[Bug 1859862] Re: networkd doesn't re-enable ipv6 unless mtu is set

[Steve Langasek]

Hello Dan, or anyone else affected,

Accepted systemd into bionic-proposed. The package will build now and be
available at
https://launchpad.net/ubuntu/+source/systemd/237-3ubuntu10.34 in a few
hours, and then in the -proposed repository.

Please help us by testing this new package.  See
https://wiki.ubuntu.com/Testing/EnableProposed for documentation on how
to enable and use -proposed.  Your feedback will aid us getting this
update out to other Ubuntu users.

If this package fixes the bug for you, please add a comment to this bug,
mentioning the version of the package you tested and change the tag from
verification-needed-bionic to verification-done-bionic. If it does not
fix the bug for you, please add a comment stating that, and change the
tag to verification-failed-bionic. In either case, without details of
your testing we will not be able to proceed.

Further information regarding the verification process can be found at
https://wiki.ubuntu.com/QATeam/PerformingSRUVerification .  Thank you in
advance for helping!

N.B. The updated package will be released to -updates after the bug(s)
fixed by this package have been verified and the package has been in
-proposed for a minimum of 7 days.

** Changed in: systemd (Ubuntu Bionic)
       Status: In Progress => Fix Committed

** Tags added: verification-needed verification-needed-bionic

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https://bugs.launchpad.net/bugs/1859862

Title:
  networkd doesn't re-enable ipv6 unless mtu is set

Status in systemd package in Ubuntu:
  Fix Released
Status in systemd source package in Bionic:
  Fix Committed
Status in systemd source package in Disco:
  Won't Fix
Status in systemd source package in Eoan:
  In Progress

Bug description:
  [impact]

  if the 'disable_ipv6' procfs setting is disabled, and networkd is
  configured with ipv6 networking, networkd will not enable ipv6 via the
  procfs setting unless the interface mtu is also set to a custom value,
  that doesn't match the current interface mtu.

  [test case]

  create networkd config similar to:

  [Match]
  MACAddress=52:54:00:0c:09:77
  Name=ens3

  [Network]
  DHCP=ipv4
  LinkLocalAddressing=ipv6
  Address=2001:db8::100/64

  manually disable ipv6 for the interface:

  ubuntu at lp1859862-b:~$ echo 1 | sudo tee /proc/sys/net/ipv6/conf/ens3/disable_ipv6
  1

  the interface should not contain any ipv6 addresses:

  ubuntu at lp1859862-b:~$ ip -6 a show ens3
  ubuntu at lp1859862-b:~$

  restart networkd; it should enable ipv6 as ipv6 addresses are
  configured, but it does not:

  ubuntu at lp1859862-b:~$ sudo systemctl restart systemd-networkd
  ubuntu at lp1859862-b:~$ ip -6 a show ens3
  ubuntu at lp1859862-b:~$

  Note that with the mtu set, it will successfully re-enable ipv6;
  modify the networkd config to set the mtu by adding this:

  [Link]
  MTUBytes=1400

  verify ipv6 is still disabled and has no addresses, then restart and
  check again:

  ubuntu at lp1859862-b:~$ cat /proc/sys/net/ipv6/conf/ens3/disable_ipv6
  1
  ubuntu at lp1859862-b:~$ ip -6 a show ens3
  ubuntu at lp1859862-b:~$ sudo systemctl restart systemd-networkd
  ubuntu at lp1859862-b:~$ cat /proc/sys/net/ipv6/conf/ens3/disable_ipv6
  0
  ubuntu at lp1859862-b:~$ ip -6 a show ens3
  2: ens3: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1400 state UP qlen 1000
      inet6 2001:db8::100/64 scope global
         valid_lft forever preferred_lft forever
      inet6 fe80::5054:ff:fe0c:977/64 scope link
         valid_lft forever preferred_lft forever

  That should be the behavior even if MTUBytes is not set.

  [regression potential]

  as this adjusts when the procfs 'disable_ipv6' setting is changed, the
  regression potential could cause failure to properly set/create ipv6
  addresses, or to leave ipv6 disabled entirely.

  [scope]

  This is needed in Bionic and Eoan.

  the commit to fix this is 482efedc081b0c4bf2e77a3dee6b979d9c9a5765
  which is included starting in v243, so it is included already in
  Focal.

  Ignoring Disco, as it is EOL next week.

  This problem was introduced by commit
  44b598a1c9d11c23420a5ef45ff11bcb0ed195eb, which was included starting
  in v239, but also backported to Bionic in bug 1850704.

  [other info]

  for Eoan, this bug has always existed since release, but for Bionic,
  this is a regression introduced by bug 1850704.  However, manual
  disabling of ipv6 is very unusual, so this bug is unlikely to have an
  impact to many systems.

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