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DNS: Difference between revisions

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DNS (view source)

Revision as of 20:17, 30 October 2017

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=DHCP=
 
;What are the ports used in DHCP?<ref name="bt">www.bt.com/in</ref>
 
[Client]-------------------[Server]
Request
[68] ------------------> [67]
<-----------------
Reply
 
All are UDP ports
 
;What packets are exchanged in DHCP?<ref>www.convergys.com</ref> Which packets are unicast or broadcast in DHCP?<ref name="bt"/>
 
*Usually 4 messages are involved:
 
D = Discover = Broadcast
O = Offer = May be broadcast or unicast depending upon implementation
R = Request = Broadcast
A = Acknowledge = May be broadcast or unicast depending upon implementation
 
*A DHCP Server listens on port number UDP 67 and client uses source port UDP 68.
*The message from client is encapsulated in a UDP datagram, using the destination port number 67 and the source port number 68.
*This UDP datagram is encapsulated in an IP datagram.
*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).
*The client uses all 0s as the source address and all 1s as the destination address.
*The server responds with either a broadcast or a unicast message using UDP source port number 67 and destination port number 68.
*The response can be unicast because the server knows the IP address of the client.
*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.
*However, some systems do not allow the bypassing of ARP, resulting in the use of the broadcast address.
<br />
 
== DHCP Headers ==
{{UC}}
 
 
==DHCP Relay Agent==
 
PC-----Switch-------------Router------DHCP Server
|
|
Relay Agent
|-------->|----------------->|------------->|
Broadcast Unicast Unicast
 
* The DHCP request is broadcast because the client does not know the IP address of the server.
* A broadcast IP datagram cannot pass through any router.
* To solve the problem, there is a need for an intermediary.
* One of the hosts (or a router) can be used as a relay agent.
* The relay agent knows the unicast address of a DHCP server and listens for broadcast messages on port 67.
* It encapsulates the message in a unicast datagram and sends the request to the DHCP server.
* The packet is routed by any router and reaches the DHCP server.
* 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.
* The relay agent, after receiving the reply, sends it to the DHCP client.
<br />
 
;References
Retrieved from "https://aman.awiki.org/wiki/Special:MobileDiff/1922"