You lie about your hostname, your IP address, your reverse PTR records,
or take advantage of a less restrictive domain allowed to post as the
domain in question. Or you interfere with the published DNS of the site
in question, since DNS is not overall designed for robust authentication
if other people's zone informatino.
I know that this sort of thing IS _possible_ however not practical,
indeed not trivial. And likely not profitible. I would have to guess it
would be easier to modify someone else's web site home page directly
than to carry out an effective DNS spoof.
Sheesh theese are big characters, feels like I am typing on a Trash 80.
Take care
Waitman Gobble