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DNS on a Cisco 857

Discussion in 'Routing & Switching' started by Stoney, Dec 14, 2007.

  1. Stoney

    Stoney Megabyte Poster


    I'm just after a bit of clarification regarding setting up DNS on a Cisco ADSL router.

    Currently I have my ISP's DNS server addresses added to the DHCP pool, clients connect and get assigned the ISP DNS servers addresses and can resolve host names on the internet. This is fine.

    If I don't have the ISP server addresses added to the DHCP pool, clients can ping IP's on the internet, but can not resolve host names as one would expect.

    The bit I need a some clarification on is this;

    If I add private DNS addresses to the DHCP pool (instead of the ISP's) do the private DNS servers have to be responsible for forwarding DNS requests out on to the internet, or can the router still do this?

    What I would like is for the router to handle client DNS requests, with or without private DNS servers, and without adding the ISP's DNS addresses to the DHCP pool.

    I have been looking at the dns forwarding command but the IOS I have doesn't seem to support this. Is there another way to do this?

    Certifications: 25 + 50 metre front crawl
    WIP: MCSA - Exam 70-270
  2. Spice_Weasel

    Spice_Weasel Kilobyte Poster

    Depending on the version of IOS you have, and exactly what you want to do, there are several ways to do this. The easiest is to simply use the router as a forwarding dns server - internal clients would use the router as their dns server. The router in turn would forward the requests to another dns server and send the replies back to the clients.

    Configuration is easy:

    - Enable dns lookups with ip domain-lookup (or ip domain lookup, depending on version)
    - Define an upstream dns server: ip name-server <ip address>
    - Enable dns forwarding: ip dns server

    You might also wish to configure a local domain suffix, e.g. ip domain name mydomain.com

    That should get you started - I would recommend using one name server only if you encounter problems. Also, make sure dns replies can get back through your external access-list/firewall. The IOS dns server is clearly not as good as a proper dns server, but it will do for a cheap and cheerful dns forwarder. For example, the IOS dns server will not (as far as I know) act as a recursive dns server, the upstream dns server will have to handle that.

    If you have a recent IOS release it is possible to configure dns views, which offers more control, but for a small network just using ip dns server usually is adequate.

    Certifications: CCNA, CCNP, CCIP, JNCIA-ER, JNCIS-ER,MCP

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