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As we know there are 4 types of packets involved when a client wants to get an IPv4 address from DHCP Server. These can be seen in the image bellow (from Microsoft web site). enter image description here

From the perspective of data link layer (Layer 2 of OSI model) frames sent from client to the server are always broadcast (DISCOVER & REQUEST) but the other two can be either broadcast or unicast depending on how the broadcast-flag option is set by the client.

enter image description here

I know this flag can be changed in cisco IOS using this command:

R(config)# [no] ip dhcp-client broadcast-flag

My question is how I can change this flag in Windows or Linux?

BTW I just want to know this out of curiosity and I don't have any special needs for that. I tested on Windows and Linux and know that this flag is set to 0 so those frames are sent in unicast.

AKJ88
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1 Answers1

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That's something you can easily learn by just reading the doc:

Citing man dhcpd.conf:

always-broadcast flag;

The DHCP and BOOTP protocols both require DHCP and BOOTP clients to set the broadcast bit in the flags field of the BOOTP message header. Unfortunately, some DHCP and BOOTP clients do not do this, and therefore may not receive responses from the DHCP server. The DHCP server can be made to always broadcast its responses to clients by setting this flag to 'on' for the relevant scope; relevant scopes would be inside a conditional statement, as a parameter for a class, or as a parameter for a host declaration. To avoid creating excess broadcast traffic on your network, we recommend that you restrict the use of this option to as few clients as possible. For example, the Microsoft DHCP client is known not to have this problem, as are the OpenTransport and ISC DHCP clients.

On the client side (man dhclient):

-B

Set the BOOTP broadcast flag in request packets so servers will always broadcast replies.

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Sven
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