11.2.4 Rewriting Incoming Requests

The need for rewriting the incoming requests arises from the fact that some NASes are very particular about the information they send with the requests. There are cases when the information they send is hardly usable or even completely unusable. For example, a Cisco AS5300 terminal server used as a voice-over IP router packs a lot of information into its Acct-Session-Id attribute. Though the information stored there is otherwise relevant, it makes proper accounting impossible, since the Acct-Session-Id attributes in the start and stop packets of the same session become different, and thus Radius cannot determine the session start to which the given session stop request corresponds (see section 14.2.7 Acct-Session-Id).

In order to cope with such NASes, GNU Radius is able to invoke a Rewrite function upon arrival of the packet and before processing it further. This function can transform the packet so that it obtains the form prescribed by RFCs and its further processing becomes possible.

For example, in the case of the AS5300 router, a corresponding Rewrite function parses the Acct-Session-Id attribute; breaks it down into fields; stores them into proper attributes, creating them if necessary; and finally replaces Acct-Session-Id with its real value, which is the same for the start and stop records corresponding to a single session. Thus all the information that came with the packet is preserved, but the packet itself is made usable for proper accounting.

A special attribute, Rewrite-Function, is used to trigger invocation of a Rewrite function. Its value is a name of the function to be invoked.

When used in a `naslist' profile, the attribute causes the function to be invoked when the incoming request matches the huntgroup (see section 3.4.4 Huntgroups). For example, to have a function fixup invoked for each packet from the NAS 10.10.10.11, the following huntgroup rule may be used:

DEFAULT NAS-IP-Address = 11.10.10.11 Rewrite-Function = "fixup"

The Rewrite-Function attribute may also be used in a `hints' rule. In this case, it will invoke the function if the request matches the rule (see section 3.4.3 Hints). For example, this `hints' rule will cause the function to be invoked for each request containing the user name starting with `P':

DEFAULT Prefix = "P" Rewrite-Function = "fixup"

Note that in both cases the attribute can be used either in LHS or in RHS pairs of a rule.

The packet rewrite function must be declared as having no arguments and returning an integer value:

integer fixup() { }

The actual return value from such a function is ignored, the integer return type is just a matter of convention.

The following subsection present some examples of packet rewrite functions.

11.2.4.1 Examples of Various Rewrite Functions