DHCP Working

165 pts.
Tags:
DHCP
DHCP Client
DHCP server
Microsoft Windows Server 2003
Hi All, I am working as a network Admin in a small organization, i am still confused about working of DHCP Server. DORA.
D : Discover : Client to Server Broadcast
O: Offer: Server to client Broadcast as per Microsoft
R: Request: Client to server Broadcast
A: Ack : Server to client
according to RFC all communication from client to server are broadcast and from server to client is unicast. Can anyone please help me to remove the confusion please...
urgent


Software/Hardware used:
DHCP, Windows server 2003

Answer Wiki

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This is from the RFC document:

Normally, DHCP servers and BOOTP relay agents attempt to deliver DHCPOFFER, DHCPACK and DHCPNAK messages directly to the client using unicast delivery. The IP destination address (in the IP header) is set to the DHCP ‘yiaddr’ address and the link-layer destination address is set to the DHCP ‘chaddr’ address.

Unfortunately, some client implementations are unable to receive such unicast IP datagrams until the implementation has been configured with a valid IP address (leading to a deadlock in which the client’s IP address cannot be delivered until the client has been configured with an IP address).

A client that cannot receive unicast IP datagrams until its protocol software has been configured with an IP address SHOULD set the BROADCAST bit in the ‘flags’ field to 1 in any DHCPDISCOVER or DHCPREQUEST messages that client sends. The BROADCAST bit will provide a hint to the DHCP server and BOOTP relay agent to broadcast any messages to the client on the client’s subnet. A client that can receive unicast IP datagrams before its protocol software has been configured SHOULD clear the BROADCAST bit to 0. The BOOTP clarifications document discusses the ramifications of the use of the BROADCAST bit [21].”

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  • Matt Mather
    Most vendors implementation is broadcast initially, then unicast once source and destination MAC addresses are known..
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  • Labnuke99
    See this Wireshark wiki article on DHCP (the first graphic)- the client sends to the broadcast addresses - the server sends to a unicast address. However, this is only true if the DHCP client is on the same server as the DHCP server. If a DHCP relay agent is involved, then the DHCP server will not see the traffic as being sent to the broadcast address.
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