[Bug 1964494] Re: Setting DuplicateAddressDetection=none doesn't disable DAD for link-local IPs

[Alejandro Santoyo Gonzalez]

I agree, a user-configured setting should be honored. Another thing to
consider is that according to the Jammy systemd.network man page, the 
default should be 'ipv6' so one would expect that to be a global 
default, but it gets overridden for link-local IPs due to this commit.  


[1] https://manpages.ubuntu.com/manpages/jammy/man5/systemd.network.5.html

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Title:
  Setting DuplicateAddressDetection=none doesn't disable DAD for link-
  local IPs

Status in systemd package in Ubuntu:
  New

Bug description:
  A customer reported network disconnections on their storage 
  servers when running 'netplan apply'. The culprit was that
  they have link-local addresses configured and the Duplicate 
  Address Detection (DAD) mechanism was delaying the interfaces 
  from coming back up. 

  As a workaround we tried to disable DAD for the interfaces 
  but that's not working in Ubuntu 22.04:

  I've noticed that setting DuplicateAddressDetection=none for an
  interface with a link-local address (e.g., 169.254.*) via a 
  .network file added to /etc/systemd/network/ doesn't really 
  disable Duplicate Address Detection.

  OS and package versions:
  ------------------------
   - Description:	Ubuntu Jammy Jellyfish (development branch). Release: 22.04
   - systemd 249.5-2ubuntu4

  Reproducer:
  -----------
  1- Set up Ubuntu 22.04 VM
  2- Increase systemlog level:

    mkdir -p /etc/systemd/system/systemd-networkd.service.d/
    cat > /etc/systemd/system/systemd-networkd.service.d/10-debug.conf <<EOF
    [Service]
    Environment=SYSTEMD_LOG_LEVEL=debug
    EOF
    systemctl daemon-reload && systemctl restart systemd-networkd

  3- Configure a link-local address to a network interface on the VM:
  vi /etc/systemd/network/10-netplan-enp7s0.network
  # add

  [Match]
  Name=enp7s0

  [Address]
  Address=169.254.240.10/24
  DuplicateAddressDetection=ipv4

  4- Restart services and apply

  systemctl daemon-reload && systemctl restart systemd-networkd && systemctl restart networkd-dispatcher.service
  netplan apply

  5- Check if DAD is running (it should):

  journalctl -b -u systemd-networkd --no-pager -o short-precise | grep
  -i ACD

  6- Set DuplicateAddressDetection=none in 
  /etc/systemd/network/10-netplan-enp7s0.network

  7- Restart services and apply as in step 4
  8- Check if DAD is running as in step 5 (it shouldn't, but 
  it's there)

  After step 7 DAD should be disabled as expected and it 
  shouldn't be executed when running 'netplan apply' but 
  it is actually executed.

  I think this commit [1] may be related (landed in systemd 
  v249). I've also tested the out-of-the-box systemd versions 
  for Focal and Impish but there I can effectively disable 
  DAD. If the IPs are set to non-link local addresses then 
  the problem is not observed anymore. 

  
  [1] https://github.com/systemd/systemd/commit/1cf4ed142d6c1e2b9dc6a0bc74b6a83ae30b0f8e

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