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<title>s6-dns: the skadnsd internal program</title>
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<a href="index.html">libskadns</a><br />
<a href="../index.html">s6-dns</a><br />
<a href="http://skarnet.org/software/">Software</a><br />
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<h1> The <tt>skadnsd</tt> program </h1>
<p>
<tt>skadnsd</tt> is the skadns daemon. It reads a series of
queries from the client on stdin, resolves them asynchronously,
and writes
the answers to the client as soon as it gets them. It exits 0
when its stdin closes. It exits 111 on any serious error,
writing the error message to stderr.
</p>
<p>
<tt>skadnsd</tt> is a stub resolver. It reads <tt>/etc/resolv.conf</tt>
at start looking for a "nameserver" line containing
the address of a DNS cache (aka full resolver). It will exit 111 if it cannot
find any valid cache address in <tt>/etc/resolv.conf</tt>. If the
<tt>DNSCACHEIP</tt> environment variable is set, its value overrides
what <tt>/etc/resolv.conf</tt> says.
</p>
<h2> Interface </h2>
<p>
skadnsd does not fork, does not background itself automatically,
and does not use syslog. It is not meant to be run directly by the
user: it will be invoked and spawned by the skadns library calls.
</p>
<p>
There are 2 ways to use skadnsd:
</p>
<ol>
<li> (preferred) Use the <tt>skadns_startf()</tt> library call.
A <tt>skadnsd</tt> child will then be spawned from your
calling process, and automatically reaped when you call
<tt>skadns_end()</tt>. It requires care with applications that
trap SIGCHLD. </li>
<li> Use the <tt>skadns_start()</tt> library call, together with
a <em>skadnsd service</em>. </li>
</ol>
<h3> Running skadnsd as a child process </h3>
<p>
This is the simplest and safest way of using <em>skadns</em>. Forget
about <tt>skadnsd</tt>: just start your library calls with
<tt>skadns_startf()</tt> and end them with <tt>skadns_end()</tt>.
Be careful though: if you're using SIGCHLD handlers, make sure they do
not interfere with the child processes your application has without
knowing. This is a general Unix programming rule.
</p>
<h3> Running a skadnsd as a daemon: the skadnsd service </h3>
<p>
In this mode, you set up a daemon listening on a Unix domain socket,
and clients connect to this socket to access the service. The
advantage of this setup is that it works even with badly written
clients that have trouble handling a child process; the drawback is
that it requires support from the system administrator.
</p>
<p>
skadnsd has no "standalone" mode: it is designed to work with a Unix
domain superserver, like
<a href="http://skarnet.org/software/s6-networking/s6-ipcserver.html">s6-ipcserver</a>.
skadnsd follows the <a href="http://cr.yp.to/proto/ucspi.txt">UCSPI"</a>
interface, it can be directly executed from the superserver.
</p>
<p>
You should run skadnsd (and its Unix superserver) under a specific user
and group, for elementary security reasons; and you should run its
dedicated logger as another specific user. Do NOT run skadnsd as root;
check your super-server documentation to find how
to run it under a specific account.
</p>
<h2> Notes </h2>
<ul>
<li> Users should never invoke <tt>skadnsd</tt> directly. It's an
internal program designed to be spawned by the skadns library. </li>
<li> If a poorly designed client sends a lot of queries and never reads the
answers, those will indefinitely queue up in the daemon, eating up
memory. You should run your process (or your Unix superserver, if you're
using a skadnsd service) under a program like
<a href="http://skarnet.org/software/s6/s6-softlimit.html">s6-softlimit</a> to
set a memory limit to every skadnsd instance. If skadnsd runs out of
allowed memory, it will simply die. </li>
<li> If you're using a skadnsd service: you should configure your
Unix superserver to set a maximum
number of concurrent skadnsd instances; most
Unix superserver implementations provide the
<tt>-c</tt> option for this. The s6-ipcserver program also
allows you to specify a maximum number of concurrent daemon
instances per client. </li>
<li> The only way to ensure absolute reliability of the skadnsd process is
to run it as a child, not as a daemon: use <tt>skadnsd_startf()</tt> when
possible. </li>
</ul>
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