Switching from Linux Mint 19 to Linux Mint 20.1 meant that pinging host on my local network stopped working. I could still ping them using IP-addresses, but not their host names

mm@wolfrax:~/dev/pyPDP/asm$ ping rpi2.local
ping: rpi2.local: Name or service not known
mm@wolfrax:~/dev/pyPDP/asm$ ping 192.168.1.51
PING 192.168.1.51 (192.168.1.51) 56(84) bytes of data.
64 bytes from 192.168.1.51: icmp_seq=1 ttl=64 time=10.3 ms

How come? This made me search and read on the configuration for mDNS. Until I found the right solution, I edited /etc/hosts with hard-coded node name and IP-address (which is Ok, as I have configured static IP-addresses for these nodes). These looks like so,

mm@wolfrax:~/dev/pyPDP/asm$ cat /etc/hosts
127.0.0.1   localhost
127.0.1.1   wolfrax

192.168.1.50    rpi1
192.168.1.51    rpi2
192.168.1.52    rpi3
192.168.1.53    rpi4
192.168.1.54    rpi5    

# The following lines are desirable for IPv6 capable hosts
::1     ip6-localhost ip6-loopback
fe00::0 ip6-localnet
ff00::0 ip6-mcastprefix
ff02::1 ip6-allnodes
ff02::2 ip6-allrouters
mm@wolfrax:~/dev/pyPDP/asm$ ping rpi2
PING rpi2 (192.168.1.51) 56(84) bytes of data.
64 bytes from rpi2 (192.168.1.51): icmp_seq=1 ttl=64 time=6.61 ms

mDNS

Multicast DNS (mDNS) resolve hostnames to IP-addresses for local networks. By default mDNS resolve hostnames with top-domain .local. So, rpi2.local would resolve to IP-address 192.168.1.51 in my network.

There seems to be several ways to do this in a Linux environment, 3 of them are:

Avahi
NetworkManager
systemd-resolved, see systemd and systemd-resolved

If not configured correctly, they might counteract each other. See this article where disabling Avahi is suggested.

After some searching, I found this thread where the problem I have where noted and resolved...somehwat. The core of the thread is to enable multicastDNS by editing /etc/systemd/resolved.conf and editing the file so

MulticastDNS=yes

Then issue the following command (change interface to appropriate value) to set mdns for the interface

$ sudo systemd-resolve --set-mdns=yes --interface=wlp3s0
$ ping rpi2.local
PING rpi2.local (192.168.1.51) 56(84) bytes of data.
64 bytes from rpi2 (192.168.1.51): icmp_seq=1 ttl=64 time=5.26 ms
64 bytes from rpi2 (192.168.1.51): icmp_seq=2 ttl=64 time=5.34 ms
$ resolvectl
Global
       LLMNR setting: no                  
MulticastDNS setting: yes                 
  DNSOverTLS setting: no                  
      DNSSEC setting: no                  
    DNSSEC supported: no                  
          DNSSEC NTA: 10.in-addr.arpa     
                      16.172.in-addr.arpa 
                      168.192.in-addr.arpa
                      17.172.in-addr.arpa 
                      18.172.in-addr.arpa 
                      19.172.in-addr.arpa 
                      20.172.in-addr.arpa 
                      21.172.in-addr.arpa 
                      22.172.in-addr.arpa 
                      23.172.in-addr.arpa 
                      24.172.in-addr.arpa 
                      25.172.in-addr.arpa 
                      26.172.in-addr.arpa 
                      27.172.in-addr.arpa 
                      28.172.in-addr.arpa 
                      29.172.in-addr.arpa 
                      30.172.in-addr.arpa 
                      31.172.in-addr.arpa 
                      corp                
                      d.f.ip6.arpa        
                      home                
                      internal            
                      intranet            
                      lan                 
                      local               
                      private             
                      test                

Link 3 (wlp3s0)
      Current Scopes: DNS mDNS/IPv4 mDNS/IPv6
DefaultRoute setting: yes                    
       LLMNR setting: yes                    
MulticastDNS setting: yes

Works! Until you switch the interface off, then on again. The mdns setting for the interface has been lost. :-(

But, it can be fixed! Here is how.

Create a file named for example wifi.network (it can be named anything, but the file extension must be network). Store the file in /etc/systemd/network. The content of the file is

[Match]
Name=w*

[Network]
DHCP=yes
MulticastDNS=yes
LLMR=no

Note the wildcard on the Match-section. It could also be stated as Name=wlp3s0. So, the logic is that mDNS needs to be enabled both in resolved.conf (globally) and per link in a new file with extension network to become persistent.

Make systemd read the changes and check status using networkctl

$ networkctl reload
$ networkctl list
IDX LINK      TYPE     OPERATIONAL SETUP     
  1 lo        loopback carrier     unmanaged 
  2 enp0s31f6 ether    no-carrier  unmanaged 
  3 wlp3s0    wlan     routable    configured

Now, the system should find rpi2.local always.

Note that if systemd.networkd is not enabled/running, do this through

$ sudo systemctl enable systemd-networkd
$ sudo systemctl start systemd-networkd

Using DNS stub file

Note below information from archlinux:

Using the systemd DNS stub file - the systemd DNS stub file /run/systemd/resolve/stub-resolv.conf contains the local stub 127.0.0.53 as the only DNS server and a list of search domains. This is the recommended mode of operation. The service users are advised to redirect the /etc/resolv.conf file to the local stub DNS resolver file /run/systemd/resolve/stub-resolv.conf managed by systemd-resolved. This propagates the systemd managed configuration to all the clients. This can be done by replacing /etc/resolv.conf with a symbolic link to the systemd stub:

$ sudo ln -sf /run/systemd/resolve/stub-resolv.conf /etc/resolv.conf