Comcast XFINITY → [DNS] How to diagnose strange DNS issue?

I had a problem that vimeo.com was not loading on some of my LAN machines. All other websites were working fine. I went through I tried /flushdns and /release /renew, but that didn't help. I couldn't ping or tracert vimeo.com "unable to resolve target system name", so there was no IP entry at all. I looked at my router and it was using the default Comcast DNS entries: 75.75.75.75, 75.75.76.76. So, I added Google's public DNS to my router and now vimeo.com is resolving fine on all machines.

I guess my problem is solved, but I'm curious as to why I couldn't resolve vimeo.com with the default Comcast DNS settings. Is there a way to do a more advanced query to 75.75.75.75, 75.75.76.76 to figure out why it wasn't sending an IP for vimeo.com? I'm interested to figure out whether there's still something wrong on my end.

 

Works fine for me:

$ dig @75.75.75.75 vimeo.com

;  DiG 9.4.2-P2  @75.75.75.75 vimeo.com
; (1 server found)
;; global options:  printcmd
;; Got answer:
;; -HEADER- opcode: QUERY, status: NOERROR, id: 49608
;; flags: qr rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 0, ADDITIONAL: 0

;; QUESTION SECTION:
;vimeo.com.                     IN      A

;; ANSWER SECTION:
vimeo.com.              29      IN      A       74.113.233.128

;; Query time: 31 msec
;; SERVER: 75.75.75.75#53(75.75.75.75)
;; WHEN: Sat Nov  9 09:22:50 2013
;; MSG SIZE  rcvd: 43

 

 
$ dig @75.75.76.76 vimeo.com

;  DiG 9.4.2-P2  @75.75.76.76 vimeo.com
; (1 server found)
;; global options:  printcmd
;; Got answer:
;; -HEADER- opcode: QUERY, status: NOERROR, id: 20290
;; flags: qr rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 0, ADDITIONAL: 0

;; QUESTION SECTION:
;vimeo.com.                     IN      A

;; ANSWER SECTION:
vimeo.com.              30      IN      A       74.113.233.128

;; Query time: 44 msec
;; SERVER: 75.75.76.76#53(75.75.76.76)
;; WHEN: Sat Nov  9 09:23:53 2013
;; MSG SIZE  rcvd: 43

Though there's a possibility I may not be using the same DNS servers as you.

Boy, sometimes the DNS servers can drive me crazy. I often use the Comcast ones, but sometimes use google's, or opendns... Seems like some times for me one of them will not resolve like you, and switching fixes it. I always wonder if simply changing them to another DNS server (in windows at least) forces something to reset that a /flushdns should do.
a tool at »code.google.com/p/namebench/ that says it can test DNS response times, not sure if it really speeded things up or not for me, but...

Try
nslookup www.vimeo.com 75.75.75.75
from the command prompt, and post the result back here.

for me it looks like this

C:\Users\tshirt>nslookup www.vimeo.com 75.75.75.75
Server: cdns01.comcast.net
Address: 75.75.75.75

Non-authoritative answer:
Name: vimeo.com
Address: 74.113.233.128
Aliases: www.vimeo.com

There is no way to know what happened at this point (perhaps there was actually a temporary glitch with Comcast's DNS system), but since you are using a router, that is also a possibility for the source of the problem.

Before you changed the DNS forwarders in the router from Comcast to Google, were the Comcast DNS servers simply automatically being used, or had you manually configured the router to use the Comcast servers? With some routers, if you use the ISP DHCP supplied DNS servers, the router itself becomes the default DNS server for its LAN DHCP clients, but if you manually set the DNS forwarders, the router will supply those DNS addresses to its DHCP clients (and in other routers, you may be able to manually select whether or not the router acts as the default DNS server for its LAN DHCP clients). Either way, simply changing the DNS servers being used in the router reset everything and allowed proper DNS resolution again; and possibly if you change back to using the Comcast servers again, DNS resolution might still work OK.

FWIW, if the router I am using allows it (and a router that does not allow it does not stay in service for very long on my network), I always configure it to not be a DNS server because resi/soho routers in general are very poor DNS relay/forwarding agents.

I agree 100% NetFixer. If you do an ipconfig /all and your pc shows 10.0.0.1 as the dns server, that's a bad setup. Setup your network to use the dns servers directly (I use comcast, google, opendns, and bellsouth/southeast's dns server handed out on my pfsense router).

Exclusively using ANY isps dns server is bad. There was a study a few years back that shows that 30% of all dns lookups to isps dns servers are dropped. The case may be better nowadays. Also if you can find an ISP with open dns servers (that are not commonly known on the public dns servers lists on the internet) that only serve business class customers (for example, windstream has their fiber/t1/business dns servers and their residential dsl/dialup dns servers, and the residential servers are awful because there's a magnitude more residential users. A windstream fiber installer said this himself!). Here's one to use: it's the gulftel dns server: 216.231.160.2. Gulftel was acquired by centurylink but I believe they hand out a different dns server now. As always, test with Google Namebench.

It's better to have the dns servers show on the pc as the actual dns server. Think of the memory and cpu on your computer versus the ram and cpu on the router.

Thanks to everyone for all the replies and advice. In the time since I first posted, vimeo.com stopped loading again even after adding the Google DNS entries.

So, I did nslookup and looked at the router settings. Most of my machines were only showing the router IP as the single DNS entry in ipconfig /all. Earlier today, I had set added 3 manual entries to the Router DNS settings: Google, Comcast, Google. At that point, vimeo started loading again.

Just now it wasn't loading, and nslookup was coming up empty 0.0.0.0, with the Router IP listed as server. So I changed the Router DNS settings order to: Google, Google, Comcast and then vimeo started loading again.

Taking your advice, I went to look through the Router settings and found "Use internal DNS" and I unchecked that. After /flushdns /release /renew on all the affected machines, they now show the 3 DNS entries (Google, Google, Comcast) when I do ipconfig /all. And vimeo.com is loading fine (and nslookup gives a correct result.) So things seem to be working now.

With my old routers, I had always set each machine with manual IPs and manually set DNS entries, but I've had this router 2-3 years and it has always worked with everything on automatic. This is the one exception. I still can't understand why vimeo.com was the only affected site and why it changed all of a sudden.

Thanks again for your help.

Due to the way CDNs use DNS for geolocation, it is very much not advisable to use other DNS servers, especially those from other ISPs if you want your content to load quickly.

I'm trying google DNS at the moment because Seattle comcast is not working well for me.