c# qr code generator source How to Configure HTTP Web Filter Signatures in Visual C#

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How to Configure HTTP Web Filter Signatures
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An HTTP signature can be any string of characters in the HTTP header or body. To block an application based on signatures, you must identify the specific patterns the application uses in request headers, response headers, and body, and then modify the HTTP policy to block packets based on that string. One of the difficulties in configuring the HTTP policy to block packets based on the signature is ensuring that the signature contains the specific information you need to block the chosen HTTP packet while not blocking other packets. For example, if you create an HTTP policy to block the word Mozilla, you would block most Web browsers as well as other applications. This is because most Web browsers are Mozilla-compatible and include this term in HTTP headers. In most cases, you should use a more application-specific string. For example, to block MSN Messenger, configure the rule to block User-Agent: MSN Messenger in the request header. To configure how the HTTP filter will manage signatures, follow this procedure: 1. To access the HTTP policy associated with a specific rule, right-click the rule in ISA Server Management Console and select Configure HTTP.
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Implementing Application and Web Filtering
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2. To modify the HTTP signatures settings on the HTTP policy, click the Signatures tab as shown in Figure 7-21. The Signatures tab shows the signatures that will be blocked.
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Configuring the HTTP policy signatures
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3. Click Add to add a signature to the list as shown in Figure 7-22. In the Signature dialog box, in the Name text box, type a name for the signature search.
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Figure 7-22
Configuring a specific HTTP signature
4. Under Signature search criteria, in the Search In drop-down list, select the part of the client request or server response you want the Web filter to search. Then, in the Signature text box, type the string that you want to filter. 5. When you have added signatures to the list on the Signatures tab, you can enable or disable specific signatures using the check boxes next to the signature names.
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Configuring ISA Server as a Firewall
Note When you configure the HTTP policy to search the HTTP request body or response body, you can also specify how much of the body will be scanned for the signature and what format to use when scanning. By default, the filter will scan only the first 100 bytes of the request or response body. Increasing this number can negatively affect the server performance. You can also specify whether to search using text or binary format.
You can use HTTP signatures to block access to specific applications or to specific content. For example, if you configure the policy to block User-Agent: MSN Messenger in the request header, users will not be able to use MSN Messenger through the firewall. You can also block access to sites that might contain malicious code if you are aware of common malicious code. For example, a Web page containing <iframe src=" "/> will cause Internet Explorer to use up central processing unit (CPU) resources in an infinitely nested iframe element. To prevent access to Web pages containing this code, use a signature that searches in the response body for the text <iframe src=" "/>. HTTP signature filtering assumes that all HTTP requests and responses are Uniform Transformation Format-8 (UTF-8, a transformation of Unicode character encoding) encoded. If a different encoding scheme is used, signature blocking cannot be performed. Table 7-8 shows some of the signatures used by common HTTP-based applications.
If you do not know the application signature for an application that you want to block, you can use a network sniffer, such as Network Monitor, to capture the traffic between the application client and a server. Then you can analyze the traffic to determine which signature you should block.
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