1. TLS Module

Andrei Pelinescu-Onciul

iptelorg GmbH
Revision History
Revision $Revision$ $Date$

1.1. Overview
1.2. Quick Start
1.3. Important Notes
1.4. Compiling the TLS Module
1.5. TLS and Low Memory
1.6. Known Limitations
1.7. Quick Certificate Howto
1.8. Parameters
1.8.1. tls_method (string)
1.8.2. certificate (string)
1.8.3. private_key (string)
1.8.4. ca_list (string)
1.8.5. verify_certificate (boolean)
1.8.6. verify_depth (integer)
1.8.7. require_certificate (boolean)
1.8.8. cipher_list (string)
1.8.9. send_timeout (int)
1.8.10. handshake_timeout (int)
1.8.11. connection_timeout (int)
1.8.12. tls_disable_compression (boolean)
1.8.13. tls_log (int)
1.8.14. low_mem_threshold1 (integer)
1.8.15. low_mem_threshold2 (integer)
1.8.16. tls_force_run (boolean)
1.8.17. config (string)
1.9. Functions
1.9.1. is_peer_verified()
1.10. History

1.1. Overview

This module implements the TLS transport for ser using the openssl library (http://www.openssl.org). To enable the TLS support this module must be loaded and enable_tls=yes must be added to the ser config file

1.2. Quick Start

Make sure you have a proper certificate and private key and either use the certificate and private_key module parameters, or make sure the certificate and key are in the same PEM file, named cert.pem an placed in [your-cfg-install-prefix]/etc/ser/. Don't forget to load the tls module and to enable tls (add enable_tls=yes to your config).

Example 1. quick start config

loadmodule "modules/tls/tls.so"

modparam("tls", "private_key", "./andrei-test.pem")
modparam("tls", "certificate", "./andrei-test.pem")
modparam("tls", "ca_list", "./calist.pem")


	# ....

1.3. Important Notes

The tls module needs some special options enabled when compiling ser. These options are enabled by default, however in case you're using a modified ser version or Makefile, make sure that you enable -DUSE_TLS and -DTLS_HOOKS (or compile with make TLS_HOOKS=1 which will take care of both options). To quickly check if your ser version was compiled with these options, run ser -V and look for USE_TLS and TLS_HOOKS among the flags.

This module includes several workarounds for various openssl bugs (like compression and kerberos using the wrong memory allocations functions, low memory problems a.s.o). On startup it will try to enable the needed workarounds based on the openssl library version. Each time a known problem is detected and a workaround is enabled, a message will be logged. In general it is recommended to compile this module on the same machine or a similar machine to where ser will be run or to link it statically with libssl. For example if on the compile machine openssl does not have the kerberos support enabled, but on the target machine a kerberos enabled openssl library is installed, ser cannot apply the needed workarounds and will refuse to start. The same thing will happen if the openssl versions are too different (to force ser startup anyway, see the tls_force_run module parameter).

Try to avoid using keys larger then 1024 bytes. Large keys significantly slow down the TLS connection handshake, thus limiting the maximum ser TLS connection rate.

Compression is fully supported and used by default, if you have a new enough openssl version (starting with 0.9.8). Although there are some problems with zlib compression in currently deployed openssl versions (up to and including 0.9.8d, see openssl bug #1468), the tls module will automatically switch to its own fixed version. There's no need to force-disable the compression.

The tls module includes workarounds for the following known openssl bugs: openssl #1204 (disable SS_OP_TLS_BLOCK_PADDING_BUG if compression is enabled, for versions between 0.9.8 and 0.9.8c), openssl #1468 (fix zlib compression memory allocation), openssl #1467 (kerberos support will be disabled if the openssl version is less than 0.9.8e-beta1) and openssl #1491 (stop using tls in low memory situations due to the very high risk of openssl crashing or leaking memory). The bug reports can be viewed at http://rt.openssl.org/.

1.4. Compiling the TLS Module

In most case compiling the TLS module is as simple as:

make modules modules=modules/tls


cd modules/tls

or (compiling whole ser and the tls module)

make all include_modules=tls


However in some cases the openssl library requires linking with other libraries. For example compiling the openssl library with kerberos and zlib-shared support will require linking the tls module with libkrb5 and libz. In this case just add TLS_EXTRA_LIBS="library list" to make's command line. E.g.:

make TLS_EXTRA_LIBS="-lkrb5 -lz" all include_modules=tls

In general, if ser fails to start with a symbol not found error when trying to load the tls module (check the log), it means some needed library was not linked and it must be added to TLS_EXTRA_LIBS

1.5. TLS and Low Memory

The openssl library doesn't handle very well low memory situations. If memory allocations start to fail (due to memory shortage), openssl can crash or cause memory leaks (making the memory shortage even worse). As of this writing all openssl versions were affected (includind 0.9.8e), see openssl bug #1491. The tls module has some workarounds for preventing this problem (see low_mem_treshold1 and low_mem_threshold2), however starting ser with enough shared memory is higly recommended. When this is not possible a quick way to significantly reduce openssl memory usage it to disable compression (see tls_disable_compression).

1.6. Known Limitations

The private key must not encrypted (ser cannot ask you for a password on startup).

The tls certificate verifications ignores the certificate name, altname and ip extensions, it just checks if the certificate is signed by a recognized CA. One can use the select framework to try to overcome this limitation (check in the script for the contents of various certificate fields), but this is not only slow, but also not exactly standard conforming (the verification should happen during TLS connection establishment and not after).

TLS specific config reloading is not safe, so for now better don't use it, especially under heavy traffic.

This documentation is incomplete. The select framework and rpc sections are completely missing.

1.7. Quick Certificate Howto

Revision History
Revision $Revision$ $Date$

There are various ways to create, sign certificates and manage small CAs (Certificate Authorities). If you want a GUI, try tinyca (http://tinyca.sm-zone.net/), a very nice and small CA management application. If you are in a hurry and everything you have are the installed openssl libraries and utilities, read on.

Assumptions: we run our own CA.

Warning: in this example no key is encrypted. The client and server private keys must not be encrypted (ser doesn't support encrypted keys), so make sure the corresponding files are readable only by trusted people. You should use a password for your CA private key.

Creating CA certificate
1. create CA dir
	mkdir ca
	cd ca
2. create ca dir structure and files  (see ca(1))
	mkdir demoCA #default CA name, edit /etc/ss/openssl.cnf
	mkdir  demoCA/private
	mkdir demoCA/newcerts
	touch demoCA/index.txt
	echo 01 >demoCA/serial
2. create CA private key
	openssl genrsa -out demoCA/private/cakey.pem 2048
	chmod 600 demoCA/private/cakey.pem
3. create CA self-signed certificate
	openssl req -out demoCA/cacert.pem   -x509 -new -key demoCA/private/cakey.pem

Creating a server/client certificate
1. create a certificate request (and its private key in privkey.pem)
	openssl req -out ser1_cert_req.pem -new -nodes
   WARNING: the organization name should be the same as in the ca certificate.
2. sign it with the ca certificate
	openssl ca -in ser1_cert_req.pem -out ser1_cert.pem
3. copy ser1_cert.pem to your ser config. dir

Setting ser to use the certificate
1. create the ca list file:
	for each of your ca certificates that you intend to use do:
		cat cacert.pem >>calist.pem
2. copy your ser certificate, private key and ca list file to your 
	intended machine (preferably in your ser cfg. directory, this is the 
	default place ser searches for)
3. set up ser.cfg to use the certificate
	if your ser certificate name is different from cert.pem or it is not
	placed in ser cfg. directory, add to your ser.cfg:
		modparam("tls", "certificate", "/path/cert_file_name")
4. set up ser to use the private key
	if your private key is not contained in the certificate (or the
	 certificate name is not the default cert.pem), add to your ser.cfg:
		modparam("tls", "private_key", "/path/private_key_file")
5. set up ser to use the ca list (optional)
	add to your ser.cfg:
		modparam("tls", "ca_list", "/path/ca_list_file")
6. set up tls authentication options:
		modparam("tls", "verify_certificate", 1)
		modparam("tls", "require_certificate", 1) 
	(for more information see the module parameters documentation)

1.8. Parameters

Revision History
Revision $Revision$ $Date$

1.8.1. tls_method (string)

Sets the SSL protocol method. Possible values are:

  • TLSv1 - only TLSv1 connections are accepted. This is the default and recommended method (if you want to be rfc3261 conformant don't change it).

  • SSLv3 - only SSLv3 connections are accepted

  • SSLv2 - only SSLv2 connections, for old clients. Note: you shouldn't use SSLv2 for anything which should be highly secure.

  • SSLv23 - any of the above methods will be accepted, with the following limitation: the initial SSL hello message must be V2 (in the initial hello all the supported protocols are advertised enabling switching to a higher and more secure version). This means connections from SSLv3 or TLSv1 clients will not be accepted.

If rfc3261 conformance is desired, TLSv1 must be used. For compatibility with older clients SSLv23 is a good option.

Example 2. Set tls_method parameter

modparam("tls", "tls_method", "TLSv1")

1.8.2. certificate (string)

Sets the certificate file name. The certificate file can also contain the private key.

Warning: try not to use certificate with keys longer then 1024 bytes. Longer keys will severely impact performance, in particular the tls connection rate.

The default value is [SER_CFG_DIR]/cert.pem.

Example 3. Set certificate parameter

modparam("tls", "certificate", "/usr/local/etc/ser/my_certificate.pem")

1.8.3. private_key (string)

Sets the private key file name.

Note: the private key can be contained in the same file as the certificate (just append it to the certificate file, e.g.: cat pkey.pem >> cert.pem)

The default value is [SER_CFG_DIR]/cert.pem.

Example 4. Set private_key parameter

modparam("tls", "private", "/usr/local/etc/ser/my_pkey.pem")

1.8.4. ca_list (string)

Sets the CA list file name. This file contains a list of all the trusted CAs certificates. If a signature in a certificate chain belongs to one of the listed CAs, the authentication will succeed. See also verify_certificate, verify_depth and require_certificate.

By default the CA file is not set.

An easy way to create the CA list is to append each trusted trusted CA certificate in the PEM format to one file, e.g.: for f in trusted_cas/*.pem ; do cat "$f" >> ca_list.pem ; done .

Example 5. Set ca_list parameter

modparam("tls", "ca_list", "/usr/local/etc/ser/ca_list.pem")

1.8.5. verify_certificate (boolean)

If enabled it will force certificate verification. For more information see the verify(1) openssl man page.

Note: the certificate verification will always fail if the ca_list is empty.

See also: ca_list, require_certificate, verify_depth.

By default the certificate verification is off.

Example 6. Set verify_certificate parameter

modparam("tls", "verify_certificate", 1)

1.8.6. verify_depth (integer)

Sets how far up the certificate chain will the certificate verification go in the search for a trusted CA.

See also: ca_list, require_certificate, verify_certificate,

The default value is 9.

Example 7. Set verify_depth parameter

modparam("tls", "verify_depth", 9)

1.8.7. require_certificate (boolean)

When enabled it will require a certificate from a client. If the client does not offer a certificate and verify_certificate is on, the certificate verification will fail.

The default value is off.

Example 8. Set require_certificate parameter

modparam("tls", "require_certificate", 1)

1.8.8. cipher_list (string)

Sets the list of accepted ciphers. The list consists of cipher strings separated by colons. For more information on the cipher list format see the cipher(1) openssl man page.

The default value is not set (all the openssl supported ciphers are enabled).

Example 9. Set cipher_list parameter

modparam("tls", "cipher_list", "HIGH")

1.8.9. send_timeout (int)

Sets the maximum interval of time after which ser will give up trying to send a message over tls (time after a tls send will be aborted and the corresponding tls connection closed). The value is in seconds.

The default value is 120 s.

Example 10. Set send_timeout parameter

modparam("tls", "send_timeout", 1)

1.8.10. handshake_timeout (int)

Sets the maximum interval of time after which ser will give up trying to accept a tls connection or connect to a tls peer. The value is in seconds.

The default value is 120 s.

Example 11. Set handshake_timeout parameter

modparam("tls", "handshake_timeout", 1)

1.8.11. connection_timeout (int)

Sets the amount of time after which an idle tls connection will be closed. This is similar to tcp_connection_lifetime. The value is expressed in seconds.

The default value is 10 min.

If the value set is -1, the connection will never be close on idle.

Example 12. Set connection_timeout parameter

modparam("tls", "connection_timeout", 60)

1.8.12. tls_disable_compression (boolean)

If set compression over SSL/TLS will be disabled.

By default compression is enabled.

Example 13. Set tls_disable_compression parameter

modparam("tls", "tls_disable_compression", 1)

1.8.13. tls_log (int)

Sets the log level at which tls related messages will be logged.

The default value is 3.

Example 14. Set tls_log parameter

# ignore tls messages if ser is started with debug less than 10
modparam("tls", "tls_log", 10)

1.8.14. low_mem_threshold1 (integer)

Sets the minimal free memory from which new tls connection will start to fail. The value is expressed in KB.

The default value depends on whether the openssl library used handles well low memory situations (openssl bug #1491). As of this writing this is not true for any openssl version (including 0.9.8e).

If an ill-behaved openssl version is detected, a very conservative value is choosed, which depends on the maximum possible number of simultaneously created tls connections (and hence on the process number).

The following values have a special meaning:

  • -1 - use the default value

  • 0 - disable (tls connections will not fail preemptively)

See also low_mem_threshold2.

Example 15. Set low_mem_threshold1 parameter

modparam("tls", "low_mem_threshold1", -1)

1.8.15. low_mem_threshold2 (integer)

Sets the minimal free memory from which tls operations on already established tls connections will start to fail preemptively. The value is expressed in KB.

The default value depends on whether the openssl library used handles well low memory situations (openssl bug #1491). As of this writing this is not true for any openssl version (including 0.9.8e).

If an ill-behaved openssl version is detected, a very conservative value is choosed, which depends on the maximum possible number of simultaneously created tls connections (and hence on the process number).

The following values have a special meaning:

  • -1 - use the default value

  • 0 - disable (tls operations will not fail preemptively)

See also low_mem_threshold1.

Example 16. Set low_mem_threshold2 parameter

modparam("tls", "low_mem_threshold2", -1)

1.8.16. tls_force_run (boolean)

If enabled ser will start even if some of the openssl sanity checks fail (turn it on at your own risk).

Currently failing any of the following sanity checks will not allow ser to start:

  • the version of the library the tls module was compiled with is "too different" from the library used at runtime. The versions should have the same major, minor and fix level (e.g.: 0.9.8a and 0.9.8c are ok, but 0.9.8 and 0.9.9 are not)

  • the openssl library used at compile time and the one used at runtime have different kerberos options

By default tls_force_run is disabled.

Example 17. Set tls_force_run parameter

modparam("tls", "tls_force_run", 11)

1.8.17. config (string)

Sets the name of the TLS specific config file.

If set the tls module will load a special config file, in which different tls parameters can be specified on a per role (server or client) and domain basis (for now only IPs). The corresponding module parameters will be ignored.

By default no config file is specified.

The following parameters can be set in the config file, for each domain:

  • tls_method

  • verify_certificate

  • require_certificate

  • private_key

  • certificate

  • verify_depth

  • ca_list

  • cipher_list

ser acts as a server when it accepts a connection and as a client when it initiates a new connection by itself (it connects to something).

Example 18. Short config file

method = TLSv1
verify_certificate = no
require_certificate = no
private_key = default_key.pem
certificate = default_cert.pem
ca_list = default_ca.pem

verify_certificate = yes
require_certificate = yes

#more relaxed for connection on the loopback interface
method = SSLv23
verify_certificate = yes
require_certificate = no
private_key = local_key.pem
certificate = local_cert.pem
verify_depth = 3
ca_list = local_ca.pem

For a more complete example check the tls.cfg distributed with the ser source (sip_router/modules/tls/tls.cfg).

Example 19. Set config parameter

modparam("tls", "config", "/usr/local/etc/ser/tls.cfg")

1.9. Functions

Revision History
Revision $Revision$ $Date$

1.9.1. is_peer_verified()

Returns true if the connection on which the message was received is TLS , the peer presented an X509 certificate and the certificate chain verified ok. It can be used only in a request route.

Example 20. is_peer_verified usage

	if (proto==TLS && !is_peer_verified()){
		sl_send_reply("400", "No certificate or verification failed");

1.10. History

Revision History
Revision $Revision$ $Date$

This module was put together by Jan Janak from code from the experimental tls core addon (http://cvs.berlios.de/cgi-bin/viewcvs.cgi/ser/experimental/tls/), code originally written by Peter Griffiths and later maintained by Cesc Santasusana and from an iptelorg tls code addon, written by Andrei Pelinescu-Onciul . Jan also added support for multiple domains, a tls specific config, config reloading and a tls specific select framework.

The code is currently maintained by Andrei Pelinescu-Onciul .