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I was trying to do some reverse DNS lookups on some IP addresses I see in my traceroute (ultimately to figure out roughly geographically where my path is going through), but I often get results that look like:

> nslookup 209.85.251.242 
Server:     192.168.1.1
Address:    192.168.1.1#53

** server can't find 242.251.85.209.in-addr.arpa.: NXDOMAIN

Similarly, dig gives:

dig -x 209.85.251.242

; <<>> DiG 9.8.5-P1 <<>> -x 209.85.251.242
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NXDOMAIN, id: 14592
;; flags: qr rd ra; QUERY: 1, ANSWER: 0, AUTHORITY: 1, ADDITIONAL: 0

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

;; AUTHORITY SECTION:
251.85.209.in-addr.arpa. 60 IN  SOA ns1.google.com. dns-admin.google.com. 1540420 21600 3600 1209600 10800

;; Query time: 67 msec
;; SERVER: 192.168.1.1#53(192.168.1.1)
;; WHEN: Sat Dec 07 02:13:46 CET 2013
;; MSG SIZE  rcvd: 105

This suggested to me that no PTR record exists for this IP address, but I am able to ping the address.

I think I don't understand well enough how DNS works to see what's going on here. In what circumstances can this happen?

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2 Answers 2

up vote 5 down vote accepted

PTR records are entirely optional, and have no bearing on the ability to communicate with a specific address or netblock. Expanding on that thought, DNS as a whole doesn't have any bearing on this ability either. Basic DNS queries (A, CNAME, PTR, etc.) function solely to map a name to an IP address.

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Ah right of course. Learned that in my networking class before. The routers will direct ip addresses based on their routing tables, without going through dns. –  bhh1988 Dec 7 '13 at 7:15

The IP indeed does not have a PTR record. I can't ping it at the moment. Another tool to try to glean some information is whois.

$ whois 209.85.251.242

NetRange:       209.85.128.0 - 209.85.255.255
CIDR:           209.85.128.0/17
OriginAS:
NetName:        GOOGLE
NetHandle:      NET-209-85-128-0-1
Parent:         NET-209-0-0-0-0
NetType:        Direct Allocation
RegDate:        2006-01-13
Updated:        2012-02-24
Ref:            http://whois.arin.net/rest/net/NET-209-85-128-0-1

You can also try mtr.

$ mtr 209.85.251.242 -r -w -c 1

HOST: myhost                                             Loss%   Snt   Last   Avg  Best  Wrst StDev
  1.|-- oscilloscope                                     0.0%     1    4.9   4.9   4.9   4.9   0.0
  2.|-- 67.188.232.1                                     0.0%     1  132.7 132.7 132.7 132.7   0.0
  3.|-- te-0-2-0-3-ur05.santaclara.ca.sfba.comcast.net   0.0%     1  113.4 113.4 113.4 113.4   0.0
  4.|-- te-1-1-0-3-ar01.sfsutro.ca.sfba.comcast.net      0.0%     1  123.6 123.6 123.6 123.6   0.0
  5.|-- he-3-9-0-0-cr01.sanjose.ca.ibone.comcast.net     0.0%     1   23.8  23.8  23.8  23.8   0.0
  6.|-- pos-0-1-0-0-pe01.529bryant.ca.ibone.comcast.net  0.0%     1  206.6 206.6 206.6 206.6   0.0
  7.|-- ???                                             100.0     1    0.0   0.0   0.0   0.0   0.0

Dying for me at Comcast Equinix SV 8 in Palo Alto, CA.

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Care to explain the random downvote? –  dmourati Dec 7 '13 at 4:18
1  
I would guess it's because you haven't attempted to answer the question. The OP says he can ping it and wants to know why he can. –  Ladadadada Dec 7 '13 at 8:45
    
@Ladadadada - that was precisely the reason for the downvote. –  EEAA Dec 7 '13 at 16:51
    
I was answering the ultimate question: "(ultimately to figure out roughly geographically where my path is going through)" –  dmourati Dec 8 '13 at 6:46

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