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Disable SIP ALG on Fortigate Firewalls

On Fortigate firewalls SIP Application Layer Gateway (SIP ALG) is enabled by default. This will cause problems with SIP VoIP phones registration and call processing.

We observed following problems when SIP ALG is active on Fortigate firewalls:

  1. SIP phones are unable to register on a remote phone system
  2. Calls are dropped after 5-15 min
  3. Incoming phone calls are not reaching the SIP phone(s)



IMPORTANT - Backup configuration of your firewall before making any changes


Run following commands from Fortigate firewall CLI

config system settings

set sip-helper disable

set sip-nat-trace disable

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


If you see an error while entering “set default-voip-alg-mode kernel-helper-based” , just ignore it.

Next we need to locate SIP entry in session helper list and delete it

config system session-helper


Scroll down until you see an entry for SIP, in our example it was number 13 but this may be different depending on model and software release. Now execute  following commands:

delete 13


The last set of commands disables processing of RTP protocol on the firewall

config voip profile

edit default

config sip

set rtp disable



Normally Fortigate firewalls do not require a reboot when you change configuration, but , it seems, in this case we need reboot it to activate session helper changes.

Last step – restart or power cycle all your SIP phones and devices.