[Bridge] Incoming packets not always traversing the bridge

Brad Hudson hudson at pythian.com
Wed Jan 20 13:29:36 PST 2010


I looked over your site and could not find the document you reference in
your links.  Can you provide the url to get to it?

I'll be happy to pass along anything I find and would appreciate it if
you would do the same.  As the client having the issue is using it for
production we may need to move to a non-bridged variety of transparent
firewall with proxy_arp to get them back up quickly.  Ideally I would
like to avoid that, but it's prod and needs to work.



Robert LeBlanc wrote:
> On Wed, Jan 20, 2010 at 1:04 PM, Brad Hudson <hudson at pythian.com
> <mailto: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.
> Thanks,
> Robert LeBlanc
> Life Sciences & Undergraduate Education Computer Support
> Brigham Young University 

Brad Hudson
SA Team Lead
The Pythian Group - love your data
Desk: 613-565-8696 x202
IM: pythianhudson

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