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<title>s6: what is a local service</title>
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<p>
<a href="index.html">s6</a><br />
<a href="http://skarnet.org/software/">Software</a><br />
<a href="http://skarnet.org/">skarnet.org</a>
</p>
<h1> Local services </h1>
<p>
A <em>local service</em> is a daemon that listens to incoming connections
on a Unix domain socket. Clients of the service are programs connecting to
this socket: the daemon performs operations on their behalf.
</p>
<p>
The service is called <em>local</em> because it is not accessible to
clients from the network.
</p>
<p>
A widely known example of a local service is the <tt>syslogd</tt> daemon.
On most implementations, it listens to the <tt>/dev/log</tt> socket.
Its clients connect to it and send their logs via the socket. The
<tt>openlog()</tt> function is just a wrapper arround the <tt>connect()</tt>
system call, the <tt>syslog()</tt> function a wrapper around <tt>write()</tt>,
and so on.
</p>
<h2> Benefits </h2>
<h3> Privileges </h3>
<p>
The most important benefit of a local service is that it permits
<strong>controlled privilege gains without using setuid programs</strong>.
The daemon is run as user S; a client running as user C and connecting to
the daemon asks it to perform operations: those will be done as user S.
</p>
<p>
Standard Unix permissions on the listening socket can be used to implement
some basic access control: to restrict access to clients belonging to group
G, change the socket to user S and group G, and give it 0420 permissions.
This is functionally equivalent to the basic access control for setuid
programs: a program having user S, group G and permissions 4750 will be
executable by group G and run with S rights.
</p>
<p>
But modern systems implement the
<a href="http://www.superscript.com/ucspi-ipc/getpeereid.html">getpeereid()</a>
system call or library function. This function allows the server to know the
client's credentials: so fine-grained access control is possible. On those
systems, <strong>local services can do as much authentication as setuid programs,
in a much more controlled environment</strong>.
</p>
<h3> fd-passing </h3>
<p>
The most obvious difference between a local service and a network service
is that a local service does not serve network clients. But local services
have another nice perk: while network services usually only provide you
with a single channel (a TCP or UDP socket) of communication between the
client and the server, forcing you to multiplex your data into that
channel, local services allow you to have as many
communication channels as you want.
</p>
<p>
(The SCTP transport layer provides a way for network services to use
several communication channels. Unfortunately, it is not widely deployed
yet, and a lot of network services still depend on TCP.)
</p>
<p>
The <em>fd-passing</em> mechanism is Unix domain socket black magic
that allows one peer of the socket to send open file descriptors to
the other peer. So, if the server opens a pipe and sends one end of
this pipe to a client via this mechanism, there is effectively a
socket <em>and</em> a pipe between the client and the server.
</p>
<h2> UCSPI </h2>
<p>
The <a href="http://cr.yp.to/proto/ucspi.txt">UCSPI</a> protocol
is an easy way of abstracting clients and servers from the network.
A server written as a UCSPI server, just as it can be run
under inetd or
<a href="http://skarnet.org/software/s6-networking/s6-tcpserver.html">s6-tcpserver</a>,
can be run under
<a href="s6-ipcserver.html">s6-ipcserver</a>: choose a socket
location and you have a local service.
</p>
<p>
Fine-grained access control can be added by inserting
<a href="s6-ipcserver-access.html">s6-ipcserver-access</a> in
your server command line after s6-ipcserver.
</p>
<p>
A client written as an UCSPI client, i.e. assuming it has descriptor
6 (resp. 7) open and reading from (resp. writing to) the server socket,
can be run under <a href="s6-ipcclient.html">s6-ipcclient</a>.
</p>
<h2> Use in skarnet.org software </h2>
<p>
skarnet.org libraries often use a separate process to handle
asynchronicity and background work in a way that's invisible to
the user. Among them are:
</p>
<ul>
<li> <a href="libs6/s6-ftrigrd.html">s6-ftrigrd</a>,
managing the reception of notifications and only waking up the client process
when the notification pattern matches a regular expression. </li>
<li> <a href="libs6/s6lockd.html">s6lockd</a>,
handling time-constrained lock acquisition on client behalf. </li>
<li> <a href="http://skarnet.org/software/s6-dns/skadns/skadnsd.html">skadnsd</a>,
performing asynchronous DNS queries and only waking up the client process
when an answer arrives. </li>
</ul>
<p>
Those processes are usually spawned from a client, via the corresponding
<tt>*_startf*()</tt> library call. But they can also be spawned from a
s6-ipcserver program in a local service configuration. In both cases, they
need an additional control channel to be passed from the server to
the client: the main socket is used for synchronous commands from the client
to the server and their answers, whereas the additional channel, which is
now implemented as a socket as well (but created by the server on-demand
and not bound to a local path), is used for asynchronous
notifications from the server to the client. The fd-passing mechanism
is used to transfer the additional channel from the server to the client.
</p>
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