Understanding DHCP IP Address Assignment for RAS Clients

- Microsoft Windows NT Workstation versions 3.5, 3.51, and 4.0 - Microsoft Windows NT Server versions 3.5, 3.51, and 4.0

SUMMARY

When Remote Access Service (RAS) uses Dynamic Host Configuration Protocol (DHCP) to obtain IP addresses for dial-in clients, only the IP address from the DHCP lease is passed to the RAS client. Other options in the DHCP scope are not. This article describes the behavior that occurs.

MORE INFORMATION

The RAS help file states:

   Use DHCP to assign remote TCP/IP client addresses.

When the RAS server starts up with the option to "Use DHCP to assign remote TCP/IP addresses," it makes several DHCP requests in advance and caches the DHCP leases that it will need for dial-in clients. The RAS Server will request a number of addresses equal to the number of RAS ports set to receive calls plus one. For example, if the RAS Server has 2 analog modems and 2 ISDN adapters all set to receive calls, then the RAS Server will request 5 IP addresses from the DHCP Server. The first four are to assign to RAS clients that will be dialing into the RAS ports and the fifth one is for the RAS Server itself so that the clients can connect to it.

The RAS server records the address of the DHCP server, the leased IP address, when the lease was obtained, when the lease expires, and the lease length. All other DHCP options defined for that scope are discarded. When the client dials into the RAS server and requests an IP address ("Server Assigned IP Address" is selected) the RAS server will use one of these cached leases. The IP address is then given to the dial-in client, which is unaware that the IP address has been obtained through DHCP. The RAS server maintains the lease on behalf of the client. Therefore, the only information that the client receives from the DHCP lease is the IP address.

When a RAS client obtains an IP address Lease from a RAS server, Winipcfg.exe (for Windows 95) or Ipconfig.exe (for Windows NT) will show the following information about the Lease:

Lease Obtained:      Tue Jan 1 80 12:00:00 AM
Lease Expires:       Tue Jan 1 80 12:00:00 AM

However, RAS clients still have the ability to receive additional TCP/IP configuration information from the RAS server. WINS server assignments and Domain Name Service (DNS) server assignments can be delegated to the client when it connects.

Again, this information is not taken from the Options given in the DHCP lease. Rather these are taken directly from the RAS server's settings. If a RAS server has WINS or DNS entries, these will be passed to the client.

Here is a list of the DHCP Options that Microsoft DHCP clients will support and how a RAS client will obtain them:

IP

As described above, the RAS server obtains an IP address from the DHCP server. The RAS server then gives the IP address to the dial-in client and manages its lease. This is the only information from the DHCP server that the RAS client receives.

WINS

This is taken from the RAS server if the RAS server is configured with WINS addresses. The client will acquire the list of WINS servers that are configured on the RAS server.

DNS

This is taken from the RAS server if the RAS server is configured with DNS server addresses. The client will acquire the first DNS server address listed in the RAS server's DNS Service Search Order.

Subnet Mask

The subnet mask corresponds to the standard mask associated with the standard class type of the given IP address.

NetBIOS Scope ID

NetBIOS scope ID information is not passed to the client. If you need to modify this setting it will have to be changed directly on the client.

Node Type

Node Type is not taken from the DHCP lease but can change on the RAS client depending on WINS information. If the RAS server has no WINS servers defined locally, a b-node Windows NT RAS client will remain a b-node client. If the RAS server has WINS servers defined locally, a b-node Windows NT RAS client will switch to h-node for the duration of the connection.

NOTE: Windows 95 clients will not automatically switch between node-types if the RAS server supplies WINS addresses, this will have to be done manually. Click Control Panel and double-click Network. On the Selection tab, select TCP/IP from the list of installed network components and then click Properties. Click the WINS Configuration tab and either manually specify WINS addresses or select "Use DHCP for WINS Resolution".

For additional information, please see the following article(s) in the Microsoft Knowledge Base:

   ARTICLE-ID: Q121005
   TITLE     : DHCP Options Supported by Clients

   ARTICLE-ID: Q124358
   TITLE     : RAS Server Assigns Cached IP Addresses to RAS Clients

   ARTICLE-ID: Q142303
   TITLE     : DNS Address from RAS Server Not Passed to RAS Client

   ARTICLE-ID: Q160177
   TITLE     : Default Node Type For Microsoft Clients

Keywords          : ntnetserv NTRAS NTSrvWkst nttcp kbnetwork
Version           : WinNT:3.5,3.51,4.0
Platform          : winnt
Issue type        : kbinfo