CURLing to a UNIX socket
In deployed environments, I often come across two Ruby application servers - Unicorn, and Puma.
These servers are usually configured to listen on a UNIX socket file, usually located in the
tmp/sockets directory of a Capistrano install, e.g.
It’s quite often useful to be able to connect to a UNIX socket to see if the application server is behaving correctly,
especially if the instance is not accessible via a public IP address. A little-known feature of the
curl command line is that it accepts a
--unix-socket argument to send requests to. This allows
application servers and any other process that listens on a socket and supports HTTP to be tested
easily with a command like:
All the usual features of cURL are available when the UNIX file is provided, such as making POST/HEAD/OPTIONS/anything else requests, sending form data or files, and using cookies.
I’ve found that this technique is very handy for diagnosing problems that lie between the web/proxy server (usually Nginx), and the application itself.