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> Chapter 33 - VoIP Solutions: SIP > How the SIP ALG performs NAT > Adding the original IP address and port to the SIP message header after NAT

Adding the original IP address and port to the SIP message header after NAT

In some cases your SIP configuration may require that the original IP address and port from the SIP contact request is kept after NAT. For example, the original SIP contact request could include the following:

Contact: <sip:0150302438@172.20.120.110:5060>;

After the packet goes through the FortiGate and NAT is performed, the contact request could normally look like the following (the IP address translated to a different IP address and the port to a different port):

Contact: <sip:0150302438@10.10.10.21:33608>;

You can enable register-contact-trace in a VoIP profile to have the SIP ALG add the original IP address and port in the following format:

Contact: <sip:0150302438@<nated-ip>:<nated-port>;o=<original-ip>: <original-port>>;

So the contact line after NAT could look like the following:

Contact: <sip:0150302438@10.10.10.21:33608;o=172.20.120.110:5060>;

Enter the following command to enable keeping the original IP address and port:

config voip profile

edit Profile_name

config sip

set register-contract-trace enable

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