[Sipsak] using sipsak to send NOTIFY message for MWI - IP address and port masquerading

[Monideth Pen]

Hi all,

I am looking to use sipsak to send SIP NOTIFY messages to our SIP
clients to provide support for MWI on our softswitch. This is required
because the softswitch dooes not have a built in voicemail system. The
voicemail service is provided by a different system but we need a
mechanism to alert users that a new voicemail message is available -
hence the need to using sipsak to send a SIP NOTIFY message.

I have figured out how to achieve this but the main problem is that the
SIP clients will be behind NAT and thus the NAT device will only have
sessions opened specific to the softswitch IP and port. Thus, when I
send the SIP NOTIFY messagefrom sipsak it needs to be seen as coming
from the softswitch IP address and port. If I tell sipsak to  use a
specific local port then I get the following error:

"unconnected UDP socket binding failed: Address already in use"

Which I assume is because the softswitch has already binded with the UDP
port - in this case 5060.

OK - so I thought that I would run sipsak from a different machine - but
then how do you spoof/masquerade the IP address of the softswitch?

I am using sipsak on a Linux system - CentOS 4.3. I understand the Linux
firewall can perform masquerading - but how do you  get it to masquerade
only the sipsak  process/traffic?

I appreciate any help anybody can give on this subject.

Regards,

Mon