strange ARP problem

[Glenn Dawson]

At 08:34 PM 3/17/2006, ray at redshift.com wrote:
>I'm having a strange issue here and thought maybe someone on this list might
>have some ideas.  I have tried to figure it out for a couple of days, but no
>luck yet.  The problem seems to be around reporting of arp information.
>
>Here is my basic config.  I have my workstation (a windows XP box) with 2 IP's
>on a private network segment (both with /24 subnet masks)
>
>192.168.10.250
>192.168.20.250
>
>the 10.250 and 20.250 are connected out to a small switch. Also connected to
>that small switch is a mail server as shown below.
>
>[ workstation  ]                         [ mail server  ]
>[192.168.10.250]-------[ small  ]--------[ 192.168.10.15]
>[192.168.20.250]-------[ switch ]--------[ 192.168.20.15]
>                            |
>                            |
>                   [router 192.168.10.1]
>                            |
>                        public IP
>
>10.15 handles SMTP to the public, 20.15 is for admin and POP to/from the
>workstation on 20.250
>
>Okay, so the problem is that when I fire up the Workstation (it's running
>Windows XP), the arp data for 192.168.20.15 comes back with the incorrect Mac
>address.  It ends up with the Mac address for 10.15, instead of 20.15 - which
>keeps the machines from talking correctly.  If you delete the ARP table and
>re-arp, then it's perfectly fine from then on.  Totally odd.
>
>Then the other night I noticed the following errors (see below) from the mail
>server.  It seems to be related, but I can't pin point the source or 
>what might
>cause something like this.
>
>Does anyone have any ideas what could be causing this?
>
> > arp: 192.168.10.1 is on fxp0 but got reply from 00:30:48:52:08:03 on bge0
> > arp: 192.168.20.250 is on bge0 but got reply from 00:e0:81:32:e0:a0 on fxp0
> > arp: 00:30:48:51:ce:f0 is using my IP address 192.168.20.15!
> > arp: 00:30:48:51:ce:f0 is using my IP address 192.168.20.15!
> > arp: 192.168.10.1 is on fxp0 but got reply from 00:30:48:52:08:03 on bge0
> > arp: 00:30:48:51:ce:f0 is using my IP address 192.168.20.15!
> > arp: 192.168.10.15 is on lo0 but got reply from 00:30:48:51:ce:f0 on bge0
> > arp: 192.168.10.1 is on fxp0 but got reply from 00:30:48:52:08:03 on bge0
> > arp: 192.168.10.15 is on lo0 but got reply from 00:30:48:51:ce:f0 on bge0
> > arp: 192.168.20.250 is on bge0 but got reply from 00:e0:81:32:e0:a0 on fxp0
> > arp: 192.168.10.15 is on lo0 but got reply from 00:30:48:51:ce:f0 on bge0
> > arp: 192.168.10.1 is on fxp0 but got reply from 00:30:48:52:08:03 on bge0
>
>here is the ifconfig from the mail server:
>
>[ray at mail ray]$ ifconfig
>fxp0: flags=8843<UP,BROADCAST,RUNNING,SIMPLEX,MULTICAST> mtu 1500
>         inet 192.168.10.15 netmask 0xffffff00 broadcast 192.168.10.255
>         ether 00:30:48:51:ce:f0
>         media: Ethernet autoselect (100baseTX <full-duplex>)
>         status: active
>bge0: flags=8843<UP,BROADCAST,RUNNING,SIMPLEX,MULTICAST> mtu 1500
>         options=1b<RXCSUM,TXCSUM,VLAN_MTU,VLAN_HWTAGGING>
>         inet 192.168.20.15 netmask 0xffffff00 broadcast 192.168.20.255
>         ether 00:30:48:51:ce:f1
>         media: Ethernet autoselect (100baseTX <full-duplex>)
>         status: active
>lo0: flags=8049<UP,LOOPBACK,RUNNING,MULTICAST> mtu 16384
>         inet 127.0.0.1 netmask 0xff000000
>
>If anyone has any idea, please let me know.  Thanks!

This is exactly why it's ill-advised to have two network interfaces 
on different networks connected to the same physical network.

If you actually need two different networks (although from your 
description I don't see a reason why you would) then use a single 
physical interface and assign it an IP from each network.  Or, get a 
switch that has VLAN capabilities and keep the two networks separated.

-Glenn


>Ray
>
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