General use cases

There are three scenarios in which the FortiOS session initiation protocol (SIP) solution is usually deployed:

1. The SIP server is in a private network, protected from the internet by a FortiOS device.
2. The SIP clients are in a private network, protected from the internet by a FortiOS device.
3. The SIP server is in a private network, such as a corporation's internal network or an ISP’s network, protected from the Internet by a FortiOS device. The SIP clients are in a remote private network, such as a SOHO network, and behind a NAT device that is not aware of SIP applications.

The following VIP, NAT, and HNT examples show configurations for each of the three common scenarios.

VIP

A FortiGate with SIP Application Layer Gateway (ALG) or SIP Session Helper protects the SIP server from the internet, while SIP phones from the internet need to register to the SIP server and establish calls through it.

A VIP needs to be configured for the SIP server, and the VIP must be applied in a firewall policy for the phones to send REGISTER messages through the FortiGate from port1 to port2.

Only one firewall policy needs to be configured for all SIP phones on both the internet and private network to register to the SIP server through Port1 and set up SIP calls.

Assuming either SIP ALG or SIP Session Helper is enabled, configure the FortiGate with the following CLI commands:

config firewall vip
    edit "VIP_for_SIP_Server"
        set extip 172.20.120.50
        set extintf "port1"
        set mappedip "10.11.101.50"
    next
end
config firewall policy
    edit 1
        set srcintf "port1"
        set dstintf "port2"
        set srcaddr "all"
        set dstaddr "VIP_for_SIP_Server"
        set action accept
        set schedule "always"
        set service "SIP"
    next
end

Setting service to SIP and not All in the firewall policy can improve protection by restricting the data traffic passing through the FortiGate to the SIP call traffic only.

NAT

A FortiGate with SIP ALG or SIP Session Helper protects the SIP phones and the internal network from the internet, while SIP phones in the internal network need to register to the SIP server installed on the internet and establish calls through it.

One firewall policy needs to be configured with NAT enabled for SIP phones to send REGISTER messages through the FortiGate from port2 to port1.

Assuming either SIP ALG or SIP Session Helper is enabled, configure the FortiGate with the following CLI commands:

config firewall policy
    edit 1
        set srcintf "port2"
        set dstintf "port1"
        set srcaddr "all"
        set dstaddr "all"
        set action accept
        set schedule "always"
        set service "SIP"
        set nat enable
    next
end

HNT

A FortiGate with SIP ALG or SIP Session Helper protects the SIP server from the internet, while SIP phones are in remote private networks behind NAT devices that are not aware of the SIP application.

For example, the SIP server is located in an ISP's service cloud that is protected by the FortiGate SIP ALG, and the SIP phones are installed in the home networks of the ISP's customers.

The SIP messages traversing the remote NAT devices might have their IP addresses translated by the NAT device at the network layer, but untranslated at the SIP application layer because those NAT devices are not aware of the SIP applications. This causes problems in a SIP session initiated process. Special configurations for the Hosted NAT Traversal (HNT) are required to resolve this issue.

To configure the FortiGate with HNT support for SIP phones A and B to set up calls with each other:

1. Identify port1 as the external interface:

   config system interface
       edit "port1"
           set external enable
       next
   end

2. Configure VIP for the SIP server:

   config firewall vip
       edit "VIP_for_SIP_Server"
           set extip 10.21.101.10
           set extintf "port1"
           set mappedip "10.30.120.20"
       next
   end

3. Configure a VoIP profile with HNT enabled:

   config voip profile
       edit "hnt"
           config sip
               set hosted-nat-traversal enable
               set hnt-restrict-source-ip enable
           end
       next
   end

   hosted-nat-traversal must be enabled. hnt-restrict-source-ip does not have to be enabled, but can be enabled to restrict the RTP packets’ source IP to be the same as the SIP packets’ source IP.

4. Apply the VoIP profile and VIP in a firewall policy for phone A and B to register and set up SIP calls through the FortiGate and SIP server:

   config firewall policy
       edit 1
           set srcintf "port1"
           set dstintf "port2"
           set srcaddr "all"
           set dstaddr "VIP_for_SIP_Server"
           set action accept
           set schedule "always"
           set service "SIP"
           set utm-status enable
           set voip-profile “hnt”
           set nat enable
       next
   end

   nat must be enabled in the firewall policy.