This thread is very old (2005-2020), but still easily googleable.
If a dhcp is used, dhcp will supply the hostname and the domain name, and will override the default settings. In a bad situation, a misbehaving ISP can supply garbage as a hostname or a domainname, believing that only IPs and DNS servers are important to the user.
This can be partially overriden in {/etc/resolv.conf.head, /etc/resolv.conf.tail}
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