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Posted by prox, from Seattle, on August 19, 2017 at 11:39 local (server) time

For about two weeks, I've been trying to troubleshoot some odd host-only networking issues with VirtualBox.  It turned out to be a configuration-related bug in a file marked DO NOT EDIT THIS FILE, which, of course, I ultimately had to edit.

I previously had two VirtualBox VMs, dax (FreeBSD) and adria (Linux) connected together using intnet networking.  The setup worked, but I wanted to convert the adria VM to an LXC instance since I didn't actually need full VM emulation.  I spun up the LXC with the following in its configuration file:

# Networking
lxc.network.type = veth
lxc.network.name = eth0
lxc.network.link = lxcbr0
lxc.network.hwaddr = 00:02:bc:56:cc:99

Then, I put the following in /etc/network/interfaces on the host:

# lxcbr0: bridges vboxnet1/vtnet2 on dax to adria
# This cannot be auto because vboxnet1 does not exist on boot
#auto lxcbr0
iface lxcbr0 inet manual
        bridge_ports vboxnet1
        bridge_fd 0
        bridge_maxwait 0

Since VirtualBox's bridging doesn't play nice directly with LXC veth interfaces (explained here), I decided to convert dax's interface to host-only networking (vboxnet1) and use Linux's native bridging on the host to connect the VM and LXC.  I ended up with the following:

(excalibur:14:09:EDT)% ifconfig vboxnet1
vboxnet1: flags=4163<UP,BROADCAST,RUNNING,MULTICAST>  mtu 1500
        inet 192.168.57.1  netmask 255.255.255.0  broadcast 192.168.57.255
        ether 0a:00:27:00:00:01  txqueuelen 1000  (Ethernet)
        RX packets 0  bytes 0 (0.0 B)
        RX errors 0  dropped 13  overruns 0  frame 0
        TX packets 19796  bytes 29052799 (27.7 MiB)
        TX errors 0  dropped 0 overruns 0  carrier 0  collisions 0

(excalibur:14:09:EDT)% ifconfig vethGEVFJP
vethGEVFJP: flags=4163<UP,BROADCAST,RUNNING,MULTICAST>  mtu 1500
        inet6 fe80::fca5:45ff:fee5:ecb2  prefixlen 64  scopeid 0x20<link>
        ether fe:a5:45:e5:ec:b2  txqueuelen 1000  (Ethernet)
        RX packets 15215  bytes 1209437 (1.1 MiB)
        RX errors 0  dropped 0  overruns 0  frame 0
        TX packets 19678  bytes 29042119 (27.6 MiB)
        TX errors 0  dropped 0 overruns 0  carrier 0  collisions 0

(excalibur:14:09:EDT)% brctl show         
bridge name	bridge id		STP enabled	interfaces
lxcbr0		8000.0a0027000001	no		vboxnet1
							vethGEVFJP
(excalibur:14:09:EDT)% sudo lxc-ls -f     
NAME  STATE   AUTOSTART GROUPS IPV4       IPV6                               
adria RUNNING 0         -      10.3.7.217 2620:6:2000:12e:202:bcff:fe56:cc99 
(excalibur:14:10:EDT)% 

Looks good, right?  Wrong.  Connectivity worked, but only in one direction, and it seemed to be stopping on vboxnet1.  After lots of tcpdumps, I figured out the following.

LXC ---> VM  (GOOD)
VM  ---> LXC (DROPPED)

More specifically:

LXC -- vethGEVFJP --> lxcbr0 -- vboxnet1   --> VM  (GOOD)
VM  -- vboxnet1   --> lxcbr0 -- vethGEVFJP --> LXC (DROPPED)
     |
     |---- DROPPED HERE

Here are all of the things I tried, but none worked:

It seemed that the problem was on the VirtualBox side between vboxnet1 and the actual VM (vtnet2 on FreeBSD).  This was odd, because I also have vboxnet0 connected to the host and the configuration is identical:

(excalibur:14:18:EDT)% VBoxManage showvminfo dax|grep NIC|head -n3
NIC 1:           MAC: 0002BC56CB39, Attachment: Bridged Interface 'eth0', Cable connected: on,\
Trace: off (file: none), Type: virtio, Reported speed: 0 Mbps,\
Boot priority: 0, Promisc Policy: allow-all, Bandwidth group: none
NIC 2:           MAC: 0002BC56CB99, Attachment: Host-only Interface 'vboxnet0', Cable connected: on,\
Trace: off (file: none), Type: virtio, Reported speed: 0 Mbps,\
Boot priority: 0, Promisc Policy: allow-all, Bandwidth group: none
NIC 3:           MAC: 0002BC56CB29, Attachment: Host-only Interface 'vboxnet1', Cable connected: on,\
 Trace: off (file: none), Type: virtio, Reported speed: 0 Mbps,\
Boot priority: 0, Promisc Policy: allow-all, Bandwidth group: none
(excalibur:14:18:EDT)% 

So, what's the problem, here?  I started looking into configuration files.  Here's what I found in ~/VirtualBox VMs/dax/dax.vbox:

      <Network>
        <Adapter slot="0" enabled="true" MACAddress="0002BC56CB39" cable="true" promiscuousModePolicy="AllowAll" type="virtio">
          <DisabledModes>
            <InternalNetwork name="intnet"/>
            <NATNetwork name="NatNetwork"/>
          </DisabledModes>
          <BridgedInterface name="eth0"/>
        </Adapter>
        <Adapter slot="1" enabled="true" MACAddress="0002BC56CB99" cable="true" promiscuousModePolicy="AllowAll" type="virtio">
          <DisabledModes>
            <InternalNetwork name="intnet"/>
            <NATNetwork name="NatNetwork"/>
          </DisabledModes>
          <HostOnlyInterface name="vboxnet0"/>
        </Adapter>
        <Adapter slot="2" enabled="true" MACAddress="0002BC56CB29" cable="true" promiscuousModePolicy="AllowAll" type="virtio">
          <DisabledModes>
            <BridgedInterface name="tap0"/>
            <InternalNetwork name="intnet"/>
            <NATNetwork name="NatNetwork"/>
          </DisabledModes>
          <HostOnlyInterface name="vboxnet1"/>
        </Adapter>
        <Adapter slot="3" cable="true" type="82540EM"/>
        <Adapter slot="4" cable="true" type="82540EM"/>
        <Adapter slot="5" cable="true" type="82540EM"/>
        <Adapter slot="6" cable="true" type="82540EM"/>
        <Adapter slot="7" cable="true" type="82540EM"/>

The only thing that was different between vboxnet0 and vboxnet1 was the BridgedInterface under the DisabledModes tag.  It must have been added during my original try to get LXC connectivity working and then modified when I used the TAP device during troubleshooting.  It shouldn't matter because it's disabled.  Also, there's a big warning on the top of the file stating:

<!--
** DO NOT EDIT THIS FILE.
** If you make changes to this file while any VirtualBox related application
** is running, your changes will be overwritten later, without taking effect.
** Use VBoxManage or the VirtualBox Manager GUI to make changes.
-->

Well, I made changes to it and removed that one BridgedInterface line, but when the VM was stopped.  Bingo, that fixed it.  I now have bidirectional networking through the Linux bridge between my VM and LXC instance.

This smells like a bug.  VirtualBox must erroneously apply some networking state when it reads through DisabledModes when it really shouldn't.  As a result, I opened ticket 17022.

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