D-Link DFL-1660 User Manual page 232

6.2.7. The SIP ALG
The exchanges illustrated are as follows:
• 1,2 - An initial INVITE is sent to the outbound local proxy server on the DMZ.
• 3,4 - The proxy server sends the SIP messages towards the destination on the Internet.
• 5,6 - A remote client or proxy server replies to the local proxy server.
• 7,8 - The local proxy forwards the reply to the local client.
This scenario can be implemented in a topology hiding setup with DMZ (Solution A below) as well
as a setup without NAT (Solution B below).
Solution A - Using NAT
The following should be noted about this setup:
• The IP address of the SIP proxy must be a globally routable IP address. The NetDefend Firewall
does not support hiding of the proxy on the DMZ.
• The IP address of the DMZ interface must be a globally routable IP address. This address can be
the same address as the one used on the external interface.
The setup steps are as follows:
1. Define a single SIP ALG object using the options described above.
2. Define a Service object which is associated with the SIP ALG object. The Service should have:
• Destination Port set to 5060 (the default SIP signalling port)
• Type set to TCP/UDP
3. Define four rules in the IP rule set:
• A NAT rule for outbound traffic from the clients on the internal network to the proxy
located on the DMZ interface. The SIP ALG will take care of all address translation needed
by the NAT rule. This translation will occur both at the IP level and at the application level.
Note
Clients registering with the proxy on the DMZ will have the IP address of the
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Chapter 6. Security Mechanisms