docker-network.log

root@bradd:~# docker network inspect pterodactyl_nw      
[
    {
        "Name": "pterodactyl_nw",
        "Id": "33d78172783fcbca4b35042fa7305b9e0a8e5c558616f92423e22e3567958339",
        "Created": "2019-07-23T18:02:06.288369825+02:00",
        "Scope": "local",
        "Driver": "bridge",
        "EnableIPv6": false,
        "IPAM": {
            "Driver": "default",
            "Options": null,
            "Config": [
                {
                    "Subnet": "172.18.0.0/16",
                    "Gateway": "172.18.0.1"
                },
                {
                    "Subnet": "fdba:17c8:6c94::/64",
                    "Gateway": "fdba:17c8:6c94::1011"
                }
            ]
        },
        "Internal": false,
        "Attachable": false,
        "Ingress": false,
        "ConfigFrom": {
            "Network": ""
        },
        "ConfigOnly": false,
        "Containers": {},
        "Options": {
            "com.docker.network.bridge.default_bridge": "false",
            "com.docker.network.bridge.enable_icc": "true",
            "com.docker.network.bridge.enable_ip_masquerade": "true",
            "com.docker.network.bridge.host_binding_ipv4": "0.0.0.0",
            "com.docker.network.bridge.name": "pterodactyl0",
            "com.docker.network.driver.mtu": "1500",
            "encryption": "false"
        },
        "Labels": {}
    }
]

nslookup-1 (ipv6 answer)

root@bradd:~# docker run --rm --network pterodactyl_nw busybox nslookup archive.ubuntu.com
Server:         127.0.0.11
Address:        127.0.0.11:53

Non-authoritative answer:
Name:   archive.ubuntu.com
Address: 2001:67c:1560:8001::11
Name:   archive.ubuntu.com
Address: 2001:67c:1360:8001::17
Name:   archive.ubuntu.com
Address: 2001:67c:1360:8001::21
Name:   archive.ubuntu.com
Address: 2001:67c:1560:8001::14

*** Can't find archive.ubuntu.com: No answer

nslookup-2 (ipv4 answer)

root@bradd:~# docker run --rm --network pterodactyl_nw busybox nslookup archive.ubuntu.com
Server:         127.0.0.11
Address:        127.0.0.11:53

Non-authoritative answer:
Name:   archive.ubuntu.com
Address: 91.189.88.24
Name:   archive.ubuntu.com
Address: 91.189.88.31
Name:   archive.ubuntu.com
Address: 91.189.88.149
Name:   archive.ubuntu.com
Address: 91.189.88.162

*** Can't find archive.ubuntu.com: No answer

nslookup-3 (ipv4 answer) (explicit type)

root@bradd:~# docker run --rm --network pterodactyl_nw busybox nslookup -type=A archive.ubuntu.com
Server:         127.0.0.11
Address:        127.0.0.11:53

Non-authoritative answer:
Name:   archive.ubuntu.com
Address: 91.189.88.24
Name:   archive.ubuntu.com
Address: 91.189.88.149
Name:   archive.ubuntu.com
Address: 91.189.88.162
Name:   archive.ubuntu.com
Address: 91.189.88.31

root@bradd:~#

nslookup-3 (ipv6 answer) (explicit type)

root@bradd:~# docker run --rm --network pterodactyl_nw busybox nslookup -type=AAAA archive.ubuntu.com
Server:         127.0.0.11
Address:        127.0.0.11:53

Non-authoritative answer:
Name:   archive.ubuntu.com
Address: 2001:67c:1560:8001::11
Name:   archive.ubuntu.com
Address: 2001:67c:1360:8001::17
Name:   archive.ubuntu.com
Address: 2001:67c:1360:8001::21
Name:   archive.ubuntu.com
Address: 2001:67c:1560:8001::14

root@bradd:~#

Questions.md

1. Why is the IPv6 subnet registered although it is disabled?
2. Why does the nslookup return IPv6 addresses when that shouldn't be available inside the container 2.1 Why does the lookup return two different sets of addresses before failing?
3. Why does the lookup succeed when the request type is explicitly specified?
4. Since applications mostly use the equivalent of the non-explicit lookup, how can we debug or even fix the issue?