Your ffmpeg binary was built with --enable-librtmp, therefore you should be doing all of this in ffmpeg, and not using rtmpdump at all.
Code:
ffmpeg -i "rtmp://1.2.3.4/live/feed1 pageUrl=http://website.net swfurl=http://xyz.net/mediaplayer.swf live=1" -re -isync -vcodec libx264 -b 600k -acodec libfaac -ab 128000 -ar 32000 -f flv "rtmp://5.6.7.8/live/feed1relay"