LAST_ACK hanging around / reaping?

[Karl Pielorz]

Hi,

We've a number of FreeBSD 9.2 amd64 systems - recently we've noticed a 
large number of TCP sessions ending up stuck in 'LAST_ACK' (sometimes this 
can creep up to many thousands per box).

Having dug around - it appears some kind of load balancer at the 'other 
ends' of these connections isn't handling connection closes too well or 
something [I don't think it's a FreeBSD issue - it looks like a 'them' 
issue].

The questions are

  a) Should we be concerned by constantly having several thousand 
connections in LAST_ACK

  b) Is there a sysctl to change how long they'll hang around for?

And, more importantly,

  c) If the system needs these resources - will it reap some of them? (i.e. 
oldest first or something) - or could they potentially just keep growing in 
a worst case scenario until something runs out?

Thanks,

-Karl