LinuxBabu ………..Linux System Admin Blog….

Here is a command line to run on your server if you think your server is under attack. It prints our a list of open connections to your server and sorts them by ammount.

RedHat: netstat -ntu | awk ‘{print $5}’ | cut -d: -f1 | sort | uniq -c | sort -n

BSD: netstat -na |awk ‘{print $5}’ |cut -d “.” -f1,2,3,4 |sort |uniq -c |sort -n

You can also check for connections by running the following command.
netstat -plan | grep :80 | awk ‘{print $4 }’ | sort -n | uniq -c | sort

These are few step to be taken when you feel the server is under attack:
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Step 1: Check the load using the command “w”.
Step 2: Check which service is utilizing maximum CPU by “nice top”.
Step 3: Check which IP is taking maximum connection by netstat -anpl|grep :80|awk {’print $5′}|cut -d”:” -f1|sort|uniq -c|sort -n
Step 4: Then block the IP using firewall (APF or iptables “apf -d < IP>” )
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You can also implement security features in your server like:

1) Install apache modules like mod_dosevasive and mod_security in your server.
2) Configure APF and IPTABLES to reduce the DDOS
3) Basic server securing steps :
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http://www.linuxdevcenter.com/pub/a/linux/2006/03/23/secure-your-server.html?page=1
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4) Configure sysctl parameters in your server to drop attacks.

You can block the IP which is attacking your server using Ipsec from command prompt.
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>> netsh ipsec static add filterlist name=myfilterlist
>> netsh ipsec static add filter filterlist=myfilterlist srcaddr=a.b.c.d dstaddr=Me
>> netsh ipsec static add filteraction name=myaction action=block
>> netsh ipsec static add policy name=mypolicy assign=yes
>> netsh ipsec static add rule name=myrule policy=mypolicy filterlist=myfilterlist filteraction=myaction
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Denial of Service attacks are among the oldest yet most common form of attacking a server. Most system administrators have had to deal with DOS attacks taking down a server, router, or other networking device and know how difficult they can be to prevent.

Mod_evasive is an Apache module that limits the number of Apache connections to the server at once, and blocks an offending IP for a specified amount of time. This tutorial will show you how to install mod_evasive on your system.

1. Install:

wget http://www.zdziarski.com/projects/mod_evasive/mod_evasive_1.10.1.tar.gz
tar -xvzf mod_evasive_1.10.1.tar.gz
cd mod_evasive_1.10.1
/usr/local/apache/bin/apxs -cia mod_evasive.c

2. Configure:

Once the module is compiled, add these lines to httpd.conf and stop Apache completely before starting it up again:

<IfModule mod_evasive.c>
DOSHashTableSize 3097
DOSPageCount 6
DOSSiteCount 50
DOSPageInterval 2
DOSSiteInterval 2
DOSBlockingPeriod 600
</IfModule>

Below is an explanation of these settings:
DOSHashTableSize - Size of the hash table. The greater this setting, the more memory is required - faster

DOSPageCount - Max number of requests for the same page within the ‘DOSPageInterval’ interval

DOSSiteCount - Max number of requests for a given site, uses the ‘DOSSiteInterval’ interval.

DOSPageInterval - Interval for the ‘DOSPageCount’ threshold in second intervals.

DOSSiteInterval- Interval for the ‘DOSSiteCount’ threshold in second intervals.

DOSBlockingPeriod - Blocking period in seconds if any of the thresholds are met. The user will recieve a 403 (Forbidden) when blocked, and the timer will be reset each time the site gets hit when the user is still blocked.