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<title>Dillo RFC 003 - UNIX sockets in URLs</title>
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<h1>Dillo RFC 003 - UNIX sockets in URLs</h1>
<header>
<dl>
<dt>State</dt><dd>Draft</dd>
<dt>Date</dt><dd>Draft on 2024-12-30</dd>
<dt>Author</dt><dd>Rodrigo Arias Mallo
<<a href="mailto:rodarima@gmail.com">rodarima@gmail.com</a>></dd>
</dl>
</header>
<h2>Abstract</h2>
<p>This document adds UNIX sockets support to URLs by using host names with the TLD
“.unix” and a unix.hosts file that describe where in the file system
the socket file is located for a given host.</p>
<h2>Proposal</h2>
<p>UNIX sockets allow permissions to be established on a socket so that only a user
or a group of users can connect to it. However, the HTTP protocol doesn’t make
provisions to specify how to connect to UNIX sockets, and instead it uses TCP
to connect to a given host, which is resolved via DNS to an IP address.</p>
<p>To allow connecting to UNIX sockets, we propose using a special top level
domain (TLD) “.unix” which can be mapped to a local UNIX socket. In
this way, an URL like the following:</p>
<pre><code>http://example.unix/index.html
</code></pre>
<p>Will search for a socket file mapped to “example.unix” and
perform a HTTP query over that socket to retrieve the “/index.html”
file. Similarly, other protocols like Gopher or Gemini can work with UNIX
sockets in the same way.</p>
<p>This scheme allows a user to place the sockets in any path of their choosing,
which doesn’t need to be revealed in the URL.</p>
<p>To support this scheme, a unix.hosts file is be used with a similar syntax as
the hosts file, in such a way that a user can define its own aliases:</p>
<pre><code>unix:~/.dillo/example.sock example.unix
</code></pre>
<p>Similarly, a system-wide configuration could be made available, and still
inherit the benefits from UNIX permissions:</p>
<pre><code>unix:/var/lib/foobar.sock foobar.unix
</code></pre>
<p>This proposal fails gracefully when a URL referring to a UNIX socket is
opened by a program that doesn’t support UNIX hosts, as the top level
domain “.unix” doesn’t exist
[<a href="https://data.iana.org/TLD/tlds-alpha-by-domain.txt">1</a>].
Additionally, a program may determine that a host ending with .unix will use a
UNIX connection, so it is not needed to query any DNS server.</p>
<p>If no entry is found on the unix.hosts file for a given .unix host a default
set of locations for the UNIX socket could be attempted, but this is left out of
the current proposal.</p>
<h2>Implementation details</h2>
<p>This proposal could be implemented by any program that performs network
operations, it is not specific to Dillo. The following sections apply only to
Dillo itself, but may serve as a reference to other implementations.</p>
<h3>The unix.hosts file</h3>
<p>The format of the hosts file extends the syntax of /etc/hosts to allow hosts
that begin with the “unix:” prefix and are followed by a path. The
rest of the line defines aliases to that socket separated by white-spaces.</p>
<pre><code>unix:/var/lib/foobar.sock foobar.unix
</code></pre>
<p>The symbol ~ is expanded by the value of the <code>$HOME</code> variable of
the user performing the lookup. Therefore, to define sockets in the user home
directory:</p>
<pre><code>unix:~/foobar.sock foobar.unix foo-bar.unix
</code></pre>
<h3>UNIX host resolution</h3>
<p>The process to resolve a UNIX host is quite simple. First, determine if the
URL host name ends in “.unix”. If so, follow the steps below.
Otherwise, proceed with the current host resolution, possibly querying DNS
servers.</p>
<p>To find how to connect to a .unix host, identify a matching entry in the
hosts.unix file by looking at all the aliases for each UNIX socket. If there is
a match, use the unix socket patch of that line and the socket() interface with
the <code>AF_UNIX</code> family.</p>
<h3>Multiple entries</h3>
<p>In order to give users the ability to redefine aliases to their own UNIX
sockets, the entries in the unix.hosts file have precedence over the ones in the
system-wide unix.hosts configuration file.</p>
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