VoIP calls and TLS security

After a while, here I am again, talking about VoIP.
The last article is about audio problems and the quality of a VoIP call. In this new article, I talk about VoIP over TLS and in particular how to configure a SIP server with self-renewing TLS certificates.

This is an article accompanying code found in this github repo.

The idea and the purpose

The purpose of this article is to explain how to deal with SIP over TSL. In the this github repo the evoseed team explains a simple way to deploy a Kamailio server in a docker container with TLS enabled using Let's Encrypt.

Thanks to Fred Posner's article we have understood that there are 2 ways to deal with TLS in Kamailio:

quoting Fred Posner:

1. Use a purchased certificate

   • Pro: easy to deploy on the client side

   • Cons: expensive, painful acquisition model

2. Use a self-signed certificate

   • Pro: free

   • Cons: difficult to deploy on the client side

• if you use a self-signed certificate (not certified by any authority) on your SIP Server, necessarily on the client side you must use its calist.pem; so free but difficult to deploy on the client side.
  If you want to go down this road here you can find my guide. You can also find the procedure here and here on Kamailio's site):

• if you use a certificate trusted by a guarantor authority recognized by the client then client-side should not need to load the calist.pem; so easy to deploy on the client side but expensive.
  If you want to go down this road you have to buy a certificate and then put it in your Kamailio installation following the same procedure describe for self-signed certificates.

There is, however, an alternative that is free, automatable, easy to use and requires no client-side configuration: Let’s Encrypt.

I wrote this article and created this github repo to take a step forward from Fred's excellent article https://www.fredposner.com/1836/kamailio-tls-and-letsencrypt/.
Fred explains how to install Let's Encrypt and create a certificate (using letsencrypt-auto, which unfortunately is no longer supported) and how to configure the certificate in kamailio.
I want to go one step further and explain how to have a recipe and a ready-to-use docker-compose that allows you to:

• pre-configure the TLS certificate for your domain

• pre-configure kamailio to use the certificate

• have a container that checks the validity of the certificate and autonomously performs the renewal when necessary

• have, ready-to-use, an nginx reverse proxy that uses the certificate: HTTP 80 -> HTTPS

---

The project structure

/doc
/modules/db
/modules/kamailio` 
/modules/nginx
env-template
docker-compose.yml

---

Step to deploy

1. clone the repository in your deploy machine git clone https://github.com/evoseed/kamailio-tls-letsencrypt .

2. pre-configure the TLS certificate for our domain

   • Install docker-compose.

   • Modify configuration of nginx
     Of course change your.domain.com with you real domain and change 1.2.3.4 with your real IP address.

     • Linux users

         cd modules/nginx/nginx-certbot
         sed -i 's/${DOMAIN}/your.domain.com/g' init-letsencrypt.sh
         sed -i 's/${DOMAIN}/your.domain.com/g' data/nginx/app.conf
         sed -i 's/${IP}/1.2.3.4/g' data/nginx/app.conf
       

     • MacOS users

         cd modules/nginx/nginx-certbot
         sed -i '' 's/${DOMAIN}/your.domain.com/g' init-letsencrypt.sh
         sed -i '' 's/${DOMAIN}/your.domain.com/g' data/nginx/app.conf
         sed -i '' 's/${IP}/1.2.3.4/g' data/nginx/app.conf
       

   • Run the init script and follow the Let's encrypt instructions:

       ./init-letsencrypt.sh
     

     NB It's quite important to compile correctly the conf in this way; in particular Organization Name and Common Name with your domain

3. return in the root of the project and compile the .env file following the env-template file

    cd ../../.. && cp env-template .env
   

   and complete the .env file

4. start all the containers

     docker-compose up -d
   

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The running project

When everything has started, what you will see is more or less this:

CONTAINER ID   IMAGE                    COMMAND                  CREATED       STATUS       PORTS     NAMES
48838aee0057   kamailio-test_kamailio   "/etc/kamailio/boots…"   2 hours ago   Up 1 hour                kamailio
8103e16bd845   kamailio-test_db         "docker-entrypoint.s…"   2 hours ago   Up 1 hour                db
e212e6c8241f   certbot/certbot          "/bin/sh -c 'trap ex…"   2 hours ago   Up 1 hour                certbot
3c101597b160   nginx:1.15-alpine        "/bin/sh -c 'while :…"   2 hours ago   Up 1 hour                nginx

The kamailio container, your SIP Server
the db container
the nginx container as reverse proxy for HTTP to HTTPS and the certbot container who will check every 12 hours whether certificates are still valid or need to be renewed (and will renew them independently if necessary)

---

Technical details

1. NB this project is designed for dev purpose, so it is not optimized for production.

   • docker-compose for example use the build stage

       ...
       build:
         context: ./modules/db
         dockerfile: Dockerfile
       ...
     

     so if you want to have a production ready version you can use built image:

       ...
       image: your.registry.com/project-name/db:latest
       ...
     

2. NB this project is designed to run on a VM to which the domain you configured in init-letsencrypt.sh and in the .env points and with a correct domain.

   If you run the init-letsencrypt.sh on your local private machine and you set ${DOMAIN} = your.domain.com you'll receive an error like this

    Requesting a certificate for your.domain.com
   
    Certbot failed to authenticate some domains (authenticator: webroot). The Certificate Authority reported these problems:
    Domain: your.domain.com
    Type:   dns
    Detail: no valid A records found for your.domain.com; no valid AAAA records found for your.domain.com
   
    Hint: The Certificate Authority failed to download the temporary challenge files created by Certbot. Ensure that the listed domains serve their content from the provided --webroot-path/-w and that files created there can be downloaded from the internet.
   
    Some challenges have failed.
    Ask for help or search for solutions at https://community.letsencrypt.org. See the logfile /var/log/letsencrypt/letsencrypt.log or re-run Certbot with -v for more details.
    ERROR: 1
   

   This happens because Let's Encrypt need to test if you are the admin of the domain.

3. How is the creation of certificates and their use automated in kamailio?

   As you can see in the docker-compose.yaml file, the certbot container and the kamailio container mount both volumes with the same source directory: ./module/nginx/nginx-certbot/data/certbot/conf/.
   So the certbot create (and renew) the certificates and kamailio use them.

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Thanks to

• Fred Posner for your article

• Kamailio SIP Server and all your community

• Philipp Schmieder for your nginx-certbot repo

• my evoseed team