How To Get The Remote Computer Name From IP Address

[sevenyb420]

For all of you out there wondering how to use the gethostbyaddr functionality shown in this vb 6 example: Here is the updated code that works properly in a VB.NET environment. After Several days of searching, reading about API and Data Marshalling, etc, This is the shortest and sweetest working code I have come up with. If you have questions about how this works or why things are the way they are, e-mail me and I will be happy to explain!

Option Explicit On
Imports System.Runtime.InteropServices
Class HNResolver
 
    Public Shared Sub Main()
        Dim resolve As New HNResolver()
        Console.WriteLine(resolve.GetHostNameFromIP("your dotted quad ip address here"))
 
    End Sub
 
    Private Const WSADescription_Len As Long = 256
    Private Const IP_SUCCESS As Integer = 0
    Private Const SOCKET_ERROR As Long = -1
    Private Const AF_INET As Long = 2
    Dim Initialized As Boolean
 
    <StructLayout(LayoutKind.Sequential)> Public Structure WSADATA
        Dim wVersion As Short
        Dim wHighVersion As Short
        <MarshalAs(UnmanagedType.ByValTStr, sizeConst:=WSADescription_Len + 1)> Dim szDescription As String
        <MarshalAs(UnmanagedType.ByValTStr, sizeConst:=WSADescription_Len + 1)> Dim szSystemStatus As String
        Dim iMaxSockets As Integer
        Dim iMaxUdpDg As Integer
        Dim lpVenderInfo As IntPtr
    End Structure
 
    <StructLayout(LayoutKind.Sequential)> Public Class HOSTENT
        Public h_name As IntPtr
        Dim h_aliases As Long
        Dim h_addrtype As Integer
        Dim h_length As Integer
        Dim h_addr_list As Long
    End Class
 
    Private Declare Function WSAStartup Lib "wsock32" _
      (ByVal VersionReq As Int16, _
    ByRef WSADataReturn As WSADATA) As Integer
 
    Private Declare Function WSACleanup Lib "wsock32" () As Long
 
    Private Declare Function inet_addr Lib "wsock32" _
      (ByVal s As String) As Long
 
    Private Declare Function gethostbyaddr Lib "wsock32" _
      (ByRef haddr As Long, _
       ByVal hnlen As Integer, _
       ByVal addrtype As Integer) As IntPtr
 
    Private Declare Function lstrlen Lib "kernel32" _
       Alias "lstrlenA" _
      (ByVal lpString As IntPtr) As Integer
 
    Public Sub New()
        Dim WSAD As New WSADATA()
        If Not WSAStartup(&H201, WSAD) = IP_SUCCESS Then
            Console.WriteLine("Wsa Startup failed")
            Initialized = False
        Else
            Initialized = True
        End If
    End Sub
 
    Private Sub SocketsCleanup()
        If WSACleanup() <> 0 Then
            MsgBox("Windows Sockets error occurred in Cleanup.", vbExclamation)
        End If
    End Sub
 
    Private Function GetHostNameFromIP(ByVal sAddress As String) As String
        Dim host As New HOSTENT
        Dim ptrHost As IntPtr
        Dim hAddress As Long
        Dim nbytes As Integer
 
        If Initialized Then
            hAddress = inet_addr(sAddress)
            If hAddress <> SOCKET_ERROR Then
                ptrHost = gethostbyaddr(hAddress, 4, AF_INET)
                System.Runtime.InteropServices.Marshal.PtrToStructure(ptrHost, host)
                If ptrHost <> IntPtr.Zero Then
                    nbytes = lstrlen(host.h_name)
                    Dim temp(nbytes) As Byte
                    System.Runtime.InteropServices.Marshal.Copy(host.h_name, temp, 0, nbytes - 1)
                    Return System.Text.Encoding.Default.GetString(temp)
                Else
                    Return "Call to gethostbyaddr failed."
                End If 'If ptrHosent
                SocketsCleanup()
            Else
                Return "String passed is an invalid IP."
            End If 'If hAddress
        Else
            Return "Sockets failed to initialize."
        End If  'If SocketsInitialize
    End Function
End Class

[siva_vandanapu]

Hi Sevenyb420,
I am sivakumar a new user, looked at your code, which matched my requirements. it worked for most of the ip addresses to resolve or get the computer name. But very few of the systems in my network i am getting the argument null exception:

"Value cannot be null. Parameter name: ptr"

this exception occured at line :

System.Runtime.InteropServices.Marshal.PtrToStructure(ptrHost, host)
in GetHostNameFromIP method.

Can you please let me know the solution. If you need any more details please feel free to ask.

Thanks in advance
Siva kumar V

[chris128]

From the MSDN documentation on the gethostbyaddr function:

Note The capability to perform reverse lookups using the gethostbyaddr function is convenient, but such lookups are considered inherently unreliable, and should be used only as a hint.

So I would say you should probably look for another way to do this reverse lookup you need to do. One way would be to just use Process.Start to launch the NSLOOKUP program that is included in Windows and parse the output. Another way would be to build a DNS query and send it to your DNS server via UDP but that would be quite a lot of work.

In response to your actual problem though, the MSDN documentation also says this:

If no error occurs, gethostbyaddr returns a pointer to the hostent structure. Otherwise, it returns a null pointer, and a specific error code can be retrieved by calling WSAGetLastError.

MSDN article in question: http://msdn.microsoft.com/en-us/libr...21(VS.85).aspx

[siva_vandanapu]

Hi chris128,
Thanks for your reply. I am working on that. Before that i found one Weird thing from Sevenyb420 code, when ever i execute the code ( GetHostNameFromIP method) i am getting the system name with out last letter. for example system name is "sivakumar.xx.com", but code returning the system name as "sivakumar.xx.co" . Do you have any clue why it is showing like that?

Thanks
Siva kumar V

[Mathiaslylo]

There is another way of doing this!

I recently developed an online API (if you can call it that) that displays the IP. I use it myself, so it will always be there.

Try going to http://flamefusion.net/API/InternetProtocolAddress.aspx to see what I mean.

The page only contains your IP, and nothing else. Just download that file with a System.Net.HttpWebRequest or a System.Net.WebClient depending on what you prefer, and you've got the IP!

[chris128]

That is an external IP, I dont think that is what Siva is after

[Mathiaslylo]

That is an external IP, I dont think that is what Siva is after

Oh! My bad. I got confused by the word "Remote" in the title of the topic.

[siva_vandanapu]

Hi Chris 128,
Thanks for immediate reply. I got the solution for this problem.
In the line number 83 following code has been written:
System.Runtime.InteropServices.Marshal.Copy(host.h_name, temp, 0, nbytes - 1)

i donot know why they have written nbytes-1, after removing "-1" I got the system name comng properly.
Any how thanks alot for your helping hand.

Regards
Siva Kumar V