User Tools

Site Tools


Frequently Asked Questions

This page is open for contributions, you must create an user account via registration form:

If you have a question suitable for FAQ and you don't know the answer, add it and set the answer to 'TBA', others can read it and give the right answer.


Kamailio is a SIP server, implementing the specifications from RFC3261. Its root functionality is routing any kind of SIP packets. On top of that, many conceptual features are implemented, see more at:

No, Kamailio is a flexible SIP proxy. Many people integrate Asterisk, FreeSWITCH, SEMS, or other products with Kamailio for a B2BUA.

If you use native scripting language, you must restart after you update the configuration file.

But note that many global parameters can be changed via RPC/MI commands without restart (e.g., TCP connecting timeout, debug level). Applying changes related to loaded modules or routing block require always a restart.

If you use a KEMI scripting language (Lua, Python, JavaScript, Ruby, Squirrel), then you can reload the routing logic script without restarting Kamailio by issuing an RPC command (see KEMI interpreter modules documentation for more details: app_lua, app_python, app_python3, app_jsdt, app_ruby, app_sqlang).

Kamailio is an open source application licensed under GNU Public License version 2 (aka GPLv2). It can be used for free “as in beer” on your infrastructure. Keep in mind that you need also distribute the source code of your changes, if you distribute it as a binary to your customer. For more information have a look to the GPLv2 FAQ.

Starting with end of 2008, contributions to core and several modules are done under BSD license. That means parts of it can be extracted and used under BSD license terms. But over all, when used as one application, the use must follow the terms of GPLv2, because GPLv2 is viral.

No. There is no official development pushed in this direction, Linux/Unix being the de facto for running servers. But it might not be big effort to make it compile using the Linux-like environments for Windows (such as cygwin).

Transport Layers

At network layer, Kamailio supports both IPv4 and IPv6.

At transport layer, Kamailio support UDP, TCP, TLS and SCTP. Transmission can be done in asynchronous mode (configuration option), inclusive for TCP and TLS.

Kamailio can be used to bridge between any combinations of these protocols, e.g., receiving SIP packets on UDP over IPv4 and sending out on TLS over IPv6.

Starting with version 4.0.0, Kamailio supports WebSocket transport protocol, allowing modern web browsers to call between them directly through Kamailio.

Yes, starting with version 4.0.0. It supports WebSockets over TCP (WS) and over TLS (WSS), on both IPv4 and IPv6.

Yes. You can use Kamailio to receive SIP message on one transport and send out on a different transport. For example, receive TLS on IPv6 and send out UDP on IPv4.

Yes, from signaling point of view. Remember that Kamailio is SIP packet router, not handling media plane at all. Simply, Kamailio can do gatewaying from WebSocket to UDP, TCP, TLS or SCTP for all SIP messages.

However, note that media plane (RTP streams) requires special secure encoding of the RTP packets, which may not be supported by the old SIP phones.


A module is an extension that compiles in a separate object file and can be loaded at Kamailio startup. Usually a module exports new functions that can be used in configuration file routing blocks.

Inside the source tree, there are three directories:

  • modules/
  • modules_k/ - no longer used since version 4.0.0
  • modules_s/ - no longer used since version 4.0.0

Each module is located in its own sub-folder. The documentation for modules that are bundled in Kamailio packages is available at:

The modules that have the same dependencies as the core of Kamailio. Those that are not compiled by default are kept in the variable 'exclude_modules' of Makefile. To print the list of excluded modules:

make printvar v=exclude_modules

Add it to the variable 'include_modules' to the command 'make cfg'. For example, compiling and installing the modules app_lua and db_mysql:

make FLAVOUR=kamailio include_modules="app_lua db_mysql" cfg
make all
make install

The module is not compiled by default, because depends on libmysqlclient. Use following commands to get in compiled and installed:

make modules include_modules="db_mysql"

make install include_modules="db_mysql" 

First be sure that the core compiles fine with the command:


To compile a single module, use:

make modules modules=modules/modname

Replace modname with the real name of the module you want to compile – for example, compiling only tls module:

make modules modules=modules/tls

SIP Message Processing

Kamailio is working always with the received SIP message (request or reply). Changes done to headers or body are kept as a list of operations to be applied before writing to the network. If you want changes to be applied immediately, use msg_apply_changes() from textopsx module:

Be careful with it and don't use it after record_route(), because this function has to insert a header containing the IP address from the local socket that is going to be used for forwarding - the socket is not yet know during configuration file execution.

For example, considering that X-Hdr does not exist in incoming message, here are some cases:

  • without applying changes
append_hf("X-Hdr: xyz\r\n");
if(is_present_hf("X-Hdr")) {
   # this part is not executed
} else {
   # this part is executed
  • with applying changes
append_hf("X-Hdr: xyz\r\n");
if(is_present_hf("X-Hdr")) {
   # this part is executed
} else {
   # this part is not executed

After doing header management operations, it can result that parts of From/To header are duplicated or the result is the concatenation of many values. That is related to previous question, because the changes are not applied immediately and updates to parts of headers are not a simple set operation.

For example, if From username is “alice” and the operations in the config file are:

$fU = "bob";
$fU = "carol";

The result can be that From username is “bobcarol”.

One solution is to use msg_apply_changes() in between:

$fU = "bob";
$fU = "carol";

Another solution is to keep the value in a variable (e.g., avp or xavp) and do the operation only once, like:

$xavp(fuser) = "bob";
$xavp(fuser[0]) = "carol";
$fU = $xavp(fuser);

Updating From/To headers is recommended to be done in branch_route, specially if it is needed to have different value for outgoing branches.

The examples above were with assignments to $fU (can be other vars as well, such as $fu, $tU, $tu, ..), but it is the same behaviour when using functions such as uac_replace_from() or uac_replace_to().

It is practically the same reason as for multiple From/To parts.

The configuration file does an operation that changes parts in the headers or body many times. For example, if the body has two concatenated IP addresses for media stream, then likely the rtpproxy function is used twice, or, if the SDP appears two time, then the rtpengine function is used two times.

If the update has to be done many times, use msg_apply_changes() in between, otherwise refactor the config to perform the operation only once (e.g., the RTP relaying functions should be used in a branch_route block).

Set the value of the header inside a branch_route.

All the operations done over a SIP message inside the request_route (including from the sub-routes executed from request_route) will be common to all outgoing branches. If you want to do updates only for specific destinations, use branch routes.


  • add X-VBox header only when sending to voicemail server
  • add X-Peer-ID header when sending somewhere else
request_route {
  if(is_method("INVITE")) {
branch_route[SETHEADERS] {
   if($rd=="") {
     append_hf("X-VBox: $fU\r\n");
   } else {
     append_hf("X-Peer-ID: abc\r\n");
failure_route[REROUTE] {
  if (t_is_canceled()) {
  if (t_check_status("486|408")) {
    $du = $null;
    $ru = "";

Very important is also to be aware that doing same operation many times in request_route is not overwriting the previous value, but combines them. For example, if you do two times uac_replace_from(), the From header can become corrupt. Like:

request_route {

Results in From header having the URI: Again, use branch_route to do the operations if you need different From header for outgoing branches.

SIP allows that certain header fields may appear multiple times in a SIP message. This header fields (e.g. Via, Route, Record-Route, Contact) may be written either on a single line (with comma separated) or on multiple lines (see RFC 3261 section 7.3 for details).

The remove_hf() function from textops module always removes all header fields with a certain name, thus it can not be used in this case.

To address a certain header (regardless if headers are in a single line or in separate lines) use the @hf_value select. The trick is to load the textopsx module (this select used to be in ser's textops module). Note, header names must use '_' instead of '-'. To delete or manipulate a certain header also use the functions from the textopsx module, e.g: remove_hf_value(). Note, here '-' is used for headers with '-'

For example, incoming message:

Record-Route: <sip:;lr=on;nat=yes>
Record-Route: <sip:;lr;transport=udp>,

Dump all headers:


Dump the first header (<sip:;lr=on;nat=yes>):


Dump the second last header (<sip:;lr;transport=udp>):


Remove the last header (<sip:;lr;transport=udp>):

#or in above case:

In both cases, the reason is probably an error in request routing script which caused an infinite loop.

You can easily verify whether this happens by watching SIP traffic on loopback interface, for example using ngrep:

ngrep -d lo -qt -W byline port 5060

A typical reason for misrouting is a failure to match local domain correctly. If a server fails to recognize a request for itself, it will try to forward it to current URI in believe it would forward them to a foreign domain.

Alas, it forwards the request to itself again. This continues to happen until value of the max_forwards header field reaches zero or the request grows too big.

The solution: make sure that domain matching is correctly configured.

A quick way to achieve that is to introduce a config option to kamailio.cfg:


where domainname has to be replaced with name of domain, which you wish to serve by Kamailio and which appears in request-URIs.

Check if you have a firewall rule dropping traffic on SIP port (5060). Note that network sniffing tools have hooks in kernel before the firewall, so even if you see the SIP packets with ngrep or wireshark, they may be dropped by the firewall.

If you have the pike module loaded, double check to see if you don't block valid trusted traffic with it.

You can increase the debug parameter value to 3 in the configuration file, restart kamailio and watch syslog messages to see if there is any text printed by kamailio. For each SIP packet received on the networks, Kamailio is printing at least few debug messages.

Media Streams

No, however Kamailio can be configured to proxy media if needed.

Kamailio supports all codecs (even codecs that haven't been created yet).

Since Kamailio is a SIP proxy, it does not handle the media streams. Codecs are negotiated between the two endpoints.

Yes, Kamailio can be used for video calls. The two SIP phones must support video codecs/calling.

Getting Started

To be familiar with Linux or UNIX (FreeBSD, OpenBSD, OS/X) and understand Session Initiation Protocol (SIP - RFC3261).


Yes, there are few of them, each addressing a particular category of topics, see the list at:

You must subscribe to be able to post questions. Subscription is free, instructions are presented at the link related to each mailing list.

The most important rules are to use a decent vocabulary and post about something related to Kamailio.

If the question is related to a stable version of Kamailio, then email to:

If the question is related to development version of Kamailio, then email to:

If the question is related to commercial aspects of Kamailio, then email to:


Yes. See enum module.

Kamailio can do next DNS query types: A-Name, C-Name, AAAA (IPv6), SRV, NAPTR and TXT.

Do not enclose the IP address between quotes. Use like:

if(src_ip== { ...

The next example results in reverse DNS operation, because src_ip is compared with a string:

if(src_ip=="") { ...

Instant Messaging and Presence

Yes. There is nothing special to do for that in configuration file, MESSAGE requests are simply routed as any other SIP request.

Yes. Kamailio offers an embedded MSRP relay function via msrp module. You don't need any external application for relaying MSRP packets.

Yes. There is nothing special to do for that in configuration file, SUBSCRIBE and NOTIFY requests are simply routed as any other SIP request.

Yes. You have to use presence and presence_xml modules to handle PUBLISH and SUBSCRIBE requests.

Yes. You have to use rls module.

Yes. See pua module and its extensions named with 'pua_' prefix (such as pua_usrloc, pua_dialoginfo, etc.).

Yes. You have to use dialog module together with presence_dialoginfo and pua_dialoginfo modules.

Yes. Kamailio offers an embedded XCAP server via xcap_server module. It can receive and handle directly the XCAP requests without using any external application.

The xcap_client module can be used when you want to interact with an external XCAP server (that could be another instance of Kamailio).

NAT Traversal

Yes, Kamailio supports the so called “far-end NAT traversal”. One solution is to use nathelper for NAT traversal of SIP signaling, and rtpproxy modules with the rtpproxy ( application for relaying RTP packets.

Read the comments at top of default configuration file shipped with Kamailio, to enable NAT traversal logic there, one has to enable the define WITH_NAT:

#!define WITH_NAT

Yes. You have to set the public IP address as advertised attribute to listen parameter:

  listen=_localsocket_ advertise _publicaddr_


  listen=udp: advertise

Configuration File Processing

Yes. Use '-c' command line parameter, like:

/usr/local/sbin/kamailio -c -f /usr/local/etc/kamailio/kamailio.cfg

Note that even configuration file is reported to be ok, there might still be troubles starting kamailio with the respective configuration file. For example, the database URL can be syntactically correct, but the access itself is not working due to wrong username or password, causing kamailio to fail starting.

No. The AVPs are automatically deleted when associated SIP transaction is destroyed.

If you want to do it on purpose, you can delete the AVP using assignment to $null, like:

$avp(name) = $null;

A pseudo-variable is a special token that is expanded at runtume with a specific value. It can be used inside the parameters of some functions or in configuration file expressions.

A pseudo-variable can refer to the value of an avp, body of a header, part of a SIP message or other variables from system.

For more see:

If you have a variable holding a string like “a,b,c,d” and want to get each character separately, you have to use a WHILE loop with {s.count,separator} and {,index,separator} transformations.

Here is an example:

$var(x) = "a,b,c,d";
$var(i) = 0;
$var(n) = $(var(x){s.count,,});
while( $var(i) <= $var(n) ) {
   xlog("token at position $var(i) is: $(var(x){,$var(i),,})\n");
   $var(i) = $var(i) + 1;

Configuration file interpreter evaluates the return code of a function as follow:

  • <0 (negative value) - it is evaluated to false
  • >0 (positive value) - it is evaluated to true
  • =0 (zero) - it is evaluated as exit (stop execution of configuration file)


if(function_returns_one()) {
   # it goes here
} else {
   # it doesn't go here
if(function_returns_minus_one()) {
   # it doesn't go here
} else {
   # it goes here
if(function_returns_zero()) {
   # it doesn't go here
} else {
   # it doesn't go here

Note that you can use $rc to get the return code value, like:

xlog("returned code by function() is $rc\n");

However, if return code is 0, the next action after function() is not executed. It can be used only of positive or negative response code.

On the other hand, the pseudo-variables (including $rc) have to be compared as an integer, the rules for evaluating return code of the functions do not apply, for example:

$var(x) = 0;
if($var(x) == 0) {
  # do something

To compare the return code of a function with a number is recommended to use the value stored in $rc, because the priority of the operators can convert function return code to internal-true or internal-false. Therefore the next approach should be used:

$var(rc4) = $rc;
if($var(rc4)==4) {
   # it goes here
} else {
   # it doesn't go here
$var(rc2) = $rc;
if($var(rc4)==$var(rc2)) {
   # it doesn't go here
} else {
   # it goes here
if (function_returns_four() && $rc ieq 6 ) {
   # it doesn't go here
} else if ($rc ieq 4) {
   # it goes here
} else {
   # it doesn't go here

Note that $rc is overwritten by a new function call, therefore store its value in another variable (like $var(…)) if it is needed for longer time.

For pseudo-variables, the non-zero value is evaluated to true and zero to false, for example:

$var(x) = 1;
if($var(x)) {
  # do something

The next snippet is detecting retransmissions:

    # handle retransmissions
      if (!is_method("ACK")) {
          if(t_precheck_trans()) {

The ACK request is skipped because it doesn't have a SIP response, therefore it doesn't create a transaction, being forwarded in stateless mode.

The function t_precheck_trans() returns true if the same SIP request is processed at that time by another Kamailio process, so in this case it is a retransmission. The inner t_check_trans() has the role of detecting if the SIP transaction was created by the other process, and if yes, send again the last SIP response of the transaction, when one was already sent out (a requirement from SIP specs). The exit is then used to finish the execution of the config file for that request.

If the function t_precheck_trans() returns false, the SIP request is not under processing by another Kamailio process, but it can be the case that the request processing was finished, request being sent out in stateful mode, therefore t_check_trans() is used to see if a transaction is found in shared memory corresponding to the same request. If such transaction is found in shared memory, then t_check_trans() triggers internally the exit for configuration file execution.


Most probably you run out of memory allocated for Kamailio. The application does not use the entire system memory, but up to a configured size.

There are two memory pools used by Kamailio:

  • shared memory - shm - common at application level, where most of the data needed for long time is stored (such as location records, least cost routing records, transactions, etc.). The default size is 32MB.
  • private memory - pkg - allocated per process, used for local storage and temporary operations. The default size is 4MB.

To increase the sizes for memory pools you have to give following command line parameters:

  • -m SIZE - specify the shared memory size in MB
  • -M SIZE - specify the private memory size in MB

For example, start Kamailio with 512MB of shared memory and 8MB of private memory:

/usr/local/bin/kamailio -m 512 -M 8 ...


Use following command:

echo -n 'user:realm:password' | md5sum | awk '{ print $1 }'

Use following command:

echo -n 'user@realm:realm:password' | md5sum | awk '{ print $1 }'

Kamctl couldn't connect to FIFO file for sending MI commands - default FIFO file is /tmp/kamailio_fifo.

Check if your configuration loads the mi_fifo module and configures a fifo name.

Example for the configuration syntax:

loadmodule ""

modparam("mi_fifo", "fifo_name", "/tmp/kamailio_fifo")

Check also the access privileges of the FIFO file in order to be sure that the user running kamctl can read and write to the file.

There are situations when a kamcmd RPC command fails because the parameters that are provided are auto-converted to integer numbers. This happens when the implementation of the RPC commands expects a string parameter (e.g., username).

To prevent kamcmd to do the auto-conversion, you can prefix its parameter with s:.


# - next command fails
kamcmd uac.reg_refresh 1001

# use instead
kamcmd uac.reg_refresh s:1001
tutorials/faq/main.txt · Last modified: 2021/08/27 08:11 by miconda