Note: For information about configuring the Citrix NetScaler to perform IP-HTTPS preauthentication, click here. For information about configuring Windows Server 2012 R2 to perform IP-HTTPS preauthentication natively, click here.

Introduction

Recently I wrote about security challenges with DirectAccess and the IP-HTTPS IPv6 transition technology. Specifically, IP-HTTPS transition tunnel connections are not authenticated by the DirectAccess server, only the client. This allows an unauthorized device to obtain an IPv6 address on the DirectAccess client network. With it, an attacker can perform network reconnaissance using ICMPv6 and potentially launch a variety of Denial-of-Service (DoS) attacks. For more details, click here.

Note: DirectAccess IPsec data connections not at risk. Data is never exposed at any time with the default configuration.

Mitigation

To mitigate these issues, it is recommended that an Application Delivery Controller (ADC) be used to terminate SSL connections and enforce client certificate authentication. Doing this will ensure that only authorized connections will be accepted by the DirectAccess server. In addition, there are some scalability and performance benefits to implementing this configuration when supporting Windows 7 clients.

Important Considerations

Performing IP-HTTPS preauthentication on the F5 BIG-IP is formally unsupported by Microsoft. In addition, terminating IP-HTTPS on the F5 appliance breaks OTP authentication.

F5 BIG-IP Configuration

To configure the F5 BIG-IP to perform SSL offload for DirectAccess IP-HTTPS, follow the guidance documented here. In addition, to configure the F5 BIG-IP to perform preauthentication for DirectAccess clients, when creating the client SSL profile, click Custom above the Client Authentication section and choose Require from the Client Certificate drop-down list and Always from the Frequency drop-down list. In addition, choose your internal PKI’s root Certification Authority (CA) certificate from the Trusted Certificate Authorities drop-down list and from the Advertised Certificate Authorities drop-down list.

Summary

Enabling client certificate authentication for IP-HTTPS connections ensures that only authorized DirectAccess clients can establish a connection to the DirectAccess server and obtain an IPv6 address. It also prevents an unauthorized user from performing network reconnaissance or launching IPv6 Denial-of-Service (DoS) attacks.