How to Troubleshoot Firewall Problems

Many attacks are initiated across network connections. To reduce the impact of those attacks, Windows Firewall by default blocks unrequested, unapproved incoming traffic and unapproved outgoing traffic. Although Windows Firewall will not typically cause application problems, it has the potential to block legitimate traffic if not properly configured. When troubleshooting application connectivity issues, you will often need to examine and possibly modify the client's or server's Windows Firewall configuration.

Misconfiguring Windows Firewall can cause several different types of connectivity problems. On a computer running Windows 7 that is acting as the client, Windows Firewall might block outgoing communications for the application (though blocking outgoing communications is not enabled by default). On a computer running Windows 7 that is acting as the server (for example, a computer that is sharing a folder), Windows Firewall misconfiguration might cause any of the following problems:

• Windows Firewall blocks all incoming traffic for the application.
• Windows Firewall allows incoming traffic for the LAN but blocks incoming traffic for other networks.
• Windows Firewall allows incoming traffic when connected to a domain network but blocks incoming traffic when connected to a public or private network.

The symptoms of client- or server-side firewall misconfiguration are the same: application communication fails. To make troubleshooting more complex, network firewalls can cause the same symptoms. Answer the following questions to help identify the source of the problem:

1. Can you connect to the server from other clients on the same network? If the answer is yes, you have a server-side firewall configuration problem that is probably related to the configured scope of a firewall exception. If adjusting the scope of the firewall exception does not solve the problem, it is probably caused by a network firewall, and you should contact your network administrators for further assistance.
2. Can you connect to the server when the client is connected to one type of network location (such as a home network or a domain network), but not when it is connected to a different type of network location? If the answer is yes, you have a client-side firewall configuration problem that is probably caused by having an exception configured for only one network location type.
3. Can other clients on the same network connect to the server using the same application? If the answer is yes, you have a client-side firewall configuration problem that is probably caused by having a rule that blocks outgoing traffic for the application.
4. Can the client connect to other servers using the same application? If the answer is yes, you have a server-side firewall configuration problem, and the server needs a firewall exception added. If adding an exception does not solve the problem, it is probably caused by a network firewall, and you should contact your network administrators for further assistance.