Re: [PATCH] TCP: add option for silent port knocking with integrity protection

[Stephen Hemminger <stephen@networkplumber.org>]

On Wed, 11 Dec 2013 21:19:00 +0100
Christian Grothoff <grothoff@in.tum.de> wrote:

> On 12/11/2013 09:01 PM, David Miller wrote:
> > From: Christian Grothoff <grothoff@in.tum.de>
> > Date: Tue, 10 Dec 2013 19:35:36 +0100
> > 
> >> Only NAT implementations that change the SQN are not supported
> >> (those should be rare, but we have no hard data on this).
> > 
> > Even Linux's netfilter can and does do this, it is absolutely necessary
> > for tracking SIP and FTP protocols, and it's also used in our virtual
> > server load balancing modules.
> > 
> 
> We're aware that Linux _can_ do this.  I was not aware it was doing this
> for
> SIP and FTP specifically; regardless, what implementations can do is less
> important than what they are configured to do most of the time, and that's
> what we'd need hard data on.  Anyway, I'd be very interested to learn how
> you use this for SIP/FTP to evaluate the impact.  Do you have documentation
> on this?
> 
> As for server load balancing, I suspect that those are not the kinds of
> services that one would typically use port knocking for.  Still, again a
> good hint as to where trouble might lurk (and we will definitively include
> those points in the next revision of the documentation).

The point is that doing it outside of TCP core is safer, less error prone
and more flexible.