large organization nameservers sending icmp packets to dns servers.

Valdis.Kletnieks at Valdis.Kletnieks at
Tue Aug 7 19:45:22 UTC 2007

On Tue, 07 Aug 2007 14:38:06 EDT, "Patrick W. Gilmore" said:
>> In addition, any UDP truncated response needs to be retried via  
>> TCP- blocking it would cause a variety of problems.

> Since we are talking about authorities here, one can control the size  
> of ones responses.


% dig txt

; <<>> DiG 9.5.0a6 <<>> txt
;; global options:  printcmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 57320
;; flags: qr rd ra; QUERY: 1, ANSWER: 2, AUTHORITY: 4, ADDITIONAL: 2

;                       IN      TXT

;; ANSWER SECTION:                218     IN      TXT     "v=spf1 ip4: ip4: ip4: ip4: ip4: ip4: ip4: ?all"                218     IN      TXT     "spf2.0/pra ip4: ip4: ip4: ip4: ip4: ip4: ip4: ?all"

;; AUTHORITY SECTION:                439     IN      NS                439     IN      NS                439     IN      NS                439     IN      NS

;; ADDITIONAL SECTION:      158598  IN      A      158598  IN      A

;; Query time: 4 msec
;; SERVER: 2001:468:c80:6101:213:72ff:fefc:d5cc#53(2001:468:c80:6101:213:72ff:fefc:d5cc)
;; WHEN: Tue Aug  7 15:32:36 2007
;; MSG SIZE  rcvd: 512

So tell me what they should do if they wanted to add 1 more byte to those TXT
records. Say they want to start announcing IPv6 addresses too... :)  They're
running just a bit tight on 'authority and 'additional' they can heave over the
side - they *already* don't answer with the txt records if you try to do a
'dig any' because that 512 and the 497 returned on a 'dig mx'
won't fit in one 512-byte packet.

> Unless, of course, you are so incredibly stupid you can't figure out  
> the difference between an authority and a caching server.

I wish people would keep straight what direction they're doing the measurement,
and for who's benefit.

If *XYZ* wants to find which of their servers I'm closest to, they'll most
likely be poking at my *caching* nameservers, because that's where my recursive
query arrived from[1]. 

So we're *not* talking about authorities here.  We're talking about DNS servers
that are quite possibly configured to not talk, or give only partial results
via UDP, to queries coming from outside the provider's network (after all,
those people probably *should* be using *their* provider's caching DNS, right?)

[1] If they *want* to go to the added effort of digging up my PTR, and from
that finding the SOA and NS and ask my *authoritative* DNS for timing info,
that's their right.  Of course, all 3 of my caching servers are *really close*
to me netwise - but of the 5 NS entries we list, 3 are intentionally *far away*,
being off-campus secondaries. Hell, they're not even in our AS. :)

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