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<title>s6-networking: the s6-tcpclient 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>
</p>
<h1> The <tt>s6-tcpclient</tt> program </h1>
<p>
<tt>s6-tcpclient</tt> is an
<a href="https://cr.yp.to/proto/ucspi.txt">UCSPI client tool</a> for
INET domain sockets. It establishes a TCP connection to a server,
then executes into a program.
</p>
<h2> Interface </h2>
<pre>
s6-tcpclient [ -q | -Q | -v ] [ -4 | -6 ] [ -d | -D ] [ -r | -R ] [ -h | -H ] [ -n | -N ] [ -t <em>timeout</em> ] [ -l <em>localname</em> ] [ -T <em>timeoutconn</em> ] [ -i <em>localip</em> ] [ -p <em>localport</em> ] <em>host</em> <em>port</em> <em>prog...</em>
</pre>
<ul>
<li> s6-tcpclient establishes a TCP connection to host <em>host</em>
port <em>port</em>. </li>
<li> It executes into <em>prog...</em> with descriptor 6 reading from
the network and descriptor 7 writing to it. </li>
</ul>
<h2> Host address determination </h2>
<ul>
<li> <em>host</em> may be an IP address, in which case s6-tcpclient will
connect to that IP address. If the underlying skalibs has been
compiled with IPv6 support, <em>host</em> can be an IPv6 address as
well as an IPv4 one. </li>
<li> <em>host</em> may be a domain name, in which case a DNS
resolution will be performed on it, and a connection will be tried to
all the resulting IP addresses in a round-robin fashion, twice:
first with a small timeout, then with a longer timeout. The first
address to answer wins. The connection attempt fails if no address
in the list is able to answer. </li>
</ul>
<h2> Environment variables </h2>
<p>
<em>prog...</em> is run with the following variables set:
</p>
<ul>
<li> PROTO: always set to TCP </li>
<li> TCPREMOTEIP: set to the chosen IP address of <em>host</em>. </li>
<li> TCPREMOTEPORT: set to <em>port</em>. </li>
<li> TCPREMOTEHOST: if the <tt>-H</tt> option has been given, set to the
name obtained by a reverse DNS resolution of the IP address chosen
for <em>host</em>. Else unset. </li>
<li> TCPLOCALHOST: if the <tt>-l</tt> option has been given, set to
<em>localname</em>. Else set to the name obtained by a reverse DNS
resolution of the IP address chosen for the local host. </li>
<li> TCPREMOTEINFO: if the <tt>-r</tt> option has been given, set
to the information given by an IDENT server on <em>host</em> about
the current connection (very unreliable). Else unset. </li>
</ul>
<h2> Options </h2>
<ul>
<li> <tt>-q</tt> : be quiet. </li>
<li> <tt>-Q</tt> : be normally verbose. This is the default. </li>
<li> <tt>-v</tt> : be verbose. </li>
<li> <tt>-4</tt> : (only valid if the underlying skalibs has
IPv6 support) Interpret <em>host</em> as an IPv4 address or make A
queries to determine its addresses. </li>
<li> <tt>-6</tt> : (only valid if the underlying skalibs has
IPv6 support) Interpret <em>host</em> as an IPv6 address or make
AAAA queries to determine its addresses. This option and the previous
one are not mutually exclusive: if both are given, both IPv6 and
IPv4 addresses will be tried and IPv6 addresses will be given priority.
If neither option is given, only IPv4 address will be tried. </li>
<li> <tt>-d</tt> : don't use the TCP_NODELAY socket option. This
is the default. </li>
<li> <tt>-D</tt> : use the TCP_NODELAY socket option, which disables
Nagle's algorithm. </li>
<li> <tt>-r</tt> : try and obtain a TCPREMOTEINFO string via the
IDENT protocol. This is obsolete and unreliable, and should only be used for
compatibility with legacy programs. </li>
<li> <tt>-R</tt> : do not use the IDENT protocol. This is the
default. </li>
<li> <tt>-h</tt> : try and obtain the remote host name via DNS.
This is the default. </li>
<li> <tt>-H</tt> : do not try and obtain the remote host name
via DNS. </li>
<li> <tt>-n</tt> : qualify <em>host</em> when resolving it to
find suitable IP addresses. This is the default. </li>
<li> <tt>-N</tt> : do not qualify <em>host</em>. </li>
<li> <tt>-t :<em>timeout</em></tt> : put a global timeout
on the connection attempt. If no fully functional connection has been
established after <em>timeout</em> seconds, abort the program. By
default, <em>timeout</em> is 0, which means no timeout. </li>
<li> <tt>-i <em>localip</em></tt> : use <em>localip</em> as
the local socket address for the connection. By default, address selection
is left to the operating system. </li>
<li> <tt>-p <em>localport</em></tt> : use <em>localport</em>
as the local socket port for the connection. By default, port selection
is left to the operating system. </li>
<li> <tt>-l <em>localname</em></tt> : use <em>localname</em>
as the value of the TCPLOCALPATH environment variable instead of
looking it up via the DNS. </li>
<li> <tt>-T :<em>timeoutconn</em></tt> : configure the
connection timeouts. <em>timeoutconn</em> must be of the form
<em>x</em><tt>+</tt><em>y</em>, where <em>x</em> and <em>y</em> are
integers. <em>x</em> is the first timeout and <em>y</em> is the
second one: all suitable addresses for <em>host</em> are first
tried with a timeout of <em>x</em> seconds, and if all of them
fail, then they are tried again with a timeout of <em>y</em>
seconds. (Be aware that the timeout specified with the <tt>-t</tt>
option overrides everything.) By default, <em>x</em> is 2 and
<em>y</em> is 58. </li>
</ul>
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