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<h1><a name="top">nss-mdns 0.10</a></h1>

<p><i>Copyright 2004-2007 Lennart Poettering &lt;mzaffzqaf (at) 0pointer (dot) de&gt;</i></p>

<ul class="toc">
    <li><a href="#license">License</a></li>
    <li><a href="#news">News</a></li>
    <li><a href="#overview">Overview</a></li>
    <li><a href="#status">Current Status</a></li>
    <li><a href="#documentation">Documentation</a></li>
    <li><a href="#requirements">Requirements</a></li>
    <li><a href="#installation">Installation</a></li>
    <li><a href="#acks">Acknowledgements</a></li>
    <li><a href="#download">Download</a></li>
</ul>

<h2><a name="license">License</a></h2>

<p>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.</p>

<p>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.</p>

<p>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.</p>

<h2><a name="news">News</a></h2>

<div class="news-date">Sat May 12 2007: </div>

<p class="news-text"><a
href="http://0pointer.de/lennart/projects/nss-mdns/nss-mdns-0.10.tar.gz">Version 0.10</a>
released. Changes include: Ported to FreeBSD; alignment fixes for SPARC.</p>

<div class="news-date">Mon Jan 1 2007: </div>

<p class="news-text"><a
href="http://0pointer.de/lennart/projects/nss-mdns/nss-mdns-0.9.tar.gz">Version 0.9</a>
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.</p>

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

<div class="news-date">Sat Apr 29 2006: </div> <p class="news-text"><a
href="http://0pointer.de/lennart/projects/nss-mdns/nss-mdns-0.8.tar.gz">Version 0.8</a>
released. Changes include: Build time option to disable "legacy unicast" mDNS
requests, i.e. resolve exclusively with Avahi; build a special
<tt>_minimal</tt> flavour of the shared objects to minimize
unnecessary name lookup timeouts; fix IPv6 resolving when using
Avahi.</p>

<p class="news-text"><b>Please note that starting with nss-mdns 0.8 we encourage you to use
a different <tt>/etc/nsswitch.conf</tt> configuration line. See below
for more information!</b></p>

<div class="news-date">Sat Nov 19 2005: </div> <p class="news-text"><a
href="http://0pointer.de/lennart/projects/nss-mdns/nss-mdns-0.7.tar.gz">Version 0.7</a>
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</p>

<div class="news-date">Sun Aug 21 2005: </div> <p class="news-text"><a
href="http://0pointer.de/lennart/projects/nss-mdns/nss-mdns-0.6.tar.gz">Version 0.6</a>
released. Changes include: honour search list in
<tt>/etc/resolv.conf</tt>; try to contact <a
href="http://avahi.org/">Avahi</a> for
resolving.</p>

<div class="news-date">Sat Jun 4 2005: </div> <p class="news-text"><a
href="http://0pointer.de/lennart/projects/nss-mdns/nss-mdns-0.5.tar.gz">Version 0.5</a>
released. Changes include: only lookup hostnames ending in
<tt>.local</tt>; add support for a configuration file
(<tt>/etc/mdns.allow</tt>) to allow lookups for other names.</p>

<div class="news-date">Sun May 15 2005: </div> <p class="news-text"><a
href="http://0pointer.de/lennart/projects/nss-mdns/nss-mdns-0.4.tar.gz">Version 0.4</a>
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)</p>

<div class="news-date">Jan Sun 16 2005: </div> <p class="news-text"><a
href="http://0pointer.de/lennart/projects/nss-mdns/nss-mdns-0.3.tar.gz">Version 0.3</a>
released. Changes include: add Debianization; use <tt>ip6.arpa</tt> instead of <tt>ip6.int</tt> for reverse IPv6 lookups.</p>

<div class="news-date">Fri Dec 17 2004: </div> <p class="news-text"><a
href="http://0pointer.de/lennart/projects/nss-mdns/nss-mdns-0.2.tar.gz">Version 0.2</a>
released. Changes include: send mDNS queries on every interface that
supports multicasts, instead of only the one with the default route,
making <tt>nss-mdns</tt> more robust on multi-homed hosts; gcc 2.95
compatiblity.</p>

<div class="news-date">Mon Dec 6 2004: </div> <p class="news-text"><a
href="http://0pointer.de/lennart/projects/nss-mdns/nss-mdns-0.1.tar.gz">Version 0.1</a> released</p>

<h2><a name="overview">Overview</a></h2>

<p><tt>nss-mdns</tt> is a plugin for the GNU Name Service
Switch (NSS) functionality of the GNU C Library (<tt>glibc</tt>)
providing host name resolution via <a
href="http://www.multicastdns.org/">Multicast DNS</a> (aka
<i>Zeroconf</i>, aka <i>Apple Rendezvous</i>, aka <i>Apple Bonjour</i>), effectively allowing
name resolution by common Unix/Linux programs in the ad-hoc mDNS domain <tt>.local</tt>.</p>

<p><tt>nss-mdns</tt> provides client functionality only, which
means that you have to run a mDNS responder daemon seperately
from <tt>nss-mdns</tt> if you want to register the local host name via
mDNS. I recommend <a href="http://avahi.org/">Avahi</a>.</p>

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

<p>By default <tt>nss-mdns</tt> tries to contact a running <a
href="http://avahi.org/">avahi-daemon</a> for resolving host names and
addresses and making use of its superior record cacheing. Optionally
<tt>nss-mdns</tt> 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.</p>

<h2><a name="status">Current Status</a></h2>

<p>It works!</p>

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

<h2><a name="documentation">Documentation</a></h2>

<p>After compiling and installing <tt>nss-mdns</tt> you'll find six
new NSS modules in <tt>/lib</tt>:</p>
<ul>
<li><tt>libnss_mdns.so.2</tt></li>
<li><tt>libnss_mdns4.so.2</tt></li>
<li><tt>libnss_mdns6.so.2</tt></li>
<li><tt>libnss_mdns_minimal.so.2</tt></li>
<li><tt>libnss_mdns4_minimal.so.2</tt></li>
<li><tt>libnss_mdns6_minimal.so.2</tt></li>
</ul>
<p><tt>libnss_mdns.so.2</tt>
resolves both IPv6 and IPv4 addresses, <tt>libnss_mdns4.so.2</tt> only
IPv4 addresses and <tt>libnss_mdns6.so.2</tt> only IPv6 addresses. Due
to the fact that most mDNS responders only register local IPv4
addresses via mDNS, most people will want to use
<tt>libnss_mdns4.so.2</tt> exclusively. Using
<tt>libnss_mdns.so.2</tt> or <tt>libnss_mdns6.so.2</tt> 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.</p>

<p><tt>libnss_mdns{4,6,}_minimal.so</tt> (new in version 0.8) is mostly identical to the
versions without <tt>_minimal</tt>. However, they differ in one
way. The minimal versions will always deny to resolve host names that
don't end in <tt>.local</tt> or addresses that aren't in the range
<tt>169.254.x.x</tt> (the range used by <a
href="http://files.zeroconf.org/rfc3927.txt">IPV4LL/APIPA/RFC3927</a>.)
Combining the <tt>_minimal</tt> 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.</p>

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

<pre># /etc/nsswitch.conf

passwd:         compat
group:          compat
shadow:         compat

hosts:          files <b>mdns4_minimal [NOTFOUND=return]</b> dns <b>mdns4</b>
networks:       files

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

netgroup:       nis</pre>

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

<pre>$ getent hosts <i>foo</i>.local
192.168.50.4    foo.local</pre>

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

<p>If you run a firewall, don't forget to allow UDP traffic to the the
mDNS multicast address <tt>224.0.0.251</tt> on port 5353.</p>

<p><b>Please note:</b> The line above makes <tt>nss-mdns</tt>
authoritative for the <tt>.local</tt> 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 <tt>.local</tt> are
inherently incompatible. Please contact your local admistrator and ask
him to move to a different domain name since <tt>.local</tt> is to be
used exclusively for Zeroconf technology. <a
href="http://avahi.org/wiki/AvahiAndUnicastDotLocal">Further
information</a>.</p>

<p>Starting with version 0.5, <tt>nss-mdns</tt> has a simple
configuration file <tt>/etc/mdns.allow</tt> for enabling name lookups
via mDNS in other domains than <tt>.local</tt>. 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 <tt>*</tt>
only (similar to <tt>nss-mdns</tt> mode of operation of versions &lt;= 0.4):</p>

<pre># /etc/mdns.allow
*</pre>

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

<pre># /etc/mdns.allow
.local.
.local</pre>

<p>i.e. only hostnames ending with <tt>.local</tt> are resolved via
mDNS.</p>

<p>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 <tt>.local</tt>, hence you usually don't want to touch this file.</p>

<h2><a name="requirements">Requirements</a></h2>

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

<p><tt>nss-mdns</tt> 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.</p>

<h2><a name="installation">Installation</a></h2>

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

<h2><a name="acks">Acknowledgements</a></h2>

<p>SHIROYAMA Takayuki, Anand Kumria and Bastien Nocera, Sjoerd Simons for patches.</p>

<p>Sean Meiners for search list support.</p>

<p>Philipp Zabel for ARM support.</p>

<p>Bruce M Simpson for porting it to FreeBSD.</p>

<h2><a name="download">Download</a></h2>

<p>The newest release is always available from <a href="http://0pointer.de/lennart/projects/nss-mdns/">http://0pointer.de/lennart/projects/nss-mdns/</a></p>

<p>The current release is <a href="http://0pointer.de/lennart/projects/nss-mdns/nss-mdns-0.10.tar.gz">0.10</a></p>

<p>Get <tt>nss-mdns</tt>'s development sources from the <a href="http://subversion.tigris.org/">Subversion</a> <a href="svn://svn.0pointer.de/nss-mdns">repository</a> (<a href="http://0pointer.de/cgi-bin/viewcvs.cgi/?root=nss-mdns">viewcvs</a>): </p>

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

<p>You may find an up to date Debian package of <tt>nss-mdns</tt> on the <a href="http://packages.debian.org/libnss-mdns">Debian package repository</a>. Many other distributions ship it, too.</p>

<p>If you want to be notified whenever I release a new version of this software use the subscription feature of <a href="http://freshmeat.net/projects/nss-mdns/">Freshmeat</a>.</p>

<hr/>
<address class="grey">Lennart Poettering &lt;mzaffzqaf (at) 0pointer (dot) de&gt;, May 2007</address>
<div class="grey"><i>$Id: README.html.in 116 2007-05-12 14:54:24Z lennart $</i></div>

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