When Always On VPN clients connect to the VPN server, they must be assigned an IP address to facilitate network communication. When using Windows Server and Routing and Remote Access Service (RRAS) for VPN services, administrators must choose between Dynamic Host Configuration Protocol (DHCP) and static address pool assignment methods.
DHCP
DHCP is a quick and easy way to handle VPN client IP address assignment. However, there are some drawbacks and limitations associated with this option. Consider the following.
Allocation
DHCP for Always On VPN clients does not work as you might expect. For example, when a VPN client connects, it does not obtain its IP address directly from the DHCP server. Instead, the VPN server leases a block of IP addresses from the DHCP server and manages those on behalf of its clients. On the DHCP server, you will see the Unique ID column of these IP address leases indicating RAS.
Address Block Size
After configuring the VPN server to use DHCP VPN client IP address assignment, the VPN server will automatically lease a block of ten IP addresses from a DHCP server. When this initial block of ten IP addresses is exhausted, the VPN server will lease another block of ten IP addresses. Administrators can increase the size of the requested address block by creating the following registry key on each VPN server.
Key: HKLM\SYSTEM\CurrentControlSet\Services\RemoteAccess\Parameters\IP
Value: InitialAddressPoolSize
Type: DWORD
Data: <size of DHCP pool request>
Alternatively, administrators can download Update-VpnServerDhcpPoolSize.ps1 from my GitHub repository and run it on each VPN server to increase the size of the initial DHCP address pool request.
DHCP Options
The VPN server discards all DHCP option information returned by the DHCP server. The VPN server uses only the IP address from the DHCP lease. The client is unaware of any other information in the DHCP lease.
Subnet
By default, the VPN server will only request DHCP addresses from a scope that matches the same subnet as the IP address assigned to the VPN server’s network adapter. If the VPN server has more than one network interface, it will send DHCP requests from the network interface listed on the Adapter drop-down list, as shown here.
Note: This option is only available on servers configured with multiple network interfaces. Also, if the value is set to Allow RAS to select adapter, it is best to specifically define the network interface where DHCP and DNS requests are made.
Scope Size
When using the DHCP assignment method, ensure the DHCP scope contains enough IP addresses to support the number of concurrent connections expected on all VPN servers.
IPv6
DHCPv6 is not supported on RRAS for VPN client IP address assignment. The only option for IPv6 is prefix assignment.
RRAS in Azure
DHCP is not supported when deploying RRAS in Azure. Administrators deploying RRAS in Azure to support Always On VPN must use the static address pool assignment method. More details here.
Known Issues
When using DHCP with Windows Server 2019 RRAS servers, a known issue prevents this from working correctly. Administrators can download Update-VpnServerDhcpPrivileges.ps1 from my GitHub repository and run it on each VPN server to ensure proper DHCP operation.
Increased Complexity
Since the VPN server leases IP addresses on behalf of clients and discards DHCP option information included in the lease, there’s no real benefit to using DHCP. Using DHCP only adds complexity and introduces another dependency, making the solution more brittle and difficult to manage. Using the static address pool assignment method is a better choice.
Static Pool
Implementation best practices dictate using the static address pool assignment method instead of DHCP. The following is guidance for configuring RRAS to support the static address pool option for VPN client IP address assignment.
Unique Subnet
Using a unique IP subnet is best when using the static address pool assignment method. However, this also requires configuring internal network routing to return traffic for that subnet to the individual VPN server where that subnet is assigned. Each server must have a unique IP address pool assigned. Define static address pools using subnet boundaries when configuring multiple VPN servers. Assigning IP address pools along subnet boundaries simplifies internal network routing configuration. Ensure that assigned IP address pool subnets are large enough to accommodate the total number of concurrent connections expected on each server. Be sure to overprovision to handle failover scenarios.
Same Subnet
Alternatively, administrators can assign VPN client IP addresses from the same subnet as the VPN server’s network interface. Assigning VPN client IP addresses from the same subnet as the VPN server eliminates the need for any internal network routing configuration, simplifying deployment. However, server subnets are often small and may not have enough IP address space to support numerous concurrent VPN connections. Be sure to plan accordingly.
Static IP Addresses
It is possible to assign a static IP address to an individual user. However, assigning a static IP address to a specific device is not. I will discuss static IP address assignments for Always On VPN clients in a future blog post.
Other Limitations
Here are some additional things to consider when creating a VPN client IP addressing strategy.
DNS
Always On VPN clients can be configured to register their IP address in DNS. However, the VPN client configuration controls this setting. The DHCP server does not register IP addresses in DNS when using DHCP. The client registers its IP address in DNS directly after it connects. In addition, a VPN client will receive a different IP address each time it connects to the VPN server. DNS propagation can delay hostname resolution on-premises for remote-connected VPN clients.
Selective Addressing
Regardless of which assignment method is selected, assigning different IP addresses to different types of connections is not possible. For example, a common ask is to assign user connections from one IP address pool and device connections from another. The only option to support this is to use different servers for each type of connection.
Summary
The best practice for IPv4 VPN client addressing is to use the static address pool method with a unique IPv4 subnet per server. Using static address pool assignment provides the most flexible configuration options and eliminates the dependency on internal services, making the solution more resilient and easier to manage. A unique address pool per server ensures that a large enough subnet can be defined to support the expected number of concurrent connections, regardless of the subnet size the VPN server is assigned to. Also, a unique IP subnet for VPN clients makes configuring internal firewall rules to control VPN client access easier.
Additional Information
Always On VPN Client DNS Server Configuration
