Network Puzzle with 2 NICs

Hi,

"Now you see it, now you don't". I have 2 PCs connected on a HUB.
One is a Windows box, the other one is a RH 9.1.

The Setup:
----------
PC1:
RH9.1
eth0 (e100) 192.168.0.100, 255.255.255.0
eth1 (3c59x) 192.168.0.101, 255.255.255.0

PC2:
Windows, one NIC, 192.168.0.10, 255.255.255.0

The Test:
---------
I can ping any nic from any other nic. If I type "ifdown eth0",
Windows can no longer ping eth0 but it can PING eth1 and eth1 can PING
Windows. Same thing with "ifdown eth1". Life is good.

The Problem:
------------
If I disconnect the eth1 network cable, I can't PING any NIC from
anywhere. Strange, this should be the same result as "ifdown eth1"
right? So I put back the eth1 network cable and I disconnect eth0.
This time I can PING any NIC from anywhere. Geeezzz

Question: Why? Is this something about "routing tables"? Any help
will be appreciated.

Thanks.

I bet 2 cents
o
circuit_breaker wrote:
> PC1:
> RH9.1
> eth0 (e100) 192.168.0.100, 255.255.255.0
> eth1 (3c59x) 192.168.0.101, 255.255.255.0

You have two interfaces on two physical networks but they belong
to the _same_ logical network 192.168.0.0/24.

For that to work your PC1 has to be a _bridge_
>
> PC2:
> Windows, one NIC, 192.168.0.10, 255.255.255.0

I'm not saying that this is impossible, but need more knowledge
about networking ;-)

My suggestion create two logical networks like 192.168.0.0/24 and
192.168.1.0/24

Have fun!

Arun

circuit_breaker wrote:
> The Setup:
> ----------
> PC1:
> RH9.1
> eth0 (e100) 192.168.0.100, 255.255.255.0
> eth1 (3c59x) 192.168.0.101, 255.255.255.0
>
> PC2:
> Windows, one NIC, 192.168.0.10, 255.255.255.0
>
> The Test:
> ---------
> I can ping any nic from any other nic. If I type "ifdown eth0",
> Windows can no longer ping eth0 but it can PING eth1 and eth1 can PING
> Windows. Same thing with "ifdown eth1". Life is good.
>
> The Problem:
> ------------
> If I disconnect the eth1 network cable, I can't PING any NIC from
> anywhere. Strange, this should be the same result as "ifdown eth1"
> right? So I put back the eth1 network cable and I disconnect eth0.
> This time I can PING any NIC from anywhere. Geeezzz
>
> Question: Why? Is this something about "routing tables"? Any help
> will be appreciated.

Please send the routing tables of Your linux box ("route -n"), with
both interfaces activated.

You run Yourself into trouble when connecting two NICs in one box to
the same logical subnet.

In Your case, I predict that if You pull the cable _and_ ifdown eth1,
You will be able to ping eth0 again.

The reason for that is simple: When adding a NIC with ifconfig, the
kernel routing table will be updated accordingly. Say, when You add
one NIC with IP 192.168.0.100/24, You will get a route to that subnet
via that NIC. So far, so good. - If now You add a second NIC with, say,
192.168.0.101/24, You'll get the same result, i. e. You now have two
routes to that same subnet. But, only the first matching rule in the
table will be used by the kernel (with no respect to whether the link
is functional or not).

What You're seeing is, in fact, that when both NICs are up and connec-
ted, Your ICMP echo request ("ping") will get to the respective inter-
face in Your linux box, but from there, the ICMP echo reply ("pong")
will always come from one and the same NIC, namely that one that the
route is set for first. - So even if You ping eth0, the reply may come
from eth1, and vice versa.


Cheers, Jack.

--
----------------------------------------------------------------------
My personal reading of the string "MicroSoft" expands to "NanoWeak"...

Arun Dev <nospam@pleaz.xy> wrote in message news:<40769e7b$1_1@news.tiscalinet.ch>...
> I bet 2 cents
>
> My suggestion create two logical networks like 192.168.0.0/24 and
> 192.168.1.0/24

Do you mean that each NIC should be on its own network? The subnet
mask is not enough specific in that case?

Thanks.

jack <not@all.org> wrote in message news:<c56e2p$sup$07$1@news.t-online.com>...
>
> Please send the routing tables of Your linux box ("route -n"), with
> both interfaces activated.
>
> You run Yourself into trouble when connecting two NICs in one box to
> the same logical subnet.
>
> In Your case, I predict that if You pull the cable _and_ ifdown eth1,
> You will be able to ping eth0 again.
>
> The reason for that is simple: When adding a NIC with ifconfig, the
> kernel routing table will be updated accordingly. Say, when You add
> one NIC with IP 192.168.0.100/24, You will get a route to that subnet
> via that NIC. So far, so good. - If now You add a second NIC with, say,
> 192.168.0.101/24, You'll get the same result, i. e. You now have two
> routes to that same subnet. But, only the first matching rule in the
> table will be used by the kernel (with no respect to whether the link
> is functional or not).
>
> What You're seeing is, in fact, that when both NICs are up and connec-
> ted, Your ICMP echo request ("ping") will get to the respective inter-
> face in Your linux box, but from there, the ICMP echo reply ("pong")
> will always come from one and the same NIC, namely that one that the
> route is set for first. - So even if You ping eth0, the reply may come
> from eth1, and vice versa.
>
>
> Cheers, Jack.

Here's my routing tables:
-------------------------
Destination Gateway Genmask Flags Metric Ref
Use Iface
192.168.0.0 0.0.0.0 255.255.255.0 U 0 0
0 eth1
192.168.0.0 0.0.0.0 255.255.255.0 U 0 0
0 eth1
169.254.0.0 0.0.0.0 255.255.0.0 U 0 0
0 eth1
127.0.0.0 0.0.0.0 255.0.0.0 U 0 0
0 lo
0.0.0.0 192.168.0.1 0.0.0.0 UG 0 0
0 eth1

This morning, it behaves differently. While both NICs are active,
"ifdown eth1" makes both interfaces unable to communicate with PC2.
Then I did "ifdown eth0" and of course, still no reply from any
interface, and then "ifup eth1". Guess what, eth1 was replying... :)

Looks like there's something to add or remove in those routing tables.
Thanks for your help

circuit_breaker wrote:
> Here's my routing tables:
> -------------------------
> Destination Gateway Genmask Flags Metric Ref
> Use Iface
> 192.168.0.0 0.0.0.0 255.255.255.0 U 0 0
> 0 eth1
> 192.168.0.0 0.0.0.0 255.255.255.0 U 0 0
> 0 eth1
> 169.254.0.0 0.0.0.0 255.255.0.0 U 0 0
> 0 eth1
> 127.0.0.0 0.0.0.0 255.0.0.0 U 0 0
> 0 lo
> 0.0.0.0 192.168.0.1 0.0.0.0 UG 0 0
> 0 eth1

Is that Your routing table? Did You "cut and paste"?

See that there is a duplicate entry at the top. So everything that goes
into that 192.168.0/24 subnet will be sent via eth1. If the second entry
had eth0 as device, it would not change things because only the first
matching entry will be used. From the kernels point of view: It gets a
packet that shall be sent to, say, 192.168.0.20. The kernel will look at
its routing table and find that 192.168.0/24, which .20 belongs to, can
be accessed via eth1. Good, so that packet will be shoved out via eth1,
done. All other entries in that routing table will not be used, because
in a properly configured system, there's exactly one route to any given
destination. So if one route matches the target, all other rules will
not match, so there's no need to even bother.


> This morning, it behaves differently. While both NICs are active,
> "ifdown eth1" makes both interfaces unable to communicate with PC2.
> Then I did "ifdown eth0" and of course, still no reply from any
> interface, and then "ifup eth1". Guess what, eth1 was replying... :)

Yes, have a look at the routing table after taking down eth1. Normally,
ifconfig will remove all routes from the table that refer to the inter-
face that is being taken down. In Your case, all routes to 192.168.0/24
plus the default route. Since there was no route to Your subnet via eth0
anymore, that subnet is unreachable for Your box. You still can receive
incoming packets via eth0, no problem, but You cannot send replies. Bad
thing in a two-way communication...


> Looks like there's something to add or remove in those routing tables.

Normally, when bringing up a NIC, a route will be added to the routing
table based on the information You provide to configure the card. If
You specify its IP to be 192.168.0.100, and the netmask 255.255.255.0,
this information is sufficient to know that everybody in that subnet
can be reached via that respective NIC.

Try this: After adding or removing ("up"-ing and "down"-ing) the NICs,
do a "route -n" as a first thing. Try to find how the kernel will try
to send data into that subnet of Yours. Then, generate such traffic and
see whether Your guess was right or not.

Again, if the route to Your subnet is through eth1, You cann still send
things to eth0 from that other box. In any case, the reply will come
from eth1, and hence things will fail when You pull the plug from eth1.
eth0 will not send away anything at all, except for ARP, perhaps.


Hope that clears things a bit, Jack.

--
----------------------------------------------------------------------
My personal reading of the string "MicroSoft" expands to "NanoWeak"...

jack <not@all.org> wrote in message news:<c591a6$kbe$02$1@news.t-online.com>...

> Is that Your routing table? Did You "cut and paste"?

Yes.

> Hope that clears things a bit, Jack.

You did more than I expected. It makes sense to me now, Thanks a lot!