I am a complete Linux newbie.

I work on HP NonStop (aka Tandem) servers which have Linux servers attached that provide the TCP/IP stack/connectivity.

The question I have (and I've spent hours trawling the net) is:
If I issue a "sysctl -update net.ipv4.tcp_keepalive_probes 3" I assume that it will apply to new sessions established after the command is issued but, will it apply to currently active sockets/sessions?

Regards
Don

It shouldn't affect existing session because they already connected.

I work in a 24x7 environment (99.999%+ availability - ATM etc) and need to know precisely.
I know nearly all of the answers regarding my core machine but not the Linux boxes that have been bolted on to my machine (in this incarnation) to provide IP comms.

Does shouldn't = will not?

Regards
Don

You can know precisely by testing. Do following things and watch new and old session, how long these sessions are declared dead session.

- net.ipv4.tcp_keepalive_time - time of connection inactivity after which
the first keep alive request is sent
- net.ipv4.tcp_keepalive_probes - number of keep alive requests retransmitted
before the connection is considered broken
- net.ipv4.tcp_keepalive_intvl - time interval between keep alive probes

You can manipulate with these settings using the following command:

sysctl -w net.ipv4.tcp_keepalive_time=60 net.ipv4.tcp_keepalive_probes=3 net.ipv4.tcp_keepalive_intvl=10

This sample command changes TCP keepalive timeout to 60 seconds with 3 probes,
10 seconds gap between each. With this, your application will detect dead TCP
connections after 90 seconds (60 + 10 + 10 + 10).

I fully understand that I can establish the answer to my own question by testing but human and computing resources are at a premium. Naively, I expected that someone in the Linux world could simply and definitively answer my question.
Regards Don