Why would I receive input/output error with testsr

Question

I am trying to write an integration test which requires actually RTMP streaming to a 3rd party service. How to generate an RTMP test stream using ffmpeg command? seems like the right answer, however I can't get it to work.

As a baseline, without RTMP, ffmpeg -f lavfi -i testsrc -t 30 -pix_fmt yuv420p test.mp4 works.

ffmpeg -f lavfi -i testsrc -t 30 -pix_fmt yuv420p -f flv rtmp://mylocation gives me the error rtmp://mylocation: Input/output error

I should note that the URL is valid, otherwise I get an error saying it can't open a connection.

I also tried ffmpeg -f lavfi -i testsrc -pix_fmt yuv420p -f flv rtmp://mylocatiom. I had read that if you don't supply time, it goes on forever and thought I might need that for an RTMP stream. I got the same error.

Here is full output from last attempt, with added params as suggested by @Gyan below, along with verbose logging.

ffmpeg -loglevel verbose -f lavfi -i testsrc -pix_fmt yuv420p -f flv -c:v libx264 -g 50 rtmp://myhost:1935/identifier
ffmpeg version 4.1 Copyright (c) 2000-2018 the FFmpeg developers
  built with Apple LLVM version 10.0.0 (clang-1000.11.45.5)
  configuration: --prefix=/usr/local/Cellar/ffmpeg/4.1 --enable-shared --enable-pthreads --enable-version3 --enable-hardcoded-tables --enable-avresample --cc=clang --host-cflags= --host-ldflags= --enable-ffplay --enable-gpl --enable-libmp3lame --enable-libopus --enable-libsnappy --enable-libtheora --enable-libvorbis --enable-libvpx --enable-libx264 --enable-libx265 --enable-libxvid --enable-lzma --enable-opencl --enable-videotoolbox
  libavutil      56. 22.100 / 56. 22.100
  libavcodec     58. 35.100 / 58. 35.100
  libavformat    58. 20.100 / 58. 20.100
  libavdevice    58.  5.100 / 58.  5.100
  libavfilter     7. 40.101 /  7. 40.101
  libavresample   4.  0.  0 /  4.  0.  0
  libswscale      5.  3.100 /  5.  3.100
  libswresample   3.  3.100 /  3.  3.100
  libpostproc    55.  3.100 / 55.  3.100
[Parsed_testsrc_0 @ 0x7fddf34022c0] size:320x240 rate:25/1 duration:-1.000000 sar:1/1
Input #0, lavfi, from 'testsrc':
  Duration: N/A, start: 0.000000, bitrate: N/A
    Stream #0:0: Video: rawvideo, 1 reference frame (RGB[24] / 0x18424752), rgb24, 320x240 [SAR 1:1 DAR 4:3], 25 tbr, 25 tbn, 25 tbc
[tcp @ 0x7fddf3404880] Starting connection attempt to {theIP} port 1935
[tcp @ 0x7fddf3404880] Successfully connected to {theIP} port 1935
rtmp://myhost:1935/identifier: Input/output error

Updating to debug added some more data like this before error:

[tcp @ 0x7fc4e1601a00] Starting connection attempt to 34.224.30.155 port 1935
[tcp @ 0x7fc4e1601a00] Successfully connected to 34.224.30.155 port 1935
[rtmp @ 0x7fc4e1411200] Handshaking...
[rtmp @ 0x7fc4e1411200] Type answer 3
[rtmp @ 0x7fc4e1411200] Server version 3.0.1.1
[rtmp @ 0x7fc4e1411200] Proto = rtmp, path = /identifier, app = identifier, fname = 
[rtmp @ 0x7fc4e1411200] Window acknowledgement size = 2500000
[rtmp @ 0x7fc4e1411200] Max sent, unacked = 2500000
[rtmp @ 0x7fc4e1411200] New incoming chunk size = 4096
[rtmp @ 0x7fc4e1411200] Releasing stream...
[rtmp @ 0x7fc4e1411200] FCPublish stream...
[rtmp @ 0x7fc4e1411200] Creating stream...
[rtmp @ 0x7fc4e1411200] Sending publish command for ''
rtmp://myhost:1935/identifier: Input/output error

Sending publish command for '' is suspect, but not sure what to do about that.

Answer 1

rtmp://localhost:1935/app/streamkey is usually the endpoint for rtmp based streaming... in your examples you only show rtmp://host:1935/app, and depending on what type of service your publishing to, may not work. however, i did test this against a vanilla install of wowza streaming engine to rtmp://wowza:1935/app and in fact it did work, and i was able to view the stream at the same endpoint with ffplay

Answer 2

I didn't notice - fname was blank in the debug output. I was missing the stream name/key. I needed to do rtmp://myhost:1935/identifier/streamkey. The logs showed my stream name was empty. I had been copy pasting the URL without the key since I had absentmindendly forgotten about it - my OBS is setup to always have the same key but the URL changes and that was what I was pasting again and again.