[Bug 1499827] Re: [ipv6] Network manager incorrectly lowers MTU when IPv6 is enabled

[Mathieu Trudel-Lapierre]

Uploaded network-manager 1.0.4-0ubuntu5.2 to wily-proposed for review
and approval by the SRU team.

** Description changed:

+ [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
+ 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
+  # 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
+  [main]
+  NetworkingEnabled=true
+  WirelessEnabled=true
+  WWANEnabled=true
+  WimaxEnabled=true
  RfKill:
-  0: phy0: Wireless LAN
-  	Soft blocked: no
-  	Hard blocked: no
+  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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https://bugs.launchpad.net/bugs/1499827

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

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