[SOLVED] sshd stopped responding to external connections

suicidaleggroll · 07-20-2012, 09:43 PM

Alright, this is a pretty weird situation.

I have several Linux boxes behind a router with a single public static IP. All machines have static IPs on the LAN, all are set up in a headless config so they are remotely managed through SSH, all work great. For several years, I have been forwarding port 22 to one of the machines (call it box3), and it's been working great. A couple of weeks ago we had a power outage, the UPS on box3 failed and box3 experienced a hard shutdown. The UPS's on the other machines were fine, no problems there. When box3 came back up, the following problems started.

Box3 is accessible through SSH within the LAN like normal. However, I can no longer connect to box3 from outside the LAN using the public IP like before. It's as if the forwarding of port 22 to box3 isn't working like it should. Box3's LAN IP has not changed, and if I change port 22 to point to a new machine on the LAN in the router (eg: box4, box5, etc), I can access them without a problem from outside the LAN. But as soon as I switch port 22 to point to box3, the connections time out again.

Summary:
SSH connections to box3 within the LAN work fine
SSH connections to box3 from outside the LAN with the router set to forward port 22 to box3 fail
SSH connections to box4 from outside the LAN with the router set to forward port 22 to box4 work fine

Any ideas?

All machines are running Fedora.

pingu · 07-21-2012, 03:05 AM

Sounds like a firewall issue.
Check with "iptables -L" or whatever software you might have for firewalling.

suicidaleggroll · 07-29-2012, 07:43 PM

So, inexplicably, another reboot of the machine has fixed the problem. Now responds fine to both internal and external SSH connections. Still not sure what the cause of the problem was, never made any changes and it's back to normal. Sounds like Windows behavior to me...odd

suicidaleggroll · 03-04-2015, 11:00 AM

Quote:

Originally Posted by suicidaleggroll

So, inexplicably, another reboot of the machine has fixed the problem. Now responds fine to both internal and external SSH connections. Still not sure what the cause of the problem was, never made any changes and it's back to normal. Sounds like Windows behavior to me...odd

I finally found the solution to this. For some reason the troublesome box lost its default route ("ip route" showed no default). While this didn't affect incoming SSH connections on the LAN it did prevent incoming SSH connections from outside the network.

A simple "ip route add default via 192.168.1.1" fixes it temporarily. Still not sure why it vanished in the first place, but it happens rarely enough that having to run this command manually to fix it isn't so bad.