Regarding EDNS Responses.

[Ashwin]

In message <001501ca5785$257c7220$2101120a at china.huawei.com>, Ashwin writes:
> 
> Hi All,
> 
>  RFC 2671 mentions in Section 5.3
> 
> Responders who do not understand these protocol extensions are
> expected to send a response with RCODE NOTIMPL, FORMERR, or
> SERVFAIL.
> 
> However the above mentioned error codes are shared [SERVFAIL, NOTIMPL] are
> shared, so how do we ascertain that the error code returned is an
indication
> that a particular server is non-EDNS, since the error might be returned
due
> to some other reason also.
> 
> So essentially my query is how do we decide that a particular server is
EDNS
> or not? Can it be assumed that each time the above three error codes are
> returned , it signifies that the DNS server is not EDNS capable?


Hi Mark,
>> You assume it is EDNS if it is in response to a EDNS query and retry
>> w/o EDNS.  It the problem is EDNS the plain DNS query will succeed.
>> If it is not EDNS the plain EDNS query will fail.
  
Thanks for you response. I have a doubt though.

  I send out an EDNS query, for the response the following possibilities
	a) Success, with OPT RR, I assume server is EDNS capable
	b) Failure with RCODE NOTIMPL, FORMERR, or SERVFAIL with or without
OPT RR.

In b) I do not know whether server is EDNS or not, since server might return
NOTIMPL & SERVFAIL error codes for some other reason also. If I consider the
case that retry with plain DNS query is success and assume EDNS was problem,
I think maybe its not correct because SERVFAIL might happen for some other
reason at the time EDNS query is sent, but that error is resolved by the
time the plain DNS query is sent. So even though server is EDNS i would
assume it is non-EDNS.

The idea is to identify whether a server supports EDNS through a first
query, and then subsequent requests we send based on this identification.
One could call it a pseudo-caching of the EDNS feature for servers.

I hope I made myself clear :(
    
> Regards
> 
> Ashwin

-- 
Mark Andrews, ISC
1 Seymour St., Dundas Valley, NSW 2117, Australia
PHONE: +61 2 9871 4742                 INTERNET: marka at isc.org