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<title>s6-networking: the s6-ucspitlsd 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-ucspitlsd</tt> program </h1>
<p>
<tt>s6-ucspitlsd</tt> is a server-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 server itself.
</p>
<h2> Interface </h2>
<pre>
s6-ucspitlsd [ -S | -s ] [ -J | -j ] [ -Y | -y ] [ -Z | -z ] [ -v <em>verbosity</em> ] [ -K kimeout ] [ -k snilevel ] [ -- ] <em>prog...</em>
</pre>
<ul>
<li> s6-ucspitlsd expects to have an open TCP connection it
can talk to on its stdin and stdout. </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-ucspitlsd, 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-ucspitlsd child will exec into a
<a href="s6-tlsd-io.html">s6-tlsd-io</a>
process that will initialize the TLS connection, perform the
handshake (expecting a TLS client 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-ucspitlsd process execs into <em>prog...</em>
and the child process execs into <a href="s6-tlsd-io.html">s6-tlsd-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-ucspitlsd does not expect to have any particular
environment variables, but it spawns a
<a href="s6-tlsd-io.html">s6-tlsd-io</a> program that does.
So it should pay attention to the following variables:
</p>
<ul>
<li> <tt>CERTFILE</tt> and <tt>KEYFILE</tt>. Also (or alternatively),
if the <tt>-k</tt> option is given: a series of
<tt>KEYFILE:<em>x</em></tt> and <tt>CERTFILE:<em>x</em></tt> variables,
for every <em>x</em> in the set of server names </li>
<li> (if the <tt>-Y</tt> or <tt>-y</tt> option has been given) <tt>CADIR</tt> or <tt>CAFILE</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, KEYFILE:<em>x</em> and CERTFILE:<em>x</em> for
every <em>x</em>, TLS_UID and TLS_GID. The variables are passed to
the <a href="s6-tlsd-io.html">s6-tlsd-io</a> child but
not to <em>prog...</em>.
The <tt>-Z</tt> option prevents that behaviour and keeps them
accessible in the child.
</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. </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-tlsd-io.html">s6-tlsd-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>-J</tt> : make <a href="s6-tlsd-io.html">s6-tlsd-io</a>
exit with a nonzero code if the peer sends EOF without a close_notify first </li>
<li> <tt>-j</tt> : treat EOF from the peer as a normal exit condition </li>
<li> <tt>-Y</tt> : Request an optional client certificate. </li>
<li> <tt>-y</tt> : Request a mandatory client certificate.
The default, with neither the <tt>-Y</tt> nor the <tt>-y</tt> option,
is not to request a client certificate at all. </li>
<li> <tt>-K <em>kimeout</em></tt> : close the connection if
the handshake takes more than <em>kimeout</em> milliseconds to complete.
The default is 0, which means infinite timeout: let the handshake complete
at its own pace, no matter how slow. </li>
<li> <tt>-k <em>snilevel</em></tt> : support alternative
certificate chains for SNI. If <em>snilevel</em> is nonzero, private
key file names are read from every environment variable of the form
<tt>KEYFILE:<em>x</em></tt>, where <em>x</em> is a server name that
the client may require, and a corresponding certificate chain for the name
<em>x</em> should exist in the file named after the contents of the
<tt>CERTFILE:<em>x</em></tt> environment variable. If <em>snilevel</em>
is 2 or more, <em>only</em> those files are read, and the generic
<tt>KEYFILE</tt> and <tt>CERTFILE</tt> variables are ignored.
If <em>snilevel</em> is 0, or if the option is not given, which is the
default, <tt>KEYFILE</tt> and <tt>CERTFILE</tt> are the only private
key / certificate chain pair that are loaded, no other environment
variable is read for keypairs. </li>
</ul>
<h2> Notes </h2>
<ul>
<li> It only makes sense to run s6-ucspitlsd 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-tlsd.html">s6-tlsd</a>. </li>
</ul>
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