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IPv6 stateless address autoconfiguration of the WAN interface on a Unifi Security Gateway

The WAN interface on a Unifi Security Gateway (USG) supports static IPv6 address configuration and IPv6 prefix delegation. However it does not officially support the scenario of IPv6 stateless address autoconfiguration. This mode is deployed by some ISPs, notably NTT in Japan.

This note describes the configuration of a USG to operate in this environment. It assumes familarity with USG administration, including config.gateway.json files, and basic Linux knowledge.

Configure IPv6 stateless address autoconfiguration on eth0

This requires the creation or modification of a config.gateway.json file on the Unifi controller. On a Unifi Cloud Key controller, this file is located at:


The config.gateway.json should include the following nodes:

    "interfaces": {
        "ethernet": {
            "eth0": {
                "ipv6": {
                    "address": "autoconf"

Configure private local IPv6 addresses for internal use

  1. Select a random RFC 4193 IPv6 prefix of the form fdxx:xxxx:xxxx:/48. There are online tools to make this easy.
  2. Use the configuration GUI to assign addresses and subnets to each internal interface. For example, fdxx:xxxx:xxxx:1::1/64

Configure NATv6

SSH to the USG, become root and create the file:



# enable ipv6 NAT
/sbin/ip6tables -t nat -A POSTROUTING -o eth0 -j MASQUERADE

Make sure it is executable:

chmod 755 /config/scripts/post-config.d/