What is SSH jump server

It's common that you have a protected Linux/BSD server that isn’t publicly accessible. Typically, you may have what is commonly referred to as a jump server or bastion server which is accessible from a public network (sometimes this jump server would be in a DMZ, and also Linux/BSD), you connect to this jump server first, then connect to the protected server from jump server.

Sample setup:

+--------+       +-------------+      +------------------+
| Laptop | <---> | Jump server | <--> | Protected server |
+--------+       +-------------+      +------------------+

You can connect to the protected server through jump server via ssh with command like below:

ssh -v -J user1@jump-server user2@protected-server