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Date:	Wed, 8 Oct 2008 13:54:02 +0200
From:	Stephen Hemminger <>
To:	Willy Tarreau <>
Cc:	David Miller <>,
Subject: Re: [PATCH] add a sysctl to disable TCP simultaneous connection

On Wed, 8 Oct 2008 10:11:09 +0200
Willy Tarreau <> wrote:

> Hi David,
> I hope you had a pleasant journey in Paris last week. It was nice to meet you.
> In 2005, I submitted a patch for 2.6.11 which we finally did not decide
> on, it was about the ability to disable TCP simultaneous connection open.
> For the last few years, we've been bothered several times by newbies
> reading "TCP/IP for dummies" then trying to get a name by discovering
> the ultimate vulnerability which will get a lot of press coverage. Of
> course those attacks are often pointless or just proofs of concepts
> with no application in real life, but it's nonetheless annoying to have
> to deal with the issues, especially to explain to customers why they
> shouldn't have to worry.
> I would not be surprized that a next one will exploit TCP's ability to
> perform simultaneous connections between two clients. It's very easy
> to trigger, there's no SEQ to guess, just a port, and the effect is
> simply a poor DoS on the service trying to connect outside. In other
> times we would have found it very minor, but judging by the consideration
> given to harder and less effective "attacks" these days, this trivial
> one may finally get picked and annoy us again.
> As a reminder (especially for those who are not aware of this feature),
> it is possible with TCP to connect two clients together if both send
> crossed SYNs, then SYN-ACKs, then ACKs. This implies that each side
> accepts the sequence number of the other one without any ability to
> check that it matches its SYN. So it's trivial for an attacker to
> prevent one client from establishing a connection from a known port
> to a known address/port by sending it a SYN to that port. The client
> will then send a SYN-ACK and will not accept the expected server's
> SYN-ACK because the SYN SEQ will be different. The server might also
> send an RST on the client's SYN-ACK if it's not firewalled. The
> connection will eventually timeout in a SYN-RECV state or simply be
> aborted.
> The theorical DoS effect on some predictable address/port destinations
> is easy to understand. Services with very few destination IP/ports such
> as software/signature updates, SMTP relaying, DNS clients for zone
> transfers, or SSH remote accesses are easy targets. In practice, the
> SYN would have to be sent after the client's SYN and before the server's
> SYN-ACK, which leaves a small time window limiting the attack to far
> remote, unfirewalled communications.
> This is very easy to test, I'm used to do it between two netcats, and
> preventing the initial RST by unplugging the cable before sending the
> connects. I remember it also worked on Solaris 8, and I don't remember
> about BSDs (though I would not be surprized they support it too).
> IMHO this feature is totally useless nowadays, because :
>   - if one of the machines is firewalled, the firewall will block it
>     (none of the firewalls I've tested among Netfilter, Checkpoint,
>      Cisco, Fortinet, Juniper supports simultaneous connect, and it
>      would cause a big security issue).
>   - if neither machines are firewalled, then the SYN to a closed port
>     will immediately trigger an RST, making it very difficult to
>     establish a working connection.
> For this reason, I'd like that we plan on merging the attached patch
> (or any variant) for 2.6.28 or 2.6.29 before a new random junkie comes
> aroung screaming loud he uncovered a big DoS hole in Linux TCP stack. The
> patch provides a sysctl allowing the user to enable or disable the feature.
> I've been running all my kernels with the code ifdef'ed out for the last
> 4-5 years, but I certainly can understand that some people want to be able
> to enable it for any reason (even for educationnal purposes), hence the
> sysctl.
> I have rediffed the patch for 2.6.27-rc9 and successfully tested it (both
> with feature enabled and disabled). It disables the feature by default,
> but I have no problem with leaving it on and expecting that distros will
> ship it off.
> By the way, during the tests I noticed something strange. While the
> socket is in SYN_RECV on the first side to receive the other one's
> SYN, it has a huge receive queue :
> Active Internet connections (servers and established)
> Proto Recv-Q     Send-Q Local Address        Foreign Address      State
> tcp   3964220580      1  SYN_RECV
> The value corresponds to the ACK value emitted, which is equal to the
> other end's SYN+1. I don't know if this is just an artefact of the way
> the queue size is reported (probably because the first ACK has not yet
> been considered since we're not in ESTABLISHED state) or if this can
> have any further impact (eg: unexpected memory freeing on termination,
> etc...).
> Best regards,
> Willy

Does this break NAT traversal via STUNT used by applications like Skype?
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