Module Interface

RLM Module Interface (for developers).

Overview

FreeRADIUS is an authentication server. It does RADIUS authorization, authentication, and accounting. It does NOT do database management, user configuration updates, or email. All of those functions may be more easily (and correctly) performed in programs outside of the server.

The only functionality performed by the server is:

receive a RADIUS request
process the request
look up information one or more databases
store information in one or more databases (proxying can be viewed this way)
respond to the request

There is no room, or need, for timers, listening on sockets, or anything else in the server. Adding those functions to the server means that it will become more complex, more unstable, more insecure, and more difficult to maintain.

Intent of the modules

The intent of modules is that they do small, simple, well-defined things when RADIUS packets are received. If the module does things when RADIUS packets are NOT received, then it has no business being in the server. Similarly, the module infrastructure is NOT intended to allow servers, applications, timed events, or anything other than handling incoming RADIUS packets.

Modules perform an action when RADIUS packets are received. Modules which do more (creating threads, forking programs) will NOT be added to the server, and the server core will NOT be modified to enable these kinds of modules. Those functions more properly belong in a separate application.

Modules ARE permitted to open sockets to other network programs, and to send and receive data on those sockets. Modules are NOT permitted to open sockets, and to listen for requests. Only the server core has that functionality, and it only listens for RADIUS requests.

Module outline

The fundamental concepts of the rlm interface are module, instance, and component.

A module is a chunk of code that knows how to deal with a particular kind of database, or perhaps a collection of similar databases.

Examples:

Module	Description
rlm_sql	Contains code for talking to MySQL or Postgres, and for mapping RADIUS records onto SQL tables.
rlm_unix	Contains code for making radiusd fit in well on unix-like systems, including getpw* authentication and utmp/wtmp-style logging.

Module

Description

rlm_sql

Contains code for talking to MySQL or Postgres, and for mapping RADIUS records onto SQL tables.

rlm_unix

Contains code for making radiusd fit in well on unix-like systems, including getpw* authentication and utmp/wtmp-style logging.

An instance specifies the actual location of a collection data that can be used by a module. Examples:

/var/log/radutmp
the MySQL database on bigserver.theisp.com.example

A module can have multiple components which act on RADIUS requests at different stages. A module declares which components it supports by putting function pointers in its module_dl_t rlm_* structure.

Module configuration

The administrator requests the creation of a module instance by adding it inside the modules { … } block in radiusd.conf. The instance definition looks like this::

module_name [instance_name] {
  param1 = value1
  param2 = value2
  param3 = value3
  ...
}

module_name: The name used to load the module. To see the names of the available modules, look for the rlm_*.so files in $installprefix/lib. The module_name is that, minus the rlm_ prefix and the .so suffix.
instance_name: an identifier for distinguishing multiple instances of the same module. If you are only loading a module once, you can leave out the instance_name and it will be assumed to be the same as the module_name.

The parameters inside the module block are passed without interpretation to the module and generally point to the exact location of a database or enable optional features of the module. Each module should document what parameters it accepts and what they do.

For each Access-Request that comes to the server, the recv Access-Request { … } section is called. Then one of the authenticate <type> { … } sections called, depending on the Auth-Type attribute that was chosen by the previous section.`.

Finally, the send Access-Accept { … } section is called, or another one, depending on the reply packet type.

For each Accounting-Request that comes to the server, the recv Accounting-Request { … } ` section is called, followed by the `accounting <Acct-Status-Type> { … } ` section. Finally, the `send Accounting-Response { … } section is called.

The lifetime of a module

When the server is starting up, or reinitializing itself as a result of a SIGHUP, it reads the modules{} section. Each configured module will be loaded and its init() method will be called::

int init(void)

The init() method should take care of any setup that is not tied to a specific instance. It will only be called once, even if there are multiple instances configured.

For each configured instance, after the init() method, the instantiate() method is called. It is given a handle to the configuration block holding its parameters, which it can access with cf_section_parse().

int instantiate(CONF_SECTION *cs, void **instance)

The instantiate() function should look up options in the config section, and open whatever files or network connections are necessary for the module to do its job. It also should create a structure holding all of the persistent variables that are particular to this instance (open file descriptors, configured pathnames, etc.) and store a pointer to it in *instance. That void * becomes a handle (some would call it a cookie) representing this instance. The instance handle is passed as a parameter in all subsequent calls to the module’s methods, so they can determine which database they are supposed to act on.

The authorize(), authenticate(), preaccounting(), and accounting() functions are all called the same way:

int authorize(void *instance, request_t *request)
int authenticate(void *instance, request_t *request)
int preaccounting(void *instance, request_t *request)
int accounting(void *instance, request_t *request)

they each receive the instance handle and the request, and are expected to act on the request using the database pointed to by the instance handle (which was set by the instantiate() function).

When the server is being shut down (as the first part of SIGHUP for example) detach() is called for each module instance.

int detach(void *instance)

The detach() method should release whatever resources were allocated by the instantiate() method.

After all instances are detached, the destroy() method is called.

int destroy(void)

It should release resources that were acquired by the init() method.