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 anuragbhatia.com