[dns-wg] Question about use of cname in Google's services

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 at 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 at time-travellers.org>
> wrote:
>
>> Anurag,
>>
>> At 2016-06-04 02:52:46 +0530
>> Anurag Bhatia <me at 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
>
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