Using Routing and Remote Access Servers with DHCP
Applies To: Windows Server 2008
The DHCP Server service can be deployed along with the Routing and Remote Access service to provide remote access clients with a dynamically assigned IP address during connection. When these services are used together on the same server computer, the information provided during dynamic configuration is provided in a way that is different from typical DHCP configuration for LAN-based clients.
In local area network (LAN) environments, DHCP clients negotiate and receive the following configuration information, based entirely on settings configured in the DHCP console for the DHCP server:
A leased IP address provided from the available address pool of an active scope on the DHCP server. The DHCP server directly manages and distributes the address to the LAN-based DHCP client.
Additional parameters and other configuration information provided through assigned DHCP options in the address lease. The values and list of options correspond to option types configured and assigned on the DHCP server.
When a Routing and Remote Access server provides dynamic configuration for dial-up clients, it first performs the following steps:
When the server running Routing and Remote Access starts with the Use DHCP to assign remote TCP/IP addresses option, it instructs the DHCP client to obtain 10 IP addresses from a DHCP server.
The remote access server uses the first of these 10 IP addresses obtained from the DHCP server for the remote access server interface.
The remaining nine addresses are allocated to TCP/IP-based clients as they dial in to establish a session with the remote access server.
IP addresses that are freed when remote access clients disconnect are reused. When all 10 IP addresses are used, the remote access server obtains 10 more from a DHCP server. When the Routing and Remote Access service is stopped, all IP addresses obtained through DHCP are released.
When the Routing and Remote Access server uses this type of proactive caching of DHCP address leases for dial-up clients, it records the following information for each lease response it obtains from the DHCP server:
The IP address of the DHCP server.
The client leased IP address (for later distribution to the Routing and Remote Access client).
The time at which the lease was obtained.
The time at which the lease expires.
The duration of the lease.
All other DHCP option information returned by the DHCP server (such as server, scope, or reservation options) is discarded. When the client dials in to the server and requests an IP address (that is, when Server Assigned IP Address is selected), it uses a cached DHCP lease to provide the dial-up client with dynamic IP address configuration.
When the IP address is provided to the dial-up client, the client is unaware that the IP address has been obtained through this intermediate process between the DHCP server and the Routing and Remote Access server. The Routing and Remote Access server maintains the lease on behalf of the client. Therefore, the only information that the client receives from the DHCP server is the IP address lease.
In dial-up environments, DHCP clients negotiate and receive dynamic configuration using the following modified behavior:
A leased IP address from the Routing and Remote Access server cache of DHCP scope addresses. The Routing and Remote Access server obtains and renews its cached address pool with the DHCP server.
If additional parameters and other configuration information provided through assigned DHCP options in the address lease are normally provided by the DHCP server, this information is returned to the Routing and Remote Access client based on TCP/IP properties configured on the Routing and Remote Access server.
- DHCP servers running Windows Server® 2008 provide a predefined user class, the Default Routing and Remote Access Class, for assigning options that are provided only to Routing and Remote Access clients.