We are currently trying out this Fail2Ban rule on one of our server, to block simple (but very upsetting) DOS attacks on Nginx automatically (after 30 seconds).

New filter in /etc/fail2ban/filter.d/nginx-dos.conf:

# Fail2Ban configuration file
#
# Generated on Fri Jun 08 12:09:15 EST 2012 by BeezNest
#
# Author: Yannick Warnier
#
# $Revision: 1 $
#

[Definition]
# Option:  failregex
# Notes.:  Regexp to catch a generic call from an IP address.
# Values:  TEXT
#
failregex = ^<HOST> -.*"(GET|POST).*HTTP.*"$

# Option:  ignoreregex
# Notes.:  regex to ignore. If this regex matches, the line is ignored.
# Values:  TEXT
#
ignoreregex =

In our jail.local, we have (at the end of the file):

[nginx-dos]
# Based on apache-badbots but a simple IP check (any IP requesting more than
# 240 pages in 60 seconds, or 4p/s average, is suspicious)
# Block for two full days.
# @author Yannick Warnier
enabled = true
port    = http,8090
filter  = nginx-dos
logpath = /var/log/nginx/*-access.log
findtime = 60
bantime  = 172800
maxretry = 240

Of course, in case you would be logging all resources of your site (images, css, js, etc), it would be really easy to get to those numbers as a normal user. To avoid this, use the access_log off directive of Nginx, like so:

 # Serve static files directly
        location ~* .(png|jpe?g|gif|ico)$ {
                expires 1y;
                access_log off;
                try_files $uri $uri/ @rewrite;
                gzip off;
        }
        location ~* .(mp3)$ {
                expires 1y;
                access_log off;
                gzip off;
        }
        location ~* .(css)$ {
                expires 1d;
                access_log off;
        }
        location ~* .(js)$ {
                expires 1h;
                access_log off;
        }

We’ll see how that works for us… (and report here)