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Re: Comcast DNS servers are dropping constantly (75.75.75.75 and 75.75.76.76) ... Revisited 2020

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New Poster

Re: Comcast DNS servers are dropping constantly (75.75.75.75 and 75.75.76.76) ... Revisited 2020

I just signed up for Xfinity last Tuesday and puschased my own modem (Motorola MG7540). I've been getting DNS server errors for the last three days now. I've tried riunning IPCONFIG to flush the DNS, nosing around on the configuration manager of my router, and turing it on/off multiple times, but nothing has worked thus far.

 

I saw on the configuration manager that there was an option to customize the IPv4 DNS server addresses. The current ones are the infamous 75.75.75.75 and 75.75.76.76. I'm VERY much interested in if you have found a solution to this. I happen to like my router; it's got a lot of nice bells and whistles to play with!

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Silver Problem Solver

Re: Comcast DNS servers are dropping constantly (75.75.75.75 and 75.75.76.76) ... Revisited 2020

If you do a tracert to the IP's, it will give a clue as to which DNS server you are actually connecting to. The next-to-last hop is the actual server.

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Regular Contributor

Re: Comcast DNS servers are dropping constantly (75.75.75.75 and 75.75.76.76) ... Revisited 2020

How do you know this?   Traditionally traceroute /tracert is used to determine the *routers* between two endpoints, thus the next-to-last name/address reported would be a router not a server.   My years of experience in the industry has demonstrated this to always be the case even for virtualized services; I have *never* seen the host system of a virtualized service respond in a traceroute; it is always the end system.

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Regular Contributor

Re: Comcast DNS servers are dropping constantly (75.75.75.75 and 75.75.76.76) ... Revisited 2020

I can not tell you why this is the case but DNS service seems to be particularly sensitive to a bad cable connection.   You need to inspect your modem for cable problems.   Look at the power levels and signal to noise ratios (SNR) for all channels.    Look at both upstream and downstream and check the correctable and uncorrectable error counters.    You will not have error counters or SNRs for upstream channels but you should see your power levels.  Odds are you have a problem on the cable.     Most likely you will see SNR ratios are out of spec.

 

If you do find this to be the case you should then check your line in the house for poor quality splitters or extenders.   Eliminate/replace them as you can.   Also be aware that the cable has a bend radius limit and if you have exceeded it you could be causing problems on the line.   This page lists a minimum bend radius of 2.5" for RG6 cable which means a loop of cable needs to be at least 5" across; 6" would be better.

 

If your home is clean then it is time to call for a line check.   I always have the technician start at the cable box and measure from the box to the CMTS.   100% of my problems have been between the box and the CMTS.

 

Good luck!

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Gold Problem Solver

Re: Comcast DNS servers are dropping constantly (75.75.75.75 and 75.75.76.76) ... Revisited 2020


@ewbuck1 wrote: ... I've been getting DNS server errors ...

You can certainly try other DNS servers, but be aware that devices often report a "DNS Error" when they are actually having trouble reaching the Internet at all. This is because when you click a link the first thing the device needs to do is look up the IP address of the site using DNS. Typically as the page loads this needs to be repeated many times to load all of the page components. If these lookups fail because of a connection problem the device won't be able to access DNS and may incorrectly announce a "DNS Error".

 

If you want to troubleshoot this yourself, please see Connection Troubleshooting Tips. If you still need help, please post your modem or gateway's signal information, especially downstream power levels and SNR, upstream power levels, and error log. There are usually found at http://10.0.0.1 or http://192.168.100.1.

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Gold Problem Solver

Re: Comcast DNS servers are dropping constantly (75.75.75.75 and 75.75.76.76) ... Revisited 2020


@cxrider wrote: ... the next-to-last name/address reported would be a router not a server ...

Perhaps it would be more accurate to say that the next-to-last IP indicates which server you are reaching. I do not know exactly how Comcast has this set up.

 

I do know that Comcast VP Jason Livingood once asked me to run a traceroute to 75.75.75.75 and post it so he could see which server I was having problems with.

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Expert

Re: Comcast DNS servers are dropping constantly (75.75.75.75 and 75.75.76.76) ... Revisited 2020


@BruceW wrote:

but be aware that devices often report a "DNS Error" when they are actually having trouble reaching the Internet at all.


Very true @BruceW  Many operating systems generate a *generic* DNS error when the problem is actually a line / connection quality impairment.



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Silver Problem Solver

Re: Comcast DNS servers are dropping constantly (75.75.75.75 and 75.75.76.76) ... Revisited 2020

But each of the addresses is routed to a unique path that is local to you. The final address will ALWAYS be cdns01.comcast.net and cdns02.comcast.net. The hop before shows the router that the actual server is connected to.

This is what I see in the Chicago area:

c:\downloads>tracert 75.75.75.75

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

  1    <1 ms    <1 ms    <1 ms  RT-AC68U-F288 [192.168.1.1]
  2     9 ms     9 ms     9 ms  96.120.25.213
  3     9 ms     8 ms     9 ms  68.86.185.21
  4    11 ms    12 ms    11 ms  be-124-ar01.area4.il.chicago.comcast.net [162.151.45.109]
  5    12 ms    12 ms    11 ms  ae100-ur02-d.area4.il.chicago.comcast.net [68.87.210.6]
  6    11 ms    11 ms    11 ms  dns-sw01.area4.il.chicago.comcast.net [68.86.188.70]
  7    11 ms    11 ms    10 ms  cdns01.comcast.net [75.75.75.75]

Trace complete.
c:\downloads>tracert 75.75.76.76

Tracing route to cdns02.comcast.net [75.75.76.76]
over a maximum of 30 hops:

  1    <1 ms    <1 ms    <1 ms  RT-AC68U-F288 [192.168.1.1]
  2     9 ms     8 ms     7 ms  96.120.25.213
  3     9 ms     8 ms     8 ms  68.86.185.21
  4    12 ms    12 ms    11 ms  be-124-ar01.area4.il.chicago.comcast.net [162.151.45.109]
  5    12 ms    12 ms    12 ms  be-32221-cs02.350ecermak.il.ibone.comcast.net [96.110.40.53]
  6    13 ms    13 ms    13 ms  be-1202-cr02.350ecermak.il.ibone.comcast.net [96.110.36.102]
  7    13 ms    14 ms    12 ms  ae-0-0-ar02-d.northlake.il.ndcchgo.comcast.net [68.86.87.70]
  8    13 ms    12 ms    13 ms  et-6-3-1-as02-d.northlake.il.ndcchgo.comcast.net [96.108.162.94]
  9    13 ms    13 ms    13 ms  et-3-12-1-ss03a-d.northlake.il.ndcchgo.comcast.net [96.108.162.82]
 10    13 ms    13 ms    12 ms  dns-sw01.northlake.il.ndcchgo.comcast.net [162.151.211.82]
 11    13 ms    13 ms    12 ms  cdns02.comcast.net [75.75.76.76]

Trace complete.

 

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Gold Problem Solver

Re: Comcast DNS servers are dropping constantly (75.75.75.75 and 75.75.76.76) ... Revisited 2020


@andyross wrote: But each of the addresses is routed to a unique path that is local to you. The final address will ALWAYS be cdns01.comcast.net and cdns02.comcast.net. The hop before shows the router that the actual server is connected to.

Similar for me in South-Central PA:

 

C>tracert -4 75.75.75.75
Tracing route to cdns01.comcast.net [75.75.75.75] over a maximum of 30 hops:
 ...
6 24 ms 24 ms 25 ms 162.151.21.114 7 24 ms 23 ms 24 ms dns-sw02.summitpark.pa.pitt.comcast.net [69.139.167.234] 8 * 23 ms 23 ms cdns01.comcast.net [75.75.75.75] Trace complete. C>tracert -4 75.75.76.76 Tracing route to cdns02.comcast.net [75.75.76.76] over a maximum of 30 hops: ...
14 28 ms 27 ms 25 ms eth-2-4-ss01a-d.ashburn.va.ndcasbn.comcast.net [162.151.66.146] 15 27 ms 28 ms 27 ms dns-sw01.ashburn.va.ndcasbn.comcast.net [162.151.198.50] 16 35 ms 25 ms 25 ms cdns02.comcast.net [75.75.76.76] Trace complete.

 

 

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Expert

Re: Comcast DNS servers are dropping constantly (75.75.75.75 and 75.75.76.76) ... Revisited 2020


@cxrider wrote:

How do you know this?   .


FWIW, the 75.xx.xx DNS IP addresses are *anycast* addresses. You can Google for more info.



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I am a Customer Expert volunteering my time to help other customers here in the Forums.
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