I tried
ffmpeg -i rtsp://172.19.1.34/live.sdp -acodec copy -vcodec copy b.mp4 2>>log.txt 2>&1
to keep stderr output and also redirect it to file.
But it failed.
However, I can keep the stderr output by
ffmpeg -i rtsp://172.19.1.34/live.sdp -acodec copy -vcodec copy b.mp4 2>log.txt