Title: Create a dedicated user for ssh tunneling only

	Title: Create a dedicated user for ssh tunneling only
	Author: Solène
	Date: 17 April 2019
	Tags: openbsd ssh
	Description:

	I use ssh tunneling A LOT, for everything. Yesterday, I removed the
	public access of my IMAP server, it's now only available through ssh
	tunneling to access the daemon listening on localhost. I have plenty
	of daemons listening only on localhost that I can only reach through a
	ssh tunnel. If you don't want to bother with ssh and redirect ports you
	need, you can also make a VPN (using ssh, openvpn, iked, tinc...)
	between your system and your server. I tend to avoid setting up VPN for
	the current use case as it requires more work and more maintenance than
	running ssh server and a ssh client.

	The last change, for my IMAP server, added an issue. I want my phone
	to access the IMAP server but I don't want to connect to my main
	account from my phone for security reasons. So, I need a dedicated
	user that will only be allowed to forward ports.

	This is done very easily on OpenBSD.

	The steps are:
	1. generate ssh keys for the new user
	2. add a user with no password
	3. allow public key for port forwarding

	**Obviously, you must allow users (or only this one) to make port
	forwarding in
	your sshd_config**.


	### Generating ssh keys

	Please generate the keys in a safe place, using
	[ssh-keygen](https://man.openbsd.org/ssh-keygen)

	$ ssh-keygen
	Generating public/private rsa key pair.
	Enter file in which to save the key (/home/user/.ssh/id_rsa):
	Enter passphrase (empty for no passphrase):
	Enter same passphrase again:
	Your identification has been saved in /home/user/.ssh/id_rsa.
	Your public key has been saved in /home/user/.ssh/id_rsa.pub.
	The key fingerprint is:
	SHA256:SOMETHINGSOMETHINSOMETHINSOMETHINSOMETHING user@myhost
	The key's randomart image is:
	+---[RSA 3072]----+
	\| \|
	\| ** \|
	\| * ** . \|
	\| * * \|
	\| **** * \|
	\| **** \|
	\| \|
	\| \|
	\| \|
	+----[SHA256]-----+

	This will create your public key in ~/.ssh/id_rsa.pub and the private
	key in
	~/.ssh/id_rsa


	### Adding a user

	On OpenBSD, we will create a user named tunnel, this is done with
	the
	following command as root:

	# useradd -m tunnel

	This user has no password and can't login on ssh.


	### Allow the public key to port forward only

	We will use the command restriction in the authorized_keys file
	to
	allow the previously generated key to only forward.

	Edit /home/tunnel/.ssh/authorized_keys as following

	command="echo 'Tunnel only!'" ssh-rsa PUT_YOUR_PUBLIC_KEY_HERE

	This will tell "Tunnel only" and abort the connection if the user
	connects and
	with a shell or a command.


	### Connect using ssh

	You can connect with [ssh(1)](https://man.openbsd.org/ssh.1) as usual
	but you
	will require the flag -N to not start a shell on the remote server.

	$ ssh -N -L 10000:localhost:993 tunnel@host

	If you want the tunnel to stay up in the most automated way possible,
	you can
	use autossh from ports, which will do a great job at keeping ssh
	up.

	$ autossh -M 0 -o "ExitOnForwardFailure yes" -o
	"ServerAliveInterval 30" -o "ServerAliveCountMax 3" -o "TCPKeepAlive
	yes" -N -v -L 9993:localhost:993 tunnel@host

	This command will start autossh, restart if forwarding doesn't work
	which is
	likely to happens when you lose connectivity, it takes some time for
	the remote
	server to disable the forwarding effectively. It will make a keep alive
	check
	so the tunnel stays up and ensure it's up (this is particularly useful
	on
	wireless connection like 4G/LTE).

	The others flags are also ssh parameters, to not start a shell, and for
	making
	a local forwarding. Don't forget that as a regular user, you can't bind
	on
	ports less than 1024, that's why I redirect the port 993 to the local
	port
	9993 in the example.


	### Making the tunnel on Android

	If you want to access your personal services from your Android phone,
	you can
	use ConnectBot ssh client. It's really easy:

	1. upload your private key to the phone
	2. add it in ConnectBot from the main menu
	3. create a new connection the user and your remote host
	4. choose to use public key authentication and choose the registered
	key
	5. uncheck "start a shell session" (this is equivalent to -N ssh flag)
	6. from the main menu, long touch the connection and edit the forwarded
	ports

	Enjoy!