hostname

When you change hostname with hostnamectl set-hostname new_name, you need to edit /etc/hosts file accordingly.

If not, sudo will try to resolve the new hostname, but won’t due to security.

Reason

The reason is:

🔒 sudo logs the command with your hostname

When you run a sudo command, it:

1. Resolves the current hostname to an IP (using /etc/hosts or DNS),

2. Logs the command like this:

   Aug 4 12:00:00 maindebian sudo: user : TTY=... ; COMMAND=...

If the system can’t resolve the hostname to an IP, it prints:

sudo: unable to resolve host maindebian: Name or service not known

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🧠 Why the resolution fails:

When you changed the hostname (e.g., from ubuntu to maindebian), you didn’t update /etc/hosts, which still contains:

127.0.1.1 ubuntu

So now, when sudo looks up maindebian, it doesn’t exist in /etc/hosts, and DNS isn’t used for local hostnames like this — hence the error.

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✅ Moral of the story:

sudo relies on the hostname → /etc/hosts must map that hostname to an IP (usually 127.0.1.1).