My guess is the *.l.google.com runs on a separate geo-ip aware backend DNS cluster rather than *.google.com, even though the authoritative public dns records are ns[1-4].google.com

C:\>dig www.gmail.com @ns1.google.com

;; QUESTION SECTION:
;www.gmail.com.                 IN      A

;; ANSWER SECTION:
www.gmail.com.          86400   IN      CNAME   mail.google.com.
mail.google.com.        604800  IN      CNAME   googlemail.l.google.com.
googlemail.l.google.com. 300    IN      A       172.217.1.101



The TTL for the mail.google.com record CNAME is 604800 seconds, which is 7 days, as opposed to the googlemail.l.google.com A record which expires in 300 seconds, allowing Google to quickly take servers in and out of rotation as required.



On Sat, Jun 4, 2016 at 4:48 AM, Anurag Bhatia <me@anuragbhatia.com> wrote:
Hi Shane



Sure, they route googlemail.l.google.com. to nearest datacenter but when prevents them from doing same with mail.google.com instead? 

They return Geographically closer A record for googlemail.l.google.com. but why not for mail.google.com itself? 




Thanks. 

On Sat, Jun 4, 2016 at 10:31 AM, Shane Kerr <shane@time-travellers.org> wrote:
Anurag,

At 2016-06-04 02:52:46 +0530
Anurag Bhatia <me@anuragbhatia.com> wrote:

> Someone asked me question on why google uses cname for their services
> anyways? I mean I get it that for Google Apps customers it makes sense to
> have mail.domain.com pointed to a cname rather then A record to a host
> which may die.
>
> But why for their own services? Like e.g "mail.google.com" is cname to
> googlemail.l.google.com. and googlemail.l.google.com. eventually returns A
> record. This adds up one extra step in resolution and I wonder why Google
> does it this way? What advantage they get ? or What advantage they miss if
> they simply return record which I am getting for googlemail.l.google.com.
> directly as A record for mail.google.com ?

I guess that this is a CDN trick, to give different answers based on
the resolver's originating IP address (or client-subnet EDNS0
information, if available).

In Beijing I get this:

$ host mail.google.com
mail.google.com is an alias for googlemail.l.google.com.
googlemail.l.google.com is an alias for mail-china.l.google.com.
mail-china.l.google.com has address 74.125.203.19
mail-china.l.google.com has address 74.125.203.18
mail-china.l.google.com has address 74.125.203.17
mail-china.l.google.com has address 74.125.203.83
mail-china.l.google.com has IPv6 address 2404:6800:4005:802::2005

The CNAME chain can send users to servers closer to where they are, and
allows operators to redirect traffic to less-busy servers or even take
sites offline easily.

Cheers,

--
Shane



--


Anurag Bhatia