Hello
Is possible blocking web with content for adults with iptables?
Thank you very much for your help

Sure. But will it be all-encompassing, manageable or efficient? No. It'll be as much a kludge as blocking websites using /etc/hosts.

Thanks for your quickly reply
My english is basic
I want blocked websites for keywords or blacklist. As I can do?
I have linksys with dd-wrt

Does it come with a proxy server application? That would be the best tool for the job, IMHO. For example, you can filter by site/URL with Squid, and by web page content (as well as by site/URL) with DansGuardian. Also, these don't necessarily need to run on the Linksys box itself. As hinted at by unSpawn, iptables is simply not the right tool for the job, although it can play an important role in the overall solution (such as preventing the bypass of the content filter, for example).

EDIT: At the time of this post, the website for DG was down, but there's several resources to help you get started.