Mitigate DDoS attack with ngx_http_limit_req_module and fail2ban

The fail2ban do have comprehensive collection of scripts that scan log files and ban IPs that match malicious activities. But we are going to look on how to use ngx_http_limit_req_module logs to ban IPs that shows sign of Distributed Denial of Service (DDoS) attack on your website.

It is assumed in this tutorial that Nginx server is installed in your server. The following procedures are tested on my Linode server running Centos 7 64-bit Linux distribution.

1. Enable ngx_http_limit_req_module by adding the following script in your Nginx configuration:

     
   http {
     limit_req_zone $binary_remote_addr zone=one:10m rate=1r/s;
     ...
     server {
       ...
       limit_req zone=one burst=5;
     }
   }
     
   

2. Restart Nginx server:

     
   systemctl restart  nginx.service
     
   

   You will see entry something like this in Nginx error log if there's abuse detected:

     
   2015/08/27 02:18:05 [error] 21235#21235: *326 limiting requests, excess: 5.297 by zone "one", client: 91.214.169.44, server: www.webfoobar.com, request: "GET /node/8 HTTP/1.1", host: "www.webfoobar.com", referrer: "https://www.webfoobar.com/archive/201502"
     
   

   We will use this sample log entry for our fail2ban filter script.

3. Install fail2ban:

     
   yum install -y fail2ban
     
   

4. Create fail2ban filter script based on the Nginx error log entry:

     
   vi /etc/fail2ban/filter.d/nginx-ddos.conf
     
   

   The content of this filter file:

     
   [Definition]
   failregex = limiting requests, excess:.* by zone.*client: 
   ignoreregex = 
     
   

5. We will use the /etc/hosts.deny to block the IP of the DDoS attacker so we will need to create new fail2ban action script:

     
   vi /etc/fail2ban/action.d/hostsdeny.conf
     
   

   Add the following script as its content:

     
   [Definition]
   actionstart = 
   actionstop = 
   actioncheck = 
   actionban = IP= &&
               printf %%b ": $IP\n" >> 
   actionunban = IP= && sed -i.old /ALL:\ $IP/d 
   
   [Init]
   file = /etc/hosts.deny
   daemon_list = ALL
     
   

6. Enable the newly created fail2ban filter:

     
   vi /etc/fail2ban/jail.local
     
   

   Append the following script:

     
   [nginx-ddos]
   enabled = true
   port    = http,https
   banaction = hostsdeny
   findtime = 120
   bantime  = 7200
   maxretry = 30
   logpath = %(nginx_error_log)s
     
   

7. Start the fail2ban service:

     
   systemctl start fail2ban
   systemctl enable fail2ban.service
   systemctl list-unit-files | grep fail2ban
     
   

8. To check the status of this fail2ban filter:

     
   fail2ban-client status nginx-ddos
     
   

   You will see something like this:

     
   Status for the jail: nginx-ddos
   |- Filter
   |  |- Currently failed: 18
   |  |- Total failed:     770
   |  `- File list:        /var/log/nginx/nginx_error_log
   `- Actions
      |- Currently banned: 1
      |- Total banned:     8
      `- Banned IP list:   91.214.169.44
     
   

9. To test if the fail2ban nginx-ddos filter working:

     
   fail2ban-regex /var/log/nginx/nginx_error_log /etc/fail2ban/filter.d/nginx-ddos.conf
     
   

10. You can use apache-bench to test the whole system:

      
    ab -n 20 -c 10 http://www.webfoobar.com
      
    

    Execute the following command to monitor the fail2ban log:

      
    watch -n 1 tail -n 20 /var/log/fail2ban.log
      
    

    And you will something like this while testing with apache-bench:

      
    2015-08-29 09:56:45,535 fail2ban.filter         [16931]: INFO    [nginx-ddos] Found 112.209.203.176
    2015-08-29 09:56:45,536 fail2ban.filter         [16931]: INFO    [nginx-ddos] Found 112.209.203.176
    2015-08-29 09:56:45,537 fail2ban.filter         [16931]: INFO    [nginx-ddos] Found 112.209.203.176
    2015-08-29 09:56:45,538 fail2ban.filter         [16931]: INFO    [nginx-ddos] Found 112.209.203.176