[ntp:hackers] Protocol specification modification for MS-SNTP

David Mills mills at udel.edu
Tue Jul 7 03:41:40 UTC 2009


Amdrew.

Ah well, I was having a funky dream. The suggestion on digest algorithm 
stands.

Dave

Andrew Bartlett wrote:

>On Mon, 2009-07-06 at 21:28 +0000, David Mills wrote:
>  
>
>>Folks,
>>
>>The Samba folks have modified ntpd to support Microsoft authentication, 
>>which requies some bending of the protocol specification and 
>>implementation. I'd like to make it much simpler, reduce the code 
>>clutter and support alternative digest algorithms. The present protocol 
>>specification requires a nonzero key ID and MD5 digest algorithm by 
>>default. The ntpd never generates a MAC with a zero key ID. 
>>    
>>
>
>The Key ID is not zero in MS-SNTP authentication.  It is the little
>endian RID of the machine account. 
>
>What is zero is the MAC (16 bytes).  (How could the key ID be zero?
>What otherwise would determine what key to use to sign the reply?)
>
>  
>
>>The current 
>>MS-SNTP code uses a 20-octet all-zero MAC, causing ntpd to hand the 
>>packet off to another program which fills in the MAC. This makes
>>MS-SNTP 
>>"legal" and much more likely to be supported by "official" servers. I 
>>propose to add something like this, either to the protocol
>>specification 
>>or as an addendum:
>>
>>"A received MAC with a zero key ID is a special case. By default, the 
>>packet is treated as if the MAC were not present, so is not 
>>authenticated. On output the MAC is returned exactly as received and
>>the 
>>packet is not authenticated. As a configurable option, an output
>>packet 
>>with zero key ID can be passed to another program that fills in the
>>MAC."
>>    
>>
>
>This simply needs to be reworded to be 'an all Zero MAC (with a non-zero
>key id) is a special case...'
>
>Thanks,
>
>
>Andrew Bartlett
>  
>



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