AH or ESP

Merike Kaeo kaeo at merike.com
Mon May 25 21:51:11 CDT 2009


Coming from someone who is somewhat jaded.....politics.

Realistically there are some folks who believe that not having the IP  
header (and with v6 also the option headers) integrity protected is  
an issue.  It's not.  You have more risk of operation issues from  
adding complexity of AH.....note that the fields that are modified in  
transit in the headers are NOT included in the integrity protection.   
So it really becomes an issue of is the IP address protected and  
basically, yes that's done via IKE and the way security associations  
work anyhow. [if you change IP address of header you will not have  
appropriate security association match]

Once the technology is there to quickly differentiate ESP-Null from  
ESP-encrypted packets the argument of "but you can inspect AH  
packets" becomes irrelevant.

- merike

On May 25, 2009, at 5:23 PM, Glen Kent wrote:

> Just a quick question: Why do we need AH when we have ESP-NULL? Is AH
> now being supported only for legacy reasons? The only negative with
> ESP-NULL afaik was that it could not be filtered (since packets could
> not be inspected), however, this changes with the "wesp" proposal.
> Also, the fact that AH is NAT unfriendly should be the final nail in
> its coffin.
>
> Any reasons why we still see it around?
>
> Thanks,
> Glen
>
> On Tue, May 26, 2009 at 5:44 AM, Jack Kohn <kohn.jack at gmail.com>  
> wrote:
>> Not really.
>>
>> Currently, you cant even look at the ESP trailer to determine if  
>> its an
>> encrypted or an integrity protected packet, because the trailer  
>> itself could
>> be encrypted.
>>
>> A router, by reading the next-header field from the ESP trailer  
>> can never be
>> sure that its an OSPFv3 packet inside since it wouldnt know  
>> whether the
>> packet is encrypted or not. One could have an encrypted packet  
>> inside, for
>> which the next-header field turns out to be 89, but that may not  
>> necessarily
>> mean that its an OSPFv3 packet. It could be a VoIP packet that  
>> just happens
>> to look like OSPFv3 once encrypted. There is no indication sent on  
>> the wire
>> that says that the packet is encrypted.
>>
>> So, there is no way to identify/deep inspect/filter ESP packets  
>> unless
>> you're the recipient, which imo is the root cause of all heart  
>> burn in the
>> intermediate devices like firewalls, transit routers, etc.
>>
>> A couple of solutions were thrown at the WG and the current one  
>> (wesp) was
>> selected as the best.
>>
>> I agree that we should just throw out AH and stick to one protocol  
>> which has
>> been extensively tested. A quick scan through some of vendors data  
>> sheets
>> quickly reveals that most of them dont even provide support for AH.
>>
>> Jack
>>
>> On Tue, May 26, 2009 at 2:33 AM, Merike Kaeo <kaeo at merike.com> wrote:
>>
>>> Yeah - the main issue with using ESP is that there's a trailer at  
>>> end of
>>> packet that tells you more info to determine whether you can  
>>> inspect the
>>> packet.  So you have to look at the end of the packet to see  
>>> whether ESP is
>>> using encryption or null-encryption (i.e. just integrity  
>>> protection).  Some
>>> vendors do have proprietary mechanisms in software for now which  
>>> doesn't
>>> scale.  The work below will hopefully lock into a solution where  
>>> hw can be
>>> built to quickly determine if ESP is used for integrity only.
>>>
>>> AH is not really widely used (except for OSPFv3 since early  
>>> implementations
>>> locked in on AH when the standard said to use IPsec for integrity
>>> protection).  Note that a subsequent standard now exists which  
>>> explicitly
>>> states that ESP-Null MUST be supported and AH MAY be supported.   
>>> But how
>>> many folks are actually running OSPF for a v6 environment and  
>>> using IPsec to
>>> protect the communicating peers?  Some but not many (yet).
>>>
>>> Personally, I'd stick with ESP.  AH complicates matters  
>>> (configuration,
>>> nested environments when you do decide to also use ESP for  
>>> encryption maybe
>>> later, NAT) and while is isn't officially deprecated vendors  
>>> don't test it
>>> as much as ESP - at interoperability tests it's not stressed, at  
>>> least the
>>> ones I've been to.  Ask your vendor(s) what they think of the  
>>> work below and
>>> see where they stand with implementing it.
>>>
>>> Be happy to answer any more questions offline.
>>>
>>> - merike
>>>
>>> On May 25, 2009, at 6:24 AM, Jack Kohn wrote:
>>>
>>>  Glen,
>>>>
>>>> IPSECME WG <http://www.ietf.org/html.charters/ipsecme- 
>>>> charter.html> at
>>>> IETF
>>>> is actually working on the exact issue that you have described  
>>>> (unable to
>>>> deep inspect ESP-NULL packets).
>>>>
>>>> You can look at
>>>> draft-ietf-ipsecme-traffic-visibility-02<http://tools.ietf.org/ 
>>>> html/
>>>> draft-ietf-ipsecme-traffic-visibility-02>for
>>>> more details.
>>>>
>>>> Jack
>>>>
>>>> On Sat, May 23, 2009 at 5:06 AM, Glen Kent <glen.kent at gmail.com>  
>>>> wrote:
>>>>
>>>>> Yes, thats what i had meant !
>>>>>
>>>>> On Fri, May 22, 2009 at 10:46 PM, Christopher Morrow
>>>>> <morrowc.lists at gmail.com> wrote:
>>>>>
>>>>>>
>>>>>> On Fri, May 22, 2009 at 1:04 PM, Glen Kent  
>>>>>> <glen.kent at gmail.com> wrote:
>>>>>>
>>>>>>> Hi,
>>>>>>>
>>>>>>> It is well known in the community that AH is NAT unfriendly  
>>>>>>> while ESP
>>>>>>> cannot
>>>>>>> be filtered, and most firewalls would not let such packets  
>>>>>>> pass. I am
>>>>>>> NOT
>>>>>>>
>>>>>>
>>>>>> 'the content of the esp packet can't be filtered in transit' I  
>>>>>> think
>>>>>> you mean... right?
>>>>>>
>>>>>>  interested in encrypting the data, but i do want origination
>>>>>>> authentication
>>>>>>> (Integrity Protection). Do folks in such cases use AH or ESP- 
>>>>>>> NULL,
>>>>>>>
>>>>>> given
>>>>
>>>>> that both have some issues?
>>>>>>>
>>>>>>> Thanks,
>>>>>>> Glen
>>>>>>>
>>>>>>>
>>>>>
>>>>>
>>>
>>





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