maxfwd Module

Bogdan-Andrei Iancu

Voice Sistem SRL

Edited by

Bogdan-Andrei Iancu

Table of Contents

1. Admin Guide
1. Overview
2. Dependencies
2.1. Kamailio Modules
2.2. External Libraries or Applications
3. Parameters
3.1. max_limit (integer)
4. Functions
4.1. maxfwd_process(max_value)
4.2. mf_process_maxfwd_header(max_value)
4.3. process_maxfwd(max_value)
4.4. is_maxfwd_lt(max_value)
4.5. maxfwd_at_least(max_value)
4.6. mf_lowlimit(max_value)

List of Examples

1.1. Set max_limit parameter
1.2. maxfwd_process usage
1.3. is_maxfwd_lt usage

Chapter 1. Admin Guide

1. Overview

The module implements all of the operations that handle the Max-Forward SIP header field, like adding it (if not present) or decrementing and checking the value of the existent one.

The SIP Max-Forward header is used to prevent loops in a SIP network. Every server that process and forward a SIP request lowers the Max-Forward value with one. When the value reaches zero, the request is not forwarded and an error response is sent to the UAC.

2. Dependencies

2.1. Kamailio Modules

The following modules must be loaded before this module:

  • No dependencies on other Kamailio modules.

2.2. External Libraries or Applications

The following libraries or applications must be installed before running Kamailio with this module loaded:

  • None.

3. Parameters

3.1. max_limit (integer)

Set an upper limit for the max-forward value in the outgoing requests. If the header is present, the decremented value is not allowed to exceed this max_limits - if it does, the header value will by decreased to max_limit.

Note: This check is done when calling the maxfwd_process() function.

The range of values stretches from 1 to 256, which is the maximum MAX-FORWARDS value allowed by RFC 3261. The value can be changed at runtime.

Default value is 70.

Example 1.1. Set max_limit parameter

modparam("maxfwd", "max_limit", 32)

4. Functions

4.1.  maxfwd_process(max_value)

If no Max-Forward header is present in the received request, a header will be added having the original value equal with max_value. If a Max-Forward header is already present, its value will be decremented (if not 0). The parameter can be a variable.

Return codes:

  • 2 (true) - header was not found and a new header was successfully added.

  • 1 (true) - header was found and its value was successfully decremented (had a non-0 value).

  • -1 (false) - the header was found and its value is 0 (cannot be decremented).

  • -2 (false) - error during processing.

The return code may be extensively tested via script variable retcode (or $?).

Meaning of the parameters is as follows:

  • max_value - Value to be added if there is no Max-Forwards header field in the message.

This function can be used from REQUEST_ROUTE.

Example 1.2. maxfwd_process usage

# initial sanity checks -- messages with
# max_forwards==0, or excessively long requests
if (!maxfwd_process("10") && $retcode==-1) {
	sl_send_reply("483","Too Many Hops");

4.2.  mf_process_maxfwd_header(max_value)

Same as maxfwd_process(max_value).

4.3.  process_maxfwd(max_value)

Same as maxfwd_process(max_value).

4.4.  is_maxfwd_lt(max_value)

Checks if the Max-Forward header value is less then the max_value parameter value. It considers also the value of the new inserted header (if locally added). The parameter can be a variable.

Retuning codes:

  • 1 (true) - header was found or set and its value is strictly less than max_value.

  • -1 (false) - the header was found or set and its value is greater or equal to max_value.

  • -2 (false) - header was not found or not set.

  • -3 (false) - error during processing.

The return code may be extensively tested via script variable retcode (or $?).

Meaning of the parameters is as follows:

  • max_value - value to check the Max-Forward.value against (as less than).

Example 1.3. is_maxfwd_lt usage

# next hop is a gateway, so make no sense to
# forward if MF is 0 (after decrement)
if ( is_maxfwd_lt("1") ) {
	sl_send_reply("483","Too Many Hops");

4.5.  maxfwd_at_least(max_value)

Same as is_maxfwd_lt(max_value).

4.6.  mf_lowlimit(max_value)

Same as is_maxfwd_lt(max_value).