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The configuration is in /etc/avahi/avahi-daemon.conf, and the following changes are recommended (assume again the App host-name is 'ubuntu-4321-http-server'). If there is no other mDNS services already been enabled, this 'publish-workstation=yes' must be configured:
host-name=ubuntu-4321-http-server
use-ipv6=no
publish-workstation=yes
then restart the 'avahi-daemon' if it is already running. Note that even if the host-name is already defined in the Linux, make this 'host-name' explicit in 'Avahi' to match the data-store FQDN domain name will make sure it will still work even the host-name is changed by other services in the App. The 'use-ipv6=yes' is the default, and people have reported issues when 'Avahi' sometimes only gets the IPv6 address without wait for an IPv4 address and returns the reply, so this 'no' makes the reply have the IPv4 address always. The 'publish-workstation=yes' is to support the service of 'workstation'.
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