Title: Faster SSH with multiplexing

	Title: Faster SSH with multiplexing
	Author: Solène
	Date: 22 May 2018
	Tags: unix ssh
	Description:

	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.


	Host *
	ControlMaster auto
	ControlPath ~/.ssh/sessions/%h%p%r.sock
	ControlPersist 60

	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.