Closing a stale SSH connection

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Suppose you’re connected to a remote host with SSH and after a while the SSH session goes stale. The terminal is unresponsive and no keypress seem to take effect. There might be something with the network, the remote host is restarting or maybe your machine has been in hibernation, there could be multiple reasons for a stale session.

 

The first solution that might come to mind is to just close the terminal emulator and create another one, but there is a better way.

SSH escape sequences

Before I show the trick we take a quick detour and explore a kind of hidden feature that is implemented in many of the available SSH clients.

 

Built in to the SSH client are multiple hidden commands that can be triggered with a so called escape sequence. These commands can be access by a combination of the tilde prefix (~) followed by the command.

 

For example ~? print the help message containing all of the supported escape sequences:

 

david@remote-host:~$ ~?
Supported escape sequences:
 ~.   - terminate session
 ~B   - send a BREAK to the remote system
 ~R   - request rekey
 ~#   - list forwarded connections
 ~?   - this message
 ~~   - send the escape character by typing it twice
(Note that escapes are only recognized immediately after newline.)

Pay extra attention to the last line;

(Note that escapes are only recognized immediately after newline.)

This means that for the escape sequence to take effect a preceding newline is required.

 

Also, a small note for people using a keyboard with a nordic layout; To type the tilde character, press AltGr+^ <Space>. I know that this tripped me up when I first learned about escape sequences.

The “terminate session” escape sequence

So, back to the initial problem.

 

As the help message stated above we can close the session with ~.. And remember, press enter a couple of times before initiating the sequence:

 

david@remote-host:~$ ~.
david@remote-host:~$ Shared connection to remote-host.davidisaksson.dev closed.
david@local:~$ echo $?
255

From here we can try to initiate the connection again, or just reuse the terminal.

 

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