PhilsWiki

• https://goteleport.com/blog/how-to-ssh-properly/
  • A great guide on setting up host and user certificates.
• RHEL Guide

• SSH certificates

• StackExchange

[Unit]
Description=SSH key agent

[Service]
Type=simple
Environment=SSH_AUTH_SOCK=%t/ssh-agent.socket
ExecStart=/usr/bin/ssh-agent -D -a $SSH_AUTH_SOCK

[Install]
WantedBy=default.target

cat ~/.config/environment.d/ssh_auth_socket.conf
SSH_AUTH_SOCK="${XDG_RUNTIME_DIR}/ssh-agent.socket"

systemctl --user enable --now ssh-agent

Optional

echo 'AddKeysToAgent  yes' >> ~/.ssh/config

https://www.everythingcli.org/ssh-tunnelling-for-fun-and-profit-autossh/

Now that you are able to create various forward or reverse SSH tunnels with lots of options and even simplify your live with `~/.ssh/config` you probably also want to know how make a tunnel persistent. By persistent I mean, that it is made sure the tunnel will always run. For example, once your ssh connection times out (By server-side timeout), your tunnel should be re-established automatically.

I know there are plenty of scripts out there which try to do that somehow. Some scripts use a while loop, others encourage you to run a remote command (such as tail) to make sure you don’t run into timeout and various others. But actually, you don’t want to re-invent the wheel and stick to bullet-proof already existing solutions. So the game-changer here is AutoSSH.

autossh -M 0 -o "ServerAliveInterval 30" -o "ServerAliveCountMax 3" -L 5000:localhost:3306 cytopia@everythingcli.org

or fully configured (via `~/.ssh/config`) for background usage

autossh -M 0 -f -T -N cli-mysql-tunnel

http://www.harding.motd.ca/autossh/README

Autossh is a program to start a copy of ssh and monitor it, restarting it as necessary should it die or stop passing traffic.

usage: autossh [-V] [-M monitor_port[:echo_port]] [-f] [SSH_OPTIONS]

gnore `-M` for now. `-V` simply displays the version and exits. The important part to remember is that `-f` (run in background) is not passed to the ssh command, but handled by `autossh` itself. Apart from that you can then use it just like you would use ssh to create any forward or reverse tunnels.

Let’s take the basic example from part one of this article series (forwarding a remote MySQL port to my local machine on port 5000):

ssh -L 5000:localhost:3306 cytopia@everythingcli.org

This can simply be turned into an autossh command:

autossh -L 5000:localhost:3306 cytopia@everythingcli.org

This is basically it. Not much magic here.

Note 1: Before you use `autossh`, make sure the connection works as expected by trying it with ssh first. Note 2: Make sure you use public/private key authentification instead of password-based authentification when you use `-f`. This is required for `ssh` as well as for `autossh`, simply because in a background run a passphrase cannot be entered interactively.

With `-M` AutoSSH will continuously send data back and forth through the pair of monitoring ports in order to keep track of an established connection. If no data is going through anymore, it will restart the connection. The specified monitoring and the port directly above (+1) must be free. The first one is used to send data and the one above to receive data on.

Unfortunately, this is not too handy, as it must be made sure both ports (the specified one and the one directly above) a free (not used). So in order to overcome this problem, there is a better solution:

`ServerAliveInterval` and `ServerAliveCountMax` – they cause the SSH client to send traffic through the encrypted link to the server. This will keep the connection alive when there is no other activity and also when it does not receive any alive data, it will tell AutoSSH that the connection is broken and AutoSSH will then restart the connection.

The AutoSSH man page also recommends the second solution:

-M [:echo_port],
…
In many ways this [ServerAliveInterval and ServerAliveCountMax options] may be a better solution than the monitoring port.

You can disable the built-in AutoSSH monitoring port by giving it a value of 0:

Additionally you will also have to specify values for ServerAliveInterval and ServerAliveCountMax

autossh -M 0 -o "ServerAliveInterval 30" -o "ServerAliveCountMax 3"

So now the complete tunnel command will look like this:

autossh -M 0 -o "ServerAliveInterval 30" -o "ServerAliveCountMax 3" -L 5000:localhost:3306 cytopia@everythingcli.org

In the previous article we were able to simplify the tunnel command via `~/.ssh/config`. Luckily autossh is also aware of this file, so we can still keep our configuration there.

This was our very customized configuration for ssh tunnels which had custom ports and custom rsa keys:

$ vim ~/.ssh/config
 Host cli-mysql-tunnel
    HostName      everythingcli.org
    User          cytopia
    Port          1022
    IdentityFile  ~/.ssh/id_rsa-cytopia@everythingcli
    LocalForward  5000 localhost:3306

We can also add the `ServerAliveInterval` and `ServerAliveCountMax` options to that file in order to make things even easier.

$ vim ~/.ssh/config
 Host cli-mysql-tunnel
    HostName      everythingcli.org
    User          cytopia
    Port          1022
    IdentityFile  ~/.ssh/id_rsa-cytopia@everythingcli
    LocalForward  5000 localhost:3306
    ServerAliveInterval 30
    ServerAliveCountMax 3

If you recall all the ssh options we had used already, we can now simply start the autossh tunnel like so:

autossh -M 0 -f -T -N cli-mysql-tunnel

AutoSSH can also be controlled via a couple of environmental variables. Those are useful if you want to run AutoSSH unattended via cron, using shell scripts or during boot time with the help of systemd services. The most used variable is probably AUTOSSH_GATETIME:

AUTOSSH_GATETIME
How long ssh must be up before we consider it a successful connection. Default is 30 seconds. If set to 0, then this behaviour is disabled, and as well, autossh will retry even on failure of first attempt to run ssh.

Setting `AUTOSSH_GATETIME` to 0 is most useful when running AutoSSH at boot time.

All other environmental variables including the once responsible for logging options can be found in the AutoSSH Readme.

If you want a permanent SSH tunnel already created during boot time, you will (nowadays) have to create a systemd service and enable it. There is however an important thing to note about systemd and AutoSSH: `-f` (background usage) already implies `AUTOSSH_GATETIME=0`, however `-f` is not supported by systemd.

http://www.freedesktop.org/software/systemd/man/systemd.service.html
[…] running programs in the background using “&”, and other elements of shell syntax are not supported.

So in the case of systemd we need to make use of `AUTOSSH_GATETIME`. Let’s look at a very basic service:

$ vim /etc/systemd/system/autossh-mysql-tunnel.service
[Unit]
Description=AutoSSH tunnel service everythingcli MySQL on local port 5000
After=network.target

[Service]
Environment="AUTOSSH_GATETIME=0"
ExecStart=/usr/bin/autossh -M 0 -o "ServerAliveInterval 30" -o "ServerAliveCountMax 3" -NL 5000:localhost:3306 cytopia@everythingcli.org -p 1022

[Install]
WantedBy=multi-user.target

systemctl daemon-reload
systemctl start autossh-mysql-tunnel.service
systemctl enable autossh-mysql-tunnel.service