Second case is very useful in situation when example.com can connect to [google.com] host while your box can't.
For example, you have VPN connection which is restricted to a number of boxes, while you want to access host not in list.
ssh -L 123:target.host.com:456 user@vpn.host.com.
So, basic usage is to jump INSIDE the network or jump OUTSIDE the network (ssh to some kind of proxy/gateway).
And finally, there may be firewall restrictions on target server which accepts connections only from given hosts.
What you are describing is known as SSH multiplexing.
I use that setup in a devops setting for caching my connections to any VMs.
In that way I reuse the same connection for up to 30 minutes/cache the connection, without renegotiating the entire SSH connection (and authenticating the user) in each new command.
It gives me an huge boost in speed, when sending (multiple) commands in a row to a VM/server.
The setup is done on the client side, and for a cache of 30 minutes, the setup can be done in /etc/ssh/ssh_config as:
ControlPath ~/.ssh/cm-%r@%h:%p
ControlMaster auto
ControlPersist 30m
The MaxSessions parameter, also in ssh_config also defines how many multiplexed connections simultaneous connections are allowed; the default value is 10. If you need more simultaneous cached connections, you might want to change it.
For instance, for a maximum of 20 cached connections:
MaxSessions 20
For more information, see OpenSSH/Cookbook/Multiplexing
An advantage of SSH multiplexing is that the overhead of creating new
TCP connections is eliminated.
...
The second and later connections will reuse the established TCP connection >over and over and not need to create a new TCP connection for each new SSH connection.
Also see Using SSH Multiplexing
SSH multiplexing is the ability to carry multiple SSH sessions over a
single TCP connection
Without multiplexing, every time that command is executed your SSH
client must establish a new TCP connection and a new SSH session with
the remote host. With multiplexing, you can configure SSH to establish
a single TCP connection that is kept alive for a specific period of
time, and SSH sessions are established over that connection. This can
result in speed increases
that can add up when repeatedly running commands against remote SSH hosts.
Lastly, as the multiplexing keeps the TCP connection open between the client and the server, you will have the guarantee that you are talking with the same machine in the load balancer, as long as the cache is open/active.
Best Answer
The feature is called
ControlMasterwhich does multiplexing over one existing channel. It causes ssh to do all of the key exchanges and logging in only once; thus, the later commands will go through much faster. You activate it using these three lines in your.ssh/config:You can adjust it to your needs; one alternative is that you could open one master connection that stays open during your other commands; then you would not need
ControlPersist.There are many possibilities with this feature to tweak, but make sure you store your
ControlPathsocket in a safe place, not readable by other users, otherwise it could be misused.More info can be found in the
ssh_config(5)manual page.