The UniFi Controller has no UI configuration to assign an additional IP for the UniFi Security Gateway (USG). Hopefully this will be added from Ubiquiti in the future.

Date: 25.12.2017
Controller Version: 5.6.26
Firmware: 4.4.12.5032482

After many hours of reading, try and error I was able to solve this task. This guide is focused on people with network basics, therefore I will not cover technically explanations.

UPDATE:  Important, set only the values for the second IP inside config.gateway.json the settings like Port Forwarding for the first IP have to configured on the webinterface.

Attention: After following this guide you will see the first IP inside the USG Overview/Details pane and the second IP inside the Config pane in the webinterface. This looks like a bug UniFi Controller which can be ignored.

Goal:

NAT Public IP 10.0.0.2 on Port 80/TCP to internal server 192.168.0.2 on Port 8080/TCP.

Solution:

Overview:

1. Test if the Port is closed
2. Add the additional IP to the gateway
3. Create a DNAT rule from WAN to LAN
4. Create a SNAT rule from LAN to WAN
5. Create a Firewall rule to allow traffic from WAN to LAN
6. Apply changes
7. Test if the Port is now open

NOTE: You can also add this with CLI commands but it will not persist after changes made from the Web-Interface.

Step 1

Test with NMAP from WAN/Internet

nmap -n -Pn -p 80 10.0.0.2

If this Port is open you should check your network setup because something is responding to 80/TCP and the next steps will potentially lead to undesired results.

Step 2-4

We need to create or append config.gateway.json inside the UniFi Controller. Place this file inside the site configuration, e.g. for the default page but the file inside “data/sites/default”

Content of config.gateway.json:

{
    "interfaces": {
        "ethernet": {
            "eth0": {
                "address": [
                    "10.0.0.1/29",
                    "10.0.0.2/29"
                ],
                "firewall": {
                    "in": {
                        "name": "WAN_IN"
                    },   
                    "local": {
                        "name": "WAN_LOCAL"
                    },   
                    "out": {
                        "name": "WAN_OUT"
                    }
                }
            }
        }
    },
    "service": {
        "nat": {
            "rule": {
                "3000": {
                    "description": "DNAT 10.0.0.2 TCP/8080 to 192.168.0.2",
                    "destination": {
                        "address": "10.0.0.2",
                        "port": "80"
                    },   
                    "inbound-interface": "eth0",
                    "inside-address": {
                        "address": "192.168.0.2",
                        "port": "8080"
                    },   
                    "log": "enable",
                    "protocol": "tcp",
                    "type": "destination"
                },
                "5000": {
                    "description": "SNAT 192.168.0.2 TCP/8080 to 10.0.0.2",
                    "log": "enable",
                    "outbound-interface": "eth0",
                    "outside-address": {
                        "address": "10.0.0.2",
                        "port": "80"
                    },   
                    "protocol": "tcp",
                    "source": {
                        "address": "192.168.0.2",
                        "port": "8080"
                    },   
                    "type": "source"
                }
            }
        }
    },
    "firewall": {
        "name": {
            "WAN_IN": {
                "default-action": "drop",
                "rule": {
                    "1000": {
                        "action": "accept",
                        "description": "NAT 10.0.0.2 TCP/8080 to 192.168.0.2",
                        "destination": {
                            "address": "192.168.0.2",
                            "port": "8080"
                        },
                        "log": "enable",
                        "protocol": "tcp"
                    },   
                }
            }
        }
    }
}

Step 5

Now it’s time to apply these rules to the USG. To do this log in to your UniFi Controller and force provisioning

Step 6

Test with NMAP from WAN/Internet

nmap -n -Pn -p 80 10.0.0.2