import re

def validate_hostname(value):
    value = value.strip().rstrip(".")
    if len(value) > 253 or not value or any(
        not re.fullmatch(r"[A-Za-z0-9](?:[A-Za-z0-9-]{0,61}[A-Za-z0-9])?", label)
        for label in value.split(".")
    ):
        raise ValueError("Enter a hostname or FQDN using letters, digits, hyphens and dots (no URLs or commands).")
    return value.lower()