Chris Pietschmann

While working on the Virtual Router project, I’ve come across a need to be able to retrieve the IP Address and Host Name of a given machine on the local network when only the machines MAC Address is known. This took a bit of research to figure out, and eventually I stumbled across the “arp.exe” utility within Windows.

“arp.exe” uses the Address Resolution Protocol to provide functionality to add, delete or display the IP address for MAC (Media Access Control) address translation.

To see the IP / MAC Address translations, just open up the Command Prompt in Windows, and type “arp –a” and press enter.

Now, you may be asking, how do we use this utility from within a .NET application? Well, it’s rather simple. All you need to do is use the System.Diagnostics.Process class to execute the “arp –a” call and retrieve the results. Then just parse through the results and get out the IP Address for the desired MAC Address.

I’m not going to give full detail on how to call “arp” from .NET. Below is an example on using the “IPInfo” class I created to perform the previously mentioned method of retrieving the IP Address for a specified MAC Address. I also added in the ability to get the machines Host Name using the the “Dns.GetHostEntry” method.

var ipinfo = IPInfo.GetIPInfo(txtMAC.Text);

if (ipinfo == null)
{
    // Machine Not Found!
}
else
{
    var ip = ipinfo.IPAddress;
    var hostname = ipinfo.HostName;
}

Below is the full code of my “IPInfo” class:

using System;
using System.Collections.Generic;
using System.Diagnostics;
using System.Linq;
using System.Net;

/// <summary>
/// This class allows you to retrieve the IP Address and Host Name for a specific machine on the local network when you only know it's MAC Address.
/// </summary>
public class IPInfo
{
    public IPInfo(string macAddress, string ipAddress)
    {
        this.MacAddress = macAddress;
        this.IPAddress = ipAddress;
    }

    public string MacAddress { get; private set; }
    public string IPAddress { get; private set; }

    private string _HostName = string.Empty;
    public string HostName
    {
        get
        {
            if (string.IsNullOrEmpty(this._HostName))
            {
                try
                {
                    // Retrieve the "Host Name" for this IP Address. This is the "Name" of the machine.
                    this._HostName = Dns.GetHostEntry(this.IPAddress).HostName;
                }
                catch
                {
                    this._HostName = string.Empty;
                }
            }
            return this._HostName;
        }
    }


    #region "Static Methods"

    /// <summary>
    /// Retrieves the IPInfo for the machine on the local network with the specified MAC Address.
    /// </summary>
    /// <param name="macAddress">The MAC Address of the IPInfo to retrieve.</param>
    /// <returns></returns>
    public static IPInfo GetIPInfo(string macAddress)
    {
        var ipinfo = (from ip in IPInfo.GetIPInfo()
                  where ip.MacAddress.ToLowerInvariant() == macAddress.ToLowerInvariant()
                  select ip).FirstOrDefault();

        return ipinfo;
    }

    /// <summary>
    /// Retrieves the IPInfo for All machines on the local network.
    /// </summary>
    /// <returns></returns>
    public static List<IPInfo> GetIPInfo()
    {
        try
        {
            var list = new List<IPInfo>();

            foreach (var arp in GetARPResult().Split(new char[] { '\n', '\r' }))
            {
                // Parse out all the MAC / IP Address combinations
                if (!string.IsNullOrEmpty(arp))
                {
                    var pieces = (from piece in arp.Split(new char[] { ' ', '\t' })
                                  where !string.IsNullOrEmpty(piece)
                                  select piece).ToArray();
                    if (pieces.Length == 3)
                    {
                        list.Add(new IPInfo(pieces[1], pieces[0]));
                    }
                }
            }

            // Return list of IPInfo objects containing MAC / IP Address combinations
            return list;
        }
        catch(Exception ex)
        {
            throw new Exception("IPInfo: Error Parsing 'arp -a' results", ex);
        }
    }

    /// <summary>
    /// This runs the "arp" utility in Windows to retrieve all the MAC / IP Address entries.
    /// </summary>
    /// <returns></returns>
    private static string GetARPResult()
    {
        Process p = null;
        string output = string.Empty;

        try
        {
            p = Process.Start(new ProcessStartInfo("arp", "-a")
            {
                CreateNoWindow = true,
                UseShellExecute = false,
                RedirectStandardOutput = true
            });

            output = p.StandardOutput.ReadToEnd();

            p.Close();
        }
        catch(Exception ex)
        {
            throw new Exception("IPInfo: Error Retrieving 'arp -a' Results", ex);
        }
        finally
        {
            if (p != null)
            {
                p.Close();
            }
        }
        
        return output;
    }

    #endregion
}

C# c#, .net framework, networking