Introduction

 

Consider a scenario where the ISA Server administrator has dial-in VPN correctly configured and working through ISA Server 2006. Now he needs to use RADIUS as the credentials authority and he makes the appropriate changes to the RADIUS configuration on the VPN settings as shown in Figures 1, 2 and 3:

 

 

Figure 1 – RADIUS correctly enabled on ISA.

 

 

Figure 2 – RADIUS Server using the default authentication port.

 

 

Figure 3 – RADIUS Server with the shared secret set.

 

After applying the changes, the RRAS service stops and the following event appears in Event Viewer when RRAS tries to restart:

 

Event Type: Error

Event Source:     Microsoft Firewall

Event Category:   None

Event ID:   21098

Date:       12/15/2009

Time:       4:16:46 PM

User:       N/A

Computer:   ISACONTN1

Description:

The RADIUS server list is empty. As a result, the Remote Access Service may fail to start.

 

Understanding the Behavior

 

Clearly the RADIUS server list is not empty as you can see in Figure 2 and 3. However if you observe other events in the event viewer you will see that the RADIUS server failure event is almost immediately preceded by this one:

 

Event Type: Warning

Event Source:     Microsoft Firewall

Event Category:   None

Event ID:   21301

Date:       12/15/2009

Time:       4:16:43 PM

User:       N/A

Computer:   ISACONTN1

Description:

The server name dccont cannot be resolved by DNS to a valid IP address.

 

Using Network Monitor during the moment of the failure, you can see that the Windows OS (where ISA server is installed) sends the DNS query for this name:

10.20.20.1  dccont.contoso.com      DNS   DNS:QueryId = 0xDE85, QUERY (Standard query), Query  for dccontn1.contoso.com of type Host Addr on class Internet

 

The DNS Server reply with the following answer:

 

dnsrv.contoso.com 10.20.20.1  DNS   DNS:QueryId = 0xDE85, QUERY (Standard query), Response - Name Error

 

What happens is that if ISA Server is not able to resolve the name of the RADIUS Server it can’t really proceed with this configuration because it has no credentials authority for the VPN client connections.

 

Common errors that can cause this are:

1.       “dccont” was specified as an unqualified name (as shown in Figures 2 & 3). This leaves the server having to rely on the domain suffix devolution to resolve the name to an IP address.  If the ISA has no domain suffix or the domain where the “dccont” RADIUS server registers its name is unknown to the ISA server or its DNS servers, the ISA server won’t resolve that name. You should always use fully-qualified names to avoid this behavior.

2.       dccont.contoso.com is not known to the DNS server used by the ISA server. This may be due to an incorrect DNS configuration at the ISA server or the RADIUS server is not properly registered in DNS

3.       DNS forwarding or recursion is failing at the DNS server used by the ISA server.

 

There are no doubt many other reasons for name resolution to fail at the chosen DNS server; you’ll just have to put on your Sherlock Holmes hat and get sleuthing.

 

Author

Yuri Diogenes

Sr Security Support Escalation Engineer

Microsoft CSS Forefront Edge Team

 

Technical Reviewer

Jim Harrison

Program Manager

Microsoft CSS Forefront Edge CS Team