Active Directory Federation Services (AD FS) provides Web single-sign-on to authenticate a user to related external hosted Web applications. AD FS performs this by securely sharing digital identity and entitlement rights or claims across security and enterprise boundaries.
AD FS supports distributed authentication and authorization over the Internet to provide access to resources that are offered by trusted partners.
Another aspect of ADFS technology can be found in providing external access from Internet connections to internal resources. In that case the ADFS server can provide an additional layer of security by offering various pre-authentication methods, while the second part of the ADFS technology, the Web Application Proxy server (WAP) acts as a Reverse Proxy by terminating the incoming SSL connections.
In version 3.0 of the ADFS technology, the WAP server cannot be run without ADFS server in the backend, which stores the configuration of the WAP servers. The WAP servers themselves are “stateless” and therefore easy to scale up behind a Layer 4 Load Balancer.
ADFS Server Farms
The Active Directory Federation Services technology can be scaled out by deploying multiple ADFS servers in a farm model. The servers share the same configuration information which is stored in a database on each server (Windows Internal Database (WID) model) or in a central SQL store. In most of the implementations, the WID model is used.
For using the WID model, the configuration can only be modified on the Primary ADFS server and is then replicated to all other ADFS servers of the same farm.
To find the Primary Server use the command “get-adfssyncproperties” on one of the ADFS servers:
Web Application Proxy server
The Web Application Proxy Server is typically the Internet facing component of the Active Directory Federation Services technology. Located in the DMZ, the Web Application Proxy (WAP) servers act as reverse proxy server and terminate the incoming SSL connects from the Internet to the published applications.
Web Application Proxy servers are N to 1 connected to a specific ADFS server (farm). Multiple WAP servers can be easily configured for Layer 4 Loadbalancing.
Since WAP servers are “stateless”, they do not store any persistent configuration information, but load the information from the Primary ADFS server. Therefore a WAP server cannot exist without underlying ADFS server and needs to be installed after the ADFS farm has been deployed.
When acting as reverse proxy for client access using IWA (Integrated Windows Authentication) or when serving non claims aware application access based on Kerberos, the WAP servers must be able to perform Kerberos Constrained Delegation. The WAP server presents a Kerberos token on behalf of the accessing client or user, which in consequence requires the WAP server to be a member of an Active Directory domain. Unfortunately the domain membership of the WAP server means to open a lot more ports from the DMZ to the internal network, which is a disadvantage from network security perspective.
For that reason, applications that do not require Kerberos Constrained Delegation should always be published on non domain based WAP servers, where the exception is to publish applications on domain based WAP servers.
Example for ADFS Farm structure:
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