proto_tls module

Table of Contents

1. Admin Guide
1.1. Overview
1.2. History
1.3. Scenario
1.4. Dependencies
1.4.1. OpenSIPS Modules
1.4.2. Dependencies of external libraries
1.5. OpenSIPS Exported parameters
1.5.1. listen=interface
1.5.2. tls_port (integer)
1.5.3. tls_crlf_pingpong (integer)
1.5.4. tls_crlf_drop (integer)
1.5.5. tls_max_msg_chunks (integer)
1.5.6. cert_check_on_conn_reusage (integer)
1.5.7. trace_destination (string)
1.5.8. trace_on (int)
1.5.9. trace_filter_route (string)
1.5.10. tls_handshake_timeout (integer)
1.5.11. tls_send_timeout (integer)
1.5.12. tls_async (integer)
1.5.13. tls_async_max_postponed_chunks (integer)
1.5.14. tls_async_local_connect_timeout (integer)
1.5.15. tls_async_handshake_timeout (integer)
1.6. Exported MI Functions
1.6.1. tls_trace
2. Frequently Asked Questions
3. Contributors
3.1. By Commit Statistics
3.2. By Commit Activity
4. Documentation
4.1. Contributors

List of Tables

3.1. Top contributors by DevScore(1), authored commits(2) and lines added/removed(3)
3.2. Most recently active contributors(1) to this module

List of Examples

1.1. Set listen variable
1.2. Set tls_port variable
1.3. Set tls_crlf_pingpong parameter
1.4. Set tls_crlf_drop parameter
1.5. Set tls_max_msg_chunks parameter
1.6. Set cert_check_on_conn_reusage parameter
1.7. Set trace_destination parameter
1.8. Set trace_on parameter
1.9. Set trace_filter_route parameter
1.10. Set tls_handshake_timeout variable
1.11. Set tls_send_timeout variable
1.12. Set tls_async variable
1.13. Set tls_async_max_postponed_chunks parameter
1.14. Set tls_async_local_connect_timeout parameter
1.15. Set tls_async_handshake_timeout parameter

Chapter 1. Admin Guide

1.1. Overview

TLS, as defined in SIP RFC 3261, is a mandatory feature for proxies and can be used to secure the SIP signalling on a hop-by-hop basis (not end-to-end). TLS works on top of TCP. DTLS, or TLS over UDP is already defined by IETF and may become available in the future.

1.2. History

The TLS support was originally developed by Peter Griffiths and posted as a patch on SER development mailing list. Thanks to Cesc Santasusana, several problems were fixed and some improvements were added.

The TLS support was simultaneously added in both projects. In SER, the support was committed in a separate experimental CVS tree, as patch to the main CVS tree. In OpenSIPS, the support was integrated directly into the CVS tree, as a built-in component, and is part of stable OpenSIPS since release >=1.0.0.

Starting with OpenSIPS 2.1, the TLS has been moved to a separate transport module, that implements the more generic Transport Interface.

1.3. Scenario

By the increased number of providers the SIP world is continuously growing. More users means more calls and more calls means a high probability for a user to receive calls from totally unknown people or, in the worst case, to receive unwanted calls. To prevent this, a defense mechanism must be adopted by the SIP provider. Since only the called user is fully able to classify a call as being unwanted, the SIP server, based on all information regarding the call should notify the user about the desirability of the call. Information like the caller domain, the received source or the incoming protocol can be very useful for a SIP server to establish the nature of the call.

As this information is quite limited, is very improbable for a server to be able detect the unwanted calls - there are many calls that it cannot predict anything about its status (neutral calls). So, instead on alerting the called user about unwanted calls, the server can notify the user about calls that are considered trusted - calls for which the server is 100% sure there are not unwanted.

So, a trust concept must be defined for SIP servers. Which calls are trusted and which are not? A call is trusted if the caller can be identify as a trustable user - a user about we have reliable information.

Since all the user from its domain are authenticated (or should be), a SIP server can consider all the calls generated by its user as trusted. Now we have to extend the trust concept to the multi-domain level. A mutual agreement, between several domains, can establish a trusting relationship. So, a domain (called A) will consider also as trusted calls all the calls generated by user from a different domain (called B) and vice-versa. But just an agreement is not enough; since the authentication information is strictly limited to a domain (a domain can authenticate only its own user, not the user from other domains), there is still the problem of checking the authenticity of the caller - he can impersonate (by a false FROM header) a user from a domain that is trusted.

The answer to this problem is TLS (Transport Layer Security). All calls via domain A and domain B will be done via TLS. Authentication in origin domain plus TLS transport between domains will make the call 100% trusted for the target domain.

For such a mechanism to work, the following requirements must be met:

  • all UA must have set as outbound proxy their home server.

  • all SIP servers must authenticated all the calls generated by their own users.

  • all SIP servers must relay the calls generated be their user to a trusted domain via TLS.

Based on this, a server can classify as trusted a call for one of its user only if the call is also generated by one of its users or is the call is received from a trusted domain ( which is equivalent with a call received via TLS). Untrusted call will be calls received from users belonging to untrusted domains or from users from trusted domains, but whose calls are not routed via their home server (so, they are not authenticated by there home servers).

Once the server is able to tell if the call is trusted or not, the still open issue is about the mechanism used by server to notify the called user about the nature of the incoming call.

One way to do it is by remotely changing the ringing type of the called user's phone. This can be done by inserting special header into the INVITE request. Such feature is supported by now by several hardphones like CISCO ATA, CISCO 7960 and SNOM. This phones can change their ringing tone based on the present or content of the "Alert-Info" SIP header as follows:

  • CISCO ATA - it has 4 pre-defined ringing types. The Alert-Info header must look like Alert-info: Bellcore-drX EOH where X can be between 1 and 4. Note that 1 is the phone default ringing tone.

  • CISCO 7960 - it has 2 pre-defined ringing types and the possibility of uploading new ones. The Alert-Info header must look like Alert-info: X EOH where X can be whatever number. When this header is present, the phones will not change the ringing tone, but the ringing pattern. Normally, the phone rings like [ring.........ring..........ring] where [ring] is the ringing tone; if the header is present, the ringing pattern will be [ring.ring.........ring.ring........]. So, to be able to hear some difference between the two patterns (and not only as length), its strongly recommended to have a highly asymmetric ringing type (as the pre-defined are not!!).

  • SNOM - The Alert-Info header must look like Alert-info: URL EOH" where URL can be a HTTP URL (for example) from where the phone can retrieve a ringing tone.

1.4. Dependencies

1.4.1. OpenSIPS Modules

The following modules must be loaded before this module:

  • tls_openssl or tls_wolfssl, depending on the desired TLS library

  • tls_mgm.

1.4.2. Dependencies of external libraries

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

  • None.

1.5. OpenSIPS Exported parameters

All these parameters can be used from the opensips.cfg file, to configure the behavior of OpenSIPS-TLS.

1.5.1. listen=interface

Not specific to TLS. Allows to specify the protocol (udp, tcp, tls), the IP address and the port where the listening server will be.

Example 1.1. Set listen variable

socket= tls:

1.5.2. tls_port (integer)

The default port to be used for all TLS related operation. Be careful as the default port impacts both the SIP listening part (if no port is defined in the TLS listeners) and the SIP sending part (if the destination URI has no explicit port).

If you want to change only the listening port for TLS, use the port option in the SIP listener defintion.

Default value is 5061.

Example 1.2. Set tls_port variable

modparam("proto_tls", "tls_port", 5062)

1.5.3. tls_crlf_pingpong (integer)

Send CRLF pong (\r\n) to incoming CRLFCRLF ping messages over TLS. By default it is enabled (1).

Default value is 1 (enabled).

Example 1.3. Set tls_crlf_pingpong parameter

modparam("proto_tls", "tls_crlf_pingpong", 0)

1.5.4. tls_crlf_drop (integer)

Drop CRLF (\r\n) ping messages. When this parameter is enabled, the TLS layer drops packets that contains a single CRLF message. If a CRLFCRLF message is received, it is handled according to the tls_crlf_pingpong parameter.

Default value is 0 (disabled).

Example 1.4. Set tls_crlf_drop parameter

modparam("proto_tls", "tls_crlf_drop", 1)

1.5.5. tls_max_msg_chunks (integer)

The maximum number of chunks that a SIP message is expected to arrive via TLS. If a packet is received more fragmented than this, the connection is dropped (either the connection is very overloaded and this leads to high fragmentation - or we are the victim of an ongoing attack where the attacker is sending the traffic very fragmented in order to decrease server performance).

Default value is 4.

Example 1.5. Set tls_max_msg_chunks parameter

modparam("proto_tls", "tls_max_msg_chunks", 8)

1.5.6. cert_check_on_conn_reusage (integer)

This parameter turns on or off the extra checking/matching of the TLS domain (SSL certificate) when comes to reusing an existing TLS connection. Without this extra check, only IP and port of the connections will be check (in order to re-use an existing connection). With this extra check, the connection to be reused must have the same SSL certificate as the one set for the current signaling operation.

This checking is done only when comes to send SIP traffic via TLS and it is applied only against connections that were created / initiated by OpenSIPS (as TLS client). Any accepte connection (as TLS server) will automatically match (the extra test will be skipped).

Default value is 0 (disabled).

Example 1.6. Set cert_check_on_conn_reusage parameter

modparam("proto_tls", "cert_check_on_conn_reusage", 1)

1.5.7. trace_destination (string)

Trace destination as defined in the tracing module. Currently the only tracing module is proto_hep. Network events such as connect, accept and connection closed events shall be traced along with errors that could appear in the process. For each connection that is created an event containing information about the client and server certificates, master key and network layer information shall be sent.

WARNING: A tracing module must be loaded in order for this parameter to work. (for example proto_hep).

Default value is none(not defined).

Example 1.7. Set trace_destination parameter

modparam("proto_hep", "hep_id", "[hep_dest];transport=tcp;version=3")

modparam("proto_tls", "trace_destination", "hep_dest")

1.5.8. trace_on (int)

This controls whether tracing for tls is on or not. You still need to define trace_destinationin order to work, but this value will be controlled using mi function tls_trace.

Default value is 0(tracing inactive).

Example 1.8. Set trace_on parameter

modparam("proto_tls", "trace_on", 1)

1.5.9. trace_filter_route (string)

Define the name of a route in which you can filter which connections will be trace and which connections won't be. In this route you will have information regarding source and destination ips and ports for the current connection. To disable tracing for a specific connection the last call in this route must be drop, any other exit mode resulting in tracing the current connection ( of course you still have to define a trace_destination and trace must be on at the time this connection is opened.

IMPORTANT Filtering on ip addresses and ports can be made using $si and $sp for matching either the entity that is connecting to OpenSIPS or the entity to which OpenSIPS is connecting. The name might be misleading ( $si meaning the source ip if you read the docs) but in reality it is simply the socket other than the OpenSIPS socket. In order to match OpenSIPS interface (either the one that accepted the connection or the one that initiated a connection) $socket_in(ip) (ip) and $socket_in(port) (port) can be used.

WARNING: IF trace_on is set to 0 or tracing is deactived via the mi command tls_trace this route won't be called.

Default value is none(no route is set).

Example 1.9. Set trace_filter_route parameter

modparam("proto_tls", "trace_filter_route", "tls_filter")
/* all tls connections will go through this route if tracing is activated
 * and a trace destination is defined */
route[tls_filter] {
	/* all connections opened from/by ip will be traced
	   on interface listener)
	   all the other connections won't be */
	 if ( $si == "" && $sp == 8000 &&
		$socket_in(ip) == ""  && $socket_in(port) == 5060)

1.5.10. tls_handshake_timeout (integer)

Sets the timeout (in milliseconds) for the SSL handshake sequence to complete. It may be necessary to increase this value when using a CPU intensive cipher for the connection to allow time for keys to be generated and processed.

The timeout is invoked during acceptance of a new connection (inbound) and during the wait period when a new session is being initiated (outbound).

Default value is 100.

Example 1.10. Set tls_handshake_timeout variable

modparam("proto_tls", "tls_handshake_timeout", 200) # number of milliseconds

1.5.11. tls_send_timeout (integer)

Sets the timeout (in milliseconds) for the send operations to complete

The send timeout is invoked for all TLS write operations, excluding the handshake process (see: tls_handshake_timeout)

Default value is 100.

Example 1.11. Set tls_send_timeout variable

modparam("proto_tls", "tls_send_timeout", 200) # number of milliseconds

1.5.12. tls_async (integer)

If the TLS connect and write operations should be done in an asynchronous mode (non-blocking connect and write). If disabled, OpenSIPS will block and wait for TLS operations like connect and write.

Default value is 0 (disabled).

Example 1.12. Set tls_async variable

modparam("proto_tls", "tls_async", 1) # enable async TLS

1.5.13. tls_async_max_postponed_chunks (integer)

If tls_async is enabled, this specifies the maximum number of SIP messages that can be stashed for later/async writing. If the connection pending writes exceed this number, the connection will be marked as broken and dropped.

Default value is 32.

Example 1.13. Set tls_async_max_postponed_chunks parameter

modparam("proto_tls", "tls_async_max_postponed_chunks", 16)

1.5.14. tls_async_local_connect_timeout (integer)

If tls_async is enabled, this specifies the number of milliseconds that a connect will be tried in blocking mode (optimization). If the connect operation lasts more than this, the connect will go to async mode and will be passed to tls MAIN for polling.

Default value is 100 ms.

Example 1.14. Set tls_async_local_connect_timeout parameter

modparam("proto_tls", "tls_async_local_connect_timeout", 200)

1.5.15. tls_async_handshake_timeout (integer)

If tls_async is enabled, this specifies the number of milliseconds that a TLS handshake should be tried in blocking mode (optimization). If the handshake operation lasts more than this, the write will go to async mode and will be passed to tls MAIN for polling.

Default value is 10 ms.

Example 1.15. Set tls_async_handshake_timeout parameter

modparam("proto_tls", "tls_async_handshake_timeout", 100)

1.6. Exported MI Functions

1.6.1.  tls_trace

Name: tls_trace


  • trace_mode(optional): set tls tracing on and off. This parameter can be missing and the command will show the current tracing status for this module( on or off ); Possible values:

    • on

    • off

MI FIFO Command Format:

			opensips-cli -x mi tls_trace on

Chapter 2. Frequently Asked Questions


Where can I post a question about TLS?

Use one (the most appropriate) of the OpenSIPS mailing lists:

Remember: first at all, check if your question wasn't already answered.


How can I report a bug?

Accumulate as much as possible information (OpenSIPS version, opensips -V output, your OS (uname -a), OpenSIPS logs, network dumps, core dump files, configuration file) and send a mail to

Also you may try OpenSIPS's bug report web page:


How can I debug ssl/tls problems?

Increase the log level in opensips.cfg (log_level=4) and watch the log statements in syslog.

Install the ssldump utility and start it. This will give you a trace of the ssl/tls connections.


What is the difference between the TLS directory and the TLSOPS module directory?

The code in the TLS directory implements the TLS transport layer. The TLSOPS module implements TLS related functions which can be used in the routing script.


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:

Chapter 3. Contributors

3.1. By Commit Statistics

Table 3.1. Top contributors by DevScore(1), authored commits(2) and lines added/removed(3)

 NameDevScoreCommitsLines ++Lines --
1. Bogdan-Andrei Iancu (@bogdan-iancu)58311548812
2. Razvan Crainea (@razvancrainea)5434922708
3. Eseanu Marius Cristian (@eseanucristian)415552133
4. Ionut Ionita (@ionutrazvanionita)35141232576
5. Vlad Patrascu (@rvlad-patrascu)1913184213
6. Liviu Chircu (@liviuchircu)11830107
7. Bogdan Chifor521411
8. Vlad Paiu (@vladpaiu)42843
9. Maksym Sobolyev (@sobomax)42318
10. Dan Pascu (@danpascu)3124

All remaining contributors: Nick Altmann (@nikbyte), Julián Moreno Patiño, Peter Lemenkov (@lemenkov), Zero King (@l2dy).

(1) DevScore = author_commits + author_lines_added / (project_lines_added / project_commits) + author_lines_deleted / (project_lines_deleted / project_commits)

(2) including any documentation-related commits, excluding merge commits. Regarding imported patches/code, we do our best to count the work on behalf of the proper owner, as per the "fix_authors" and "mod_renames" arrays in opensips/doc/ If you identify any patches/commits which do not get properly attributed to you, please submit a pull request which extends "fix_authors" and/or "mod_renames".

(3) ignoring whitespace edits, renamed files and auto-generated files

3.2. By Commit Activity

Table 3.2. Most recently active contributors(1) to this module

 NameCommit Activity
1. Liviu Chircu (@liviuchircu)Mar 2015 - May 2024
2. Bogdan-Andrei Iancu (@bogdan-iancu)Feb 2015 - Dec 2023
3. Vlad Patrascu (@rvlad-patrascu)May 2017 - Oct 2022
4. Razvan Crainea (@razvancrainea)Feb 2015 - Feb 2022
5. Nick Altmann (@nikbyte)May 2021 - May 2021
6. Maksym Sobolyev (@sobomax)Jul 2017 - Sep 2020
7. Zero King (@l2dy)Mar 2020 - Mar 2020
8. Dan Pascu (@danpascu)Jan 2020 - Jan 2020
9. Peter Lemenkov (@lemenkov)Jun 2018 - Jun 2018
10. Ionut Ionita (@ionutrazvanionita)Mar 2015 - Apr 2017

All remaining contributors: Julián Moreno Patiño, Eseanu Marius Cristian (@eseanucristian), Bogdan Chifor, Vlad Paiu (@vladpaiu).

(1) including any documentation-related commits, excluding merge commits

Chapter 4. Documentation

4.1. Contributors

Last edited by: Vlad Patrascu (@rvlad-patrascu), Razvan Crainea (@razvancrainea), Bogdan-Andrei Iancu (@bogdan-iancu), Zero King (@l2dy), Liviu Chircu (@liviuchircu), Peter Lemenkov (@lemenkov), Ionut Ionita (@ionutrazvanionita), Eseanu Marius Cristian (@eseanucristian), Vlad Paiu (@vladpaiu).

Documentation Copyrights:

Copyright © 2015

Copyright © 2013 Secusmart GmbH

Copyright © 2006

Copyright © 2005 Voice Sistem SRL

Copyright © 2005 Cesc Santasusana