Mike I can also have a single DNS > server give 192.0.2.80 out to queries sourced from a US IP Address, > 198.51.100.80 for queries sourced from a German IP Address and > 203.0.113.80 to queries sourced from a Chinese address (djbdns has a > module for this for example).
I have never did such setup, but I assume it works as you say. I wonder how it finds a US based system from IP quickly (since it's DNS server)? Thanks. On Thu, Feb 9, 2012 at 1:21 AM, Anurag Bhatia <m...@anuragbhatia.com> wrote: > Nice explanation! > > > Thanks Mike. > > Appreciate it. > > On Thu, Feb 2, 2012 at 6:08 AM, Mike Jones <m...@mikejones.in> wrote: > >> On 1 February 2012 20:25, Anurag Bhatia <m...@anuragbhatia.com> wrote: >> <snip> >> > 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? >> >> You are confusing anycasting with offering different results. >> >> I can have an anycast DNS setup where all my servers give the same >> response (example: most DNS providers), I can also have a single DNS >> server give 192.0.2.80 out to queries sourced from a US IP Address, >> 198.51.100.80 for queries sourced from a German IP Address and >> 203.0.113.80 to queries sourced from a Chinese address (djbdns has a >> module for this for example). >> >> I would guess that google probably have a highly customised algorithm >> which uses a combination of source IP and the node that your query >> arrived at as part of the process for deciding what answer to give >> you, along with dozens of other internal factors. >> >> Although I do sometimes wonder why they use CNAME chains in cases >> where the same servers are authoritative for the target name anyway. >> >> If you were wondering why they direct you to the unicast addresses for >> the local datacentre instead of just giving an anycast address which >> your nearest datacentre would answer, well their algorithm might >> decide that it wants to serve you content from the second closest >> datacentre because the closest one is near capacity, anycast can't do >> that. >> >> - Mike >> > > > > -- > > 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 > > -- 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
