pfSense

# RTP - Real-time Transport Protocol - RFC 3550

# Pattern attributes: ok overmatch undermatch fast fast

# Protocol groups: streaming_video ietf_internet_standard

# Wiki: http://www.protocolinfo.org/wiki/RTP

# Copyright (C) 2008 Matthew Strait, Ethan Sommer; See ../LICENSE

#

# RTP headers are *very* short and compact.  They have almost nothing in 

# them that can be matched by l7-filter.  As RTP connections take place 

# between even numbered ports, you should probably check for that before 

# applying this pattern.  If you want to match them along with their 

# associated SIP packets, you might try setting up some iptables rules 

# that watch for SIP packets and then also match any other UDP packets 

# that are going between the same two IP addresses.

#

# I think we can count on the first bit being 1 and the second bit being 

# 0 (meaning protocol version 2). The next two bits could go either way, 

# but in the example I've seen, they are zero, so I'll assume they are 

# usually zero.  The next four bits are a count of "contributing source 

# identifiers".  I'm not sure how big that could be, but in the example 

# I've seen, they're zero, so I'll assume they're usually zero. So that 

# gives us ^\x80.  The next bit is a tossup. Next is the payload type, 7 

# bits.  I've taken likely values from the WireShark code: 0-34, 96-127 

# (decimal). The rest of the header is random numbers (sequence number, 

# timestamp, synchronization source identifier), so that's no help at 

# all.

rtp

^\x80[\x01-"`-\x7f\x80-\xa2\xe0-\xff]?..........*\x80

# Might also try this.  It's a bit slower (one packet and not too much extra

# regexec load) and a bit more accurate:

#^\x80[\x01-"`-\x7f\x80-\xa2\xe0-\xff]?..........*\x80.*\x80