Creating a Port Scanner in Visual Basic
Introduction
I created this because when I read HTRegz’s tutorial about PyScan, I thought I should apply my modest VB skills and create a port scanner of my own.
This tutorial is aimed at VB programmers who are comfortable with the basics of the language. If there’s a lot of stuff you don’t understand here, you might want to read some more basic tutorials then come back to this one.
Attached is a ZIP file containing all the VB files. Download it and follow along; it will make it a lot easier. Enjoy!
Extra Components
Aside from all the normal frames, text boxes, command buttons, etc. you will need the Winsock control and the (optional) ProgressBar control. To insert these:
1. On the menu bar click Project -> Components…
2. Scroll down in the list and insert a check box into “Microsoft Windows Common Controls 6.0” and “Microsoft Winsock Control 6.0”
Now you can insert the socket control and the progress bar. I’m not going to go into the details of setting up all the objects on the form. I’m assuming you already know how to do that.
The Code Explained
Now I’ll explain most of the code to you.
Code:
Private Sub cmdScan_Click()
'This Sub creates the socket objects and initializes everything.
'---------------------------------------------------------------
Dim I As Integer 'Counter for the For loop.
NextPort = txtMin.Text 'Set the next port to the first port to be scanned.
'Set the progress bar values:
prgProgress.Min = txtMin.Text
prgProgress.Max = txtMax.Text
prgProgress.Value = txtMin.Text
txtOutput.Text = "" 'Clear the text box.
For I = 1 To txtMaxConn.Text 'Loop from 1 to the value in the Maximum Connections textbox.
Load sckScan(I) 'Load a socket with index of I.
sckScan(I).Connect txtIP.Text, NextPort 'Connect the socket.
NextPort = NextPort + 1 'Increment next port.
Next I
End Sub
This is called when the user clicks the Scan button. The code is pretty self-explanatory down to the For loop. Let’s look at the loop:
Code:
For I = 1 To txtMaxConn.Text
Load sckScan(I)
sckScan(I).Connect txtIP.Text, NextPort
NextPort = NextPort + 1
Next I
This is what sets off everything. As you can see, it loops from 1 to the user-specified amount in the maximum connections text box. Each time, it loads the next socket object. It connects then adds 1 NextPort so that another socket can scan the next port.
When the socket objects are initialized in Sub cmdScan_Click(), they attempt to connect to the remote machine. There are two possibilities: it connects successfully, or it can’t connect. When it connects successfully sckScan_Connect() is called:
Code:
Private Sub sckScan_Connect(Index As Integer)
'This Sub is called when the socket makes a successful connection. It prints the port to
'the text box and calls TryNext to connect the socket to the next port.
'---------------------------------------------------------------------------------------
txtOutput.Text = txtOutput.Text & "Port " & sckScan(Index).RemotePort & vbCrLf
TryNext (Index)
End Sub
This prints “Port #” to the text box (where “#” is the remote port). Then it calls TryNext(), a sub that will connect the socket again to another port.
Code:
Sub TryNext(Index)
'This Sub is called when a socket finishes trying a port. It closes the connection then
'connects the socket to the next open port.
'--------------------------------------------------------------------------------------
On Error Resume Next 'I had some errors with the progress bar so I added this statement.
sckScan(Index).Close 'Close the conection.
prgProgress.Value = prgProgress.Value + 1 'Increment the progress bar.
'This If statement unloads the socket if there are no more ports to scan.
If NextPort > txtMax.Text Then
Unload sckScan(Index)
Exit Sub
End If
sckScan(Index).Connect txtIP.Text, NextPort 'Connect to the target with the next port.
NextPort = NextPort + 1 'Increment the next port.
End Sub
Let’s walk through this step by step.
On Error Resume Next – If an error occurs, it will skip to the next line of code. I inserted this because I was having errors with the progress bar. Sloppy programming, no doubt :D. It works fine now, though.
sckScan(Index).Close – This closes the connection. If this statement wasn’t there, it wouldn’t allow the socket to connect. It can’t connect when it’s already connected! :p
prgProgress.Value = prgProgress.Value + 1 – This adds 1 to the value of the progress bar.
If NextPort > txtMax.Text Then
Unload sckScan(Index)
Exit Sub
End If
This determines if there are no more ports to scan. If the next port is higher than the last port to scan, it unloads the socket object since it has no more to do.
sckScan(Index).Connect txtIP.Text, NextPort – Connects to next port.
NextPort = NextPort + 1 – adds 1 to NextPort for the use other sockets.
Last but not least, we have the code for the Stop button:
Code:
Private Sub cmdStop_Click()
'This Sub stops the scan by closing the connection then unloading each socket
'----------------------------------------------------------------------------
'I is the counter for the For loop
Dim I As Integer
'Loop from 1 to the last Winsock control:
For I = 1 To sckScan.UBound
sckScan(I).Close 'Close the connection
Unload sckScan(I) 'Unload the control
Next I
End Sub
When the user clicks the Stop button, this loops from sckScan(1) to sckScan(UBound), (where UBound is the last object). It disconnects them then unloads them.
Conclusion
I think that’s about it. If you have any questions (I know sometimes I explain things in a way that only I understand) just ask and I’ll try to help you out. This is my first security tutorial so if I did something wrong, TELL ME so that I won’t do it in the future. I always welcome constructive criticism.
Look out for more VB tutorials from mjk in the future :D
mjk
EDIT:
New zip file uploaded 5/25/04. I fixed all the bugs I could find.
