Sockets

Core Idea

A socket is an endpoint a process uses to communicate, often over the network with an address, port, and protocol.

Why Sockets Matter In This Course

• ssh opens a client socket to the SSH server.
• curl opens a client socket to an HTTP server.
• python3 -m http.server --bind 127.0.0.1 opens a listening socket on your service port.
• Caddy connects to your user service through a local socket endpoint.

Client And Listening Sockets

client socket -> network -> listening socket

A listening socket waits for incoming connections. A client socket initiates a connection. For your personal service, Python listens on 127.0.0.1:<your-port>; Caddy connects to that local endpoint when handling the public service URL.

Address And Port

127.0.0.1:14567
|         |
address   port

127.0.0.1 means localhost from the machine’s point of view. The port selects the service endpoint on that address.

Commands To Try

Run ss to inspect listening and connected sockets:

ss -ltnp
ss -tnp

Use curl to test an HTTP listening socket:

PORT="$((10000 + $(id -u)))"
curl -I "http://127.0.0.1:$PORT/"

Common Failures

• Connection refused: no process is listening on that address and port.
• Address already in use: another process is already listening on that port.
• Public URL gives 502: the reverse proxy could not connect to the backend socket or got a bad response.
• Works on 127.0.0.1 but not publicly: local service is alive, but proxy, DNS, or routing needs checking.

Proof Check

Start your user service, compute your port with printf '%s\n' "$((10000 + $(id -u)))", then run a local curl -I against that port. Explain which process is listening and which command is the client.

Docs Pointers

• Run man ss.
• Run man 2 socket.
• Read IP Networking, Service, Server, Client, and Platform Reference.