[Bug 1533631] Re: dhclient killed when DHCPv6 lease is out-of date

[VF]

This problem has been happening to me recently with up to date Ubuntu
16.04 amd64 desktop.

Workaround is to clean up the broken state by removing all leases:

1. Disconnect from the network in Network Manager
2. Execute: sudo rm /var/lib/NetworkManager/dhclient*.lease
3. Connect to the network in Network Manager

After this dhclient -6 keeps running and IPv6 leases are renewed
correctly.

I had this problem on a second very similar installation. There it has
been verified that dhclient -6 process is killed/crashed right after
acquiring the IPv6 address, so I had a second reproduction.

I saved the contents of /var/lib/NetworkManager and can send it to the
developer fixing it on request. There must be something there crashing
dhclient in IPv6 mode.

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https://bugs.launchpad.net/bugs/1533631

Title:
  dhclient killed when DHCPv6 lease is out-of date

Status in NetworkManager:
  Confirmed
Status in isc-dhcp package in Ubuntu:
  In Progress
Status in network-manager package in Ubuntu:
  In Progress

Bug description:
  Network Manager summarily kills the IPv6 dhclient process when the
  DHCPv6 lease contained in the dhclient6-${UUID}.lease file is out-of
  date, leaving the IPv6 interface without a stateful IPv6 address, or,
  if the "require IPv6 address on this interface" option is enabled,
  causes NM to cycle continuously deactivating and reactivating the
  interface (including the IPv4 addresses).

  This is effectively a Denial Of Service. It can be trivially induced
  if, for example, the dhclient6-$(UUID}.lease file contains a lease
  that was issued before the user went away on vacation or the PC wasn't
  connected to the same network for a few days (depending on the lease
  renew/rebind/expiry times). Calculation on the old lease of

  start + preferred_lifetime < NOW

  triggers dhclient to 'DEPREFER6' the lease (withdraw the address
  record) and ask the DHCPv6 server for a new lease, but Network Manager
  will kill the dhclient because it only sees an 'EXPIRE6' state change.

  In summary, when the DHCPv6 state transitions from "bound" to
  "unknown" then "expire" to "done" Network Manager kills the 'dhclient'
  process before it has chance to request and bind a fresh lease, If
  'dhclient' is run manually with the same command-line options and
  allowed to continue running it correctly gains a new lease.

  Network Manager doesn't know how to handle "DEPREF6", which is sent
  from isc-dhcp dhclient to the helper script (set by "-sf" option).

  So it seems that to correctly solve this issue Network Manager must be
  taught how to handle DEPREF6.

  /var/log/syslog will show a message from dhclient of the form:

  dhclient: PRC: Address 2a02:8011:2007::2 depreferred.

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