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1 Q: Why does dnsmasq open UDP ports >1024 as well as port 53.
2 Is this a security problem/trojan/backdoor?
4 A: The high ports that dnsmasq opens are for replies from the upstream
5 nameserver(s). Queries from dnsmasq to upstream nameservers are sent
6 from these ports and replies received to them. The reason for doing this is
7 that most firewall setups block incoming packets _to_ port 53, in order
8 to stop DNS queries from the outside world. If dnsmasq sent its queries
9 from port 53 the replies would be _to_ port 53 and get blocked.
11 This is not a security hole since dnsmasq will only accept replies to that
12 port: queries are dropped. The replies must be to oustanding queries
13 which dnsmasq has forwarded, otherwise they are dropped too.
15 Addendum: dnsmasq now has the option "query-port" (-Q), which allows
16 you to specify the UDP port to be used for this purpose. If not
17 specified, the operating system will select an available port number
18 just as it did before.
20 Second addendum: following the discovery of a security flaw in the
21 DNS protocol, dnsmasq from version 2.43 has changed behavior. It
22 now uses a new, randomly selected, port for each query. The old
23 default behaviour (use one port allocated by the OS) is available by
24 setting --query-port=0, and setting the query port to a positive
25 value is still works. You should think hard and know what you are
26 doing before using either of these options.
28 Q: Why doesn't dnsmasq support DNS queries over TCP? Don't the RFC's specify
29 that?
31 A: Update: from version 2.10, it does. There are a few limitations:
32 data obtained via TCP is not cached, and source-address
33 or query-port specifications are ignored for TCP.
35 Q: When I send SIGUSR1 to dump the contents of the cache, some entries have
36 no IP address and are for names like mymachine.mydomain.com.mydomain.com.
37 What are these?
39 A: They are negative entries: that's what the N flag means. Dnsmasq asked
40 an upstream nameserver to resolve that address and it replied "doesn't
41 exist, and won't exist for <n> hours" so dnsmasq saved that information so
42 that if _it_ gets asked the same question it can answer directly without
43 having to go back to the upstream server again. The strange repeated domains
44 result from the way resolvers search short names. See "man resolv.conf" for
45 details.
48 Q: Will dnsmasq compile/run on non-Linux systems?
50 A: Yes, there is explicit support for *BSD and MacOS X and Solaris.
51 There are start-up scripts for MacOS X Tiger and Panther
52 in /contrib. Dnsmasq will link with uclibc to provide small
53 binaries suitable for use in embedded systems such as
54 routers. (There's special code to support machines with flash
55 filesystems and no battery-backed RTC.)
56 If you encounter make errors with *BSD, try installing gmake from
57 ports and building dnsmasq with "make MAKE=gmake"
58 For other systems, try altering the settings in config.h.
60 Q: My company's nameserver knows about some names which aren't in the
61 public DNS. Even though I put it first in /etc/resolv.conf, it
62 dosen't work: dnsmasq seems not to use the nameservers in the order
63 given. What am I doing wrong?
65 A: By default, dnsmasq treats all the nameservers it knows about as
66 equal: it picks the one to use using an algorithm designed to avoid
67 nameservers which aren't responding. To make dnsmasq use the
68 servers in order, give it the -o flag. If you want some queries
69 sent to a special server, think about using the -S flag to give the
70 IP address of that server, and telling dnsmasq exactly which
71 domains to use the server for.
73 Q: OK, I've got queries to a private nameserver working, now how about
74 reverse queries for a range of IP addresses?
76 A: Use the standard DNS convention of <reversed address>.in-addr.arpa.
77 For instance to send reverse queries on the range 192.168.0.0 to
78 192.168.0.255 to a nameserver at 10.0.0.1 do
79 server=/0.168.192.in-addr.arpa/10.0.0.1
80 Note that the "bogus-priv" option take priority over this option,
81 so the above will not work when the bogus-priv option is set.
83 Q: Dnsmasq fails to start with an error like this: "dnsmasq: bind
84 failed: Cannot assign requested address". What's the problem?
86 A: This has been seen when a system is bringing up a PPP interface at
87 boot time: by the time dnsmasq start the interface has been
88 created, but not brought up and assigned an address. The easiest
89 solution is to use --interface flags to specify which interfaces
90 dnsmasq should listen on. Since you are unlikely to want dnsmasq to
91 listen on a PPP interface and offer DNS service to the world, the
92 problem is solved.
94 Q: I'm running on BSD and dnsmasq won't accept long options on the
95 command line.
97 A: Dnsmasq when built on some BSD systems doesn't use GNU getopt by
98 default. You can either just use the single-letter options or
99 change config.h and the Makefile to use getopt-long. Note that
100 options in /etc/dnsmasq.conf must always be the long form,
101 on all platforms.
103 Q: Names on the internet are working fine, but looking up local names
104 from /etc/hosts or DHCP doesn't seem to work.
106 A: Resolver code sometime does strange things when given names without
107 any dots in. Win2k and WinXP may not use the DNS at all and just
108 try and look up the name using WINS. On unix look at "options ndots:"
109 in "man resolv.conf" for details on this topic. Testing lookups
110 using "nslookup" or "dig" will work, but then attempting to run
111 "ping" will get a lookup failure, appending a dot to the end of the
112 hostname will fix things. (ie "ping myhost" fails, but "ping
113 myhost." works. The solution is to make sure that all your hosts
114 have a domain set ("domain" in resolv.conf, or set a domain in
115 your DHCP server, see below fr Windows XP and Mac OS X).
116 Any domain will do, but "localnet" is traditional. Now when you
117 resolve "myhost" the resolver will attempt to look up
118 "myhost.localnet" so you need to have dnsmasq reply to that name.
119 The way to do that is to include the domain in each name on
120 /etc/hosts and/or to use the --expand-hosts and --domain options.
122 Q: How do I set the DNS domain in Windows XP or MacOS X (ref: previous
123 question)?
125 A: for XP, Control Panel > Network Connections > { Connection to gateway /
126 DNS } > Properties > { Highlight TCP/IP } > Properties > Advanced >
127 DNS Tab > DNS suffix for this connection:
129 A: for OS X, System Preferences > Network > {Connection to gateway / DNS } >
130 Search domains:
132 Q: Can I get dnsmasq to save the contents of its cache to disk when
133 I shut my machine down and re-load when it starts again?
135 A: No, that facility is not provided. Very few names in the DNS have
136 their time-to-live set for longer than a few hours so most of the
137 cache entries would have expired after a shutdown. For longer-lived
138 names it's much cheaper to just reload them from the upstream
139 server. Note that dnsmasq is not shut down between PPP sessions so
140 go off-line and then on-line again will not lose the contents of
141 the cache.
143 Q: Who are Verisign, what do they have to do with the bogus-nxdomain
144 option in dnsmasq and why should I wory about it?
146 A: [note: this was written in September 2003, things may well change.]
147 Versign run the .com and .net top-level-domains. They have just
148 changed the configuration of their servers so that unknown .com and
149 .net domains, instead of returning an error code NXDOMAIN, (no such
150 domain) return the address of a host at Versign which runs a web
151 server showing a search page. Most right-thinking people regard
152 this new behaviour as broken :-). You can test to see if you are
153 suffering Versign brokeness by run a command like
155 host jlsdajkdalld.com
157 If you get "jlsdajkdalld.com" does not exist, then all is fine, if
158 host returns an IP address, then the DNS is broken. (Try a few
159 different unlikely domains, just in case you picked a wierd one
160 which really _is_ registered.)
162 Assuming that your DNS is broken, and you want to fix it, simply
163 note the IP address being returned and pass it to dnsmasq using the
164 --bogus-nxdomain flag. Dnsmasq will check for results returning
165 that address and substitute an NXDOMAIN instead.
167 As of writing, the IP address in question for the .com and .net
168 domains is is 64.94.110.11. Various other, less prominent,
169 registries pull the same stunt; there is a list of them all, and
170 the addresses to block, at http://winware.org/bogus-domains.txt
172 Q: This new DHCP server is well and good, but it doesn't work for me.
173 What's the problem?
175 A: There are a couple of configuration gotchas which have been
176 encountered by people moving from the ISC dhcpd to the dnsmasq
177 integrated DHCP daemon. Both are related to differences in
178 in the way the two daemons bypass the IP stack to do "ground up"
179 IP configuration and can lead to the dnsmasq daemon failing
180 whilst the ISC one works.
182 The first thing to check is the broadcast address set for the
183 ethernet interface. This is normally the adddress on the connected
184 network with all ones in the host part. For instance if the
185 address of the ethernet interface is 192.168.55.7 and the netmask
186 is 255.255.255.0 then the broadcast address should be
187 192.168.55.255. Having a broadcast address which is not on the
188 network to which the interface is connected kills things stone
189 dead.
191 The second potential problem relates to firewall rules: since the ISC
192 daemon in some configurations bypasses the kernel firewall rules
193 entirely, the ability to run the ISC daemon does not indicate
194 that the current configuration is OK for the dnsmasq daemon.
195 For the dnsmasq daemon to operate it's vital that UDP packets to
196 and from ports 67 and 68 and broadcast packets with source
197 address 0.0.0.0 and destination address 255.255.255.255 are not
198 dropped by iptables/ipchains.
200 Q: I'm running Debian, and my machines get an address fine with DHCP,
201 but their names are not appearing in the DNS.
203 A: By default, none of the DHCP clients send the host-name when asking
204 for a lease. For most of the clients, you can set the host-name to
205 send with the "hostname" keyword in /etc/network/interfaces. (See
206 "man interfaces" for details.) That doesn't work for dhclient, were
207 you have to add something like "send host-name daisy" to
208 /etc/dhclient.conf [Update: the lastest dhcpcd packages _do_ send
209 the hostname by default.
211 Q: I'm network booting my machines, and trying to give them static
212 DHCP-assigned addresses. The machine gets its correct address
213 whilst booting, but then the OS starts and it seems to get
214 allocated a different address.
216 A: What is happening is this: The boot process sends a DHCP
217 request and gets allocated the static address corresponding to its
218 MAC address. The boot loader does not send a client-id. Then the OS
219 starts and repeats the DHCP process, but it it does send a
220 client-id. Dnsmasq cannot assume that the two requests are from the
221 same machine (since the client ID's don't match) and even though
222 the MAC address has a static allocation, that address is still in
223 use by the first incarnation of the machine (the one from the boot,
224 without a client ID.) dnsmasq therefore has to give the machine a
225 dynamic address from its pool. There are three ways to solve this:
226 (1) persuade your DHCP client not to send a client ID, or (2) set up
227 the static assignment to the client ID, not the MAC address. The
228 default client-id will be 01:<MAC address>, so change the dhcp-host
229 line from "dhcp-host=11:22:33:44:55:66,1.2.3.4" to
230 "dhcp-host=id:01:11:22:33:44:55:66,1.2.3.4" or (3) tell dnsmasq to
231 ignore client IDs for a particular MAC address, like this:
232 dhcp-host=11:22:33:44:55:66,id:*
234 Q: What network types are supported by the DHCP server?
236 A: Ethernet (and 802.11 wireless) are supported on all platforms. On
237 Linux all network types (including FireWire) are supported.
239 Q: What is this strange "bind-interface" option?
241 A: The DNS spec says that the reply to a DNS query must come from the
242 same address it was sent to. The traditional way to write an UDP
243 server to do this is to find all of the addresses belonging to the
244 machine (ie all the interfaces on the machine) and then create a
245 socket for each interface which is bound to the address of the
246 interface. Then when a packet is sent to address A, it is received
247 on the socket bound to address A and when the reply is also sent
248 via that socket, the source address is set to A by the kernel and
249 everything works. This is the how dnsmasq works when
250 "bind-interfaces" is set, with the obvious extension that is misses
251 out creating sockets for some interfaces depending on the
252 --interface, --address and --except-interface flags. The
253 disadvantage of this approach is that it breaks if interfaces don't
254 exist or are not configured when the daemon starts and does the
255 socket creation step. In a hotplug-aware world this is a real
256 problem.
258 The alternative approach is to have only one socket, which is bound
259 to the correct port and the wildcard IP address (0.0.0.0). That
260 socket will receive _all_ packets sent to port 53, no matter what
261 destination address they have. This solves the problem of
262 interfaces which are created or reconfigured after daemon
263 start-up. To make this work is more complicated because of the
264 "reply source address" problem. When a UDP packet is sent by a
265 socket bound to 0.0.0.0 its source address will be set to the
266 address of one of the machine's interfaces, but which one is not
267 determined and can vary depending on the OS being run. To get round
268 this it is neccessary to use a scary advanced API to determine the
269 address to which a query was sent, and force that to be the source
270 address in the reply. For IPv4 this stuff in non-portable and quite
271 often not even available (It's different between FreeBSD 5.x and
272 Linux, for instance, and FreeBSD 4.x, Linux 2.0.x and OpenBSD don't
273 have it at all.) Hence "bind-interfaces" has to always be available
274 as a fall back. For IPv6 the API is standard and universally
275 available.
277 It could be argued that if the --interface or --address flags are
278 used then binding interfaces is more appropriate, but using
279 wildcard binding means that dnsmasq will quite happily start up
280 after being told to use interfaces which don't exist, but which are
281 created later. Wildcard binding breaks the scenario when dnsmasq is
282 listening on one interface and another server (most probably BIND)
283 is listening on another. It's not possible for BIND to bind to an
284 (address,port) pair when dnsmasq has bound (wildcard,port), hence
285 the ability to explicitly turn off wildcard binding.
287 Q: Why doesn't Kerberos work/why can't I get sensible answers to
288 queries for SRV records.
290 A: Probably because you have the "filterwin2k" option set. Note that
291 it was on by default in example configuration files included in
292 versions before 2.12, so you might have it set on without
293 realising.
295 Q: Can I get email notification when a new version of dnsmasq is
296 released?
298 A: Yes, new releases of dnsmasq are always announced through
299 freshmeat.net, and they allow you to subcribe to email alerts when
300 new versions of particular projects are released. New releases are
301 also announced in the dnsmasq-discuss mailing list, subscribe at
302 http://lists.thekelleys.org.uk/mailman/listinfo/dnsmasq-discuss
304 Q: What does the dhcp-authoritative option do?
306 A: See http://www.isc.org/index.pl?/sw/dhcp/authoritative.php - that's
307 for the ISC daemon, but the same applies to dnsmasq.
309 Q: Why does my Gentoo box pause for a minute before getting a new
310 lease?
312 A: Because when a Gentoo box shuts down, it releases its lease with
313 the server but remembers it on the client; this seems to be a
314 Gentoo-specific patch to dhcpcd. On restart it tries to renew
315 a lease which is long gone, as far as dnsmasq is concerned, and
316 dnsmasq ignores it until is times out and restarts the process.
317 To fix this, set the dhcp-authoritative flag in dnsmasq.
319 Q: My laptop has two network interfaces, a wired one and a wireless
320 one. I never use both interfaces at the same time, and I'd like the
321 same IP and configuration to be used irrespective of which
322 interface is in use. How can I do that?
324 A: By default, the identity of a machine is determined by using the
325 MAC address, which is associated with interface hardware. Once an
326 IP is bound to the MAC address of one interface, it cannot be
327 associated with another MAC address until after the DHCP lease
328 expires. The solution to this is to use a client-id as the machine
329 identity rather than the MAC address. If you arrange for the same
330 client-id to sent when either interface is in use, the DHCP server
331 will recognise the same machine, and use the same address. The
332 method for setting the client-id varies with DHCP client software,
333 dhcpcd uses the "-I" flag. Windows uses a registry setting,
334 see http://www.jsiinc.com/SUBF/TIP2800/rh2845.htm
335 Addendum:
336 From version 2.46, dnsmasq has a solution to this which doesn't
337 involve setting client-IDs. It's possible to put more than one MAC
338 address in a --dhcp-host configuration. This tells dnsmasq that it
339 should use the specified IP for any of the specified MAC addresses,
340 and furthermore it gives dnsmasq permission to sumarily abandon a
341 lease to one of the MAC addresses if another one comes along. Note
342 that this will work fine only as longer as only one interface is
343 up at any time. There is no way for dnsmasq to enforce this
344 constraint: if you configure multiple MAC addresses and violate
345 this rule, bad things will happen.
347 Q: Can dnsmasq do DHCP on IP-alias interfaces?
349 A: Yes, from version-2.21. The support is only available running under
350 Linux, on a kernel which provides the RT-netlink facility. All 2.4
351 and 2.6 kernels provide RT-netlink and it's an option in 2.2
352 kernels.
354 If a physical interface has more than one IP address or aliases
355 with extra IP addresses, then any dhcp-ranges corresponding to
356 these addresses can be used for address allocation. So if an
357 interface has addresses 192.168.1.0/24 and 192.68.2.0/24 and there
358 are DHCP ranges 192.168.1.100-192.168.1.200 and
359 192.168.2.100-192.168.2.200 then both ranges would be used for host
360 connected to the physical interface. A more typical use might be to
361 have one of the address-ranges as static-only, and have known
362 hosts allocated addresses on that subnet using dhcp-host options,
363 while anonymous hosts go on the other.
366 Q: Dnsmasq sometimes logs "nameserver xxx.xxx.xxx.xxx refused
367 to do a recursive query" and DNS stops working. What's going on?
369 A: Probably the nameserver is an authoritative nameserver for a
370 particular domain, but is not configured to answer general DNS
371 queries for an arbitrary domain. It is not suitable for use by
372 dnsmasq as an upstream server and should be removed from the
373 configuration. Note that if you have more than one upstream
374 nameserver configured dnsmasq will load-balance across them and
375 it may be some time before dnsmasq gets around to using a
376 particular nameserver. This means that a particular configuration
377 may work for sometime with a broken upstream nameserver
378 configuration.
381 Q: Does the dnsmasq DHCP server probe addresses before allocating
382 them, as recommended in RFC2131?
384 A: Yes, dynmaically allocated IP addresses are checked by sending an
385 ICMP echo request (ping). If a reply is received, then dnsmasq
386 assumes that the address is in use, and attempts to allocate an
387 different address. The wait for a reply is between two and three
388 seconds. Because the DHCP server is not re-entrant, it cannot serve
389 other DHCP requests during this time. To avoid dropping requests,
390 the address probe may be skipped when dnsmasq is under heavy load.
393 Q: I'm using dnsmasq on a machine with the Firestarter firewall, and
394 DHCP doesn't work. What's the problem?
396 A: This a variant on the iptables problem. Explicit details on how to
397 proceed can be found at
398 http://lists.thekelleys.org.uk/pipermail/dnsmasq-discuss/2005q3/000431.html
401 Q: I'm using dnsmasq on a machine with the shorewall firewall, and
402 DHCP doesn't work. What's the problem?
404 A: This a variant on the iptables problem. Explicit details on how to
405 proceed can be found at
406 http://lists.thekelleys.org.uk/pipermail/dnsmasq-discuss/2007q4/001764.html
409 Q: Dnsmasq fails to start up with a message about capabilities.
410 Why did that happen and what can do to fix it?
412 A: Change your kernel configuration: either deselect CONFIG_SECURITY
413 _or_ select CONFIG_SECURITY_CAPABILITIES. Alternatively, you can
414 remove the need to set capabilities by running dnsmasq as root.
416 Q: Where can I get .rpms Suitable for Suse?
418 A: Dnsmasq is in Suse itself, and the latest releases are also
419 available at ftp://ftp.suse.com/pub/people/ug/
422 Q: Can I run dnsmasq in a Linux vserver?
424 A: Yes, as a DNS server, dnsmasq will just work in a vserver.
425 To use dnsmasq's DHCP function you need to give the vserver
426 extra system capabilities. Please note that doing so will lesser
427 the overall security of your system. The capabilities
428 required are NET_ADMIN and NET_RAW. NET_ADMIN is essential, NET_RAW
429 is required to do an ICMP "ping" check on newly allocated
430 addresses. If you don't need this check, you can disable it with
431 --no-ping and omit the NET_RAW capability.
432 Adding the capabilities is done by adding them, one per line, to
433 either /etc/vservers/<vservername>/ccapabilities for a 2.4 kernel or
434 /etc/vservers/<vservername>/bcapabilities for a 2.6 kernel (please
435 refer to the vserver documentation for more information).
438 Q: What's the problem with syslog and dnsmasq?
440 A: In almost all cases: none. If you have the normal arrangement with
441 local daemons logging to a local syslog, which then writes to disk,
442 then there's never a problem. If you use network logging, then
443 there's a potential problem with deadlock: the syslog daemon will
444 do DNS lookups so that it can log the source of log messages,
445 these lookups will (depending on exact configuration) go through
446 dnsmasq, which also sends log messages. With bad timing, you can
447 arrive at a situation where syslog is waiting for dnsmasq, and
448 dnsmasq is waiting for syslog; they will both wait forever. This
449 problem is fixed from dnsmasq-2.39, which introduces asynchronous
450 logging: dnsmasq no longer waits for syslog and the deadlock is
451 broken. There is a remaining problem in 2.39, where "log-queries"
452 is in use. In this case most DNS queries generate two log lines, if
453 these go to a syslog which is doing a DNS lookup for each log line,
454 then those queries will in turn generate two more log lines, and a
455 chain reaction runaway will occur. To avoid this, use syslog-ng
456 and turn on syslog-ng's dns-cache function.
459 Q: DHCP doesn't work with windows Vista, but everything else is fine.
461 A: The DHCP client on windows Vista (and possibly later versions)
462 demands that the DHCP server send replies as broadcasts. Most other
463 clients don't do this. The broadcasts are send to
464 255.255.255.255. A badly configured firewall which blocks such
465 packets will show exactly these symptoms (Vista fails, others
466 work).