Cisco DHCP scope and DNS server addressing

Same setup in a previous post, but different issue. My 2611 has eth0 set up as a DHCP client to my cable modem and it does receive the DNS server addresses that is is supposed to as evidenced by show hosts (and they're definitely not set static).

Eth1 is set up as a DHCP server for the LAN. It's handing out IPs and the default gateway information, but not the DNS server addresses.

I tried setting eth1's address as the DNS server in the scope, and eth0's, and I've tried leaving it as blank,...... but none will let my clients resolve names.

And I don't want to set the DNS server addresses that eth0 recieves from the cable modem into the LAN scope, since the ISP can change those DNS server addresses at any time.

Any suggestions?

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I don't think its possible to take the DNS servers received by the DHCP client, and rebroadcast them in the DHCP packets sent out by the DHCP server process on IOS.

BUT, its highly uncommon for an ISP to rotate DNS server IP addresses, those usually are pretty well set in stone for clients to use. Unless they have a fairly short lease time, they could have clients connected for sometime using the values they handed out some time ago.

If you are really concerned about the ISP really truely dropping DNS resolver servers constantly and don't want to hard-code them into the DHCP server process on IOS, use public DNS servers that are set-in-stone like OpenDNS's servers.

If anything, most of the really big ISP's anycast the same IPs for their DNS cache resolvers across their whole network such that the same DNS server IP address will work consistantly and network wide, even if that server is down, the next geographically closest in the cluster will anycast for that IP address if one goes down.

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Doug McIntyre

! ip dhcp pool whatever import all !

DHCP Server Options Import and Autoconfiguration

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