Visual Basic Internet Routines

IcmpSendEcho: Perform a Tracert (Trace Route) in VB
Posted:   Sunday October 30, 2000
Updated:   Monday December 26, 2011
Applies to:   VB4-32, VB5, VB6
Developed with:   VB6, Windows NT4
OS restrictions:   None
Author:   VBnet - Randy Birch, Jim Huff
Related:   IcmpSendEcho: Perform a Tracert (Trace Route) in VB with Host Name Resolution
gethostbyname: Determine Network Host Name and IP Address
gethostbyaddr: Obtain Host Name from IP Address
IcmpSendEcho: Ping a Machine by IP Address
IcmpSendEcho: Ping a Machine by Host Name
Network or DUN connection.

Another popular request is how to perform a network router trace via VB, without using Shell to run the DOS command line app tracert.exe. This demo shows how to perform a true tracert using Visual Basic, and is loosely based on some old VB4 code I found by Jim Huff.

Rather than attempt to explain what is going on in a tracert, I found interesting text  explaining what tracert does at Connected: An Internet Encyclopaedia. Unfortunately, the link is no longer active. However, the main portion of the page is duplicated in the explanation below.

"Traceroute'' is a network debugging utility that attempts to trace the path a packet takes through the network - its route. A key word here is "attempts'' - by no means does traceroute work in all cases.

If you've been paying attention, you already know that the only facilities TCP/IP provide for tracing packet routes are IP packet options (record route and its variants) that are poorly specified, rarely implemented in a useful way, and often disabled for security reasons. Traceroute does not depend on any of these facilities. Traceroute, to put it simply, is a hack.

How Traceroute Works
Traceroute transmits packets with small TTL values. Recall that the TTL (Time To Live) is an IP header field that is designed to prevent packets from running in loops. Every router that handles a packet subtracts one from the packet's TTL. If the TTL reaches zero, the packet has expired and is discarded. Traceroute depends on the common router practice of sending an ICMP Time Exceeded message, documented in RFC 792, back to the sender when this occurs. By using small TTL values which quickly expire, traceroute causes routers along a packet's normal delivery path to generate these ICMP messages which identify the router. A TTL value of one should produce a message from the first router; a TTL value of two generates a message from the second; etc.

      +--------+                                          +--------+   
      | SENDER |                                          | TARGET |   
      +--------+                                          +--------+   
          |                                                   ^|     
       [============( Router )=====( Router )=====( Router )==|====]
                   ^              ^              ^            |  
                   | TTL=1        | TTL=2        | TTL=3      | TTL=4  
  Traceroute       |              |              |            |        
  shows these -----+--------------+--------------+------------/       
  IP addresses                                                         

In a typical traceroute session, a group of packets with TTL=1 are sent. A single router should respond, using the IP address of the interface it transmits the ICMP Timeout messages on, which should be the same as the interface it received the original packets on. The user is told this IP address, and DNS is used to convert this into a symbolic domain address. Also, round trip times are reported for each packet in the group. Traceroute reports any additional ICMP messages (such as destination unreachables) using a rather cryptic syntax - !N means network unreachable, !H means host unreachable, etc. Once this first group of packets has been processed (this can take 10 seconds or no time at all), the second group (TTL=2) begins transmitting, and the whole process repeats.

Problems you might encounter
Since TCP/IP was not designed to support traceroute, several kinds of problems might arise.

Changing paths
Always remember - you are not tracing the path of one packet, but of many. Hopefully, all those packets will follow the same route, but this is by no means assured. What if a link fails during the traceroute? Your packets may be rerouted, and traceroute's output becomes a confused combination of two separate routes.

No sending addresses
You only see one IP address from each router - the address closest to you. To put it another way, traceroute can't tell you which interfaces routers are sending the packets on. It only shows the interfaces packets are being received on. The sending interfaces can often be deduced by matching each router with the next one in line - typically only one interface could be used between them.

Routing problems
TCP/IP's sinister and ubiquitous routing problems may cause the router not to have a route back to the sender, or to have a route through some interface other than the one it received the packet on. In these cases, you will either receive no reply at all (no route), or a reply showing an IP address that never handled the original packet (it was handled by some other interface on the same router). In short, don't completely trust traceroute.

Buggy TCP/IP implementations
Traceroute depends on a rather obscure feature that has been known to be mis-implemented. Some of the problems people have found: code that fails to decrement TTL, code that incorrectly forwards packets with zero TTL, code that does not generate ICMP Timeouts, and code that sends ICMPs with the same TTL as the original packet. This last problem, of course, results in our ICMP Timeouts being sent with zero TTL - guaranteed not to make it back to us.

 BAS Module Code
Place the following code into the general declarations area of a bas module:

 Form Code
To a form add a command button (Command1), check box (disabled - Check1), a combo (Combo1) and four text boxes (Text1 - Text 4). Text 4 is the tracert output box. Label as desired, and add the following code:

Option Explicit
' Copyright 1996-2011 VBnet/Randy Birch, All Rights Reserved.
' Some pages may also contain other copyrights by the author.
' Distribution: You can freely use this code in your own
'               applications, but you may not reproduce 
'               or publish this code on any web site,
'               online service, or distribute as source 
'               on any media without express permission.
Private Const WSADescription_Len As Long = 255  '256, 0-based
Private Const WSASYS_Status_Len As Long = 127   '128, 0-based
Private Const WS_VERSION_REQD As Long = &H101
Private Const SOCKET_ERROR As Long = -1
Private Const AF_INET As Long = 2
Private Const IP_SUCCESS As Long = 0
Private Const MIN_SOCKETS_REQD As Long = 1
Private Const EM_SETTABSTOPS As Long = &HCB

Private Type WSADATA
   wVersion As Integer
   wHighVersion As Integer
   szDescription(0 To WSADescription_Len) As Byte
   szSystemStatus(0 To WSASYS_Status_Len) As Byte
   imaxsockets As Integer
   imaxudp As Integer
   lpszvenderinfo As Long
End Type

   ttl             As Byte         'Time To Live
   Tos             As Byte         'Timeout
   Flags           As Byte         'option flags
   OptionsSize     As Long         '
   OptionsData     As Long         '
End Type

   Address         As Long         'replying address
   Status          As Long         'reply status code
   RoundTripTime   As Long         'round-trip time, in milliseconds
   datasize        As Integer      'reply data size. Always an Int.
   Reserved        As Integer      'reserved for future use
   DataPointer     As Long         'pointer to the data in Data below
   Options         As ICMP_OPTIONS 'reply options, used in tracert
   ReturnedData    As String * 256 'the returned data follows the
                                    'reply message. The data string
                                    'must be sufficiently large enough
                                    'to hold the returned data.
End Type

Private Declare Function SendMessage Lib "user32" _
   Alias "SendMessageA" _
  (ByVal hwnd As Long, _
   ByVal wMsg As Long, _
   ByVal wParam As Long, _
   lParam As Any) As Long
Private Declare Function WSAStartup Lib "wsock32" _
  (ByVal VersionReq As Long, _
   WSADataReturn As WSADATA) As Long
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" _
  (haddr As Long, _
   ByVal hnlen As Long, _
   ByVal addrtype As Long) As Long
Private Declare Function gethostname Lib "wsock32" _
   (ByVal szHost As String, _
    ByVal dwHostLen As Long) As Long
Private Declare Function gethostbyname Lib "wsock32" _
   (ByVal szHost As String) As Long

Private Declare Sub CopyMemory Lib "kernel32" _
   Alias "RtlMoveMemory" _
  (Dest As Any, _
   Source As Any, _
   ByVal nbytes As Long)

Private Declare Function inet_ntoa Lib "wsock32.dll" _
   (ByVal addr As Long) As Long

Private Declare Function lstrcpyA Lib "kernel32" _
  (ByVal RetVal As String, _
   ByVal Ptr As Long) As Long
Private Declare Function lstrlenA Lib "kernel32" _
  (ByVal Ptr As Any) As Long
Private Declare Function IcmpCreateFile Lib "icmp.dll" () As Long

Private Declare Function IcmpCloseHandle Lib "icmp.dll" _
   (ByVal IcmpHandle As Long) As Long
Private Declare Function IcmpSendEcho Lib "icmp.dll" _
   (ByVal IcmpHandle As Long, _
    ByVal DestinationAddress As Long, _
    ByVal RequestData As String, _
    ByVal RequestSize As Long, _
    RequestOptions As ICMP_OPTIONS, _
    ReplyBuffer As ICMP_ECHO_REPLY, _
    ByVal ReplySize As Long, _
    ByVal Timeout As Long) As Long

Private Sub Form_Load()

    With Combo1
      .AddItem ""
      .AddItem ""
      .AddItem ""
      .AddItem ""
      .ListIndex = 1
   End With

   Text1.Text = ""
   Text4.Text = ""
   ReDim TabArray(0 To 3) As Long
   TabArray(0) = 30
   TabArray(1) = 54
   TabArray(2) = 105
   TabArray(3) = 182
  'Clear existing tabs
  'and set the text tabstops
   Call SendMessage(Text4.hwnd, EM_SETTABSTOPS, 0&, ByVal 0&)
   Call SendMessage(Text4.hwnd, EM_SETTABSTOPS, 4&, TabArray(0))

End Sub

Private Sub Command1_Click()
   Command1.Enabled = False
   Call TraceRT
   Command1.Enabled = True
End Sub

Private Function TraceRT()

   Dim ipo As ICMP_OPTIONS
   Dim echo As ICMP_ECHO_REPLY
   Dim ttl As Integer
   Dim ttlAdjust As Integer
   Dim hPort As Long
   Dim nChrsPerPacket As Long
   Dim dwAddress As Long
   Dim sAddress As String
   Dim sHostIP As String

  'set up
   Text1.Text = ""  'the target IP
   Text2.Text = "1" 'force the no of packets = 1 for a tracert
   Text4.Text = ""  'clear the output window
   List1.Clear      'for info/debuging only
  'the chars per packet - can be between 32 and 128
   If IsNumeric(Text3.Text) Then
      If Val(Text3.Text) < 32 Then Text3.Text = "32"
      If Val(Text3.Text) > 128 Then Text3.Text = "128"
      Text3.Text = "32"
   End If
   nChrsPerPacket = Val(Text3.Text)
   If SocketsInitialize() Then
     'returns the IP Address for the Host in Combo 1
     'ie returns for
      sAddress = GetIPFromHostName(Combo1.Text)
     'convert the address into an internet address.
     'ie returns 1982874833 when passed
      dwAddress = inet_addr(sAddress)
     'open an internet file handle
      hPort = IcmpCreateFile()
      If hPort <> 0 Then
        'update the textboxes
         Text1.Text = sAddress
         Text4.Text = "Tracing Route to " + Combo1.Text + ":" & vbCrLf & vbCrLf
        'The heart of the call. See the VBnet
        'page description of the TraceRt TTL
        'member and its use in performing a
        'Trace Route.
         For ttl = 1 To 255
           'for demo/dedbugging only. The
           'list will show each TTL passed
           'to the calls. Duplicate TTL's
           'mean the request timed out, and
           'additional attempts to obtain
           'the route were tried.
            List1.AddItem ttl
           'set the IPO time to live
           'value to the current hop
            ipo.ttl = ttl
           'Call the API.
           'Two items of consequence happen here.
           'First, the return value of the call is
           'assigned to an 'adjustment' variable. If
           'the call was successful, the adjustment
           'is 0, and the Next will increment the TTL
           'to obtain the next hop. If the return value
           'is 1, 1 is subtacted from the TTL value, so
           'when the next increments the TTL counter it
           'will be the same value as the last pass. In
           'doing this, routers that time out are retried
           'to ensure a completed route is determined.
           '(The values in the List1 show the actual
           ' hops/tries that the method made.)
           'i.e. if the TTL = 3 and it times out,
           '     adjust = 1 so ttl - 1 = 2. On
           '     encountering the Next, TTL is
           '     reset to 3 and the route is tried again.
           'The second thing happening concerns the
           'sHostIP member of the call. When the call
           'returns, sHostIP will contain the name
           'of the traced host IP.  If it matches the
           'string initially used to create the address
           '(above) were at the target, so end.
            ttlAdjust = TraceRTSendEcho(hPort, _
                                        dwAddress, _
                                        nChrsPerPacket, _
                                        sHostIP, _
                                        echo, _
            ttl = ttl - ttlAdjust
           'need some processing time
            If sHostIP = Text1.Text Then

              'we're done
               Text4.Text = Text4.Text & vbCrLf + "Trace Route Complete"
               Exit For

            End If

         Next ttl

        'clean up
         Call IcmpCloseHandle(hPort)
         MsgBox "Unable to Open an Icmp File Handle", vbOKOnly, "VBnet TraceRT Demo"
      End If  'If hPort
     'clean up
      Call SocketsCleanup
     MsgBox "Unable to initialize the Windows Sockets", vbOKOnly, "VBnet TraceRT Demo"
   End If  'if SocketsInitialize()

End Function

Private Function GetIPFromHostName(ByVal sHostName As String) As String

  'converts a host name to an IP address.

   Dim ptrHosent As Long      'address of hostent structure
   Dim ptrName As Long        'address of name pointer
   Dim ptrAddress As Long     'address of address pointer
   Dim ptrIPAddress As Long   'address of string holding final IP address
   Dim dwAddress As Long      'the final IP address
   ptrHosent = gethostbyname(sHostName & vbNullChar)

   If ptrHosent <> 0 Then

     'assign pointer addresses and offset
     'ptrName is the official name of the host (PC).
     'If using the DNS or similar resolution system,
     'it is the Fully Qualified Domain Name (FQDN)
     'that caused the server to return a reply.
     'If using a local hosts file, it is the first
     'entry after the IP address.
      ptrName = ptrHosent
     'Null-terminated list of addresses for the host.
     'The Address is offset 12 bytes from the start of
     'the HOSENT structure. Addresses are returned
     'in network byte order.
      ptrAddress = ptrHosent + 12
     'get the actual IP address
      CopyMemory ptrAddress, ByVal ptrAddress, 4
      CopyMemory ptrIPAddress, ByVal ptrAddress, 4
      CopyMemory dwAddress, ByVal ptrIPAddress, 4

      GetIPFromHostName = GetIPFromAddress(dwAddress)

   End If
End Function

Private Sub SocketsCleanup()
  'only show error if unable to clean up the sockets
   If WSACleanup() <> 0 Then
       MsgBox "Windows Sockets error occurred during Cleanup.", vbExclamation
   End If
End Sub

Private Function SocketsInitialize() As Boolean

  'when the socket version returned == version
  'required, return True
   SocketsInitialize = WSAStartup(WS_VERSION_REQD, WSAD) = IP_SUCCESS
End Function

Private Function GetIPFromAddress(Address As Long) As String
   Dim ptrString As Long
   ptrString = inet_ntoa(Address)
   GetIPFromAddress = GetStrFromPtrA(ptrString)
End Function

Private Function GetStrFromPtrA(ByVal lpszA As Long) As String

   GetStrFromPtrA = String$(lstrlenA(ByVal lpszA), 0)
   Call lstrcpyA(ByVal GetStrFromPtrA, ByVal lpszA)
End Function

Private Sub ShowResults(timeToLive As Byte, _
                        tripTime As Long, _
                        sHostIP As String)
   Dim sTripTime As String
   Dim buff As String
   Dim tmp As String

  'format a string representing
  'the round trip time
   Select Case tripTime
      Case Is < 10:   sTripTime = "<10 ms"
      Case Is > 1200: sTripTime = "*"
      Case Else:      sTripTime = CStr(tripTime) & " ms"
   End Select
  'cache the textbox
   buff = Text4.Text
  'create a new entry
   tmp = "Hop #" & vbTab & _
          CStr(timeToLive) & vbTab & _
          sTripTime & vbTab & _
          sHostIP & vbCrLf

  'update textbox
   Text4.Text = buff & tmp
End Sub

Private Function TraceRTSendEcho(hPort As Long, _
                                 dwAddress As Long, _
                                 nChrsPerPacket As Long, _
                                 sHostIP As String, _
                                 echo As ICMP_ECHO_REPLY, _
                                 ipo As ICMP_OPTIONS) As Integer

   Dim sData As String
   Dim sError As String
   Dim sHostName As String
   Dim ttl As Integer
  'create a buffer to send
   sData = String$(nChrsPerPacket, "a")
   If IcmpSendEcho(hPort, _
                   dwAddress, _
                   sData, _
                   Len(sData), _
                   ipo, _
                   echo, _
                   Len(echo) + 8, _
                   2400) = 1 Then
      'a reply was received, so update the display
       sHostIP = GetIPFromAddress(echo.Address)
       ShowResults ipo.ttl, echo.RoundTripTime, sHostIP
      'return 0 to continue with retrieval
       TraceRTSendEcho = 0
      'a timeout was received, so set the
      'return value to 1. In the TraceRT
      'calling routine, the TTL will be
      'de-incremented by 1, causing the
      'for / next to retry this hop.
       TraceRTSendEcho = 1
   End If
End Function


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