Saturday, August 27, 2011

Secure your Linux Servers

October 19, 2009 by  
Filed under cPanel, Direct Admin, Ensim, General, HyperVM, Kloxo, Plesk, Webmin

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Securing a linux server is a continuous job as new types of exploits are discovered almost everday. Here, we tried to collect a number of quick guides that will help you securing your linux box.However, this guide covers only basic linux server security tips intended for linux learners. I am writing this guide assuming that you are running Centos 5 or later versions.

Steps in Securing your Linux Server

  • Install Firewall (APF or CSF Firewall with BFD)
  • ModSecurity (Web application firewall)
  • ModEvasive (Prevent DDOS attacks)
  • Harden SSH server
  • Fix Open DNS Recursion
  • Install RKhunter
  • Install ClamAV (Antivirus)
  • XInet Servers Hardening (Disable Telnet/Finger or unwanted services)
  • Securing PHP
  • PortsEntry (tool to detect portscans)
  • Harden host.conf (against IP spoofing)
  • Check User Uploaded files
  • Secure /tmp Folders (noexec, nosuid)

Install Firewall

The very first first step on securing a server is installing a firewall (atleast IP tables based) to close all unused or unwanted ports. Once the firewall is installed it is often considered 50% of work done. You can install CSF firewall or APF firewall. Often BFD (brute force detection) utilities comes with firewall.

We will install CSF (Config security firewall) as it is easy to install with plenty of features and easily integrated to CPanel (if you are running)

tar zxf csf.tar.gz
sh /csf/

Follow the installer and once installed, you can start the firewall.

csf -s
// start the firewall
csf -r
// restart the firewall
csf -f
// flush the rules or stop the firewall.

You can see the full installing tutorial here

Harden SSH server

Very often you will see SSH attacks from various bots trying to get access to your server by connected to port 22 with unlimited number of login attempts to break in to your system. Imagine attacks coming from different IPs can put lot of load in you server. You can trace those failed attempts by checking your log file

cat /var/log/secure
cat /var/log/messages

To harden your SSH server,

* Run SSH on other port rather than default port 22
* Disable Root login
* Use only protocol 2
* Enable Public key authentication.

You can see the full SSH hardening tutorial here
Disable Telnet & Other Unused Services

You may want to disable services like telnet, finger and other unwanted services running on your server with xinet.

nano /etc/xinetd.d/telnet
// OR
nano /etc/xinetd.d/krb5-telnet

look for lines disable=no and change to disable=yes

chkconfig telnet off

Hardening PHP for Security

PHP is the most popular scripting language for apache and mysql. You will need to disable system level functions in the php configuration file.

nano /usr/local/lib/php.ini

Look for the lines and make sure you have the lines as below..

disable_functions = exec,system,shell_exec,passthru
register_globals = Off
expose_php = Off
magic_quotes_gpc = On

It is best to keep magic_quotes to on as otherwise you forms using POST may be used for SQL injection attacks.

Disable Open DNS Recursion (DNS Server)

If you are running bind DNS server, then you might want to check your dns server statistics with You dont want to allow recursive lookups to performed on your server other than local IP. It can also slowdown your server.

nano /etc/named.conf

Under Options { place a line

Options {
recursion no;

Then restart the bind

service named restart

You will also need to restrict zone transfers and notifications if you are running Bind 9. Refer to: dns server hardening

Install Mod_Security

ModSecurity is a free open source web application firewall which can help you to guard against LFI (local file inclusion attacks) and SQL injection vulnerabilities.

cPanel Installation:

Just go to Cpanel WHM > Plugins > Enable Mod_Security > Save

Source Installation:

That should install mod security in your cpanel. Under apache it should show under installed modules if you run test.php with phpinfo() in it. Try adding some mod security rules. Installing mod_security could be sometimes complicated. Dont use apxs for compiling mod_security as it causes number of problems.

Note: Mod_security needs libxml2 and http-devel libraries before it can be installed. It also requires mod_unique_id enabled in apache modules. To install mod_unique_id, you have to place

LoadModule unique_id_module modules/

in your httpd.conf file.

yum install libxml2 libxml2-devel httpd-devel

Download the latest version of mod_security for apache2 from

tar zxf modsecurity-apache_2.5.4.tar.gz
cd modsecurity-apache_2.5.4
cd apache2


If you cannot find ./configure then you will need to edit Makefile and make change to top_dir = /usr/lib/httpd (for centos)

make install

Next, copy the rule files depending on which you want (you can also select minimal rules file which comes with source). Make a directory named modsecurity under /etc/httpd/conf and copy all the modsecurity rules there. Finally include those files in the httpd.conf file

# /etc/httpd/conf/httpd.conf

LoadModule unique_id_module modules/
LoadFile /usr/lib/
LoadModule security2_module modules/
Include conf/modsecurity var thickboxL10n = { next: "Next >", prev: "< Prev", image: "Image", of: "of", close: "Close" }; try{convertEntities(thickboxL10n);}catch(e){};