The SIP session helper

The SIP session helper

The SIP session-helper is a high-performance solution that provides basic support for SIP calls passing through the FortiGate unit by opening SIP and RTP pinholes and by performing NAT of the addresses in SIP messages.

 

The SIP session helper:

  • Understands SIP dialog messages.
  • Keeps the states of the SIP transactions between SIP UAs and SIP servers.
  • Translates SIP header and SDP information to account for NAT operations performed by the FortiGate unit.
  • Opens up and closes dynamic SIP pinholes for SIP signalling traffic.
  • Opens up and closes dynamic RTP and RTSP pinholes for RTP and RTSP media traffic.
  • Provides basic SIP security as an access control device.
  • Uses the intrusion protection (IPS) engine to perform basic SIP protocol checks.

 

SIP session helper configuration overview

By default FortiOS uses the SIP ALG for SIP traffic. If you want to use the SIP session helper you need to enter the following command:

config system settings

set default-voip-alg-mode kernel-helper-based

end

The SIP session helper is set to listen for SIP traffic on TCP or UDP port 5060. SIP sessions using port 5060 accepted by a security policy that does not include a VoIP profile are processed by the SIP session helper.

You can enable and disable the SIP session helper, change the TCP or UDP port that the session helper listens on for SIP traffic, and enable or disable SIP NAT tracing. If the FortiGate unit is operating with multiple VDOMs, each VDOM can have a different SIP session helper configuration.

To have the SIP session helper process SIP sessions you need to add a security policy that accepts SIP sessions on the configured SIP UDP or TCP ports. The security policies can have service set to ANY, or to the SIP pre- defined firewall service, or a custom firewall service. The SIP pre-defined firewall service restricts the security policy to only accepting sessions on UDP port 5060.

If NAT is enabled for security policies that accept SIP traffic, the SIP session helper translates addresses in SIP headers and in the RDP profile and opens up pinholes as required for the SIP traffic. This includes security policies that perform source NAT and security policies that contain virtual IPs that perform destination NAT and port forwarding. No special SIP configuration is required for this address translation to occur, it is all handled automatically by the SIP session helper according to the NAT configuration of the security policy that accepts the SIP session.

To use the SIP session helper you must not add a VoIP profile to the security policy. If you add a VoIP profile, SIP traffic bypasses the SIP session helper and is processed by the SIP ALG.

In most cases you would want to use the SIP ALG since the SIP session helper provides limited functionality. However, the SIP session helper is available and can be useful for high-performance solutions where a high level of SIP security is not a require- ment.

 

Disabling and enabling the SIP session helper

You can use the following steps to disable the SIP session helper. You might want to disable the SIP session helper if you don’t want the FortiGate unit to apply NAT or other SIP session help features to SIP traffic. With the SIP session helper disabled, the FortiGate unit can still accept SIP sessions if they are allowed by a security policy, but the FortiGate unit will not be able to open pinholes or NAT the addresses in the SIP messages.

 

To disable the sip session helper

1. Enter the following command to find the sip session helper entry in the session-helper list:

show system session-helper

edit 13

set name sip set port 5060 set protocol 17

next

This command output shows that the sip session helper listens in UDP port 5060 for SIP sessions.

2. Enter the following command to delete session-helper list entry number 13 to disable the sip session helper:

config system session-helper delete 13

end

If you want to use the SIP session helper you can verify whether it is enabled or disabled using the show system session-helper command.

You do not have to disable the SIP session helper to use the SIP ALG.

If the SIP session helper has been disabled by being removed from the session-helper list you can use the following command to enable the SIP session helper by adding it back to the session helper list:

config system session-helper edit 0

set name sip set port 5060 set protocol 17

end

 

Changing the port numbers that the SIP session helper listens on

You can use the following command to change the port number that the SIP session helper listens on for SIP traffic to 5064. The SIP session helper listens on the same port number for UDP and TCP SIP sessions. In this example, the SIP session helper is session helper 13:

config system session-helper edit 13

set port 5064 end

 

The config system settings options sip-tcp-port, sip-udp-port, and sip-ssl-port control the ports that the SIP ALG listens on for SIP sessions. See Changing the port numbers that the SIP ALG listens on on page 2764.

 

Your FortiGate unit may use a different session helper number for SIP. Enter the following command to view the session helpers:

show system session-helper

edit 13

set name sip set port 5060 set protocol 17

end


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