NetgearFVS and Comcast SMC - DNS forwarding

I recently fixed an incompatibility between a Netgear router and a Comcast modem. My question is in regards to DNS forwarding. Did the protocol for DNS forwarding change? Or did Comcast recently change the way their business modems handle DNS forwards from internal networks?

The network in question is using a Netgear FVS model router bridged to a Comcast SMC business gateway.

The Netgear also handles DHCP on the LAN. For a year or more the Netgear ran "DNS Proxy" for its local DHCP clients. In other words, the router handed-out  its own address for DNS.

It worked for months. In the last few days, somewhere between the Netgear and the SMC I found that DNS forwarding was dying and not recovering.

I applied a minor firmware version upgrade to the Netgear with no change in behavior. Comcast told me that their device had a good status with no pending updates. I still couldn't count on DNS proxy (aka forwarding).  

The Netgear actually has a DHCP log, but not a DNS log. (These devices offer emailed logging, which didn't produce enough details for me in the past).

I kiled the DNS Proxy and everything works. Fortunately, there are few local resources on the LAN, and they can all be addressed by IP address instead of name resolution. The DNS Proxy isn't essential.

Summary: Now the Netgear provides DHCP clients with the public DNS server 75.75.75.75 and everything works.

Question: Why did DNS Proxy fail after many months of service?
Thanks!
kengregAsked:
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DarinTCHSenior CyberSecurity EngineerCommented:
so yes probably COMCAST
no DNS protocol has not changed
allow there is a newer DNSv6
comcast has changed there modems and are even contacting customers and offering a free modem upgrade
the new modems have a wireless router builtin - and many 'PRESETS"
you can turn off - into bridge mode with their help ONLY
I had several similar small issues - but we worked thru most
i set up the DHCP to provide DNS with an internal DNS and their external DNS
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kengregAuthor Commented:
DarinTCH,
Thanks for the comment!
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kengregAuthor Commented:
Here's a follow-up.

After four weeks DNS stopped (again) out of the blue. Okay, that's odd. The new settings should have been foolproof. I was explicitly assigning 75.75.75.75 and 75.75.76.76 to the DHCP clients.

The Netgear FVS was/is still on the same (latest) firmware.

The Comcast SMC modem was/is still on the same (latest) firmware:

Vendor Name       SMC Networks
Hardware Version       1.01
Serial Number       XXXXXXXXXXXX
Firmware Version       3.1.6.56
Operating Mode       RG

I spoke to a Comcast rep and the modem was easily accessible with a good status and proper DNS settings. It wasn't their issue. I agreed.

So, I was curious to know if the DNS Proxy on the Netgear would start working again. Yes, it did.

Changes to Netgear router:
1. I checked  box for DNS Proxy.
2. I pointed the Netgear's DNS to the LAN IP of the SMC.

This worked. DNS resolution is fast.

DNS requests on the LAN are forwarding twice, first to the Netgear, and second to the SMC. It's not my preferred solution, but it's working.

I'll be monitoring for any problems.
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