iRules

iRules is a scripting language used with F5 Networks' BIG-IP application delivery controllers to customize traffic handling. The language is built on Tcl and adds F5-specific commands and event-driven constructs that run in the device’s data plane, enabling fine-grained control over TCP/IP and application-layer behavior. iRules provide a programmable interface for implementing complex traffic management policies without external components.

iRules execute in response to events such as CLIENT_ACCEPTED, CLIENT_CLOSED, SERVER_CONNECTED, HTTP_REQUEST, and HTTP_RESPONSE. Within an iRule, administrators write handlers that inspect and modify traffic using commands in the HTTP::, TCP::, and UDP:: namespaces, along with general LTM commands like pool, server, and log. For example, an iRule can inspect an HTTP request and rewrite headers, redirect clients, select a backend pool, or enforce rate limits.

Typical use cases include URL-based routing, content switching, A/B testing, cookie-based persistence, security controls, and other customized load-balancing decisions that go beyond standard virtual server settings. iRules are attached to a virtual server, and traffic matching the virtual server triggers the relevant iRule events, allowing tailored behavior for both inbound requests and outbound responses.

While powerful, iRules add maintenance considerations and can impact performance if overly complex. They require Tcl-like familiarity and careful testing; configurations are often version-controlled. iRules can be complemented by newer options such as iRules LX (JavaScript-based) and iApps, but core iRules remain the primary mechanism for on-device traffic control on BIG-IP devices.