Remote Access VPN Connections
For remote access VPN connections, a computer creates a remote access connection to a VPN server. During the connection process the VPN server assigns an IP address for the remote access VPN client and changes the default route on the remote client so that default route traffic is sent over the virtual interface.
IP Addresses and the Dial-Up VPN Client
For dial-up VPN clients who connect to the Internet before creating a VPN connection with a VPN server on the Internet, two IP addresses are allocated:
When creating the PPP connection, IPCP negotiation with the ISP NAS assigns a public IP address.
When creating the VPN connection, IPCP negotiation with the VPN server assigns an intranet IP address. The IP address allocated by the VPN server can be a public IP address or private IP address, depending on whether your organization is implementing public or private addressing on its intranet.
In either case, the IP address allocated to the VPN client must be reachable by hosts on the intranet and vice versa. The VPN server must have appropriate entries in its routing table to reach all the hosts on the intranet and the routers of the intranet must have the appropriate entries in their routing tables to reach the VPN clients.
The tunneled data sent through the VPN is addressed from the VPN client's VPN server-allocated address to an intranet address. The outer IP header is addressed between the ISP-allocated IP address of the VPN client and the public address of the VPN server. Because the routers on the Internet only process the outer IP header, the Internet routers forward the tunneled data to the VPN server's public IP address.
An example of dial-up client addressing is shown in Figure 9.14 where the organization uses private addresses on the intranet, and the tunneled data is an IP datagram.
Figure 9.14 Public and Private Addresses in PPTP Tunneled Data
Default Routes and Dial-Up Clients
When a typical dial-up client dials the ISP, it receives a public IP address from the ISP NAS. A default gateway address is not allocated as part of the IPCP negotiation process. Therefore, in order to reach all Internet addresses, the dial-up client adds a default route to its routing table using the dial-up interface connected to the ISP. As a result, the client can forward the IP datagrams to the ISP NAS from where they are routed to its Internet location.
For dial-up clients with no other TCP/IP interfaces, this is the wanted behavior. However, this behavior can cause confusion for dial-up clients that have an existing LAN-based connection to an intranet. In this scenario, a default route already exists pointing to the local intranet router. When the dial-up client creates a connection with their ISP, the original default route remains in the routing table but is changed to have a higher metric. A new default route is added with a lower metric using the ISP connection.
As a result, the intranet locations that are not on the dial-up client's directly attached network are not reachable for the duration of the connection to the ISP. If the new default route is not created, all intranet locations are reachable, but Internet locations are not.
A Windows 2000–based dial-up client creates the default route by default.
To prevent the default route from being created
In the properties of the TCP/IP protocol of the dial-up connection object, in the Advanced TCP/IP Settings dialog box, click the General tab, and then clear the Use default gateway on remote network check box.
To achieve connectivity to both intranet and Internet locations while the ISP connection is active, leave the Use default gateway on remote network option selected and add the routes of the intranet to the routing table of the dial-up client. The intranet routes can be added through static persistent routes using the route utility, or, if Routing Information Protocol (RIP) version 1 is being used as the intranet routing protocol, you can use the Route Listening Service to listen to RIP version 1 routing protocol traffic and dynamically add intranet routes. When connected to the ISP, all intranet locations are reachable using the intranet routes and all Internet locations are reachable using the default route.
Default Routes and VPNs over the Internet
When the dial-up client calls the ISP, it adds a default route using the connection to the ISP as shown in Figure 9.15. At this point, it can reach all Internet addresses through the router at the ISP NAS.
Figure 9.15 Default Route Created When Dialing an ISP
When the VPN client creates the VPN connection, another default route and a host route to the IP address of the tunnel server are added, as illustrated in Figure 9.16. The previous default route is saved but now has a higher metric. Adding the new default route means that all Internet locations except the IP address of the tunnel server are not reachable for the duration of the VPN connection.
Figure 9.16 Default Route Created When Initiating the VPN
Just as in the case of a dial-up client connecting to the Internet, when a dial-up VPN client using voluntary tunneling creates a VPN connection to a private intranet across the Internet, one of the following occurs:
Internet locations are reachable and intranet locations are not reachable when the VPN connection is not active.
Intranet locations are reachable and Internet locations are not reachable when the VPN connection is active.
For most Internet-connected VPN clients, this behavior does not represent a problem because they are typically engaged in either intranet or Internet communication, not both.
For VPN clients who want concurrent access to intranet and Internet resources when the VPN is connected, the solution depends on the nature of the IP addressing in the intranet. In all cases, configure the VPN connection object so that it does not add a default gateway. When the VPN connection is created, the default route remains pointed to the ISP NAS, allowing access to all Internet addresses.
Based on the type of intranet addressing you use, enable concurrent access to intranet and Internet resources as follows:
Public Addresses Add static persistent routes for the public network IDs of the intranet using the IP address of the VPN server's virtual interface as the gateway IP address.
Private Addresses Add static persistent routes for the private network IDs of the intranet using the IP address of the VPN server's virtual interface as the gateway IP address.
Overlapping or Illegal Addresses If the intranet is using overlapping or illegal addresses (IP network IDs that are not private and have not been registered by Internet Network Information Center [InterNIC] or obtained from an ISP), those IP addresses might be duplicated by public addresses on the Internet. If static persistent routes are added on the VPN client for the overlapping network IDs of the intranet, the locations on the Internet for the overlapping addresses are not reachable.
In each of these cases, static persistent routes for the network IDs of the intranet need to be added to the VPN client. When the persistent routes are added, they are saved in the registry. With Windows NT 4.0 Service Pack 3 and later and with Windows 2000, the persistent routes are not actually added to the IP routing table (and are not visible with the route print command at the Windows 2000 command prompt) until the IP address of the gateway is reachable. The IP address of the gateway becomes reachable when the VPN connection is made.
For each route, type the following route utility syntax at a Windows 2000 command prompt:
ROUTE ADD <Intranet Network ID> MASK <NetMask> <IP address of VPN server's virtual interface> -p
The gateway IP address in the route commands for each intranet route is the IP address assigned to the VPN server's virtual interface, not the IP address of the VPN server's Internet interface.
You can determine the IP address of the VPN server's virtual interface from the IP address of the Internal interface under IP Routing - General in the Routing and Remote Access snap-in. If you use DHCP to obtain IP addresses for dial-up networking and VPN clients, the IP address of the VPN server's virtual interface is the first IP address obtained when requesting DHCP addresses. If you have configured a static IP address pool, the IP address of the VPN server's virtual interface is the first IP address in the static IP address pool. You can also determine the IP address of the VPN server's virtual interface by double-clicking the virtual private networking connection object when the VPN connection is active. In the resulting Status dialog box, click the Details tab.
For all of these cases, you must add the routes very carefully to ensure that the private traffic to the intranet is forwarded using the VPN connection and not the PPP connection to the ISP. If the wrong routes are added, the traffic that you intend to forward across the VPN in an encrypted form is instead sent unencrypted across the Internet. For example, if your intranet is using the public network ID 220.127.116.11/24 (subnet mask 255.255.255.0), and you mistakenly add a persistent static route for 18.104.22.168/24, all traffic to the intranet network 22.214.171.124/24 is forwarded across the Internet in plaintext, rather than being encrypted and sent across the VPN connection.