Subject: Re: [PATCH] add a sysctl to disable TCP simultaneous connection opening

From: Willy Tarreau <[email-masked]>
Date: Thu, 9 Oct 2008 23:42:24 +0200

On Thu, Oct 09, 2008 at 07:21:03PM +0300, Rémi Denis-Courmont wrote:
> Le mercredi 8 octobre 2008 14:54:02 Stephen Hemminger, vous avez écrit :
> > Does this break NAT traversal via STUNT used by applications like Skype?
> 
> This will break the main ICE-TCP mechanism (IETF draft-ietf-mmusic-ice-tcp).
> I am not aware of any application using this _as_of_now_. Probably too many 
> NAT and firewall implementations will reject it already. And then, some TCP 
> stacks reportedly do not support it (e.g. Windows before Vista).

And opening this through firewalls would be too much dangerous as it would
allow servers to reconnect outside, pretty much defeating the initial purpose
of the firewall.

> On the other hand, if someone were to tunnel/encapsulate TCP over UDP, this 
> could actually be useful - think about peer-to-peer NATted-to-NATted file 
> transfers for instance.

This is already possible using netcat. You can force both ports. It has no
flow control but would be enough to chat or transfer small config files.

Willy