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.

Article series
SSH tunnelling for fun and profit
  1. Local vs Remote
  2. Tunnel options
  3. AutoSSH
  4. SSH Config

TL;DR

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

What is AutoSSH

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.

Install AutoSSH

How to install AutoSSH on various systems via their package manager.

OS Install method
Debian / Ubuntu $ sudo apt-get install autossh
CentOS / Fedora / RHEL $ sudo yum install autossh
ArchLinux $ sudo pacman -S autossh
FreeBSD # pkg install autossh
or
# cd /usr/ports/security/autossh/ && make install clean
OSX $ brew install autossh

Alternatively you can also compile and install AutoSSH from source:

wget http://www.harding.motd.ca/autossh/autossh-1.4e.tgz
gunzip -c autossh-1.4e.tgz | tar xvf -
cd autossh-1.4e
./configure
make
sudo make install

Note: Make sure to grab the latest version which can be found here: http://www.harding.motd.ca/autossh/.

Basic usage

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

Ignore -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.

AutoSSH and -M (monitoring port)

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:

autossh -M 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
Option Description
ServerAliveInterval ServerAliveInterval: number of seconds that the client will wait before sending a null packet to the server (to keep the connection alive).
Default: 30
ServerAliveCountMax Sets the number of server alive messages which may be sent without ssh receiving any messages back from the server. If this threshold is reached while server alive messages are being sent, ssh will disconnect from the server, terminating the session.
Default: 3

AutoSSH and ~/.ssh/config

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 environment variables

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.

AutoSSH during boot with systemd

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

Tell systemd that we have added some stuff:

systemctl daemon-reload

Start the service

systemctl start autossh-mysql-tunnel.service

Enable during boot time

systemctl enable autossh-mysql-tunnel.service

_

This is basically all I found useful about AutoSSH. If you thing I have missed some important parts or you know any other cool stuff, let me know and I will update this post.

Discussions

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18 comments on “SSH tunnelling for fun and profit: Autossh”

  1. Pingback: SSH tunnelling for fun and profit: Tunnel options

  2. Pingback: SSH tunnelling for fun and profit: local vs remote

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  5. Steve Smith Reply

    Depending on you usage you may also want to look at Mosh. It a UDP SSH alternative (but uses SSH for initial connection and authentication). The use of UDP enables true persistent roaming connections. It can also improve latency on lossy connections. Setup is mostly just a case of installing it on client and server.

    https://mosh.mit.edu/

    • Tom Li Reply

      No, I don’t think Mosh is suitable for this use case, which is focused on using SSH as a SOCKS proxy to transport data. Mosh is a great tool for remote shell and system administration, but it doesn’t have any ability to work as a proxy…

  6. Peter Tripp Reply

    When doing Remote Port forwards you’ll also want to use: -o ExitOnForwardFailure=yes

    Without this, if autossh detects the dropped connection before the remote end notices (likely) the subsequent reconnect will be unable to bind to your specified remote port because it’s still bound to the stale SSH session. So the session will be up, but the reason for the session (the port forward) will be unavailable. With this extra option SSH immediately quits if that bind fails and autossh will just retry until it succeeds (after the remote end detects the stale session).

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  13. Sam Smith Reply

    Ahh very helpful post. I’ll be back for more tips in the future. Thanks for the useful share~

    Sam Smith

    Technology Evangelist and Aspiring Chef.

    Large file transfers made easy.

    http://www.innorix.com/en/DS

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