Keepalive doubt.

Hello everybody:

I've found three server configuration parameters,
tcp_keepalives_{idle,interval,count} which control keepalive
behaviour. I know them, I know which my system defaults are, and I
know their default values, and I suppose in my systems ( linux ) they
correspond to setsockopt's of TCP_KEEP{INTVL,IDLE,CNT}.

But I've been unable to find any parameter to enable or disable
keepalives ( the equivalent of setsockopt with SO_KEEPALIVE ). I do
not know if this is because it's always turned on or if there is a
parameter for it which I've been unable to find.

So, before resorting to my usual last ditch solution ( Use the Source,
Luke ), I'll ask it here. Can anybody tell me if keepalives are
enabled by the server by default in Linux, and, if not, how do I
enable them?

Regards.
    Francisco Olarte.