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Question regarding anycasting in CDN setup
From: Anurag Bhatia <me () anuragbhatia com>
Date: Thu, 2 Feb 2012 01:55:03 +0530

Hello everyone!

I have a small question and was wondering if someone could help me with
that.

Question is - why companies like Google, Amazon are having partial
anycasting in CDN setups? E.g if we pick a random hostname from url of
Picasa picture - lh3.googleusercontent.com - this one is further a cname
string and at the end you will find different A records when checked from
different locations.

E.g when checked from my local system (in India):

;; QUESTION SECTION:
;lh3.googleusercontent.com. IN A

;; ANSWER SECTION:
lh3.googleusercontent.com. 86276 IN CNAME
googlehosted.l.googleusercontent.com.
googlehosted.l.googleusercontent.com. 176 IN A 209.85.175.132





Next, lookup from a server in Europe:

;; QUESTION SECTION:
;lh3.googleusercontent.com. IN A

;; ANSWER SECTION:
lh3.googleusercontent.com. 86400 IN CNAME
googlehosted.l.googleusercontent.com.
googlehosted.l.googleusercontent.com. 300 IN A 209.85.148.132


thus different IPs in both cases.

I understand that Google is doing anycasting on core DNS servers, and thus
we always hit nearest DNS server and all DNS servers are sort of
independent and carry different A records for CDN strings which point to
local cache server IP addresses. And here's confirmation:

anurag () laptop:~$ dig googleusercontent.com. ns +short
ns2.google.com.
ns3.google.com.
ns4.google.com.
ns1.google.com.



Picking ns1.google.com. and asking IP for
googlehosted.l.googleusercontent.com. from different locations:

anurag () laptop:~$ dig @ns1.google.com googlehosted.l.googleusercontent.com.
a +short
209.85.175.132


anurag () server7:~$ dig @ns1.google.com googlehosted.l.googleusercontent.com.
a +short
209.85.148.132



As expected - same server (which appears same but is different) giving
different values - thus I am actually hitting different servers in both
cases.

Now my question here is - why this setup and not simply using having a A
record for googlehosted.l.googleusercontent.com. which comes from any
anycasted IP address space? Why not anycasting at CDN itself rather then
only at DNS layer?


Can someone explain?


Thanks!

-- 

Anurag Bhatia
anuragbhatia.com
or simply - http://[2001:470:26:78f::5] if you are on IPv6 connected
network!

Twitter: @anurag_bhatia <https://twitter.com/#!/anurag_bhatia>
Linkedin: http://linkedin.anuragbhatia.com


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