I have Apache cache control set and TTL also in the rss feed. The problem is there are a number of clients that are ignoring these options and just hammer the server multiple times a second for the same feed. (which does not get updated that often)
The user-agents are typically bogus listed as " Mozilla/5.0". I do not see a solution other than leave it alone or block them.
I suppose the next step if I do not want to go that route is to create a php program to temporarily block after so many queries. Unless there is an apache setting to do that but I have not found it yet.