UDPRecv fails?

[dexto]

Hello guys,

Idea is to monitor UDP broadcast in the same subnet (private LAN) on port 67. In other words monitor DHCP requests.

Here is the code that logically should work (moded UDPRecv example):

;;This is the UDP Server
;;Start this first

; Start The UDP Services
;==============================================
UDPStartup()

; Bind to a SOCKET
;==============================================
$socket = UDPBind("0.0.0.0", 67)
If @error <> 0 Then
    ConsoleWrite(1)
    Exit
EndIf

While 1
    ConsoleWrite(UDPRecv($socket, 255, 1))
WEnd

Func OnAutoItExit()
    UDPCloseSocket($socket)
    UDPShutdown()
EndFunc   ;==>OnAutoItExit

Packets are there, as confirmed by packet monitor (pic attached (first 3 packets)).

What am I missing?

Edited February 6, 2010 by dexto

[martin]

Hello guys,

Idea is to monitor UDP broadcast in the same subnet (private LAN) on port 67. In other words monitor DHCP requests.

Here is the code that logically should work (moded UDPRecv example):

;;This is the UDP Server
;;Start this first

; Start The UDP Services
;==============================================
UDPStartup()

; Bind to a SOCKET
;==============================================
$socket = UDPBind("0.0.0.0", 67)
If @error <> 0 Then
    ConsoleWrite(1)
    Exit
EndIf

While 1
    ConsoleWrite(UDPRecv($socket, 255, 1))
WEnd

Func OnAutoItExit()
    UDPCloseSocket($socket)
    UDPShutdown()
EndFunc ;==>OnAutoItExit

Packets are there, as confirmed by packet monitor (pic attached (first 3 packets)).

What am I missing?

What version of AutoIt are you using?

[dexto]

What version of AutoIt are you using?

3.3.0.0

I do not know if this anything to do with this but its worth mentioning that it is UDP broadcast send from unknown IP 0.0.0.0 and to broadcast IP 255.255.255.255

[dexto]

Using 3.3.4.0 outcome is the same.

Tried binding to adapter separately, using binary and auto mode - no difference.

[martin]

Using 3.3.4.0 outcome is the same.

Tried binding to adapter separately, using binary and auto mode - no difference.

Sorry, I see you're missing UDPOpen which will need the flag at 1.

[jchd]

It reminds me of a fix that had to do with UDP broadcast _sent_ to 255.255.255.255, but I may be off my head. No I still have a bit of brain: this was ticket #938, closed with version 3.3.1.1 so it's pretty old story. It may be related to your issue, or not at all.

[dexto]

Sorry, I see you're missing UDPOpen which will need the flag at 1.

Like so?

; Start The UDP Services
;==============================================
UDPStartup()

; Bind to a SOCKET
;==============================================
#cs
    $socket = UDPBind("0.0.0.0", 67)
    If @error <> 0 Then
    ConsoleWrite(@error)
    Exit
    EndIf
#ce

$socket = UDPOpen("0.0.0.0", 67, 1)
If @error <> 0 Then
    ConsoleWrite(@error)
    Exit
EndIf

While 1
    $out = UDPRecv($socket, 255, 0)
    If $out <> "" Then
        ConsoleWrite($out & @CRLF)
    EndIf
WEnd

Func OnAutoItExit()
    UDPCloseSocket($socket)
    UDPShutdown()
EndFunc   ;==>OnAutoItExit

I do not know if its right to use $socket from UDPOpen with UDPRecv

but its acting the same.

[dexto]

Since I could not do it with UDPRecv here is the same using WinPcap:

DHCP monitoring

[martin]

Ok, I'm paying proper attention now!

I tried between 2 PCs like this:

PC 1 had this code

expandcollapse popup

;;Start the other script first

; Start The UDP Services
;==============================================
UDPStartup()

; Open a "SOCKET"
;==============================================

$ThisPCIP = "192.168.1.104";running this script1
$socket2 = UDPBind($ThisPCIP, 65532)

If @error <> 0 Then
 ConsoleWrite("error opening $socket2 , @error = " & @error & @CRLF)
 Exit
 EndIf

$ThatPCIP = "255.255.255.255";
 $socket = UDPOpen($ThatPCIP, 65532,1)
If @error <> 0 Then
 ConsoleWrite("error opening $socket" & @CRLF)
 Exit
 EndIf


$n=0
While 1
    $data = UDPRecv($socket2, 50)
    If $data <> "" Then
    MsgBox(0, "UDP DATA", $data, 1)
    EndIf
    sleep(100)
    $n = $n + 1
    $status = UDPSend($socket, "Message #" & $n)
    If $status = 0 then
    MsgBox(0, "ERROR", "Error while sending UDP message: " & @error)
    Exit
    EndIf
WEnd

Func OnAutoItExit()
    UDPCloseSocket($socket)
    UDPCloseSocket($socket2)
    UDPShutdown()
EndFunc

PC2 had this code which I started running first.

;Script2

; Start The UDP Services
;==============================================
UDPStartup()

; Bind to a SOCKET
;==============================================
HotKeySet("{ESC}","xit")
$ThisPCIP = "192.168.1.17";running script2
$socket = UDPBind($ThisPCIP, 65532)
If @error <> 0 Then Exit

While 1
    $data = UDPRecv($socket, 50)
    If $data <> "" Then
    MsgBox(0, "UDP DATA", $data, 1)
        ;UDPSend($socket2,"Got number " & $data)
    EndIf
    sleep(100)
WEnd

Func OnAutoItExit()
    UDPCloseSocket($socket)
    UDPShutdown()
EndFunc

func xit()
    Exit
EndFunc

Both PCs received the messages as they should.

[dexto]

The only difference comes when computer that sends it and has no IP (Requesting DHCP services) (as in Source IP is 0.0.0.0 target IP is 255.255.255.255) its not showing anything.

This might not be important at all because for the most part IP has to be present. In fact DHCP request is the only case where source computer broadcasting UDP from 0.0.0.0 IP.

[Ozgur]

Hi All,

I hope it's not too frowned up on when old topics such as this are revived with answers months/years later. It's just that, people (like me) may end up hitting them through web-searches, and I though the following addition to this thread may be useful.

The only difference comes when computer that sends it and has no IP (Requesting DHCP services) (as in Source IP is 0.0.0.0 target IP is 255.255.255.255) its not showing anything.

This might not be important at all because for the most part IP has to be present. In fact DHCP request is the only case where source computer broadcasting UDP from 0.0.0.0 IP.

Dexto,

You may have figured this out long time ago, but in case you haven't; another difference is the packet size. DHCP packets have usually been >300 bytes in the networks I've monitored. When you put that 255 limit, UDPRecv() just ignores them. It doesn't do "partial" loads. You need to make sure that the "maxlen" parameter you use is bigger than your expected packet size.

Ozgur