Disabling SIP ALG on a Fortigate Firewall

Updated at April 27th, 2023

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Table of Contents

Disabling SIP ALG

Scope:

The following article will show you how to disable the SIP ALG setting on a Fortigate Firewall.

Requirements:

CLI access to the Fortigate Firewall

Disabling SIP ALG

Open the CLI interface for your Fortigate Firewall
1. Before making any changes be sure to backup your configuration
In the CLI enter the following commands
1. Use the following commands for a device on FortiOS starting at 6.2.2
2. config system settings
3. set sip-expectation disable
4. set sip-nat-trace disable
5. set default-voip-alg-mode kernel-helper-based
6. end
For devices below FortiOS version 6.2.2 use the following commands
1. config system settings
2. set sip-helper disable
3. set sip-nat-trace disable
4. set default-voip-alg-mode kernel-helper-based

end

If you encounter and error while entering set default-voip-alg-mode kernel-helper-based go ahead and ignore it
The rest of the configuration will be the same for all FortiOS versions
Run the following commands
1. config system session-helper
2. show
  1. Here you will want to find the entry for SIP, this is typically 12 but it may differ depending on software version and model
3. delete 12
  1. Alternatively use the entry you found in the previous step
4. end
Enter the following commands in the CLI to disable RTP processing
1. config voip profile
2. edit default
3. config sip
4. set rtp disable
5. end
6. end
Once done go ahead and reboot the device, Fortigate firewalls do not require a reboot when you change configuration but in this case, we will need the reboot to activate the session helper changes
Lastly, reboot all of your SIP Devices/Phones

fortigate sip alg