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