A rudimentary dissector for Wireshark (Lua)

Wireshark offers a simple but effective option to extend its capabilities, by using Lua dissectors.

Just to give an example, recently received a pcap file containing some traffic (it was RTP) encapsulated inside a UDP header. The reason for encapsulation was transporting over a VPN.

I'm in such a habit to look into RTP streams on Wireshark, that I have a setting that tells it to try an interpret any UDP packet automatically as carrying RTP (I wrote how here). That also failed.

So Wireshark was not able to interpret those frames as RTP (or anything else, for what matters), and I remembered time ago writing a custom dissector. Wireshark allows to do that simply in Lua and add it as a plugin.

The code is available here. I just had to make it available inside '$HOME/.local/lib/wireshark/plugins/`.

Troubleshooting TURN

WebRTC applications use the ICE negotiation to discovery the best way to communicate with a remote party. I t dynamically finds a pair of candidates (IP address, port and transport, also known as “transport address”) suitable for exchanging media and data. The most important aspect of this is “dynamically”: a local and a remote transport address are found based on the network conditions at the time of establishing a session. For example, a WebRTC client that normally uses a server reflexive transport address to communicate with an SFU. when running inside the home office, may use a relay transport address over TCP when running inside an office network which limits remote UDP targets. The same configuration (defined as “iceServers” when creating an RTCPeerConnection will work in both cases, producing different outcomes.

Extracting RTP streams from network captures

I needed an efficient way to programmatically extract RTP streams from a network capture. In addition I wanted to: save each stream into a separate pcap file. extract SRTP-negotiated keys if present and available in the trace, associating them to the related RTP (or SRTP if the negotiation succeeded) stream. Some caveats: In normal conditions the negotiation of SRTP sessions happens via a secure transport, typically SIP over TLS, so the exchanged crypto information may not be available from a simple network capture. There are ways to extract RTP streams using Wireshark or tcpdump; it’s not necessary to do it programmatically. All this said I wrote a small tool ( https://github.com/giavac/pcap_tool ) that parses a network capture and tries to interpret each packet as either RTP/SRTP or SIP, and does two main things: save each detected RTP/SRTP stream into a dedicated pcap file, which name contains the related SSRC. print a summary of the crypto information exchanged, if available. With ...

Testing SIP platforms and pjsip

There are various levels of testing, from unit to component, from integration to end-to-end, not to mention performance testing and fuzzing. When developing or maintaining Real Time Communications (RTC or VoIP) systems, all these levels (with the exclusion maybe of unit testing) are made easier by applications explicitly designed for this, like sipp . sipp has a deep focus on performance testing, or using a simpler term, load testing. Some of its features allow to fine tune properties like call rate, call duration, simulate packet loss, ramp up traffic, etc. In practical terms though once you have the flexibility to generate SIP signalling to negotiate sessions and RTP streams, you can use sipp for functional testing too. sipp can act as an entity generating a call, or receiving a call, which makes it suitable to surround the system under test and simulate its interactions with the real world. What sipp does can be generalised: we want to be able to simulate the real world tha...

Giacomo Vacca

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