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Router MAC address changing?
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Thread: Router MAC address changing?

  1. #1
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    Router MAC address changing?

    I've been struggling for some time with a router problem. When I (or the PC) try to connect, it fails with the error message that it cannot find the router 192.168.1.1 on subnet 192.168.1.0. Since it has the IP address, it obviously did find it! The cure is to run ifdown, switch the router off, switch it on again, and repeat ifup. But why? The router is a British Telecom 210, and I found someone who had had problems with another model in the same range. In his case it was using two MACs, as revealed by using arping. I only get one MAC from arping, but it is different to the one returned by ifconfig. The one found by arping turns out to be the one printed on the bottom of the router, so why does the initial probe get the other one? Can anyone shed some light on all this?

  2. #2
    Administrator Advisor peter's Avatar
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    You probably have another device / router on the network with the same IP address. Is this a new router? Is the old one turned off?

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  3. #3
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    The "network" is just two items, the PC and the router. As I said, it's one and the same router which is returning two MACs and confusing my poor PC, not to mention me.

  4. #4
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    After a close read of the current kernel changelog I've stumpled uppon something which might be causing your problems:
    Code:
    commit 5eb4efe242d2fbd36551f6016707ee280bb30f31
    Author: Stephen Hemminger <shemminger@linux-foundation.org>
    Date:   Wed Mar 5 14:44:01 2008 -0800
    
        IPCONFIG: The kernel gets no IP from some DHCP servers
        Upstream commit: dea75bdfa57f75a7a7ec2961ec28db506c18e5db
        
        From: Stephen Hemminger <shemminger@linux-foundation.org>
        
        Based upon a patch by Marcel Wappler:
        
           This patch fixes a DHCP issue of the kernel: some DHCP servers
           (i.e.  in the Linksys WRT54Gv5) are very strict about the contents
           of the DHCPDISCOVER packet they receive from clients.
        
           Table 5 in RFC2131 page 36 requests the fields 'ciaddr' and
           'siaddr' MUST be set to '0'.  These DHCP servers ignore Linux
           kernel's DHCP discovery packets with these two fields set to
           '255.255.255.255' (in contrast to popular DHCP clients, such as
           'dhclient' or 'udhcpc').  This leads to a not booting system.
        
        Signed-off-by: David S. Miller <davem@davemloft.net>
        Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>
        Signed-off-by: Chris Wright <chrisw@sous-sol.org>
    Don't worry Ma'am. We're university students, - We know what We're doing.
    'Ruiat coelum, fiat voluntas tua.'
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  5. #5
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    Thanks for the reply. I don't pretend to understand it, but it seems to mean that all will be well with Fedora 9.

  6. #6
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    A litle description to give hindsight into what is going on.

    When your machine request an IP, it looks something like this:
    • Send a DHCP-discover
      • requesting Where is my DHCP-server and can I keep my last known IP
    • Recieves a DHCP-offer from the DHCP-server
      • I'm your DHCP-server, yes you may keep your old IP or No another on this subnet owns your IP, but use this instead
    • Send a DHCP-request
      • You offered this IP, bee it the previus known or a new one, I'm using it if it's OK with you
    • Recieves a DHCP-acknowledge from DHCP-server
      • I see your request to use the IP, it's fine with me
    • Your mahine says, lets use that then...
    Basicaly what is happening in previus kernels befor 2.6.24.4, the DHCP-discover, the package holding the information where's the DHCP-server and wether your machine can use its old IP or use a new one, has info of a ciaddr (Caller subnet) and siaddr (Sender subnet), which according to the standard should hold info regarding broadcast on any subnet (ciaddr = 255.255.255.255 and siaddr = 255.255.255.255), but some DHCP-servers will only reply with usefull info regarding them beeing the controler, if these are set to 0.0.0.0

    So from kernel 2.6.24.4 and onward there has been a change, so the packages sent will be what is described in Technical specifications of the DHCP-ACK.

    (notice Src set to 0.0.0.0 in both DHCP-DISCOVER and DHCP-REQUEST)

    So a quick fix for you, would be to get the 2.6.24.4 vanilla (configuration file should be located as /boot/config) and compile it, else you'll have to wait till FC comes with an update.
    Don't worry Ma'am. We're university students, - We know what We're doing.
    'Ruiat coelum, fiat voluntas tua.'
    Datalogi - en livsstil; Intet liv, ingen stil.

  7. #7
    Advisor Outlaw's Avatar
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    If it truly is a problem with getting an address through DHCP, I would think the quick fix is assigning your own address.

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