using socat to multiplex incoming tcp connection

Question

An external data provider makes a tcp connection to one of our servers.

I would like to use socat to 'multiplex' the incoming data so that multiple programs can receive data sent from the external data provider.

socat -u TCP4-LISTEN:42000,reuseaddr,fork OPEN:/home/me/my.log,creat,append

happily accepts incoming data and puts it into a file.

What I'd like to do is something that will allow local programs to connect to a TCP port and begin to receive data that arrives from connections to the external port. I tried

socat -u TCP4-LISTEN:42000,reuseaddr,fork TCP4-LISTEN:43000,reuseaddr 

but that doesn't work. I haven't been able to find any examples in the socat doco that seem relevant to back to back TCP servers.

Can someone point me in the right direction?

Answer 1

With Bash process-substitution

Multiplexing from the shell can in general be achieved with coreutils tee and Bash process-substitution. So for example to have the socat-stream multiplexed to multiple pipelines do something like this:

socat -u tcp-l:42000,fork,reuseaddr system:'bash -c \"tee >(sed s/foo/bar/ > a) >(cat > b) > /dev/null\"'

Now if you send foobar to the server:

socat - tcp:localhost:42000 <<<fobar

Files a and b will contain:

a

barbar

b

foobar

With named pipes

If the pipelines are complicated and/or you want to avoid using Bash, you can use named pipes to improve readability and portability:

mkfifo x y

Create the reader processes:

sed s/foo/bar/ x > a &
cat y > b &

Start the server:

socat -u tcp-l:42000,fork,reuseaddr system:'tee x y > /dev/null'

Again, send foobar to the server:

echo foobar |  socat - tcp:localhost:42000

And the result is the same as in the above.