Its not something you can do from a server, it need to be done at your Data center's interface.
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If you are using shared web hosting on a dedicated IP you should contact your hosting provider and ask then to setup the rDNS record, as ajonate says, some provide an self service web interface for you to do it for yourself. We do for all our VPS clients, but not for shared hosting.
If you are on a shared IP you won't have the option of setting up an rDNS record, because the IP is not used solely by yourself.
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If you are hosting your own servers, you will need to contact your internet service provider that if you have dedicate IP from them. If you are hosting somewhere contact your hosting provider they will be able to help setup your rDNS (not available on shared hosting with shared IP)