[NZLUG] Friday finking: DNS 'map'

DL Neil nzlug at etelligence.info
Fri Jan 16 17:28:47 NZDT 2015


Mark,

Thanks for this, but not quite what I had in mind. Please allow me to
try to explain further:


On 16/01/15 16:06, Mark Foster wrote:
> dig +trace?

=is close but lists (only) RootServers

=similarly +showsearch doesn't, nor +nssearch
(yes, I'd like to think I did?dug my research)


=I'll debate/differ from the following (just a little):

> Assuming you're on DSL you should be directing queries at

=yes

> a) your DSL router (which receives DNS allocation from your ISP at connect)

=no. I'm on a sub-net with its own DHCP/DNS and need for local dvc
inter-connection.

=the DSL router consolidates a number of sub-nets and connects to the
outside world, as you say. However this to a Telecom svc (not my choice,
no recommendation, no comment...), and we have found that (rare
occasional drop-outs aside) their DNSvc is usually slower than OpenDNS
(despite the round-tripping)!

> b) The recursive DNS servers offered by your ISP

=the more of which there are, the slower will be the response...
(but irrelevant to our discussion)

> c) manually move up the query chain (from the root zone, to the TLD and
> upward from there) and see when the various queries fail.

=exactly - but am seeking something starting from the very 'bottom'!

> ... is this what you mean?


=if you look at a dig report, eg

> ;; Received 131 bytes from 192.31.80.30#53(192.31.80.30) in 221 ms
> blakjak.net.            300     IN      A 210.48.13.237
...

=it tells you the Answer one (apparently) requested = the A record.

=above which it advises that resolution was provided by a particular DNS
server (192.31.80.30).


=what I would like is something like:
(maybe think traceroute...)

0 127.0.0.1 hosts file - not found - referring to DNS
1 192.168.1.251 - not found - forwarding
2 192.168.1.1 - not found - forwarding
3 208.67.220.220 - success


Notes:
0 I don't use hosts so would be happy to ignore - here for
'completeness' of the resolution process only
1 = the local sub-net's in-house DNSvr
2 = the local router - per your notes, above
3 possibly including 'the answer', eg ...blakjak.net A 210.48.13.237


=Why?
The objective is to be able to see the (actual) succession of DNSvrs, to
be sure that one is indeed forwarding to the next (per network design),
that the forwarding-succession?chain includes all of the necessary svrs,
eg the local sub-net - and thereby identifying if any problem is totally
in-house or limited to the sub-net (without resort to sneaker-net).

=clearer than mud?



> On 16/01/2015 3:53 p.m., DL Neil wrote:
>> Is there a tool which will show the DNS/named servers which will be/may
>> be/were consulted to resolve an hostName?
>>
>>
>> I've had some trouble resetting my home/office network after replacing
>> the router and access point(s), and upgrading server software. The most
>> intractable issue was DNS not working for DHCP but working happily for
>> static-IP (and defined DNS) clients...
>>
>>
>> It would have been very useful to be able to sit at a client PC and
>> confirm that it resolved a hostName by first consulting the LAN DNS
>> server, then forwarding (and external) request to the router (not), then
>> (perhaps) an ISP's DNS server, thence OpenDNS, and maybe ...
>>
>> Am envisioning a report somewhat akin to traceroute (which spans/links
>> routers rather than DNS servers), or even an RDBMS EXPLAIN-ing the plan
>> of a complex multi-table SQL query...
>>
>> ...or should I have adopted a different approach?

-- 
Regards,
=dn


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