Set RTMP Stream Key and Publishing Mode in FFmpeg

This article provides a straightforward guide on how to configure the stream key and publishing mode when sending an RTMP stream using FFmpeg. By understanding the composition of the RTMP destination URL and utilizing FFmpeg’s dedicated RTMP flags, you can control stream authentication, stream targeting, and determine whether the remote server treats the broadcast as a real-time live event or records it directly to disk.

RTMP URL Structure

An RTMP URL is generally structured into three components: the server address, the application name, and the stream key:

rtmp://[server-address]:[port]/[application]/[stream-key]
  • Server Address: The hostname or IP address of the ingestion server (e.g., live.example.com).
  • Application: The RTMP application defined on the server (e.g., live, app).
  • Stream Key: The unique identifier for your stream, which acts as both the stream's name and its access token.

1. Setting the Stream Key

The stream key is placed directly after the application name in the RTMP path.

ffmpeg -i input.mp4 -c:v libx264 -c:a aac -f flv rtmp://live.example.com/app/mySecretStreamKey

If your streaming provider separates the RTMP URL and the Stream Key (common with platforms like YouTube, Twitch, or Facebook), concatenate them with a slash:

  • RTMP URL: rtmp://a.rtmp.youtube.com/live2
  • Stream Key: xxxx-xxxx-xxxx-xxxx
  • FFmpeg Target: rtmp://a.rtmp.youtube.com/live2/xxxx-xxxx-xxxx-xxxx

If the stream key requires query parameters (e.g., for tokens or authentication), append them using standard URL query syntax:

rtmp://live.example.com/app/mySecretStreamKey?token=abc123xyz

2. Setting the Publishing Mode

RTMP publishing modes instruct the receiving server how to handle the stream. The three primary modes defined in the RTMP specification are:

  • live: Broadcasts content in real-time without writing it to server storage (default).
  • record: Writes the incoming stream to a file on the server, overwriting existing files with the same name.
  • append: Appends the incoming stream data to an existing recorded file.

Method A: Using FFmpeg’s Native -rtmp_live Option

FFmpeg’s built-in RTMP protocol handler allows you to specify whether the stream should be treated as live or playable via the -rtmp_live option:

ffmpeg -i input.mp4 -c:v libx264 -c:a aac -rtmp_live live -f flv rtmp://live.example.com/app/streamKey

Values for -rtmp_live:

  • live: Default behavior; specifies real-time transmission.
  • recorded: Informs the server that the source is pre-recorded.

Method B: Specifying Mode in the RTMP URL

Many media servers (such as NGINX-RTMP, Wowza, and Red5) allow or require the publishing mode to be declared as an inline parameter directly in the stream key path:

  • Record Mode:

    ffmpeg -i input.mp4 -c copy -f flv "rtmp://live.example.com/app/streamKey?publish_type=record"

    (Some servers use ?mode=record or append the mode directly to the stream name, such as streamKey?record)

  • Append Mode:

    ffmpeg -i input.mp4 -c copy -f flv "rtmp://live.example.com/app/streamKey?publish_type=append"

Method C: Using librtmp Syntax

If FFmpeg was compiled with librtmp support (--enable-librtmp), connection parameters, including the publishing mode, can be passed as space-delimited options in the URL string:

ffmpeg -i input.mp4 -c copy -f flv "rtmp://live.example.com/app/streamKey live=1"

  • For recording:

    ffmpeg -i input.mp4 -c copy -f flv "rtmp://live.example.com/app/streamKey record=1"

Complete Command Example

To capture a video file, encode it to standard H.264/AAC, assign the stream key live_user_123, and enforce live broadcast mode:

ffmpeg -re -i source.mp4 \
  -c:v libx264 -preset veryfast -b:v 3000k -maxrate 3000k -bufsize 6000k \
  -c:a aac -b:a 128k -ar 44100 \
  -rtmp_live live \
  -f flv "rtmp://live.example.com/live/live_user_123"