ipfw dynamic rule timeout

[Antoine Jacoutot]

Hi !

I'm having a problem with ipfw and dynamic rules timeout.
For exemple, when I ssh to a distant machine, if I don't type anything for 
like 30 seconds, the connexion is dropped.
I read this in ipfw man page:

"Dynamic rules expire after some time, which depends on the status of the
flow and the setting of some sysctl variables.  See Section SYSCTL
VARIABLES for more details.  For TCP sessions, dynamic rules can be
instructed to periodically send keepalive packets to refresh the state of
the rule when it is about to expire."

So I tried to following command and got this error:
# sysctl net.inet.ip.fw.dyn_keepalive=1
sysctl: unknown oid 'net.inet.ip.fw.dyn_keepalive'

Anyone has an idea how to increase the default timeout value.

Thanks in advance.

Antoine