Re: 2 nics, 1 network, puzzle?
On Tue, 26 Mar 2002, Shawn Yarbrough wrote:
> I have an x86 server computer containing two network cards:
>
> eth0 --> 192.168.1.130
> eth1 --> 192.168.1.131
>
> Both cards work fine alone. As you can see, both cards are on the same
> network. The subnet mask is 255.255.255.0. I can disable either card
> with ifdown and sucessfully ping the other card from elsewhere on the
> network.
>
> The puzzle comes when both cards are up at the same time.
>
> The kernel seems to pick one of the cards (somewhat arbitrarily) and
> starts answering pings to BOTH addresses on the ONE card! Could this be
> some kind of ARP bug? I can physically pull the plug out of the other
> card (the one that the kernel appears to not be using anymore) and the
> system still answers pings to both addresses!
>
>
> I first noticed this strange behavior by watching the indicator lights
> during pings. Pings to both addresses are clearly going thru only one of
> the cards. The other card remains idle.
>
First, an observation:
I don't think your installation is considered to be "standard" or
"customary" in the IP world. I think I understand what you are trying to
achieve, but I believe IP philosophically is based on having one network
interface per IP network. When one starts to do things outside of the
standard or customary, then differences in the implementations of IP
stacks begin to show up. In other words, there is no "tight" specification
for an IP implementation, and the many different IP stacks do things a
little differently. There is a wonderful book "TCP/IP Illustrated, Volume
1", by Rich Stevens that shows how differently the leading (at the time of
the writing of the book) OS's IP stacks handle different situations.
In order to really find out what is happening, I think you need to use
some better diagnostic tools, like 'tcpdump'. This will allow you to
capture the datagrams that each network interface is seeing and you can
better understand what is happening. I don't think the lights on the
Ethernet cards are really useful for diagnosing what is going on.
For a machine that is multihomed (has more than one IP network interface
assigned an IP address), any of the IP addresses for the machine are
valid and equally useable. I think your Linux system is responding in a
reasonable way as both network interfaces are receiving ICMP echo
requests, and both are apparently answering. I don't think there is a bug
here.
> This is a Debian Woody system (freshly installed and not currently in use
> for anything). The behavior is identical running either kernel 2.2.20 or
> kernel 2.4.18. In /etc/network/options the ip_forward setting is no.
>
> This is NOT a router-type computer. It's just a server that I really want
> to have on the same network, twice. And I'd rather not use IP aliasing +
> one card. I really want two cards for convienence and redundancy. (ex:
> in theory if one card failed or was misconfigured I should still be able
> to reach the machine through the other card).
>
>
> Anyone got a clue what's going on here? Thanks,
>
I'm not sure how to achieve the redundancy that you desire. Perhaps with
the load-sharing facility (I think there is a HOWTO on this), but that may
be only for point-to-point links. My experience is that Ethernet cards are
one of the more reliable components of a PC and if there is going to be a
failure, something else like a power supply, harddisk, or even a
motherboard will go before an Ethernet card. Of course, Ethernet cards do
fail, so I'm not trying to say your quest is in vain. Also, people do a
lot of things that aren't "standard" IP so I'm not trying to discourage
you there either.
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