Broadcast fails to local machine

[holmfred]

I have a client and a server which sends a UDP broadcast to 255.255.255.255 to notice their existence. On some computers, when the client and the server is running on the same machine, the UDP broadcast will fail. It works if the client and the server is on different machines.

However, if WinDump (http://www.winpcap.org/) is running with WinPcap installed on the computer, the UDP broadcast works when the client and the server is on the same computer.

Why does the broadcast work on the local machine when I run WinDump? Can I make the broadcast work without this third-party software? Is it related to the network card since it works flawless on some computers?


Fredrik

[mabr]

It is interesting that the sniffer windump makes the broadcast go through. One would like to know what it does. As you say, the normal thing is that a broadcast is also received by the local machine, but on some XP computers this fails.

[hoxsiew]

WinDump may put the ethernet card into "promiscuous" mode. That might have something to do with it.

[mabr]

Good point. But the thing is that broadcast from other machines may go through (, wich means that the network card is in promiscous mode as I understand it,) at the same time as broadcast from the local machine do not.

But as you say there could be some manipulation of the network card involved.

[Richard.J]

"promiscuous mode" to my understanding means that the incoming packets are delivered to the sniffer even though it is not listening to a specific port. That has nothing to do with a broadcast (to a specific port). Maybe the IP stack discards broadcasts when the origin and the destination are the same address.

[holmfred]

I have noticed that the computers that do not see local broadcast has a Broadcom network card. On another machine which works with the broadcast, I have another brand (I cannont remember the name right now). Could this issue depend on the manufacturer of the NIC. Does some manufacturer really put the network card in promiscuous mode by default?

[mabr]

Ok I was wrong. The promiscuous mode is acceptance of data intended for other machines and should not be confused with acceptance of broadcast data. I have read that the winpcap installs a driver in parallell with the ordinary drivers who receive data from the nic driver. It should then not affect packet delivery by replacing a driver. The only possibility that remains is that it configures the nic driver in such a way that makes it deliver broadcast packages from the local machine. It could be that the promiscuous mode erronously is linked with this 'feature'. It seems likely that we are facing a bug in the driver associated with the particular network card.