curl: (7) Failed to connect to localhost port 8080: Connection refused

curl: (7) Failed to connect to HOST port N: Connection refused means curl resolved the host but the TCP connection was refused: no program accepted connections on that address and port. Exit code 7 is CURLE_COULDNT_CONNECT. Check that the server is running and listening on the port and address you are calling (ss -ltn on Linux), then retry.

Reproduce it

Nothing listens on port 4300:

curl -sS http://127.0.0.1:4300/ 2>&1 | sed -E 's/after [0-9]+ ms/after N ms/'
echo "exit status: ${PIPESTATUS[0]}"

curl: (7) Failed to connect to 127.0.0.1:4300 after N ms: Could not connect to server
exit status: 7

The wording changes between curl versions: older 7.x releases print Failed to connect to localhost port 8080: Connection refused, curl 8 adds the elapsed time (after T ms), and the reason text depends on the build and operating system. The curl 8.21 Windows build used here writes HOST:PORT and Could not connect to server. The exit code is 7 in every case.

Fix: connect to a listening port

Start a server, then call the port it listens on:

node -e 'require("http").createServer((q, r) => r.end("ok\n")).listen(4301, "127.0.0.1"); setTimeout(() => process.exit(0), 3000)' &
sleep 1
curl -sS http://127.0.0.1:4301/
wait

ok

Checklist

A server that prints listening on 127.0.0.1:8080 inside a Docker container accepts connections only from inside that container. Make it listen on 0.0.0.0 and publish the port.