[Bug 2003025] Re: systemd/resolved can't enable mdns permanently

[Hadmut Danisch]

# journalctl -b 0 --grep eno1 | cut -c22-
 kernel: igb 0000:02:00.0 eno1: renamed from eth1
 systemd-networkd[763]: eno1: Link UP
 kernel: igb 0000:02:00.0 eno1: igb: eno1 NIC Link is Up 1000 Mbps Full Duplex, Flow Control: RX
 systemd-networkd[763]: eno1: Gained carrier
 kernel: IPv6: ADDRCONF(NETDEV_CHANGE): eno1: link becomes ready
 avahi-daemon[742]: Joining mDNS multicast group on interface eno1.IPv6 with address fe80::201:c0ff:fe21:3400.
 avahi-daemon[742]: New relevant interface eno1.IPv6 for mDNS.
 systemd-networkd[763]: eno1: Gained IPv6LL
 avahi-daemon[742]: Registering new address record for fe80::... on eno1.*.
 avahi-daemon[742]: Leaving mDNS multicast group on interface eno1.IPv6 with address fe80::....
 avahi-daemon[742]: Joining mDNS multicast group on interface eno1.IPv6 with address 2a02:....
 avahi-daemon[742]: Registering new address record for 2a02:... on eno1.*.
 avahi-daemon[742]: Withdrawing address record for fe80::... on eno1.
 avahi-daemon[742]: Registering new address record for fdfc:... on eno1.*.
 systemd-networkd[763]: eno1: DHCPv4 address 192.168..../24 via 192.168....
 avahi-daemon[742]: Joining mDNS multicast group on interface eno1.IPv4 with address 192.168.....
 avahi-daemon[742]: New relevant interface eno1.IPv4 for mDNS.
 avahi-daemon[742]: Registering new address record for 192.168.... on eno1.IPv4.
 systemd-networkd-wait-online[809]: managing: eno1


# resolvectl mdns                        
Global: yes
Link 2 (enp1s0): no
Link 3 (eno1): no
...

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Title:
  systemd/resolved can't enable mdns permanently

Status in systemd package in Ubuntu:
  Incomplete

Bug description:
  I currently ran into the problem, that systemd (i.e. its resolved)
  cannot properly configured to run with mdns.

  
  I'm currently setting up some machines in a network with a poor router which routes and offers DHCP, but does not put the DHCP clients into a DNS domain, so the machines need to use mdns to find each other. 

  
  In order to have systemd/resolved use mdns, two steps are necessary: 

  1. MulticastDNS=yes must be put into /etc/systemd/resolved.conf and
  resolved restarted to enable it in general.

  
  2. It must be enabled per interface. On the command line this can be done with
  resolvectl mdns eth0 yes

  result:
  # resolvectl mdns 
  Global: yes
  Link 63 (eth0): yes


  and it works.

  
  Problem: It cannot be done automatically in a server/LXD image. Im having this problem inside the LXD 22.04 image. 

  
  a) man pages suggest that /etc/systemd/network/eth0.network 
  [Match]
  Name=eth0

  [Network]
  MulticastDNS=yes


  would do the job, but it doesn't. It seems to be completely ignored.

  
  The reason seems to be that 
  networkctl status eth0 
  shows 

                    Network File: /run/systemd/network/10-netplan-
  eth0.network

  
  so it seems to be overridden by netplan. 



  b) netplan itself does not seem to support the MulticastDNS settings.


  
  c) On Desktop Machines I found a workaround with NetworkManager, but on server and LXD images, Network Manager does not exist. 


  
  So I do not see a way to have this configured permanently (other than writing a systemd unit). 

  
  But:

  
  I need to configure the machines / LXD machines using cloud-init, in order to have the installation pull packages from a server to be found with mdns. 

  I tried to run 
  resolvectl mdns eth0 yes

  as a bootcmd in Cloud-init, but this has no effect, because it is too
  early.

  
  I tried to run it as a runcmd, and it works but it is too late, because package installation has already failed. 


  So where in this scrub between systemd and netplan could mdns be
  enabled in a way, that the automatic installation would work?

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