The ISA Firewall resolves names on behalf of Web proxy and Firewall clients. When the ISA Firewall resolves names for these clients, it caches these entries for six hours. Some ISA Firewall admins might have problems with accessing certain Internet sites that change their IP addresses more frequently than every six hours (typically small business sites that use DDNS services for name resolution).
One way to dump the DNS cache on the ISA Firewall is to restart the Firewall service, but there should be a less disruptive way to remove old entries from the DNS cache. The solution is the ISA Firewall DNS Cache Tool. You can download the DNS cache tools at: http://www.microsoft.com/technet/isa/downloads/2006/tools/default.mspx
For example, if you want to delete the DNS cache entry for www.microsoft.com, you would enter:
dnstools /C /SN www.microsoft.com