The s6-tcpserver4d program
s6-tcpserver4d is the serving part of the
It assumes that its stdin is a bound and listening TCP/IPv4 socket,
and it accepts connections from clients connecting to it,
forking a program to handle each connection.
s6-tcpserver4d [ -1 ] [ -v verbosity ] [ -c maxconn ] [ -C localmaxconn ] prog...
- s6-tcpserver4d accepts connections from clients to an already
bound and listening TCP socket which is its standard input.
- For every TCP connection to this socket, it
forks. The child sets some environment variables, then
executes prog... with stdin reading from the network
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.
For each connection, an instance of prog... is spawned with
the following variables set:
- PROTO: always set to TCP
- TCPREMOTEIP: set to the originating address
- TCPREMOTEPORT: set to the originating port
- TCPCONNNUM: set to the number of connections originating from
the same IP address
- -1 : write a newline to stdout, and close stdout,
right before entering the client-accepting loop.
If stdout is suitably redirected, this can be used by monitoring
programs to check when the server is accepting connections. This can be
used with s6's readiness
notification mechanism to make sure subsequent services do not start
before the server is ready.
- -v verbosity : be more or less verbose.
By default, verbosity is 1: print warning messages to stderr.
0 means only print fatal error messages ; 2 means print status and
connection information for every client.
- -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 IP address.
Default is 40. It is impossible to set it higher than maxconn.
- 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.
- Unlike its ancestor
s6-tcpserver4d performs just the bare minimum: the point is to have a
very small and very fast process to serve connections with the least
possible overhead. Features such as additional environment variables,
access control and DNS resolution are provided
via the s6-tcpserver-access
- s6-tcpserver4d is meant to be execve'd into by a program that gets
the listening socket. That program is normally
which creates the socket itself; but it can be a different one if the
socket is to be retrieved by another means, for instance by fd-passing
from a fd-holding daemon (some people call this "socket activation").