netdev - Re: "Listenless" TCP over NAT?

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Message-ID: <5f1c7b80099fb4aa3ad56e830aa525d1@chewa.net>
Date:	Fri, 05 Sep 2008 14:51:17 +0200
From:	Rémi Denis-Courmont <rdenis@...phalempin.com>
To:	xerces8 <xerces8@...n.net>
Cc:	Linux Netdev <netdev@...r.kernel.org>,
	Linux Networking <linux-net@...r.kernel.org>
Subject: Re: "Listenless" TCP over NAT?

On Fri, 05 Sep 2008 13:45:13 +0200, "xerces8" <xerces8@...n.net> wrote:
> Recently someone mentioned on this (or maybe other...) list a seldom used
> technique of establishing a TCP connection without one host listening on
a
> port, but but acting as "clients" and connecting to the peers port.
>
> My question: Can this be used to establish a TCP connection between two
> hosts that are both behind a (P)NAT router ?

Linux TCP/IP does allow this at the end nodes.

However, whether it works on the NAT model. Many NAPT and firewalling boxes
will reset the TCP session if you try to do TCP simultaneous open. Besides,
you need to be able to predict your source port number, which again is not
always possible depending on the NAPT implementation.

As far as I can tell, this is simply way too failure prone in real life.
Using some form of UDP encapsulation, such as ICE, Teredo, IPsec-in-UDP,
etc works A LOT better, although not always either.

--
Rémi Denis-Courmont

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