DHCP server and Mac OSX clients
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  1. #1
    lavigj Guest

    DHCP server and Mac OSX clients

    On our university network, we recently deployed a Fedora Core 4 based DHCP server for the student segment. We have run into a problem though with Mac clients running OSX - they don't receive an address via DHCP. Linux and Windows clients can receive a DHCP address just fine from this server, the issue is only with Macs.

    Here's our DHCP version info:
    DHCP Version information: 
    Installed Packages
    Name   : dhcp
    Arch   : i386
    Version: 3.0.2
    Release: 14.FC4
    Size   : 1.3 M
    Repo   : installed
    Summary: A DHCP (Dynamic Host Configuration Protocol) server and relay agent.
     DHCP (Dynamic Host Configuration Protocol) is a protocol which allows
    individual devices on an IP network to get their own network
    configuration information (IP address, subnetmask, broadcast address,
    etc.) from a DHCP server. The overall purpose of DHCP is to make it
    easier to administer a large network. The dhcp package includes the
    DHCP server and a DHCP relay agent.
    You should install dhcp if you want to set up a DHCP server on your
    network. You must also install the dhclient package, which provides the
    DHCP client, on machines that will obtain their IP address via DHCP.

    What is puzzling is the behavior of the client.

    When on the linux based network, it looks something like this:
    [mac client] DHCP DISCOVER
    [linux server] DHCP OFFER
    [mc] nothing

    When on the windows based network, it looks something like:
    [ls] DHCP ACK
    [mc] receives an IP via dhcp

    It is possible I just missed the Discover/Offer sequence on the windows network, but I tried the packet capture a coupe times and just didn't see it. It is possible that it happens during the boot sequence, but I am not sure.

    Has anyone experienced similar problems with Macs/DHCP before? Any input would be greatly appreciated.

  2. #2
    fsck Guest
    I know nothing about the Mac, but I do know BSD which is nearly the same thing. First thing, the dhcp sequence is like this (simplified):

    Client: Do I have an outstanding DHCP lease to renew? If yes, send DHCP-REQUEST together with old lease.
    Server: DHCP server will ACK or NACK depending on availability of requested least (i.e. wrong IP range will get NACK)
    Client: If NACK, or If no lease to renew, send DHCP-DISCOVER
    Server: If allowed, send DHCP-OFFER, else NACK/IGNORE depending on config
    Client: Compare OFFER with required settings and apply to interface if correct

    So the windows machine already has a lease, asks to renew using REQUEST and gets ACK.
    The Mac has no existing workable lease, so broadcasts with DISCOVER, gets an OFFER and nothing happens.
    My guess would be that the Mac "require"s some DHCP options the server is not providing. Depending on the DHCP client, you need to find "dhclient.conf" and check the "require" line. Mine looks like this:
    request subnet-mask, broadcast-address, time-offset, routers,
            domain-name, domain-name-servers, host-name;
    require subnet-mask, domain-name-servers, routers;
    There are some other options in there, but I don't think they concern us here..
    If you use dhcpcd instead, I think it's all on the command line, man dhcpcd will tell you what the options mean.

    Then you need to either change the DHCP server to provide the required options (probably best) or change the clients so they no longer require things the server does not provide.

    Any of that make sense? I hope so!


  3. #3
    lavigj Guest
    That makes sense. Thanks for the clarification on the DHCP sequence too. I was close, but no cigar.

    I could not find the file dhclient on a mac running OSX 10.4.2.

    This problem does appear to be limited to OSX 10.4.3 because a G5 running 10.4.2 was able to connect to the same* DHCP server just fine. I am going to try to get a 10.4.3 to connect in the same way.

    *Note: I say the same version of the DHCP server, because it is the same version of the dhcp server, and the same dhcpd.conf, just put on my laptop for portable testing. In this case, it is more practical to take the server and a hub to the afflicted computers, than vice versa.

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