[Winpcap-users] Monitoring multiple network interfaces

soulstone soulstone at gmx.de
Fri Dec 4 07:02:57 PST 2009

Gianluca Varenni wrote:
> ----- Original Message ----- 
> From: "soulstone" <soulstone at gmx.de>
> To: <winpcap-users at winpcap.org>
> Sent: Thursday, December 03, 2009 8:49 AM
> Subject: [Winpcap-users] Monitoring multiple network interfaces
>> Hi,
>> I've tried to monitor multiple networ interfaces installed on
>> a system.
>> The reason is that I need to monitor network traffic to determine 
>> whether a user navigates to a given url.
>> I'd rather capture only packages from the interface which is connected 
>> to the internet but I don't know how to find out which one I need.
>> So I tried to monitor all interfaces.
>> I did this by creating one thread per interface which calls pcap_loop.
>> But this doesn't work.
> What do you mean by "it doesn't work"?

1. I use pcap_findalldevs to get all devices.
2. I open every interface by pcap_open_live.
3. I use a loop to itter through all interfaces and spawn
a seperated thread for each call of pcap_loop(Desc, 0, @PacketHandler, Err).

Usually after that the method PacketHandler would be called
if I only invoke pcap_look for the correct interface with internet access.
But if I try to monitor multiple interfaces the function PacketHandler 
isn't called anymore.

I also tried to reverse the loop (the first interface is coincidental 
the one with internet access) it works.

Example code:
   // doesn't work
   for i := 0 to Length(Interfaces) - 1 do
	// creates a thread which calls pcap_loop

   // works, because our (now last) item is the correct NIC
   for i := Length(ValidP) - 1 downto 0 do

So the problem is that PacketHandler isn't called for all devices.

Kind regards,

> GV
>> Specifying only one interface everything works fine.
>> Can I only monitor one interface at once or do I miss something?
>> Maybe someone can give me an advice.
>> Kind regards,
>>   dy 
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