On a Forefront Threat Management Gateway (TMG) 2010 firewall you may encounter a Configuration Error alert like this:
The alert description states:
“The routing table for the network adapter Internal includes IP address ranges that are not defined in the array-level network Internal, to which it is bound. As a result, packets arriving at this network adapter from the IP address ranges listed below or sent to these IP address ranges via this network adapter will be dropped as spoofed. To resolve this issue, add the missing IP address ranges to the array network.
The following IP address ranges will be dropped as spoofed:
This alert is the result of the Forefront TMG firewall’s routing table and network definition being out of sync with each other. In this example, the routing table looks like this:
However, the Forefront TMG Internal network definition looks like this:
As you can see, the Forefront TMG firewall is configured with an Internal network IP address range of 172.16.1.0/24. However, the routing table contains additional static routes that also make the 172.16.2.0/24 and 172.16.3.0/24 networks reachable.
To resolve this issue, highlight the Networking node in the navigation tree, select the Networks tab in the center window, then highlight the network that corresponds to the IP address range contained in the alert. In our example the address range 172.16.2.0-172.16.3.255 also belongs to the Internal network. Right-click the Internal network and choose properties, choose the Addresses tab, then remove all address ranges previously configured. Next, choose Add Adapter and choose the network adapter for this network.
Using this method the IP address range for this network is built using the routing table for the network interface. This is the preferred method for defining IP address ranges for Forefront TMG networks. Save the changes and apply the configuration.
For more information on configuring network interfaces for Forefront TMG 2010 firewalls, please refer to Jason Jones’ excellent documentation on the subject here: