help me to find rtmp url

[parkjs]

in the past time, i was guided way to find rtmp url by ngrep but in this time i am forget it, i'm vaguely remember: first, put rtmpdumphelper and ngrep into a directory, then run cmd and enter the command ngrep -d 3 sniff.txt and url will be available at the file sniff.txt, and now i am do that but the file sniff.txt doesn't exist, where is my mistake? please. p/s: my english is not good i hope everyone to hep me, thank a lot!