L4 module for Postgres

[metafeather]

This a simple module to detect and match on Postgres connections in a similar manner that the SSH module uses.

Currently it can be used to multiplex a single database connection along with HTTP/S and SSH to avoid needed to open additional ports:

            "routes": [
              {
                "match": [
                  {
                    "postgres": {}
                  }
                ],
                "handle": [
                  {
                    "handler": "proxy",
                    "upstreams": [
                      {
                        "dial": [
                          "postgres.localhost:5432"
                        ]
                      }
                    ]
                  }
                ]
              }
            ]

[mholt]

Thanks for the contribution! Looks like a great start.

Can we add the Apache license notice to the beginning of the file? https://github.com/mholt/caddy-l4?tab=Apache-2.0-1-ov-file#readme

Then, let's just unexport identifiers that aren't used outside the package. (I've highlighted some examples but other code changes will be needed to compile of course.)

Then we can probably merge this in. Thanks!

[metafeather]

Thanks for the feedback, all done

[mholt]

Thank you! Looks good for now. Let's give it a try :)

[coolaj86]

The first 8 bytes are the Postgres' bespoke SSLRequest.

The server (caddy) needs to respond with S, after which the very next bytes will be the normal TLS hello containing the SNI so that the connection can be matched and routed to the correct server.

At that time, the SSLRequest needs to be sent to the Postgres Server, the S response byte should be eaten, and the connection can handed off completely at that point.

Without sending S, there's no way to know which Postgres server the request should be routed to.

It looks like we could add the cx.Write('S')... but I'm not clear on how we'd pass that off to the TLS handler and then hand control back for the SSLRequest to be repeated before letting go of control.

Thoughts?