If you installed your rtmpdump in this way:
sudo apt-get install rtmpdump
you have an old version of rtmpdump package which won't be of much use today.
Follow steps from this thread to update your rtmpdump:
http://stream-recorder.com/forum/str...mp-t22689.html
This is how my rtmpdump directory looks like:
j@j:~$ cd rtmpdump
j@j:~/rtmpdump$ ls -l
total 2048
-rw-rw-r-- 1 j j 9520 Mar 30 00:00 ChangeLog
-rw-rw-r-- 1 j j 17987 Mar 30 00:00 COPYING
-rw-r--r-- 1 root root 40 Mar 30 00:03 description-pak
drwxr-xr-x 2 root root 4096 Mar 30 00:03 doc-pak
drwxrwxr-x 2 j j 4096 Mar 30 00:03 librtmp
-rw-rw-r-- 1 j j 2074 Mar 30 00:02 Makefile
-rw-rw-r-- 1 j j 139950 Dec 15 2015 Patch.diff
-rw-rw-r-- 1 j j 8516 Mar 30 00:00 README
-rwxrwxr-x 1 j j 36376 Mar 30 00:02 rtmpdump
-rw-rw-r-- 1 j j 8174 Mar 30 00:00 rtmpdump.1
-rw-rw-r-- 1 j j 11179 Mar 30 00:00 rtmpdump.1.html
-rw-r--r-- 1 root root 135258 Mar 30 00:03 rtmpdump_2.4+git20170330-1_amd64.deb
-rw-rw-r-- 1 j j 1254144 Dec 15 2015 rtmpdump-2.4.zip
-rw-rw-r-- 1 j j 41605 Mar 30 00:02 rtmpdump.c
-rw-rw-r-- 1 j j 39192 Mar 30 00:02 rtmpdump.o
-rwxrwxr-x 1 j j 32696 Mar 30 00:02 rtmpgw
-rw-rw-r-- 1 j j 7649 Mar 30 00:00 rtmpgw.8
-rw-rw-r-- 1 j j 10379 Mar 30 00:00 rtmpgw.8.html
-rw-rw-r-- 1 j j 30646 Mar 30 00:02 rtmpgw.c
-rw-rw-r-- 1 j j 31512 Mar 30 00:02 rtmpgw.o
-rwxrwxr-x 1 j j 34928 Mar 30 00:02 rtmpsrv
-rw-rw-r-- 1 j j 37630 Mar 30 00:02 rtmpsrv.c
-rw-rw-r-- 1 j j 30400 Mar 30 00:02 rtmpsrv.o
-rwxrwxr-x 1 j j 33720 Mar 30 00:02 rtmpsuck
-rw-rw-r-- 1 j j 41105 Mar 30 00:02 rtmpsuck.c
-rw-rw-r-- 1 j j 27816 Mar 30 00:02 rtmpsuck.o
-rw-rw-r-- 1 j j 1583 Mar 30 00:02 thread.c
-rw-rw-r-- 1 j j 1235 Mar 30 00:00 thread.h
-rw-rw-r-- 1 j j 2240 Mar 30 00:02 thread.o
j@j:~/rtmpdump$
Type on command prompt anywhere in your terminal: rtmpdump, and this is what you should see:
j@j:~$ rtmpdump
RTMPDump v2.4 KSV-2015-12-14
(c) 2010 Andrej Stepanchuk, Howard Chu, The Flvstreamer Team; license: GPL
ERROR: You must specify a hostname (--host) or url (-r "rtmp://host[
ort]/playpath") containing a hostname
j@j:~$