Am I allowed to use underscores for hostname names?

Question

I am trying to give a hostname along the lines of PiComputer_User1 however I am encountering the following error: unable to resolve host (hostname) : Name or service not known

I have added static ip routes and the appropriate hostname to /etc/hostname and /etc/hosts

I am able to ping and connect via ssh using the ip address and routes which I added, however I still get the error whenever I try to install anything

Additionally when I try to use hostname -i I get the error Name or service not known

In particular I am noticing that if I use hostname it returns the hostname as PiComputerUser1 where there should be an underscore between computer and user

To confirm one more time I am using the correct naming convention in hosts and hostname

Does anyone know why I am getting errors, and if I am allowed to use underscores in hostnames to begin with? Thank you!

Answer 1

The text string in a hostname must be from ASCII characters. It includes the upper or lower case (A to Z or a to z), digits (0 to 9), the dot symbol (.), and the hyphen symbol (-).

NOTE it is inadvisable to edit /etc/hostname or /etc/hosts manually.
Change using raspi-config.

Getting this wrong can cause networking errors on Debian systems (due to attempts to use FQDN).