Re: Multiple NICS on same subnet
- From: floyd@xxxxxxxxxx (Floyd L. Davidson)
- Date: Wed, 15 Mar 2006 06:28:57 -0900
andrew.bell.ia@xxxxxxxxx wrote:
Hi,
I'm trying to understand the behavior of a configuration where I have
multiple NICs on the same subnet. (I know, its wrong, I'm just trying
to understand.)
Here's the configuration:
eth0 : 172.16.11.4
eth1: 129.143.4.5
eth2: 129.143.4.6
$ netstat -rn
Kernel IP routing table
Destination Gateway Genmask Flags MSS Window
irtt Iface
129.143.4.0 0.0.0.0 255.255.255.0 U 0 0
0 eth1
The fact that this entry for eth1 comes before the next entry, for
eth2, means that (since the routing is identical), all packets to
the 129.143.4.x subnet are routed via eth1, and none are routed to
eth2.
129.143.4.0 0.0.0.0 255.255.255.0 U 0 0
0 eth2
No packets are *ever* route to this interface.
172.16.11.0 0.0.0.0 255.255.255.0 U 0 0
0 eth0
All packets to the 172.16.11.x subnet are routed to this interface
0.0.0.0 129.143.4.254 0.0.0.0 UG 0 0
0 eth1
This is a "default" route, which sends anything not matched by the
above routes to IP address 129.143.4.254 via the eth1 interface.
eth2 is not cabled. eth0 and eth1 are cabled.
That will be fine, given the above route table.
When you ping the address of eth2 (129.143.4.6), you get a response.
If sent on the same machine, the packet actually goes to the lo
device, not the eth2 device! The internals of the TCP/IP stack
are of course aware of any address assigned to an interface on
this machine, and any packet routed to an IP on the machine goes
to the lo device. (Do and "ifconfig lo" command, note the
counters, do a ping to any local IP address, and then check the
couters again with ifconfig. You'll see that they increment
nicely...)
If you look at the arp entry on the machine that originated the ping
for 129.143.4.6, you get the MAC of eth0, which isn't even on the same
logical network. No proxy ARP is turned on.
Can someone explain what is happening inside the stack to make this
happen?
Sounds as if you have IP forwarding enabled, and the machine
from which you sent the ping is gatewayed to 129.143.4.6 via
eth0 (172.16.11.0).
Ping that address again, and check to lo device counts as described
above. Interesting...
--
Floyd L. Davidson <http://www.apaflo.com/floyd_davidson>
Ukpeagvik (Barrow, Alaska) floyd@xxxxxxxxxx
.
- References:
- Multiple NICS on same subnet
- From: andrew . bell . ia
- Multiple NICS on same subnet
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