DHCP

DHCP client first tries to renew the lease

When half the lease duration time is up. If it fails then the client will try another renew once it hits 87.5% of the lease time. If it can't be released at that time then the client immediately drops the up address and attempts to get a new ip address

DHCP client

A computer that gets its IP configuration from a DHCP server

DHCP relay agent

Because most of the messages that are sent out to a DHCP server are broadcast messages they will not make it past a router so a relay agent is used. A relay agent can act as a router if it is configured properly.

DHCP lease process

Between the client and DHCP server Dora Discover, offer request acknowledgment 1. client sends a dhcpdiscover request. Because it doesn't have an IP address or know where the DHCP server is it will send a broadcast with a source of 0.0.0.0 and a 255.255.255.255 destination address. The discover message is sent with the MAC address of the client and the clients name so that the DHCP server knows who sent the broadcast. After the message the client waits 1 second to get an offer if not heard then it will send 3 more discover messages at intervals of 9,13,16. If it is still not received a client will send requests every 5 minutes. If the DHCP process fails the client will utilize an alternative IP address or an apippa address. 2. DHCP server receives the dhcpoffer message that contains a.source ip address(DHCP server) B.mac address of the requesting client c.offered IP address D. Subnet mask E. Length of lease F. A server identifier 3. After the client receives an offer it will send a broadcast dhcprequest message to all DHCP servers with a source address of 0.0.0.0 and a destination address of 255.255.255.255. Dchprequest message contains- IP address of the DHCP server chosen, IP address for the client, a list of requested parameters. Such as subnetmask, router, dns server list, domain name, vendor-specific information, wins server list, NetBIOS node type, NetBIOS scope 4. DHCP server accepts the request and sends out an acknowledgement message. This message contains the valid lease times and the duration of the lease in seconds

Benifits of DHCP

Centralized admin of ip configuration gives a one stop shop to manage all devices connected on a network. Allows you to configure and track reservations of ip addresses Dynamic host configuration- DHCP removes the need to manually configure each client with an IP address. Seamless ip host configuration- with a DHCP server clients get an ip from DHCP, subnet mask and gateway as well as the addresses of the dns servers Flexibility and scalability- a DHCP server allows an admin. To make changes to the infrastructure without having to contact each machine individually. DHCP can service clients from a small as 10 to more than thousands. DHCP scales very well with the amount of possible users available.

DHCP INFORM

Client to Server Message, client asking only for local configuration parameters

Backup and restoring the database

DHCP database is a jet database and can be very large depending on the number of clients on the network. Typically stored in %systemroot%\system32\DHCP. When you do a backup the following items are saved. - scopes - reservations - leases - options The DHCP server will do a backup automatically every 60 minutes.

DHCP and dynamic dns updates

Dynamic dns is a feature that allows hosts to register their records in dns, thus removing the need for administrators to manually create records. Secure dynamic updates can be required for zones that are ad integrated which allows only members of the authenticated users group to register records. Those records have an acl on them to stop registered records from being hijacked by other hosts

DHCP renewal

Each DHCP has a an end time the client has to renew the lease or obtain a new ip when the lease runs out. One of the times that a computer will try to renew its lease is at boot up time. If that works then the client sends a DHCPrequest message as a broadcast message and the server sends a dhcpack message as a broadcast message. If it is not done at boot time then the process will be the same but it will send out a unicast message.

Not authorized

If a DHCP server comes online and can't find an ad server it will start to service clients. It will continue to check for member servers every 5 minutes and if it does happen to find one it will stop servicing clients until it gets authorized

Authorizing a DHCP server- authorized

If a DHCP server is a member server it contacts ad to see if it is authorized. Ad confirms that the server is authorized and the DHCP server send out a dhcpinform message to see if other ad servers are out there and it will authorize them. Once that is complete then it will start to service a client

Apipa and alternate configuration

If a client can't contact a DHCP server it will configure itself with either and apipa address or use a predefined alternate configuration. Apipa address scheme is 169.254.0.0/16. The client will randomly assign itself an address from this name space and it will do a gratuitous arp to see if others have chosen this address. If someone else has it then it will release the address and pick another one.

DHCP discover

Send by clients as a broadcast to discover DHCP servers

DHCP request

Sent by a DHCP client as a broadcast with the DHCP servers ip and the configuration offered by the DHCP server acknowledging that it has accepted it as an offer. This is done as a broadcast so that other DHCP servers know that there offer has been rejected.

DHCP decline

Sent by a DHCP client to a DHCP server, informing the server that the offered IP address has been declined. This will happen if the client determines that the up addresses offered was already in use. The client needs to start the lease or renewal process again.

DHCP ack

Sent by a DHCP server to a DHCP client to confirm the up address and to provide the client with those configuration parameters that the client has requested and the server is configured to provide

DHCP nack

Sent by a DHCP server to a DHCP client to deny the client requests. This may happen if the IP address requested by the client is not correct. This may be the case if a client is moved to another subnet and it retained its old ip configuration. After this message the client must start the lease or renew process again.

DHCP offer

Sent by one or more DHCP servers in response to the DHCP discover, along with offered configuration parameters

DHCP release

Sent to the DHCP server from there DHCP client to let the server know that it is relinquishing the ip config it currently has. It will send this call to the server where it got it's ip configuration from.

If during a renewal a client has been moved to another subnet and sends a dhcprequest message to renew

The DHCP server will reply with a dhcpnack message saying that the lease can't be renewed. The client will start the process again for getting a new up address

If renew is not done on boot up

The client will send an arp to the gateway it was configured with and if it receives a response it will assume that it has not been moved and will retain its ip config. If the arp fails then it will assume it has been moved and drop the ip config.

Reconciling the DHCP database

The process where your DHCP database is validated against the registry settings. You would do a reconciliation in the following scenarios- DHCP database values are configured properly but they are not displayed correctly in the console. - you have restored the DHCP server database, but the DHCP database doesn't have the most recent values

DHCP lease

This defines the duration the client can have its ip addresses assigned from the server. Default time is 8 days.

DHCP communication

Through the UDP protocol through ports 67 on server and 68 on client

DHCP Server

a computer that provides DHCP configuration information to multiple clients