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rlm_backcounter manages various counters which count back and are periodically resetted to some value. These counters can be defined eg. for whole user group, but with values and reset intervals specific for each individual user.

There is also additional "prepaid" counter, which can be set to any value from outside of rlm_backcounter - the module only decreases it.

This module can be used to e.g.

  • limit monthly bandwidth usage, with uniform distribution of counter resets during month (ie. to overcome excessive network usage when all users have their counters resetted on the same day),
  • implement prepaid accounts,
  • put your clients' network traffic into various QoS/SLA classes depending on their network usage.

How it works


Note: Why to do anything related to counters in this section? Because it's far more easier in FreeRADIUS to react as soon as possible, and to do user updates, etc. basing on attributes in request packets (instead of reply packets).

  • if the counter of the user we're handling request for should be resetted:
    • set the counter to the value defined in user's reply attributes (fetched from database), or if these don't exist, in group's reply attributes
    • set reset time to current value + period defined in module instance-wide configuration, making sure it's greater than current time moment
  • fetch current counter values and check if their sum is greater than zero:
    • if user is under limit, set session "guard VAP" to the counter value (if configured to do so)
    • otherwise, add a new VAP to the request packet or simply send back an access rejection


Only Accounting-Stop packets are considered.

  • decrease counters in configurable manner:
    • use sum of values in VAPs given in configuration as the decrement
    • let the administrator choose whether to decrement the prepaid or the "monthly" counter first


  1. Download and unpack FreeRADIUS - e.g. to freeradius/
  2. Put source code of this module in freeradius/src/modules/rlm_backcounter/
  3. Add "rlm_backcounter" at the top of freeradius/src/modules/stable file
  4. Change directory into freeradius/src/modules/rlm_backcounter/
  5. Run autoconf (you may need to install it on your system)
  6. Proceed with the standard FreeRADIUS installation procedure.
  7. Verify if you have a file named in your libraries (usually /usr/lib or /usr/local/lib).


An example on how to create a monthly transfer limit.

In radiusd.conf, create module configuration:

modules {
    # (...)

    backcounter transfer-limit {
        # name of rlm_sql module instance to connect to
        sqlinst_name = "sql"

        # what to count in accounting packets
        count_names = "Acct-Input-Octets, Acct-Output-Octets"

        # VAP in which to send the amount which is left
        guardvap = "Session-Octets-Limit"
        #guardvap = "Mikrotik-Total-Limit"

        # VAP to use if the value in guardvap is greater than 2^32
        # it will hold the bits 32-63 of the number
        #giga_guardvap = "Mikrotik-Total-Limit-Gigawords"

        # reset counters each 30 days
        period = 2592000

        # monthly transfer limit, in bytes
        limitvap = "Monthly-Transfer-Limit"

        # if enabled, the counters will not be resetted
        #noreset = "yes"

        # time of next reset for this particular user
        resetvap = "Monthly-Transfer-Reset"

        # VAP to set in *request* to 1 if the user has reached his limits
        overvap = "Monthly-Transfer-Exceeded"

        # transfer left - monthly, prepaid
        leftvap = "Monthly-Transfer-Left"
        prepaidvap = "Monthly-Transfer-Prepaid"

        # which counter to decrease first
        prepaidfirst = yes

Then, instantiate it after rlm_sql:

instantiate {
    # (...)
    # (...)

Put it in the authorize section after rlm_sql:

authorize {
    # (...)
    # some module to restrict bandwidth if overvap is set, eg.:
    # users
    # (...)

And anywhere in the accounting section:

accounting {
    # (...)

Add new attributes in the dictionary file:

# (...)

# custom, local attributes that will not get sent over the network
ATTRIBUTE Monthly-Transfer-Limit    3100 string
ATTRIBUTE Monthly-Transfer-Reset    3101 integer
ATTRIBUTE Monthly-Transfer-Exceeded 3102 integer
ATTRIBUTE Monthly-Transfer-Left     3103 string
ATTRIBUTE Monthly-Transfer-Prepaid  3104 string

# a session traffic limit for pppd
# for some reason it's not in the official FreeRADIUS dictionary
ATTRIBUTE Session-Octets-Limit       227 integer

# Mikrotik session traffic limits
ATTRIBUTE Mikrotik-Total-Limit                    17      integer
ATTRIBUTE Mikrotik-Total-Limit-Gigawords          18      integer

If you wish to limit bandwidth usage on eg. a Lintrack router, add this to your users file (some modifications depending on local configuration may be required):

DEFAULT Monthly-Transfer-Exceeded == 1
    Reply-Message = "You have reached your monthly bandwidth limit",
    ASN-Kbps-Down := 64,
    ASN-Kbps-Up := 64


Version 0.2 brings a feature of counter rates dependent on time - for example it can count two times slower during weekends. This functionality is configured using levels keyword.

Example: Lets say that today just after midnight the UNIX timestamp was 1279670400. Its a time reference point. You want to make a special level which repeats each week = 7 days = 168 hours = 604800 seconds. You want to count half data from midnight till 6.00 AM = 6 hours = 21600 seconds. So in radiusd.conf you'll write:

levels = "from 1279670400 each 604800 for 21600 use 0.5"

If you want to implement your "half data is counted at evenings and weekends", you write:

levels = "from 1279753200 each 86400 for 21600 use 0.5,   \
          from 1279947600 each 604800 for 172800 use 0.5"

If you have at least one level defined, the server will automatically set Session-Timeout VAP to the closest level change event minus 15 seconds. The server will also ensure that the Session-Timeout is not set to a value smaller than 60 seconds.

Levels are used in the order they appear in config file. First matching level wins.

Current limitations (maybe a TODO list)

  • probably works only with MySQL
  • a bit too "hardcoded"
    • queries and table names not configurable
    • low-level access to database


A FreeRADIUS module for pre-paid services



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