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ISA 2006 Flood Mitigation Strategies

October 18, 2009

The flood mitigation features included in Microsoft ISA Server 2006 were one of many improvements over previous versions of ISA. Enabled by default, this enhanced network protection functionality allows the ISA firewall to withstand direct attacks (e.g. DoS or SYN flood) and provides resiliency in the event of a worm breakout. There are times, however, when this feature can impede valid network communication. If, for example, a host protected by the ISA firewall is very busy it may run in to connection limits imposed by the firewall. When this happens you may see the following error in the event log: ‘TCP connections per minute from one IP address limit exceeded’.


When legitimate network communication is dropped for this reason, it is possible to configure the firewall to allow more connections for this host. This is accomplished by opening the ISA management console, expanding the ‘Configuration’ node in the console tree, then clicking on the ‘Configure Flood Mitigation Settings’ link in the ‘Additional Security Policy’ section.


Too often I see administrators disable flood mitigation altogether. This is strongly discouraged. I also see administrators raise connection limits for ALL hosts by clicking on the ‘Edit…’ button and entering a new limit. This is also a bad practice. The best way to resolve this problem is to add the host(s) to a computer set, then add that computer set to the ‘IP Exceptions’ list.


In my experience this often needs to be done for DNS servers and for busy mail servers. Your alerts will tell you which systems are good candidates for the exception list though, so be sure to monitor your ISA firewalls closely.