Denial-of-service attack (DoS attack) or Distributed Denial-of-service attack (DDoS attack) is an attempt to make a server or network resource unavailable to its users. This attack generally target sites or services hosted on web servers. DoS attacks are implemented by either forcing the targeted computer to reset or consuming its resources so that it can no longer provide its services or obstructs the communication media between the users and the victim so that they can no longer communicate adequately.
The best command to detect the DoS attack is 'netstat' and we have plenty of options are available to check the details of the attack.
Some examples provided below with the example commands:
netstat -na
This display all active Internet connections to the server and only established connections are included.
netstat -an | grep :80 | sort
Show only active Internet connections to the server on port 80, this is the HTTP port and so it’s useful if you have a web server, and sort the results. Useful in detecting a single flood by allowing you to recognize many connections coming from one IP.
netstat -n -p|grep SYN_REC | wc -l
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 the system, so a high value may be average on another server.
netstat -n -p | grep SYN_REC | sort -u
List out the all IP addresses involved instead of just count.
netstat -n -p | grep SYN_REC | awk '{print $5}' | awk -F: '{print $1}'
List all the unique IP addresses of the node that are sending SYN_REC connection status.
netstat -ntu | awk '{print $5}' | cut -d: -f1 | sort | uniq -c | sort -n
Use netstat command to calculate and count the number of connections each IP address makes to the server.
netstat -anp |grep 'tcp|udp' | awk '{print $5}' | cut -d: -f1 | sort | uniq -c | sort -n
List count of number of connections the IPs are connected to the server using TCP or UDP protocol.
netstat -ntu | grep ESTAB | awk '{print $5}' | cut -d: -f1 | sort | uniq -c | sort -nr
Check on ESTABLISHED connections instead of all connections, and displays the connections count for each IP.
netstat -plan|grep :80|awk {'print $5'}|cut -d: -f 1|sort|uniq -c|sort -nk 1
Show and list IP address and its connection count that connect to port 80 on the server. Port 80 is used mainly by HTTP web page request.
How to mitigate a DOS attack
Once that you have found the IP's that are attacking your server you can try using the following commands to block their connection to your server:
iptables -A INPUT 1 -s $IPADRESS -j DROP/REJECT
Please note that you have to replace $IPADRESS with the IP numbers that you have found with netstat. If the attack is from different ranges of IP's and if it is in a huge volume, you might need to get help from the support team in order to get it fixed.