I work for a site that often get's attacked by bot networks. We have started to use this tool: http://deflate.medialayer.com/ which auto-bans ip's that have more open connections than the set value. By default it's set to 150, we are currently using 250.
I would like to know, how low is safe so that search bots and normal visitors do not get blocked?
Modern browsers may open up to 250 connections in total (Firefox on Windows is limited to 48 in FF 8, and by default somewhere between 4 and 16 for a single server.
In Firefox the setting is named Network.http.max-connections-per-server and defaults to 8. AFAIK Chrome has a default of 6. However, because of delays on connection timeouts the number of open connections that DDOS Deflate gets from netstat might be higher, maybe up to 30-40.
So from what I've read from various search results like this one on Lighttpd is that 100 should be a safe number that won't ban regurlar users.