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Comcast DNS Outages

Frequent Visitor

Comcast DNS Outages

It appears that DNS is not always working on Comcast. I experience outages of 20 or 30 minutes or more. The last one was 7/29/2019 6:10 pm to 6:40 pm eastern time.

I cannot access mail.comcast.net, xfinity.com, google.com or any other website using the domain name. However, I can access websites or FTP sites using the IP address only.

These outages have been going on for over a month.

When will they be fixed?

Silver Problem Solver

Re: Comcast DNS Outages

Roughly what area are you in? I haven't noticed any issues here in Chicago. Comcast has many DNS servers around the country. They all have the same IP address, but the local routers will connect you to the closest. Also, are you using a Comcast gateway, your own gateway, or your own modem and router?

 

The addresses used are:

75.75.75.75

75.75.76.76

2001:558:feed::1

2001:558:feed::2

Frequent Visitor

Re: Comcast DNS Outages

I am in the Pittsburgh PA area and my DNS servers are exactly the same. I tried to respond to this post several times logged on, then Server Not Found.  the outage this time was 24 minutes overall.

Frequent Visitor

Re: Comcast DNS Outages

I forgot, I use all Comcast equipment.

New Poster

Re: Comcast DNS Outages

Same issue here in Folsom, PA.
Silver Problem Solver

Re: Comcast DNS Outages

I don't know if it will actually return a unique name, but from a command prompt, you can do an "NSLOOKUP 75.75.75.75" to see what name is tied to that address. Here in Chicago, it says cdns01.comcast.net.

Expert

Re: Comcast DNS Outages

FWIW, AFAIK, they all have that reverse DNS / host name: "cdns01.comcast.net."



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Silver Problem Solver

Re: Comcast DNS Outages

That is what I was afraid of. About the only way to tell would be to do a "TRACERT 75.75.75.75" and looks at the next-to-last hop. For me, 75.75.75.75 is area4.il.chicago, and 75.75.76.76 is Northlake IL. The IPV6 versions point to the same.

Frequent Visitor

Re: Comcast DNS Outages

I have not been able to override the DNS server though Windows and the Comcast router will only display the the servers.

 

This is mine from the Pittsburgh, PA area:

TRACERT  75.75.75.75

Tracing route to cdns01.comcast.net [75.75.75.75]
over a maximum of 30 hops:

1  11 ms  3 ms  3 ms  Docsis-Gateway.hsd1.pa.comcast.net [10.0.0.1]
2  12 ms  15 ms  11 ms  96.120.62.149
3  12 ms  13 ms  12 ms  68.86.102.217
4  20 ms  19 ms  17 ms  be-50-ar01.pittsburgh.pa.pitt.comcast.net [68.85.75.161]
5  17 ms  18 ms  17 ms  162.151.21.114
6  19 ms  17 ms  15 ms  dns-sw01.summitpark.pa.pitt.comcast.net [69.139.167.226]
7  16 ms  23 ms  19 ms  cdns01.comcast.net [75.75.75.75]

Trace complete.

Official Employee

Re: Comcast DNS Outages

As was pointed out, we have an anycast DNS resolver infrastructure with sites distributed across our footprint.  All are monitored continuously and none have had failures in recent history.  Every customer  is provided both primary (75.75.75.75/2001:558:feed::1) and secondary (75.75.76.76/2001:558:feed::2) IPv4/IPv6 resolvers that are in diverse locations, so you’ll retry and get a response from the other if the first doesn’t respond.  If any of our sites are fully down, you’ll be re-routed to the next closest.  

 

DNS queries are performed early and often by many applications.  If you are seeing DNS failures, they may be acting as the canary in the coal mine for connectivity or configuration issues close to or on the computer performing the DNS lookup.  This could be anything from weak wifi or interference, a loose coax connection, or a local configuration issue.   The list of possibilities is nearly endless, but it’s unlikely to actually be a DNS problem.

 


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Silver Problem Solver

Re: Comcast DNS Outages

Comcast gateways will not allow you to alter the DNS. You should be able to alter it in Windows and try somthing like Google's DNS for testing (8.8.8.8 and 8.8.4.4). If your computer has multiple network interfaces (WiFi and Ethernet), then you need to alter the settings on both, or at least the one you are currently using.

Expert

Re: Comcast DNS Outages


@andyross wrote:

Comcast gateways will not allow you to alter the DNS. You should be able to alter it in Windows and try somthing like Google's DNS for testing (8.8.8.8 and 8.8.4.4). If your computer has multiple network interfaces (WiFi and Ethernet), then you need to alter the settings on both, or at least the one you are currently using.


FWIW, I've seen posts indicating that doing that doesn't even work for them anymore as it appears that they no longer will act as DNS forwarders / DNS relays.



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Silver Problem Solver

Re: Comcast DNS Outages

So they are actively blocking all attempts at DNS other than their own? Are they blocking or redirecting port 53 to their own DNS? Yet another reason to dump Comcast gateways or at least put it in bridge mode (assuming they don't block it then, too.)

Expert

Re: Comcast DNS Outages


@andyross wrote:

So they are actively blocking all attempts at DNS other than their own?

 

Seems that way from what I've read.

 

Are they blocking or redirecting port 53 to their own DNS?

 

Not certain of the exact method that they are using. Something in the firmware load.

 

Yet another reason to dump Comcast gateways or at least put it in bridge mode (assuming they don't block it then, too.)

 

Agreed !


 



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Frequent Visitor

Re: Comcast DNS Outages

If DNS is not the problem, then why is it I can access my server in Chicago by IP Address without any issues, and at the same time cannot access it by domain name, or other sites like comcast.net,  mail.comcast.net,  xfinity.com,  google.com or any other site by domain name. When the problem clears I can access all the above sites by domain name.  This also occurs to other devices connecting to the xfinity router (DPC3941T) at both 5GHz and 2.4GHz. The internet is only used by 2 people.

Frequent Visitor

Re: Comcast DNS Outages

At one time a while back, I used OpenDNS without any problems. When I started to have these outages, I tried using OpenDNS but could not access their servers. They have a test site for registered users to make sure it works (https://welcome.opendns.com/) but it dosen't. Their other set of DNS servers for non-registered use does not work either, not does Google DNS.

Expert

Re: Comcast DNS Outages

Please read back few posts. You can't change the DNS servers anymore when using Comcast rented gateway devices. Not even in the individual network clients. The gateways no longer act as DNS forwarders.


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Frequent Visitor

Re: Comcast DNS Outages

Dear ComcastDNS,

 

Sorry but your response was not helpful at all, and I am still having the same DNS problems.  You said that it was "weak wifi or interference, a loose coax connection".  As it turns out I us mostly Ethernet. Also could you explain how a loose coax connection could effect only Port 53 (DNS) as the rest of the Internet runs perfectly using just IP addresses durning the DNS outage.

 

Starting off your reply with "As was pointed out, we have an anycast DNS"  when anycast DNS was never mentioned before in this thread makes it sound like a canned answer. Also, how did your reply get marked as "Best Answer".  I know I did not do it, but I did remove it.

 

Can you explain why durning my last DNS outage a "nslookup xfinity.com" failed?

nslookup xfinity.com

DNS request timed out.
timeout was 2 seconds.
Server: UnKnown
Address: 2001:558:feed::1

DNS request timed out.
timeout was 2 seconds.
DNS request timed out.
timeout was 2 seconds.
DNS request timed out.
timeout was 2 seconds.
DNS request timed out.
timeout was 2 seconds.
*** Request to UnKnown timed-out

 

When I say DNS outage I am referring to the most dominant indication I see from my end. But on your end, it could be a bad line or router which is connected to your Anycast DNS resolver or even an DNS attack and not a DNS Server problem. Or simply overload. But I would expect a better answer than I received.

 

Thanks

 

Contributor

Re: Comcast DNS Outages

Not sure if this is related but I have been having connectivity issues with my internet over the past several months. It is only occasional but after some debugging with Wireshark, I found that the Comcast DNS servers had significant response delay (>3 secs) when these issues surfaced. When I switch the modem to use Google's Public DNS servers, the issues went away (<<100 ms delay).

Frequent Visitor

Re: Comcast DNS Outages

Thanks for your reply and I think you may be on to something. Perhaps the Comcast DNS servers are overloaded. But unfortunately I am using a Comcast gateway and it allows you to only view the DNS servers but not alter them. And they override any attempt by Windows to access alternate servers.

 

If you know a way around the Comcast gateway PLEASE let me know. What modem/router are you using?

 

THANKS!

 

Contributor

Re: Comcast DNS Outages

I am using the Motorola MG7700. 

 

Normally, the NIC card DNS setting should take priority over the modem when set properly.