I'm working on some software that regularly pings another server's IP addresses to detect whether that IP (and therefore the server) is on the network. One means of testing this software is to physically unplug the NIC from the network. However, when I have multiple NICs (eth0, eth1, eth2...) if I unplug 1 or 2, the associated addresses only become unavailable momentarily and then come back alive even while the cable is still unplugged. Unplugging only eth0 causes all addresses to become unavailable. It's almost as if the other addresses get re-routed through eth0, but eth0's address never re-routes through the other NICs.

The addresses of the NICs are all on the same subnet, in my test.
I can simulate the behavior of the software with a couple of ping sessions to each address, so this isn't a problem with the software itself. I'm trying to understand the underlying network config.

Can anyone explain why Linux would behave like this? Is this some kind of high-availability feature? Is there a way to disable this functionality so that the cards act independently?

Are all the cards the same, and therefore using the same driver? If so, it might be a driver bug.

It happens on virtual machines as well as physical. In the case of the physical, the cards are not the same. I have an Intel 82540EM Gigabit and 3Com 3c905B cards in the physical machine I am testing on. We've tried this on RedHat, SLES10, and CentOS and seen the same results.

Here are some results from further testing we've done.
· 2 NICs on the same network and subnet – Unplug eth0 and pings to both NICs report “Destination Unreachable”.

· 2 NICs on the same network and subnet – Unplug eth1 and the unplugged NIC cease to return pings for about 70 seconds then resumes returning pings, even though eth1 is still unplugged. ßTwilight Zone music here.

· 2 NICs on different physical networks – Unplug eth0 or 1 and the unplugged NIC reports “Destination Unreachable” while the one still connected ping stops updating. It doesn’t error out it just stops responding with no ping errors.