Server security is a huge issue specially when you do care about your data and your integrity as a host master. One of the things you can do in order to increase server security (if you can't afford a hardware firewall) is software firewall.
In this case CSF is one of the free but still very helpful and secure software that you can use free of charge ( or with a minimal donation ) to increase server security.
To download the latest version you can use the download link here : Homepage: http://www.configserver.com/cp/csf.html
Although it's up to you if you want to donate and help development of CSF we strongly recommend doing so because it actually worth every single penny you give them!
Step by step install CSF on your Centos Linux Server
Login to your server using SSH (something like putty)
First you have to download the CSF and make sure you have perl modules installed, so here are the step by step command's you have to run.
yum install -y perl-libwww-perl
The next step is to download, unpack and install CSF from APF source.
Since you installed the CSF there's no longer needed to keep the install files on your server so let's clean them up.
rm -Rf /usr/local/src/csf* $$ cd
Now let's create a backup file of the original CSF configuration file.
cp /etc/csf/csf.conf /etc/csf/csf.conf.bak
It's about time to edit the configuration of your CSF and tune it according to your needs...
nano -w /etc/csf/csf.conf
Just make sure you won't lock yourself out side your server