News

Fix Available for Windows XP SP2 Problem

• By Scott Bekker
• 09/22/2004

In loopback, an application sends a test message to the loopback address on a machine that is not delivered to the network but is instead returned to the sending application. Applications often use the feature to test the behavior of the local network adapter.

Internet Protocol officially reserves the range from 127.0.0.0 through 127.255.255.255 for loopback proposes. With SP2, Microsoft chose to reject all messages in the loopback range except for the 127.0.0.1 address.

"Programs that connect to IP addresses in the loopback address range may not work as expected and you may receive an error message indicating you cannot establish a connection," Microsoft notes on the download page for the configuration change.

VPN users were among the most likely to experience problems because of the loopback rejection. Microsoft is not the first firewall provider to cause problems for VPN users with its loopback settings. VPN users have had loopback rejection problems when using high security settings with third-party firewall software, such as ZoneAlarm.

Microsoft released a hotfix for the problem in August, but the new version is available without having to go through Microsoft support. A restart may be required after installing the new fix.

The fix is a 507 KB download that is available here.