How do you prevent SSH sessions from hanging or freezing when the network is unstable?

1 Enable keepalive packets

One way to prevent SSH sessions from hanging or freezing is to enable keepalive packets. These are small messages that are sent periodically between the client and the server to check if the connection is still active. If the server does not respond to a keepalive packet, the client will assume that the connection is broken and close the session. To enable keepalive packets, you can use the

-o ServerAliveInterval=n        

option when you initiate the SSH connection, where n is the number of seconds between each packet. For example,

ssh -o ServerAliveInterval=60 user@host        

will send a keepalive packet every 60 seconds.

2 Use screen or tmux

Another way to prevent SSH sessions from hanging or freezing is to use screen or tmux. These are terminal multiplexers that allow you to create and manage multiple windows and sessions within a single SSH connection. If the network is unstable, you can detach from your screen or tmux session and reattach later without losing your work. To use screen or tmux, you need to install them on the remote server and start them before running any commands. For example,

ssh user@host        

followed by

screen        

or

tmux        

will create a new session. To detach from the session, press

Ctrl-a d        

for screen or

Ctrl-b d        

for tmux. To reattach to the session, use

screen -r        

or

tmux attach        

.

3 Use mosh

A third way to prevent SSH sessions from hanging or freezing is to use mosh. Mosh stands for mobile shell and is a replacement for SSH that uses UDP instead of TCP. UDP is a protocol that does not require a stable connection and can handle packet loss and latency better than TCP. Mosh also supports roaming, which means that you can switch between different network interfaces or locations without losing your session. To use mosh, you need to install it on both the client and the server and run it instead of SSH. For example,

mosh user@host        

will start a mosh session.

4 Use escape sequences

A fourth way to prevent SSH sessions from hanging or freezing is to use escape sequences. Escape sequences are special commands that you can enter after pressing

~        

followed by a newline. For example,

~.        

will terminate the SSH connection,

~^Z        

will suspend the SSH connection and return to the local shell, and

~#        

will show the list of available escape sequences. Escape sequences can be useful when you need to quickly exit or pause a SSH session that is not responding.

5 Use timeout settings

A fifth way to prevent SSH sessions from hanging or freezing is to use timeout settings. Timeout settings are parameters that control how long the client or the server will wait for a response before closing the connection. You can configure timeout settings on both the client and the server side, depending on your preference and access. To configure timeout settings on the client side, you can use the

-o ConnectTimeout=n        

and

-o ConnectionAttempts=n        

options when you initiate the SSH connection, where n is the number of seconds or attempts. For example,

ssh -o ConnectTimeout=10 -o ConnectionAttempts=3 user@host        

will try to connect for 10 seconds and retry 3 times before giving up. To configure timeout settings on the server side, you can edit the

/etc/ssh/sshd_config        

file and set the

ClientAliveInterval        

and

ClientAliveCountMax        

options, where the former is the number of seconds between each keepalive packet sent by the server and the latter is the number of keepalive packets that can be missed before closing the connection. For example,

ClientAliveInterval 60        

and

ClientAliveCountMax 3        

will close the connection if no response is received for 180 seconds.

7 Here’s what else to consider

This is a space to share examples, stories, or insights that don’t fit into any of the previous sections. What else would you like to add?