DHCP Broadcast - Notes

In my last post on DHCP, DHCP Offer and DHCP ACK were unicast frames.

DHCP Unicast Flag

In the Bootp Flags, the broadcast bit is set to 0(means unicast).  It means DHCP Client tells that it is expecting unicast responses from DHCP server.  So, the reply from DHCP server(DHCP Offer and DHCP ACK) will have Unicast MAC and IP Addresses. 

On a linux machine, this can be seen using command 'dhclient eth0'

Broadcast

There are cases when they will be broadcast.  From linux machine, we can use 'dhclient -B eth0' to request Broadcast responses.  Most of the times, DHCP starts from DHCP Offer, instead of Discovery.  So, by googling I found that we had to release the existing IP address for the transaction to happen from Discovery.  Like this

# dhclient -r  eth0
# dhclient -B  eth0

DHCP Broadcast

Observe that the Bootp flag here is '1'(means Broadcast flag set).  Client is asking the server to give broadcast responses.  Therefore, the DHCP Offer and ACK from the server has broadcast MAC and IP addresses.