nathelper Module

Maxim Sobolev


Edited by

Maxim Sobolev

Edited by

Bogdan-Andrei Iancu

Revision History
Revision $Revision: 4473 $$Date: 2008-07-11 23:16:22 +0200 (Fri, 11 Jul 2008) $

Table of Contents

1. Admin Guide
1.1. Overview
1.2. NAT pinging types
1.3. Multiple RTPProxy usage
1.4. Dependencies
1.4.1. OpenSIPS Modules
1.4.2. External Libraries or Applications
1.5. Exported Parameters
1.5.1. natping_interval (integer)
1.5.2. ping_nated_only (integer)
1.5.3. natping_processes (integer)
1.5.4. natping_socket (string)
1.5.5. received_avp (str)
1.5.6. rtpproxy_sock (string)
1.5.7. rtpproxy_disable_tout (integer)
1.5.8. rtpproxy_tout (integer)
1.5.9. rtpproxy_retr (integer)
1.5.10. force_socket (string)
1.5.11. sipping_bflag (integer)
1.5.12. sipping_from (string)
1.5.13. sipping_method (string)
1.5.14. nortpproxy_str (string)
1.6. Exported Functions
1.6.1. fix_nated_contact()
1.6.2. fix_nated_sdp(flags [, ip_address])
1.6.3. set_rtp_proxy_set()
1.6.4. force_rtp_proxy([flags [, ip_address]])
1.6.5. unforce_rtp_proxy()
1.6.6. add_rcv_param([flag]),
1.6.7. fix_nated_register()
1.6.8. nat_uac_test(flags)
1.6.9. start_recording()
1.7. MI Commands
1.7.1. nh_enable_ping
1.7.2. nh_enable_rtpp
1.7.3. nh_show_rtpp
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 rtpproxy_sock parameter
1.7. Set rtpproxy_disable_tout parameter
1.8. Set rtpproxy_tout parameter
1.9. Set rtpproxy_retr parameter
1.10. Set force_socket parameter
1.11. Set sipping_bflag parameter
1.12. Set sipping_from parameter
1.13. Set sipping_method parameter
1.14. Set nortpproxy_str parameter
1.15. fix_nated_contact usage
1.16. fix_nated_sdp usage
1.17. fix_nated_contact usage
1.18. force_rtp_proxy usage
1.19. unforce_rtp_proxy usage
1.20. add_rcv_paramer usage
1.21. fix_nated_register usage
1.22. start_recording usage
1.23. nh_enable_ping usage
1.24. nh_enable_rtpp usage
1.25. nh_show_rtpp 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).

Known devices that get along over NATs with nathelper are ATAs (as clients) and Cisco Gateways (since 12.2(T)) as servers. See">

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. Multiple RTPProxy usage

Currently, the nathelper module can support multiple rtpproxies for balancing/distribution and control/selection purposes.

The module allows the definition of several sets of rtpproxies - load-balancing will be performed over a set and the user has the ability to choose what set should be used. The set is selected via its id - the id being defined along with the set. Refer to the “rtpproxy_sock” module parameter definition for syntax description.

The balancing inside a set is done automatically by the module based on the weight of each rtpproxy from the set.

The selection of the set is done from script prior using [un]force_rtp_proxy() functions - see the set_rtp_proxy_set() function.

For backward compatibility reasons, a set with no id take by default the id 0. Also if no set is explicitly set before [un]force_rtp_proxy(), the 0 id set will be used.

IMPORTANT: if you use nultiple sets, take care and use the same set for both force_ and unforce_rtpproxy()!!

1.4. Dependencies

1.4.1. OpenSIPS Modules

The following modules must be loaded before this module:

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

1.4.2. External Libraries or Applications

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

  • None.

1.5. Exported Parameters

1.5.1. 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)

1.5.2. 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)

1.5.3. 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)

1.5.4. 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", "")

1.5.5. 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(i:42)")

1.5.6. rtpproxy_sock (string)

Definition of socket(s) used to connect to (a set) RTPProxy. It may specify a UNIX socket or an IPv4/IPv6 UDP socket.

Default value is “NONE” (disabled).

Example 1.6. Set rtpproxy_sock parameter

# single rtproxy
modparam("nathelper", "rtpproxy_sock", "udp:localhost:12221")
# multiple rtproxies for LB
modparam("nathelper", "rtpproxy_sock",
	"udp:localhost:12221 udp:localhost:12222")
# multiple sets of multiple rtproxies
modparam("nathelper", "rtpproxy_sock",
	"1 == udp:localhost:12221 udp:localhost:12222")
modparam("nathelper", "rtpproxy_sock",
	"2 == udp:localhost:12225")

1.5.7. rtpproxy_disable_tout (integer)

Once RTPProxy was found unreachable and marked as disable, nathelper will not attempt to establish communication to RTPProxy for rtpproxy_disable_tout seconds.

Default value is “60”.

Example 1.7. Set rtpproxy_disable_tout parameter

modparam("nathelper", "rtpproxy_disable_tout", 20)

1.5.8. rtpproxy_tout (integer)

Timeout value in waiting for reply from RTPProxy.

Default value is “1”.

Example 1.8. Set rtpproxy_tout parameter

modparam("nathelper", "rtpproxy_tout", 2)

1.5.9. rtpproxy_retr (integer)

How many times nathelper should retry to send and receive after timeout was generated.

Default value is “5”.

Example 1.9. Set rtpproxy_retr parameter

modparam("nathelper", "rtpproxy_retr", 2)

1.5.10. force_socket (string)

Socket to be forced in communicating to RTPProxy. It makes sense only for UDP communication. If no one specified, the OS will choose.

Default value is “NULL”.

Example 1.10. Set force_socket parameter

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

1.5.11. sipping_bflag (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.

Default value is -1 (disabled).

Example 1.11. Set sipping_bflag parameter

modparam("nathelper", "sipping_bflag", 7)

1.5.12. 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 1.12. Set sipping_from parameter

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

1.5.13. 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 1.13. Set sipping_method parameter

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

1.5.14. 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 1.14. Set nortpproxy_str parameter

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

1.6. Exported Functions

1.6.1.  fix_nated_contact()

Rewrites Contact HF to contain request's source address:port.

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

Example 1.15. fix_nated_contact usage

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

1.6.2.  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 1.16. fix_nated_sdp usage

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

1.6.3.  set_rtp_proxy_set()

Sets the Id of the rtpproxy set to be used for the next [un]force_rtp_proxy() command.

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

Example 1.17. fix_nated_contact usage


1.6.4.  force_rtp_proxy([flags [, ip_address]])

Rewrites SDP body to ensure that media is passed through an RTP proxy. It can have optional parameters to force additional features. If ip_address is provided, it will be used to replace the one in SDP.

Meaning of the parameters is as follows:

  • flags - flags to turn on some features.

    • a - flags that UA from which message is received doesn't support symmetric RTP.

    • l - force “lookup”, that is, only rewrite SDP when corresponding session is already exists in the RTP proxy. By default is on when the session is to be completed (reply in non-swap or ACK in swap mode).

    • i - flags that message is received from UA in the LAN (internal network). Only makes sense when RTP proxy is running in the bridge mode.

    • e - flags that message is received from UA in the WAN (external network). Only makes sense when RTP proxy is running in the bridge mode.

    • f - instructs nathelper to ignore marks inserted by another nathelper in transit to indicate that the session is already goes through another proxy. Allows creating chain of proxies.

    • r - flags that IP address in SDP should be trusted. Without this flag, nathelper ignores address in the SDP and uses source address of the SIP message as media address which is passed to the RTP proxy.

    • o - flags that IP from the origin description (o=) should be also changed.

    • c - flags to change the session-level SDP connection (c=) IP if media-description also includes connection information.

    • s - flags to swap creation with confirmation between requests and replies. By default, a request creates the RTP session and a reply confirms it. If swapped, a reply will create the RTP session and a request will confirm it.

    • w - flags that for the UA from which message is received, support symmetric RTP must be forced.

  • ip_address - new SDP IP address.


Example 1.18. force_rtp_proxy usage

if (search("User-Agent: Cisco ATA.*") {force_rtp_proxy();};
if (src_ip= {force_rtp_proxy("i");};
if (search("User-Agent: Cisco ATA.*") {force_rtp_proxy("","");};

1.6.5.  unforce_rtp_proxy()

Tears down the RTPProxy session for the current call.


Example 1.19. unforce_rtp_proxy usage


1.6.6.  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 1.20. 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

1.6.7.  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 1.21. fix_nated_register usage


1.6.8.  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

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


1.6.9.  start_recording()

This command will send a signal to the RTP-Proxy to record the RTP stream on the RTP-Proxy.

This function can be used from REQUEST_ROUTE and ONREPLY_ROUTE.

Example 1.22. start_recording usage


1.7. MI Commands

1.7.1. nh_enable_ping

Enables natping if parameter value greater than 0. Disables natping if parameter value is 0.

The function takes only one parameter - a number in decimal format.

Example 1.23. nh_enable_ping usage

$ opensipsctl fifo nh_enable_ping 1

1.7.2. nh_enable_rtpp

Enables a rtp proxy if parameter value is greater than 0. Disables it if a zero value is given.

The first parameter is the rtp proxy url (exactly as defined in the config file).

The second parameter value must be a number in decimal.

NOTE: if a rtpproxy is defined multiple times (in the same or diferente sete), all its instances will be enables/disabled.

Example 1.24.  nh_enable_rtpp usage

$ opensipsctl fifo nh_enable_rtpp udp: 0

1.7.3. nh_show_rtpp

Displays all the rtp proxies and their information: set and status (disabled or not, weight and recheck_ticks).

No parameter.

Example 1.25.  nh_show_rtpp usage

$ opensipsctl fifo nh_show_rtpp 

Chapter 2. Frequently Asked Questions


What happend with “rtpproxy_disable” parameter?

It was removed as it became obsolete - now “rtpproxy_sock” can take empty value to disable the rtpproxy functionality.


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: