NEVERMIND! figured out how to do it, just put a ip in localnet that is not your localnet. Kind of faking asterisk thinking your intranet is public.
With CentOS Ethernet card being on private 192.168.1.5 and with the following in sip.conf
[general]
externip=209.10.10.10
localnet=192.168.1.0/255.255.255.0
When asterisk sent 200OK back out to public side, it does put the public IP (209.10.10.10) in the SDP correctly. BUT when it sent INVITE out to the private side 192.168.1.xxx, it will use the private IP in the SDP of INVITE. Anyway to FORCE it to use public IP in the SDP of INVITE and 200 OK no matter which side it goes out on?
Or is the only way is to have public in in the CentOS?
Thx!