Common operational misconceptions

Mark Andrews marka at isc.org
Thu Feb 16 08:28:36 CST 2012


In message <20120216134437.GB65401 at macbook.bluepipe.net>, Phil Regnauld writes:
> Mark Andrews (marka) writes:
> > If you want to know if your resolver talks IPv6 to the world and
> > supports 4096 EDNS UDP messages the following query will tell you.
> > 
> > 		dig edns-v6-ok.isc.org txt
> > 
> > Similarly for IPv4.
> > 
> > 		dig edns-v4-ok.isc.org txt
> > 
> 
> 	9.8.1P1 on a dual stacked native v6 host: I'm seeing TC on both answers,
> 	the difference is the TCP answer comes through on v4 but v6 gives SERVFAIL.

You will see TC between dig and the resolver.  If you see TC between the resolver
and the server it will fail as neither server answers over TCP.  If you are seeing
TC between the resolver and the server and the TCP query is being answers then
something in the path is intercepting the DNS queries.

> 	Don't see any v6 fragments (that'd be a problem since PF doesn't handle
> 	them on this host).

You should see something like this on the wire.  The second query is to answer
dig's query over TCP.

01:19:30.421959 IP6 2001:470:1f00:820:6965:eba7:eff6:1242.64345 > 2001:4f8:0:2::8.53: 44698% [1au] TXT? edns-v6-ok.isc.org. (47)
01:19:30.591828 IP6 2001:4f8:0:2::8 > 2001:470:1f00:820:6965:eba7:eff6:1242: frag (0|1232) 53 > 64345: 44698*- 1/0/1 TXT[|domain]
01:19:30.592851 IP6 2001:4f8:0:2::8 > 2001:470:1f00:820:6965:eba7:eff6:1242: frag (1232|1232)
01:19:30.593889 IP6 2001:4f8:0:2::8 > 2001:470:1f00:820:6965:eba7:eff6:1242: frag (2464|1232)
01:19:30.593963 IP6 2001:4f8:0:2::8 > 2001:470:1f00:820:6965:eba7:eff6:1242: frag (3696|408)
01:19:30.596552 IP6 2001:470:1f00:820:6965:eba7:eff6:1242.61500 > 2001:4f8:0:2::8.53: 60740% [1au] TXT? edns-v6-ok.isc.org. (47)
01:19:30.767351 IP6 2001:4f8:0:2::8 > 2001:470:1f00:820:6965:eba7:eff6:1242: frag (0|1232) 53 > 61500: 60740*- 1/0/1 TXT[|domain]
01:19:30.768362 IP6 2001:4f8:0:2::8 > 2001:470:1f00:820:6965:eba7:eff6:1242: frag (1232|1232)
01:19:30.769399 IP6 2001:4f8:0:2::8 > 2001:470:1f00:820:6965:eba7:eff6:1242: frag (2464|1232)
01:19:30.769473 IP6 2001:4f8:0:2::8 > 2001:470:1f00:820:6965:eba7:eff6:1242: frag (3696|408)

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



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