Forwarding DNS queries to upstream ISP?

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Nov 11th, 2009
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The site in question has a 3620 running IOS 12.1. Its upstream ISP uses DHCP to transfer things like IP address and DNS server address(es), so the router will have its upstream Ethernet interface configured as a DHCP client.


The downstream LAN, connected to the other Ethernet interface, will use static /24 addressing (192.168.1.x) and NAT in the usual manner.


Question: How do we configure the router to forward DNS queries to the ISP's dynamically provided DNS server addresses? We don't necessarily want the router acting as a DNS server, or even a DNS cache. It just needs to transparently forward the queries and return the responses - like a $20 wireless router from Netgear will do.


I found the "dns forwarding" command but it is associated with DNS views which don't appear until IOS 12.4. Besides, this capability must have been around for a lot longer than 12.4. I just can't find how to do it in the IOS docs.


I'm happy to RTFM if someone will point me in the correct direction. Thanks!


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cpubob Thu, 11/12/2009 - 14:39
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ios DNS server became available in ios 12.3. If you would like to use that function, you must upgrade. the config would be something like this:


ip dns server

ip name-server x.x.x.x <-replace with your ISP's name servers. Without this, the requests won't be forwarded upstream.


If upgrading your router is not an option, then your left with two alternatives, statically configuring the isp's name servers into your hosts or setting up a local dns server.


HTH,


-rob

richardhartman Thu, 11/12/2009 - 16:42
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Upgrading is no problem, though I notice that the latest version available for the 3620 is 12.3.26 (i.e. no 12.4 versions). Looks like that's barely late enough to get the features you mention.


This leaves one unanswered question: What about the upstream interface running as a DHCP client (ip address dhcp)? I don't necessarily know the DNS IP's and therefore cannot statically include them in "ip name-server w.x.y.z" commands. They come in dynamically when the interface requests a DHCP lease. How do they get configured into the DNS server within IOS?


Thank you!

cpubob Thu, 11/12/2009 - 14:56
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ios DNS server became available in ios 12.3. If you would like to use that function, you must upgrade. the config would be something like this:


ip dns server

ip name-server x.x.x.x <-replace with your ISP's name servers. Without this, the requests won't be forwarded upstream.


If upgrading your router is not an option, then your left with two alternatives, statically configuring the isp's name servers into your hosts or setting up a local dns server.


HTH,


-rob

wsanders1 Thu, 11/12/2009 - 17:50
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A third possible option, if the ISP's DNS servers don't change and the 3620 supports it, is to use dynamic addressing and set up your clients to get their DNS parameters along with their IP address from the 3620.

richardhartman Thu, 11/12/2009 - 21:29
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I do believe it's possible to configure the DHCP server in IOS to transfer static DNS addresses, and I'll do that if I can confirm the ISP's DNS server addresses are consistent. But if they're not, I'll still need a way to convey those dynamic addresses to the downstream DHCP clients.


I can't be the first Cisco user to need this ability, so I'm hopeful someone will chime in and provide details.


Thanks!

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