Security Basics mailing list archives
RE: 2 NIC's on same network, possible?
From: "Max Harvey" <IT () smc ac nz>
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. --------------------------------------------------------------------------- ----------------------------------------------------------------------------
Current thread:
- Re: 2 NIC's on same network, possible?, (continued)
- Re: 2 NIC's on same network, possible? Ansgar Wiechers (Jul 29)
- RE: 2 NIC's on same network, possible? David Gillett (Jul 29)
- Re: 2 NIC's on same network, possible? Ansgar Wiechers (Jul 29)
- RE: 2 NIC's on same network, possible? David Gillett (Jul 30)
- Re: 2 NIC's on same network, possible? Ansgar Wiechers (Jul 31)
- RE: 2 NIC's on same network, possible? David Gillett (Jul 29)
- Re: 2 NIC's on same network, possible? Ansgar Wiechers (Jul 29)
- Re: 2 NIC's on same network, possible? thedistance (Jul 30)
- RE: 2 NIC's on same network, possible? Burton M. Strauss III (Jul 29)
- RE: 2 NIC's on same network, possible? Max Harvey (Jul 30)
- RE: 2 NIC's on same network, possible? Shane Lahey (Jul 29)
- RE: 2 NIC's on same network, possible? David Gillett (Jul 30)
- Re: 2 NIC's on same network, possible? skate (Jul 30)