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How to Detect and Stop DDos/syn Attack in Linux

The below command will show the number of active connections open in your server. Many of the attacks typically work by starting a connection to the server and then not sending any reply making the server wait for it to time out. The number of active connections from the first command is going to vary widely but if you are much above 500 you are probably having problems. If the second command is over 100 you are having trouble with a syn attack.

# netstat -n | grep :80 |wc -l

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

Then  check your log for the ip address with maximum connections. The below command will show the list of hits in your access log…

# cd /var/log/httpd

# tail -n 10000 access_log|cut -f 1 -d ' '|sort|uniq -c|sort -nr|more

This will look at the currently active connections to see if there are any IPs connecting to port 80. You might need to alter the cut -c 45- as the IP address may not start at column 45. If someone was doing a UDP flood to your webserver, this would pick it up as well.

# netstat -n|grep :80|cut -c 45-|cut -f 1 -d ':'|sort|uniq -c|sort -nr|more

Then block the ip address with below command…

# route add ipaddress  reject

#killall -KILL httpd

#service httpd start


  1. Hi,
    Some addition ....
    You can install (D)DoS-Deflate to protect your self.
    (D)DoS-Deflate script basically monitors and tracks the IP addresses are sending and establishing large amount of TCP network connections such as mass emailing, DoS pings, HTTP requests) by using “netstat” command, which is the symptom of a denial of service attack. When it detects number of connections from a single node that exceeds certain preset limit, the script will automatically uses APF or IPTABLES to ban and block the IPs. Depending on the configuration, the banned IP addresses would be unbanned using APF or IPTABLES (only works on APF v 0.96 or better).


    chmod 0700


    chmod 0700 uninstall.ddos


    1. Also you can install mod_evasive for Apache to prevent DDOS attacks.

      consult the following link for more details..

  2. Stop ICMP the following command..

    iptables -A OUTPUT -p icmp --icmp-type echo-request -j DROP


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Virus f Opyum Team

Symptoms :: 
Several processes running named f or i. 
Network choked
Apache Dead
Unknown entry  * * * * * root f Opyum Team in crontab

Investigate ::
Try to kill the f process and even root will be unable to kill it. Check file named f and i in /etc or /bin or /usr/sbin directory. Try to delete, the file f  will not delete.  Some unknown .jpg files will be there in the same folder. 

Solve :: The root is unable to delete the file f because of the immutable bit in f
To remove it use ses or chattr
Follow the below steps to remove it completely.....

remove the unknown .jpg files in system folders

(kill process f) option k

#ses -i /bin/f
#rm /bin/f
#ses -i /etc/crontab
#vi /etc/crontab (and delete last line)

Precautions ::  In my case the server was hacked bu Brute-force attack
So set a good password with a combination of upper,lower case letters and special characters.

Post your feedback or any issues you may face removing the virus.  Very less number of good threads are there explaining …