nss-mdns 0.10

Copyright 2004-2007 Lennart Poettering <mzaffzqaf (at) 0pointer (dot) de>


This program is free software; you can redistribute it and/or modify it under the terms of the GNU Lesser General Public License as published by the Free Software Foundation; either version 2 of the License, or (at your option) any later version.

This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU Lesser General Public License for more details.

You should have received a copy of the GNU Lesser General Public License along with this program; if not, write to the Free Software Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.


Sat May 12 2007:

Version 0.10 released. Changes include: Ported to FreeBSD; alignment fixes for SPARC.

Mon Jan 1 2007:

Version 0.9 released. Changes include: Make most shared library symbols private to not conflict with any symbols of the program we're loaded into. Fix a potential endless loop in the mDNS packet parsing code.

Please note that due to security reasons from this release on the minimal mDNS stack included in nss-mdns (dubbed "legacy") is no longer built by default. Thus, nss-mdns will not work unless Avahi is running! That makes Avahi essentially a hard dependency of nss-mdns. Pass --enable-legacy to reenable the mini mDNS stack again. Please note as well that this release does not honour /etc/resolv.conf domain search lists by default anymore. It created a lot of problems and was never recommended anyway. You may reenable this functionality by passing --enable-search-domains.

Sat Apr 29 2006:

Version 0.8 released. Changes include: Build time option to disable "legacy unicast" mDNS requests, i.e. resolve exclusively with Avahi; build a special _minimal flavour of the shared objects to minimize unnecessary name lookup timeouts; fix IPv6 resolving when using Avahi.

Please note that starting with nss-mdns 0.8 we encourage you to use a different /etc/nsswitch.conf configuration line. See below for more information!

Sat Nov 19 2005:

Version 0.7 released. Changes include: Portability patch for ARM from Philipp Zabel; make sure not to print any messages to STDERR; deal with OOM situations properly; if multiple addresses are assigned to the same interface make sure to send a query packet only once; other cleanups

Sun Aug 21 2005:

Version 0.6 released. Changes include: honour search list in /etc/resolv.conf; try to contact Avahi for resolving.

Sat Jun 4 2005:

Version 0.5 released. Changes include: only lookup hostnames ending in .local; add support for a configuration file (/etc/mdns.allow) to allow lookups for other names.

Sun May 15 2005:

Version 0.4 released. Changes include: small portability fix for big endian architectures; send "legacy unicast" packets instead of normal mDNS packets (this should reduce traffic and improve response time)

Jan Sun 16 2005:

Version 0.3 released. Changes include: add Debianization; use ip6.arpa instead of ip6.int for reverse IPv6 lookups.

Fri Dec 17 2004:

Version 0.2 released. Changes include: send mDNS queries on every interface that supports multicasts, instead of only the one with the default route, making nss-mdns more robust on multi-homed hosts; gcc 2.95 compatiblity.

Mon Dec 6 2004:

Version 0.1 released


nss-mdns is a plugin for the GNU Name Service Switch (NSS) functionality of the GNU C Library (glibc) providing host name resolution via Multicast DNS (aka Zeroconf, aka Apple Rendezvous, aka Apple Bonjour), effectively allowing name resolution by common Unix/Linux programs in the ad-hoc mDNS domain .local.

nss-mdns provides client functionality only, which means that you have to run a mDNS responder daemon seperately from nss-mdns if you want to register the local host name via mDNS. I recommend Avahi.

nss-mdns is very lightweight (9 KByte stripped binary .so compiled with -DNDEBUG=1 -Os on i386, gcc 4.0), has no dependencies besides the glibc and requires only minimal configuration.

By default nss-mdns tries to contact a running avahi-daemon for resolving host names and addresses and making use of its superior record cacheing. Optionally nss-mdns can be compiled with a mini mDNS stack that can be used to resolve host names without a local Avahi installation. Both Avahi support and this mini mDNS stack are optional, however at least one of them needs to be enabled. If both are enabled a connection to Avahi is tried first, and if that fails the mini mDNS stack is used.

Current Status

It works!

If the mini MDNS stack is used, nss-mdns supports resolving IPv6 addresses but does so via IPv4 multicasts only. If Avahi is used for resolving IPv6 is supported properly.


After compiling and installing nss-mdns you'll find six new NSS modules in /lib:

libnss_mdns.so.2 resolves both IPv6 and IPv4 addresses, libnss_mdns4.so.2 only IPv4 addresses and libnss_mdns6.so.2 only IPv6 addresses. Due to the fact that most mDNS responders only register local IPv4 addresses via mDNS, most people will want to use libnss_mdns4.so.2 exclusively. Using libnss_mdns.so.2 or libnss_mdns6.so.2 in such a situation causes long timeouts when resolving hosts since most modern Unix/Linux applications check for IPv6 addresses first, followed by a lookup for IPv4.

libnss_mdns{4,6,}_minimal.so (new in version 0.8) is mostly identical to the versions without _minimal. However, they differ in one way. The minimal versions will always deny to resolve host names that don't end in .local or addresses that aren't in the range 169.254.x.x (the range used by IPV4LL/APIPA/RFC3927.) Combining the _minimal and the normal NSS modules allows us to make mDNS authoritative for Zeroconf host names and addresses (and thus creating no extra burden on DNS servers with always failing requests) and use it as fallback for everything else.

To activate one of the NSS modules you have to edit /etc/nsswitch.conf and add mdns4 and mdns4_minimal (resp. mdns, mdns6) to the line starting with "hosts:". On Debian this looks like this:

# /etc/nsswitch.conf

passwd:         compat
group:          compat
shadow:         compat

hosts:          files mdns4_minimal [NOTFOUND=return] dns mdns4
networks:       files

protocols:      db files
services:       db files
ethers:         db files
rpc:            db files

netgroup:       nis

That's it. You should now be able to resolve hosts from the .local domain with all your applications. For a quick check use glibc's getent tool:

$ getent hosts foo.local    foo.local

Replace foo whith a host name that has been registered with an mDNS responder. (Don't try to use the tools host or nslookup for these tests! They bypass the NSS and thus nss-mdns and issue their DNS queries directly.)

If you run a firewall, don't forget to allow UDP traffic to the the mDNS multicast address on port 5353.

Please note: The line above makes nss-mdns authoritative for the .local domain. If you have a unicast DNS domain with the same name you will no longer be able to resolve hosts from it. mDNS and a unicast DNS domain named .local are inherently incompatible. Please contact your local admistrator and ask him to move to a different domain name since .local is to be used exclusively for Zeroconf technology. Further information.

Starting with version 0.5, nss-mdns has a simple configuration file /etc/mdns.allow for enabling name lookups via mDNS in other domains than .local. The file contains valid domain suffixes, seperated by newlines. Empty lines are ignored as are comments starting with #. To enable mDNS lookups of all names, regardless of the domain suffix add a line consisting of * only (similar to nss-mdns mode of operation of versions <= 0.4):

# /etc/mdns.allow

If the configuration file is absent or unreadable nss-mdns behaves as if a configuration file with the following contents is read:

# /etc/mdns.allow

i.e. only hostnames ending with .local are resolved via mDNS.

If the configuration file is existent but empty, mDNS name lookups are disabled completely. Please note that usually mDNS is not used for anything but .local, hence you usually don't want to touch this file.


Currently, nss-mdns is tested on Linux only. A fairly modern glibc installation with development headers (2.0 or newer) is required. Not suprisingly nss-mdns requires a kernel compiled with IPv4 multicasting support enabled. Avahi is recommended for its superior cacheing capabilities and for security reasons. Unless you compile nss-mdns with --enable-legacy Avahi is a hard dependency when nss-mdns is used, however not a build-time requirement.

nss-mdns was developed and tested on Debian GNU/Linux "testing" from December 2004, it should work on most other Linux distributions (and maybe Unix versions) since it uses GNU autoconf and GNU libtool for source code configuration and shared library management.


As this package is made with the GNU autotools you should run ./configure inside the distribution directory for configuring the source tree. After that you should run make for compilation and make install (as root) for installation of nss-mdns.


SHIROYAMA Takayuki, Anand Kumria and Bastien Nocera, Sjoerd Simons for patches.

Sean Meiners for search list support.

Philipp Zabel for ARM support.

Bruce M Simpson for porting it to FreeBSD.


The newest release is always available from http://0pointer.de/lennart/projects/nss-mdns/

The current release is 0.10

Get nss-mdns's development sources from the Subversion repository (viewcvs):

svn checkout svn://svn.0pointer.de/nss-mdns/trunk nss-mdns

You may find an up to date Debian package of nss-mdns on the Debian package repository. Many other distributions ship it, too.

If you want to be notified whenever I release a new version of this software use the subscription feature of Freshmeat.

Lennart Poettering <mzaffzqaf (at) 0pointer (dot) de>, May 2007
$Id: README.html.in 116 2007-05-12 14:54:24Z lennart $