commandliners

There is a server I manage (called alex) which has not a fixed IP. As you know, I have a shell at the best Unix server out there (by the way, it is almost free. The problem is to keep an up-to-date record of the first server’s IP address. I do it as follows (and yes, I know timtowtdi).

What I did was:

1. Create an RSA public/private key pair at alex:

   alex $ ssh-keygen -N'' -f 'id_alex' -t rsa
   

   which creates the files id_alex and id_alex.pub.

2. Create a cron job for my account at alex which looks like

   5,10,15,20,25,30,35,40,45,50,55,0 * * * * /usr/bin/ssh -i \
   /home/pera/.ssh/id_rsa.sdf pfortuny@sdf-eu.org
   

3. Copy the file at sdf:

   alex $ scp id_alex.pub pfortuny@sdf-eu.org:.ssh/id_alex.pub
   

4. At sdf, concatenate the public key at the end of ~/.ssh/authorized_keys:

   sdf $ cd ~/.ssh
   sdf $ cat id_alex.pub >> authorized_keys
   

5. Edit the authorized_keys file so that the line corresponding to id_alex says:

   command="/bin/echo $SSH_CLIENT | cut -f1 -d' '>
   /arpa/p/pfortuny/alex.IP",no-port-forwarding,no-agent
   -forwarding,no-X11-forwarding,no-pty ssh-rsa AAAAB3....
   

   (newlines are spurious, the above is a single line). All the parameters are required to get as secure a connection as possible.

The above steps end up making alex able to automatically log in to sdf-eu.org with my username to just echo the $SSH_CLIENT variable into the file alex.IP, stored at my home directory. That variable has no more than the IP of the ‘active’ ssh client: in this specific case, alex’s IP.

Thus, I get an update of the IP every 5 minutes (which may be overkill, but I really need to be sure I can connect to alex at almost any time).

And here come the inverted commas. Whenever I want to log in to alex, I ssh sdf and connect from there:

 sdf $ ssh myself@`cat alex.IP`

the command inside the inverted commas gets replaced by its output, which is alex’s current IP.

Hope this helps.

Improvements are welcome.