Large ipnat tables

[Jeff Wyman <wysoft_(_at_)_wysoft_(_dot_)_tzo_(_dot_)_com>]

Is there any way, aside from cron, to prevent ipnat tables from growing
too large? I have rdr set to proxy to a Half Life game server, and the
ipnat list becomes rather large (in excess of 300 lines). Does ipnat
naturally clear this out after a certain amount of time? I'm sure it does,
but it doesn't seem to be doing it too quickly. 

Maybe I could increase a mapping timeout? I can't seem to find the value
for it anywhere though. Thanks for any help.

-Jeff