Re: "problem" with Google DNS and CDN sites

[Lei Wang]

Hi Will, thanks for the report. The problem you described looked like that CDN didn't return the right server -- the one closest to your location.

CDNs usually have many servers on the internet. In the best case, a server closest to user should be returned as the result of a DNS lookup. Some CDNs achieve this by supporting EDNS client subnet. Generally speaking, Google Public DNS passed users' masked  IP subnet to the CDN's authorities, and the authorities return the best server based on that. However, some CDNs don't support EDNS client subnet. They simply return the server which is closest to whoever sends the DNS queries to them. In your case, that will be either Google Public DNS servers' IPs (if you use 8.8.4.4) or your ISP's DNS servers' IPs.

Google Public DNS has servers in many countries. (Here is a list of IP subnet/Airport code of our server's locations https://developers.google.com/speed/public-dns/faq#locations) My guess is, there is some distance from your location to the nearest  Google Public DNS servers. On the other hand, ISP DNS servers are usually very close to your IP. In this case, your ISP DNS will return nicer answer for CDNs that don't support EDNS client subnet.

If you like, you could do the following little experiment: Let's say you have problem visiting some domain xxx.com, do

nslookup -debug xxx.com. 8.8.8.8

then switch to your ISP DNS server, do

 nslookup -debug xxx.com.

Now you can compare two results by using ping or tracert. If one is much slower to reach than the other, then that's the problem. (Here is some instructions about how to test 8.8.8.8 https://developers.google.com/speed/public-dns/docs/using#troubleshooting)

Hope this helps. --var

On Monday, October 1, 2012 11:22:08 AM UTC-4, will.p...@gmail.com wrote:

Folks, this was a strange one (unless I am mistaken...).

I could browse to many web sites, including Google, Facebook, and many

small sites.  However, for (some?) sites using a content-delivery

network (CDN) -- e.g. amazon.com, linkedin.com, ... -- it would load

the "main" page, then stall (for many minutes) waiting for the .js,

.css, etc., files.  In other words, everything was fine except many

"big" sites didn't work!

This applied across multiple browsers, even when started "fresh" (no

add-ons, no customizations of any kind).  My router was working --

remember, lots of things worked (both web and other).  DNS seemed OK

-- it was giving back (plausible) answers for everything, including

the CDN sites.

I was using one of the Google DNS servers (8.8.4.4) at the time.  When

I switched to using the ISP's (AT&T's) DNS servers, everything

(including the "big" sites) started working.

What could have been wrong?  (a) something at my end? - possible but

now seems unlikely; (b) Google's 8.8.4.4 giving me back useless

answers, for CDN sites but no others? -- implausible; (c) AT&T not

routing packets to CDN hosts that it deems "far away" [after all, the

whole point of CDNs is to use "nearby" hosts] -- wouldn't there have

been lots of howling if they were doing that?  (d) something I haven't

thought of :-(

I thought the symptoms were weird enough to merit a mention; my

Googling didn't find anything similar (which doesn't say much).

Will