1. ##root account hidden:
/usr/sbin/adduser -u 0 -o -g 0 -G 0,1,2,3,4,6,10 -M <accountname>
Start a firewall
The first thing you want to do is to setup the linux iptables firewall. The
setup will be a bash script with iptables rules, and you will have to run it as
a deamon service (you could write rules line by line in your terminal and then
save them as a ruleset, as described here, but the service method below is
easier to maintain imo).
First, use your favorite console text editor to create a new file in your
/etc/rc.d/init.d/ service directory (CentOS should have vim already installed),
you can name it firewall.
#Create a service owned by root
sudo vim /etc/rc.d/init.d/firewall
As a bash script service, it will need some mandatory header attributes: shell
type, runlevels, priorities and a description.
#! /bin/bash
#chkconfig: 2345 95 20
#description: iptables rules to prevent communication on unused ports.
#Reset all rules (F) and chains (X), necessary if have already defined iptables
rules
iptables -t filter -F
iptables -t filter -X
#Start by blocking
iptables -t filter
iptables -t filter
iptables -t filter
all traffic, this will allow secured, fine grained filtering
-P INPUT DROP
-P FORWARD DROP
-P OUTPUT DROP
#Keep established connexions
iptables -A INPUT -m state --state RELATED,ESTABLISHED -j ACCEPT
iptables -A OUTPUT -m state --state RELATED,ESTABLISHED -j ACCEPT
#Allow loopback
iptables -t filter
iptables -t filter
#HTTP
iptables -t filter
iptables -t filter
#HTTPS
iptables -t filter
iptables -t filter
#FTP
iptables -t filter
iptables -t filter
#SMTP
iptables -t filter
iptables -t filter
#POP3
iptables -t filter
iptables -t filter
#IMAP
iptables -t filter
iptables -t filter
#ICMP
iptables -t filter
iptables -t filter
-A INPUT -i lo -j ACCEPT
-A OUTPUT -o lo -j ACCEPT
-A OUTPUT -p tcp --dport 80 -j ACCEPT
-A INPUT -p tcp --dport 80 -j ACCEPT
-A OUTPUT -p tcp --dport 443 -j ACCEPT
-A INPUT -p tcp --dport 443 -j ACCEPT
-A OUTPUT -p tcp --dport 20:21 -j ACCEPT
-A INPUT -p tcp --dport 20:21 -j ACCEPT
-A INPUT -p tcp --dport 25 -j ACCEPT
-A OUTPUT -p tcp --dport 25 -j ACCEPT
-A INPUT -p tcp --dport 110 -j ACCEPT
-A OUTPUT -p tcp --dport 110 -j ACCEPT
-A INPUT -p tcp --dport 143 -j ACCEPT
-A OUTPUT -p tcp --dport 143 -j ACCEPT
-A INPUT -p icmp -j ACCEPT
-A OUTPUT -p icmp -j ACCEPT

2. #SSH
iptables -t filter
iptables -t filter
#SSH NEW PORT
iptables -t filter
iptables -t filter
#IRC
iptables -t filter
iptables -t filter
iptables -t filter
iptables -t filter
#IRC SERVER
iptables -t filter
iptables -t filter
iptables -t filter
iptables -t filter
#DNS
iptables -t filter
iptables -t filter
iptables -t filter
iptables -t filter
#NTP
iptables -t filter
-A INPUT -p tcp --dport 22 -j ACCEPT
-A OUTPUT -p tcp --dport 22 -j ACCEPT
-A INPUT -p tcp --dport 60125 -j ACCEPT
-A OUTPUT -p tcp --dport 60125 -j ACCEPT
-A
-A
-A
-A
OUTPUT -p tcp --dport 6667 -j ACCEPT
OUTPUT -p tcp --dport 6697 -j ACCEPT
INPUT -p tcp --dport 6667 -j ACCEPT
INPUT -p tcp --dport 6697 -j ACCEPT
-A
-A
-A
-A
OUTPUT -p tcp --dport 9784 -j ACCEPT
INPUT -p tcp --dport 9784 -j ACCEPT
OUTPUT -p tcp --dport 7000 -j ACCEPT
INPUT -p tcp --dport 7000 -j ACCEPT
-A
-A
-A
-A
OUTPUT -p tcp --dport 53 -j ACCEPT
OUTPUT -p udp --dport 53 -j ACCEPT
INPUT -p tcp --dport 53 -j ACCEPT
INPUT -p udp --dport 53 -j ACCEPT
-A OUTPUT -p udp --dport 123 -j ACCEPT
I made a text file with the lines above available to download here.
Save the script file under /etc/rc.d/init.d, make it executable and apply it, so
you will be able to launch it as a service.
chmod +x /etc/rc.d/init.d/firewall
bash /etc/rc.d/init.d/firewall
Now, if you used a debian like distro, you would have issue the update-rc.d
command to add your script to the list of services starting at boot time,
instead on CentOs, RHEL or Fedora, you have to use chkconfig.
chkconfig --add /etc/rc.d/init.d/firewall
chkconfig /etc/rc.d/init.d/firewall on
Just to be sure your firewill service is registered and will start at boot, use
the ntsysv command to open a graphical interface and "firewall" should appear in
the list of services starting at boot:
ntsysv
Harden your SSH access
In a few simple steps, you will be able to diminish risks of unauthorized ssh
accesses Your ssh settings can be found in /etc/ssh/sshd_config, this is where
you will have to modify the configuration settings below.
sudo vim /etc/ssh/sshd_config
1. Change your ssh port
By default, ssh run on port 22. You will need to change this default value to an
arbitrary port number (it must be between 1 and 65535, but prefer the unassigned
49152–65535 range, for more information about port numbers, read the wiki).
Search for the port setting, and remove the sharp to uncomment it and thus
remove default :

3. #
#
#
#
The strategy used for options in the default sshd_config shipped with
OpenSSH is to specify options with their default value where
possible, but leave them commented. Uncommented options change a
default value.
#This will require ssh connexions to use the 60125 port
Port 60125
By changing this setting, you can make a hacker drop an attack by making him
think your ssh is disable or at least force him to scan your ports in order to
find ssh access.
2. Disable root login
If the hacker still gets to connect to your ssh port, he will need
authentication. Obvisously he will try the root account which grant maximum
priviledge on the server, so you want to disable direct root ssh access.
# Authentication:
#LoginGraceTime 2m
#Find this line in your /etc/ssh/sshd_config and change its value to "no"
PermitRootLogin no
Once it's done, you will need another account to connect, so add a new password
protected user
sudo adduser bob
sudo passwd bob
Changing password for user bob.
New password: "enter bob password here"
To push this a little further, you want bob to be the only user allowed to
connect via ssh, so add the AllowUsers setting :
#Multiple users can be specified, separated by spaces.
AllowUsers bob
3. Apply new settings
Now restart your ssh service so the system will take changes into account.
Before restarting ssh, double check and make sure you didn't make any
modifications which could prevent you to reconnect ssh after you logout.
sudo /etc/rc.d/init.d/sshd restart
If you read the first part of this tutorial (setting iptables), you might want
to change iptables as follow :
#SSH (replace 22 with your custom port number, for instance 60125)
iptables -t filter -A INPUT -p tcp --dport 60125 -j ACCEPT
iptables -t filter -A OUTPUT -p tcp --dport 60125 -j ACCEPT
Check your new settings, first you will try to connect to the new ssh port you
configured, using the -p argument
ssh -p 60125 bob@server_address
4. Test against unauthorized access
If you have successfully harden ssh, you won't be able to connect as root (or
any other user than bob for that matter) :
ssh -p 60125 root@server_address

4. root@server_address's password:
Permission denied, please try again.
Likewise, any connexion on a port other than the one defined in
/etc/ssh/sshd_config will be timed out
#Connect ssh on default port
ssh bob@server_address
ssh: connect to host port 22: Connection timed out
Prevent bruteforce and DoS
Bruteforce and Denial Of Service are both automated attacks that you can prevent
by using tools specially made for this purpose.
Fail2ban
Fail2ban is designed to ban users which fail to login correctly on your server,
its main purpose is to prevent malicious users to bruteforce your password.
To install fail2ban under CentOS 6, you need to add the EPEL repository :
rpm -ivh http://download.fedoraproject.org/pub/epel/6/i386/epel-release-67.noarc...
rpm –import https://fedoraproject.org/static/0608B895.txt
yum install fail2ban
Then edit the configuration file as you wish :
vim /etc/fail2ban/jail.conf
[DEFAULT]
# "ignoreip" can be an IP address, a CIDR mask or a DNS host. Fail2ban will not
# ban a host which matches an address in this list. Several addresses can be
# defined using space separator.
ignoreip = 127.0.0.1
# "bantime" is the number of seconds that a host is banned.
bantime = 240
# A host is banned if it has generated "maxretry" during the last "findtime"
# seconds.
findtime = 240
# "maxretry" is the number of failures before a host get banned.
maxretry = 10
Don't forget to start fail2ban service :
service fail2ban start
DDOS Deflate
DDos Deflate automatically detects and blocks denial of service attempts. Switch
to a folder where you will download the DDoS Deflate script:
wget http://www.inetbase.com/scripts/ddos/install.sh
chmod 0700 install.sh
chmod 0700 install.sh
./install.sh
A ddos.conf configuration file has been created under /usr/local/ddos/ddos.conf,
have a look inside, it's commented well. A software cron job is installed and

5. will regurlarly to the DoS checking.
ls -l /etc/cron.d
-rw-r--r-- 1 root root
74 Jun 20 00:15 ddos.cron
# /usr/local/ddos/ddos.sh --help
DDoS-Deflate version 0.6
Copyright (C) 2005, Zaf <zaf@vsnl.com>
Usage: ddos.sh [OPTIONS] [N]
N : number of tcp/udp
connections (default 150)
OPTIONS:
-h | --help: Show
this help screen
-c | --cron: Create cron job to run this script regularly (default 1 mins)
-k | --kill: Block the offending ip making more than N connections

6. will regurlarly to the DoS checking.
ls -l /etc/cron.d
-rw-r--r-- 1 root root
74 Jun 20 00:15 ddos.cron
# /usr/local/ddos/ddos.sh --help
DDoS-Deflate version 0.6
Copyright (C) 2005, Zaf <zaf@vsnl.com>
Usage: ddos.sh [OPTIONS] [N]
N : number of tcp/udp
connections (default 150)
OPTIONS:
-h | --help: Show
this help screen
-c | --cron: Create cron job to run this script regularly (default 1 mins)
-k | --kill: Block the offending ip making more than N connections