I'm using VPS server with ubuntu and webmin. And in one of my domain I'd like to use google apps, but neither in webmin panel nor in conf file I see a place to add this google spf :
v=spf1 include:_spf.google.com ~all
You need to create a TXT record and set it to that value.
Webmin help page: http://doxfer.webmin.com/Webmin/BINDDNSServer#Record_types
In webmin you go to your master zone (in BIND DNS Server) and set it under Sender Permitted From records.