Re: [PATCH] tcp: sysctl to disable TCP simultaneous connect

[Stephen Hemminger]

On Thu, 7 Feb 2013 09:52:40 -0800
Kees Cook <keescook@xxxxxxxxxxxx> wrote:

> This is based on Willy Tarreau's patch from 2008[1]. The goal is to
> close a corner-case of TCP that isn't used and poses a small DoS risk.
> For systems that do not want to take any risk at all, this is a desirable
> configuration knob.
> 
> It is possible for two clients to connect with crossed SYNs without
> checking sequence numbers. As such, it might be possible to guess a source
> port number to block a system from making connections to well-known
> ports and IP addresses (e.g. auto-update checks) without requiring a
> MiTM position.
>

This patch probably also breaks TCP STUNT that is used by some applications for NAT
traversal.
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