External NAT IP mapping support

[tkunicki]

Support external IP mappings for host interface IPs

The external IP must not be behind a CGNAT and port-forwarding for incoming UDP packets from WgPort on from the external IP to WgPort on the host interface IP must be present. This can take form of:

• single port-forwarding rule from external IP to host interface IP
• 1:1 DNAT mapping ExternalIP to host interface IP
• NAT DMZ to host interface IP.

Configuration

A special value of stun will use the STUN server's XORMappedAddress to set the External IP value; this is for installations behind dynamic DHCP assuming the above restrictions are met.

A single mapping will take the form of external[/internal]

• external (required):
  • external IP address or stun to use STUN to determine the external IP address
• internal (optional)
  • the host interface IP address or an interface name
  • if the "internal" argument is present and "stun" is specified for external, the STUN request will be generated by a connection originating from the specified host interface IP

Examples:

 "stun"                 => all interface IPs will be mapped to external IP reported by stun
 "12.34.56.78"          => all interfaces IPs will be mapped to external IP of 12.34.56.78
 "stun/eth0"            => IPv4 assigned to interface eth0 will be mapped to external IP reported by stun
 "stun/10.1.2.3"        => interface IP 10.1.2.3 will be mapped to external IP reported by stun
 "12.34.56.78/eth0"     => IPv4 assigned to interface eth0 will be mapped to external IP of 12.34.56.78
 "12.34.56.78/10.1.2.3" => interface IP 10.1.2.3 will be mapped to external IP of 12.34.56.78

[CLAassistant]

All committers have signed the CLA.

[mlsmaycon]

Thanks @tkunicki for the contribution. I left some notes.

I will ping you on slack regarding tests and the Stun comments.

[mlsmaycon]

The Release checks failed because of updated go.sum file. This usually means that go.mod was updated and your local go.sum wasn't pushed

[mlsmaycon]

Thanks @tkunicki for this contribution. @braginini I will be adding unit tests here before release