How many times have your SSH session suddenly interrupted while using SSH, and then you have to reconnect it. Of course, the simplest solution is to use the arrow keys to retrieve the previous command, and then press the Enter key.

This trick you have enjoyed for many years, is there a more elegant solution? Today, I will introduce you a better solution to solve this problem, that is a CLI tool named autossh.

What’s AutoSSH?

autossh is a program to start a copy of ssh and monitor it, restarting it as necessary should it die or stop passing traffic. The idea is from rstunnel (Reliable SSH Tunnel), but implemented in C.

Connection monitoring is done by using ssh to construct a loop of ssh forwardings (one from local to remote, one from remote to local), and then sends test data that it expects to get back. It backs off on the rate of connection attempts when experiencing rapid failures such as connection refused.

Install AutoSSH

If you don’t have this program, please install it at first.

# For ArchLinux
$ sudo pacman -S autossh

# For Debian/Ubuntu
$ sudo apt-get install autossh

# For CentOS/Fedora/RHEL
$ sudo yum install autossh

# For FreeBSD
$ pkg install autossh

# For OSX
$ brew install autossh

Basic usage

$ autossh -M <port>[:echo_port] [-f] [SSH OPTIONS]
  • -M: To specify the base monitoring port to use, or alternatively, to specify the monitoring port and echo service port to use.
    • When no echo service port is specified, this port and the port immediately above it (port# + 1) should be something nothing else is using.
    • -M 0 will turn the monitoring off, and autossh will only restart ssh on ssh exit.
  • -f: Causes autossh to drop to the background before running ssh.

Accordingly, we can use below command to automatically restart SSH tunnels:

$ autossh -M 20000 -L 5000:localhost:5000 -C -N -T user@example.com
  • -C: It forces compression of the connection, it’s useful on modem lines and slow internet connections but if a fast connection is in place it will actually slow things down. Nowadays it should most likely be not used, unless your connection speed is terribly slow.
  • -N: Do not execute a remote command. This is useful for just forwarding ports (protocol version 2 only).
  • -T: Disable pseudo-tty allocation.

For monitoring port, 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.

Alternative command:

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

Booting as a systemd service

For systemd service, there is however an important thing to note from the document.

Running programs in the background using “&”, and other elements of shell syntax are not supported.

The AutoSSH flag -f (background usage) already implies AUTOSSH_GATETIME=0, however the flag -f is not supported by systemd.

Here is an example:

Create a systemd service configuration:

$ sudo vim /etc/systemd/system/autossh-tunnel.service

Content as below:

[Unit]
Description=AutoSSH tunnel service
After=network.target

[Service]
Environment="AUTOSSH_GATETIME=0"
ExecStart=/usr/bin/autossh -M 0 -o "ServerAliveInterval 30" -o "ServerAliveCountMax 3" -N -L 5000:localhost:5000 -p 2222 user@example.com

[Install]
WantedBy=multi-user.target

Reload systemd for the new adding stuff:

$ sudo systemctl daemon-reload

Start the service

$ sudo systemctl start autossh-tunnel.service

Enable the service on booting:

$ sudo systemctl enable autossh-mysql-tunnel.service

References