Re: Re; [Xen-users] Ethernet has Alzheimers

[Matthew Crocker <mcrocker@xxxxxxxxxxx>]

IPTables rules blocking ARP?
Is the same IP address being used by multiple hosts?

If you have access to the switch you can do 'show mac-address- 
table' (assuming cisco) and see what port has that MAC address.
Also, FE:FF:FF:FF:FF:FF is not a valid MAC address, do you have  
something that is generating bogus MACs?  FF:FF:FF:FF:FF:FF is the  
Ethernet broadcast address (frames go to all ports on the switch/ 
vlan).  For obvious reasons you don't want to use the Ethernet  
broadcast address for your machines MAC.
-Matt

On Jan 7, 2008, at 6:11 PM, Gareth Bult wrote:

> Ok, I've managed to pin it down and you are quite right - it's ARP.
>
> Now the question is, how do I fix it.
>
> here's what I have
>
> Dom0 :: 10.0.0.1
> DomU :: 10.0.0.12
>
> Both machines work fine for 40 mins .. then;
>
> DomU reports Dom0 unreachable.
> Sure enough ping 10.0.0.1 gives no response.
> However, ping 10.0.0.12 from Dom0 responds fine.
> A one-way ping!
>
> arp -na on Dom0 reports as expected.
> arp -na on the broken DomU shows;
> ? (10.0.0.1) at FE:FF:FF:FF:FF:FF [ether] on eth0
>
> It's picking up FE:EE ... instead of the desired MAC address ?!
>
> How can it do this ?!
>
> On Dom0:
> eth0      Link encap:Ethernet  HWaddr 00:15:C5:5D:C0:BE
>          inet addr:10.0.0.1  Bcast:10.0.0.255  Mask:255.255.255.0
>          inet6 addr: fe80::215:c5ff:fe5d:c0be/64 Scope:Link
>          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
>          RX packets:205397 errors:0 dropped:0 overruns:0 frame:0
>          TX packets:413848 errors:0 dropped:0 overruns:0 carrier:0
>          collisions:0 txqueuelen:0
>          RX bytes:41267633 (39.3 MB)  TX bytes:95050228 (90.6 MB)
>
> On DomU:
> eth0      Link encap:Ethernet  HWaddr 00:00:10:00:00:0C
>          inet addr:10.0.0.12  Bcast:10.0.0.255  Mask:255.255.255.0
>          inet6 addr: fe80::200:10ff:fe00:c/64 Scope:Link
>          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
>          RX packets:6297 errors:0 dropped:0 overruns:0 frame:0
>          TX packets:11662 errors:0 dropped:0 overruns:0 carrier:0
>          collisions:0 txqueuelen:1000
>          RX bytes:1157351 (1.1 MB)  TX bytes:907972 (886.6 KB)
>
> Help!
>
>
>
>
> --
> Managing Director, Encryptec Limited
> Tel: 0845 25 77033, Mob: 07853 305393, Int: 00 44 1443205756
> Email: gareth@xxxxxxxxxxxxx
> Statements made are at all times subject to Encryptec's Terms and  
> Conditions of Business, which are available upon request.
> ----- Original Message -----
> From: "Matthew Crocker" <mcrocker@xxxxxxxxxxx>
> To: "Gareth Bult" <gareth@xxxxxxxxxxxxx>
> Cc: xen-users@xxxxxxxxxxxxxxxxxxx
> Sent: Monday, January 7, 2008 10:00:40 PM (GMT) Europe/London
> Subject: Re: Re; [Xen-users] Ethernet has Alzheimers
>
> > > Is the machine answering ARP replies?
> > Honestly, I don't know .. the machine tends to lock up for other
> > reasons when it dies hence it's not easy to track ..
> >
> > > Does the upstream router have the IP & MAC in its ARP table?
> > > Does the upstream switch have the MAC in its mac-address-table?
> > > Assigned to the correct port?
> > > Sounds to me like an ARP timeout problem.
> > This occurs between DomU's and Dom0 in addition to external
> > addresses ... so I don't think it's linked to anything outside of
> > Xen .. I've experienced the same problem on 4 different machines,
> > all different HW config .. so again I think faulty HW is out.
> >
> > For what it's worth;
> >
> > I'm using Ubuntu Gutsy (7.10) with the stock Xen 3.1 kernel all on
> > AMD64 and Intel/Xeon machines all running 64 bit kernels and distros.
> >
> > All machines are using bridging with two physical ethernet ports.
> > All DomU's are running two matching virtual ports.
> > I'm using IPTABLES (firehol) fairly heavily for port filtering.
> Bridging is Layer2,  IP is Layer 3, you are having a problem at layer
> 3 so you need to look to make sure your layer 2 stuff is working
> properly.
>
> If Xen is bridging only then you won't really have visibility into the
> Layer 3 problem from Dom0.  You could look at the bridging config and
> see if it knows about the MAC address properly in the switch.   At
> some point upstream from the Xen hardware you have another Layer 3
> device,  most likely a router.  You need to get into that router and
> see if it has the IP -> MAC entry in its ARP table.  If it doesn't
> have it then there is your problem.  Something is stopping the DomU
> from answering the ARP queries from the router.  The route loses track
> of the MAC address and can no longer send Ethernet frames to your
> DomU.  If your router does have the ARP entry then I would look into
> your switches and see if they are dropping the MAC address from their
> table.
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