Re: arp table - same mac address shows two ip addresses

Hi Leroy,

On Thu, Oct 18, 2018 at 02:05:20PM +0000, Leroy Tennison wrote:
> Erik Auerswald wrote "One example I experienced are misconfigured end-systems ..." - Could you explain this a little more thoroughly?  Are you saying that, given a system with 172.16.30.1 address on network A and 10.20.30.1 address on network B that an application is sending to the A network using the 10... address rather than the 172... address?  If so was the application configured to use a particular IP address?

What I meant was a host configured for network A but connected to
network B:

ES 10.20.30.47/24 <-----> GW Iface 172.16.30.1/24

The gateway (GW) did have the IP address 10.20.30.1/24 on another
interface, and the end-system (ES) used that IP address as its default
gateway. ES and GW could ping each other. The GW was a PC with Linux
kernel and GNU userland. Thus the "application" on the multi-NIC PC
was Linux.

> You also wrote "The gateway for both networks was based on the Linux kernel."  Could you be a little more specific?  Were multiple routing tables being used along with 'ip rule' entries or was it a "nexthop with weights" situation or something else?

Just one routing table. A PC with several NICs, GNU/Linux, and
ipforwarding acting as a "router".

> I'm just trying to better understand the situations you encountered so I can recognize them in the future.

In the above mentioned situation, of which I have forgotten most of
the details, end-systems that should no longer have been able to reach
their old Linux-based default gateway, because IP addresses and VLANs
were changed, but the end-systems used the pre-change configuration,
still had full network connectivity.

When using Linux on a PC with several NICs, expect the unexpected. ;)

Thanks,
Erik
-- 
Do things that have never been done before.
                        -- Russell Kirsch