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<title>s6-networking: the s6-tcpserver4-socketbinder program</title>
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<a href="index.html">s6-networking</a><br />
<a href="//skarnet.org/software/">Software</a><br />
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<h1> The <tt>s6-tcpserver4-socketbinder</tt> program </h1>
<p>
<tt>s6-tcpserver4-socketbinder</tt> binds an INET domain
socket to an IPv4 address and port, then executes a program.
</p>
<h2> Interface </h2>
<pre>
s6-tcpserver4-socketbinder [ -d | -D ] [ -b <em>backlog</em> ] [ -M | -m ] [ -B ] <em>ip</em> <em>port</em> <em>prog...</em>
</pre>
<ul>
<li> s6-tcpserver4-socketbinder creates an TCP socket
and binds it to IPv4 address <em>ip</em>, port <em>port</em>.
It prepares the socket to accept connections by calling
<a href="https://pubs.opengroup.org/onlinepubs/9699919799/functions/listen.html">listen()</a>. </li>
<li> It then execs into <em>prog...</em> with the open socket
as its standard input. </li>
</ul>
<h2> Options </h2>
<ul>
<li> <tt>-d</tt> : allow instant rebinding to the same IP and port
even if it has been used not long ago - this is the SO_REUSEADDR flag to
<a href="https://pubs.opengroup.org/onlinepubs/9699919799/functions/setsockopt.html">setsockopt()</a>
and is generally used with server programs. This is the default. </li>
<li> <tt>-D</tt> : disallow instant rebinding to the same path. </li>
<li> <tt>-b <em>backlog</em></tt> : set a maximum of
<em>backlog</em> backlog connections on the socket - extra
connection attempts will rejected by the kernel. The default is SOMAXCONN,
i.e. the maximum number allowed by the system. If <em>backlog</em>
is 0, then the socket will be created, but it <strong>will not be
listening</strong>. </li>
<li> <tt>-M</tt> : create a TCP socket. This is the default. </li>
<li> <tt>-m</tt> : create a UDP socket. Note
that by default UDP sockets are not connection-mode, and <tt>listen()</tt>
will fail - so you should always give the <tt>-b0</tt> option to
s6-tcpserver4-socketbinder along with <tt>-m</tt>. </li>
<li> <tt>-B</tt> : create a blocking socket. Default is non-blocking. </li>
</ul>
<h2> Notes </h2>
<ul>
<li> The socket is provided <strong>non-blocking by default</strong>. </li>
<li> s6-tcpserver4-socketbinder is part of a set of basic blocks used to
build a flexible TCP/IPv4 super-server. It normally should be given a
command line crafted to make it execute into
<a href="s6-tcpserver4d.html">s6-tcpserver4d</a> to accept connections
from clients, or into a program such as
<a href="//skarnet.org/software/s6/s6-applyuidgid.html">s6-applyuidgid</a>
to drop privileges before doing so. </li>
<li> The <a href="s6-tcpserver4.html">s6-tcpserver4</a> program does
exactly this. It implements
a full TCP/IPv4 super-server by building a command line starting with
s6-tcpserver4-socketbinder and ending with s6-tcpserver4d followed by the
application program, and executing into it. </li>
</ul>
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