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6.9 Multiple Login Checking

The number of sessions a user can have open simultaneously can be restricted by setting Simultaneous-Use attribute in the user's profile LHS (see section Simultaneous-Use). By default the number of simultaneous sessions is unlimited.

When a user with limited number of simultaneous logins authenticates himself, Radius counts the number of the sessions that are already opened by this user. If this number is equal to the value of Simultaneous-Use attribute the authentication request is rejected.

This process is run in several stages. First, Radius retrieves the information about currently opened sessions from one of its accounting databases. Then, it verifies whether all these sessions are still active. This pass is necessary since an open entry might be a result of missing Stop request. Finally, the server counts the sessions and compares their count with the value of Simultaneous-Use attribute.

The following subsections address each stage in detail.

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6.9.1 Retrieving Session Data

Radius retrieves the list of sessions currently opened by the user either from the system database (see section System Accounting), or from the SQL database (see section sql Accounting). The system administrator determines which method to use.

By default, system accounting database is used. Its advantages are simplicity and ease of handling. It has, however, a serious deficiency: the information is kept in the local files. If you run several radius servers, each of them has no easy way of knowing about the sessions initiated by other servers.

This problem is easy to solve if you run SQL accounting (see section sql Accounting). In this case, each radius server stores the data in your SQL database and can easily retrieve them from there.

To enable use of SQL database for multiple login checking, do the following:

In your ‘raddb/config’ file set:

mlc {
    method sql;

In your ‘raddb/sqlserver’ file, specify the queries for retrieving the information about open sessions and, optionally, a query to close an existing open record.

There are two queries for retrieving the information: mlc_user_query returns the list of sessions opened by the user, mlc_realm_query returns the list of sessions opened for the given realm. Each of them should return a list of 4-element tuples(4):

user-name, nas-ip-address, nas-port-id, acct-session-id

Here is an example of mlc_user_query and mlc_realm_query:

mlc_user_query SELECT user_name,nas_ip_address,\
                      nas_port_id,acct_session_id \
               FROM calls \
               WHERE user_name='%C{User-Name}' \
               AND status = 1

mlc_realm_query SELECT user_name,nas_ip_address,\
                       nas_port_id,acct_session_id \
                FROM calls \
                WHERE realm_name='%C{Realm-Name}'     

Apart from these two queries you may also wish to provide a query for closing a hung record. By default, radiusd will use acct_stop_query. If you wish to override it, supply a query named mlc_stop_query, for example:

mlc_stop_query UPDATE calls \
               SET status=4,\
                                  unix_timestamp(event_date_time) \
               WHERE user_name='%C{User-Name}' \
                 AND status = 1 \
                 AND acct_session_id='%C{Acct-Session-Id}' 

See section Writing SQL Accounting Query Templates, for detailed information on how to write these queries.

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6.9.2 Verifying Active Sessions

Whatever database radiusd uses, an open entry in it does not necessary mean that the corresponding session is still being active. So, after retrieving the information about user sessions, Radius verifies on corresponding NASes whether these are actually active.

For each entry in the session list, if its NAS acknowledges the session, the session count is incremented. Otherwise, such entry is marked as closed in the database and is not counted.

There may also be cases when the NAS is unreachable due to some reasons. In such cases the Radius behavior is determined by the value of checkrad-assume-logged in ‘config’ file auth statement (raddb/config). If the value is yes, Radius assumes the session is still active and increases the session count, otherwise it proceeds as if the NAS returned negative reply.

To query a NAS, Radius first looks up its type and additional parameters in ‘naslist’ file (see section NAS List — ‘raddb/naslist). There are two predefined NAS types that cause Radius to act immediately without querying tne NAS: the special type ‘true’ forces Radius to act as if the NAS returned 1, the type ‘false’ forces it to act as if the NAS returned 0. If the type is neither of this predefined types, Radius uses it as a look up key into the ‘nastypes’ file (see section NAS Types — ‘raddb/nastypes) and tries to retrieve an entry which has matching type. If such entry does not exist, Radius issues the error message and acts accordingly to the value of configuration variable checkrad-assume-logged. Otherwise, Radius determines the query method to use from the second field of this entry, and constructs method arguments by appending arguments from the ‘naslist’ entry to those of nastypes entry. Note, that the former take precedence over the latter, and can thus be used to override default values specified in ‘nastypes’.

Having determined the query method and its argument, Radius queries NAS and analyzes its output by invoking a user-supplied Rewrite function. The function to use is specified by the function= argument to the method. It is called each time a line of output is received from the NAS (for finger queries) or a variable is received (for SNMP queries). The process continues until the function returns 1 or the last line of output is read or a timeout occurs whichever comes first.

If the user-function returns 1 it is taken to mean the user's session is now active at the NAS, otherwise, if it replies 0 or if the end of output is reached, it is taken to mean the user's session is not active.

The syntax conventions for user-supplied functions are described in detail in Login Verification Functions.

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