What is dora process in DHCP and how it works?

Showing Answers 1 - 26 of 26 Answers

puneet chawla

  • Nov 4th, 2007
 

1) Client makes a UDP Broadcast to the server about the DHCP discovery.

2) DHCP offers to the client.

3) In response to the offer Client requests the server.

4)Server responds all the Ip Add/mask/gty/dns/wins info along with the acknowledgement packet.

Thanks.

  Was this answer useful?  Yes

 When the term "broadcast" is used, there are two types of broadcasts: 1) Network Layer broadcast and 2) Datalink Layer broadcast. For Internet Protocol, the standard broadcast DESTINATION address is 255.255.255.255. For Ethernet, the broadcast DESTINATION address is FF:FF:FF:FF:FF:FF. You have to think of the DORA process in terms of encapsulation and decapsulation, as well as switch behavior with frames it receives.

So let's say you've got two computers, a client host and a server host. The client host is the DHCP client and the server host is the DHCP server. Let's call the client host PC and the DHCP server DHCP.

Now, when the PC is turned on, the NIC has a MAC address but no IP address. So the PC tries to "Discover" the DHCP server by sending out a "Discover" packet. In that packet, the destination IP address is 255.255.255.255 and source IP address of 0.0.0.0 (because the PC doesn't have an IP address yet). That Discover packet is encapsulated into an Data Link layer Ethernet Frame. That Ethernet Frame has a destination address of FF:FF:FF:FF:FF:FF and the source address of whatever the MAC address of the PC's nic is, let's call it PC:MA:CA:DD:RR:SS (I know this is not a valid MAC address). Now, since the destination address is FF:FF:FF:FF:FF:FF, the first switch that receives the frame will, by definition, send that frame out of every single switch port, except the switch port that received the frame. Since the switch doesn't know (from it's CAM table), who FF:FF:FF:FF:FF:FF is, it forwards the Discover packet out of every single switch port, except the switch port that received the frame. Let's say the DHCP server is directly connected to the switch. So the DHCP server first receives the Ethernet frame with the destination address of FF:FF:FF:FF:FF:FF (which by definition, means itself), and a source address of PC:MA:CA:DD:RR:SS. So when the DHCP server receives that Discover Packet, it replies to it by sending the Offer Packet. Since the DHCP server has no idea who was contacting them (since PC currently has an IP address of 0.0.0.0), the Offer packet has the destination address of 255.255.255.255. But the DHCP server remembered the MAC address of whoever it was trying to contact them, so the DHCP server sends out the Offer packet in a frame with a destination address of PC:MA:CA:DD:RR:SS and uses it's own source address of DH:CP:SE:RV:ER:MC (again, I know this is not a valid MAC address). So the Offer packet is sent from the DHCP server to the PC as a unicast not a broadcast. So the switch receives the frame from the DHCP server and forwards it to the PC which receives the Offer packet. Then the rest of the DHCP process of R and A occurs.

So that's why the "...the first two messages in the DHCP are broadcast messages..." refers to Network Layer IP broadcast messages (destination IP address of 255.255.255.255). Then "...only the first message (the Discover message) is a LAN Broadcast..." because only the Discover packet's frame had the Ethernet destination address of FF:FF:FF:FF:FF:FF (therefore a LAN broadcast) and the Offer packet's frame had the Ethernet destination address of PC:MA:CA:DD:RR:SS (therefore a LAN unicast).

I hope this helps.

MotoRooter

  • Jul 27th, 2008
 

DHCP (D)iscover
DHCP (O)ffer
DHCP (R)equest
DHCP (A)ck

Step 1:

Your computer sends a "Discovery" request, asking for it's IP information from any listening DHCP servers.

Step 2:

Any listening DHCP servers will "Offer" their configuration information to your workstation.

Step 3:

You workstation chooses the best lease then "Requests" that lease from the corresponding DHCP server.

Step 4:

The DHCP server you requested the IP configuration information from then "Acknowledges" your request and leases you the IP configuration information.

The DORA process in DHCP works as folows.

D ----------> Discover
O ----------> Offer
R ----------> Request
A ----------> Acknowledgement

1. DHCP automatically discovers the system
2. Server sends offer to the client system
3. Client sends request to the server system
4. At lats Acknowledgement is send by server to client system

MotoRooter

  • Jul 1st, 2011
 

1) Client makes a UDP Broadcast to the server with a DHCPDiscover, or Discover packet.

2) DHCP offers to the client.
The server sends a DHCPOffer including other configuration parameters (DHCP Options) for the client per the servers configuration file

3) In response to the offer Client requests the server.
The client replies DHCPRequest, unicast to the server, requesting the offered address.

4) The server sends DHCPAck acknowledging the request which is the clients final permission to take the address as offered. Before sending the ack the server double checks that the offered address is still available, that the parameters match the clients request and (if so) marks the address taken.

A good reference for DHCP information is Droms and Lemon "The DHCP Handbook".

  Was this answer useful?  Yes

sritharan

  • Jul 18th, 2011
 

DHCP (D)iscover
DHCP (O)ffer
DHCP (R)equest
DHCP (A)ck

Step 1:

Your computer sends a "Discovery" request, asking for it's IP information from any listening DHCP servers.

Step 2:

Any listening DHCP servers will "Offer" their configuration information to your workstation.

Step 3:

You workstation chooses the best lease then "Requests" that lease from the corresponding DHCP server.

Step 4:

The DHCP server you requested the IP configuration information from then "Acknowledges" your request and leases you the IP configuration information.

  Was this answer useful?  Yes

arun

  • Aug 2nd, 2011
 

Dora=
d-discover
o-offer
r-request
a-acknowledgement
=========
dhcp first discovers the computer and it then offers the computer to assign an ip address to i.then the computer request for the ip address when it accepts the offer from dhcp..when dhcp assigns the iip to the computer then it sends an acknowledgement that it had been assigned the ip address ...this is process of DORA....

  Was this answer useful?  Yes

Rupesh

  • Aug 23rd, 2011
 

The DORA process in DHCP works as follows.

D ----------> Discover
O ----------> Offer
R ----------> Request
A ----------> Acknowledgement

1. DHCP automatically discovers the system

2. Server sends offer to the client system

3. Client sends request to the server system

4. At lats Acknowledgement is send by server to client system

  Was this answer useful?  Yes

sanjay

  • Mar 24th, 2013
 

check this vedio this might help:
http://www.youtube.com/watch?v=0Dp7YoR0SLE&feature=g-user-u

  Was this answer useful?  Yes

ASWINI DEY

  • May 28th, 2013
 

HCP (D)Discover
DHCP (O)ffer
DHCP (R)equest
DHCP (A)ck

Step 1:

Your computer sends a "Discovery" request, asking for its IP information from any listening DHCP servers.

Step 2:

Any listening DHCP servers will "Offer" their configuration information to your workstation.

Step 3:

You workstation chooses the best lease then "Requests" that lease from the corresponding DHCP server.

Step 4:

The DHCP server you requested the IP configuration information from then "Acknowledges" your request and leases you the IP configuration information.


DHCP server send the IP address to the request client and this IP is already assigned in another p.c in static ip then user 1 agent sent a discovery message to the server. Then all process are again going.

  Was this answer useful?  Yes

Kiran

  • Jan 8th, 2015
 

DORA Discover,Offer,Request,Acknowledge.
You want to buy hdd.You go to computer market and find many hdd sellers(Servers).You look for a best seller and Discover a seller.You approach him and request for hdd(data/ip info).Seller Offer s a deal.You accept the deal and Request him for the price and pay him.Seller(Server) Acknowledge s the price and gives an ack or receipt.

  Was this answer useful?  Yes

g.rakesh kumar

  • Feb 11th, 2015
 

dynamic host configuration protocal.

  Was this answer useful?  Yes

Gaurav Kumar

  • May 7th, 2015
 

DHCP (Dynamic Host Configuration Protocol) is a protocol use to allot IP address,Subnet Mask,Default Gateway,DNS to the legitimate client. The process of allocating these things are called DORA. A client sends DHCP Discover message to DHCP server, Server Offers IP address,Client Request for the details and finally Server sends Acknowledgement. This whole process takes 16 sec to complete after that if client doesn't receive any info from server it goes to APIPA.

  Was this answer useful?  Yes

Mahavir Singh Negi

  • Jul 11th, 2015
 

What is PIPA IP?

  Was this answer useful?  Yes

Matthias Esoimeme

  • Aug 8th, 2015
 

DHCP is Dynamic Host Corporation Protocol
It work data link layers broadcast, In respond offer to client request and help in sending messages or receives it assign IP addresses by DHCP server which is know as DORA (Discovery Offer Request and Acknowledgement)

  Was this answer useful?  Yes

Deepak jha

  • Dec 1st, 2015
 

Discovery Offer Request Acknowledgement

  Was this answer useful?  Yes

Robin

  • Feb 2nd, 2016
 

It is not PIPA. It is APIPA and APIPA is stands for Automatic Private IP Addressing.

When our PC is not connected with DHCP Server or we didn't configure a IP address manually its take a IP itself and it is called APIPA.. :)

  Was this answer useful?  Yes

don omer

  • Feb 7th, 2016
 

I understood DORA concept so thank you so much

  Was this answer useful?  Yes

Amit Routh

  • Jun 24th, 2016
 

Hello All,
Thanks for such a beaitiful explanation.
My question is when we connect a new pc ic a LAN having DHCP server which process will happen first ARP or DORA.Is DORA will will allone enough there.?

  Was this answer useful?  Yes

ayush

  • Jul 26th, 2016
 

DORA refers to it first discover new device then offer IP to that device then new device can request for other IP and finally it accept IP.

  Was this answer useful?  Yes

balraje

  • Nov 27th, 2016
 

It will start DHCP process only... Arp is to get the remote hosts mac address by having remote host IP address. In this scenario a client dont know the IP address of the DHCP.. its keep the dest IP as 255.255.255.255 and dest mac as ff:ff:ff:ff:ff:ff.... and once it got ip only then it will use ARP if situation comes....

  Was this answer useful?  Yes

Saumik Shah

  • Dec 2nd, 2016
 

Automatic private Invalid IP address- If you have lost your connection and when you write ipconfig /all the dhcp server will give you an address which is invalid as the job of the dhcp is to give an address. This address is APIPA address and not PIPA

  Was this answer useful?  Yes

Tausif

  • Dec 28th, 2016
 

Does DORA process happens again once the lease period expires?

  Was this answer useful?  Yes

Shiv prakash

  • Jan 6th, 2017
 

What is APIPA?

  Was this answer useful?  Yes

@net-worker

  • Apr 18th, 2017
 

DORA: Discover-Offer-Reply-Acknowledge.

When PC talks to DHCP requesting for a IP address, it discovers DHCP, DHCP offers IP address to PC, PC replies back to DHCP and DHCP marks this IP address and releases to PC lastly PC send Acknowledgement.

  Was this answer useful?  Yes

Aravindraj

  • Apr 30th, 2017
 

PIPA-Private IP Address.
DHCP clients can automatically self-configuration an IP address and subnet mask when a DHCP server isn’t available.
The IP address range is 169.254.0.1 through 169.254.255.254.
The client also configures itself with a default class B subnet mask of 255.255.0.0.

  Was this answer useful?  Yes

Sandhiya P

  • May 26th, 2017
 

1) Client makes a UDP Broadcast to the server about the DHCP discovery.
2) DHCP offers to the client.
3) In response to the offer Client requests the server.
4)Server responds all the Ip Add/mask/gty/dns/wins info along with the acknowledgement packet.

  Was this answer useful?  Yes

Swap

  • Jul 14th, 2017
 

Can you please explain what is subnet not subnetting?

  Was this answer useful?  Yes

jaspreet singh

  • Feb 23rd, 2018
 

DHCP operation divides into 4 phases.
Server Discovery
IP Lease Offer
IP Lease Request
IP lease Acknowledgement
This DHCP operation called DORA Process.

First I am telling you about DORA process in DHCP
Discovery ( DHCP discover ):
By this message interaction start between server and client. Discovery sent by a client that is connected to a local subnet. While send discovery destination address is broadcast 255.255.255.255 and source address is 0.0.0.0.

OFFER ( DHCP offer ):
Offer is a response to Discovery message by DHCP server to DHCP client. It contains network configuration setting for the client like an IP address offer to client 10.1.1.1.

REQUEST ( DHCP request ):
A response to Offer is indicating that client has accepted the network configuration. It means to accept the offer by DHCP server with IP 10.1.1.1. this message sent by the client with destination address 255.255.255.255 and source address is 10.1.1.1.

ACKNOWLEDGE ( DHCP ack ):
After the request message or accept the IP by DHCP SERVER, server sent an ACK to the client. This message clear to the client that now you can start using the network.

  Was this answer useful?  Yes

Niveditha

  • May 8th, 2018
 

I hope it is APIPA (AUTOMATIC PRIVATE IP ADDRESS). It is a range of IP address from 169.254.1.0 to 169.254.255.255 where host machine obtains automatically when DHCP fails to assign IP address.

  Was this answer useful?  Yes

Give your answer:

If you think the above answer is not correct, Please select a reason and add your answer below.

 

Related Answered Questions

 

Related Open Questions