[Richard Brooks <richardbuk@xxxxxxx>]

Hello Guy

Your just not getting it. 

The question is given the ip address of '74.125.127.208', how does one query
a DNS server (in this case DNS ip 8.8.8.8 = public Google DNS) to get the
reply 'bskyb-pop3-ssl.l.google.co' (which is the reply Wireshark gets), and
not the reply 'pz-in-f208.1e100.net', which is what nslookup gets back.

Regards
Richard
 
 


-----Original Message-----
From: wireshark-dev-bounces@xxxxxxxxxxxxx
[mailto:wireshark-dev-bounces@xxxxxxxxxxxxx] On Behalf Of Guy Harris
Sent: 07 January 2010 00:29
To: Developer support list for Wireshark
Subject: Re: [Wireshark-dev] How does Wireshark do name resolution?


On Jan 6, 2010, at 12:55 PM, Richard Brooks wrote:

> I think that Sky have more than one email server/mirror. Try doing an
> nslookup on '74.125.127.208', on my PC it came back as
> 'pz-inf208.1e100.net'.'

So there are at least two different A records out there in DNSland that
return 74.125.127.208.

Let's see:

$ host -a bskyb-pop3-ssl.l.google.com
Trying "bskyb-pop3-ssl.l.google.com"
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 51975
;; flags: qr rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 4, ADDITIONAL: 4

;; QUESTION SECTION:
;bskyb-pop3-ssl.l.google.com.	IN	ANY

;; ANSWER SECTION:
bskyb-pop3-ssl.l.google.com. 211 IN	A	74.125.127.208

;; AUTHORITY SECTION:
google.com.		31920	IN	NS	ns1.google.com.
google.com.		31920	IN	NS	ns3.google.com.
google.com.		31920	IN	NS	ns4.google.com.
google.com.		31920	IN	NS	ns2.google.com.

;; ADDITIONAL SECTION:
ns2.google.com.		341295	IN	A	216.239.34.10
ns1.google.com.		36532	IN	A	216.239.32.10
ns4.google.com.		150710	IN	A	216.239.38.10
ns3.google.com.		70248	IN	A	216.239.36.10

Received 197 bytes from XX.XX.XX.XX#53 in 23 ms

$ host -a pz-in-f208.1e100.net
Trying "pz-in-f208.1e100.net"
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 34359
;; flags: qr rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 13, ADDITIONAL: 13

;; QUESTION SECTION:
;pz-in-f208.1e100.net.		IN	ANY

;; ANSWER SECTION:
pz-in-f208.1e100.net.	86351	IN	A	74.125.127.208

;; AUTHORITY SECTION:
net.			17825	IN	NS	E.GTLD-SERVERS.net.
net.			17825	IN	NS	H.GTLD-SERVERS.net.
net.			17825	IN	NS	D.GTLD-SERVERS.net.
net.			17825	IN	NS	J.GTLD-SERVERS.net.
net.			17825	IN	NS	F.GTLD-SERVERS.net.
net.			17825	IN	NS	G.GTLD-SERVERS.net.
net.			17825	IN	NS	I.GTLD-SERVERS.net.
net.			17825	IN	NS	A.GTLD-SERVERS.net.
net.			17825	IN	NS	C.GTLD-SERVERS.net.
net.			17825	IN	NS	K.GTLD-SERVERS.net.
net.			17825	IN	NS	B.GTLD-SERVERS.net.
net.			17825	IN	NS	M.GTLD-SERVERS.net.
net.			17825	IN	NS	L.GTLD-SERVERS.net.

;; ADDITIONAL SECTION:
B.GTLD-SERVERS.net.	75602	IN	A	192.33.14.30
B.GTLD-SERVERS.net.	75602	IN	AAAA	2001:503:231d::2:30
F.GTLD-SERVERS.net.	85234	IN	A	192.35.51.30
J.GTLD-SERVERS.net.	78545	IN	A	192.48.79.30
H.GTLD-SERVERS.net.	79639	IN	A	192.54.112.30
L.GTLD-SERVERS.net.	150475	IN	A	192.41.162.30
I.GTLD-SERVERS.net.	155091	IN	A	192.43.172.30
A.GTLD-SERVERS.net.	57911	IN	A	192.5.6.30
A.GTLD-SERVERS.net.	57911	IN	AAAA	2001:503:a83e::2:30
D.GTLD-SERVERS.net.	55140	IN	A	192.31.80.30
K.GTLD-SERVERS.net.	109507	IN	A	192.52.178.30
M.GTLD-SERVERS.net.	41034	IN	A	192.55.83.30
C.GTLD-SERVERS.net.	75746	IN	A	192.26.92.30

Received 507 bytes from XX.XX.XX.XX#53 in 2 ms

Both of those came from an internal server caching the results.  And for the
reverse lookup:

$ host -a 74.125.127.208             
Trying "208.127.125.74.in-addr.arpa"
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 55402
;; flags: qr rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 4, ADDITIONAL: 4

;; QUESTION SECTION:
;208.127.125.74.in-addr.arpa.	IN	PTR

;; ANSWER SECTION:
208.127.125.74.in-addr.arpa. 86210 IN	PTR	pz-in-f208.1e100.net.

;; AUTHORITY SECTION:
125.74.in-addr.arpa.	44346	IN	NS	NS4.GOOGLE.COM.
125.74.in-addr.arpa.	44346	IN	NS	NS1.GOOGLE.COM.
125.74.in-addr.arpa.	44346	IN	NS	NS3.GOOGLE.COM.
125.74.in-addr.arpa.	44346	IN	NS	NS2.GOOGLE.COM.

;; ADDITIONAL SECTION:
NS1.GOOGLE.COM.		36446	IN	A	216.239.32.10
NS3.GOOGLE.COM.		70162	IN	A	216.239.36.10
NS4.GOOGLE.COM.		150624	IN	A	216.239.38.10
NS2.GOOGLE.COM.		341209	IN	A	216.239.34.10

Received 225 bytes from XX.XX.XX.XX#53 in 7 ms

Again, from the internal server.

Perhaps if some other server is queried, a different PTR record will come
back for 208.127.125.74.in-addr.arpa.  It would be interesting to see the
DNS traffic issued when you do an nslookup on 74.125.127.208 and when
Wireshark resolves 74.125.127.208.

The problem is that the difference might just reflect configuration
differences in the resolvers, or quirks of the current implementations of
those resolvers, or something else not guaranteed to be an invariant
characteristic of whatever resolver Wireshark happens to be using (which is
a function of how it was built - it might just be using the native OS's
resolver, rather than any of the asynchronous resolvers it can be configured
to use).
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