IPV6
Internet Protocol Version 6
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DHCPv6 "prepends" DNS on router

Model:CGA4131COM
Vendor:Technicolor
Hardware Revision:2.1
Serial Number:290123500
Processor Speed:1503 MHz
DRAM Total Memory:1024 MB
DRAM Used Memory:465 MB
DRAM Available Memory:559 MB
Flash Total Memory:512 MB
Flash Used Memory:2 MB
Flash Available Memory:22 MB

Trying to eliminate the "helpful" insertion of IPv6 DNS servers at the top of every host's resolv stack.

 

Even though I manually specify IPv6 DNS in the router configuration, it "prepends" the Comcast placeholder /advertising DNS (2001:558:feed::1 and ::2), which overrides all my internal DNS, disclosing internal lookups and devices to the Public Internet.

 

Is it possible to turn this "feature" off?  I have internal DNSv6 running for static IPv6 (RFC 4193) configured LAN devices and would prefer to lookup local devices before forwarding to external systems to: preserve privacy (as much as practical), speed up resolution and control my own network as much as possible.

 

I have already disabled IPv4 DHCP (for the same reason) and using the router as my gateway /masquerade IP works fine, it's just the IPv6 side of things that is causing me grief at the moment.  I have no objection to using the router's DHCPv6, as long as it honors my DNS servers first.

 

If it doesn't (or can't), then I need to figure out how to "turn off" DHCPv6 so that setting up IPv6 on my internal DHCP won't cause conflicts.

 

Searching other threads and looking at the guides published doesn't provide any guidance in this area, so I'm hoping someone here can help me with this conundrum.

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Re: DHCPv6 "prepends" DNS on router

Fair number of eyeballs, but no answers so I girded my loins and braved the Comcast Voice Menu system to talk to a tech rep.

 

From what she discovered:

 

  • DHCPv6 cannot be turned off unless you subscribe to "Static" IPv6 ($18/mo for one device, $25/mo for five devices - not sure why IPv6 costs so much).
  • Unlike the IPv4 DHCP server, you cannot assign internal reservations for IPv6 (despite this being in the spec for private IPv6).
  • The Comcast IPv6 DNS servers will be prepended regardless of any other setting, static or dynamic.  Additionally, all IPv6 DNS lookups are "intercepted" by Comcast and redirected to their IPv6 DNS servers.
  • IPv6 cannot be disabled overall - even if devices have their IPv6 "turned off", the router will forward v4 to v6 and insert itself in the mix.

Not a great set of answers.

 

And on-phone the rep was "encouraging" me even further to purchase my own equipment if I want to fully leverage my paid-for services, even though it means loss of full Comcast Support.

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Community Manager
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Re: DHCPv6 "prepends" DNS on router

Hi there, thanks so much for bringing this to our attention, and we would love to look into this more or you. Can you please send a private message with your name, the full address, and phone/account number? 

 

Anisa 

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Re: DHCPv6 "prepends" DNS on router

 
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Re: DHCPv6 "prepends" DNS on router

Thanks so much for taking the time to reach out through our business forums with your DNS on IPV6 concerns. To fully address and confirm costs on the IPV6 statics and DNS I would need to further review the account. Can you please reach out through private message with your first and last name, full service address and account number or phone number and we will dig into this further? -Comcast_Michelle

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Re: DHCPv6 "prepends" DNS on router

Account info sent via PM...

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Re: DHCPv6 "prepends" DNS on router

As far as the DHCPv6 lease time, it shouldn't be releasing and renewing 7 times a day. There is a way to change the lease time by logging into the modem. You'll want to go to "Connection" then to the "Local IP Network" tab and ensure that the "Stateful" box is checked and then under "DHCPv6 Lease Time" you can change it. We definitely cannot make these changes since it may affect your network and as a heads up it won't change any settings in your routing devices. But that's a way to do it Smiley Happy I hope that useful and if you need anything else, please let us know! 

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Re: DHCPv6 "prepends" DNS on router

Lease time is set to "1 week" for DHCPv6 and "Stateful(Use DHCP Server)" selected (in fact, it cannot be "unselected"), but even with that I notice that all devices (not just mobile devices like tablets and phones) end up changing IPv6 address upwards of five times a day.  Is this perhaps a cosmetic setting only and is not used by the DHCPv6 back-end?

 

I'm also hoping you have had the chance to confirm the other points that the on-phone technician relayed, it would be good to know whether phone and web support are in-sync ...  and if there are discrepancies to get them resolved on both ends so that we (the customers) get consistant answers.

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Re: DHCPv6 "prepends" DNS on router

I do want to submit a request to our advance team to make sure that we get everything addressed for you. Can you confirm the best phone number to reach you? 

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Re: DHCPv6 "prepends" DNS on router


@evazquez00 wrote:

Fair number of eyeballs, but no answers so I girded my loins and braved the Comcast Voice Menu system to talk to a tech rep.

 

From what she discovered:

 

  • DHCPv6 cannot be turned off unless you subscribe to "Static" IPv6 ($18/mo for one device, $25/mo for five devices - not sure why IPv6 costs so much).
  • Unlike the IPv4 DHCP server, you cannot assign internal reservations for IPv6 (despite this being in the spec for private IPv6).
  • The Comcast IPv6 DNS servers will be prepended regardless of any other setting, static or dynamic.  Additionally, all IPv6 DNS lookups are "intercepted" by Comcast and redirected to their IPv6 DNS servers.
  • IPv6 cannot be disabled overall - even if devices have their IPv6 "turned off", the router will forward v4 to v6 and insert itself in the mix.

Not a great set of answers.

 

And on-phone the rep was "encouraging" me even further to purchase my own equipment if I want to fully leverage my paid-for services, even though it means loss of full Comcast Support.


Hi there, I can address some of these concerns. Here is additional info I've found:

 

First, I want to mention the DHCP lease time of 1 week is standard/default. If you are rebooting your devices, that would cause a new IP address to be assigned. That is possibly why you're getting 4 or 5 different addresses for your devices throughout the day. Additionally, if your modem is rebooted or drops connection (perhaps you are connected via wiFi), this will also assign a new IP upon new connection. To avoid this, you can purchase a static IP or block of static IPs to keep those IPs if the modem or devices are rebooted.

 

The "IPv6 stateless (autoconfig)" option cannot be disabled in your modem, so you'll need to disable IPV6 from each connected device manually that you don't want to be assigned an IPv6 address.

 

When LAN DHCP is disabled, the gateway won't assigned IPs to conencted devices as you know. Any connected device will have to be manually configured witha private IP matching the LAN IP scope of the gateway or a manually configured public static IP if a static IP block has been configured on the gateway.

 

In your modem settings, you can choose how the DNS is assigned. If the Assign DNS Manually option is checkjed, you assign the primary and secondary DNS servers that will be assigned to all devices connected tothe LAN DHCP server as a client. Default is unchecked which obtains DNS server settings from the Comcast DHCP server. All changes go into play after the modem reboots.

 

Select gateways can utilize passthrough mode to disable DHCP and your firewall for routing purposes. DHCPv6 isn't truly disabled though (as I said before), you'd need to disabled ipv6 from each connected device manually.

 

I do think your easiest (albeit more expensive) option is to purcahse a static IPv4/6 address but I know you were concerned about costs. With that said, I've reached out to tech ops to confirm the info you were given earlier.

 

Ken

 

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