nathelper Module

Maxim Sobolev

Sippy Software, Inc.

Edited by

Maxim Sobolev

Edited by

Bogdan-Andrei Iancu

Revision History
Revision $Revision: 8740 $$Date: 2013-01-29 13:35:11 +0100 (Tue, 29 Jan 2013) $

Table of Contents

1. Admin Guide
1.1. Overview
1.2. NAT pinging types
1.3. Dependencies
1.3.1. OpenSIPS Modules
1.3.2. External Libraries or Applications
1.4. Exported Parameters
1.4.1. natping_interval (integer)
1.4.2. ping_nated_only (integer)
1.4.3. natping_processes (integer)
1.4.4. natping_socket (string)
1.4.5. received_avp (str)
1.4.6. force_socket (string)
1.4.7. sipping_bflag (string/integer)
1.4.8. sipping_from (string)
1.4.9. sipping_method (string)
1.4.10. nortpproxy_str (string)
1.4.11. natping_tcp (integer)
1.5. Exported Functions
1.5.1. fix_nated_contact([uri_params])
1.5.2. fix_nated_sdp(flags [, ip_address])
1.5.3. add_rcv_param([flag]),
1.5.4. fix_nated_register()
1.5.5. nat_uac_test(flags)
1.5.6. MI Commands
2. Frequently Asked Questions

List of Examples

1.1. Set natping_interval parameter
1.2. Set ping_nated_only parameter
1.3. Set natping_processes parameter
1.4. Set natping_socket parameter
1.5. Set received_avp parameter
1.6. Set force_socket parameter
1.7. Set sipping_bflag parameter
1.8. Set sipping_from parameter
1.9. Set sipping_method parameter
1.10. Set nortpproxy_str parameter
1.11. Set natping_interval parameter
1.12. fix_nated_contact usage
1.13. fix_nated_sdp usage
1.14. add_rcv_paramer usage
1.15. fix_nated_register usage
1.16. nh_enable_ping usage

Chapter 1. Admin Guide

1.1. Overview

This is a module to help with NAT traversal. In particular, it helps symmetric UAs that don't advertise they are symmetric and are not able to determine their public address. fix_nated_contact rewrites Contact header field with request's source address:port pair. fix_nated_sdp adds the active direction indication to SDP (flag 0x01) and updates source IP address too (flag 0x02).

Works with multipart messages that contain an SDP part, but not with multi-layered multipart messages.

1.2. NAT pinging types

Currently, the nathelper module supports two types of NAT pings:

  • UDP package - 4 bytes (zero filled) UDP packages are sent to the contact address.

    • Advantages: low bandwitdh traffic, easy to generate by OpenSIPS;

    • Disadvantages: unidirectional traffic through NAT (inbound - from outside to inside); As many NATs do update the bind timeout only on outbound traffic, the bind may expire and closed.

  • SIP request - a stateless SIP request is sent to the contact address.

    • Advantages: bidirectional traffic through NAT, since each PING request from OpenSIPS (inbound traffic) will force the SIP client to generate a SIP reply (outbound traffic) - the NAT bind will be surely kept open.

    • Disadvantages: higher bandwitdh traffic, more expensive (as time) to generate by OpenSIPS;

1.3. Dependencies

1.3.1. OpenSIPS Modules

The following modules must be loaded before this module:

  • usrloc module - only if the NATed contacts are to be pinged.

1.3.2. External Libraries or Applications

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

  • None.

1.4. Exported Parameters

1.4.1. natping_interval (integer)

Period of time in seconds between sending the NAT pings to all currently registered UAs to keep their NAT bindings alive. Value of 0 disables this functionality.


Enabling the NAT pinging functionality will force the module to bind itself to USRLOC module.

Default value is 0.

Example 1.1. Set natping_interval parameter

modparam("nathelper", "natping_interval", 10)

1.4.2. ping_nated_only (integer)

If this variable is set then only contacts that have “behind_NAT” flag in user location database set will get ping.

Default value is 0.

Example 1.2. Set ping_nated_only parameter

modparam("nathelper", "ping_nated_only", 1)

1.4.3. natping_processes (integer)

How many timer processes should be created by the module for the exclusive task of sending the NAT pings.

Default value is 1.

Example 1.3. Set natping_processes parameter

modparam("nathelper", "natping_processes", 3)

1.4.4. natping_socket (string)

Spoof the natping's source-ip to this address. Works only for IPv4.

Default value is NULL.

Example 1.4. Set natping_socket parameter

modparam("nathelper", "natping_socket", "")

1.4.5. received_avp (str)

The name of the Attribute-Value-Pair (AVP) used to store the URI containing the received IP, port, and protocol. The URI is created by fix_nated_register function of nathelper module and the attribute is then used by the registrar to store the received parameters. Do not forget to change the value of corresponding parameter in registrar module if you change the value of this parameter.


You must set this parameter if you use "fix_nated_register". In such case you must set the parameter with same name of "registrar" module to same value.

Default value is "NULL" (disabled).

Example 1.5. Set received_avp parameter

modparam("nathelper", "received_avp", "$avp(received_avp)")

1.4.6. force_socket (string)

Socket to be forced when sending pings. It makes sense only for UDP communication. If no one specified, the OS will choose.

Default value is “NULL”.

Example 1.6. Set force_socket parameter

modparam("nathelper", "force_socket", "localhost:33333")

1.4.7. sipping_bflag (string/integer)

What branch flag should be used by the module to identify NATed contacts for which it should perform NAT ping via a SIP request instead if dummy UDP package.

WARNING: Setting INT flags is deprecated! Use quoted strings instead!

Default value is "NULL" (disabled).

Example 1.7. Set sipping_bflag parameter

modparam("nathelper", "sipping_bflag", "SIP_PING_FLAG")

1.4.8. sipping_from (string)

The parameter sets the SIP URI to be used in generating the SIP requests for NAT ping purposes. To enable the SIP request pinging feature, you have to set this parameter. The SIP request pinging will be used only for requests marked so.

Default value is “NULL”.

Example 1.8. Set sipping_from parameter

modparam("nathelper", "sipping_from", "")

1.4.9. sipping_method (string)

The parameter sets the SIP method to be used in generating the SIP requests for NAT ping purposes.

Default value is “OPTIONS”.

Example 1.9. Set sipping_method parameter

modparam("nathelper", "sipping_method", "INFO")

1.4.10. nortpproxy_str (string)

The parameter sets the SDP attribute used by nathelper to mark the packet SDP informations have already been mangled.

If empty string, no marker will be added or checked.


The string must be a complete SDP line, including the EOH (\r\n).

Default value is “a=nortpproxy:yes\r\n”.

Example 1.10. Set nortpproxy_str parameter

modparam("nathelper", "nortpproxy_str", "a=sdpmangled:yes\r\n")

1.4.11. natping_tcp (integer)

If the flag is set, TCP/TLS clients will also be pinged with SIP OPTIONS messages.

Default value is 0 (not set).

Example 1.11. Set natping_interval parameter

modparam("nathelper", "natping_tcp", 1)

1.5. Exported Functions

1.5.1.  fix_nated_contact([uri_params])

Rewrites the URI Contact HF to contain request's source address:port. If a list of URI parameter is provided, it will be added to the modified contact;

This function can be used from REQUEST_ROUTE, ONREPLY_ROUTE, BRANCH_ROUTE.

Example 1.12. fix_nated_contact usage

if (search("User-Agent: Cisco ATA.*") {
} else {

1.5.2.  fix_nated_sdp(flags [, ip_address])

Alters the SDP information in orer to facilitate NAT traversal. What changes to be performed may be controled via the “flags” paramter.

Meaning of the parameters is as follows:

  • flags - the value may be a bitwise OR of the following flags:

    • 0x01 - adds “a=direction:active” SDP line;

    • 0x02 - rewrite media IP address (c=) with source address of the message or the provided IP address (the provide IP address take precedence over the source address).

    • 0x04 - adds “a=nortpproxy:yes” SDP line;

    • 0x08 - rewrite IP from origin description (o=) with source address of the message or the provided IP address (the provide IP address take precedence over the source address).

  • ip_address - IP to be used for rewritting SDP. If not specified, the received signalling IP will be used. The parameter allows pseudo-variables usage. NOTE: For the IP to be used, you need to use 0x02 or 0x08 flags, otherwise it will have no effect.


Example 1.13. fix_nated_sdp usage

if (search("User-Agent: Cisco ATA.*") {fix_nated_sdp("3");};

1.5.3.  add_rcv_param([flag]),

Add received parameter to Contact header fields or Contact URI. The parameter will contain URI created from the source IP, port, and protocol of the packet containing the SIP message. The parameter can be then processed by another registrar, this is useful, for example, when replicating register messages using t_replicate function to another registrar.

Meaning of the parameters is as follows:

  • flag - flags to indicate if the parameter should be added to Contact URI or Contact header. If the flag is non-zero, the parameter will be added to the Contact URI. If not used or equal to zero, the parameter will go to the Contact header.

This function can be used from REQUEST_ROUTE.

Example 1.14. add_rcv_paramer usage

add_rcv_param(); # add the parameter to the Contact header
add_rcv_param("1"); # add the paramter to the Contact URI

1.5.4.  fix_nated_register()

The function creates a URI consisting of the source IP, port, and protocol and stores the URI in an Attribute-Value-Pair. The URI will be appended as "received" parameter to Contact in 200 OK and registrar will store it in the user location database.

This function can be used from REQUEST_ROUTE.

Example 1.15. fix_nated_register usage


1.5.5.  nat_uac_test(flags)

Tries to guess if client's request originated behind a nat. The parameter determines what heuristics is used.

Meaning of the flags is as follows:

  • 1 - Contact header field is searched for occurrence of RFC1918 addresses.

  • 2 - the "received" test is used: address in Via is compared against source IP address of signaling

  • 4 - Top Most VIA is searched for occurrence of RFC1918 addresses

  • 8 - SDP is searched for occurrence of RFC1918 addresses

  • 16 - test if the source port is different from the port in Via

  • 32 - address in Contact is compared against source IP address of signaling

  • 64 - Port in Contact is compared against source port of signaling

All flags can be bitwise combined, the test returns true if any of the tests identified a NAT.


1.5.6. MI Commands nh_enable_ping

Enables natping if parameter value greater than 0. Disables natping if parameter value is 0. With no parameter, it returns the current natping status.

The function may takean optional parameter (for set operations) - a number in decimal format.

Example 1.16. nh_enable_ping usage

$ opensipsctl fifo nh_enable_ping
Status:: 1
$ opensipsctl fifo nh_enable_ping 0
$ opensipsctl fifo nh_enable_ping
Status:: 0

Chapter 2. Frequently Asked Questions


Where can I find more about OpenSIPS?

Take a look at


Where can I post a question about this module?

First at all check if your question was already answered on one of our mailing lists:

E-mails regarding any stable OpenSIPS release should be sent to and e-mails regarding development versions should be sent to .

If you want to keep the mail private, send it to .


How can I report a bug?

Please follow the guidelines provided at: