Configuration Iptables on linux/centos/Ubuntu

7: Set the Default Firewall Policies

To drop all traffic:
# iptables -P INPUT DROP
# iptables -P OUTPUT DROP
# iptables -P FORWARD DROP
# iptables -L -v -n
#### you will not able to connect anywhere as all traffic is dropped ###
# ping cyberciti.biz
# wget http://www.kernel.org/pub/linux/kernel/v3.0/testing/linux-3.2-rc5.tar.bz2

#7.1: Only Block Incoming Traffic

To drop all incoming / forwarded packets, but allow outgoing traffic, enter:
# iptables -P INPUT DROP
# iptables -P FORWARD DROP
# iptables -P OUTPUT ACCEPT
# iptables -A INPUT -m state --state NEW,ESTABLISHED -j ACCEPT
# iptables -L -v -n
### *** now ping and wget should work *** ###
# ping cyberciti.biz
# wget http://www.kernel.org/pub/linux/kernel/v3.0/testing/linux-3.2-rc5.tar.bz2

#8:Drop Private Network Address On Public Interface

IP spoofing is nothing but to stop the following IPv4 address ranges for private networks on your public interfaces. Packets with non-routable source addresses should be rejected using the following syntax:
# iptables -A INPUT -i eth1 -s 192.168.0.0/24 -j DROP
# iptables -A INPUT -i eth1 -s 10.0.0.0/8 -j DROP

#8.1: IPv4 Address Ranges For Private Networks (make sure you block them on public interface)

• 10.0.0.0/8 -j (A)
• 172.16.0.0/12 (B)
• 192.168.0.0/16 (C)
• 224.0.0.0/4 (MULTICAST D)
• 240.0.0.0/5 (E)
• 127.0.0.0/8 (LOOPBACK)
  

#9: Blocking an IP Address (BLOCK IP)

To block an attackers ip address called 1.2.3.4, enter:
# iptables -A INPUT -s 1.2.3.4 -j DROP
# iptables -A INPUT -s 192.168.0.0/24 -j DROP

#10: Block Incoming Port Requests (BLOCK PORT)

To block all service requests on port 80, enter:
# iptables -A INPUT -p tcp --dport 80 -j DROP
# iptables -A INPUT -i eth1 -p tcp --dport 80 -j DROP
To block port 80 only for an ip address 1.2.3.4, enter:
# iptables -A INPUT -p tcp -s 1.2.3.4 --dport 80 -j DROP
# iptables -A INPUT -i eth1 -p tcp -s 192.168.1.0/24 --dport 80 -j DROP

#11: Block Outgoing IP Address

To block outgoing traffic to a particular host or domain such as cyberciti.biz, enter:
# host -t a cyberciti.biz
Sample outputs:

cyberciti.biz has address 75.126.153.206

Note down its ip address and type the following to block all outgoing traffic to 75.126.153.206:
# iptables -A OUTPUT -d 75.126.153.206 -j DROP
You can use a subnet as follows:
# iptables -A OUTPUT -d 192.168.1.0/24 -j DROP
# iptables -A OUTPUT -o eth1 -d 192.168.1.0/24 -j DROP

#11.1: Example - Block Facebook.com Domain

First, find out all ip address of facebook.com, enter:
# host -t a www.facebook.com
Sample outputs:

www.facebook.com has address 69.171.228.40

Find CIDR for 69.171.228.40, enter:
# whois 69.171.228.40 | grep CIDR
Sample outputs:

CIDR:           69.171.224.0/19

To prevent outgoing access to www.facebook.com, enter:
# iptables -A OUTPUT -p tcp -d 69.171.224.0/19 -j DROP
You can also use domain name, enter:
# iptables -A OUTPUT -p tcp -d www.facebook.com -j DROP
# iptables -A OUTPUT -p tcp -d facebook.com -j DROP
From the iptables man page:

... specifying any name to be resolved with a remote query such as DNS (e.g., facebook.com is a really bad idea), a network IP address (with /mask), or a plain IP address ...

#12: Log and Drop Packets

Type the following to log and block IP spoofing on public interface called eth1
# iptables -A INPUT -i eth1 -s 10.0.0.0/8 -j LOG --log-prefix "IP_SPOOF A: "
# iptables -A INPUT -i eth1 -s 10.0.0.0/8 -j DROP
By default everything is logged to /var/log/messages file.
# tail -f /var/log/messages
# grep --color 'IP SPOOF' /var/log/messages

#13: Log and Drop Packets with Limited Number of Log Entries

The -m limit module can limit the number of log entries created per time. This is used to prevent flooding your log file. To log and drop spoofing per 5 minutes, in bursts of at most 7 entries .
# iptables -A INPUT -i eth1 -s 10.0.0.0/8 -m limit --limit 5/m --limit-burst 7 -j LOG --log-prefix "IP_SPOOF A: "
# iptables -A INPUT -i eth1 -s 10.0.0.0/8 -j DROP

#14: Drop or Accept Traffic From Mac Address

Use the following syntax:
# iptables -A INPUT -m mac --mac-source 00:0F:EA:91:04:08 -j DROP
## *only accept traffic for TCP port # 8080 from mac 00:0F:EA:91:04:07 * ##
# iptables -A INPUT -p tcp --destination-port 22 -m mac --mac-source 00:0F:EA:91:04:07 -j ACCEPT

#15: Block or Allow ICMP Ping Request

Type the following command to block ICMP ping requests:
# iptables -A INPUT -p icmp --icmp-type echo-request -j DROP
# iptables -A INPUT -i eth1 -p icmp --icmp-type echo-request -j DROP
Ping responses can also be limited to certain networks or hosts:
# iptables -A INPUT -s 192.168.1.0/24 -p icmp --icmp-type echo-request -j ACCEPT
The following only accepts limited type of ICMP requests:
### ** assumed that default INPUT policy set to DROP ** #############
iptables -A INPUT -p icmp --icmp-type echo-reply -j ACCEPT
iptables -A INPUT -p icmp --icmp-type destination-unreachable -j ACCEPT
iptables -A INPUT -p icmp --icmp-type time-exceeded -j ACCEPT
## ** all our server to respond to pings ** ##
iptables -A INPUT -p icmp --icmp-type echo-request -j ACCEPT

#16: Open Range of Ports

Use the following syntax to open a range of ports:
iptables -A INPUT -m state --state NEW -m tcp -p tcp --dport 7000:7010 -j ACCEPT

#17: Open Range of IP Addresses

Use the following syntax to open a range of IP address:
## only accept connection to tcp port 80 (Apache) if ip is between 192.168.1.100 and 192.168.1.200 ##
iptables -A INPUT -p tcp --destination-port 80 -m iprange --src-range 192.168.1.100-192.168.1.200 -j ACCEPT
## nat example ##
iptables -t nat -A POSTROUTING -j SNAT --to-source 192.168.1.20-192.168.1.25

#18: Established Connections and Restaring The Firewall

When you restart the iptables service it will drop established connections as it unload modules from the system under RHEL / Fedora / CentOS Linux. Edit, /etc/sysconfig/iptables-config and set IPTABLES_MODULES_UNLOAD as follows:

IPTABLES_MODULES_UNLOAD = no

#19: Help Iptables Flooding My Server Screen

Use the crit log level to send messages to a log file instead of console:
iptables -A INPUT -s 1.2.3.4 -p tcp --destination-port 80 -j LOG --log-level crit

#20: Block or Open Common Ports

The following shows syntax for opening and closing common TCP and UDP ports:

 
Replace ACCEPT with DROP to block port:
## open port ssh tcp port 22 ##
iptables -A INPUT -m state --state NEW -m tcp -p tcp --dport 22 -j ACCEPT
iptables -A INPUT -s 192.168.1.0/24 -m state --state NEW -p tcp --dport 22 -j ACCEPT
 
## open cups (printing service) udp/tcp port 631 for LAN users ##
iptables -A INPUT -s 192.168.1.0/24 -p udp -m udp --dport 631 -j ACCEPT
iptables -A INPUT -s 192.168.1.0/24 -p tcp -m tcp --dport 631 -j ACCEPT
 
## allow time sync via NTP for lan users (open udp port 123) ##
iptables -A INPUT -s 192.168.1.0/24 -m state --state NEW -p udp --dport 123 -j ACCEPT
 
## open tcp port 25 (smtp) for all ##
iptables -A INPUT -m state --state NEW -p tcp --dport 25 -j ACCEPT
 
# open dns server ports for all ##
iptables -A INPUT -m state --state NEW -p udp --dport 53 -j ACCEPT
iptables -A INPUT -m state --state NEW -p tcp --dport 53 -j ACCEPT
 
## open http/https (Apache) server port to all ##
iptables -A INPUT -m state --state NEW -p tcp --dport 80 -j ACCEPT
iptables -A INPUT -m state --state NEW -p tcp --dport 443 -j ACCEPT
 
## open tcp port 110 (pop3) for all ##
iptables -A INPUT -m state --state NEW -p tcp --dport 110 -j ACCEPT
 
## open tcp port 143 (imap) for all ##
iptables -A INPUT -m state --state NEW -p tcp --dport 143 -j ACCEPT
 
## open access to Samba file server for lan users only ##
iptables -A INPUT -s 192.168.1.0/24 -m state --state NEW -p tcp --dport 137 -j ACCEPT
iptables -A INPUT -s 192.168.1.0/24 -m state --state NEW -p tcp --dport 138 -j ACCEPT
iptables -A INPUT -s 192.168.1.0/24 -m state --state NEW -p tcp --dport 139 -j ACCEPT
iptables -A INPUT -s 192.168.1.0/24 -m state --state NEW -p tcp --dport 445 -j ACCEPT
 
## open access to proxy server for lan users only ##
iptables -A INPUT -s 192.168.1.0/24 -m state --state NEW -p tcp --dport 3128 -j ACCEPT
 
## open access to mysql server for lan users only ##
iptables -I INPUT -p tcp --dport 3306 -j ACCEPT
 

#21: Restrict the Number of Parallel Connections To a Server Per Client IP

You can use connlimit module to put such restrictions. To allow 3 ssh connections per client host, enter:
# iptables -A INPUT -p tcp --syn --dport 22 -m connlimit --connlimit-above 3 -j REJECT
Set HTTP requests to 20:
# iptables -p tcp --syn --dport 80 -m connlimit --connlimit-above 20 --connlimit-mask 24 -j DROP
Where,

1. --connlimit-above 3 : Match if the number of existing connections is above 3.
2. --connlimit-mask 24 : Group hosts using the prefix length. For IPv4, this must be a number between (including) 0 and 32.
   

#22: HowTO: Use iptables Like a Pro

For more information about iptables, please see the manual page by typing man iptables from the command line:
$ man iptables
You can see the help using the following syntax too:
# iptables -h
To see help with specific commands and targets, enter:
# iptables -j DROP -h

#22.1: Testing Your Firewall

Find out if ports are open or not, enter:
# netstat -tulpn
Find out if tcp port 80 open or not, enter:
# netstat -tulpn | grep :80
If port 80 is not open, start the Apache, enter:
# service httpd start
Make sure iptables allowing access to the port 80:
# iptables -L INPUT -v -n | grep 80
Otherwise open port 80 using the iptables for all users:
# iptables -A INPUT -m state --state NEW -p tcp --dport 80 -j ACCEPT
# service iptables save
Use the telnet command to see if firewall allows to connect to port 80:
$ telnet www.cyberciti.biz 80
Sample outputs:

Trying 75.126.153.206...
Connected to www.cyberciti.biz.
Escape character is '^]'.
^]
telnet> quit
Connection closed.

You can use nmap to probe your own server using the following syntax:
$ nmap -sS -p 80 www.cyberciti.biz
Sample outputs:

Starting Nmap 5.00 ( http://nmap.org ) at 2011-12-13 13:19 IST
Interesting ports on www.cyberciti.biz (75.126.153.206):
PORT   STATE SERVICE
80/tcp open  http
Nmap done: 1 IP address (1 host up) scanned in 1.00 seconds

I also recommend you install and use sniffer such as tcpdupm and ngrep to test your firewall settings.