Dynamically configuring DNS information in a DHCP pool on Cisco Router 851

Unanswered Question
Jun 3rd, 2007

Hi,

I just purchase my first Cisco router 851 and trying to assign automatically the DNS server information my WAN interface received from my provider to a DHCP pool on the router.

So far, I was only able to set manually the DNS info in the DHCP pool. I would like the pool to be updated automatically if my provider decide to make change to its DNS.

Any idea?

Nam

I have this problem too.
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mohammedmahmoud Sun, 06/03/2007 - 13:48

Hi,

I think that i've once got through this command "ppp ipcp dns request", please try it and feed us back.

[edit] This will help that the local DNS on the router is updated according to the provider, and you can further use DNS spoofing to solve your clients part (or just enable DNS service on your router and make your router the DNS of the DHCP pool):

http://www.cisco.com/en/US/products/sw/iosswrel/ps5207/products_feature_guide09186a00801a7605.html

HTH, please do rate all helpful replies,

Mohammed Mahmoud.

Richard Burts Sun, 06/03/2007 - 14:01

I believe that I have done this using the "import all" command. This is entered in DHCP configuration mode (after you define the pool name where you enter the network address to use you would also enter import all).

Give it a try and let us know if it works for you.

HTH

Rick

nleviet007 Sun, 06/03/2007 - 18:07

Rick,

Thank you for your reply. I actually have the import all command in my startup file but it seems it does not work import anything. I have 12.4(4)6.

My WAN interface is configured through DHCP parameters givin by my provider. I was hoping that "import all" would import all the parameters provided. It does not... perhaps a bug??

Thanks,

Nam

nleviet007 Sun, 06/03/2007 - 18:10

Mohammed,

I don't use ppp. the router is connected directly to a broadband cable modem.

Nam

mohammedmahmoud Sun, 06/03/2007 - 23:17

Hi,

We are only trying to help here :)

I guess that your "import all" problem is described here:

"This option requires that a central DHCP server be configured to provide the DHCP options. The central DHCP server should be on the same subnet as was configured using the network option. (On Cisco IOS routers, this is done using the ip dhcp database command.) If you are using the PPP/IPCP protocol on the WAN interface, or if the client on the WAN interface supports the Easy IP feature, then the central DHCP server can be on a different subnet or network."

HTH,

Mohammed Mahmoud.

nleviet007 Mon, 06/04/2007 - 10:28

Hi Mohammed,

Thank for your reply. I am not sure where my Provider's DHCP server is. What is the command to see the DHCP options assigned to the WAN interface?

I would assume that there is no addittional conversation between the router and the DHCP server beside ARP and RARP to initialize the WAN interface. The WAN interface should get all the options during the initialization, DNS, gateway address, lease etc...

Is the "import all" imports the WAN assigned DHCP options into the router's DHCP server, or do you have additional conversation between the router and the DHCP server?

I am not sure why the DHCP server should be on the same network. As long as the DHCP options are properly assigned to the WAN interface, where the DHCP server is sholud not have impact.

Thank you,

Nam

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