I don't really know your setup, or if money is an issue, but you could use a sonicwall. Though, it kind of conflicts with your ubuntu server. Sonicwalls have antivirus built into them, and if you try to download something that is infected, it will block access and alert you that the file is infected. Also, I don't know if it would be useful to you, but sonicwalls also have a content filter. I don't work for sonicwall, so this isn't a plug for them, but my brother is a reseller for them, so we have one at home.