to a local syslog, which then writes to disk,
   then there's never a problem. If you use network logging, then
   there's a potential problem with deadlock: the syslog daemon will
   do DNS lookups so that it can log the source of log messages,
   these lookups will (depending on exact configuration) go through
   dnsmasq, which also sends log messages. With bad timing, you can
   arrive at a situation where syslog is waiting for dnsmasq, and
   dnsmasq is waiting for syslog; they will both wait forever. This
   problem is fixed from dnsmasq-2.39, which introduces asynchronous
   logging: dnsmasq no longer waits for syslog and the deadlock is
   broken. There is a remaining problem in 2.39, where "log-queries"
   is in use. In this case most DNS queries generate two log lines, if
   these go to a syslog which is doing a DNS lookup for each log line,
   then those queries will in turn generate two more log lines, and a
   chain reaction runaway will occur. To avoid this, use syslog-ng
   and turn on syslog-ng's dns-cache function.