[clug] Linux routing problem - two interfaces on same media segment, one seems isolated

Paul Wayper paulway at mabula.net
Sun Aug 24 13:42:47 GMT 2008

Hash: SHA1

Daniel Pittman wrote:
| steve jenkin <sjenkin at canb.auug.org.au> writes:
|> I've setup systems before with multiple interfaces and this should be
|> easy... I'm missing something, but don't know what.
| There is probably one key thing you are missing if you have two
| interfaces on a single physical segment:
| Linux implements the "weak end host model", in which the addresses of
| the system are a property of the *host*, not of the *interface*.
| This means, for example, that it is going to ARP for all your IP
| addresses on a single physical device per physical segment.
| This also means that you are not going to get, for your machine, any of
| load balancing, improved performance, or improved reliability, by
| connecting multiple physical NICs to a single physical segment.
| (Without additional effort.)

Dan, I don't think Steve's after NIC bonding here.

As I understand it, he has two networks, public and private.  All machines
have connections to both the public and private networks via NICs.  The
complication is that the networks are on the same physical switch, and two of
the machines have one NIC (each) and are using multi-homing to provide two
interfaces, one with a public IP and one with a private one.

I'm assuming that Steve's objective here is not to have redundancy or load
balancing.  It's to allow the machines to talk to eachother on one network
that the internet connection can't see, and another to talk to the internet.
The fact that the machines are using a switch which has access to the internet
to talk privately is odd, but that's what VLANs were invented for and we can
assume that they either plan to or are using VLANs on the switch to ensure
traffic privacy.

Some day, one imagines, they might get second NICs for the ones which only
have one, and on that day they can buy a second switch and have the private
network completely separate.

This is why I don't think your suggestion here will work:

| Sure: either pull one of the physical cards, so you only have a single
| card per physical segment, or use bonding so that the cards can both
| function as if they were a single physical card for the segment.

Unfortunately, Steve, I don't have any idea why you might be having this
problem.  I would check your hardware and your kernel log, though.  I bought a
machine with some accursed Sky2-based GigE NICs on-board and they were so
dodgy I had to finally put a separate GigE PCI card in and turn the on-board
ones off.  Seriously, folks, Sky2 cards are teh crapzors.

Have fun,

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