Converting RTP in pcap to Audio Wav files

After following a lot of different tutorials (some of which worked some of which didn’t), I came up with a shell script using a couple tools to scrape a packet capture file, pull out the rtp packets, and then convert them back into audio.

For me this will be useful in automated testing.  I currently drive automated SIP calls via SIPCLI and ruby for a variety of tests at work.  But how do I know I get the right end point?  In the past, I’d have the phone number I dial, record voice mail and send me an email, and the sipcli client would send over text to speech audio.

But I dont always have the luxury of being able to configure the phone number to voice mail.

I’ve been wanting to do a packet capture during the test and convert it back to audio afterwards, then do a wav comparison on the expected audio vs. the captured audio.

Tools used:

tshark
sox

These are both linux tools.
tshark is a command line version of wireshark.  It’s installed on centos boxes using yum install wireshark-gnome.
sox via yum install sox

Sox is a audio analysis tool that is run from the command line.

Test Script:

After looking at some examples online of different tools, I pieced this together from other people’s examples, with a few modifications. It seems to work for me:

Contents of pcap_to_wav.sh:

ssrc=$(sudo tshark -n -r capture.pcap -R rtp -T fields -e rtp.ssrc -Eseparator=, | sort -u)
echo $ssrc
sudo tshark -n -r capture.pcap -R rtp -R “rtp.ssrc == $ssrc” -T fields -e rtp.payload | tee payloads
for payload in `cat payloads`; do IFS=:; for byte in $payload; do printf “\\x$byte” >> sound.raw; done; done
echo ‘sox has converted pcap to wav file’
sudo sox -t raw -r 8000 -c 1 -U sound.raw capture3d.wav
That’s it!
basically if you have sudo access, you can run this and it will take the pcap and find the rtp packets, then make that a raw audio file… sox is then used to convert the raw file to a wav file.
At this point, you can further use sox to compare one wav to another wav.

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I work for a Telecom company writing and testing software. My passion for writing code is expressed through this blog. It's my hope that it gives hope to any and all who are self-taught.