I recently build linux based bridge for packet monitoring purpose, but there is BIG problem.

the environment is,

  1. overall env.
    • target of monitoring is VMs on vSphere.
    • two vSwitches are configured on vSphere host.
    • vSwitch 1 is configured with NIC for outside-bridge communication.
    • vSwitch 2 is configured without NIC for bridge-vm connection.
    • both are configured as "Allow Promiscous mode".
  2. env. of bridge.
    • based on ubuntu 13.10, installed as minimal virtual machine.
    • br0 was configured with eth0(to vS1) and eth1(to vS2)

my problem is, when VM ping to GW, the ARP Request is made and there is a response from GW. but the response packet is shown only on eth0 and br0.

superhero@vim-firewall:~$ sudo tcpdump -i eth0 -n host
tcpdump: WARNING: eth0: no IPv4 address assigned
tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on eth0, link-type EN10MB (Ethernet), capture size 65535 bytes
12:13:45.809949 ARP, Request who-has tell, length 46
12:13:45.810060 ARP, Request who-has tell, length 46
12:13:45.810742 ARP, Reply is-at 00:00:aa:aa:aa:d9, length 46
superhero@vim-firewall:~$ sudo tcpdump -i br0 -n host
tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on br0, link-type EN10MB (Ethernet), capture size 65535 bytes
12:13:51.810928 ARP, Request who-has tell, length 46
12:13:51.811031 ARP, Request who-has tell, length 46
12:13:51.811579 ARP, Reply is-at 00:aa:aa:aa:aa:d9, length 46
superhero@vim-firewall:~$ sudo tcpdump -i eth1 -n host
tcpdump: WARNING: eth1: no IPv4 address assigned
tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on eth1, link-type EN10MB (Ethernet), capture size 65535 bytes
12:13:57.812937 ARP, Request who-has tell, length 46
12:13:57.813040 ARP, Request who-has tell, length 46

I need help!

PS. currently, only ARP is problem. if I add a GW's MAC manually, network connection is fine, except access to local subnet ofcause.

  • You haven't really described your network setup that well. I don't understand when you start talking about standby NIC or where two physical NICs come from. You say "when VM pings" but which VM, the bridge or one of the VMs behind vSW2? You could provide a much better description, with diagrams and configs. – suprjami Oct 4 '14 at 8:19
  • If you are putting two external interfaces from the same broadcast domain into a bridge, then you're creating a network loop. This is not correct. – suprjami Oct 4 '14 at 8:19
  • Thanks for comment and sorry for my bad description @suprjami. anyway, I only used the word VM for the VMs inside the bridge, connected only to vSwitch2, and all ethN or brN is on bridge-vm (or hostname vim-firewall) in this description of problem. – sio4 Oct 4 '14 at 10:12
  • and I used the word NIC for physical NICs that configured as Physical NIC of vSwitch1. and I already metioned on my answer below, It works fine without Standby NIC for vSwitch1 and there is no problem or looping on the network. – sio4 Oct 4 '14 at 10:18

Oops! I finally found the working solution!

I spent about 10+ hours spreaded on three days for this problem, but the solution is not clean, is closer to work-around. I need real solution or better work-around for this. please help.

anyway, I found below information from vmware cummunity. (root cause of the problem is related to vmware.)

  1. https://communities.vmware.com/message/1509541#1509541
  2. https://communities.vmware.com/message/2208190#2208190

URL (1) is a comment from original author who has same problem, about the reason of the problem. the problem is caused by vSwitch's behavior. if there is two or more physical NIC attatched, they made duplicated ARP request and the linux bridge receives dup.ed request from outside NIC/port. so it fall in confusion of MAC-port map.

I did test for this (dettach standby NIC from vSwitch) then networking works fine. (with single NIC vSwitch)

another comment, URL number (2) describes an work-around. if I set ageing time to 0 from linux bridge, it works as dummy hub(send all packets to all ports), so ARP response is reached to VM on internal network.

In my case, my bridge has only two ports for internal and external connection, so it is not a big problem IMHO. but there is something not clear.

If there is a way to block loop-backed/duplicated request from standby NIC, or ignore duplicate request or other clever way to handle MAC table of bridge, let me now.

Thanks to read and hope to help your same problem!


I found another working solution of this problem! https://communities.vmware.com/message/2141853#2141853

You can set Advanced System option of Esxi host: Net.ReversePathFwdCheckPromisc = 1 This option disable duplication of packet on vSwitch with multiple uplinks and portgroup in promiscuous mode.

I hope this information help you.

  • Oh! I found that I just up voted on your answer and did not make any thanks message. Thanks for your kind information! @vadim-aleksandrov – sio4 Oct 20 '17 at 13:37

In VMWARE ESXI 6.x 7.x it is necessary to leave the accept mode in the following options: "Promiscuous Mode" and "Forged Transmits"


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