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Set IPv6 DNS Manual

New Member

Set IPv6 DNS Manual

I am using Comcast with my small business, I have a new cable modem (Model: DPC3941B) which supports IPv6. I am using DHCP on the modem. I have configured the DNS server settings in DHCP for IPv4 to point at my servers. I have an AD domain and need clients to talk to my DNS servers.

I would like to do the same with IPv6. My servers have "obtain IP automatically" configured for IPv6. So they have an IPv6 from the cable modem. How do I now configure the DNS server settings in IPv6 DHCP to hand out my DNS servers?



New Member

Re: Set IPv6 DNS Manual

I also have this same question. I got nowhere trying to communicate this issue with Comcast chat support this morning.


Re: Set IPv6 DNS Manual

I have a similar problem. We're using a Peplink Balance router, a dual-WAN router that NAT-balances the office traffic between Comcast and AT&T (for IPv4 only). It also bridges IPv6 to the Comcast modem. The Balance router is also a proxy DNS server, and one if its functions is to substitute lookups for local hosts on the LAN to their local LAN address.


We have a host on our LAN that has a port forwarding through the router, so it can be accessed from both the LAN and externally. Obviously these IP address are different. The LAN address is, and the external address is one of our Comcast static IP addresses, that gets tunnelled through our router.


When a host on our LAN looks up this host, it uses the DNS server it gets from DHCP, which is our router, It will then get the LAN address of this host. When that laptop goes home to a different ISP, a lookup gives the external address. So far, so good.


The trouble is that Mac OS systems on our LAN are somehow getting the Comcast IPv6 DNS addresses (2001:558:feed::1 and 2001::558::feed::2). I assumed the only way  they could be getting those address is via DHCP6, so I made sure that is turned off on the Comcast modem. However, they still get those addresses. How are they getting them, and how do I make it stop?


I still want the Mac OS systems to do IPv6 routing (through the Comcast modem), I just don't want them to use the Comcast DNS servers.

New Member

Re: Set IPv6 DNS Manual

Hi StarNet,


You need to login to your Comcast modem.  Navigate to Gateway->Connection->Local IP Network.


Check the box for Assign DNS Manually

then enter your DNS Settings.
Occasional Visitor

Re: Set IPv6 DNS Manual

I have the same issue as the reporter--checking the Assign DNS Manually box seems to have no effect on the router advertisements.

Re: Set IPv6 DNS Manual

We recently got a new modem, a Cisco DPC3941B.

Workstations on our LAN need to use our local DNS server, because it resolves inside hostnames that are only on the LAN. This works for IPv4, because the router (a Peplink Balance) provides IPv4 DHCP service, while it bridges IPv6.


Using Wireshark, I found that the Comcast Cisco modem is ICMPv6 router advertisement packets (as expected), that include ICMPv6 option 25 (DNS Server) as 2001:558:feed::1 and 2001:558:feed::2. I want to change this to the Peplink's IPv6 address.

I logged into the modem and ended up on the "Local IP Configuration" page at

I went to the IPv6 section, checked "Assign DNS Manually", and entered a single IPv6 address for the local Peplink router, and clicked "Save Settings".


However, when I go to the "LAN Status" page at it still shows the IPV6 DNS as 2001:558:feed::1. It also is still sending this in the router advertisiment packets.


Do I need to reset the gateway?

Forum Contributor

Re: Set IPv6 DNS Manual

There are only 2 ways you can run a local nameserver on both IPv4 and IPv6


1) If you DO NOT have static IP addresses then return the Comcast modem and buy a decent cable modem that allows you to do this.  Not only will the interface be kinder to animals and plants you will save $15 a month in rental fees


2) If you DO have static IPs then run pfSense or a Unix box with 2 interfaces or a router with dd-wrt or open-wrt loaded on it.

You won't be successful trying to get the COmcast router to manage this.

Re: Set IPv6 DNS Manual

I have a local nameserver.

I need to get the Comcast Cisco modem to send the IPv6 address of this local nameserver in its DHCP6 and router advertisements.

The Comcast router always sends 2001:558:feed::1 and 2001:558:feed:2. I logged into the modem, checked "Assign DNS Manually", entered the address for my local nameserver, clicked "Save", and it appeared to save it. However, when I go to the "LAN Status" page, it still shows 2001:558:feed::1. How can I change this?

Forum Contributor

Re: Set IPv6 DNS Manual

You can't change that.  The firmware screen gives you the option to change it because the software developers who wrote the firmware thought that would be a useful option to put in.  Comcast doesen't think so and overrides all of those settings with a bootfile, they essentially treat that box as a place to display what they are sending the modem.


Keep in mind that IPv6 DHCP-PD is broken on the Cisco modem and we have been trying to get Comcast to get Cisco to fix this for 3-4 years now with no satisfaction.  I don't know if this is a Comcast foot-drag or a Cisco foot-drag but clearly, whoever is in charge of writing the firmware for the modem isn't doing their job.   It is quite likely that Comcast is overriding this local DNS setting because of some other worse bug.