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<title>s6-networking: the s6-ucspitlsc program</title>
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<p>
<a href="index.html">s6-networking</a><br />
<a href="//skarnet.org/software/">Software</a><br />
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<h1> The <tt>s6-ucspitlsc</tt> program </h1>
<p>
<tt>s6-ucspitlsc</tt> is a client-side program that establishes
communication channels according to the UCSPI-TLS protocol,
then execs into an application. Later, if the application sends
a command, a TLS tunnel will be started and the application will
be able to use it instead of communicating with the network via
cleartext.
</p>
<p>
The point of this protocol, and this program, is to make it easy
to implement commands like SMTP's STARTTLS without embedding a
TLS stack in the client itself.
</p>
<h2> Interface </h2>
<pre>
s6-ucspitlsc [ -S | -s ] [ -Y | -y ] [ -Z | -z ] [ -v <em>verbosity</em> ] [ -K kimeout ] [ -k <em>servername</em> ] [ -6 <em>rfd</em> ] [ -7 <em>wfd</em> ] [ -- ] <em>prog...</em>
</pre>
<ul>
<li> s6-ucspitlsc expects to have an open TCP connection it
can talk to on its (by default) descriptors 6 (for reading)
and 7 (for writing). </li>
<li> It forks and establishes communication channels between
the parent and the child. The parent executes into
<em>prog...</em>; the child remains and waits for a
command. </li>
<li> At any time, <em>prog...</em> can send a command to its
control socket provided by s6-ucspitlsc, following the
<a href="https://web.archive.org/web/20150311223933/http://www.suspectclass.com/sgifford/ucspi-tls/ucspi-tls.txt">UCSPI-TLS</a>
protocol. Then the s6-ucspitlsc child will exec into a
<a href="s6-tlsc-io.html">s6-tlsc-io</a>
process that will initiate the TLS connection, perform the
handshake (expecting a TLS server on the other side of the
network) and maintain a TLS tunnel. <em>prog</em> can
use that TLS tunnel instead of talking directly to the
network. </li>
</ul>
<h2> Exit codes </h2>
<ul>
<li> 100: wrong usage. </li>
<li> 111: system call failed. </li>
</ul>
<p>
Normally the parent s6-ucspitlsc process execs into <em>prog...</em>
and the child process execs into <a href="s6-tlsc-io.html">s6-tlsc-io</a>.
If the parent dies or closes its control socket before sending a
command to start TLS, the child exits 0.
</p>
<h2> Environment variables </h2>
<h3> Read </h3>
<p>
s6-ucspitlsc does not expect to have any particular
environment variables, but it spawns a
<a href="s6-tlsc-io.html">s6-tlsc-io</a> program that does.
So it should pay attention to the following variables:
</p>
<ul>
<li> <tt>CADIR</tt> or <tt>CAFILE</tt> </li>
<li> (if the -y option has been given) <tt>CERTFILE</tt> and <tt>KEYFILE</tt> </li>
<li> <tt>TLS_UID</tt> and <tt>TLS_GID</tt>
</ul>
<h3> Written </h3>
<p>
By default, <em>prog...</em> is run with all these
variables <em>unset</em>: CADIR, CAFILE,
KEYFILE, CERTFILE, TLS_UID and TLS_GID. They're passed to
the <a href="s6-tlsc-io.html">s6-tlsc-io</a> child but
not to <em>prog...</em>.
The <tt>-Z</tt> option prevents that behaviour.
</p>
<p>
However, <em>prog...</em> is run with the following additional
environment variables, following the UCSPI-TLS protocol:
</p>
<ul>
<li> <tt>SSLCTLFD</tt> contains the file descriptor number of
the control socket. </li>
<li> <tt>SSLREADFD</tt> contains the file descriptor number of
the pipe used to read data from the TLS tunnel after it
has been activated. </li>
<li> <tt>SSLWRITEFD</tt> contains the file descriptor number of
the pipe used to write data to the TLS tunnel after it
has been activated. </li>
</ul>
<p>
Since <em>prog</em> is exec'ed before the TLS handshake takes
place, it cannot get information about the TLS connection via
environment variables. However, if it starts the TLS connection
via a <tt>Y</tt> command (as opposed to a <tt>y</tt> command),
it will receive this information as a string sent over the
control socket.
</p>
<h2> Options </h2>
<ul>
<li> <tt>-v <em>verbosity</em></tt> : Be more or less
verbose. Default for <em>verbosity</em> is 1. 0 is quiet, 2 is
verbose, more than 2 is debug output. This option currently has
no effect. </li>
<li> <tt>-Z</tt> : do not clean the environment of
the variables used by <a href="s6-tlsc-io.html">s6-tlsc-io</a>
before execing <em>prog...</em>. </li>
<li> <tt>-z</tt> : clean the environment of
the variables used by <a href="s6-tlsc-io.html">s6-tlsc-io</a>
before execing <em>prog...</em>. This is the default. </li>
<li> <tt>-S</tt> : send a <tt>close_notify</tt> alert
and break the connection when <em>prog</em> sends EOF. </li>
<li> <tt>-s</tt> : transmit EOF by half-closing the TCP
connection without using <tt>close_notify</tt>. This is the default. </li>
<li> <tt>-Y</tt> : Do not send a client certificate. This is the default. </li>
<li> <tt>-y</tt> : Send a client certificate. </li>
<li> <tt>-k <em>servername</em></tt> : use Server Name
Indication, and send <em>servername</em>. The default is not to
use SNI, which may be a security risk. </li>
<li> <tt>-K <em>kimeout</em></tt> : close the connection
if <em>kimeout</em> milliseconds elapse without any data being
received from either side. The default is 0, which means
infinite timeout (never kill the connection). </li>
<li> <tt>-6 <em>fdr</em></tt> : expect an open file
descriptor numbered <em>fdr</em> to read network (ciphertext)
data from. Make sure <em>prog</em> also reads its data
from its own fd <em>fdr</em>. Default is 6. </li>
<li> <tt>-7 <em>fdw</em></tt> : expect an open file
descriptor numbered <em>fdw</em> to write network (ciphertext)
data to. Make sure <em>prog</em> also writes its data to
its own fd <em>fdw</em>. Default is 7. </li>
</ul>
<h2> Notes </h2>
<ul>
<li> It only makes sense to run s6-ucspitlsc if its application
program <em>prog</em> knows and understands the
<a href="https://web.archive.org/web/20150311223933/http://www.suspectclass.com/sgifford/ucspi-tls/ucspi-tls.txt">UCSPI-TLS</a>
protocol for opportunistic TLS. If it does not, you will not
be able to secure your connection, and what you need is a regular
immediate TLS program instead, which means
<a href="s6-tlsc.html">s6-tlsc</a>. </li>
</ul>
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