[Pkg-openssl-devel] Bug#694667: Bug#694667: Partial solution for OpenSSL 1.0.1 bugs #665452, #666051, #2771

[Zack Morris]

Hey sorry for my late reply, this email got caught in a spam black hole.
ICMP does work, I'm able to ping and traceroute and all that.  I guess I
don't understand how the relationship  between path MTU detection and
SSL works.  In my mind, SSL would work at stream level (regardless of
how the packets get swizzled in the wild).  It would handle security
purely cryptographically and not worry about weaker MITM attacks where
something about the packet (length/headers/what have you) doesn't match.

I guess another way of saying that is, we should assume that all packets
are being watched and chopped up in various ways, and SSL should still
function properly because the data is mathematically inaccessible even
by brute force.  Otherwise I worry that someone could spoof MTU issues
somehow and prevent someone from opening an SSL connection.

I could maybe see how weak MITM attack sensing might be useful for
self-signed certificates to cut down on attacks in practice (while
prototyping a webserver on linode etc).  But honestly I don't even think
it should even be turned on for real security applications because then
it's just putting another hurdle up that adds no strong security
benefit.  I lost about 8-16 hours of work tracking this down because I'm
not a network administrator.

So on that note, a quick fix would be to report an error in verbose mode
with the MTU is too low (I mean simple enough that web developers using
curl can see it) and then decide if you want to skip the checking down
the road.

Thanks,

Zack Morris

-----Original Message-----
From: Kurt Roeckx [mailto:kurt at roeckx.be] 
Sent: Wednesday, November 28, 2012 2:35 PM
To: Zack Morris; 694667 at bugs.debian.org
Subject: Re: [Pkg-openssl-devel] Bug#694667: Partial solution for
OpenSSL 1.0.1 bugs #665452, #666051, #2771

On Wed, Nov 28, 2012 at 02:15:05PM -0700, Zack Morris wrote:
> 
> sudo ifconfig eth0 mtu 1496
> 
> The issue seems to be caused by something with TLS hanging with 
> fragmented packets.  Our network's MTU is 1496 instead of 1500.  The 
> server would wait after the client sent the initial client hello 
> message.  I tried everything from upgrading to 1.0.1-4ubuntu5.5 to 
> passing CAfile and -cipher with no luck.  I am using Ubuntu 12.0.4 
> linux 3.2.0-24-generic.

This issue you're having seems to me totally unrelated to openssl, just
that for the host you're connecting to Path MTU Detection doesn't work
properly.  That's becauase some people filter too much icmp traffic.

If there is something in your network that limits it to 1496, I suggest
you set your mtu like that.  This will avoid many problems.

ping with "-s 30000" should also work if nothing gets filtered.

A tool to diagnose those PMTUD problems is tracepath.


Kurt