Manual Chapter : Writing Custom Bot Signatures
Applies To:Show Versions
- 17.1.0, 17.0.0, 16.1.3, 16.1.2, 16.1.1, 16.1.0, 16.0.1, 16.0.0, 15.1.8, 15.1.7, 15.1.6, 15.1.5, 15.1.4, 15.1.3, 15.1.2, 15.1.1, 15.1.0, 15.0.1, 15.0.0, 14.1.5, 14.1.4, 14.1.3, 14.1.2, 14.1.0
Writing Custom Bot Signatures
About custom bot signatures
If your organization has additional needs for bot defense, you can write a
custom bot signatureto identify web robots by looking for specific patterns in the headers of incoming HTTP requests.
Bot signatures are similar to attack signatures; they are written using a limited subset of allowed keywords. You can design custom bot signatures to handle emergency situations, to support security policy enforcement unique to your networking environment, or to provide an analysis of specific activity on the network.
Being able to classify bots into different classes allows you to treat each class differently. You can report, block, or do nothing when a signature matches a malicious or trusted bot class. Further, malicious and trusted bots fall into more specific bot signature categories that can be handled as needed. You can create new categories if they are needed to classify custom bot signatures.
Bot signature syntax limitations
Bot signatures are developed using Snort syntax to search for bots in either the User-Agent field of the header or the URL, or both. The User-Agent field is examined to identify the browser and operating system. The URL is searched to locate bots that access specific peculiar URLs within a site, regardless of whether the site has such a URL (in most cases it does not).
The syntax of bot signatures is similar to that of attack signatures using the general format
keyword: "value"; modifier;but bot signatures can include only the following attributes:
- uricontent(objonlyflag is not allowed)
- headercontent(useragentonlyflag is mandatory)
- ipp(for regular expressions)
The following are not allowed in bot signatures:
- norm(normalization is predefined)
Refer to the
Signature Syntax, and examples for additional details on the syntax used in bot signatures.
Creating a custom bot signature
You can write custom bot signatures to increase bot protection for your web application.
- On the Main tab, click.
- ClickCreate.The Bot Signature Properties screen opens.
- In theBot Namefield, type a unique name for the bot signature.If you attempt to create a custom bot signature with the same name as a system-supplied signature, you will receive an error and the signature will not be created.
- Select a bot category for the signature.
- In theRulesetting, develop the bot signature.
If a bot signature requires multiple search strings, a conditional text match, or a search of both the URL and User-agent contexts, you cannot use the simple edit mode.To develop more complex bot signatures, use theAdvanced Edit Modeto type expressions using Snort control. Refer to theSignature OptionsandSignature Syntaxsections for details. Refer toBot signature syntaxfor special limitations when writing bot signatures.As an example, this signature searches the header for three terms: SODA, BAR, and for a specific hexadecimal value.headercontent:\"SODA\"; useragentonly; nocase; headercontent:\"BAR\"; useragentonly; nocase; headercontent:\"0x31303235343830303522\"; useragentonly; nocase;In this example, the bot signature searches both User-Agent and the URL:headercontent: "MaliciousBot/0.1"; useragentonly; uricontent: "/settings.php";
- If the bot signature will search for patterns in the user-agent string or URL, useSimple Edit Mode.
- If searching the user-agent string, forUser-agent, select eithercontainsorregexpmatch, and then type the pattern in the text box.
- If searching requests within the normalized URI path (excluding the Query String), forURL, select eithercontainsorregexpmatch, and then type the pattern in the text box.
- In theDomainsfield, type the name of one or more domains from which the bot can send its requests, and clickAdd.This field is mandatory only for benign bots in the Search Engines category to validate their identity using reverse DNS lookup.
- For theRisksetting, select a risk level.The risk level indicates the level of potential damage an attack containing this text may cause, if it were successful.
- ClickCreateto create the new bot signature.
The new bot signature is added to the bot signature list. The signature is now available for use in your Bot Defense Profiles. After associating your Bot Defense Profiles with a virtual server, the system examines traffic for bots (using the signature you developed plus others that are enabled) as one of the first checks.
Creating a new bot signature category
You can create customized bot signature categories if the default categories do not include the one you are looking for. You can create a new malicious or benign bot category.
- On the Main tab, click.The Bot Signature Categories List screen opens.
- Enter the new bot signature category name and select a bot class for the new category.
- ClickSaveto save the new bot signature category on the system.
The bot signature category is added to the appropriate list. If you create a customized bot signature, you can assign the category you created to that signature.