Running ffmpeg as an rtmp server

Ffmpeg supports running as an rtmp server which is very useful to test/verify an incoming rtmp stream.
Run ffmpeg with -listen 1 flag as a server and set -i rtmp://ip:port. Copy the incoming stream to udp output to be able to check the stream using ffplay or vlc afterwards:

ffmpeg -f flv -listen 1 -i rtmp://127.0.0.1:9000 -c copy udp://127.0.0.1:9001

and then check the stream as follows:

ffplay -i udp:/127.0.0.1:900