Re: [Asrg] DNSSEC is NOT secure end to end

[Doug Otis <[e-mail]>]

On Jun 3, 2009, at 8:35 PM, Masataka Ohta wrote:

> The problem is that the accuracy and integrity of DNSSEC is not  
> cryptographically, but socially secure.

DNS over UDP is prone to port/transaction-id guessing,  where  
cryptography could play a protective role.  The risk of these values  
being guessed increases whenever NATs reduce port diversity, or  
operate in a predictable manner.  Protocols such as SPF that embed  
macros into DNS, allow hundreds of transactions to be chained.  The  
entire chain might result from the local-parts of a single email.  
These transactions can target otherwise uninvolved victims or evil  
domains.  When an evil domain is the target of SPF transactions,  
attackers can discover the nature of the resolver.  Afterwords, with  
only one transaction targeting the evil domain, and others targeting  
their victim, the guesswork for injecting poison is reduced, where  
even ACLs offer no protection.  The growing speed of today's Internet  
makes this an ever growing concern.

While DNSSEC might prevent caches from being poisoned, EDNS0 creates  
new concerns also aggravated by SPF.  Since the 512 byte DNS message  
size did not accommodate RSA signatures, EDNS0 is required to adjust  
message limits.  EDNS0 allows bad actors to further leverage DNS  
transactions, such as those that might emanate from cached SPF records  
to initiate more than 20 TXT record transactions when a recipient  
evaluates a single email.  The TXT records might be policy documents  
not intended for machine consumption or wildcard SPF records  
enumerating address authorizations for outbound MTAs.  The flexibility  
sought by SPF has created a sizable list of concerns, where caution  
was often countered with distain for DNS.

It might have been better to have specified limits for EDNS0, such as  
a minimum message size of 1280 and a maximum at 1424, where  
transactions that don't comply are refused.  UDP allows source  
spoofing, which could allow bad actors a means to create packet  
fragmentation by incorrectly setting message.  If addition, when  
fragmentation does occur, DNS transactional-ids offer little  
protection for packet fragments that may contained unsigned  
information.  DNS will need to be become pedantic about confirming  
information, perhaps also enforcing the use of checksums and message  
size.

While DNSSEC may not require channel security at some point when a  
trust anchor can be safely found, DNS over UDP remains a brute.  When  
an SPF process sequence prematurely times out, rather than waiting for  
exponential back-off, SPF simply begins another sequence, ignoring  
congestion avoidance.

SCTP carrying either DNS or DNSEC packets would ensure DNS remains  
tame despite much of the abuse.  Unlike TCP, SCTP does not commit  
resources without return of a cookie, but can start data exchanges  
sooner.  SCTP can carry simultaneous DNS messages and can can keep a  
large number of sparse connections per port active.  Perhaps recursive  
resolvers might be more responsive with SCTP than with UDP.   
Importantly, the source of abusive DNS behavior can be identified and  
thereby avoided.  This is not true for either TCP or UDP.

-Doug