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Broadcast flag on Windows Server 2012 R2 DHCP RRS feed

  • Question

  • Hi there!

    There's a problem with some devices not getting IP addresses from Windows DHCP. Take a look at the simplified network diagram below.

    All devices are connecting to a wireless network which lies in vlan specific for 10.0.1.0 network and they should get addresses from 10.0.1.0 pool.

    Windows phone sends a broadcast DHCP Discover, gets the offer, sends the request and gets ACK. It works OK.

    Same for Windows 10 laptop.

    Ubuntu linux however (same for MacOS on Macbook Pro) sends an unicast discover, which is properly recognized by Windows server, which then replies with an unicast offer. But the unicast offer never gets to the client machine.

    I understand this is a problem with the access point itself, but since it is unable to forward unicast offers to the clients, I thought maybe I should alter DHCP server's configuration slightly to send all offers as broadcast.

    I found this: https://technet.microsoft.com/en-us/library/cc963213.aspx

    but adding this key, no matter if its value equals 0 or 1, has absolutely no effect.

    Is there any other way to make Windows DHCP server send all offers as broadcasts? I realize this will be only a workaround, but I need to have a workaround untill the AP manufacturer resolves this.





    • Edited by Tormentor740 Wednesday, October 12, 2016 10:42 AM
    Wednesday, October 12, 2016 10:37 AM

Answers

  • Hi Tormentor,

    >>Is there any other way to make Windows DHCP server send all offers as broadcasts?

    Broadcasts is the way of DHCP server working.

    DHCP server will send offers to DHCP relay agent by point-to-point.

    For client direct obtain IP address from DHCP server, please ensure DHCP server and client are in the same LAN, and then remove DHCP relay agent.

    Please ensure Linux client did not intercept broadcast by firewall or other security software.

    Best Regards

    John


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    • Edited by John Lii Thursday, October 13, 2016 2:12 AM
    • Proposed as answer by John Lii Monday, October 31, 2016 6:41 AM
    • Marked as answer by Leo HanModerator Wednesday, November 2, 2016 5:19 AM
    Thursday, October 13, 2016 2:11 AM

All replies

  • Hi Tormentor,

    >>Is there any other way to make Windows DHCP server send all offers as broadcasts?

    Broadcasts is the way of DHCP server working.

    DHCP server will send offers to DHCP relay agent by point-to-point.

    For client direct obtain IP address from DHCP server, please ensure DHCP server and client are in the same LAN, and then remove DHCP relay agent.

    Please ensure Linux client did not intercept broadcast by firewall or other security software.

    Best Regards

    John


    Please remember to mark the replies as answers if they help and unmark them if they provide no help.
    If you have feedback for TechNet Subscriber Support, contact tnmff@microsoft.com.


    • Edited by John Lii Thursday, October 13, 2016 2:12 AM
    • Proposed as answer by John Lii Monday, October 31, 2016 6:41 AM
    • Marked as answer by Leo HanModerator Wednesday, November 2, 2016 5:19 AM
    Thursday, October 13, 2016 2:11 AM
  • I must have misunderstood the terminology there. I meant the Linux/Mac DHCP Discover packets have the broadcast flag set to unicast, and Windows devices have the same flag set to broadcast.

    I was wondering if I could make Windows DHCP server send out all offers with broadcast flag set to broadcast.

    That's Windows' discover:

    Windows device discover packet

    That's Linux discover:

    Linux device discover packet

    That's Windows Offer for Windows device:

    Windows device offer packet

    That's Windows Offer for Linux device, which in the end never gets to the client:

    Linux device offer packet

    • Edited by Tormentor740 Saturday, October 15, 2016 2:13 PM added images
    Saturday, October 15, 2016 2:09 PM
  • Hi Tormentor,

    Sorry for my misunderstanding.

    >> I meant the Linux/Mac DHCP Discover packets have the broadcast flag set to unicast, and Windows devices have the same flag set to broadcast.

    Different OSs has its own default flag of DHCP broadcast.

    You could reference article below to understand it:

    Use of BROADCAST (B) flag in dhcp…

    https://blogs.technet.microsoft.com/teamdhcp/2006/11/08/use-of-broadcast-b-flag-in-dhcp/

    You could try to fix issue by changing DHCP flag, and you could post issue to linux for DHCP flag value of Linux.

    Best Regards

    John


    Please remember to mark the replies as answers if they help and unmark them if they provide no help.
    If you have feedback for TechNet Subscriber Support, contact tnmff@microsoft.com.

    Monday, October 17, 2016 3:36 AM
  • I understand, but that would require instructing users on how to do it. Considering we have about 100 devices, 90% of which are Linux and Mac computers, having a server-side solution would be probably better.
    Monday, October 17, 2016 9:08 AM
  • Hi Tormentor,

    For windows client, you could create bat file of cmd to modify registry key on file server.

    The link below is information about batch files for your reference:

    Using batch files

    https://technet.microsoft.com/en-us/library/bb490869.aspx

    For Linux and Mac system, you could post issue to Linux forum to get effect support.

    Best Regards

    John


    Please remember to mark the replies as answers if they help and unmark them if they provide no help.
    If you have feedback for TechNet Subscriber Support, contact tnmff@microsoft.com.


    • Edited by John Lii Tuesday, October 18, 2016 8:24 AM
    Tuesday, October 18, 2016 7:45 AM
  • Hi Tormentor,

    Just want to confirm the current situations.

    Please feel free to let us know if you need further assistance.

    Best Regards,

    John


    Please remember to mark the replies as answers if they help and unmark them if they provide no help.
    If you have feedback for TechNet Subscriber Support, contact tnmff@microsoft.com.

    Monday, October 31, 2016 6:41 AM
  • Despite not finding a way to make Windows DHCP send out all DHCP Offers with broadcast flag set to broadcast, the problem has been solved. Turns out it was network switches which were causing the problem.

    We have a lot Mac laptops in our office, and every Macbook gets a thunderbolt > ethernet adapter to be able to use wired LAN. Even though we use genuine Apple adapters, switches have been reporting huge number of errors on every port where adapter was being used. In fact, the error amount was so big, that switches couldn't handle it and were constantly breaking SSH sessions.

    After we unplugged all of the adapters, the switches behaviour came back to normal and v'oila - DHCP Offers started successfully reaching all clients.

    Bottom line is - don't use Thunderbolt > Ethernet adapters.


    Sunday, January 29, 2017 6:13 PM