I discovered today an OpenSSH feature which doesn’t seem to be widely
known. The feature is called multiplexing and consists of reusing
an opened ssh connection to a server when you want to open another
one. This leads to faster connection establishment and less processes
running.
To reuse an opened connection, we need to use the ControlMaster
option, which requires ControlPath to be set. We will also set
ControlPersist for convenience.
- ControlMaster defines if we create, or use or nothing about
multiplexing
- ControlPath defines where to store the socket to reuse an opened
connection, this should be a path only available to your user.
- ControlPersist defines how much time to wait before closing a
ssh connection multiplexer after all connection using it are
closed. By default it’s “no” and once you drop all connections the
multiplexer stops.
I choosed to use the following parameters into my ~/.ssh/config file:
Host *
ControlMaster auto
ControlPath ~/.ssh/sessions/%h%p%r.sock
ControlPersist 60
This requires to have ~/.ssh/sessions/ folder restricted to my user
only. You can create it with the following command:
install -d -m 700 ~/.ssh/sessions
(you can also do mkdir ~/.ssh/sessions && chmod 700 ~/.ssh/sessions
but this requires two commands)
The ControlPath variable will creates sessions with the name
“\({hostname}\){port}${user}.sock”, so it will be unique per remote
server.
Finally, I choose to use ControlPersist to 60 seconds, so if I
logout from a remote server, I still have 60 seconds to reconnect to
it instantly.
Don’t forget that if for some reason the ssh channel handling the
multiplexing dies, all the ssh connections using it will die with it.
Benefits with ProxyJump
Another ssh feature that is very useful is ProxyJump, it’s really
useful to access ssh hosts which are not directly available from your
current place. Like servers with no public ssh server available. For
my job, I have a lot of servers not facing the internet, and I can
still connect to them using one of my public facing server which will
relay my ssh connection to the destination. Using the
ControlMaster feature, the ssh relay server doesn’t have to handle
lot of connections anymore, but only one.
In my ~/.ssh/config file:
Host *.private.lan
ProxyJump public-server.com
Those two lines allow me to connect to every servers with .private.lan
domains (which is known by my local DNS server) by typing
ssh some-machine.private.lan. This will establish a connection to
public-server.com and then connects to the next server.