I received a support call a few days ago from a customer saying that he has an environment where all users from the Front Desk need to log on to their workstations using one particular domain account called frontdesk. Those users are not allowed to access the Internet. On the customer’s ISA Server 2004 computer, Integrated Windows authentication was specified and only the users from the domain group Internet Users could access the Internet. Front Desk users who require Internet access are allowed access when they log on as users from the domain group Internet Users.
The behavior that the customer expected was:
1. User logs on to the domain with the frontdesk domain account.
2. User tries to access the Internet, and because the user doesn’t belong to the Internet Users group, ISA Server asks for credentials.
3. User has the opportunity to type the domain user account associated with the Internet Users group.
The customer was frustrated, however, because the user received an access denied message immediately after ISA Server tried to authenticate, which is the correct ISA Server action. To attempt to fix the problem, the customer changed the authentication type to Basic authentication. With Basic authentication, all users are prompted for authentication, which is also the correct ISA Server action.
How Does It Work?
Assuming that the ISA Server 2004 computer is a member server of a Windows domain, and it is using Integrated Windows authentication, the following describes the process for when a user is trying to access an Internet Web site through ISA Server:
1. The Windows credentials from the current user are used for authentication purposes.
2. During this negotiation ISA Server contacts the domain controller to present the credentials.
a. If the negotiation fails at this point, the user is prompted for authentication.
3. The domain controller responds, allowing or denying the authentication.
4. Because the user is identified, ISA Server processes the Firewall policy rule using those credentials to see if this user is allowed to access the external resource. (For more information, see the article How firewall policy works at the Microsoft TechNet Web site.)
As you can see, between steps 2 and 3, there is a conditional statement. The following are situations where ISA Server prompts the user for authentication:
- If ISA Server cannot contact the domain controller for any reason (for example, connectivity).
- If the domain controller resets the connection with ISA Server during this negotiation.
- If the user doesn’t belong to the domain where ISA Server is sending the authentication. (For example, if the user is logged on with the local user account for that particular workstation.)
In this customer’s case, the user was identified during the negotiation and the user didn’t belong to the Internet Users group, so the expected result is that the user receives an access denied response.
Proving the Case
Some customers want to see proof, rather than just hearing or reading an explanation. When this situation occurs, it is not difficult to show what is happening behind the scenes.
To collect the data, do the following:
1. Attach Network Monitor to the internal network interface card (NIC) of the ISA Server computer. (Optionally, you also can attach Network Monitor to the client workstation and to the domain controller to see all of the communication.)
2. On the ISA Server computer, enable logging using a filter for the client workstation that you are testing as shown on the following screen shot:
3. Start the Network Monitor trace on all computers involved and start logging on the ISA Server computer.
4. As soon as you receive the access denied page, stop the traces and stop logging.
Network Monitor Result on the ISA Server Computer
1. The client request:
2. The ISA Server response (including the request for authentication):
3. The client sends authentication:
4. ISA Server denies access:
ISA Server Logging (Main Fields)
Destination host name
HTTP status code
Client user name
12209 The ISA Server requires authorization to fulfill the request. Access to the Web Proxy filter is denied.
Failed Connection Attempt
12202 The ISA Server denied the specified Uniform Resource Locator (URL).
The collected evidence makes it easier for a network administrator to understand how authentication works. In this particular case, the customer asked, “Is there any workaround so I can keep using my generic account and be prompted for a user name and password?” The answer is yes. The customer can use an easy technique of opening Internet Explorer on the client workstation using the Run As option, and then providing the credentials for the domain account that belongs to the Internet Users group.
Beyond the workaround, the main objective of this post is to explain how Integrated Windows authentication works with ISA Server in the domain environment, what to expect when using Integrated Windows authentication, and how to identify what is happening if authentication fails.
Microsoft Support Engineer –