I have a dedicated server running Ubuntu and Plesk. I have a primary domain on this server but will also be running several other sites and I would like to be able to set the DNS records for those to something such as:ns1.maindomain.comns2.maindomain.comI found a couple Plesk tutorials on how to do this and follow them, but when I try to change the DNS records for a domain on GoDaddy I get an error saying that ns1.maindomain.com is not a valid record.Any idea what I might be missing? 
Use custom DNS records
dns;dns servers;plesk
Have you set up ns1.maindomain.com and ns2 at your domain registrar?  Some places call it child nameservers, some call it registry hosts.