s6-networking: the s6-tcpserverd program
Keywords: s6-networking s6-tcpserverd tcpserver ucspi tcp server super-server ipv4 ipv6 TCP/IP

s6-networking
Software
skarnet.org

# The `s6-tcpserverd` program

`s6-tcpserverd` is the serving part of the s6-tcpserver super-server. It assumes that its stdin is a bound and listening TCP/IP socket, and it accepts connections from clients connecting to it, spawning a program to handle each connection.

## Interface

         s6-tcpserverd [ -1 ] [ -v verbosity ] [ -c maxconn ] [ -C localmaxconn ] prog...

- s6-tcpserverd 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 spawns a prog... child 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.

## Environment variables

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

- PROTO: always set to TCP
- TCPLOCALIP: set to the server socket's address
- TCPLOCALPORT: set to the server socket's port
- TCPREMOTEIP: set to the client socket's address
- TCPREMOTEPORT: set to the client socket's port
- TCPCONNNUM: set to the number of connections originating from the same IP address

## Options

- `-1` : write the local port, followed by 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 16384.
- `-C localmaxconn` : accept at most localmaxconn connections from the same IP address. Default is 40. It is impossible to set it higher than maxconn.

## 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

- Unlike its ancestor tcpserver, s6-tcpserverd performs just the bare minimum: the point is to have a small and very fast process to serve connections with the least possible overhead. Features such as access control and DNS resolution are provided via the s6-tcpserver-access program.
- s6-tcpserverd is meant to be execve'd into by a program that gets the listening socket. That program is normally s6-tcpserver-socketbinder, 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").
- The option to make s6-tcpserverd verbose is `-v 2`, where the verbosity level is given as an argument, 0 being quiet, 1 normal and 2 verbose. This is different from the s6-tcpserver interface, where `-v` without an argument would indicate extra verbosity. The difference exists because s6-tcpserver follows the UCSPI tool interface, whereas s6-tcpserverd has no such constraint, and giving the verbosity level as a number is a better interface.