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Re: key change for TCP-MD5

  • From: Iljitsch van Beijnum
  • Date: Wed Jun 21 13:09:21 2006 
On 21-jun-2006, at 16:24, Ross Callon wrote:


If DOS is such a large concern, IPSEC to an extent can be used
to mitigate against it.



Well, this one comment is the one that I don't understand. I
don't see how IPSEC mitigates against DOS attacks. In fact, it
seems to make things worse, since it hides the TCP header.




Only if you use encryption. You can also use authentication only, in
two ways: with ESP, which only protects and/or encrypts the payload,
or with AH, which protects the entire packet except for fields that
may/must be changed in transit. But due to layering, it's unlikely
you'll be looking at TCP stuff before performing the authentication
check.



The reason IPsec helps against a DoS against the CPU is that it has
an anti replay counter. IPsec implementations are supposed to
maintain a window, not unlike a TCP window, that allows them to
reject packets with an anti replay counter that's too far behind or
ahead of the last seen packets. So in order to make a packet reach
the CPU an attacker has to observe or guess an acceptable value for
the anti replay counter.



An even better way to protect against spoofed BGP packets (ones that
come from "far away" at least) is GTSM (RFC 3682), as Randy remarked
in his usual concise manner. GTSM means setting the TTL to 255 when
sending and checking if it's still 255 when receiving BGP packets.
This way you know that there weren't any routers between you and the
real source of the packet.



Unfortunately, GTSM isn't used much in practice because the BGP RFC
requires routers to set and check for TTL = 1, so a router employing
GTSM can't talk to one that doesn't. This means you have to enable it
manually at both ends and suffer downtime during the period where one
end has it enabled and the other doesn't. If only someone would have
taken the extra time to define a BGP option that would allow routers
to negotiate the use of GTSM automatically...