https://bugs.koha-community.org/bugzilla3/show_bug.cgi?id=37087

--- Comment #23 from David Cook <dc...@prosentient.com.au> ---
(In reply to Fridolin Somers from comment #21)
> Mmmm, I fell like this can impact the SIP server at deep level.
> Since selfcheckout machines are a very used feature and 23.11 is now a very
> stable branch, I prefer not backport to avoid any unseen impact.

If anything isn't clear about the change, I'm happy to explain it. 

Overall, it should be a very safe change to make, since it doesn't do anything
without adding configuration manually.

And even when it's turned on (using custom_tcp_keepalive=1), it will use the
default kernel parameters. 

$ sysctl -a | grep "keepalive"
net.ipv4.tcp_keepalive_intvl = 75
net.ipv4.tcp_keepalive_probes = 9
net.ipv4.tcp_keepalive_time = 7200

With these defaults, it won't send a TCP ACK until 2 hours of idle time have
passed. If that TCP ACK isn't ACKed back, it will then send up to 9 more ACKs
in intervals of 75 seconds. 

Keep in mind that these are Transport layer packets. They don't contain any
actual data in them, so the selfcheckout application and the Koha SIP server
application have zero knowledge about them. It's just OS kernel to OS kernel
communication.

The main use case being to prevent firewalls like Azure's firewall and other
corporate firewalls (like Fortinet) from dropping idle persistent SIP TCP
connections.

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