[NBLUG/talk] Strange ethernet issue

Steve Johnson fratm at adnd.com
Tue Apr 10 15:20:26 PDT 2012


I just don't see the point in the arp query when I am sitting in the
same room as all the gear and I can see what is plugged into the
switch.  Is there a point that I am missing?

-Steve


On Tue, Apr 10, 2012 at 3:17 PM, Kyle Rankin <kyle at nblug.org> wrote:
> On Tue, Apr 10, 2012 at 09:59:46AM -0700, Steve Johnson wrote:
>> Yes, I physically checked the switch (Cisco switch.. not sure on
>> model).. Also just to be sure I ran mmap on the IP of one the boxes
>> when it was down and nothing came back.  I know pings are unreliable,
>> but nmap isn't supposed to use just IMCP, so it should have detected
>> something if someone got on my network.
>>
>> I will try the arp queries after tonights reboot.. These machines are
>> production machines, so can't be down long in the middle of the day..
>> :)
>>
>> -Steve
>
> Even if the machine is up, you might get interesting information from an
> arp query from a different host on the same subnet. Perform the arp query
> from a different host and confirm that you get back the MAC address you
> expect.
>
> -Kyle
>
>>
>>
>> On Tue, Apr 10, 2012 at 9:54 AM, Kyle Rankin <kyle at nblug.org> wrote:
>> > On Tue, Apr 10, 2012 at 09:40:31AM -0700, Steve Johnson wrote:
>> >> Hi Guys,
>> >>
>> >> I am running 3 linux boxes all on the same network, running static 10
>> >> net addresses, each on their own IP address..  A strange thing has
>> >> started happening about a month ago, if I reboot the box when the
>> >> system comes up at the point when it tries to bring up the eth0
>> >> interface I get an error "IP Address in use by another host" and then
>> >> the interface does not come up.  Loggin in from the console as root
>> >> and running ifup eth0 gives me the same error.  The only way I can get
>> >> the interface to come up is to physically unplug the ethernet, then
>> >> run ifup eth0, that brings up the eth0 correctly, and then plug the
>> >> ethernet cable back in.. Then it runs fine until another reboot (Or if
>> >> I ifdown eth0 I will have the same problem)..
>> >>
>> > <snip>
>> >>
>> >> Ay ideas, or clues would be greatly appreciated.. I've been trying to
>> >> trouble shoot this for over a month now with now luck.
>> >>
>> >> -Steve
>> >>
>> >
>> > Are you absolutely sure that only one host truly has those IP addresses on
>> > that subnet? When the host comes up and tries to assign the IP addresses to
>> > itself, it will first perform an ARP and see if another MAC address on the
>> > network claims to have that IP. What I would do is take down one of the
>> > hosts, then from a different machine run ARP queries for those 10 IPs
>> > belonging to the first host and see if the MAC address you get back is the
>> > correct one. If your networking guys are trying to do anything fancy with
>> > NAT and misconfigured something, it could be that your switch is claiming
>> > to have those IPs (it's easy to check, an arp query against one of the IPs
>> > will return back a MAC belonging to a Cisco, HP, or whatever switch you
>> > have).
>> >
>> > --
>> > Kyle Rankin
>> > NBLUG President
>> > The North Bay Linux Users' Group
>> > http://nblug.org
>> > IRC: greenfly at irc.freenode.net #nblug
>> > kyle at nblug.org
>> >
>
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