Service works on localhost but not from another machine

Likely causes

1. The program binds to 127.0.0.1 (often a default, or a --host/bind setting).
2. A firewall on the server or on the path, which shows as a timeout instead of a refusal.

Diagnose

Run these in order. The output is what each printed on a lab's real Linux machines.

appServer:~# nc 127.0.0.1 8080

On the server itself the service answers.

admin panel

laptop:~# telnet 10.0.2.10 8080

From another machine, Connection refused: nothing listens on 10.0.2.10:8080.

telnet: can't connect to remote host (10.0.2.10): Connection refused

appServer:~# ss -tulnp

ss -tulnp shows why: port 8080 listens on 127.0.0.1:8080, while 443 and 80 listen on 0.0.0.0 or *, every address.

Netid State  Recv-Q Send-Q Local Address:Port  Peer Address:PortProcess                       
udp   UNCONN 0      0         127.0.0.11:44457      0.0.0.0:*                                 
udp   UNCONN 0      0                  *:7                *:*    users:(("socat",pid=89,fd=5))
tcp   LISTEN 0      5          127.0.0.1:8080       0.0.0.0:*    users:(("socat",pid=90,fd=5))
tcp   LISTEN 0      4096      127.0.0.11:33481      0.0.0.0:*                                 
tcp   LISTEN 0      511          0.0.0.0:443        0.0.0.0:*    users:(("nginx",pid=80,fd=5))
tcp   LISTEN 0      9                  *:80               *:*    users:(("httpd",pid=74,fd=3))
tcp   LISTEN 0      5                  *:7                *:*    users:(("socat",pid=81,fd=5))

Fix

pkill -f '^socat TCP-LISTEN:7,'; sleep 0.5; setsid socat TCP-LISTEN:7,bind=0.0.0.0,fork,reuseaddr EXEC:cat >/dev/null 2>&1 < /dev/null &

Restart the service bound to 0.0.0.0. For your own program, the same: change its listen address and restart it.

The fix the lab's challenge "The echo service only talks to itself" checks.

Break it and fix it yourself

Each lab is a small network of real Linux machines in your browser, where this fault happens.

• TCP and UDP: ports, connections and datagrams Free: its challenge "The echo service only talks to itself" set the fault for you to find and fix.

Open TCP and UDP: ports, connections and datagrams