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Old 07-02-2001, 04:37 PM   #1
Registered: Feb 2001
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Question 2 Nics?

There is any way to use 2 network cards, in the same network, but work them totally independent, right now I have 2 nics, but the Eth0 always depends of the Eth1, If I disconnect the Eth0 the Eth1 will not reply the pings, even if the cable is connected. I know that this works with differents Subnets. Like this


But I want to have them in the same network and in the same computer.


Any idea? Sugesstion of routing. ipchains? iptables?
Old 07-03-2001, 12:41 PM   #2
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2 nics

You should be able to do this. If you read the "Linux Ethernet-Howto" by Paul Gortmaker, it should explain how to do this. Basically you may want to create alias's. You can find this in /usr/doc/Linux-HOWTO's or

Good luck
Old 07-03-2001, 11:16 PM   #3
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can i know what is the use ??
Old 07-04-2001, 09:06 AM   #4
Registered: May 2001
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basic networking.

So you have two network cards on the same subnet.
When you disconnect one the other does not reply.
where are you pinging this machine from? The local net (i.e. the same subnet that the cards are on) or a remote net ?

I think that you problem may be that you cannot setup two default gateways.

probably at this time you routing table looks like this:

netstat david@jing~>netstat -rn
Kernel IP routing table
Destination Gateway Genmask Flags MSS Window irtt Iface UH 0 0 0 eth0 UH 0 0 0 eth1 U 0 0 0 eth1 U 0 0 0 lo UG 0 0 0 eth0
see how there is only one "default gateway" (the line that begins ?

if you are ping from then packets would come from , go to a pile of other routers , then to and then to eth1 (the router knows how to get to eth1). but the reply to the ping will not go out eth1. All your computer knows is that it must send the reply to which , according to you router table means it must go out eth0 (which you have disconnected). I suggest testing this theory by pinging you machines two interfaces from a computer on the same subnet as those interfaces. see picture

You know, now that I think of it how does the linux box know which interface to send back a packet through if the reply should go to a machine on the same subnet as the two interfaces? answer: it doesn't (least not in 2.2 kernels).

there are two ways I can think of for fixing this problem:

1. linux 2.4 iptable "source routing" , which basically says "send packets out the interface thay came in on"

2. advanced routing (BGP with MRTD )
Old 07-04-2001, 09:08 AM   #5
Registered: Feb 2001
Location: Costa Rica
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Well the use of having two network cards, is that I could get rid of too much traffic from one server. So if I have 2 Nic's all the traffic to this server will be faster. I hope to get that FAQ and put to work this out soon.


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