The `s6-ipcserver` program

s6-ipcserver is an UCSPI server tool for Unix domain sockets, i.e. a super-server. It accepts connections from clients, and forks a program to handle each connection.

Interface

     s6-ipcserver [ -1 ] [ -q | -Q | -v ] [ -d | -D ] [ -P | -p ] [ -c maxconn ] [ -C localmaxconn ] [ -b backlog ] [ -G gidlist ] [ -g gid ] [ -u uid ] [ -U ] path prog...

s6-ipcserver binds to a Unix domain socket on path.
It closes its stdin and stdout.
For every client connection to this socket, it forks. The child sets some environment variables, then executes prog... with stdin reading from the socket and stdout writing to it.
Depending on the verbosity level, it logs what it does to stderr.
It runs until killed by a signal. Depending on the received signal, it may kill its children before exiting.

Environment variables

For each connection, an instance of prog... is spawned with the following variables set:

PROTO: always set to IPC
IPCREMOTEEUID: set to the effective UID of the client, unless credentials lookups have been disabled
IPCREMOTEEGID: set to the effective GID of the client, unless credentials lookups have been disabled
IPCREMOTEPATH: set to the path associated with the remote socket, if any. Be aware that it may contain arbitrary characters.
IPCCONNNUM: set to the number of connections originating from the same user (i.e. same uid)

If client credentials lookup has been disabled, IPCREMOTEEUID and IPCREMOTEEUID will be set, but empty.

Options

-1 : write path, followed by a newline, to stdout, before closing it, right after binding and listening to the Unix socket. If stdout is suitably redirected, this can be used by monitoring programs to check when the server is ready to accept connections.
-q : be quiet.
-Q : be normally verbose. This is the default.
-v : be verbose.
-d : allow instant rebinding to the same path even if it has been used not long ago - this is the SO_REUSEADDR flag to setsockopt() and is generally used with server programs. This is the default. Note that path will be deleted if it already exists at program start time.
-D : disallow instant rebinding to the same path.
-P : disable client credentials lookups. The IPCREMOTEEUID and IPCREMOTEEGID environment variables will be unset in every instance of prog.... This is the portable option, because not every system supports credential lookup across Unix domain sockets; but it is not as secure.
-p : enable client credentials lookups. This is the default; it works at least on Linux, Solaris, and *BSD systems. On systems that do not support it, every connection attempt will fail with a warning message.
-c maxconn : accept at most maxconn concurrent connections. Default is 40. It is impossible to set it higher than 1000.
-C localmaxconn : accept at most localmaxconn connections from the same user ID. Default is 40. It is impossible to set it higher than maxconn.
-b backlog : set a maximum of backlog backlog connections on the socket. Extra connection attempts will rejected by the kernel.
-G gidlist : change s6-ipcserver's supplementary group list to gidlist after binding the socket. This is only valid when run as root. gidlist must be a comma-separated list of numerical group IDs.
-g gid : change s6-ipcserver's groupid to gid after binding the socket. This is only valid when run as root.
-u uid : change s6-ipcserver's userid to uid after binding the socket. This is only valid when run as root.
-U : change s6-ipcserver's user id, group id and supplementary group list according to the values of the UID, GID and GIDLIST environment variables after binding the socket. This is only valid when run as root. This can be used with the s6-envuidgid program to easily script a service that binds to a privileged socket then drops its privileges to those of a named non-root account.

Signals

SIGTERM: exit.
SIGHUP: send a SIGTERM and a SIGCONT to all children.
SIGQUIT: send a SIGTERM and a SIGCONT to all children, then exit.
SIGABRT: send a SIGKILL to all children, then exit.

Notes