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.
program can be used before the s6-tcpserver
invocation to notify listeners when 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").