쫑콩아빠의 지금 이 순간...

## Dynamic source NAT without changing the source port (one-to-one source NAT)

# Problem


Some protocols or services will only function if they use a specific source port, or a source port that does not change. Normally source NAT changes the source port to allow multiple simultaneous sessions. 

# Solution

You can select the fixed port option to restrict the FortiGate unit to not translate the source port. This results in a one-to-one NAT configuration. One-to-one NAT limits the number of simultaneous sessions that are supported because one variable for tracking sessions (the source port number) is no longer available. To allow more sessions, one-to-one NAT is normally used with multiple external IPs added to an IP pool. 

In this example, you enable one-to-one NAT by enabling the fixed port option in a security policy and adding an IP pool containing three IP addresses: 172.20.120.[13-15]. The fixed port option is enabled from the CLI so this entire example is configured from the CLI.

1 Enter the following command to add the IP pool:

config firewall ippool

edit Dynamic-Source

set startip 172.20.120.13

set endip 172.20.120.15

end

2 Enter the following command to add a security policy that allows users on the private network to access the Internet.

config firewall policy

edit 0

set srcintf internal

set srcaddr all

set dstintf wan1

set dstaddr all

set schedule always

set service ANY

set action accept

set nat enable

set fixedport enable

set ippool enable

set poolname Dynamic-Source

end

If you edit this policy from the web‑based manager, you will notice that the Fixed Port option is visible and is selected.

Fortigate SIP ALG / Fortinet SIP ALG

FortiOS has two features that can modify the SIP headers and SDP parameters. The first feature is called the “SIP Session Helper”. If you are experiencing one way audio issues disable this feature first, reboot your IP phone then try making another call. If disabling the session helper does not work, disable the SIP ALG as well.

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 10
set name sip
set port 5060
set protocol 17

2 Enter the following command to delete session-helper list entry number 10 to disable the sip session helper:

config system session-helper
delete 10

To disable the SIP ALG:

There are typically two VOIP profiles on a factory shipped Fortinet firewall. You may need to disable both profiles to fully stop the ALG.

config voip profile
edit VoIP_Pro_2
config sip
set status disable
end
end

See the Fortigate Technical documentation page for further details.