Controlling how the SIP ALG NATs SIP contact header line addresses

You can enable contact-fixup so that the SIP ALG performs normal SIP NAT translation to SIP contact headers as SIP messages pass through the FortiGate.

Disable contact-fixup if you do not want the SIP ALG to perform normal NAT translation of the SIP contact header if a Record-Route header is also available. If contact-fixup is disabled, the FortiGate ALG does the following with contact headers:

• For Contact in Requests, if a Record-Route header is present and the request comes from the external network, the SIP Contact header is not translated.

Controlling NAT for addresses in SDP lines

• For Contact in Responses, if a Record-Route header is present and the response comes from the external network, the SIP Contact header is not translated.

If contact-fixup is disabled, the SIP ALG must be able to identify the external network. To identify the external network, you must use the config system interface command to set the external keyword to enable for the interface that is connected to the external network.

Enter the following command to perform normal NAT translation of the SIP contact header:

config voip profile edit VoIP_Pro_1 config sip set contact-fixup enable

end

end

Configuring SIP IP address conservation for the SIP session helper

You can use the following command to enable or disable SIP IP address conservation for the SIP session helper. IP address conservation is enabled by default for the SIP session helper.

config system settings set sip-nat-trace disable

end

If the SIP message does not include an i= line and if the original source IP address of the traffic (before NAT) was 10.31.101.20 then the FortiGate would add the following i= line.

i=(o=IN IP4 10.31.101.20)

Configuring SIP IP address conservation for the SIP ALG

You can use the following command to enable or disable SIP IP address conservation in a VoIP profile for the SIP ALG. SIP IP address conservation is enabled by default in a VoIP profile.

config voip profile edit VoIP_Pro_1 config sip set nat-trace disable

end

end

If the SIP message does not include an i= line and if the original source IP address of the traffic (before NAT) was 10.31.101.20 then the FortiGate would add the following i= line.

i=(o=IN IP4 10.31.101.20)

You can also use the preserve-override option to configure the SIP ALG to either add the original o= line to the end of the i= line or replace the i= line in the original message with a new i= line in the same form as above for adding a new i= line.

By default, preserver-override is disabled and the SIP ALG adds the original o= line to the end of the original i= line. Use the following command to configure the SIP ALG to replace the original i= line:

config voip profile edit VoIP_Pro_1 config sip set preserve-override enable

end

end

NAT with IP address conservation

In a source or destination NAT security policy that accepts SIP sessions, you can configure the SIP ALG or the SIP session helper to preserve the original source IP address of the SIP message in the i= line of the SDP profile. NAT with IP address conservation (also called SIP NAT tracing) changes the contents of SIP messages by adding the source IP address of the originator of the message into the SDP i= line of the SIP message. The SDP i= line is used for free-form text. However, if your SIP server can retrieve information from the SDP i= line, it can be useful for keeping a record of the source IP address of the originator of a SIP message when operating in a NAT environment. You can use this feature for billing purposes by extracting the IP address of the originator of the message.

Different source and destination NAT for SIP and RTP

This is a more complex scenario that a SIP service provider may use. It can also be deployed in large-scale SIP environments where RTP has to be processed by the FortiGate and the RTP server IP has to be translated differently than the SIP serverIP.

NAT with IP address conservation

Different source and destination NAT for SIP and RTP

RTP servers

192.168.0.21 – 192.168.0.23                            219.29.81.10

In this scenario, shown above, assume there is a SIP server and a separate media gateway. The SIP server is configured so that the SIP phone (219.29.81.20) will connect to 217.233.90.60. The media gateway (RTP server:

219.29.81.10) will connect to 217.233.90.65.

What happens is as follows:

1. The SIP phone connects to the SIP VIP. The FortiGate ALG translates the SIP contact header to the SIP server: 219.29.81.20 > 217.233.90.60 (> 10.0.0.60).
2. The SIP server carries out RTP to 217.233.90.65.
3. The FortiGate ALG opens pinholes, assuming that it knows the ports to be opened.
4. RTP is sent to the RTP-VIP (217.233.90.65.) The FortiGate ALG translates the SIP contact header to 192.168.0.21.

Source NAT with an IP pool

You can choose NAT with the Dynamic IP Pool option when configuring a security policy if the source IP of the SIP packets is different from the interface IP. The FortiGate ALG interprets this configuration and translates the SIP header accordingly.

This configuration also applies to destination NAT.