For registrations in a scenario like “[UAC] -> [P1] -> [REG]”, the "path" module can be used at the intermediate proxy P1 to insert a Path header into the message before forwarding it to the registrar REG. Two functions can be used to achieve this:

Note that some SIP registrars may check if header Supported includes 'path'. It can be added in Kamailio.cfg using append_hf() from textops module.

...
append_hf("Supported: path\r\n");
...

If the NAT'ed address of an UAC is passed to the registrar, the registrar routes back subsequent requests using the Path header of the registration as Route header of the current request. If the intermediate proxy had inserted a Path header including the “received” parameter during the registration, this parameter will show up in the Route header of the new request as well, allowing the intermediate proxy to route to this address instead of the one propagated in the Route URI for tunneling through NAT. This behaviour can be activated by setting the module parameter “use_received”.

The following modules must be loaded before this module:

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

If set to 1, the “received” parameter of the first Route URI is evaluated and used as destination-URI if present.

Default value is 0.

...
modparam("path", "use_received", 1)
...

If set to 0, the “received” parameter value will be in the escaped SIP URI format.

If set to 1, the “received” parameter value will be in the same format as the “alias” parameter added by set_contact_alias() from “nathelper” module (i.e., “ip~port~protid”). This is a solution with some SIP applications that incorrectly match the transport from received parameter instead of the one from the SIP URI.

Default value is 0.

...
modparam("path", "received_format", 1)
...

If set to 1, the module will add two Path headers, similar to the double Record-Route done by rr module. One Path headers corresponds to incoming network socket and the other to outgoing network socket. The URIs in the Path headers will have the 'r2=on' parameter as well.

Note: if enabled, the module adds the two Path headers even when the incoming socket is the same as outgoing socket. Improvements to skip the second Path header in this case may be introduced in the future, meanwhile, if you need to enable this parameter but also deal with same socket routing, use 'insert_hf("Path: <$Ru>\r\n")'.

Default value is 0.

...
modparam("path", "enable_r2", 1)
...

This function is used to insert a Path header in the form “Path: <sip:1.2.3.4;lr>”, where “1.2.3.4” is the address of the outgoing interface.

If the “outbound” module was loaded before this module, and outbound is required for this request, the header will be in the form “Path: <sip:flowtoken@1.2.3.4;lr;ob>”, where “flowtoken” is the RFC 5626 flow-token that can be used to identify the source and local address and transport the request was received on, and where “1.2.3.4” is the address of the outgoing interface.

This function can be used from REQUEST_ROUTE.

...
if (!add_path()) {
	sl_send_reply("503", "Internal Path Error");
	...
};
...

This function adds a Path header in the form “Path: <sip:user@1.2.3.4;lr>”.

Meaning of the parameters is as follows:

This function can be used from REQUEST_ROUTE.

...
if (!add_path("loadbalancer")) {
	sl_send_reply("503", "Internal Path Error");
	...
};
...

This function adds a Path header in the form “Path: <sip:user@1.2.3.4;lr>” and appends the given parameters as additional URI parameters.

Meaning of the parameters is as follows:

This function can be used from REQUEST_ROUTE.

...
if (!add_path("loadbalancer", "ob")) {
	sl_send_reply("503", "Internal Path Error");
	...
};
...

This function adds a Path header in the form “Path: <sip:1.2.3.4;received=sip:2.3.4.5:1234;lr>”, setting its own outgoing address as domain-part, and the address the request has been received from as received-parameter.

If the “outbound” module was loaded before this module, and outbound is required for this request, the header will be in the form “Path: <sip:flowtoken@1.2.3.4;lr;received=sip:2.3.4.5:1234;ob>”, where “flowtoken” is the RFC 5626 flow-token that can be used to identify the source and local address and transport the request was received on, and where “1.2.3.4” is the address of the outgoing interface.

This function can be used from REQUEST_ROUTE.

...
if (!add_path_received()) {
	sl_send_reply("503", "Internal Path Error");
	...
};
...

This function adds a Path header in the form “Path: <sip:user@1.2.3.4;received=sip:2.3.4.5:1234;lr>”, setting 'user' as username part of address, its own outgoing address as domain-part, and the address the request has been received from as received-parameter.

This function can be used from REQUEST_ROUTE.

...
if (!add_path_received("inbound")) {
	sl_send_reply("503", "Internal Path Error");
	...
};
...

This function adds a Path header in the form “Path: <sip:user@1.2.3.4;received=sip:2.3.4.5:1234;lr>”, setting 'user' as username part of address, its own outgoing address as domain-part, and the address the request has been received from as received-parameter.

This function can be used from REQUEST_ROUTE.

...
if (!add_path_received("inbound", "ob")) {
	sl_send_reply("503", "Internal Path Error");
	...
};
...