There are times when your machine is behind NAT and you couldn’t access easily. Unless you have VPN tunneling, it would be tricky. However, reverse ssh might be a handy tool.

| Host A | ----> NAT ---> | Host B |

For above case, you can clearly see that B can access A easily, but reversely, it’s not w/o port forwarding blah blah blah. As a result, you can just do reverse ssh by start ssh from Host B to Host A with command

$ ssh -NR 8888:localhost:22 user@host-a

Then at Host A, you can just ssh via port 8888.

$ ssh user@localhost -p 8888

Violà!