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  1. #1
    Join Date
    Jul 2012
    Location
    Hungary
    Posts
    17

    Why sents 36 character MAC address from my laptop (Fedora22) to Windows DHCP Server?

    Hello!

    At our workplace we have a Windows DHCP server dolling out IPs. On the Windows server our admin can see 36 character MAC address from my laptop (Fedora 22 Gnome, ThinkPad T540p), with the last 12 being the actual MAC address. The other client sends their MAC address right except my machine.

    I have no any idea why?

    I've disabled ipv6 like this:

    ..]# echo "net.ipv6.conf.all.disable_ipv6 = 1" >> /etc/sysctl.conf
    ..]# sysctl -p

    but it's still the same address is sent to the Windows Server.

    Does someone have any idea what can i do to send right MAC address? Thank you in advance for any help you can provide.

  2. #2
    Join Date
    Jan 2015
    Location
    Al Ain, UAE
    Posts
    735

    Re: Why sents 36 character MAC address from my laptop (Fedora22) to Windows DHCP Serv

    DHCP is handled by a program called dhclient. Read the dhclient and tcpdump man pages to figure out what is wrong.
    --
    Have fun!
    http://www.aeronetworks.ca

  3. #3
    Join Date
    Jul 2012
    Location
    Hungary
    Posts
    17

    Re: Why sents 36 character MAC address from my laptop (Fedora22) to Windows DHCP Serv

    Dear flyingdutchman!

    What should i looking for about dhclient or tcpdump? I am not a linux expert.
    Could you help me what should i try?

    Our admin see looks like this:


    After the 753bb...4cce begins my real MAC address with 5... What is those numbers before of my MAC address and how can i send only my Mac Address to the DHCP server?

  4. #4
    Join Date
    Aug 2010
    Location
    Al Ain, UAE
    Posts
    2,007

    Re: Why sents 36 character MAC address from my laptop (Fedora22) to Windows DHCP Serv

    Well, this is not a Linux problem per se. It is a DHCP and DNS networking problem. If you can debug and fix it on Linux, then you can fix it only any type of machine.

    Do you have an IP address?
    # ifconfig

    Do you have a default route?
    # route

    Check whether the DNS works with ping or nslookup:
    # nslookup www.yahoo.com

    Do you get an address or a bunch of error messages?

    See what is going on with tcpdump:
    # tcpdump -nlX -i em0

    Do you have a DNS server configured?
    # cat /etc/resolve.conf

    and so on.

    Read the man pages for details.

  5. #5
    Join Date
    Jan 2008
    Posts
    2

    Re: Why sents 36 character MAC address from my laptop (Fedora22) to Windows DHCP Serv

    edfromhadria,

    As often happens in the Linux world, dhclient evolved to become more RFC compliant which can break software which is decidedly less so, e.g., your Microsoft DHCP service. You have two options:

    1) Modify your Windows DHCP server and set aside a DHCP reservation according to the overblown MAC size address that appears in the lease entry. The downside to this approach is that it requires you to have change access to this server and the fix is a bit of a kludgey workaround as the overblown address may change as it is a superset of the actual 6 octet hardware address of your host.

    2) The better option is to create (as root) an /etc/dhcp/dhclient.conf file (it probably won't exist in F21/22) on your Linux client with the following line:

    Code:
    send dhcp-client-identifier = hardware;
    This line was present in Fedora versions prior to F21 but was removed when dhclient became RFC 4361-compliant. To make it work with older DHCP servers, this explicit line will cause the exact MAC address to be issued to the DHCP server rather than the server extracting it from the chaddr field of the DHCP packet.

    Hope this works for you...and I hope you continue to stick with Fedora as it is a great operating system. If you are curious about this issue, you can find more info here.

    --Wayne

  6. #6
    Join Date
    Jul 2012
    Location
    Hungary
    Posts
    17

    Re: Why sents 36 character MAC address from my laptop (Fedora22) to Windows DHCP Serv

    Dear sheridan!

    I've just come to write that i find (here: https://bugzilla.redhat.com/show_bug.cgi?id=560361 ) the solution of this problem but I see you wrote the right answer too!

    Thank you so much, and my apologize for my late reaction.

    ed

  7. #7
    Join Date
    Nov 2007
    Posts
    19

    Re: Why sents 36 character MAC address from my laptop (Fedora22) to Windows DHCP Serv

    Dear sheridan!

    I have Fedora 26 with the same problem. I followed you answer and that solutioned the problem.

    Thank you !

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