DHCP client receving address from one server and scope options from another

Discussion in 'Windows Networking' started by usenetposter2006, Aug 25, 2006.

  1. I am having trouble understanding something that is occuring on one of
    my client computers. I am in the process of bringing a new Windows
    2003 R2 DC online to replace an old Windows 2000 DC. Both servers are
    running DHCP (with non-overlapping scopes), DNS, WINS. There is
    another Windows 2000 server on the same network offering the same
    services. My previous setup is as follows:

    All DHCP servers have non-overlapping scopes and each scope provides
    scope options to point DHCP clients to DNS and WINS servers. When a
    DHCP server offers a client an address, the options that it sends
    always list the DHCP server as the primary server for DNS and WINS. So
    for example, Server A gives out an address and the DNS options for it
    would be Server A as the primary, and Server B as the secondary. This
    has always worked without any problems. With the addition of the new
    server I am seeing some very strange behaviour. In particular, one
    client is getting a DHCP address from Server C (the new server), but
    it's getting it's DNS scope options from Server B. The WINS scope
    options on the other hand are coming from Server C as they should be.

    If I release and renew the IP address on the affected client, it gets
    all of the proper settings. It gets all scope options from Server C.
    But, a few minutes later (5-15) when I check the client using IPCONFIG
    /ALL, the DNS options have changed to those offered by Server B.

    Does anyone have any insight into why this is happening, or any
    suggestions for how I would troubleshoot? There are no errors in the
    event logs on the clients or servers, and dns resolution on all servers
    seems to be working fine when checked with NSLOOKUP.

    usenetposter2006, Aug 25, 2006
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  2. usenetposter2006

    Bill Grant Guest

    When a client boots up, it broadcasts on the network to get its config
    from a DHCP server. The DHCP server replies with an offer.

    In addition, a client can send a DHCPINFORM message (after it gets its
    initial config) to receive additional information from a DHCP server. This
    info may come from a different source. As an example, a remote client might
    get its original config from a remote access server, then obtain additional
    LAN-based settings from the DHCP server on the LAN using DHCPINFORM.
    Bill Grant, Aug 26, 2006
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  3. usenetposter2006

    Troy F. Guest


    Thanks very much for your response, I should have thought of
    DHCPINFORM. In hopes of getting a clearer picture of what was
    happening, I used Ethereal to do a packet capture of the related
    traffic. First I released the address on the client, started the
    packet capture, did an ipconfig /renew, and then waited until the DNS
    scope options changed.

    Here is what I found:

    Client sent DHCPDISCOVER
    Client sent DHCPREQUEST
    Client sent Gratuitous ARP

    Client sent DHCPINFORM
    Server C sends ACK (Server with which the IP is registered)
    Client sent ARP reply to Server B (The server which overwrites DNS
    scope options)
    Server B sends ACK

    Client sent DHCPINFORM
    Server C sends ACK
    Server B sends ACK

    A couple things that I would like to understand, please chime in if
    anyone has any details.

    1. Why does a client send a DHCPINFORM request nearly a half hour
    after renewing the address?
    2. Why after receiving DHCPINFORM ACK notices from two servers, would
    the client send out another DHCPINFORM reuqest a few seconds later?
    3. When the client receives the secondary DHCPINFORM ACK from Server B
    which overwrites the DNS scope options, why does the order of the WINS
    options not change on the client as well?

    Troy F., Aug 28, 2006
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