SSH Server not answering connection requests

A Very Disappointing Self-Answer

Having set this problem aside for a day and come back to it, I was both relieved and perturbed (more perturbed than relieved) to find that everything was, mysteriously, working properly.

So, What Was the Issue?

No settings were changed or adjusted -- not on the router, not on the SSH server, and not on the SSH client's machine. It's fairly safe to say it was the router not handling the incoming traffic properly, in spite of proper settings. Given that dinky home router software isn't really designed to deal with port forwarding, it took the poor guy a while to implement the necessary changes.

But It's Been Like 6 Hours!!

Yeah dude, I know. I spent all day trying to figure out what was wrong -- and didn't ever find it because there wasn't anything wrong. Evidently, it can take 6 hours -- possibly more -- for the router settings to take effect.

So How Do I Know If This Is My Issue?

A nifty tool I came across during this escapade is tcpdump. This lean little guy sniffs traffic for you, offering valuable insight into what's actually going on. Plus, he's got some super filtering features that allow you to narrow down exactly what you want to look at/for. For example, the command:

tcpdump -i wlan1 port 22 -n -Q inout

Tells tcpdump to look for traffic via the wlan1 interface (-i = 'interface'), only through port 22, ignore DNS name resolution (-n = 'no name resolution'), and we want to see both incoming and outgoing traffic (-Q accepts in, out, or inout; inout is the default).

By running this command on your SSH server while attempting to connect via a remote machine, it quickly becomes clear where precisely the problem lies. There are, essentially, 3 possibilities:

1. If you're seeing incoming traffic from the remote machine, but no outgoing traffic from your local server, the problem lies with the server: there's probably a firewall rule that needs to be changed, etc.
2. If you're seeing both incoming and outgoing, but your remote machine isn't receiving the response, it's most likely the router: it's allowing the incoming traffic, but dropping your outgoing packets.
3. If there's no traffic at all, that's probably a router issue as well: the remote machine's SYN packets are being ignored and dropped by the router before they even reach your server.

And once you've discovered where the problem lies, a fix is (usually) trivial.

I'm running Mint (Ubuntu).

I'd done everything...public key in correct format in authorized_keys, chmodding, chowning, restarting ssh and sshd etc. As has all been documented all over the place.

For me, it was the ufw firewall. The lack of any ssh response at all tipped me to this, yet I could ping no problem both ways on a LAN.

I tested by:

sudo ufw service stop

...and it worked perfectly, i.e. I got a response from an ssh call.

So, start ufw up again:

sudo ufw service start

...and add the rule:

sudo ufw allow openssh

All working fine now.

If you, like me, have enabled UFW (on Linux, Ubuntu in my case), try this:

sudo ufw enable OpenSSH

This will allow OpenSSH in the firewall.