SDP header modification

I’m having troubles with 3G-based SIP clients whose service provider uses RFC1918 addresses, does not provide a SIP ALG on their firewalls and blocks STUN at the same time. I receive SDP headers with RFC1918 IPs for the Owner Address/Connection Information part so an RTP stream cannot flow properly. Is there a way to rewrite the SDP header properly either in Asterisk 11 or iptables perhaps?

Not having SIP ALG is a good thing!

You should look at the externaddr and externhost settings. I think there may be additional options for RTP. Without STUN, it would be advisable to use port mapping with no translation of port numbers,