The Dynamic Host Configuration Protocol (DHCP) is a network protocol used to configure devices that are connected to a network so they can communicate on that network using the Internet Protocol.The protocol is implemented in a client-server model,in which DHCP clients request configuration data, such as an IP address, a default route ,and one or more DNS server addresses from a DHCP server.A DHCP server maintains a database of available IP addresses and configuration information. When the server receives a request from a client, the DHCP server determines the network to which the DHCP client is connected, and then allocates an IP address or prefix that is appropriate for the client, and sends configuration information appropriate for that client. DHCP servers typically grant IP addresses to clients only for a limited interval. DHCP clients are responsible for renewing their IP address before that interval has expired, and must stop using the address once the interval has expired, if they have not been able to renew it.
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| DHCP SERVER PERFORMING DNS DYNAMIC UPDATE ON BEHALF OF DHCP CLIENT
A DHCP Client goes through six stages during the DHCP Process.These stages are.
DHCP Message Table
DHCPDISCOVER
When a client boots up for the first time, it is said to be in the Initializing state, and transmits a DHCPDISCOVER message on its local physical subnet over User Datagram Protocol (UDP) port 67 (BootP server). Since the client has no way of knowing the subnet to which it belongs, the DHCPDISCOVER is an all subnets broadcast (destination IP address of 255.255.255.255), with a source IP address of 0.0.0.0. The source IP address is 0.0.0.0, since the client does not have a configured IP address. If a DHCP server exists on this local subnet and is configured and operating correctly, the DHCP server will hear the broadcast and respond with a DHCPOFFER message. If a DHCP server does not exist on the local subnet, there must be a DHCP/BootP Relay Agent on this local subnet to forward the DHCPDISCOVER message to a subnet that contains a DHCP server.
DHCPOFFER
A DHCP server that receives a DHCPDISCOVER message may respond with a DHCPOFFER message on UDP port 68 (BootP client). The client receives the DHCPOFFER and moves into the Selecting state. This DHCPOFFER message contains initial configuration information for the client. For example, the DHCP server will fill in the yiaddr field of the DHCPOFFER message with the requested IP address. The subnet mask and default gateway are specified in the options field, subnet mask and router options, respectively. Other common options in the DHCPOFFER message include IP Address lease time, renewal time, domain name server, and NetBIOS name server (WINS). The DHCP server will send the DHCPOFFER to the broadcast address, but will include the clients hardware address in the chaddr field of the offer, so the client knows that it is the intended destination. In the event that the DHCP server is not on the local subnet, the DHCP server will send the DHCPOFFER, as a unicast packet, on UDP port 67, back to the DHCP/BootP Relay Agent from which the DHCPDISCOVER came. The DHCP/BootP Relay Agent will then either broadcast or unicast the DHCPOFFER on the local subnet on UDP port 68, depending on the Broadcast flag set by the Bootp client.
DHCPREQUEST
After the client receives a DHCPOFFER, it responds with a DHCPREQUEST message, indicating its intent to accept the parameters in the DHCPOFFER, and moves into the Requesting state. The client may receive multiple DHCPOFFER messages, one from each DHCP server that received the original DHCPDISCOVER message. The client chooses one DHCPOFFER and responds to that DHCP server only, implicitly declining all other DHCPOFFER messages. The client identifies the selected server by populating the Server Identifier option field with the DHCP server's IP address. The DHCPREQUEST is also a broadcast, so all DHCP servers that sent a DHCPOFFER will see the DHCPREQUEST, and each will know whether its DHCPOFFER was accepted or declined. Any additional configuration options that the client requires will be included in the options field of the DHCPREQUEST message. Even though the client has been offered an IP address, it will send the DHCPREQUEST message with a source IP address of 0.0.0.0. At this time, the client has not yet received verification that it is clear to use the IP address.
DHCPACK
After the DHCP server receives the DHCPREQUEST, it acknowledges the request with a DHCPACK message, thus completing the initialization process. The DHCPACK message has a source IP address of the DHCP server, and the destination address is once again a broadcast and contains all the parameters that the client requested in the DHCPREQUEST message. When the client receives the DHCPACK, it enters into the Bound state, and is now free to use the IP address to communicate on the network. Meanwhile, the DHCP server stores the lease in its database and uniquely identifies it using the client identifier or chaddr, and the associated IP address. Both the client and server will use this combination of identifiers to refer to the lease. The client identifier is the Mac address of the device plus the media type.
DHCPNAK
If the selected server is unable to satisfy the DHCPREQUEST message, the DHCP server will respond with a DHCPNAK message. When the client receives a DHCPNAK message, or does not receive a response to a DHCPREQUEST message, the client restarts the configuration process by going into the Requesting state. The client will retransmit the DHCPREQUEST at least four times within 60 seconds before restarting the Initializing state.
DHCPDECLINE
The client receives the DHCPACK and will optionally perform a final check on the parameters. The client performs this procedure by sending Address Resolution Protocol (ARP) requests for the IP address provided in the DHCPACK. If the client detects that the address is already in use by receiving a reply to the ARP request, the client will send a DHCPDECLINE message to the server and restart the configuration process by going into the Requesting state.
DHCPINFORM
If a client has obtained a network address through some other means or has a manually configured IP address, a client workstation may use a DHCPINFORM request message to obtain other local configuration parameters, such as the domain name and Domain Name Servers (DNSs). DHCP servers receiving a DHCPINFORM message construct a DHCPACK message with any local configuration parameters appropriate for the client without allocating a new IP address. This DHCPACK will be sent unicast to the client.
DHCPRELEASE
A DHCP client may choose to relinquish its lease on a network address by sending a DHCPRELEASE message to the DHCP server. The client identifies the lease to be released by the use of the client identifier field and network address in the DHCPRELEASE message. If you need to extend the current DHCP pool range, remove the current pool of addresses and specify the new range of IP addresses under the DHCP pool. In order to remove specific IP addresses or a range of addresses that you want to be in the DHCP pool, use the command ip dhcpexcluded-address .
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nice work but dont mention about DHCP port no
ReplyDeleteNice work
ReplyDeleteplease explain about DNS
good networking tip.
ReplyDeleteNice work
ReplyDeleteGreate work....keep it up...
ReplyDeletenice work......
ReplyDeletesuperb...
ReplyDelete