socat

socat A B copies data both ways between two addresses: a TCP or UDP port, a Unix socket, a file, your terminal (- or stdio). In the labs it runs small echo servers, and talks to HAProxy's control socket to enable or disable a server behind the balancer; the socat command examples below are the ones the labs run.

socat command usage

socat ADDRESS ADDRESS

Options

TCP-LISTEN:7,fork

listen on TCP 7, one child per connection

TCP:10.0.2.10:7

connect to TCP 7 on 10.0.2.10

UDP4:host:port,sourceport=5000

UDP to one peer, from a chosen port; replies from elsewhere are ignored

,reuseaddr

share the local port with another socket already bound to it

UDP4-DATAGRAM:host:port,bind=:5000

UDP to host:port from port 5000, taking datagrams from anyone

stdio / -

your terminal

/run/haproxy.sock

a Unix socket

socat examples

socat - TCP:10.0.2.10:7

Connects your terminal to TCP port 7 on 10.0.2.10.

socat TCP-LISTEN:7,fork EXEC:cat

Runs an echo server on TCP port 7 that answers every client.

echo 'show stat' | socat stdio /run/haproxy.sock

Asks HAProxy, through its control socket, for the state of every server.

socat - UDP4:10.0.4.10:3478,sourceport=5000

Sends each line you type as one UDP datagram from port 5000, and prints every answer.

echo alice bob | socat -t1 - UDP4:10.0.4.10:3478,sourceport=5000

Sends one datagram from port 5000 and prints the answer, waiting a second for it.

Pitfalls

• Without ,fork a listener serves one client and exits.
• A connected UDP socket (UDP4:) quits on the first ICMP port unreachable; UDP4-DATAGRAM: keeps going.

Run it on real machines