GetFQDN.cs

using System;
using System.Net;
using System.Net.Sockets;
using System.Net.NetworkInformation;

namespace GetFQDN
{
    internal class Program
    {
        static void Main(string[] args)
        {
            Console.WriteLine("Hello, World! Join me on an adventure to try and find the FQDN of your current machine.");

            Console.WriteLine("Dns.GetHostName: {0}", Dns.GetHostName());
            Console.WriteLine("Dns.GetHostEntry: {0}", GetFQDNViaDnsGetHostEntry());
            Console.WriteLine("IPGlobalProperties: {0}", GetFQDNViaIPGlobalProperties());

            using (var socket = new Socket(/*AddressFamily.InterNetwork,*/ SocketType.Stream, ProtocolType.Tcp)) {
                socket.Connect("www.google.com", 443);

                Console.WriteLine("Socket.LocalEndPoint: {0}", GetFQDNViaSocketLocalEndPoint(socket));
                Console.WriteLine("NetworkInterface: {0}", GetFQDNViaNetworkInterface(socket));

                socket.Disconnect(false);
            }
        }

        static string GetFQDNViaIPGlobalProperties()
        {
            var properties = IPGlobalProperties.GetIPGlobalProperties();

            if (!string.IsNullOrEmpty(properties.DomainName))
                return properties.HostName + "." + properties.DomainName;

            return properties.HostName;
        }

        static string GetFQDNViaDnsGetHostEntry()
        {
            var entry = Dns.GetHostEntry(Dns.GetHostName());
            var aliases = string.Join(", ", entry.Aliases);

            return $"HostName: {entry.HostName}; Aliases={aliases}";
        }

        static string GetFQDNViaSocketLocalEndPoint(Socket socket)
        {
            if (socket.LocalEndPoint is IPEndPoint ipEndPoint) {
                var ipAddress = ipEndPoint.Address;
                if (ipAddress.IsIPv4MappedToIPv6)
                    ipAddress = ipAddress.MapToIPv4();

                var entry = Dns.GetHostEntry(ipAddress);
                var aliases = string.Join(", ", entry.Aliases);

                return $"IP: {ipAddress}; HostName: {entry.HostName}; Aliases={aliases}";
            } else if (socket.LocalEndPoint is DnsEndPoint dnsEndPoint) {
                return dnsEndPoint.Host;
            } else {
                return "Nope.";
            }
        }

        static bool IPAddressesEqual(IPAddress ipAddress1, IPAddress ipAddress2)
        {
            if (ipAddress1.AddressFamily != ipAddress2.AddressFamily)
                return false;

            var addr1 = ipAddress1.GetAddressBytes();
            var addr2 = ipAddress2.GetAddressBytes();

            for (int i = 0; i < addr1.Length; i++) {
                if (addr1[i] != addr2[i])
                    return false;
            }

            return true;
        }

        static string GetFQDNViaNetworkInterface(Socket socket)
        {
            var networkInterfaces = NetworkInterface.GetAllNetworkInterfaces();

            if (socket.LocalEndPoint is not IPEndPoint ipEndPoint)
                return "Nope.";

            var ipAddress = ipEndPoint.Address;
            if (ipAddress.IsIPv4MappedToIPv6)
                ipAddress = ipAddress.MapToIPv4();

            foreach (var networkInterface in networkInterfaces) {
                var properties = networkInterface.GetIPProperties();
                var unicast = properties.UnicastAddresses;

                var found = unicast.FirstOrDefault(x => IPAddressesEqual(x.Address, ipAddress));
                if (found == null)
                    continue;

                return Dns.GetHostName() + "." + properties.DnsSuffix;
            }

            return "Nope.";
        }
    }
}