s6
Software
skarnet.org

# The `s6-connlimit` program

`s6-connlimit` is a small utility to perform IP-based control on the number of client connections to a TCP socket, and uid-based control on the number of client connections to a Unix domain socket.

## Interface

         s6-connlimit prog...

- `s6-connlimit` reads its environment for the PROTO environment variable, and then for ${PROTO}CONNNUM and ${PROTO}CONNMAX, which must contain integers.
- If the value of ${PROTO}CONNNUM is superior or equal to the value of ${PROTO}CONNMAX, s6-connlimit exits 1 with an error message.
- Else it execs into prog....
- If ${PROTO}CONNMAX is unset, s6-connlimit directly execs into prog... without performing any check: no maximum number of connections has been defined.

## Usage

The s6-tcpserver program defines the PROTO environment variable to "TCP", and spawns every child server with the TCPCONNNUM environment variable set to the number of connections from the same IP address. The s6-tcpserver-access program can set environment variables depending on the client's IP address. If the s6-tcpserver-access database is configured to set the TCPCONNMAX environment variable for a given set of IP addresses, and s6-tcpserver-access execs into s6-connlimit, then s6-connlimit will drop connections if there already are ${TCPCONNMAX} connections from the same client IP address.

The s6-ipcserver and s6-ipcserver-access programs can be used the same way, with "IPC" instead of "TCP", to limit the number of client connections by UID.

## Example

The following command line:

         s6-tcpserver -v2 -c1000 -C40 1.2.3.4 80 \
         s6-tcpserver-access -v2 -RHl0 -i dir \
         s6-connlimit \
         prog...

will run a server listening to IPv4 address 1.2.3.4, on port 80, serving up to 1000 concurrent connections, and up to 40 concurrent connections from the same IP address, no matter what the IP address. For every client connection, it will look up the database set up in dir; if the connection is accepted, it will run prog....

If the `dir/ip4/5.6.7.8_32/env/TCPCONNMAX` file exists and contains the string `30`, then at most 30 concurrent connections from 5.6.7.8 will execute prog..., instead of the default of 40.

## Notes

- The s6-connlimit utility was once part of the s6-networking suite, and is mostly useful with TCP connections, which is why the examples here involve TCP. Nevertheless, it can be used with connections across Unix domain sockets, and that is why it has been moved to the s6 package.