Conditional Expressions

Conditions are evaluated when parsing if and elsif statements. These conditions allow the server to make complex decisions based on one of a number of possible criteria.

if ( condition ) { ...

elsif ( condition ) { ...

Conditions are expressed using the following syntax:

Syntax Description


Check for attribute existence.


Check return code of a previous module.


Check value of data.

lhs OP rhs

Compare two kinds of data.

( condition )

Check sub-condition

( ! condition )

Negate a conditional check

( condition ) && …​

Check a condition AND the next one

( condition ) || …​

Check a condition OR the next one

if ( &User-Name == "bob" ) {

if ( &Framed-IP-Address == ) {

if ( &Calling-Station-Id == "%{sql:SELECT ...}" ) {

Load-time Syntax Checks

The server performs a number of checks when it loads the configuration files. Unlike version 2, all of the conditions are syntax checked when the server loads. This checking greatly aids in creating configurations that are correct. Where the configuration is incorrect, a descriptive error is produced.

This error contains the filename and line number of the syntax error. In addition, it will print out a portion of the line that caused the error and will point to the exact character where the error was seen. These descriptive messages mean that most errors are easy to find and fix.

Load-time Optimizations

The server performs a number of optimizations when it loads the configuration files. Conditions that have static values are evaluated and replaced with the result of the conditional comparison.

if ( 0 == 1 ) {

The condition 0 == 1 is static and will evaluate to false. Since it evaluates to false, the configuration inside of the if statement is ignored. Any modules referenced inside of the if statement will not be loaded.

This optimization is most useful for creating configurations that selectively load (or not) certain policies. If the condition above was used in version 2, then the configuration inside of the if statement would be loaded, even though it would never be used.