Archives May 2017

Configuring an RTP Proxy is one of the most confusing topic’s around setting up Kamailio. The goal of this article is to help you select the correct RTP Proxy implementation to install, discuss one common use case/pattern that RTP Proxy is used for and then setup up a RTP Proxy implementation to work with Kamailio.

Note: We offer paid support for designing, configuring and supporting VoIP infrastructures that contain Kamailio.   
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The Scenario

There are multiple use cases for using a RTP Proxy. However, in efforts to demonstrate how to use a RTP Proxy we are going to look at a simple use case where Kamailio is handling user registration and then passing the call directly to a carrier for making the outbound call, which is depicted below:

In this scenario, the User Agent is passing the SIP INVITE thru Kamailio. However, the SDP contains the local ip address of the User Agent because Kamailio is simply doing its job and passing the traffic thru without touching the requests. This means that the carrier can’t send the RTP traffic back to the User Agent because it’s sitting behind a firewall and the internal ip address is being sent in the SDP. Therefore, we need to install and configure a RTP Proxy. In the next section we will select the a RTP Proxy implementation to implement.

Select a RTP Proxy Implementation

There are 2 main versions of RTPProxy and it’s very confusing for a newbie to figure out which version you should implement.

Name	Pro’s	Con’s	REPO
RTPProxy	not sure	Doesn’t seem to be well documented	download
RTPEngine (formedly mediaproxy-ng)	Works with WebRTC clients	Not well documented	download

Since we are not going to implement WebRTC we can use RTPProxy.

Installing RTPProxy

Install pre-req’s and rtpproxy repo using yum package manager:

yum -y update
yum -y install gcc gcc-c++ bison openssl-devel libtermcap-devel ncurses-devel doxygen      \
            curl-devel newt-devel mlocate lynx tar wget nmap bzip2 unixODBC unixODBC-devel      \
            libtool-ltdl libtool-ltdl-devel mysql-connector-odbc mysql mysql-devel mysql-server \
            flex libxml2 libxml2-devel pcre pcre-devel git

From here you can either install using yum or from source.
We recommend installing from src as there are needed patches in new releases.

Installing RTPProxy from Source

Clone and compile the latest from github

cd /usr/src/
git clone --recursive https://github.com/sippy/rtpproxy.git
cd rtpproxy
./configure
make
make install

Copy the init script to init.d/ directory

/bin/cp -f rpm/rtpproxy.init /etc/rc.d/init.d/rtpproxy &&
chmod +x /etc/rc.d/init.d/rtpproxy

Create the rtpproxy user and group

mkdir -p /var/run/rtpproxy
groupadd rtpproxy
useradd -d /var/run/rtpproxy -M -s /bin/false rtpproxy
chown rtpproxy:rtpproxy -R /var/run/rtpproxy

Change the config file to use the rtpproxy binary.
You may need to replace if you installed in custom location.

sed -i 's:rtpproxy=/usr/bin/:rtpproxy=/usr/local/bin/:' /etc/rc.d/init.d/rtpproxy

I had to manually add the path of the program, as follows:

cat << 'EOF' > /etc/profile.d/rtpproxy.sh
#!/bin/sh
#
# Set an alias for compiled version of rtpproxy

# Change if installed in custom location
rtpproxy_prog="/usr/local/bin/rtpproxy"

if [ -f "$rtpproxy_prog" ] && [ -x "$rtpproxy_prog" ]; then
    export PATH=$PATH:$rtpproxy_prog
fi
EOF

Source your global shell environment configs;

source /etc/profile & source ~/.bashrc

Make rtpproxy start on boot:

chkconfig rtpproxy on

Replace startup options with whatever you want rtpproxy to start with:

local_ip=your.local.i.p
external_ip=your.external.i.p
echo 'OPTIONS="-F -s udp:127.0.0.1:7722 -l $local_ip -A $external_ip -m 10000 -M 20000 -d DBUG:LOG_LOCAL0"' > /etc/sysconfig/rtpproxy

Enabling NAT within Kamailio Configuration File

We used Flowroute for our carrier here. other carriers would be similar. If using Flowroute, you will need to login to your Flowroute account and obtain your techprefix.

We will assume that you are using the default kamailio.cfg file that ships with Kamailio.

1. vi /etc/kamailio/kamailio.cfg
2. add the following to the top of your config, beneath #!KAMAILIO

#!define WITH_MYSQL
#!define WITH_AUTH
#!define WITH_NAT
#!define WITH_USRLOCDB
#!define WITH_FLOWROUTE

1. save the configuration

Update the port kamailio listens on:

1. vi /etc/kamailio/kamailio.cfg
2. find the line that starts with: #listen=udp:10.0.0.10:5060
3. after that line add: listen=udp:yourlocalip:5060 advertise yourpublicip:5060
4. save the file

Add the carrier routing logic:

1. vi /etc/kamailio/kamailio.cfg
2. in the routing logic section find “route[PSTN] {“
3. add the following inside that function, replace with your own values:

#!ifdef WITH_FLOWROUTE
        $var(sip_prefix) = "<Flowroute carrier prefix>";  #Leave empty if your carrier doesn't require it
        $var(carrier_ip) = "216.115.69.144"; #Signaling IP for Flowroute. Replace with your carrier info

        # add prefix to rU
        $rU = $var(sip_prefix) + $rU;

        # forward to flowroute
        forward($var(carrier_ip), 5060);
        exit;
#!endif

Startup

Start rtpproxy:

service rtpproxy start

Restart kamailio:

You need to restart Kamailio if you make changes to rtpproxy configuration. In our case, we just installed it. So, you need to restart Kamailio in order for it to connect to the RTPProxy.

kamctl restart

Testing RTPProxy and Kamailio

First off make sure both Kamailio and Rtpproxy are running:

ps -ef | grep rtpproxy
ps -ef | grep kamailio

If either service fails to start check your logs while starting the service.
Open another terminal and run the following:

# terminal 2
tail -f /var/log/messages

Then in your original terminal start the services one by one.
You will see errors labeled. One of those will lead to your problem.

Add a test extension to Kamailio

If you already have a test extension in Kamailio you can skip this section.
“test” is our pw for extension “1001”:

kamctl add 1001@your.domain.com test

Now try to register a softphone with the extension you added.

Once registered run the following cmd and you should see your extension:

kamctl ul show

Try to make calls using that extension and capture the traffic. We will assume that you have either sngrep or ngrep installed already.

Your INVITE and 200 OK messages should be rewritten correctly.

Sources:

RTPProxy Revisted
RTPProxy Control Protocol