1. Nathelper Module

Maxim Sobolev

Sippy Software, Inc.

1.1. Overview
1.2. Parameters
1.2.1. natping_interval (integer)
1.2.2. ping_nated_only (integer)
1.2.3. received_avp (integer)
1.3. Functions
1.3.1. fix_nated_contact()
1.3.2. fix_nated_sdp(mode)
1.3.3. add_rcv_param()
1.3.4. fix_nated_register()
1.3.5. nat_uac_test(mode)
1.3.6. ping_contact(contact)
1.3.7. @nathelper.rewrite_contact[n]

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 http://www.cisco.com/en/US/products/sw/iosswrel/ps1839/products_feature_guide09186a0080110bf9.html">

1.2. Parameters

1.2.1. natping_interval (integer)

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

Default value is 0.

Example 1. Set natping_interval parameter

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

1.2.2. ping_nated_only (integer)

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

Default value is 0.

Example 2. Set ping_nated_only parameter

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

1.2.3. received_avp (integer)

The number 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.

Default value is 42.

Example 3. Set received_avp parameter

modparam("nathelper", "received_avp", 43)

1.3. Functions

1.3.1.  fix_nated_contact()

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

Example 4. fix_nated_contact usage

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

1.3.2.  fix_nated_sdp(mode)

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

Meaning of the parameters is as follows:

  • mode - 0x01 (add direction=active), 0x02 (rewrite media IP address with source address of the message).

Example 5. fix_nated_sdp usage

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

1.3.3.  add_rcv_param()

Add received parameter to Contact header fields. 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.

Example 6. add_rcv_paramer usage


1.3.4.  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.

Example 7. fix_nated_register usage


1.3.5.  nat_uac_test(mode)

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

  • 01 - the Contact URI host contains a private IP address (from RFC1981).

  • 02 - the address in the topmost Via header differs from the source IP address of the request.

  • 04 - the address in the topmost Via contains a private IP address (RFC1918).

  • 08 - the SDP body of the request contains a private IP address (RFC1918).

  • 16 - the port in the topmost Via differs from the source port of the request.

  • 32 - the Contact URI port differs from the source port of the request (Warning: this is might be legal or even intended combination in non natted scenarios).

All of them might be bitwise combined (which is equal to the sum of the values from the list above). If one of the test matched the function returns true.

1.3.6.  ping_contact(contact)

Ping contact specified by parameter. It enables pinging independently on usrloc. It may be processed e.g. via timer module.

Example 8. ping_contact usage

$c = @get_a_contact;

1.3.7.  @nathelper.rewrite_contact[n]

Get n-th Contact value with IP:Port rewritten to source ip:port. N is counted from 1. Only IP:port is rewritten, remaining part are left unchanged. Full nameaddr is supported.

Example 9. @nathelper.rewrite_contact usage

$c = @nathelper.rewrite_contact[1];
$c2 = @nathelper.rewrite_contact[1].nameaddr.uri;