Hosted NAT traversal

With the increase in the use of VoIP and other media traffic over the Internet, service provider network administrators must defend their networks from threats while allowing voice and multimedia traffic to flow transparently between users and servers and among users. A common scenario could involve providing SIP VoIP services for customers with SIP phones installed behind NAT devices that are not SIP aware. NAT devices that are not SIP aware cannot translate IP addresses in SIP headers and SDP lines in SIP packets but can and do perform source NAT on the source or addresses of the packets. In this scenario the user’s SIP phones would communicate with a SIP proxy server to set up calls between SIP phones. Once the calls are set up RTP packets would be communicated directly between the phones through each user’s NAT device.

The problem with this configuration is that the SIP headers and SDP lines in the SIP packets sent from the phones and received by the SIP proxy server would contain the private network addresses of the VoIP phones that would not be routable on the service provider network or on the Internet. One solution could be to for each customer to install and configure SIP aware NAT devices. If this is not possible, another solution requires implement hosted NAT traversal.

In a hosted NAT traversal (HNT) configuration, a FortiGate is installed between the NAT device and the SIP proxy server and configured with a VoIP profile that enables SIP hosted NAT traversal. Security policies that include the VoIP profile also support destination NAT using a firewall virtual IP. When the SIP phones connect to the SIP server IP address the security policy accepts the SIP packets, the virtual IP translates the destination addresses of the packets to the SIP server IP address, and the SIP ALG NAT traversal configuration translates the source IP addresses on the SIP headers and SDP lines to the source address of the SIP packets (which would be the external IP address of the NAT devices). The SIP server then sees the SIP phone IP address as the external IP address of the NAT device. As a result SIP and RTP media sessions are established using the external IP addresses of the NAT devices instead of the actual IP addresses of the SIP phones.

Configuration example: Hosted NAT traversal for calls between SIP Phone A and SIP Phone B

FortiGate SIP Hosted NAT Traversal configuration

Configuration example: Hosted NAT traversal for calls between SIP Phone A and SIP Phone B

The following address translation takes place to allow a SIP call from SIP Phone A to SIP Phone B in the above diagram.

SIP Phone A sends a SIP Invite message to the SIP server. Packet source IP address: 192.168.10.1, destination IP address: 10.21.101.10.
The SIP packets are received by the NAT device which translates the source address of the SIP packets from 192.168.10.1 to 10.11.101.20.
The SIP packets are received by the FortiGate which translates the packet destination IP address to 10.30 120.20. The SIP ALG also translates the IP address of the SIP phone in the SIP header and SDP lines from 192.168.10.1 to 10.11.101.20.
The SIP server accepts the Invite message and forwards it to SIP Phone B at IP address10.11.101.20. The SIP server has this address for SIP Phone B because SIP packets from SIP Phone B have also been translated using the hosted NAT traversal configuration of the SIP ALG.
When the SIP call is established, the RTP session is between 10.11.101.10 and 10.11.101.20 and does not pass through the FortiGate. The NAT devices translated the destination address of the RTP packets to the private IP addresses of the SIP phones.

General configuration steps

The following general configuration steps are required for this destination NAT SIP configuration. This example uses the default VoIP profile.

Add a VoIP profile that enables hosted NAT translation.
Add a SIP proxy server firewall virtual IP.
Add a firewall address for the SIP proxy server on the private network.
Add a destination NAT security policy that accepts SIP sessions from the Internet destined for the SIP proxy server virtual IP and translates the destination address to the IP address of the SIP proxy server on the private network.
Add a security policy that accepts SIP sessions initiated by the SIP proxy server and destined for the Internet.

Configuration steps – GUI

To add the SIP proxy server firewall virtual IP

Go to Policy & Objects > Virtual IPs.
Add the SIP proxy server virtual IP.

Name	SIP_Proxy_VIP
External Interface	port1
Type	Static NAT
External IP Address/Range	172.20.120.50
Mapped IP Address/Range	10.31.101.50

To add a firewall address for the SIP proxy server

Go to Policy & Objects > Addresses.
Add the following for the SIP proxy server:

Category	Address
Name	SIP_Proxy_Server
Type	Subnet
Subnet / IP Range	10.31.101.50/255.255.255.255
Interface	port2

Configuration example: Hosted NAT traversal for calls between SIP Phone A and SIP Phone B

To add the security policies

Go to Policy & Objects > IPv4 Policy.
Add a destination NAT security policy that includes the SIP proxy server virtual IP that allows Phone B (and other SIP phones on the Internet) to send SIP request messages to the SIP proxy server.

Incoming Interface	port1
Outgoing Interface	port2
Source	all
Destination Address	SIP_Proxy_VIP
Schedule	always
Service	SIP
Action	ACCEPT

Turn on NAT and select Use Outgoing Interface Address.
Turn on VoIP and select the default VoIP profile.
Select OK.
Add a source NAT security policy to allow the SIP proxy server to send SIP request messages to Phone B and the

Internet:

Incoming Interface	port2
Outgoing Interface	port1
Source	SIP_Proxy_Server
Destination Address	all
Schedule	always
Service	SIP
Action	ACCEPT

Turn on NAT and select Use Outgoing Interface Address.
Turn on VoIP and select the default VoIP profile.
Select OK.

Configuration steps – CLI

To add a VoIP profile that enables hosted NAT translation

Enter the following command to add a VoIP profile named HNT that enables hosted NAT traversal. This command shows how to clone the default VoIP profile and enable hosted NAT traversal.

config voip profile

Hosted NAT traversal Configuration example: Hosted NAT traversal for calls between SIP Phone A and SIP Phone B

clone default to HNT edit HNT config sip set hosted-nat-traversal enable

end

To add the SIP proxy server firewall virtual IP and firewall address

Enter the following command to add the SIP proxy server firewall virtual IP. config firewall vip edit SIP_Proxy_VIP set type static-nat set extip 10.21.101.10 set mappedip 10.30.120.20 set extintf port1

end

Enter the following command to add the SIP proxy server firewall address. config firewall address edit SIP_Proxy_Server set associated interface port2 set type ipmask

set subnet 10.30.120.20 255.255.255.255

end

To add security policies

Enter the following command to add a destination NAT security policy that includes the SIP proxy server virtual IP that allows Phone A to send SIP request messages to the SIP proxy server.

config firewall policy edit 0 set srcintf port1 set dstintf port2 set srcaddr all set dstaddr SIP_Proxy_VIP set action accept set schedule always set service SIP set nat enable set utm-status enable

set profile-protocol-options default set voip-profile HNT

end

Enter the following command to add a source NAT security policy to allow the SIP proxy server to send SIP request messages to Phone B:

config firewall policy edit 0 set srcintf port2 set dstintf port1 set srcaddr SIP_Proxy_Server

set dstaddr all set action accept set schedule always set service SIP

Hosted NAT traversal for calls between SIP Phone A and SIP Phone C

set nat enable set utm-status enable

set profile-protocol-options default set voip-profile default end