 | | | Oddity with Ping or routing under AS3 | Oddity with Ping or routing under AS3 2004-01-21 - By j.lane
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Hey everyone... I have an odd question that I cant seem to figure out...
I am not sure that this is specific to Taroon or not, but here goes:
First, the servers in question are different makes and models... all are
running RHEL 3 AS. I have not tested this on any other release... but
architectures are x86, IA-64, and AMD64.
For setup, there are multiple NICs in each machine. This is so far
reproducable on every machine I try it on.
on the test lan, we use different Class C subnets with the standard
Mask, so for example, the ifconfig would look something like this:
eth0 192.168.0.170 255.255.255.0
eth1 192.168.1.170 255.255.255.0
eth2 192.168.2.170 255.255.255.0
and so on (one test machine currently has 10 ethernet interfaces on it.)
So, the problem is with ping. I was under the impression that with that
netmask, .0.XXX packets would only go to .0.XXX machines, and .1.XXX
packets would only go to .1.XXX machines, and so on.
On the test side (the machines that do the testing) each one has a
single ethernet port that has multiple IPs assigned as such:
eth0 192.168.0.XX 255.255.255.0
eth0:1 192..168.1.XX 255.255.255.0
eth0:2 192.168.2.XX 255.255.255.0
and so on.
Now, if I ping from the server to 192.168.0.XX without specifying an
interface, packets go out the way they should... (i.e. the .0.170
interface sends packets out to .0.XX, .1.170 sends packets out to .1.XX
and so on)
If I ping using -I like so: ping -I eth1 192.168.1.XX things go as they
should again. Packets go out the .1.170 (eth1) interface.
Now, I bring eth0 down so I only have eth1 up at 192.168.1.170. if I
ping without -I, packets are NOT sent out, since there is no route to
.0.XX with eth0 down.
BUT, if I ping -I eth1 like this: ping -I eth1 192.168.0.XX, packets are
not only sent out the .1.170 interface, but they are recieved AND
returned by .0.XX.
So if I understand the setup correctly, I am sending packets out of an
interface on the 192.168.1.XXX subnet, and those packets are being
recieved by the client test machine, but are being returned wiht a from
address of 192.168.0.XX.
It was my understanding that if I have an interface configured to
192.168.1.XXX, and i try sending packets to a different network (in this
case 192.168.0.XXX,) the packets should never be recieved, because the
.0.XXX net should not be able to see packets send from a machine on the
.1.XXX net.
So I guess the real question is, why is this happeneing? Is this an
artifact of aliasing ethernet addresses on the recieving NIC? Or am I
missing something here in the subnetting???
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