Sunday, 15 January 2017

Analysing Opus media from network traces

VoIP/RTC platforms have typically many elements processing audio. When an issue is reported it's important to be able to restrict the investigation field, to save time and resources.

A typical scenario is bad or missing audio perceived on the client side. As I've done previously (here for Opus and here for SILK) I'd like to share some practical strategies to extract audio from a pcap trace (to verify the audio received/sent was "correct") and to "re-play" the call inside a test bed (to verify that the audio was good but also carried correctly by the RTP stream). Of course a lot can be inferred by indirect data, for example the summary of RTCP reports showing the number of packets exchanged, packets lost, the latency. But sometimes those metrics are perfect while the issue is still there.

Focusing in this case on Opus audio, and starting from a pcap file with the network traces for a call under investigation, let's see how to decode the Opus frames carried by the RTP packets into an audible WAV file.

You don't even need to have captured the signalling: it's sufficient to have the UDP packets carrying the RTP. If signalling is not visible by Wireshark it may not recognize that the UDP packets carry RTP, but you give it a hint by right-clicking on a frame and "Decode as..." and selecting "RTP".

It's typically easy to find the relevant RTP stream in Wireshark ("Telephony -> RTP -> RTP Streams"), select it, and prepare a filter. Then you can Export the packets belonging to that stream into a dedicated pcap file ("File --> Export Specified Packets...").

I've then modified opusrtp from a fork of opus-tools in order to be able to extract the payload from a given pcap, creating an Opus file. e.g.:

./opusrtp --extract trace.pcap

This will output a rtpdump.opus file, which can be converted into a WAV file directly with opusdec, still part of opus-tools:

./opusdec --rate 8000 rtpdump.opus audio.wav

You can listen to the wav file and verify whether at least the carried RTP payload was valid.

The network trace with the RTP can also be used to re-play the call, injecting the same RTP as in the call under investigation. With the help of sipp you can set up a rudimentary but very powerful test bed. Use the standard UAS scenario (e.g. in uas.xml), but with an additional part:

right after the ACK is received. If you launch sipp with a command like:

sipp -sf uas.xml -i MEDIA_IP_ADDRESS

you'll be able to call sipp. It will answer the call, as the scenario mandates, and will play the RTP contained in rtp_opus.pcap. The stream SSRC, timestamps, even Marker bits will be preserved. This will give you quite an accurate simulation of the stream received by the client in the original call.

It should be straightforward to reach all these components. For opus-tools, on a debian-based machine, you can just:

sudo apt-get install libogg-dev libpcap-dev
git clone https://github.com/giavac/opus-tools.git
cd opus-tools
./autogen.sh
./configure
make

For sipp:
sudo apt-get install sip-tester

I hope this will save the reader some time in future investigations.

UPDATE: The fork of opus-tools was merged into the original repo, so you don't need my repo.

UPDATE 2: This only works if the opus payload in the RTP is not encrypted. Also it may need a patch when the extension header for volume indications are used (e.g. 'urn:ietf:params:rtp-hdrext:ssrc-audio-level', see RFC-6464). Don't forget that at the moment the payload type is harcoded to 120. You may need to rebuild opusrtp with the type your trace has, e.g. 96 (It should be easy to pass it as command line argument, something for a quiet moment).



14 comments:

  1. Hi,

    I am trying to covert a pcap and I have the following error:

    Got 132 byte packet (132 bytes captured)
    skipping packet: unrecognized linktype 12
    Got 127 byte packet (127 bytes captured)
    skipping packet: unrecognized linktype 12


    Here the capture:
    https://drive.google.com/file/d/0BySOpdix-JH6b2Z2Q2xnbFVnOUk/view?usp=sharing

    BR,
    Martín.



    ReplyDelete
  2. Hi Martin,
    the tool as is now expects data captured with ethernet (DLT_EN10MB) or null (DLT_NULL) type, while yours is a raw packet capture.

    Is there any chance you can have a capture with the ethernet layer?
    Otherwise the tool should be changed to support raw packet data (I don't think I'll be able to work on this any time soon).

    Giacomo

    ReplyDelete
    Replies
    1. HI Giacomo,

      I am using rtpengine to save the videocall, and right I save the pcap with the eth header, and the .opus is generating but is no audible.

      I am attaching the pcap here:
      https://drive.google.com/file/d/0BySOpdix-JH6djB0T01ncVpEblE/view?usp=sharing

      I think the log is ok:
      Got 155 byte packet (155 bytes captured)
      eth 0x0800 00:00:00:00:00:00 -> 00:00:00:00:00:00
      ipv4 protocol 17 195.77.235.141 -> 195.77.235.141 header 20 bytes
      udp 121 bytes 40268 -> 40282 crc 0x0000
      rtp 0xfca33a6e 111 23426 985204741 v2 .X. CC 0 101 bytes
      Got 157 byte packet (157 bytes captured)
      eth 0x0800 00:00:00:00:00:00 -> 00:00:00:00:00:00
      ipv4 protocol 17 195.77.235.141 -> 195.77.235.141 header 20 bytes
      udp 123 bytes 40268 -> 40282 crc 0x0000

      I don't know what I am doing wrong, maybe when I have exported from the original pcap to the only opus.pcap

      Delete
    2. After you have the .opus file, decode it into a wav file, e.g.:

      ./opusdec --rate 8000 rtpdump.opus output.wav

      Delete
    3. also the wav file is not audible, has the following errors:
      Decoding to 8000 Hz (2 channels)
      Encoded with opus rtp packet dump
      Decoding error: corrupted stream
      Decoding error: corrupted stream
      Decoding error: corrupted stream
      Decoding error: corrupted stream
      Decoding error: corrupted stream
      Decoding error: corrupted stream
      Decoding error: corrupted stream
      Decoding error: corrupted stream
      Decoding error: corrupted stream
      Decoding error: corrupted stream
      Decoding error: corrupted stream
      Decoding error: corrupted stream
      Decoding error: corrupted stream
      Decoding error: corrupted stream
      Decoding error: corrupted stream
      Decoding error: corrupted stream
      Decoding error: corrupted stream
      Decoding error: corrupted stream
      Decoding error: corrupted stream
      Decoding error: corrupted stream
      Decoding error: corrupted stream
      Decoding error: corrupted stream
      Decoding error: corrupted stream
      Decoding error: corrupted stream
      Decoding error: corrupted stream
      Decoding error: corrupted stream
      Decoding error: corrupted stream
      Decoding error: corrupted stream
      Decoding complete.

      Delete
    4. Hi Giacomo,

      Could you help me to see what happens? Do you need the capture?

      Thanks,
      Martín.

      Delete
    5. OK Martin, feel free to send me the capture (gmail email address).

      Delete
  3. This comment has been removed by the author.

    ReplyDelete
  4. I have a pcap with 2 VoIP calls(the first is a 1-way audio call). Installed your fork opus-tools to make th eopus payload to listen if the sound got to the devices.
    Unfortunately running into the issues below
    Running Kali Linux after .\configure I get Type "make; make install" then after make I get the following errors:
    /usr/bin/ld: src/opusenc-resample.o: undefined reference to symbol 'sin@@GLIBC_2.2.5'
    /usr/bin/ld: /lib/x86_64-linux-gnu/libm.so.6: error adding symbols: DSO missing from command line collect2: error: ld returned 1 exit status.
    Thanks

    ReplyDelete
    Replies
    1. Hi Unknown, are you sure the configure command hasn't complained about missing libraries?

      Regards,
      Giacomo

      Delete
  5. It says is OK, log below. Thanks for the response:
    root@hostname:/home/git_repo/opus-tools# ./configure
    checking whether make supports nested variables... yes
    checking for a BSD-compatible install... /usr/bin/install -c
    checking whether build environment is sane... yes
    checking for a thread-safe mkdir -p... /usr/bin/mkdir -p
    checking for gawk... gawk
    checking whether make sets $(MAKE)... yes
    checking whether to enable maintainer-specific portions of Makefiles... yes
    checking build system type... x86_64-pc-linux-gnu
    checking host system type... x86_64-pc-linux-gnu
    checking whether make supports the include directive... yes (GNU style)
    checking for gcc... gcc
    checking whether the C compiler works... yes
    checking for C compiler default output file name... a.out
    checking for suffix of executables...
    checking whether we are cross compiling... no
    checking for suffix of object files... o
    checking whether we are using the GNU C compiler... yes
    checking whether gcc accepts -g... yes
    checking for gcc option to accept ISO C89... none needed
    checking whether gcc understands -c and -o together... yes
    checking dependency style of gcc... gcc3
    checking for gcc option to accept ISO C99... none needed
    checking how to run the C preprocessor... gcc -E
    checking for grep that handles long lines and -e... /usr/bin/grep
    checking for egrep... /usr/bin/grep -E
    checking for ANSI C header files... yes
    checking for sys/types.h... yes
    checking for sys/stat.h... yes
    checking for stdlib.h... yes
    checking for string.h... yes
    checking for memory.h... yes
    checking for strings.h... yes
    checking for inttypes.h... yes
    checking for stdint.h... yes
    checking for unistd.h... yes
    checking whether byte ordering is bigendian... no
    checking for an ANSI C-conforming const... yes
    checking for inline... inline
    checking for C/C++ restrict keyword... __restrict
    checking for C99 variable-size arrays... yes
    checking if gcc supports -O3 -g -ffast-math... yes
    ./configure: line 4992: LT_LIB_M: command not found
    checking for main in -lwinmm... no
    checking for pkg-config... yes
    checking for pkg-config... /usr/bin/pkg-config
    checking pkg-config is at least version 0.9.0... yes
    checking for OGG... yes
    checking for OPUS... yes
    checking sys/soundcard.h usability... yes
    checking sys/soundcard.h presence... yes
    checking for sys/soundcard.h... yes
    checking for sio_open in -lsndio... no
    checking for FLAC... yes
    checking for pcap_open_live in -lpcap... no
    checking if gcc supports -fstack-protector-all... yes
    checking for PIE support... yes
    checking if gcc supports -Wall -Wextra -Wcast-align -Wnested-externs -Wshadow -Wstrict-prototypes... yes
    checking for _LARGEFILE_SOURCE value needed for large files... no
    checking for lrintf... no
    checking for fminf... no
    checking for fmaxf... no
    checking that generated files are newer than configure... done
    configure: creating ./config.status
    config.status: creating Makefile
    config.status: creating config.h
    config.status: config.h is unchanged
    config.status: executing depfiles commands
    configure:
    ------------------------------------------------------------------------
    opus-tools 0.1.9-40-g64925e0: Automatic configuration OK.

    Compiler support:

    C99 var arrays: ................ yes
    C99 lrintf: .................... no
    Stack protector: ............... yes
    PIE: ........................... yes

    General configuration:

    Assertion checking: ............ no
    FLAC input: .................... yes

    ------------------------------------------------------------------------

    Type "make; make install" to compile and install

    ReplyDelete
    Replies
    1. I'm suspicious about

      > ./configure: line 4992: LT_LIB_M: command not found

      Delete
  6. Would you recommend a specific Linux Distro? I do not require to Kali Linux.
    Thanks

    ReplyDelete
    Replies
    1. I'd recommend Debian or Ubuntu, just because that's where I was working on this project.
      Regards,
      Giacomo

      Delete