SSH public key issue?

Question

Hi all,
I've been trying to set up public key authentication for SSH recently, and have come across a problem which has left me stumped. I want to be able to SSH into computer A (iBook G4, 10.5.1) from computer B (iMac G4, 10.5.0), and vice versa. At the moment, both these machines are on the same LAN, and SSH-ing to their respective local addresses works fine - A can connect to B, and B can connect to A (e.g. ssh -l username computerA.local). So far so good.
The end goal is to allow SSH access between my two machines over the web, using No-ip.com's dynamic DNS app. Both machines have this daemon installed and running. When SSh-ing to the machines using their no-ip DNS names (e.g. ssh -l username computerA.no-ip.org) for some reason connecting from B to A works fine, but from A to B throws up a "Permission denied (publickey)" error.
As far as I'm aware, I've set up all the ssh_config and sshd_config files on both machines correctly, specifying the correct protocols and key files, and neither machine is firewalled. Both are running OpenSSH 4.5p1.
Can anyone think of a reason why this is happening?
Thanks in advance,
Pete

Answer 1

Are both A and B behind the same home router? If so, it is likely that both the computerA.no-ip.org IP address is the same as the computerB.no-ip.org IP address, and you have only configured your home router to forward ssh connections to computerA.no-ip.org.
You home router would typically only have 1 internet WAN IP address. The no-ip client is going to figure out the router's WAN IP address and give that address to computerA.no-ip.org and computerB.no-ip.org DNS names.
If my guess is correct, then when telling A to connect to B, the name lookup for B gives an IP address which is your router, and your router then forwards port 22 traffic to back to A, and since you most likely have not put A's ssh .pub key into A's .ssh/authorized_key2 file, it fails to connect.
One way to verify my guess is to ask http://whatismyip.com from both A and B. If you get the same IP address, then computerA.no-ip.org and computerB.no-ip.org DNS names will have the same IP address and thus from the DNS name level there is no way to tell the difference between computerA.no-ip.org and computerB.no-ip.org.
Again, if I am correct, then what you want to do is configure your router to
forward port 22100 to A port 22
forward port 22101 to B port 22
Then when you want to make an ssh connection use
ssh -p 22100 [email protected]
ssh -p 22101 [email protected]