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Manual Chapter: Introduction to iRules
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What is an iRule?

An iRule is a powerful and flexible feature within BIG-IP Local Traffic Manager that you can use to manage your network traffic. Using syntax based on the industry-standard Tools Command Language (Tcl), the iRules feature not only allows you to select pools based on header data, but also allows you to direct traffic by searching on any type of content data that you define. Thus, the iRules feature significantly enhances your ability to customize your content switching to suit your exact needs.

Important: For complete and detailed information on iRules syntax, see the F5 Networks DevCentral web site, Note that iRules must conform to standard Tcl grammar rules; therefore, for more information on Tcl syntax, see

An iRule is a script that you write if you want individual connections to target a pool other than the default pool defined for a virtual server. iRules allow you to more directly specify the destinations to which you want traffic to be directed. Using iRules, you can send traffic not only to pools, but also to individual pool members, ports, or URIs. The iRules you create can be simple or sophisticated, depending on your content-switching needs.

when CLIENT_ACCEPTED { if { [IP::addr [IP::client_addr] equals] } { pool my_pool } }

This iRule is triggered when a client-side connection has been accepted, causing Local Traffic Manager to send the packet to the pool my_pool, if the client’s address matches

Using a feature called the Universal Inspection Engine, you can write an iRule that searches either a header of a packet, or actual packet content, and then directs the packet based on the result of that search. iRules can also direct packets based on the result of a client authentication attempt.

iRules can direct traffic not only to specific pools, but also to individual pool members, including port numbers and URI paths, either to implement persistence or to meet specific load balancing requirements.

The syntax that you use to write iRules is based on the Tool Command Language (Tcl) programming standard. Thus, you can use many of the standard Tcl commands, plus a robust set of extensions that Local Traffic Manager provides to help you further increase load balancing efficiency.

Important: When referencing an object within an iRule, you must include the full path name of the object.

iRule commands

An iRule command within an iRule causes Local Traffic Manager to take some action, such as querying for data, manipulating data, or specifying a traffic destination. The types of commands that you can include within iRules are:

Statement commands
These commands cause actions such as selecting a traffic destination or assigning a SNAT translation address. An example of a statement command is pool <name>, which directs traffic to the named load balancing pool.
Commands that query or manipulate data
Some commands search for header and content data, while others perform data manipulation such as inserting headers into HTTP requests. An example of a query command is IP::remote_addr, which searches for and returns the remote IP address of a connection. An example of a data manipulation command is HTTP::header remove <name>, which removes the last occurrence of the named header from a request or response.
Utility commands
These commands are functions that are useful for parsing and manipulating content. An example of a utility command is decode_uri <string>, which decodes the named string using HTTP URI encoding and returns the result.

Event declarations

iRules are event-driven, which means that Local Traffic Manager triggers an iRule based on an event that you specify in the iRule. An event declaration is the specification of an event within an iRule that causes Local Traffic Manager to trigger that iRule whenever that event occurs. Examples of event declarations that can trigger an iRule are HTTP_REQUEST, which triggers an iRule whenever the system receives an HTTP request, and CLIENT_ACCCEPTED, which triggers an iRule when a client has established a connection.

when HTTP_REQUEST { if { [HTTP::uri] contains "aol" } { pool aol_pool } else { pool all_pool } }


An iRule operator compares two operands in an expression.

For example, you can use the contains operator to compare a variable operand to a constant. You do this by creating an if statement that represents the following: "If the HTTP URI contains aol, send to pool aol_pool."

Creating an iRule

You create an iRule to customize the way that the BIG-IP system processes traffic.
  1. On the Main tab, click Local Traffic > iRules.
  2. Click Create.
  3. In the Name field, type a name, such as my_irule. The full path name of the iRule cannot exceed 255 characters.
  4. In the Definition field, type the syntax for the iRule using Tool Command Language (Tcl) syntax. For complete and detailed information iRules syntax, see the F5 Networks DevCentral web site
  5. Click Finished.
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