RE: 2 NIC's on same network, possible?
From: Max Harvey (IT_at_smc.ac.nz)
Date: 07/30/03
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To: <security-basics@securityfocus.com> Date: Wed, 30 Jul 2003 11:17:20 +1200
> From: Vineet Mehta [mailto:vineet@linux.com.kw]
> My collegue has a Linux machine which has 2 NIC's on it. What
> he did was assign the IP's 192.168.0.6/24 and 192.168.0.7/24
> to the NIC's. And he was trying to ping the network but was
> getting errors (i dont know the errors).
>
> -----------------
> | Switch |
> |_________________|
> | |
> | |
> | |
> -------------------------------
> | NIC1 NIC2 |
> |192.168.0.6/24 192.168.0.7/24|
> | Machine |
> |-----------------------------|
>
> From: Burton M. Strauss III [mailto:BStrauss@acm.org]
> Now it gets weird if somebody is trying to reach YOU on the
> 2nd NIC. Why? Because the routing decision is address based,
> not NIC based.
>
> So a packet TO the address of the 2nd NIC (192.168.0.7) is
> received on the 2nd NIC. The reply, addressed say to
> 192.168.0.5 again, is sent VIA the 1st NIC (1st match in the
> routing rules wins!). Unless the sender also has this kind
> of funky routing table.
>
> THIS is what leads to the 'rule' that you can't have two NICs
> with the same network portions, because if they're really NOT
> connected identically, you'll lose traffic, and if you're not
> really, really careful with routine rules (wait for it) (yes)
> you'll lose traffic.
>
Does this mean, that if both eth0 and eth1 are connected directly to the
same switch as in the original post I believe, therefore having the same
connection to the network, that it would work?
I am wondering as I had a network cable die on one of my servers
yesterday, so I am considering sticking a backup Ethernet card in the
server, and having it connected into the exactly same switch that the
current card/cable connects to. I am just trying to confirm that I have
read this right, and that since they are connected to the same
connection, that it will work.
Hopefully this means that if I lose a cable/card, it will determine that
the route isn't working, and move onto the next match?
Max.
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