Meaning of SSH command with 2 IPs

There's an SSH feature called ProxyJump:

ssh -J [email protected]:port1,[email protected]:port2 [email protected]

which sounds like what you're referring to.

More information on this in eg Section 6 on ProxyJump here or this page on ProxyJump from RedHat. In particular note that there are alternative formats, eg when the user:passwd are the same on each host (don't do that!). You can also put the info into a config file to avoid typing it out each time (though history is useful there).

Finally there are other ways to achieve the same thing, in SSH: -o proxycommand=, and -q -W (the later explained here on ExplainShell).

As many of the comments suggest, the syntax ssh user1@user2@<one-ip-address>@<second-ip-address does not lead to a useful ssh behavior and it has nothing to do with the jump host function.

Try the following:

• On any system, connect with your current user to localhost:

  $ ssh localuser@localhost
  

  This will, as expected, open a ssh session on localhost

• Now try the same with the proposed syntax:

  $ ssh localuser@localhost@localuser@localhost
  localuser@localhost@localuser@localhost's password: 
  Permission denied, please try again.
  ...
  

  You will be asked for a password, but the system will not let you in, since the user localuser@localhost@localuser does not exist on the system.

  Also check the output of /var/log/auth.log:

  Aug 17 08:45:46 somehost sshd[73042]: Invalid user localuser@localhost@localuser from 127.0.0.1 port 44292
  Aug 17 08:45:48 somehost sshd[73042]: pam_unix(sshd:auth): check pass; user unknown
  Aug 17 08:45:48 somehost sshd[73042]: pam_unix(sshd:auth): authentication failure; logname= uid=0 euid=0 tty=ssh ruser= rhost=127.0.0.1 
  Aug 17 08:45:50 somehost sshd[73042]: Failed password for invalid user localuser@localhost@localuser from 127.0.0.1 port 44292 ssh2
  

Unless you create a user localuser@localhost@localuser on the system you want to connect to, this syntax won't work.