Linux: 20 Iptables Examples For New SysAdmins

by nixCraft on December 13, 2011 · 52 comments· Last updated March 20, 2012

Linux comes with a host based firewall called Netfilter. According to the official project site:

netfilter is a set of hooks inside the Linux kernel that allows kernel modules to register callback functions with the network stack. A registered callback function is then called back for every packet that traverses the respective hook within the network stack.

This Linux based firewall is controlled by the program called iptables to handles filtering for IPv4, and ip6tables handles filtering for IPv6. I strongly recommend that you first read our quick tutorial that explains how to configure a host-based firewall called Netfilter (iptables) under CentOS / RHEL / Fedora / Redhat Enterprise Linux. This post list most common iptables solutions required by a new Linux user to secure his or her Linux operating system from intruders.

IPTABLES Rules Example

Most of the actions listed in this post are written with the assumption that they will be executed by the root user running the bash or any other modern shell. Do not type commands on remote system as it will disconnect your access.
For demonstration purpose I've used RHEL 6.x, but the following command should work with any modern Linux distro.
This is NOT a tutorial on how to set iptables. See tutorial here. It is a quick cheat sheet to common iptables commands.

#1: Displaying the Status of Your Firewall

Type the following command as root:
# iptables -L -n -v
Sample outputs:

Chain INPUT (policy ACCEPT 0 packets, 0 bytes)
 pkts bytes target     prot opt in     out     source               destination
Chain FORWARD (policy ACCEPT 0 packets, 0 bytes)
 pkts bytes target     prot opt in     out     source               destination
Chain OUTPUT (policy ACCEPT 0 packets, 0 bytes)
 pkts bytes target     prot opt in     out     source               destination

Above output indicates that the firewall is not active. The following sample shows an active firewall:
# iptables -L -n -v
Sample outputs:

Chain INPUT (policy DROP 0 packets, 0 bytes)
 pkts bytes target     prot opt in     out     source               destination
    0     0 DROP       all  --  *      *       0.0.0.0/0            0.0.0.0/0           state INVALID
  394 43586 ACCEPT     all  --  *      *       0.0.0.0/0            0.0.0.0/0           state RELATED,ESTABLISHED
   93 17292 ACCEPT     all  --  br0    *       0.0.0.0/0            0.0.0.0/0
    1   142 ACCEPT     all  --  lo     *       0.0.0.0/0            0.0.0.0/0
Chain FORWARD (policy DROP 0 packets, 0 bytes)
 pkts bytes target     prot opt in     out     source               destination
    0     0 ACCEPT     all  --  br0    br0     0.0.0.0/0            0.0.0.0/0
    0     0 DROP       all  --  *      *       0.0.0.0/0            0.0.0.0/0           state INVALID
    0     0 TCPMSS     tcp  --  *      *       0.0.0.0/0            0.0.0.0/0           tcp flags:0x06/0x02 TCPMSS clamp to PMTU
    0     0 ACCEPT     all  --  *      *       0.0.0.0/0            0.0.0.0/0           state RELATED,ESTABLISHED
    0     0 wanin      all  --  vlan2  *       0.0.0.0/0            0.0.0.0/0
    0     0 wanout     all  --  *      vlan2   0.0.0.0/0            0.0.0.0/0
    0     0 ACCEPT     all  --  br0    *       0.0.0.0/0            0.0.0.0/0
Chain OUTPUT (policy ACCEPT 425 packets, 113K bytes)
 pkts bytes target     prot opt in     out     source               destination
Chain wanin (1 references)
 pkts bytes target     prot opt in     out     source               destination
Chain wanout (1 references)
 pkts bytes target     prot opt in     out     source               destination

Where,

-L : List rules.
-v : Display detailed information. This option makes the list command show the interface name, the rule options, and the TOS masks. The packet and byte counters are also listed, with the suffix 'K', 'M' or 'G' for 1000, 1,000,000 and 1,000,000,000 multipliers respectively.
-n : Display IP address and port in numeric format. Do not use DNS to resolve names. This will speed up listing.

#1.1: To inspect firewall with line numbers, enter:

# iptables -n -L -v --line-numbers
Sample outputs:

Chain INPUT (policy DROP)
num  target     prot opt source               destination
1    DROP       all  --  0.0.0.0/0            0.0.0.0/0           state INVALID
2    ACCEPT     all  --  0.0.0.0/0            0.0.0.0/0           state RELATED,ESTABLISHED
3    ACCEPT     all  --  0.0.0.0/0            0.0.0.0/0
4    ACCEPT     all  --  0.0.0.0/0            0.0.0.0/0
Chain FORWARD (policy DROP)
num  target     prot opt source               destination
1    ACCEPT     all  --  0.0.0.0/0            0.0.0.0/0
2    DROP       all  --  0.0.0.0/0            0.0.0.0/0           state INVALID
3    TCPMSS     tcp  --  0.0.0.0/0            0.0.0.0/0           tcp flags:0x06/0x02 TCPMSS clamp to PMTU
4    ACCEPT     all  --  0.0.0.0/0            0.0.0.0/0           state RELATED,ESTABLISHED
5    wanin      all  --  0.0.0.0/0            0.0.0.0/0
6    wanout     all  --  0.0.0.0/0            0.0.0.0/0
7    ACCEPT     all  --  0.0.0.0/0            0.0.0.0/0
Chain OUTPUT (policy ACCEPT)
num  target     prot opt source               destination
Chain wanin (1 references)
num  target     prot opt source               destination
Chain wanout (1 references)
num  target     prot opt source               destination

You can use line numbers to delete or insert new rules into the firewall.

#1.2: To display INPUT or OUTPUT chain rules, enter:

# iptables -L INPUT -n -v # iptables -L OUTPUT -n -v --line-numbers

#2: Stop / Start / Restart the Firewall

If you are using CentOS / RHEL / Fedora Linux, enter:
# service iptables stop # service iptables start # service iptables restart
You can use the iptables command itself to stop the firewall and delete all rules:
# iptables -F # iptables -X # iptables -t nat -F # iptables -t nat -X # iptables -t mangle -F # iptables -t mangle -X # iptables -P INPUT ACCEPT # iptables -P OUTPUT ACCEPT # iptables -P FORWARD ACCEPT
Where,

-F : Deleting (flushing) all the rules.
-X : Delete chain.
-t table_name : Select table (called nat or mangle) and delete/flush rules.
-P : Set the default policy (such as DROP, REJECT, or ACCEPT).

#3: Delete Firewall Rules

To display line number along with other information for existing rules, enter:
# iptables -L INPUT -n --line-numbers # iptables -L OUTPUT -n --line-numbers # iptables -L OUTPUT -n --line-numbers | less # iptables -L OUTPUT -n --line-numbers | grep 202.54.1.1
You will get the list of IP. Look at the number on the left, then use number to delete it. For example delete line number 4, enter:
# iptables -D INPUT 4
OR find source IP 202.54.1.1 and delete from rule:
# iptables -D INPUT -s 202.54.1.1 -j DROP
Where,

-D : Delete one or more rules from the selected chain

#4: Insert Firewall Rules

To insert one or more rules in the selected chain as the given rule number use the following syntax. First find out line numbers, enter:
# iptables -L INPUT -n --line-numbers
Sample outputs:

Chain INPUT (policy DROP)
num  target     prot opt source               destination
1    DROP       all  --  202.54.1.1           0.0.0.0/0
2    ACCEPT     all  --  0.0.0.0/0            0.0.0.0/0           state NEW,ESTABLISHED

To insert rule between 1 and 2, enter:
# iptables -I INPUT 2 -s 202.54.1.2 -j DROP
To view updated rules, enter:
# iptables -L INPUT -n --line-numbers
Sample outputs:

Chain INPUT (policy DROP)
num  target     prot opt source               destination
1    DROP       all  --  202.54.1.1           0.0.0.0/0
2    DROP       all  --  202.54.1.2           0.0.0.0/0
3    ACCEPT     all  --  0.0.0.0/0            0.0.0.0/0           state NEW,ESTABLISHED

#5: Save Firewall Rules

To save firewall rules under CentOS / RHEL / Fedora Linux, enter:
# service iptables save
In this example, drop an IP and save firewall rules:
# iptables -A INPUT -s 202.5.4.1 -j DROP # service iptables save
For all other distros use the iptables-save command:
# iptables-save > /root/my.active.firewall.rules # cat /root/my.active.firewall.rules

#6: Restore Firewall Rules

To restore firewall rules form a file called /root/my.active.firewall.rules, enter:
# iptables-restore < /root/my.active.firewall.rules
To restore firewall rules under CentOS / RHEL / Fedora Linux, enter:
# service iptables restart

#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,

--connlimit-above 3 : Match if the number of existing connections is above 3.
--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.

Conclusion:

This post only list basic rules for new Linux users. You can create and build more complex rules. This requires good understanding of TCP/IP, Linux kernel tuning via sysctl.conf, and good knowledge of your own setup. Stay tuned for next topics:

Stateful packet inspection.
Using connection tracking helpers.
Network address translation.
Layer 2 filtering.
Firewall testing tools.
Dealing with VPNs, DNS, Web, Proxy, and other protocols.

You should follow me on twitter here or grab rss feed to keep track of new changes.

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{ 52 comments… read them below or add one }

1 Happysysadm December 13, 2011 at 10:10 am

This is a nice breakdown of IPTABLES indeed! Thank you for taking the time for such a comprehensive explaination… I shall bookmark this!

2 logicos December 13, 2011 at 11:56 am

Try ferm, “for Easy Rule Making” .

In file like “ferm.conf” :

chain ( INPUT OUTPUT FORWARD ) policy DROP;
chain INPUT proto tcp dport ssh ACCEPT;

And next:
ferm -i ferm.conf

Source: http://ferm.foo-projects.org/

3 LeftMeAlone December 13, 2011 at 1:58 pm

Can any one tell me the difference between the DROP vs REJECT? Which one is recommended for my mail server?

4 Worked December 13, 2011 at 2:59 pm: LeftMeAlone, “drop” does not send anything to the remote socket while “reject” sending the following message to the remote socket: (icmp destination port unrechable).
Make clean… “drop” maybe the service does not exists. “reject” you can not access to the service.
Reply

5 Joeman1 December 13, 2011 at 3:07 pm

@LeftMeAlone

DROP will silently drop a packet, not notifying the remote host of any problems, just won’t be available. This way, they will no know if the port is active and prohibited or just not used.

REJECT will send an ICMP packet back to the remote host explaining (For the lack of better words) that the host is administratively denied.

The former is preferred as a remote host will not be able to determine if the port is even up.

The latter is not recommended unless software requires the ICMP message for what ever reason. Its not recommended because the remote host will know that the port is in use, but will not be able to connect to it. This way, they can still try to hack the port and get into the system,

Hope this helps!
Joe

6 Prabal Mishra December 13, 2011 at 3:36 pm

thanks !

help for Iptables…………..

7 smilyface December 13, 2011 at 4:11 pm

Thankssss..

8 noone December 13, 2011 at 7:28 pm

how about you try
host -t a http://www.facebook.com
a few times, just to see how dns round-rbin works…

9 noone December 13, 2011 at 7:37 pm

also, you can try this

#!/bin/bash
# Clear any previous rules.
/sbin/iptables -F
# Default drop policy.
/sbin/iptables -P INPUT DROP
/sbin/iptables -P OUTPUT ACCEPT
# Allow anything over loopback and vpn.
/sbin/iptables -A INPUT -i lo -s 127.0.0.1 -d 127.0.0.1 -j ACCEPT
/sbin/iptables -A OUTPUT -o lo -s 127.0.0.1 -d 127.0.0.1 -j ACCEPT
/sbin/iptables -A INPUT -i tun0 -j ACCEPT
/sbin/iptables -A OUTPUT -o tun0 -j ACCEPT
/sbin/iptables -A INPUT -p esp -j ACCEPT
/sbin/iptables -A OUTPUT -p esp -j ACCEPT
# Drop any tcp packet that does not start a connection with a syn flag.
/sbin/iptables -A INPUT -p tcp ! --syn -m state --state NEW -j DROP
# Drop any invalid packet that could not be identified.
/sbin/iptables -A INPUT -m state --state INVALID -j DROP
# Drop invalid packets.
/sbin/iptables -A INPUT -p tcp -m tcp --tcp-flags FIN,SYN,RST,PSH,ACK,URG NONE -j DROP
/sbin/iptables -A INPUT -p tcp -m tcp --tcp-flags SYN,FIN SYN,FIN              -j DROP
/sbin/iptables -A INPUT -p tcp -m tcp --tcp-flags SYN,RST SYN,RST              -j DROP
/sbin/iptables -A INPUT -p tcp -m tcp --tcp-flags FIN,RST FIN,RST              -j DROP
/sbin/iptables -A INPUT -p tcp -m tcp --tcp-flags ACK,FIN FIN                  -j DROP
/sbin/iptables -A INPUT -p tcp -m tcp --tcp-flags ACK,URG URG                  -j DROP
# Reject broadcasts to 224.0.0.1
/sbin/iptables -A INPUT -s 224.0.0.0/4 -j DROP
/sbin/iptables -A INPUT -d 224.0.0.0/4 -j DROP
/sbin/iptables -A INPUT -s 240.0.0.0/5 -j DROP
# Blocked ports
/sbin/iptables -A INPUT -p tcp -m state --state NEW,ESTABLISHED,RELATED --dport 8010 -j DROP
# Allow TCP/UDP connections out. Keep state so conns out are allowed back in.
/sbin/iptables -A INPUT  -p tcp -m state --state ESTABLISHED     -j ACCEPT
/sbin/iptables -A OUTPUT -p tcp -m state --state NEW,ESTABLISHED -j ACCEPT
/sbin/iptables -A INPUT  -p udp -m state --state ESTABLISHED     -j ACCEPT
/sbin/iptables -A OUTPUT -p udp -m state --state NEW,ESTABLISHED -j ACCEPT
# Allow only ICMP echo requests (ping) in. Limit rate in. Uncomment if needed.
/sbin/iptables -A INPUT  -p icmp -m state --state NEW,ESTABLISHED --icmp-type echo-reply -j ACCEPT
/sbin/iptables -A OUTPUT -p icmp -m state --state NEW,ESTABLISHED --icmp-type echo-request -j ACCEPT
# or block ICMP allow only ping out
/sbin/iptables -A INPUT  -p icmp -m state --state NEW -j DROP
/sbin/iptables -A INPUT  -p icmp -m state --state ESTABLISHED -j ACCEPT
/sbin/iptables -A OUTPUT -p icmp -m state --state NEW,ESTABLISHED -j ACCEPT
# Allow ssh connections in.
#/sbin/iptables -A INPUT -p tcp -s 1.2.3.4 -m tcp --dport 22 -m state --state NEW,ESTABLISHED,RELATED -m limit --limit 2/m -j ACCEPT
# Drop everything that did not match above or drop and log it.
#/sbin/iptables -A INPUT   -j LOG --log-level 4 --log-prefix "IPTABLES_INPUT: "
/sbin/iptables -A INPUT   -j DROP
#/sbin/iptables -A FORWARD -j LOG --log-level 4 --log-prefix "IPTABLES_FORWARD: "
/sbin/iptables -A FORWARD -j DROP
#/sbin/iptables -A OUTPUT  -j LOG --log-level 4 --log-prefix "IPTABLES_OUTPUT: "
/sbin/iptables -A OUTPUT  -j ACCEPT
iptables-save > /dev/null 2>&1

10 Coolm@x December 13, 2011 at 7:38 pm

Nice examples, but missing one. Commonly searched rule is one for masquerade.

11 Roy December 13, 2011 at 10:19 pm

This is extremely useful, somekind of magic and quick recipe…
(Of course now i can’t send mail on my remote server (to strict rate limit …))

12 3y3lop December 14, 2011 at 3:00 am

Nice examples & thanks.

13 Jani December 15, 2011 at 9:00 am

.. I’m anxiously awaiting similar translated to ip6tables. :-)

14 Howard December 22, 2011 at 3:24 am

A most excellent presentation of iptables setup and use. Really Superior work. Thanks kindly.

15 Linus Gasser December 22, 2011 at 7:32 pm

Point 8:

And for the private address ranges to block on public interfaces, you’ll also want to block

169.254/16 – zeroconf

16 Pieter December 23, 2011 at 5:44 pm

Nice post, thanks! In example #19 there is an error in the last line:

## open access to mysql server for lan users only ##
iptables -I INPUT -p tcp --dport 3306 -j ACCEPT

Should probably be:

## open access to mysql server for lan users only ##
iptables -I INPUT -p tcp -s 192.168.1.0/24 --dport 3306 -j ACCEPT

17 shawn cao February 24, 2012 at 4:33 am: that is right.
Reply

18 Alejandro December 23, 2011 at 11:15 pm

Thanks for this post, I hope you don’t mind if I translate this to spanish and post it on my blog, Mentioning the original source, of course.

Regards

19 strangr December 24, 2011 at 12:41 am

Simple rules to share your connection to internet (interface IFNAME) with other hosts on your local LAN (NATTED_SUBNET).
In other words how to do NAT and MASQEURADEing.

IFNAME=ppp0
NATTED_SUBNET=192.168.2.0/24

# 1) load appropriate kernel module

modprobe iptable_nat

# 2) make sure IPv4 forwarding is enabled

echo 1 > /proc/sys/net/ipv4/ip_forward

# 3) the appropriate rules

iptables -A POSTROUTING -t nat -o $IFNAME -s $NATTED_SUBNET -d 0/0 \
         -j MASQUERADE
iptables -A FORWARD -t filter -o $IFNAME -s $NATTED_SUBNET -m state \
        --state NEW,ESTABLISHED,RELATED -j ACCEPT
iptables -A FORWARD -t filter -i $IFNAME -d $NATTED_SUBNET -m state \
        --state ESTABLISHED,RELATED -j ACCEPT

20 JD December 31, 2011 at 2:27 am

## open access to mysql server for lan users only ##
iptables -I INPUT -p tcp -s 192.168.1.0/24 –dport 3306 -j ACCEPT

This should be like this:

-s 192.168.1.0/24 -d 192.168.2.2 -i eth0 -p tcp -m state –state NEW -m tcp –dport 3306 -j ACCEPT

a rule like this should go under RELATED,ESTABLISHED in the INPUT chain

21 JD December 31, 2011 at 2:39 am

For email servers, I have rate limiting rules in place for all service ports.

In the INPUT chain I have the spam firewall ip(s), allowed via port 25.

Then for the email ports, I impose a hit count of 10 in 60 seconds, smart phones, email clients do not poll every second. Anything more than this is dropped and they can continue on a rampage with no affect on the server(s). It took me a while to come up with the rate-limiting chains to work with the email server. Since the Watch Guard XCS devices needed to be exempt from the rules. They have rate-limits on incoming connections as well, a lot better than Barracuda.

I always specify the source/destination interface, state then the port.

22 MB January 3, 2012 at 8:17 am: How do i open the port 25 on a public ip (eg. 1.2.3.4) because it is close, I can only send email but can’t receive email?
But on my localhost it’s open, when I test I able to send and receive only on 127.0.0.1. This is my rule
iptables -A INPUT -p tcp -m tcp –dport 25 -j ACCEPT
when i check netstat -tulpn | grep :25
tcp 0 0 127.0.0.1:25 0.0.0.0:* LISTEN 2671/exim4
tcp6 0 0 ::1:25 :::* LISTEN 2671/exim4
Hope you can help me on this matter. I really confused on this one.
Reply

23 Badr Najah January 2, 2012 at 6:55 pm

Very useful.
Thanks

24 dilip January 5, 2012 at 7:36 am

Wooooooooooowwwwww. thats coooool…
very usefull link….

Thanks yar….

25 nbasileu January 9, 2012 at 10:19 am

Rule #14

## *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

–destination-port 8080 not 22

Anyway, this is a fu**** good website with fully nice articles.
Very big thx dudes.

Happy new year everyone.

26 Atul Modi March 11, 2012 at 10:16 am

Excellent Stuff Guys!!!

Everyone is putting their part. Great to see this kind of community flourish further.

I am thankful to the ppl who started this website.

27 Daniel Vieceli March 13, 2012 at 2:38 pm

Excellent thanks.

28 jm April 1, 2012 at 3:48 am

Good info and well written.Easy to understand for everyone… I will be back to learn more needed security rules.. Oh and yes I’m a human but I hate to say the definition of human is ( MONSTER) don’t believe me ? Look it up on the net ! Ha ha ha ha
Thank you for this page….

29 rw1 April 5, 2012 at 7:45 am

thank you! for the information on how to delete a firewall rule! priceless! thanks!

30 Eli May 11, 2012 at 12:19 am

How can i use iptable rules to use multiple internet connections for the same bit torrent download?
Actually, i have two broadband connections. I want to combine them. I am told to get load balancing hardware and i cant afford that. So, i did some experimenting. On first DSL modem, i set its IP to be 192.168.1.1
On second modem, i set its IP to be 192.168.2.1
Then in windows network adapter settings, i set Metric value of each adapter to 1. Thats about it. My bit torrent downloads/uploads use both my internet connections at the same time which gives effect of combined speed.
Can i do something like that in Linux?
Or, how can i combine two internet connections by using iptables? I dont want any hardware changes. All i have is two DSL modems and two network interface cards. Precise help would be greatly appreciated.

31 kolya May 13, 2012 at 6:55 pm

Hi, got a question to the author of the article. I have tried different kind of commands from the command line, edited the file /etc/sysconfig/iptables directly with following saving and restarting iptables/rebooting system. Nothing helps, my rules get overwritten by the system flushing my new rules or editing them. I tried to open ports (22,21 etc). The goal why I edit my firewall is to get connected to ftp server via FileZilla. Would you recommend me how to open ports? Tell me please if you need any system outputs or something. Cheers

32 Vivek Gite May 13, 2012 at 8:35 pm

> my rules get overwritten by the system flushing my new rules or editing them

I think you got some sort of script or other firewall product running that is overwriting your rules. Check your cron job and you find the source for the same. If you need further assistance head over to the nixcraft Linux Support forum.

33 kolya May 14, 2012 at 12:21 pm

thanks for your respond, as I am not a specialist I didn’t any changes to my crontab yet, anyway I checked it, also /cron.d and everything connected to cron in /var/spool/…. Nothing about iptables or something. What I noticed there are several iptables files in /etc/sysconfig/: iptables.old written by system-config-firewall, iptables generated by iptables-save with some changes what I didn’t entered.
Here is what I entered from wiki.centos.org/HowTos/Network/IPTables:

# iptables -P INPUT ACCEPT
# iptables -F
# iptables -A INPUT -i lo -j ACCEPT
# iptables -A INPUT -m state –state ESTABLISHED,RELATED -j ACCEPT
# iptables -A INPUT -p tcp –dport 22 -j ACCEPT
# iptables -P INPUT DROP
# iptables -P FORWARD DROP
# iptables -P OUTPUT ACCEPT
# iptables -L -v

Here is what I got in the iptables’s file:

:INPUT DROP [1:40]
:FORWARD DROP [0:0]
:OUTPUT ACCEPT [526:43673]
-A INPUT -i lo -j ACCEPT
-A INPUT -m state –state RELATED,ESTABLISHED -j ACCEPT
-A INPUT -p tcp -m tcp –dport 22 -j ACCEPT
COMMIT
Don’t know why it changes, probably it is aplying kind of default settings, but analyzing this settings the port 22 should be open. Nmap says it is closed, telnet outputs connection refused. Was trying to set samba server with the same result due to my firewall. What to do?

34 Sigma May 25, 2012 at 6:53 am

Thanks a lot for this article, which is extremely easy to understand and follow for beginners as me!

35 dima June 9, 2012 at 10:38 am

Hi
Regarding the block #7.1: Only Block Incoming Traffic
The rule
# iptables -A INPUT -m state –state NEW,ESTABLISHED -j ACCEPT
looks dubious to me
Why would you want to allow NEW connections?
In my view it should read
# iptables -A INPUT -m state –state RELATED,ESTABLISHED -j ACCEPT

36 BiBi June 21, 2012 at 3:24 am

Thank you very much, this site is very useful. I love all of you.

37 Juan July 14, 2012 at 1:53 pm

Hi.
Excellent tutorial. My desire is to block social networking in my job, I did it with squid in transparent mode but skipped to enter https. I did the tests on a virtual pc and it worked fine. The issue is that I is working on the production server. This has two network cards, eth0 traffic where it enters the Internet and eth1 to connect to the network. For the case of Facebook do the following:

# We block Facebook
iptables-A OUTPUT-p tcp-d 69.63.176.0/20-dport 443-j DROP
iptables-A OUTPUT-p tcp-d 66.220.144.0/20-dport 443-j DROP
iptables-A OUTPUT-p tcp-d 69.171.224.0/19-dport 443-j DROP
iptables-A OUTPUT-p tcp-d http://www.facebook.com-dport 443-j DROP
iptables-A OUTPUT-p tcp-d facebook.com-dport 443-j DROP

Any suggestions?.

Greetings.

38 jaydatt August 30, 2012 at 10:47 am

really helpful article

39 Borislav Bozhanov September 11, 2012 at 11:13 pm

Hi,

Here is how to BLOCK FACEBOOK with single line command and iptables:

for i in $(nslookup facebook.com|grep Address|grep -v “#53″|awk ‘{print $2}’); do iptables -I FORWARD -m tcp -p tcp -d $i/24 –dport 443 -j DROP; done

You can replace the website with any other secure (https) you want.

For http websites (non-secure) – use the following line, replacing yahoo.com with the desired domain name:
for i in $(nslookup yahoo.com|grep Address|grep -v “#53″|awk ‘{print $2}’); do iptables -I FORWARD -m tcp -p tcp -d $i/24 –dport 80 -j DROP; done

Don’t forget to save your iptables configuration.

Regards,
Borislav Bozhanov

40 Łukasz Bodziony September 13, 2012 at 7:37 pm

Thank you!!!

41 Rogier October 23, 2012 at 5:48 am

Hi, I have two interfaces: eth0 (for internal network) and eth1 (WAN). The server does the routing to the clients with the following IPtables:

# Generated by iptables-save v1.4.12 on Fri Oct 19 21:14:26 2012
*nat
:PREROUTING ACCEPT [14:1149]
:INPUT ACCEPT [6:625]
:OUTPUT ACCEPT [4:313]
:POSTROUTING ACCEPT [0:0]
-A POSTROUTING -o eth1 -j MASQUERADE
COMMIT
# Completed on Fri Oct 19 21:14:26 2012
# Generated by iptables-save v1.4.12 on Fri Oct 19 21:14:26 2012
*filter
:INPUT ACCEPT [505:53082]
:FORWARD ACCEPT [0:0]
:OUTPUT ACCEPT [247:29622]
-A FORWARD -s 192.168.1.0/24 -i eth0 -o eth1 -m conntrack --ctstate NEW -j ACCEPT
-A FORWARD -m conntrack --ctstate RELATED,ESTABLISHED -j ACCEPT
COMMIT
# Completed on Fri Oct 19 21:14:26 2012

This works fine, however I have no other rules set up. Can anyone help me in deciding what rules I need? the server (who does the NAT) is also running a webserver on port 80, SSH server on 22. All other ports can may be blocked.. how can I achieve this?

42 Jorge Robles October 24, 2012 at 2:37 pm: I use fwbuilder to create my rules, this interface “looks like” checkpoint’s fw1 client to edit rules. Very graphical, and good to work with.
Reply

43 sahil November 8, 2012 at 10:14 am

very nice and informative article
it really helped to work for my VPS server

44 bussy November 9, 2012 at 8:09 pm

how i do give access ip ex 192.168.0.2 only for facebook .

45 Sayajin December 19, 2012 at 7:27 am: fb_address=$(dig facebook.com +tcp +short);
iptables -A OUTPUT -p tcp -s !192.168.0.2/32 -d $fb_address -j DROP;
Reply

46 bahathir November 25, 2012 at 3:17 am

For tip #2, it is advisable to run the -P chain ACCEPt first, before flushing it.
Exampes
# iptables -P INPUT ACCEPT
# iptables -P OUTPUT ACCEPT
# iptables -P FORWARD ACCEPT
# iptables -F
# iptables -t nat -F
# iptables -X
#iptables -t nat -X

Why? If the current chain’s policy is DROP, and you are remotely accessing to the server via SSH, and the rule “-A INPUT -p tcp –dport 22 -j ACCEPT” is still opens the “-P INPUT DROP”. you may disconnected as soon as you flush *-F* the rules, and the default policy “-P INPUT DROP” kicks in. :) If you are working on the local console, it is fine.

Thank you.

47 levi November 27, 2012 at 4:24 am

Could it be you are using iptables save after directly editing? This will overwrite your work. Do a restart to load your newly edited table.

48 KeepEnEmUp December 8, 2012 at 2:32 am

Great Thx for awesome site and for awesome reading,tutos.
Respect And KeepUp dude!

49 rashid Iqbal December 13, 2012 at 11:43 am

from graphical user and groups If I add or delete any user I can’t see any reference log nor in messages or in /var/log/secure file,

Kindly please advise on this that from GUI if we run/execute any command where does the log message will go.

50 Gangadhar February 27, 2013 at 2:50 pm

thank you very much for such a wonderful explanation….. very clear and had nice experience with iptables…

51 haidt March 10, 2013 at 9:04 am

Hi there,

i have a problem, i have got a server and LAN network, and this’s feature

internet (eth0) server (eth1) clients -> 10.0.0.2
-> 10.0.0.3
now, i can config to iptables accept all client connect internet, but in this situation, i want to allow only one client (assume: 10.0.0.3), i try but not completed. pls help me :)

Thanks

52 Manish Narnaware April 24, 2013 at 5:33 am

Thanks a lot.