The &Attribute-Name Operator

The &Attribute-Name operator returns a reference to the named attribute.

When used as an existence check, the condition evaluates to true if the attribute exists. Otherwise, the condition evaluates to false.

Older versions of the server did not use the & character. Version 3 does not require it, but a warning will be printed in debugging mode if the & is omitted.

The & character was added in version 3 in order to disambiguate the grammar of unlang. Previously, older versions of the server could interpret a string such as hello-there either as a literal string "hello-there", or as a reference to an attribute named hello-there. This ambiguity caused problems.

Adding the leading & character means that attribute references are now easily distinguishable from literal strings.

Examples:

&User-Name
&NAS-IP-Address