How to Keep Alive SSH Sessions

Translations

German

Abstract

Many NAT firewalls time out idle sessions after a certain period of time to keep their trunks clean.

There are two options:

We can tell the SSH client to send keepalive packets to the server
We can make the SSH server keep alive all connections with its clients

The settings below will make the SSH client or server send a null packet to the other side every 300 seconds (5 minutes), and give up if it doesn’t receive any response after 2 tries, at which point the connection is likely to have been discarded anyway.

Clientsetting

PuTTY

In your session properties, go to Connection and under Sending of null packets to keep session active, set Seconds between keepalives (0 to turn off) to e.g. 300 (5 minutes).

keepalive settings in PuTTY

OpenSSH

To enable the keep alive system-wide, edit /etc/ssh/ssh_config

To set the settings for a user only, edit ~/.ssh/config

Insert the following in the appropriate file:

Host *
    ServerAliveInterval 300
    ServerAliveCountMax 2

Serverseting

OpenSSH

Add to the file /etc/ssh/sshd_config:

ClientAliveInterval 300
ClientAliveCountMax 2