Re: conntrack, idle TCP connection and keep-alives

[Patrick McHardy <kaber@trash.net>]

On Sun, Oct 27, 2013 at 06:01:30PM +0000, Patrick McHardy wrote:
> On Sun, Oct 27, 2013 at 08:34:09AM -0700, Phil Oester wrote:
> > On Sun, Oct 27, 2013 at 12:14:48AM +0400, WGH wrote:
> > > It seems that, when masquerading, conntrack silently drops idle
> > > connection after nf_conntrack_tcp_timeout_established seconds. This's
> > > pretty terrible, as application inside the network, if it never sends
> > > anything, will never know that connection was dropped.
> > 
> > If this is a problem for you, then increase nf_conntrack_tcp_timeout_established
> > to an insanely high value.  You do realize, of course, that the conntrack
> > table has a finite number of entries.
> >  
> > > RFC 5382 gives us a solution to this:
> > > > A NAT can check if an endpoint for a session has crashed by sending a
> > > > TCP keep-alive packet and receiving a TCP RST packet in response.
> > > 
> > > However, it I couldn't find such feature in netfilter. It would be
> > > pretty nice to have.
> > 
> > Keepalives should be done in the application, not in the firewall.
> 
> Actually I think its a pretty nice idea to reduce breakage introduced
> by NATs. There a millions of embedded devices that use very small timeout
> values to reduce memory usage, at the cost of frequently breaking idle
> connections.

The downside seems to be that we'd need to keep track of timestamp values
to send valid keepalives, which also costs extra memory. I don't think that
cost is justifiable for NAT keepalives alone.