How can I communicate with a Unix domain socket via the shell on Debian Squeeze?

I’m running a Debian Squeeze web server. I’ve installed memcached on it, and configured memcached to listen on a Unix domain socket (at /tmp/memcached.sock), as it only needs to receive messages from the website, which lives on the same server.

It seems to be working fine, but I’d also like to communicate with memcached via the shell, to check that it’s doing what I think it’s doing.

memcached accepts messages via a simple ASCII protocol (if I understand correctly). If it was listening on TCP/IP, I could send messages to it via e.g. nc:

$ echo "stats settings" | nc localhost 11211

But I can’t figure out how to send that text to the domain socket instead.

On my laptop (which runs OS X Lion), both nc and telnet have options (-U and -u respectively) to use domain sockets. However, on my Debian Squeeze web server, these options aren’t present.

Asked By: Paul D. Waite

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You can use socat on Debian.
To install it:

# apt-get install socat
Answered By: sendmoreinfo

With netcat-openbsd, there is a -U option. If you don’t have it, you probably have netcat-traditional installed instead; I’d suggest switching.

Example command:
nc -U /var/run/socket

Answered By: derobert

With socat (a ‘bidirectional data relay between two data channels’) you can connect to the unix domain socket like this:

$ socat - UNIX-CONNECT:/tmp/memcached.sock
Answered By: maxschlepzig

netcat-openbsd supports connecting to UNIX-domain sockets. Using this you can connect to either a UNIX-domain stream socket or a UNIX-domain datagram socket, and therefore you have to tell the socket’s type to netcat.

for example, /dev/log file in Linux is a UNIX-domain datagram socket socket, thus nc -U /dev/log won’t work. Instead use nc -uU /dev/log. Using -u along with -U tells netcat that it is a UNIX-domain datagram socket.

nc -U /tmp/socket  #Connect to UNIX-domain stream socket
nc -uU /tmp/socket #Connect to UNIX-domain datagram socket

Similarly, while using socat, use UNIX-CLIENT option. Using this option you can connect to both UNIX-domain stream and UNIX-domain datagram sockets. From its man page (man socat), “It first tries to connect and, if that fails, assumes it is a
datagram socket, thus supporting both types
“.

socat - UNIX-CLIENT:/dev/socket #connect to UNIX-domain socket, irrespective of its type
Answered By: Sahil Singh

If you happen to be on a system that only has busybox installed, it may look like it doesn’t support unix domain sockets (the -U option doesn’t exist) but I found the following works:

$ echo "stats slabs" | nc local:/tmp/memcached.sock
STAT active_slabs 0
STAT total_malloced 0
END

This can be really helpful for creating health checks for alpine containers.

Answered By: Greg Bray

Send the request "GET" to the socket "/tmp/server.sock" and print the result:

printf GET | socat UNIX-CONNECT:/tmp/server.sock -
Answered By: ceving
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