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<title>s6-dns: the s6-dns-hosts-compile program</title>
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<h1> The s6-dns-hosts-compile program </h1>
<p>
s6-dns-hosts-compile compiles the system's <tt>/etc/hosts</tt> file into a
CDB file for more efficient later use by s6-dns programs that may use the
data contained in the hosts file.
</p>
<h2> Interface </h2>
<pre>
s6-dns-hosts-compile [ -i <em>ifile</em> ] [ -o <em>ofile</em> ]
</pre>
<ul>
<li> s6-dns-hosts-compile reads the <tt>/etc/hosts</tt> file, which must follow the
<a href="https://man7.org/linux/man-pages/man5/hosts.5.html">traditional hosts format</a>. </li>
<li> It outputs a CDB database in the <tt>/etc/hosts.cdb</tt> file. </li>
<li> It exits 0. </li>
</ul>
<h2> Options </h2>
<ul>
<li> <tt>-i</tt> <em>ifile</em> : read from <em>ifile</em> instead of <tt>/etc/hosts</tt>. </li>
<li> <tt>-o</tt> <em>ofile</em> : write to <em>ofile</em> instead of <tt>/etc/hosts.cdb</tt>. </li>
</ul>
<h2> Notes </h2>
<ul>
<li> The <tt>/etc/hosts</tt> file is an old way of specifying IP from/to domain
name correspondences. It's convenient in a pinch when a user needs to locally
override DNS data - for instance, to redirect common advertising sites to 127.0.0.1 -
but it does not map exactly to the DNS view of domain names. </li>
<li> Due to this, and to its line-by-line text syntax, lookups using this file
are terribly inefficient and rely on approximate qualification heuristics. </li>
<li> To avoid that, s6-dns tools prefer having the <tt>/etc/hosts</tt> data in
a format that's both more DNS-like and more efficient; the file output by
s6-dns-hosts-compile is the result. </li>
<li> When asked to take <tt>/etc/hosts</tt> data into account, s6-dns tools will
first look for a <tt>/etc/hosts.cdb</tt> file and use the data from it. If
absent, they will read the <tt>/etc/hosts</tt> file and compile its contents
into a temporary cdb file under <tt>/tmp</tt> (which they immediately unlink),
and use that cdb to query hosts data. </li>
<li> As a consequence, lookups are always efficient, but there's an initial
compilation step. Using s6-dns-hosts-compile to produce a <tt>/etc/hosts.cdb</tt>
file in advance saves the cost of that compilation step in subsequent
invocations of s6-dns tools. </li>
<li> If s6-dns tools detect that <tt>/etc/hosts</tt> has been modified more recently
than <tt>/etc/hosts.cdb</tt>, they will not use <tt>/etc/hosts.cdb</tt> and will
perform the compilation step into a temporary file. So, remember to run
s6-dns-hosts-compile after modifying your <tt>/etc/hosts</tt> if you want to keep
the tiny performance improvement. </li>
<li> Some tools write link-local addresses in <tt>/etc/hosts</tt> i.e. IPv6 addresses
followed by a <tt>%</tt> (percent) and a network interface name. This is nonstandard
and unusable with the DNS protocol, which does not take interface names into
account. <tt>s6-dns-hosts-compile</tt> will ignore such lines if they appear in
your hosts file. </li>
</ul>
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