How to Secure Linux Servers

Basic Linux Server Security

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)


This tutorial 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.

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,

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/*.conf


/etc/init.d/httpd restart

Log Files

Watch for log files to detect any errors or intrusion activity


If you get any errors, i have compiled a list of errors while compiling. see here

Install Mod_Evasive

ModEvasive module for apache offers protection against DDOS (denial of service attacks) in your server.

tar zxf mode_evasive-1.10.1.tar.gz
cd mod_evasive

then run the following command for apache2...

> /usr/sbin/apxs -cia mod_evasive20.c

Once mod evasive is installed, place the following lines in your /etc/httpd/conf/httpd.conf

<IfModule mod_evasive20.c>
DOSHashTableSize 3097
DOSPageCount 2
DOSSiteCount 50
DOSPageInterval 1
DOSSiteInterval 1
DOSBlockingPeriod 10

Follow the instructions in the README for more tuning of mod_evasive. This will compile, install and activate the module in your server.

Install RkHunter (Rootkit)

RkHunter is a rootkit scanner scans for vulnerabilities, insecure files, backdoors in your system and reports it so that you can further harden the server. Installing RkHunter is very easy!

yum install rkhunter

To run checks in your system

rkhunter --checkall
rkhunter -c

You can find what command options are available under rkhunter by issuing this help command

> rkhunter --help

Install PortsEntry

Portsentry is a tool to detect port scans and log it. Download the sorce package of portsentry from

wget http://path/to/portsentry-1.2.tar.gz
tar zxf portsentry-1.2.tar.gz
make linux
make install

If you get errors like while compiling

make linux
gcc -O -Wall -DLINUX -DSUPPORT_STEALTH -o ./portsentry ./portsentry.c \
./portsentry_io.c ./portsentry_util.c
./portsentry.c: In function 'PortSentryModeTCP':
./portsentry.c:1187: warning: pointer targets in passing argument 3 of 'accept' differ in signedness
./portsentry.c: In function 'PortSentryModeUDP':
./portsentry.c:1384: warning: pointer targets in passing argument 6 of 'recvfrom' diffe r in signedness
./portsentry.c: In function 'Usage':
./portsentry.c:1584: error: missing terminating " character
./portsentry.c:1585: error: 'sourceforget' undeclared (first use in this function)
./portsentry.c:1585: error: (Each undeclared identifier is reported only once
./portsentry.c:1585: error: for each function it appears in.)
./portsentry.c:1585: error: expected ')' before 'dot'
./portsentry.c:1585: error: stray '\' in program
./portsentry.c:1585: error: missing terminating " character
./portsentry.c:1595: error: expected ';' before '}' token
make: *** [linux] Error 1

To fix:

Open portsentry.c and look for the following line. There will be a extra carriage return breaking the line and you have to delete the carriage return and make single line. It should look like below.

printf ("Copyright 1997-2003 Craig H. Rowland <craigrowland at users dot sourceforget dot net>\n");

Then run make and make install. That should fix it!

To launch portsentry

/usr/local/psionic/portsentry/portsentry -stcp
/usr/local/psionic/portsentry/portsentry -sudp

check the log files /var/log/secure on what portsentry is active or not.

Prevent IP Spoofing

IP spoofing is a security exploit and can be prevented from placing nospoof on in host.conf file. Edit the host.conf file and place the following lines. If you run dns bind, give it preference.

order bind,hosts
nospoof on

Install ClamAV

Antivirus protection is the last thing you need for your security to protect against worms and trojans invading your mailbox and files! Just install clamav (a free open source antivirus software for linux). More information can be found on clamav website

yum install clamav

Once you have installed clamav in your centos…here are some of the basic commands using the software..

1. To update the antivirus database

> freshclam

2. To run antivirus

clamav -r /home

3. Running as Cron Daily Job

To run antivirus as a cron job (automatically scan daily) just run crontab -e from your command line. Then add the following line and save the file.

02 1 * * * root clamscan -R /var/www

This will run the cron job daily @ 1.02 AM by scanning the public html. You can change the folder to whatever you want for mail etc.

Thats it! Always keep an eye for log files for any attacks or error messages!

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