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New Visitor
Posts: 1
Registered: ‎02-22-2006

Dynamic DNS and router

Just received notice that Comcast is going to dynamic DNS servers. I think this means that the DNS address will change from time to time. I have a hardware firewall/router with couple PCs behind it. The router does DHCP with Comcast, but the PCs are manually assigned private addresses and are manually told what the DNS address is. How will the new dynamic DNS scheme goof up my setup? How do I get around it?
Email Expert
Posts: 18,241
Registered: ‎04-27-2004

Re: Dynamic DNS and router

Most routers will automatically redirect DNS. Configure your PCs to use the router as their DNS server, and the router will forward to the DNS servers it got from Comcast via DHCP.

This is how it has been working for years in most Comcast markets.
Bronze Star Contributor
Posts: 102
Registered: ‎09-15-2003

Re: Dynamic DNS and router

Or possibly use public domain DNS servers (i.e 4.2.2.1 through 4.2.2.6) which I find more reliable than Comcasts, if you want to maintain your static configuration.
Email Expert
Posts: 18,241
Registered: ‎04-27-2004

Re: Dynamic DNS and router

Those aren't public DNS servers, they belong to another ISP (Level 3) and are for their customers' use (Verizon Online is one of their customers, so they're used by Verizon DSL customers).

It's common to use them temporarily during DNS outages, but it's inappropriate to recommend that anyone switch to them permanently -- you're advocating theft of services.
Contributor
Posts: 7
Registered: ‎09-22-2004

Re: Dynamic DNS and router

Good info on the class A public DNS's. Didn't know about that.

Okay, so it's easy to have all your internal LAN computers have DNS point to the gateway router. But the router (linksys in my case) requires at least one DNS IP. There is no setup for dynamic DNS. I too use static IP's for all my boxes (except wireless laptops). So how do we setup the router for the dynamic DNS?

Then again, those public DNS IP's are probably the best answer?
New Visitor
Posts: 1
Registered: ‎04-02-2004

Re: Dynamic DNS and router

Barmer has it right--at least for me (using a Linksys Model RT31P2 Router.) But I get your point about there being no setup option to specify dynamic DNS. There is no such option on my router either--just fields for three static DNS IP Addresses and one WINS address. Here's what I'd suggest you try:

1) Router's "Internet Connection Type: Obtain an IP Automatically" I assume that you've always had it set this way unless you've bought a static IP Address from Comcast. This should cause the router to get an IP Address for itself AND IP Addresses for DNS purposes from Comcast's DHCP servers.

2) Router's Local DHCP Server: Enable This tells your router to allocated IP Addresses as requested. Again, many people are already doing this. Just as Comcast's DHCP server gives your router an IP and DNS IPs, turning on this feature should allow your router to do likewise for devices on your private network.

3) Specify 0.0.0.0 for each of the three static DNS IP Addresses (assuming your router has foelds for three.) Before getting Comcast's email the other day, I had static DNS IP Addresses in these fields.

Save your changes and exit the administration page. Power-down your cable modem and router for 2-3 minutes then power up the modem. Once you've got a green light for "Online," power-up the router. Once you're donw with that, follow Comcast's directions for DHCP config for your O.S. Finally, to prove to yourself that all is working well, you might do the following:

1) Open a command window.
2) Type IPCONFIG /RELEASE This fill flush all cached networking information.
3) Type IPCONFIG /ALL You should note that you have no IP Address (or any IPs for DNS.)
4) Type IPCONFIG /RENEW This should cause windows to ask the DHCP server (your router) for an IP and for DNS IPs.
5) Type IPCONFIG /ALL Now you should have an IP and DNS IPs.

Failing to flush cached configuration state after making changes might cause it to appear not to work (that's how it looked to me last night,) but following the steps I outlined above worked perfectly for me.

What gets me is that Comcast is so petty as to provide no meaningful help on this subject. I got one of their support people in an online chat last night and he basically said "if Comcast didn't setup your network, we won't help you." What a joke. Considering what we pay for the service you'd think that they could afford to pay some third world helpdesk mope to write a general FAQ, but NO... Anyway, hope this helps.
Most Valued Poster
rickmac
Posts: 1,419
Registered: ‎12-09-2004

Re: Dynamic DNS and router

With my D-Link router, I can tell it to assign (map) certain IP addresses to certain MAC addresses. That way my DNS and IP can be set to "automatic" in Windows, but I still know what IP each computer is using when hooked up to my network.

Maybe you can do the same with your Linksys. :smileyhappy:
"Neo, you must understand, there's a difference between knowing the path and walking the path." Morpheus - The Matrix
Contributor
Posts: 7
Registered: ‎09-22-2004

Re: Dynamic DNS and router

I can't tell you how many times I've echoed the exact same feelings right out of my own mouth! The one's you refer to in the last paragraph - he he. Very true. Comcast is the best deal in my area right now. With limited service, I won't shed a tear when I find a better option to connect with the Internet! Kinda the same thing with oil companies. It's great they make money, but I won't cry when we finally move to alternative fuels. I digress.

Yeah, I have the same router (from Vonage). I have mine configured exactly as you stated, however, I limited the scope of its DHCP as an extra layer of security.

I'll try that 0.0.0.0 address scheme in all three DNS fields later tonight and see if that works. Otherwise, hopefully Comcast will give us legitimate address's for their DNS servers. Would that be asking too much?

Also, don't forget the IPCONFIG /FLUSHDNS and /DISPLAYDNS commands. They are useful from time to time.

Cheers.
New Visitor
Posts: 1
Registered: ‎11-07-2004

Re: Dynamic DNS and router

Similar situation to everyone else - Windows XP with assigned IP addresses on my Linksys WRT54G network. The router has always used the Automatic DHCP setting. Tried switching a computer to "automatic DNS" and that isn't allowed with manual IP addresses.

My solution was to log onto the router. The last page (status tab) shows the IP and DNS addresses passed from Comcast. Set these manually instead of the old numbers in WinXP and it seems to work fine. Denver DNS numbers are:

68.87.85.98
68.87.69.146

If these quit working I can certainly check the router again.
Still will try the 0 0 0 0 trick, though. Thanks!
Networking Expert
kevj
Posts: 4,818
Registered: ‎10-03-2003

Re: Dynamic DNS and router

As BArmar has previously noted, almost all 'home networking' routers use a technique called 'DNS masq' which allows them to forward DNS requests to a valid internet DNS server, which (in our case) Comcast provides during the DHCP assignment of your router's public IP address.

The undeniably simplest way to provide yourself proper DNS service if you use a router and static IPs on your internal network, is to assign the same IP for your DNS as is assigned for your default gateway (i.e. the LAN IP of your router).

Comcast will usually assigne more than one DNS IP during DHCP negotiation with your router, and the likelihood that both those IPs will be down at the same time is probably pretty low.

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New Visitor
Posts: 3
Registered: ‎02-25-2004

Re: Dynamic DNS and router

Linksys Answer ID 534, "How do I setup a static IP address that will work with my router?" , indeed shows using the Gateway IP as the only IP address in the DNS Server section to be the proper setup. I tried it on my WRT54GS, with a static local IP address of course, and it worked fine; no problems.
New Visitor
Posts: 3
Registered: ‎07-13-2004

Re: Dynamic DNS and router

Anon569468 - Thank you for your great instruction. I've been banging my head for a week now. I did get an error when I did "ipconfig /renew". The error was, "An error occurred while renewing interface Local Area Connection : Acess Denied". But, everything else work. My local area connection does not disconnect every 30 seconds anymore. The event viewer on XP explained that the lease address has been denied. But, with your method everything seems well. I test the new connection bu downloading a large linux image file. The connection is still going as we speak. So, again. Thanks.