DHCP Interoperability with Routing and Remote Access
Updated: November 10, 2008
Applies To: Windows Server 2008
The DHCP Server service can be deployed along with Routing and Remote Access to provide remote access clients with a dynamically assigned IP address during connection. When these services are used together on the same server, the information provided during dynamic configuration is provided in a way different from a typical DHCP configuration for LAN-based clients.
In 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 is 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 are 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.
RRAS servers provide dynamic configuration for dial-up clients by performing the following steps:
When the RRAS server 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 RRAS server uses the first of these 10 IP addresses for the remote access server interface.
The remaining 9 addresses are allocated to TCP/IP-based clients as they dial in to establish a session with the RRAS server.
IP addresses that are freed when remote access clients disconnect are reused. When all 10 IP addresses are used up, the remote access server obtains 10 more IP addresses from a DHCP server. When the Routing and Remote Access service stops, all IP addresses obtained through DHCP are released.
When the RRAS 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:
IP address of the DHCP server.
Client leased IP address (for later distribution to the RRAS client).
Time at which the lease was obtained.
Time at which the lease expires.
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 RRAS server, which 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.
DHCP servers running on Windows Server 2008, Windows Server 2003, and Windows 2000 Server provide a predefined user class, the Default Routing and Remote Access Class, for assigning options that are specific and provided only to RRAS clients.