How to debug SSH port forwarding

Question

This is an extension to ssh port forward to access my home machine from anywhere

I tried things mentioned there but I am not able to ssh to my machine.

Netgear router settings for port forwarding:

Start Port: 22
End Port: 22
Server IP Address: IP address of by FreeBSD box

/etc/ssh/sshd_conf of my FreeBSD box:

PasswordAuthentication yes
AllowUsers root
X11Forwarding yes
AllowTcpForwarding yes

How I am trying to connect:

I've signed up for dyndns.com and got a URL that maps to my external IP address.

From another machine in my home network, I do: ssh -l root my_dyndns_ip Which is just timing out.

On the other hand, I can ping my_dyndns_url successfully.

Debugging:

How can I fix this so that I can ssh from anywhere to my_dnydns_url ?

I tried to look into logs of my Netgear router but on failed ssh attempt, no log in generated on the router.

I also looked into /var/log/messages but could not finding anything.

Edit:0 Running ssh in verbose mode (as per @jasonwryan suggested):

I am noticing a weird thing: When I do ssh to the dyndns.com provided url, its trying to connect to other IP address than what I get from whatismyip.com. Isn't that wrong?

Edit:1 Problem with dyndns is now solved.

Now when I try to ssh, it times out and I get error: Connection timed out

Edit:2 Does iptables or NAT has anything to do with it?

Edit:3 I started ssh with -d option to capture debug messages.

When I ssh to the freebsd box from any other machine in wlan, It works fine and I can see logs.

But when I ssh to external ip (which should forward that to my freebsd box), I do not even see any logs - that means, request is not even reaching freebsd box. And it times out.

Answer 1

I am thinking you problem is not in the port forwarding, but another option in the NAT config in the router.

First, ensure if you use your LAN IP, you can successfully SSH from another machine on the network. This ensures SSH works at all.

Second, test from another machine outside the network using the public IP. This ensures that port forwarding works.

Third, test from that same machine outside the network and use the DynDNS URL. This ensures that DynDNS is working properly.

If all of those succeed, then nothing is wrong with your configuration (which I'll assume is correct) and you problem is only accessng the public IP (either directly or through DynDNS) from inside the network. This means that your router needs to have NAT reflection enabled (if possible) to route internal requests as if they were external requests for the public IP.

Answer 2

Is your sshd server listening on port 22, try adding:

Port 22

to your config.

If that doesn't work try forwarding a port above 1024 (Some ISP's don't allow non business subscriber traffic on ports below 1024). Also make sure if you do this you change the line in the conf I just told you about to reflect the same value.

Answer 3

try to change ssh port to 443 - if you will be able to connect to this port, so there is a proxy between client and your home PC

log all dropped and forwarded packets in your firewall it can allow you to find incorrect forwarding rules

try to scan you external IP with nmap (or another tool) (ports <1024) - firewall logging should show dropped connections and on client side you should see opened ports

make traceroute to you home PC from client - is there any unexpected hosts between, are these hosts forward ports correctly?

Answer 4

Try with tcptraceroute to debug if you can go to the remote port first. If some machine is blocking the port you can see in the output. Try to use netstat -atun the see the open ports/connections on both machines.