Cannot find IP from Hostname

Question

I have a wifi sensor module (SN8200 EVK) running a webserver. The board is configured to work as a STA and automatically join a WiFi router. The sensor board uses the Hostname: "sensor" and a DHCP interface to get its IP address.

I am developing a visual studio C# application to connect to this webserver. The application runs on a laptop which is connected to the same WiFi router.

Sensor Board <-----> WiFi Router <-----> Laptop

The application can connect to the sensor board if I use the board IP address. As the IP address is not static, I would like to use the Hostname to retrieve the IP.

I tried to use GetHostEntry but got an unknown host result.

IPHostEntry host;
host = Dns.GetHostEntry("sensor");

Something strange is that if I use GetHostEntry with the IP address it cannot resolved the hostname neither.

The GetHostEntry works fine if I try with the laptop hostname or IP. It looks like GetHostEntry cannot resolve the hostname behind the router.

Any help is welcome, Thanks.

Answer 1

Static IP Solution

First :

Configure in your Wifi Router a fixed IP Address inside the DHCP Server.
PS : I'm pretty sure the Mac address is not dynamic

Second :

Add the hostname in your DNS Server with the previously fixed IP Address.

Scan the network for IPs and check Mac Address Solution

Before using the following code you will have to ping the whole range of IPs the sensor board can use in order to refresh your ARP Cache.

using System;
using System.Runtime.InteropServices;
using System.ComponentModel;
using System.Net;
using System.Collections.Generic;

namespace GetIpNetTable
{
    class Program
    {
        static void Main(string[] args)
        {
            string[] ips = searchIpsFromMac("ff-ff-ff-ff-ff-ff");
            if (ips.Length != 0)
            {
                foreach (string ip in ips)
                {
                    // TODO - Do what you want with your IP address
                    Console.WriteLine(ip);
                }
            }
            Console.ReadKey();
        }

        // The max number of physical addresses.
        const int MAXLEN_PHYSADDR = 8;

        // Define the MIB_IPNETROW structure.
        [StructLayout(LayoutKind.Sequential)]
        struct MIB_IPNETROW
        {
            [MarshalAs(UnmanagedType.U4)]
            public int dwIndex;
            [MarshalAs(UnmanagedType.U4)]
            public int dwPhysAddrLen;
            [MarshalAs(UnmanagedType.U1)]
            public byte mac0;
            [MarshalAs(UnmanagedType.U1)]
            public byte mac1;
            [MarshalAs(UnmanagedType.U1)]
            public byte mac2;
            [MarshalAs(UnmanagedType.U1)]
            public byte mac3;
            [MarshalAs(UnmanagedType.U1)]
            public byte mac4;
            [MarshalAs(UnmanagedType.U1)]
            public byte mac5;
            [MarshalAs(UnmanagedType.U1)]
            public byte mac6;
            [MarshalAs(UnmanagedType.U1)]
            public byte mac7;
            [MarshalAs(UnmanagedType.U4)]
            public int dwAddr;
            [MarshalAs(UnmanagedType.U4)]
            public int dwType;
        }

        // Declare the GetIpNetTable function.
        [DllImport("IpHlpApi.dll")]
        [return: MarshalAs(UnmanagedType.U4)]
        static extern int GetIpNetTable(
           IntPtr pIpNetTable,
           [MarshalAs(UnmanagedType.U4)]
         ref int pdwSize,
           bool bOrder);

        [DllImport("IpHlpApi.dll", SetLastError = true, CharSet = CharSet.Auto)]
        internal static extern int FreeMibTable(IntPtr plpNetTable);

        // The insufficient buffer error.
        const int ERROR_INSUFFICIENT_BUFFER = 122;

        static string[] searchIpsFromMac(string macSearched)
        {
            List<string> ips = new List<string>(1);

            // The number of bytes needed.
            int bytesNeeded = 0;

            // The result from the API call.
            int result = GetIpNetTable(IntPtr.Zero, ref bytesNeeded, false);

            // Call the function, expecting an insufficient buffer.
            if (result != ERROR_INSUFFICIENT_BUFFER)
            {
                // Throw an exception.
                throw new Win32Exception(result);
            }

            // Allocate the memory, do it in a try/finally block, to ensure
            // that it is released.
            IntPtr buffer = IntPtr.Zero;

            // Try/finally.
            try
            {
                // Allocate the memory.
                buffer = Marshal.AllocCoTaskMem(bytesNeeded);

                // Make the call again. If it did not succeed, then
                // raise an error.
                result = GetIpNetTable(buffer, ref bytesNeeded, false);

                // If the result is not 0 (no error), then throw an exception.
                if (result != 0)
                {
                    // Throw an exception.
                    throw new Win32Exception(result);
                }

                // Now we have the buffer, we have to marshal it. We can read
                // the first 4 bytes to get the length of the buffer.
                int entries = Marshal.ReadInt32(buffer);

                // Increment the memory pointer by the size of the int.
                IntPtr currentBuffer = new IntPtr(buffer.ToInt64() +
                   Marshal.SizeOf(typeof(int)));

                // Allocate an array of entries.
                MIB_IPNETROW[] table = new MIB_IPNETROW[entries];

                // Cycle through the entries.
                for (int index = 0; index < entries; index++)
                {
                    // Call PtrToStructure, getting the structure information.
                    table[index] = (MIB_IPNETROW)Marshal.PtrToStructure(new
                       IntPtr(currentBuffer.ToInt64() + (index *
                       Marshal.SizeOf(typeof(MIB_IPNETROW)))), typeof(MIB_IPNETROW));
                }

                for (int index = 0; index < entries; index++)
                {
                    MIB_IPNETROW row = table[index];
                    IPAddress ip = new IPAddress(BitConverter.GetBytes(row.dwAddr));
                    string macAddr = row.mac0.ToString("X2") + '-' +
                        row.mac1.ToString("X2") + '-' +
                        row.mac2.ToString("X2") + '-' +
                        row.mac3.ToString("X2") + '-' +
                        row.mac4.ToString("X2") + '-' +
                        row.mac5.ToString("X2");
                    if (macAddr.ToUpper() == macSearched.ToUpper())
                    {
                        ips.Add(ip.ToString());
                    }
                }
            }
            finally
            {
                // Release the memory.
                FreeMibTable(buffer);
            }
            return ips.ToArray();
        }
    }
}

References : https://stackoverflow.com/a/1148861/3635715