Port forwarding stopped working

Likely causes

1. The server got a new address from DHCP; the forward still points at the old one.
2. The DNAT rule was lost (a reset or a firmware update): the router refuses the port itself.
3. The firewall blocks the forwarded port even though the DNAT rule exists.

Diagnose

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

outsider:~# ssh -p 2222 admin@10.0.2.2 hostname

From outside: Host is unreachable. The router forwards to an address nobody has now.

ssh: connect to host 10.0.2.2 port 2222: Host is unreachable

nas:~# ip -br addr

The server's address is 10.0.1.20, not the 10.0.1.10 the forward was made for.

…
eth0@if60262     UP             10.0.1.20/24 

outsider:~# curl -sS -m 4 http://10.0.2.2:8080/

A different case: a forward the firewall blocks times out.

curl: (28) Connection timed out after 4004 milliseconds

Fix

nas:~# ip addr replace 10.0.1.10/24 dev homeNetwork

Give the server back the address the forward uses (a fixed address, or a DHCP reservation).

nas:~# ip -br addr show homeNetwork
eth0@if60262     UP             10.0.1.20/24 10.0.1.10/24 

outsider:~# ssh -p 2222 admin@10.0.2.2 hostname

The forward works again.

nas

iptables -t nat -A PREROUTING -p tcp --dport 2222 -j DNAT --to-destination 10.0.1.10:22

When the rule itself is gone: add the DNAT back on the router.

The fix the lab's challenge "The NAS vanished from outside" checks.

Break it and fix it yourself

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

• A home router: NAT, one port forward, and the firewall Free: its challenge "The NAS vanished from outside" set the fault for you to find and fix.

Open A home router: NAT, one port forward, and the firewall