arp - How are DHCP Replies unicasted by DHCP Server or DHCP Relay Agent? -


while dhcp request messages allways sent via broadcast during dhcp client initialization phase, dhcp replies can sent both broadcast , unicast. method of sending used depends on dhcp server or dhcp relay agent , requirement of dhcp client.

dhcp client can request dhcp replies broadcasted in case it's own ip protocol implementation software can not receive unicast ip packets untill has valid ip address configured , case in initialization phase. in case request dhcp server or relay agent sent dhcp replies via broadcast setiing broadcast bit of flags fleld in dhcp request messages 1. in case when dhcp client can receive unicast ip packets though not have configured ip address, broadcast bit unset (0) , dhcp server or relay agent decide wheather send dhcp replies via unicast or broadcast. unicast offcourse preffered method reduces network traffic , proccessing power of clients.

in order send packet/frame both ip address , mac address need known sender. dhcp server or relay agent knows both ip , mac address, ip address offering 1 , mac address contained in dhcp request messages. however, standard sending packet procedure require mac address obtained arp protocol , can not work dhcp client can not respond arp packets not yet have ip address configured. question then, how can dhcp server or relay agent send dhcp replies via unicast? there mechanism accessing arp table , adding arp entries directly?

the dhcp server can generate raw packet , put mac , ip addresses that. when sending raw packets, os doesn't checks arp entries or anything. packet valid unicast udp packet though client can't answer arp yet , server doesn't have in arp tables.


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