1. Chris Webb
  2. skd


skd - a lightweight socket daemon

skd is a small daemon which binds to a udp, tcp or unix-domain socket, waits
for connections and runs a specified program to handle them. It is ideal as a
secure, efficient replacement for traditional inetd as well as being an
easy-to-use tool for non-privileged users wanting to run their own network

Datagram and stream sockets are available in both the internet and unix
namespaces, each with the expected inetd behaviour. In the internet domain,
IPv6 is supported in addition to IPv4.

skd also supports connection limits, verbose logging of connections, dropping
of privileges, forking into the background with a pidfile and redirecting
stderr to syslog or a file. Some of these facilities (such as forking into the
background, privilege dropping and logging) are also useful for standalone,
non-network services and can be used without binding any socket.

Building and installing

Unpack the source tar.gz file and change to the unpacked directory.

Run 'make', then 'make install' to install in /bin. Alternatively, you can set
DESTDIR and/or BINDIR to install in a different location, or strip and copy
the compiled skd binary into the correct place manually.

skd was developed on GNU/Linux and FreeBSD, but should be reasonably portable.
In particular, it is expected to compile on most modern unix platforms. Please
report any problems or bugs to Chris Webb <chris@arachsys.com>.


Usage: skd [OPTIONS] PROG [ARGS]...
  -i [INTERFACE:]PORT   bind a listening socket in the internet namespace
  -l PATH, -x PATH      bind a listening socket in the local unix namespace
  -s                    create a stream socket (default socket style)
  -d                    create a datagram socket instead of a stream socket
  -t [INTERFACE:]PORT   create a TCP socket: equivalent to -s -i
  -u [INTERFACE:]PORT   create a UDP socket: equivalent to -d -i
  -b BACKLOG            set the listen backlog for a stream socket
  -c MAXCLIENTS         set the maximum number of concurrent connections
                          accepted by a stream socket (default is unlimited)
  -n                    set TCP_NODELAY to disable Nagle's algorithm for TCP
                          stream connections
  -v                    report information about every connection accepted
                          or initial datagram received to stderr or the log
  -B                    fork, establish new session id, redirect stdin and
                          stdout to/from /dev/null if they are attached to a
                          terminal, and run as a daemon in the background
  -L TAG[:FAC.PRI]      start a logger subprocess, redirecting stderr to the
                          system log with tag TAG, facility FAC and priority
                          PRI (defaulting to daemon.notice if unspecified)
  -L >LOGFILE           redirect stderr to create and write to LOGFILE
  -L >>LOGFILE          redirect stderr to append to LOGFILE
  -P PIDFILE            write pid to PIDFILE (after forking if used with -B)
  -U                    after binding the socket, drop privileges to those
                          specified by $UID and $GID, and if $ROOT is set,
                          chroot into that directory

When a stream socket is specified, listen on it and accept all incoming
connections, executing the given program in a child process with stdin and
stdout attached to the connection socket. Do not wait for the child to exit
before accepting another connection on the listening socket.

When a datagram socket is specified, wait for an initial datagram to arrive
before launching the given program with stdin and stdout attached to the
listening socket. Until this program exits, don't attempt to check for more
datagrams or spawn another child.

If none of -i, -l, -u is used, no socket is bound and the given program is
executed immediately, after any background, logging, pidfile and privilege
dropping actions have been completed. This allows use of these facilities
for standalone and non-network services.


A unix domain echo server running in the foreground, reporting connections to

  skd -vl /dev/cat.sock cat

An motd server running in the background with a pidfile /var/run/motd.pid,
reporting connections to syslog with tag 'testsrv', facility 'daemon' and
priority 'info':

  skd -vt 3000 -BP /var/run/motd.pid -L testsrv:daemon.info \
    cat /etc/motd

Uwe Ohse's uscheduled running in the background, logging errors from stderr to

  skd -BL uschedule:daemon.notice -- uscheduled -d /var/lib/uschedule

The last example demonstrates how skd can be useful as a daemontools
replacement in a more standard unix environment. I use it to daemonise
uschedule, dnscache and tinydns with logs sent to syslog.


skd was written by Chris Webb <chris@arachsys.com> and is distributed as Free
Software under the terms of the MIT license in COPYING.