How DHCP Works
When a DHCP client computer is started, the TCP/IP software is loaded into memory and starts to operate. However, because TCP/IP has not been given an IP address yet, it is incapable of sending or receiving directed datagrams. TCP/IP can, however, transmit and listen for broadcasts. This capability to communicate via broadcasts is the basis for how DHCP works. The process of leasing an IP address from the DHCP server involves four steps:
DHCPDISCOVER—
The DHCP client initiates the process by broadcasting a datagram destined for UDP port 68 (used by BOOTP and DHCP servers). This first datagram is known as a DHCP Discover message, which is a request to any DHCP server that receives the datagram for configuration information. The DHCP discover datagram contains many fields, but the one that is most important contains the physical address of the DHCP client.
DHCPOFFER—
A DHCP server configured to lease addresses for the network on which the client computer resides constructs a response datagram known as a DHCP offer and sends it via broadcast to the computer that issued the DHCP discover. This broadcast is sent to UDP port 67 and contains the physical address of the DHCP client. Also contained in the DHCP offer are the physical and IP addresses of the DHCP server, as well as the values for the IP address and subnet mask that are being offered to the DHCP client.
By the Way
At this point it is possible for the DHCP client to receive several DHCP offers, assuming there are multiple DHCP servers with the capability to offer the DHCP client an IP address. In most cases, the DHCP client accepts the first DHCP offer that arrives.
DHCPREQUEST—
The client selects an offer and constructs and broadcasts a DHCP request datagram. The DHCP request datagram contains the IP address of the server that issued the offer and the physical address of the DHCP client. The DHCP request performs two basic tasks. First it tells the selected DHCP server that the client requests it to assign the DHCP client an IP address (and other configuration settings). Second, it notifies all other DHCP servers with outstanding offers that their offers were not accepted.
DHCPACK—
When the DHCP server from which the offer was selected receives the DHCP request datagram, it constructs the final datagram of the lease process. This datagram is known as a DHCP ack (short for acknowledgement). The DHCP ack includes an IP address and subnet mask for the DHCP client. Optionally, the DHCP client is often also configured with IP addresses for the default gateway, several DNS servers, and possibly one or two WINS servers. In addition to IP addresses, the DHCP client can receive other configuration information such as a NetBIOS node type, which can change the order of NetBIOS name resolution.
Three other key fields are contained in the DHCP ack, all of which indicate time periods. One field identifies the length of the lease. Two other time fields, known as T1 and T2, are used when the client attempts to renew its lease. The use of these three time fields is explained later in this hour.
Relay Agents
If both the DHCP client and the DHCP server reside on the same network segment, the process proceeds exactly as previously indicated. If the DHCP client and DHCP server reside on different networks separated by one or more routers, the process becomes more complicated. Routers typically do not forward broadcasts to other networks. For DHCP to work, a middleman must be configured to assist the DHCP process. The middleman can be another host on the same network as the DHCP client, but often it is the router itself. In any case, the process that performs this middleman function is called either a BOOTP relay agent or a DHCP relay agent.
A relay agent is configured with a fixed IP address and also contains the IP address of the DHCP server. Because relay agents have configured IP addresses, they can always send and receive directed datagrams to the DHCP server. Because the relay agent resides on the same network as the DHCP client, it can communicate with the DHCP client via broadcasts.
Relay agents listen for broadcasts destined for UDP port 68; when the relay agent detects a DHCP request, it retransmits the request to the DHCP server. When the agent receives a response from the DHCP server, the response is rebroadcast on the local segment. This explanation has eliminated a few details for brevity but conveys the essence of the function performed by a relay agent. For more on relay agents, you can read RFC 1542.
By the Way
Not all routers are capable of providing BOOTP/DHCP relay agent services. Routers that do have this capability are said to be RFC 1542-compliant.
DHCP Time Fields
DHCP clients lease IP addresses from DHCP servers for a fixed period of time, with the actual lease length being configured on the DHCP server. The T1 and T2 time values sent with the DHCP ack message are used during the lease renewal process. The T1 value indicates to the client when it should begin the process of renewing its lease. T1 is typically set to one-half of the actual lease time. Assume in the following example that leases are issued for a period of eight days.
Four days into the lease, the client sends a DHCP request to attempt to renew its IP address lease with the DHCP server that issued the lease. Assuming the DHCP server is online, the lease typically is renewed using a DHCP ack. Unlike the DHCP request and ack explained earlier in the four-step process, these two datagrams are not broadcast but are sent as directed datagrams. This is possible because both computers at this time contain valid IP addresses.
If the DHCP server is not available when the DHCP client issues the first request at 50% (four days), the client waits and attempts to renew the lease at 75% of the lease period, or six days into the lease. If this request also fails, the DHCP client tries a third time at 87.5%, or seven-eighths of the lease. Up to this point the DHCP client has attempted to renew its lease with the DHCP server that issued the lease by sending directed datagrams. If the DHCP client is incapable of renewing its lease by 87.5% of the total lease, the T2 time period comes into effect. The T2 time allows the DHCP client to begin broadcasting requests for any DHCP server. If the DHCP client is incapable of either renewing its lease or obtaining a new lease from another DHCP server by the time the lease expires, the client must stop using the IP address and stop using TCP/IP for normal network operations.
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