At work I have a bunch of devices that communicate to and from a computer using raw 802.3 ethernet. Communication to the devices is mediated by a little program a coworker wrote, which uses the winPcap library to generate the ethernet packets.

The network topology is simple: an ethernet cable connects one NIC on the computer to an ethernet switch, and several of my devices attach to the various sockets on the switch. Everything was working fine, but then I had to reformat the computer and now I cannot communicate with my devices.

If I connect the NIC directly to one of the devices, it works fine. When I try to go through the switch, it seems that the devices do not receive packets sent out from the computer.

This is a D-Link switch with no user accessible settings as far as I can tell. Could this be an issue with NIC settings? Is there some way to reset the switch?

  • how sure are you that the destination mac-addr are correct in the non-functional case? – Mike Pennington Jul 14 '12 at 16:59
  • Everything was working fine before you reformatted one of the computers, now it doesn't work fine, and you're blaming the switch? I think your notions of cause and effect need recalibrating. – womble Jul 14 '12 at 20:54
  • Er, thanks, womble, but that doesn't really help me at all. – Martinis Group Jul 14 '12 at 23:23

It could be that your ethernet card/driver is throwing away packets not destined for your pc. Try enabling promiscuous mode* in your device settings for it to recieve all packets.

Otherwise, try checking the mac addresses (was any of them changed on the old install?), and if the packets are going in/out (you can use wireshark for that).

Also, were there any cables replaced? If it's an old NIC, it is important to use correct (straight/cross) cables - new NICs can detect the wrong cable, and swap tx/rx pins.

*sometimes it's writen "promisc mode"

  • I will check into promiscuous mode for the NIC, thanks. I will give wireshark a shot. I actually already installed it and am in the process of figuring out how it works. Thanks for that too. Same cables, so I doubt that's the issue. – Martinis Group Jul 14 '12 at 23:23

This is a D-Link switch with no user accessible settings as far as I can tell. Could this be an issue with NIC settings?

Is this an unmanaged off the shelf commodity switch? I can't count how many weird "the network isn't working" problems I have eventually traced to an old Linksys hiding underneath someone's desk or up in the drop ceiling. Their power supplies especially like to slowly die, meaning the device powers off and on randomly or pukes CRC errors or all kinds of ugly. Of course, being strictly unmanaged it can be difficult to tell that switch is at fault. Either way commodity devices don't belong in most business settings.

Could this be an issue with NIC settings?

Since everything worked before you formatted your computer it is likely that there was some network setting that made this setup all hang together because you are still using the same hardware. As mulaz rightly pointed out the only way to really know what's happening is to sniff the network traffic using a tool like wireshark.

Also a quick troll through the documentation for your devices might yield valuable information - look for a section on networking requirements.

  • Since you mention the distinction between managed and unmanaged, I'm going to ask, what exactly is the difference? I've been googling this and still am fuzzy. I believe that what I have is an unmanaged switch, because there is no way to interface with the switch and edit settings. – Martinis Group Jul 14 '12 at 23:26

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