Read IP address from exists tun+ interface

[avengermsoft]

Some routers have an additional interface for tunnel configuration TUN.
Add the ability to read IP addresses from an existing TUN adapter.

sing-box config

  "inbounds": [
    {
      "type": "tun",
      "interface_name": "opkgtun0",
      "domain_strategy": "ipv4_only",
      "auto_route": false,
      "strict_route": false,
      "sniff": true
   }
  ]

ifconfig opkgtun0

opkgtun0  Link encap:UNSPEC  HWaddr 00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00
          inet addr:172.16.0.2  P-t-P:172.16.0.2  Mask:255.255.255.255
          inet6 addr: fadd:4700:110:::2/128 Scope:Global
          POINTOPOINT NOARP MULTICAST  MTU:65535  Metric:1
          RX packets:219 errors:0 dropped:0 overruns:0 frame:0
          TX packets:254 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:150

Now

 ~  # sing-box run --config /tmp/1.json --directory /tmp
INFO[0000] network: updated default interface eth3, index 7
TRACE[0000] inbound/tun[0]: creating stack
FATAL[0000] start service: start inbound/tun[0]: missing interface address