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[giox069 <1313796@xxxxxxxxxxxxxxxxxx>]

I can confirm the problem in Ubuntu 14.04.1 server: I configured a
bridge for openvpn:

iface br0 inet static
	address 192.168.1.6
	netmask 255.255.255.0
	gateway 192.168.1.1
	bridge_ports eth0 tap0
	dns-nameservers 192.168.1.1
	dns-search mydomain.com
	pre-up openvpn --mktun --dev tap0
	post-down openvpn --rmtun --dev tap0

and for 50% of computer boots (20% on a slower pc) the br0 setup fails
and contains only tap0.

After some debugging, here are my guesses: I found that "brctl addbr br0" is called twice: from /lib/bridge-utils/ifupdown.sh and from /lib/udev/bridge-network-interface (the second one via udev due to openvpn --mktun ???).
When ifupdown.sh is called after bridge-network-interface, "brctl addbr br0" fails and returns nonzero, so the script exits and no interface are added to br0.
When the two scripts are called in the reverse order, all seems to work fine.

I used the proposed workaround to put BRIDGE_HOTPLUG=no /etc/default
/bridge-utils in and now my system is working.

-- 
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https://bugs.launchpad.net/bugs/1313796

Title:
  [No Network] ifupdown/bridge-utils & udev/bridge-utils race condition
  preventing the addition of non-pre-up ports to a bridge

Status in “bridge-utils” package in Ubuntu:
  Confirmed

Bug description:
  Problem:

  I have configured a bridge (br0) between a physical port (eth0) and a
  tap device (tap0) using the ifupdown package. About 75% of the time
  eth0 fails to be added to br0 during boot, leaving networking
  inoperable. This suggests a race condition between ifupdown/bridge-
  utils and some other component which I believe to be udev/bridge-
  utils.

  
  Layout:

  Files involved:
  /etc/network/interfaces
  /etc/network/if-pre-up.d/bridge
  /lib/bridge-utils/bridge-utils.sh
  /var/log/upstart/networking.log
  /var/log/upstart/network-interface-br0.log
  /lib/udev/bridge-network-interface
  /lib/udev/rules.d/40-bridge-network-interface.rules
  /etc/default/bridge-utils

  
  This rules out conflict between ifupdown's sysvinit and upstart integration:
  $ find /etc/rc?.d/ -iname '*network*'
  /etc/rc0.d/S35networking
  /etc/rc6.d/S35networking

  $ find /etc/init* -iname '*network*' | sort
  /etc/init.d/networking
  /etc/init/networking.conf
  /etc/init/network-interface.conf
  /etc/init/network-interface-container.conf
  /etc/init/network-interface-security.conf

  
  This rules out conflict between ifupdown/bridge-control and other ifupdown scripts
  ...:/etc/network$ tree
  .
  +-- if-down.d
  ¦   +-- bind9
  ¦   +-- openvpn
  ¦   +-- postfix
  ¦   +-- resolvconf
  ¦   +-- upstart
  ¦   +-- wpasupplicant -> ../../wpa_supplicant/ifupdown.sh
  +-- if-post-down.d
  ¦   +-- avahi-daemon -> ../if-up.d/avahi-daemon
  ¦   +-- bridge -> /lib/bridge-utils/ifupdown.sh
  ¦   +-- vde2
  ¦   +-- wireless-tools
  ¦   +-- wpasupplicant -> ../../wpa_supplicant/ifupdown.sh
  +-- if-pre-up.d
  ¦   +-- bridge -> /lib/bridge-utils/ifupdown.sh
  ¦   +-- ethtool
  ¦   +-- vde2
  ¦   +-- wireless-tools
  ¦   +-- wpasupplicant -> ../../wpa_supplicant/ifupdown.sh
  +-- if-up.d
  ¦   +-- 000resolvconf
  ¦   +-- avahi-daemon
  ¦   +-- bind9
  ¦   +-- ethtool
  ¦   +-- ntpdate
  ¦   +-- openssh-server
  ¦   +-- openvpn
  ¦   +-- postfix
  ¦   +-- upstart
  ¦   +-- wpasupplicant -> ../../wpa_supplicant/ifupdown.sh
  +-- interfaces
  +-- interfaces.d
  +-- run -> /run/network

  
  Software Versions:
  Ubuntu 14.04 LTS
  ifupdown 		0.7.47.2ubuntu4
  bridge-utils 	1.5-6ubuntu2
  upstart 		1.12.1-0ubuntu4
  udev 			204-5ubuntu20

  
  Logs:
  /var/log/upstart/networking.log
  Sun Apr 27 17:27:03 2014 TUN/TAP device tap0 opened
  Sun Apr 27 17:27:03 2014 Persist state set to: ON
  device br0 already exists; can't create bridge with the same name
  run-parts: /etc/network/if-pre-up.d/bridge exited with return code 1
  Internet Systems Consortium DHCP Client 4.2.4
  ...
  Listening on LPF/br0/46:33:ab:47:a7:05
  Sending on   LPF/br0/46:33:ab:47:a7:05
  Sending on   Socket/fallback
  DHCPDISCOVER on br0 to 255.255.255.255 port 67 interval 3 (xid=0x5d7de88)
  ...
  <<Fails to obtain lease>>>

  
  /var/log/upstart/network-interface-br0.log
  ifup: interface br0 already configured

  
  My ifupdown configuration:
  /etc/init.d/networking
  auto lo
  iface lo inet loopback

  auto eth0
  iface eth0 inet manual

  auto br0
  iface br0 inet dhcp
          bridge_ports eth0 tap0
          pre-up openvpn --mktun --dev tap0

  
  Demonstrating that "pre-up openvpn --mktun --dev tap0" triggers /lib/udev/rules.d/40-bridge-network-interface.rules, which in turn runs /lib/udev/bridge-network-interface:
  $ brctl show
  bridge name     bridge id               STP enabled     interfaces
  br0             8000.000129f5f026       no              eth0

  $ sudo ip tuntap del mode tap tap0

  KERNEL[59526.262768] remove   /devices/virtual/net/tap0 (net)
  UDEV  [59526.263236] remove   /devices/virtual/net/tap0 (net)

  $ pre-up openvpn --mktun --dev tap0
  Mon Apr 28 10:00:32 2014 TUN/TAP device tap0 opened
  Mon Apr 28 10:00:32 2014 Persist state set to: ON

  KERNEL[59622.299324] add      /devices/virtual/net/tap0 (net)
  UDEV  [59622.323388] add      /devices/virtual/net/tap0 (net)

  $ brctl show
  bridge name     bridge id               STP enabled     interfaces
  br0             8000.000129f5f026       no              eth0
                                                          tap0

  
  Confirming the problem:
  By default, BRIDGE_HOTPLUG=yes in /etc/default/bridge-utils. Once disabled, however, both interfaces (eth0, tap0) are consistently added to br0 during boot, tested across five reboots.

  Note that this resolves the problem and the errors logged to
  /var/log/upstart/networking.log ("device br0 already exists; can't
  create bridge with the same name..."), however the warning logged to
  /var/log/upstart/network-interface-br0.log (ifup: interface br0
  already configured) remains (these logfiles are appended to every
  startup). This is not a problem but it does suggest ifup is being
  called twice (once from /etc/init/network-interface.conf at least)
  during startup.

  
  Discussion:
  Disabling BRIDGE_HOTPLUG is only a temporary hack. I have a third interface I would like to bridge which does require hotplug support.

  
  Possible Solution
  * Disable hotplugging for virtual adapters
  * Disable hotplugging for a user-defined list of interfaces
  * Fix the race condition?
      Not sure how, both /etc/network/if-pre-up.d/bridge and /lib/udev/rules.d/40-bridge-network-interface.rules test for the interface's presence:
      "if ... [ ! -d /sys/class/net/$IFACE ]; then"

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