DNS: Difference between revisions
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=DHCP= |
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;What are the ports used in DHCP?<ref name="bt">www.bt.com/in</ref> |
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[Client]-------------------[Server] |
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Request |
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[68] ------------------> [67] |
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<----------------- |
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Reply |
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All are UDP ports |
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;What packets are exchanged in DHCP?<ref>www.convergys.com</ref> Which packets are unicast or broadcast in DHCP?<ref name="bt"/> |
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*Usually 4 messages are involved: |
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D = Discover = Broadcast |
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O = Offer = May be broadcast or unicast depending upon implementation |
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R = Request = Broadcast |
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A = Acknowledge = May be broadcast or unicast depending upon implementation |
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*A DHCP Server listens on port number UDP 67 and client uses source port UDP 68. |
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*The message from client is encapsulated in a UDP datagram, using the destination port number 67 and the source port number 68. |
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*This UDP datagram is encapsulated in an IP datagram. |
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*Now the question is how a client can send an IP datagram when it knows neither its own IP address nor the server’s IP address (the destination address). |
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*The client uses all 0s as the source address and all 1s as the destination address. |
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*The server responds with either a broadcast or a unicast message using UDP source port number 67 and destination port number 68. |
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*The response can be unicast because the server knows the IP address of the client. |
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*It also knows the physical address of the client, which means it does not need the services of ARP for logical to physical address mapping. |
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*However, some systems do not allow the bypassing of ARP, resulting in the use of the broadcast address. |
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== DHCP Headers == |
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{{UC}} |
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==DHCP Relay Agent== |
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PC-----Switch-------------Router------DHCP Server |
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Relay Agent |
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|-------->|----------------->|------------->| |
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Broadcast Unicast Unicast |
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* The DHCP request is broadcast because the client does not know the IP address of the server. |
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* A broadcast IP datagram cannot pass through any router. |
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* To solve the problem, there is a need for an intermediary. |
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* One of the hosts (or a router) can be used as a relay agent. |
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* The relay agent knows the unicast address of a DHCP server and listens for broadcast messages on port 67. |
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* It encapsulates the message in a unicast datagram and sends the request to the DHCP server. |
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* The packet is routed by any router and reaches the DHCP server. |
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* The DHCP server knows the message comes from a relay agent because one of the fields in the request message defines the IP address of the relay agent. |
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* The relay agent, after receiving the reply, sends it to the DHCP client. |
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;References |
;References |