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How do I add a rdns ptr record for all domains in whm/cpanel?
Can't be done using the cPanel interface. Possibly try editing the BIND zone file.
Can be done in WHM via the DNS zone editor. Could edit the default template.
I use WHM side for this also. Works wonders.
Some information regarding this can be found -> http://docs.cpanel.net/twiki/bin/view/AllDocumentation/WHMDocs/RdnsForBind
Edit: This can be used for IPv6 also with some minor tweaks.