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[Stéphane Graber <stgraber@xxxxxxxxxxxx>]

** Changed in: bridge-utils (Ubuntu)
     Assignee: Stéphane Graber (stgraber) => (unassigned)

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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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