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<title>s6-networking: the s6-tcpserver4d program</title>
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<p>
<a href="index.html">s6-networking</a><br />
<a href="//skarnet.org/software/">Software</a><br />
<a href="//skarnet.org/">skarnet.org</a>
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<h1> The <tt>s6-tcpserver4d</tt> program </h1>
<p>
<tt>s6-tcpserver4d</tt> is the serving part of the
<a href="s6-tcpserver4.html">s6-tcpserver4</a> super-server.
It assumes that its stdin is a bound and listening TCP/IPv4 socket,
and it accepts connections from clients connecting to it,
forking a program to handle each connection.
</p>
<h2> Interface </h2>
<pre>
s6-tcpserver4d [ -1 ] [ -v <em>verbosity</em> ] [ -c <em>maxconn</em> ] [ -C <em>localmaxconn</em> ] <em>prog...</em>
</pre>
<ul>
<li> s6-tcpserver4d accepts connections from clients to an already
bound and listening TCP socket which is its standard input. </li>
<li> For every TCP connection to this socket, it
forks. The child sets some environment variables, then
executes <em>prog...</em> with stdin reading from the network
socket and stdout writing to it. </li>
<li> Depending on the verbosity level, it logs what it does to stderr. </li>
<li> It runs until killed by a signal. Depending on the received
signal, it may kill its children before exiting. </li>
</ul>
<h2> Environment variables </h2>
<p>
For each connection, an instance of <em>prog...</em> is spawned with
the following variables set:
</p>
<ul>
<li> PROTO: always set to TCP </li>
<li> TCPREMOTEIP: set to the originating address </li>
<li> TCPREMOTEPORT: set to the originating port </li>
<li> TCPCONNNUM: set to the number of connections originating from
the same IP address </li>
</ul>
<h2> Options </h2>
<ul>
<li> <tt>-1</tt> : write a newline to stdout, and close stdout,
right before entering the client-accepting loop.
If stdout is suitably redirected, this can be used by monitoring
programs to check when the server is accepting connections. This can be
used with s6's <a href="//skarnet.org/software/s6/notifywhenup.html">readiness
notification mechanism</a> to make sure subsequent services do not start
before the server is ready. </li>
<li> <tt>-v <em>verbosity</em></tt> : be more or less verbose.
By default, <em>verbosity</em> is 1: print warning messages to stderr.
0 means only print fatal error messages ; 2 means print status and
connection information for every client. </li>
<li> <tt>-c <em>maxconn</em></tt> : accept at most
<em>maxconn</em> concurrent connections. Default is 40. It is
impossible to set it higher than 1000. </li>
<li> <tt>-C <em>localmaxconn</em></tt> : accept at most
<em>localmaxconn</em> connections from the same IP address.
Default is 40. It is impossible to set it higher than <em>maxconn</em>. </li>
</ul>
<h2> Signals </h2>
<ul>
<li> SIGTERM: exit. </li>
<li> SIGHUP: send a SIGTERM and a SIGCONT to all children. </li>
<li> SIGQUIT: send a SIGTERM and a SIGCONT to all children, then exit. </li>
<li> SIGABRT: send a SIGKILL to all children, then exit. </li>
</ul>
<h2> Notes </h2>
<ul>
<li> Unlike its ancestor
<a href="https://cr.yp.to/ucspi-tcp/tcpserver.html">tcpserver</a>,
s6-tcpserver4d performs just the bare minimum: the point is to have a
very small and very fast process to serve connections with the least
possible overhead. Features such as additional environment variables,
access control and DNS resolution are provided
via the <a href="s6-tcpserver-access.html">s6-tcpserver-access</a>
program. </li>
<li> s6-tcpserver4d is meant to be execve'd into by a program that gets
the listening socket. That program is normally
<a href="s6-tcpserver4-socketbinder.html">s6-tcpserver4-socketbinder</a>,
which creates the socket itself; but it can be a different one if the
socket is to be retrieved by another means, for instance by fd-passing
from a fd-holding daemon (some people call this "socket activation"). </li>
</ul>
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