If you're having problems with your Linux box, and you know someone who can fix it, but you're behind a NAT device, and they can't access your box, you can forward your SSH port to them.
They will need to create you an account on their box
This assumes that you're running SSH on port 22.
ssh email@example.com -R3333:127.0.0.1:22This will forward your port 22 to them, so that they can access it at 127.0.0.1:3333 on their.box.com.
Then you just need to give them access - assuming you trust them.
If they're fixing a problem of yours, then I'm assuming they need root access. Either temporarily change the root password, or add their SSH key to /root/.ssh/authorized_keys2.
Of course, if you're both behind NAT, you're in trouble. Get IPv6 working, and let's get rid of NAT forever.
This works for other TCP services - not just SSH. You can forward http, ftp, telnet, whatever you like. Just change the port 22 for the correct port number.