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[Launchpad Bug Tracker <1499827@xxxxxxxxxxxxxxxxxx>]

This bug was fixed in the package network-manager - 1.0.4-0ubuntu5.2

---------------
network-manager (1.0.4-0ubuntu5.2) wily; urgency=medium

  * debian/patches/lp1499827_device_avoid_undefined_mtu_c44069c1.patch
    Cherry-pick commit c44069c1 from upstream: avoid changing the MTU if it's
    undefined in NM code. (LP: #1499827)
  * debian/patches/lp1499827_dont_change_unset_mtu_a92d8b0c.patch:
    Cherry pick upstream commit a92d8b0c: don't change the MTU if we have a
    value of 0 for it; it means "default", not "0 size packets". (LP: #1499827)

 -- Mathieu Trudel-Lapierre <mathieu-tl@xxxxxxxxxx>  Fri, 27 Nov 2015
14:03:32 -0500

** Changed in: network-manager (Ubuntu Wily)
       Status: Fix Committed => Fix Released

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https://bugs.launchpad.net/bugs/1499827

Title:
  [ipv6] Network manager incorrectly lowers MTU when IPv6 is enabled

Status in NetworkManager:
  Fix Released
Status in network-manager package in Ubuntu:
  Fix Released
Status in network-manager source package in Wily:
  Fix Released

Bug description:
  [Impact]
  Users with IPv6-supporting networks will see the messages below in the bug description appear repeatedly as NM mistakes a 0 value for MTU in internal NM code with 0-size packets instead of meaning "use default MTU".

  [Test case]
  - Run network-manager on a network supporting IPv6.
  - Check /var/log/syslog for <warn> messages from NetworkManager.

  [Regression Potential]
  This changes behavior for handling the MTU value for IPv6, which should be no higher than the device MTU. Since 0 is an invalid value for MTU anyway, this should not otherwise affect MTU handling for devices.
  Possible regressions from the change might include invalid (too high) set MTU for a device due to the failure to recognize cases where the MTU needs to be adjusted, leading to packet loss on IPv6.

  --

  Network manager is sending out a lot of warnings about changing the MTU:
  Sep 25 14:13:03 desktop NetworkManager[703]: <warn>  (wlan0): Lowering IPv6 MTU (1480) to match device MTU (1280)
  Sep 25 14:13:21 desktop NetworkManager[703]: <warn>  (wlan0): Lowering IPv6 MTU (1480) to match device MTU (0)
  Sep 25 14:13:21 desktop NetworkManager[703]: <warn>  (wlan0): IPv6 MTU (0) smaller than 1280, adjusting
  Sep 25 14:13:21 desktop NetworkManager[703]: <warn>  (wlan0): Raising device MTU (0) to match IPv6 MTU (1280)
  Sep 25 14:13:29 desktop NetworkManager[703]: <warn>  (wlan0): Lowering IPv6 MTU (1480) to match device MTU (1280)

  Lowering IPv6 MTU (1480) to match device MTU (1280) repeats about 16
  times between the other messages (about 2 mins round).

  This doesn't appear to be Ubuntu specific -
  https://bbs.archlinux.org/viewtopic.php?pid=1552751

  ProblemType: Bug
  DistroRelease: Ubuntu 15.10
  Package: network-manager 1.0.4-0ubuntu3
  ProcVersionSignature: Ubuntu 4.2.0-11.13-generic 4.2.1
  Uname: Linux 4.2.0-11-generic x86_64
  NonfreeKernelModules: nvidia
  ApportVersion: 2.19-0ubuntu1
  Architecture: amd64
  CurrentDesktop: Unity
  Date: Fri Sep 25 14:09:20 2015
  IfupdownConfig:
   # interfaces(5) file used by ifup(8) and ifdown(8)
   auto lo
   iface lo inet loopback
  InstallationDate: Installed on 2015-01-08 (259 days ago)
  InstallationMedia: Ubuntu 15.04 "Vivid Vervet" - Alpha amd64 (20150108)
  NetworkManager.state:
   [main]
   NetworkingEnabled=true
   WirelessEnabled=true
   WWANEnabled=true
   WimaxEnabled=true
  RfKill:
   0: phy0: Wireless LAN
    Soft blocked: no
    Hard blocked: no
  SourcePackage: network-manager
  UpgradeStatus: Upgraded to wily on 2015-08-12 (43 days ago)
  nmcli-nm: Error: command ['nmcli', '-f', 'all', 'nm'] failed with exit code 2: Error: Object 'nm' is unknown, try 'nmcli help'.

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