[Bridge] Incoming packets not always traversing the bridge

Robert LeBlanc robert at leblancnet.us
Wed Jan 20 12:30:09 PST 2010

On Wed, Jan 20, 2010 at 1:04 PM, Brad Hudson <hudson at pythian.com> wrote:

> Hi all;
> I have an odd problem that I have been dealing with for a week.  I was
> hoping someone could help, or point me in the right direction for clues.
> I have a standard bridge setup.  br0 is composed of eth0 and eth1.
> # brctl show bro
> bridge name     bridge id               STP enabled     interfaces
> br0             8000.000c292280b9       no              eth0
>                                                        eth1
> Eth0 and eth1 both have (no) address assigned and are up.  br0
> is assigned the proper IP and the routing table is correct.  STP is off.
> I have been losing connectivity to hosts inside the local segment of the
> bridge.  Some investigation has revealed that the problem is related to
> arp not working correctly.  Arp packets going this way
> eth1->br0->eth0->network/internet
> have no problems at all.  The replies coming back the other way all get
> to br0, but only 33% (approx, it varies) make it to the eth1 side of the
> bridge.  I have verified this traffic pattern by tcpdump of arp packets
> through each of these devices while doing an nmap -sP of the /24 network
> to generate both arp and icmp.  We are not able to arp any host outside
> our local segment, including the default gateway (which is owned by the
> co-lo).  nmapping from the bridging server itself from interface br0
> gets the correct number of arp replies.
> ebtables and arp_tables are not running, and adding them in has had no
> change in result.  There was a server with 2 NICs, each with an IP on
> the same subnet, that was causing some MAC flapping but that has been
> fixed and no change to the described behaviour.  All items in
> /proc/sys/net/bridge are set to '1', but setting them to '0' has no
> effect.  The server hosting the bridge has been rebooted several times
> with no effect.  proxy_arp does not help at all.  I also tried
> parprouted with no success.
> A couple other notes.
> - This behaviour suddenly appeared about a week ago.  I think this is
> probably related to an increase in network traffic but it's hard to say,
> the client does not buy into that statement.  If it was a matter of 0
> work or all work then there's places to look for that, but in this case
> the problem is intermittent and the lost arp replies are not the same
> every time.
> - In another test we found that if we ping the inside server from the
> firewall and also from an external machine the connectivity to the
> inside server dies.  Once the pings are stopped, the connectivity
> eventually returns.  If I ping out from the inside server while doing
> that test, the session keeps going through without hanging.
> - The firewall is a Vm running under ESX.  The vmxnet driver has been
> reinstalled and the pcnet32 driver is not loaded.  Both NICs are virtual
> so there is no chance of failed hardware, though I suppose the problem
> could be on the ESX layer.  I have made some attempt to diagnose the WSX
> layer but nothing jumps out at me.
> I have been watching tcpdumps and do not see any sign of frags, dupes,
> or anything that would cause lost packets.  I have combed the
> newsgroups, google and even irc looking for clues or similar situations,
> but nothing I have found fits the profile.
> The workaround we currently have in place is to make a static arp entry
> for the gateway on all servers on the inside.  This is not ideal because
> the co-lo controls the router and it could fail over to another device
> which would kill our route again.
> Can anyone suggest anyplace I can look for clues, settings I should
> check or other?  I am out of ideas at this point.
> Your help is very much appreciated.
> Regards;
> Brad
> --
> Brad Hudson
> SA Team Lead
> The Pythian Group - love your data
> Desk: 613-565-8696 x202
> IM: pythianhudson
I assume you have multiple physical NICs connected to your virtual switch.
If so I've posted my finding on my web page http://robert.leblancnet.us and
I've posted a message to this form two days ago entitled "Need help writing
ebtables rules". I'm not sure my messages are getting through as I've sent a
few messages with no one responding. If we can work together to solve the
problem, we can both benefit.


Robert LeBlanc
Life Sciences & Undergraduate Education Computer Support
Brigham Young University
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