[Bug 1609367] Re: ifupdown does not set ipv6-only large mtu

[Dan Streetman]

> While we could see about getting the script included in yakkety (or y+1);
> curtin will still need to
> handle this unless we want to SRU ifdowndown in precise, trusty and
> xenial. This has an enormous
> impact; so it's not something we should take lightly.

As long as curtin handles it correctly, I don't see any need to SRU
ifupdown; just adding the scripts in y and forward should be enough.
Anyone manually configuring ifupdown in X or earlier can manually setup
both inet and inet6 sections to get the mtus right.

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https://bugs.launchpad.net/bugs/1609367

Title:
  ifupdown does not set ipv6-only large mtu

Status in ifupdown package in Ubuntu:
  Confirmed

Bug description:
  ifupdown changes a device's mtu differently, between "inet" section
  mtu and "inet6" section mtu.  For "inet", ifupdown changes the
  device's mtu, using 'ip link set DEV mtu NNN'.  However for "inet6",
  ifupdown changes the interface's IPv6 mtu, using 'sysctl -q -e -w
  net.ipv6.conf.DEV.mtu=NNN'.

  The problem is that a device's ipv6 mtu cannot be larger than the
  device mtu.  Normally this is not a problem; the main reason a device
  needs a separate ipv6 mtu is to support ipv6 tunneling inside ipv4
  (e.g. 6in4 or 6rd), so ifupdown would also have a ipv4 "inet" section
  that contained a larger mtu (or, the ipv6 section would have a lower-
  than-default mtu).

  But, in the case of an interface that is configured only for ipv6
  *and* large frames (e.g. 9000 mtu), there is only an "inet6" section
  in the ifupdown config, and so ifupdown fails to set the device's mtu,
  which causes its attempt to set the ipv6 mtu to fail.

  I believe the correct behavior is for ifupdown to check if there is
  any "inet" config section for the same device as the "inet6" section,
  and if there is not then it should change the device mtu as well as
  the ipv6 mtu.  Additionally, the "inet6" configuration must be done
  after the "inet" configuration for the same device, since the ipv6 mtu
  can't be higher than the device mtu.

  I don't see any easy way to add that logic to inet6.defn, however...

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