Hi Mike, you've got some useful stuff to try from others in the thread.
>
> When you're changing sshd_config you're restarting sshd afterwards?
>
> >From an Internet-based Linux host, does "nc some.dyndns.org
> <http://some.dyndns.org> 2222" produce an openssh banner?
>
> My next steps would be ssh -v on client side and a simultaneous
> multitail on /var/log/auth.log on the server.
>
> When running Internet-facing ssh it would be prudent to look into
> running some means of defence against the inevitable intrusion
> attempts. Look at DenyHosts and fail2ban - both are packages in
> Ubuntu/Debian.
>
> Security through obscurity may be scoffed at but imho it's a good way
> to counter unstructured attacks.
>
Thanks Imran and to others for the great advice. I will be going through
all the posts. However, what I would really like to understand is why
does simply changing the port number and nothing else render the server
inaccessible?
Kind regards
Mike
