RRAS and DHCP

Updated: February 13, 2009

Applies To: Windows 7, Windows Server 2008 R2

The DHCP Server service can be deployed on the same or different computer as RRAS to provide remote access clients with a dynamically assigned IP address during connection.

When an RRAS server provides dynamic configuration for dial-up clients, it performs the following steps:

  • When the RRAS server starts with the Use DHCP to assign remote TCP/IP addresses option, it instructs the DHCP client on the RRAS server to obtain 10 IP addresses from its configured DHCP server.

  • The RRAS server uses the first of these 10 IP addresses obtained from the DHCP server for the RRAS server’s interface.

  • The remaining 9 addresses are allocated to TCP/IP-based clients as they connect 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, 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 RRAS server uses this type of proactive caching of DHCP address leases for remote access 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 connects to the server and requests an IP address (that is, when Server Assigned IP Address is selected), the server uses one of the cached DHCP-leased addresses to provide the client with dynamic IP address configuration.

When the IP address is provided to the client, the client is unaware that the IP address has been obtained through this intermediate process between the DHCP server and the RRAS server. The RRAS server maintains the lease on behalf of the client. Therefore, the only information that the client receives from the DHCP server is what the RRAS server provides, the IP address lease.

In remote access environments, DHCP clients negotiate and receive dynamic configuration using a leased IP address from the RRAS server cache of DHCP scope addresses. The RRAS 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 RRAS client based on TCP/IP properties configured on the RRAS server.

  • The Windows DHCP service provides a predefined user class named Default Routing and Remote Access Class for assigning options that are provided only to RRAS clients.

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