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Re: IKE negotiation for fragmentation controls in IPsec

Imagine this scenario:


As I understand from the description, once SG1 and SG2 negotiate
and come to understanding, then SG2 can drop fragmented secured
packets. But the routers in between can fragment the packets. 
To avoid this situation, SG1 has to set the DF bit and PMTU
processing should be MUST in this case.  Is it right to assume that
all core and edge routers/firewalls in Internet and enterprise support passing
the ICMP error messages? I keep hearing that most (may be some) firewalls
can be explicitly configured to discard ICMP error messages. 


Intoto Inc. 
Enabling Security Infrastructure
3160, De La Cruz Blvd #100
Santa Clara, CA 95054
----- Original Message ----- 
From: "Stephen Kent" <kent@bbn.com>
To: <ipsec@lists.tislabs.com>
Sent: Wednesday, June 25, 2003 3:46 PM
Subject: IKE negotiation for fragmentation controls in IPsec

> A few folks have observed that the current processing requirements 
> for AH and ESP mandate ciphertext (post IPsec encapsulation) 
> fragmentation and that this poses DoS vulnerabilities for receivers. 
> (An attacker can create what appear to be legitimate, non-initial 
> fragments and cause reassembly problems for the receiver).
> As we revise 2401, we may choose to allow (or even recommend) 
> plaintext (pre-IPsec encapsulation) fragmentation. If so, we need to 
> be able to negotiate use of this capability on a per-SA basis, and to 
> notify the receiver that NO ciphertext fragments should be accepted 
> for this SA, because none will be sent by this transmitter. So, I 
> suggest that we add a paylod to IKE to allow an initiator to indicate 
> the intent to never send ciphertext fragments. The responder can take 
> advantage of this info to better protect itself, or it can ignore the 
> info, but it needs to be told to be able to take advantage of the 
> capability. I would also like to see the responder be able to notify 
> the initiator of its intent re the companion (reverse) SA, if 
> appropriate.
> A logical (but admittedly separable) companion to this feature is to 
> allow the initiator to request the responder to accept fragments on 
> an SA where port fields are used as selectors. The issue here is that 
> a host may send fragments to an IPsec device that requires port field 
> examination for the SA to which the fragments will be mapped. It 
> seems reasonably safe to allow fragments (with a suitable, minimum 
> offset) to pass through such as SA, with only the initial fragment 
> having the port fields examined. This is a separate negotiation 
> because the fragments arise from hosts behind the IPsec device, but 
> it is related, because if one fragments at the sending IPsec device, 
> it would be nice to be able to use this feature to allow the receiver 
> to pass on fragments, not reassemble them (in the case of a SG).
> Steve