Finding and preventing ddos attack on Linux Server

A distributed denial-of-service (DDoS) attack occurs when multiple systems flood the bandwidth or resources of a targeted system, usually one or more web servers. Such an attack is often the result of multiple compromised systems flooding the targeted system with traffic.A denial-of-service (DoS) attack is an attempt to make a machine or network resource unavailable to its intended users, such as to temporarily or indefinitely interrupt or suspend services of a host connected to the Internet.

check ddos attack

netstat -anp|grep 'tcp\|udp' | awk '{print $5}' | cut -d: -f1 | sort | uniq -c | sort –n

This command will show you the list of IP's which have logged in is maximum number of connections to your server.

ddos becomes more complex as attackers  use fewer connections with more number of attacking IP's.In such cases, you should get less number of connections even when your server is under ddos.One important thing that you should check is the number of active connections that your server currently has by executing below command:

netstat -n | grep :80 |wc –l

netstat -n  | grep :80 | grep SYN |wc –l

Result of active connections from the first command will vary but if it shows connections more than 500, then you will be definitely having problems. If the result after you fire second command is 100 or above then you are having problems with sync attack.
This command is useful to find out how many active SYNC_REC are occurring on the server. The number should be pretty low, preferably less than 5. On DoS attack incidents or mail bombs, the number can jump to pretty high. However, the value always depends on system, so a high value may be average on another server.

netstat -n -p|grep SYN_REC | wc -l

netstat -n -p | grep SYN_REC | sort -u

Once you find the ip attacking your server, you can easily block it.Fire the following command to block that ip or any other specific ip:

check ip blocked or not by using the following command:

route -n |grep IPaddress

You can also block a IP with iptables on the server by using the following command.

iptables -A INPUT 1 -s IPADRESS -j DROP/REJECT

service iptables restart

service iptables save

After firing the above command, KILL all httpd connection and than restart httpd service by
using following command:

killall -KILL httpd

service httpd startssl