I'm trying to get Linux bonding working over a VPN (GRE-TAP). The funny thing is, that it only works when I have tcpdump running on both hosts, but more on that later...

There are two machines, called pxn1 and pxn2. They are connected together using a simple switch via eth1.

pxn1 has IP address
pxn2 has IP address


To get a secure connection, all IP traffic is encrypted using IPsec. This works, I can ping between the two machines without any problem and tcpdump shows only encrypted packets.


A GRE-TAP (tunnels Ethernet frames over IP) interface is then set up in both directions, because I will need a virtual network interface later on:

ip link add vpn_gre_pxn2 type gretap local remote dev eth1

ifconfig shows:

vpn_gre_pxn2 Link encap:Ethernet  HWaddr 1a:73:32:7f:36:5f
          inet6 addr: fe80::1873:32ff:fe7f:365f/64 Scope:Link
          RX packets:19 errors:0 dropped:0 overruns:0 frame:0
          TX packets:26 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000
          RX bytes:1294 (1.2 KiB)  TX bytes:1916 (1.8 KiB)

This is on pxn1. On the other host the same interface is set up in the other direction.


A bridge is set up that currently uses only the GRE-TAP device.

I need the bridge because later on I want to add more machines (my plan is to bridge all GRE tunnels together). The end result should become a VPN mesh network (with a dedicated GRE-TAP interface for each host-host combination). But since for now I'm just doing a first test with two machines, the bridge is of course somewhat useless, but nonetheless important for the test itself.

brctl addbr vpn_br
brctl addif vpn_br vpn_gre_pxn2

The bridge works because when I activate the vpn_br interface and set up some IP addresses (just for testing the bridge), ICMP PINGs work perfectly.

vpn_br    Link encap:Ethernet  HWaddr 02:00:0a:01:01:c5
          RX packets:11 errors:0 dropped:0 overruns:0 frame:0
          TX packets:6 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:0
          RX bytes:448 (448.0 B)  TX bytes:468 (468.0 B)


A Linux Bonding interface is now set up. Again, since this is just a first proof of concept test, I'll only add a single slave to the bond.

Later on there will also be a real separate Gbit NIC with a dedicated switch that will act as the primary slave (with the VPN being just a backup), but for now the bonding interface will use the VPN only.

modprobe bonding mode=1 miimon=1000
ifconfig bond0 hw ether 02:00:0a:01:01:c5  # some dummy MAC
ifconfig bond0 up
ifconfig bond0 mtu 1462
ifenslave bond0 vpn_br   # as said, only a single slive at the moment
ifconfig bond0 up

The other host is set up as with HWaddr 02:00:0a:01:01:c7.

This results in a theoretically working bonding interface:

bond0     Link encap:Ethernet  HWaddr 02:00:0a:01:01:c5
          inet addr:  Bcast:  Mask:
          inet6 addr: fe80::aff:fe01:1c5/64 Scope:Link
          RX packets:11 errors:0 dropped:0 overruns:0 frame:0
          TX packets:6 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:0
          RX bytes:448 (448.0 B)  TX bytes:468 (468.0 B)

The status also looks good to me:

# cat /proc/net/bonding/bond0
Ethernet Channel Bonding Driver: v3.6.0 (September 26, 2009)

Bonding Mode: fault-tolerance (active-backup)
Primary Slave: None
Currently Active Slave: vpn_br
MII Status: up
MII Polling Interval (ms): 1000
Up Delay (ms): 0
Down Delay (ms): 0

Slave Interface: vpn_br
MII Status: up
Speed: Unknown
Duplex: Unknown
Link Failure Count: 0
Permanent HW addr: 1a:73:32:7f:36:5f
Slave queue ID: 0

...as does the routing table:

# ip route show dev eth0  proto kernel  scope link  src dev bond0  proto kernel  scope link  src dev eth1  proto kernel  scope link  src
default via dev eth1

NB: eht0 is a separate active NIC (Ethernet cross cable) but that should not matter IMHO.

The problem

The setup looks good to me, however, PING does not work (this was run on pxn1):

# ping
PING ( 56(84) bytes of data.
From icmp_seq=2 Destination Host Unreachable
From icmp_seq=3 Destination Host Unreachable
From icmp_seq=4 Destination Host Unreachable

While pinging, tcpdump on the other machine (pxn2) says:

# tcpdump -n -i bond0
tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on bond0, link-type EN10MB (Ethernet), capture size 65535 bytes
17:45:13.013791 ARP, Request who-has tell, length 28
17:45:13.013835 ARP, Reply is-at 02:00:0a:01:01:c7, length 28
17:45:14.013858 ARP, Request who-has tell, length 28
17:45:14.013875 ARP, Reply is-at 02:00:0a:01:01:c7, length 28
17:45:15.013870 ARP, Request who-has tell, length 28
17:45:15.013888 ARP, Reply is-at 02:00:0a:01:01:c7, length 28

However, when I also run tcpdump on pxn1 in a separate terminal, I suddenly get my ICMP replies!

From icmp_seq=19 Destination Host Unreachable
From icmp_seq=20 Destination Host Unreachable
64 bytes from icmp_req=32 ttl=64 time=0.965 ms
64 bytes from icmp_req=33 ttl=64 time=0.731 ms
64 bytes from icmp_req=34 ttl=64 time=1.00 ms
64 bytes from icmp_req=35 ttl=64 time=0.776 ms
64 bytes from icmp_req=36 ttl=64 time=1.00 ms

This only works as long as both machines have tcpdump running. I can start/stop tcpdump and consistenly only see replies while the program is running on both machines at the same time. It doesn't matter on which machine I try.

Is this a kernel bug or (more probable) has my configuration some problem?

Is it normal, that the bridge and bonding interface both show the same MAC address? I only configure it manually for the bonding interface, which apparently changes also the bridge..

FYI, config overview:


I get a working setup when I set the bridge interface into promiscous mode (ifconfig vpn_br promisc ). I'm not quite sure if that is normally needed. OTOH I don't think it has any downsides...

BTW, there a similar RedHat bug report exists, but setting bond0 down/up doesn't help in my case..


Does it work without the bonding piece? I suspect the issue is that the LACP messages aren't getting across the bridge until you put it in promiscuous mode.

If you're using a 3.5 or higher kernel, it might also help to enable transmission of IGMP queries for the bridge interface. This could help the bridge subscribe to the LACP multicast group.

echo -n 1 > /sys/devices/virtual/net/vpn_br/bridge/multicast_querier
  • Yes, as described, the bridge itself (without bonding) works. Indeed, activating promiscous mode helps (see 'Update' section). This is a 2.6.32 kernel, which might be the reason there is no /sys/devices/virtual/net/vpn_br/ directory (only lo and venet0 in there). – Udo G Jan 7 '15 at 8:57

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.