DHCP Leases Expanded

DHCP provides IP addresses to clients. DHCP is critical to a number of Mac OS X Server technologies, most notably with NetBoot. In doing so, communications are comprised of 4 steps: Discovery, Offer, Acceptance, and Acknowledgment. In the Discovery step, a computer that needs an IP address sends a broadcast request to the environment. These typically remain local, although most routers will allow for configuring the gateway in such a way that UDP traffic is forwarded on to other subnets. The request also includes all of the options that the client will need, with options being anything beyond an IP address, each potential option with a numerical identifier per this list (defined in various RFPs).

In the second step, any DHCP servers that received the request will issue an offer, which includes a number of DHCP options, such as a subnet mask (option 1), a gateway (option 3), DNS servers (option 6), amount of time a lease is valid for (option 51), the IP of the DHCP server making the offer (option 54). For example, WINS is two options, 44 & 46 (server and type respectively) that can be provided to clients as is LDAP (option 95). Available options are determined based on any reservations that may have been filed. For example, if an IP address has been reserved for a specific MAC address then the IP will always be the IP reserved.

Because environments can have multiple DHCP servers the Transaction ID will determine which offer to accept. The servers that issued an offer will hold the IP address from the offer until they receive the response that another offer is being accepted and then move those back into their pool of available IP addresses. In step 3, Acceptance, the DHCP client will notify the server whose lease it accepts in the form of a DHCP Request, and those whose lease it will pass on. The Acceptance is actually a request for the IP address that is being held for the MAC address in question.

Based on the Acceptance, the options are then applied in an acknowledgement sent back to the client from the server that it indeed has the IP address and all of the pertinent options required. All of this typically happens in under a second and therefore, you plug in your computer and it gets an IP address; unless you’re running wireshark to look at what’s happening beneath the scene you typically just assume that that’s all there is to it… The most powerful part of DHCP though is in the options, which shows that great thought was given to the protocol when it was conceived. These extensions provide for anything from NTP servers to SMTP servers provided that the client and the server support the implementation.

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