Why does it get the hostname of the machine when ROS_HOSTNAME is not specified?

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I tried to connect to another machine by using the machine option in the <node> tag of the launch file.

I noticed that if the destination machine does not have ROS_HOSTNAME in its env.sh file, it uses the machine's hostname as URI. For example, if the hostname of the machine is pi, it will try to use http://pi:[port].

There are a few questions here.
1. is this a function that expects name resolution to be written in /etc/hosts?
2. env.sh is erased when I run catkin clean. Is there any good solution?
2. sorry, this may be an unnecessary suggestion, but how about using avahi-daemon (expecting avahi-daemon to be installed) to access pi.local by default?

Thank you.