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I went through various stack exchange post to understand the concept of DHCP offer when it comes as a unicast to client from the DHCP server.

A server can send a unicast DHCP offer since it knows the host's MAC address. Ok I understood this but what happens at IP layer? Why host did not dropped the unicast DHCP offer at IP layer?

As per my understanding, an IPv4 host will drop layer-3 traffic delivered to it that is not destined to its IPv4 address, a subscribed IPv4 multicast address, or an IPv4 broadcast address, even if it is delivered via a layer-2 valid mac address frame.

Now since my host does not have any ip address and DHCP offer is not a broadcast address then how come host accepted the unicast DHCP offer? Why host did not dropped the DHCP offer at IP layer? I hope you got my question. Overall I want to understand what happens to the unicast DHCP offer at IP layer? Can anyone please explain me

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  • Here's a hint: when the client sends a DHCPDISCOVER message, what is the source IP of that packet?
    – Ron Trunk
    Jan 22 at 13:19
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As per my understanding, an IPv4 host will drop layer-3 traffic delivered to it that is not destined to its IPv4 address, a subscribed IPv4 multicast address, or an IPv4 broadcast address, even if it is delivered via a layer-2 valid mac address frame.

That's true for normal operation. While DHCP is in progress on an interface, the stack doesn't care too much about the layer-3 destination field as long as the DHCPOFFER matches a current request. As of RFC 2131, "each DHCP server may respond with a DHCPOFFER message". Broadcast is only mandatory for the client for DHCPDISCOVER, DHCPREQUEST and DHCPINFORM messages unless the client already knows the server's address, and for DHCPDECLINE.

What exactly a client regards as acceptable depends on its implementation which isn't on-topic here.

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