Debug Logging

Kurento Media Server generates log files that are stored in /var/log/kurento-media-server/. The content of this folder is as follows:

  • media-server_<timestamp>.<log_number>.<kms_pid>.log: Output log of a currently running instance of KMS.
  • media-server_error.log: Errors logged by third-party libraries.
  • logs: Folder that contains older KMS logs. The logs in this folder are rotated, so they don’t fill up all the space available in the disk.

Each line in a log produced by KMS has a fixed structure:

[timestamp] [pid] [memory] [level] [component] [filename:loc] [method] [message]
  • [timestamp]: Date and time of the logging message (e.g. 2017-12-31 23:59:59,493295).
  • [pid]: Process Identifier of kurento-media-sever (e.g. 17521).
  • [memory]: Memory address in which the kurento-media-sever component is running (e.g. 0x00007fd59f2a78c0).
  • [level]: Logging level. This value typically will be INFO or DEBUG. If unexpected error situations happen, the WARN and ERROR levels will contain information about the problem.
  • [component]: Name of the component that generated the log line. E.g. KurentoModuleManager, webrtcendpoint, or qtmux, among others.
  • [filename:loc]: Source code file name (e.g. main.cpp) followed by the line of code number.
  • [method]: Name of the function in which the log message was generated (e.g. loadModule(), doGarbageCollection(), etc).
  • [message]: Specific log information.

For example, when KMS starts correctly, this trace is written in the log file:

[timestamp] [pid] [memory]  info  KurentoMediaServer  main.cpp:255  main()  Kurento Media Server started

Logging levels and components

Each different component of KMS is able to generate its own logging messages. Besides that, each individual logging message has a severity level, which defines how critical (or superfluous) the message is.

These are the different message levels, as defined by the GStreamer logging library:

  • (1) ERROR: Logs all fatal errors. These are errors that do not allow the core or elements to perform the requested action. The application can still recover if programmed to handle the conditions that triggered the error.
  • (2) WARNING: Logs all warnings. Typically these are non-fatal, but user-visible problems that are expected to happen.
  • (3) FIXME: Logs all “fixme” messages. Fixme messages are messages that indicate that something in the executed code path is not fully implemented or handled yet. The purpose of this message is to make it easier to spot incomplete/unfinished pieces of code when reading the debug log.
  • (4) INFO: Logs all informational messages. These are typically used for events in the system that happen only once, or are important and rare enough to be logged at this level.
  • (5) DEBUG: Logs all debug messages. These are general debug messages for events that happen only a limited number of times during an object’s lifetime; these include setup, teardown, change of parameters, etc.
  • (6) LOG: Logs all log messages. These are messages for events that happen repeatedly during an object’s lifetime; these include streaming and steady-state conditions.
  • (7) TRACE: Logs all trace messages. These messages for events that happen repeatedly during an object’s lifetime such as the ref/unref cycles.
  • (8) MEMDUMP: Log all memory dump messages. Memory dump messages are used to log (small) chunks of data as memory dumps in the log. They will be displayed as hexdump with ASCII characters.

Logging categories and levels can be set by two methods:

  • Use the specific command-line argument while launching KMS. For example, run:

    /usr/bin/kurento-media-server \
      --gst-debug-level=3 \
  • Use the environment variable GST_DEBUG. For example, run:

    export GST_DEBUG="3,Kurento*:4,kms*:4"

Suggested levels

Here are some tips on what logging components and levels could be most useful depending on what is the issue to be analyzed. They are given in the environment variable form, so they can be copied directly into the KMS configuration file, /etc/default/kurento-media-server:

  • Default suggested levels:

    export GST_DEBUG="3,Kurento*:4,kms*:4"
  • COMEDIA port discovery:

    export GST_DEBUG="3,rtpendpoint:4"
  • ICE candidate gathering:

    export GST_DEBUG="3,kmsiceniceagent:5,kmswebrtcsession:5,webrtcendpoint:4"


    • kmsiceniceagent shows messages from the Nice Agent (handling of candidates).
    • kmswebrtcsession shows messages from the KMS WebRtcSession (decision logic).
    • webrtcendpoint shows messages from the WebRtcEndpoint (very basic logging).
  • Event MediaFlow{In|Out} state changes:

    export GST_DEBUG="3,KurentoMediaElementImpl:5"
  • Player:

    export GST_DEBUG="3,playerendpoint:5"
  • Recorder:

    export GST_DEBUG="3,KurentoRecorderEndpointImpl:4,recorderendpoint:5,qtmux:5"
  • REMB congestion control:

    export GST_DEBUG="3,kmsremb:5"


    • kmsremb:5 (debug level 5) shows only effective REMB send/recv values.
    • kmsremb:6 (debug level 6) shows full handling of all source SSRCs.
  • RPC calls:

    export GST_DEBUG="3,KurentoWebSocketTransport:5"
  • RTP Sync:

    export GST_DEBUG="3,kmsutils:5,rtpsynchronizer:5,rtpsynccontext:5,basertpendpoint:5"
  • SDP processing:

    export GST_DEBUG="3,kmssdpsession:4"
  • Transcoding of media:

    export GST_DEBUG="3,Kurento*:5,kms*:4,agnosticbin*:7"
  • Unit tests:

    export GST_DEBUG="3,check:5"

3rd-party libraries


libnice is the GLib implementation of ICE, the standard method used by WebRTC to solve the issue of NAT Traversal.

This library has its own logging system that comes disabled by default, but can be enabled very easily. This can prove useful in situations where a developer is studying an issue with the ICE process. However, the debug output of libnice is very verbose, so it makes sense that it is left disabled by default for production systems.

Run KMS with these environment variables defined: G_MESSAGES_DEBUG and NICE_DEBUG. They must have one or more of these values, separated by commas:

  • libnice
  • libnice-stun
  • libnice-tests
  • libnice-socket
  • libnice-pseudotcp
  • libnice-pseudotcp-verbose
  • all


export G_MESSAGES_DEBUG="libnice,libnice-stun"