Linux: 20 Iptables Examples For New SysAdmins

Posted on in Categories Iptables, Linux, Linux distribution, Linux Embedded devices, Linux laptop last updated December 13, 2011

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.

Install Squid Proxy Server on CentOS / Redhat enterprise Linux 5

Posted on in Categories CentOS, Linux, RedHat/Fedora Linux, Squid caching server, Suse Linux, Sys admin, Tips last updated August 30, 2007

I’ve already wrote about setting up a Linux transparent squid proxy system. However I’m getting lots of questions about Squid basic installation and configuration:

How do I install Squid Proxy server on CentOS 5 Liinux server?

Sure Squid server is a popular open source GPLd proxy and web cache. It has a variety of uses, from speeding up a web server by caching repeated requests, to caching web, name server query , and other network lookups for a group of people sharing network resources. It is primarily designed to run on Linux / Unix-like systems. Squid is a high-performance proxy caching server for Web clients, supporting FTP, gopher, and HTTP data objects. Unlike traditional caching software, Squid handles all requests in a single, non-blocking, I/O-driven process. Squid keeps meta data and especially hot objects cached in RAM, caches DNS lookups, supports non-blocking DNS lookups, and implements negative caching of failed requests. Squid consists of a main server program squid, a Domain Name System lookup program (dnsserver), a program for retrieving FTP data (ftpget), and some management and client tools.

Install Squid on CentOS / RHEL 5

Use yum command as follows:
# yum install squid

Loading "installonlyn" plugin
Setting up Install Process
Setting up repositories
Reading repository metadata in from local files
Parsing package install arguments
Resolving Dependencies
--> Populating transaction set with selected packages. Please wait.
---> Package squid.i386 7:2.6.STABLE6-4.el5 set to be updated
--> Running transaction check

Dependencies Resolved

 Package                 Arch       Version          Repository        Size
 squid                   i386       7:2.6.STABLE6-4.el5  updates           1.2 M

Transaction Summary
Install      1 Package(s)
Update       0 Package(s)
Remove       0 Package(s)

Total download size: 1.2 M
Is this ok [y/N]: y
Downloading Packages:
Running Transaction Test
Finished Transaction Test
Transaction Test Succeeded
Running Transaction
  Installing: squid                        ######################### [1/1]

Installed: squid.i386 7:2.6.STABLE6-4.el5

Squid Basic Configuration

Squid configuration file located at /etc/squid/squid.conf. Open file using a text editor:
# vi /etc/squid/squid.conf
At least you need to define ACL (access control list) to work with squid. The defaults port is TCP 3128. Following example ACL allowing access from your local networks and Make sure you adapt to list your internal IP networks from where browsing should be allowed:
acl our_networks src
http_access allow our_networks

Save and close the file. Start squid proxy server:
# chkconfig squid on
# /etc/init.d/squid start


init_cache_dir /var/spool/squid... Starting squid: .       [  OK  ]

Verify port 3128 is open:
# netstat -tulpn | grep 3128

tcp        0      0      *                   LISTEN      20653/(squid)

Open TCP port 3128

Finally make sure iptables is allowing to access squid proxy server. Just open /etc/sysconfig/iptables file:
# vi /etc/sysconfig/iptables
Append configuration:
-A RH-Firewall-1-INPUT -m state --state NEW,ESTABLISHED,RELATED -m tcp -p tcp --dport 3128 -j ACCEPT
Restart iptables based firewall:
# /etc/init.d/iptables restart

Flushing firewall rules:                                   [  OK  ]
Setting chains to policy ACCEPT: filter                    [  OK  ]
Unloading iptables modules:                                [  OK  ]
Applying iptables firewall rules:                          [  OK  ]
Loading additional iptables modules: ip_conntrack_netbios_n[  OK  ]

Client configuration

Open a webbrowser > Tools > Internet option > Network settings > and setup Squid server IP address and port # 3128.

See also

You may find our previous squid tips useful:

Squid Security and blocking content Related Tips

Squid Authentication Related Tips

Squid Other Tips

Linux Iptables: How to specify a range of IP addresses or ports

Posted on in Categories CentOS, Debian Linux, Gentoo Linux, Iptables, Linux, RedHat/Fedora Linux, Security, Suse Linux, Sys admin, Ubuntu Linux last updated September 18, 2006

Someone recently asked me a question:

How can I save time and script size by specifying a range of IP addresses or ports using iptables?

In old version of iptables IP address ranges are only valid in the nat table (see below for example). However newer version does support option that allows you to specify a range of IP addresses or ports for regular tables such as input.

Iptables set range of IP addresses

You need to use following options with match extensions (-m Ext).

iprange : This matches on a given arbitrary range of IPv4 addresses.

  • [!]–src-range ip-ip: Match source IP in the specified range.
  • [!]–dst-range ip-ip: Match destination IP in the specified range.


-m iprange –src-range IP-IP -j ACTION
-m iprange –dst-range IP-IP -j ACTION

For example, allow incoming request on a port 22 for source IP in the range only. You need to add something as follows to your iptables script:

iptables -A INPUT -p tcp --destination-port 22 -m iprange --src-range -j ACCEPT  

Port range

if –protocol tcp (-p tcp) is specified, you can specify source port range with following syntax:

  • –source-port port:port
  • –sport port:port

And destination port range specification with following option :

  • –destination-port port:port
  • –dport port:port

For example block lock all incoming ssh access at port 22, for source port range 513:65535:

iptables -A INPUT -p tcp -s 0/0 --sport 513:65535 -d --dport 22 -m state --state NEW,ESTABLISHED -j DROP

On the other hand, just allow incoming ssh request with following port range:

iptables -A INPUT -p tcp -s 0/0 -d --sport 513:65535 --dport 22 -m state --state NEW,ESTABLISHED -j ACCEPT
iptables -A OUTPUT -p tcp -s -d 0/0 --sport 22 --dport 513:65535 -m state --state ESTABLISHED -j ACCEPT

NAT table – range option

If you are using NAT table use options –to-source and –to-destination. For example IP address range:

iptables -t nat -A POSTROUTING -j SNAT --to-source

ALTERNATIVELY, try range of ports:

iptables -t nat -A POSTROUTING -j SNAT --to-source

Read man page of iptables for more information.

Linux Iptables block remote X Window server connection

Posted on in Categories Iptables, Linux, Security, X server last updated July 10, 2006

It is true that connections to remote X Window servers should be always made over SSH. SSH supports X windows connections. So my task was allow X over ssh but block unprivileged X windows mangers TCP ports.

The first running server (or display) use TCP port 6000. Next server will use 6001 and so on upto 6063 (max 64 X managers are allowed from 6000-6063).

So assuming that you are going to force user to use ssh for remote connections, here are rules for IPTABLES (add to your firewall script):

iptables -A OUTPUT -o eth0 -p tcp --syn --destination-port 6000:6063 -j REJECT
iptables -A INPUT -i eth0 -p tcp --syn --destination-port 6000:6063 -j DROP

a) The first rules blocks outgoing connection attempt to remove X windows manger.

b) The second rule block incoming request for X windows manger. By using –syn flag you are blocking only connection establishments to the server port.

This is the good way to disallow unprivileged X windows mangers – TCP 6000:6063 ports 🙂

See also:

Connecting Linux or UNIX system to Network attached storage device

Posted on in Categories Backup, CentOS, Data recovery, Debian Linux, File system, FreeBSD, Gentoo Linux, Howto, Linux, Troubleshooting, Ubuntu Linux, UNIX last updated May 20, 2006

Network attached storage (NAS) allows using TCP/IP network to backup files. This enables multiple servers in IDC to share the same storage for backup at once, which minimizes overhead by centrally managing hard disks. NAS is scalable, high performance network solution. The main advantage is more hard disk storage space added to a network that already utilizes servers without shutting them down for maintenance and upgrades.

Please note that NAS are not just common in IDC or offices but you can use it for file sharing and backup at home. You can purchase 200+GB NAS for less than $200 these days. Personally, I am using Maxtor ShareStorage 200GB Network Attached Storage at home. This is a step-by-step guide on connecting Linux or UNIX systems to SAN for backup or sharing files.

The protocol used with NAS is a file-based protocol such as NFS or Microsoft’s Common Internet File System (CIFS). Both of them allow storing backups using UNIX and Linux servers or Windows 2003 server.

However many new Linux or UNIX sys admin find it difficult to use NAS backup. Here are quick handy tips most newbie will find useful.

(A) Use IP address of NAS. If you do not have properly configured SAMBA server it is difficult to resolve hostnames. IP address will save your time.

(B) If you are using IPTABLES or PF firewall then make sure the following UDP/TCP ports are open between your firewall and the NAS Backup Server:

  1. TCP 21 (ftp)
  2. TCP 20 (ftp-data)
  3. TCP/UDP 137 (NETBIOS Name Service aka netbios-ns)
  4. TCP/UDP 138 (NETBIOS Datagram Service aka netbios-dgm)
  5. TCP/UDP 139 (NETBIOS session service aka netbios-ssn )
  6. TCP/UDP 445 (Microsoft Naked CIFS aka microsoft-ds )

Sample network diagram

Following is sample network diagram for our setup:

+-------------+               +-------------+
|             |               |             |
|   N A S     |<=============>|   Linux/    |
|             |               |   UNIX      |
IP:              IP:

Iptables configuration

FTP outgoing client request using iptables (assuming that your server IP is and NAS IP is Append following iptables rules to your script:

iptables -A OUTPUT -p tcp -s –sport 1024:65535 -d –dport 21 -m state –state NEW,ESTABLISHED -j ACCEPT
iptables -A INPUT -p tcp -s –sport 21 -d –dport 1024:65535 -m state –state ESTABLISHED -j ACCEPT
iptables -A OUTPUT -p tcp -s –sport 1024:65535 -d –dport 1024:65535 -m state –state ESTABLISHED,RELATED -j ACCEPT
iptables -A INPUT -p tcp -s –sport 1024:65535 -d –dport 1024:65535 -m state –state ESTABLISHED -j ACCEPT

NETBIOS/CIFS outgoing client request

Please add following rules to your iptables script:

iptables -A OUTPUT -p udp -s –sport 137 -d 0/0 –dport 137 -j ACCEPT
iptables -A OUTPUT -p udp -s –sport 138 -d 0/0 –dport 138 -j ACCEPT
iptables -A OUTPUT -p tcp -s –sport 1024:65535 -d –dport 139 -m state –state NEW,ESTABLISHED -j ACCEPT
iptables -A INPUT -p udp -s –sport 137 -d –dport 137 -j ACCEPT
iptables -A INPUT -p udp -s –sport 138 -d –dport 138 -j ACCEPT
iptables -A INPUT -p tcp -s –sport 139 -d –dport 1024:65535 -m state –state ESTABLISHED -j ACCEPT

Please note that when configuring a firewall, the high order ports (1024-65535) are often used for outgoing connections and therefore should be permitted through the firewall. It is prudent to block incoming packets on the high order ports except for established connections. This is what you are doing in above FTP and CIFS client request.

How do I access NAS server using FTP?

You need to use Internet file transfer program (FTP) that comes with UNIX/Linux or windows. Most service provider will provide you:

  • NAS Server IP (e.g. /
  • NAS FTP Username (e.g. nixcraft)
  • NAS FTP Password (e.g. mySecret)

Let us assume you have file called mysqldump.tar.gz. You can put this file to NAS backup server using following ftp command:

$ ftp


$ ftp


Username: nixcraft
Password: mySecret
ftp> bin
200 Type set to I.
ftp> prom
Interactive mode off.
ftp> put mysqldump.tar.gz
ftp> quit

How do I access NAS server using SAMBA client?

Make sure you have samba client installed. Use apt-get or up2date command to install SAMBA client.

a) Create a directory

# mkdir /backup

b) Mount remote NAS share (NOTE: you must type following command on a single line)

# mount -t smbfs -o username=nixcraft,password=mySecret // /backup


# smbmount -o username=nixcraft,password=mySecret // /backup

You can skip password option for security reason (samba will prompt you for password).

c) Copy files using cp command:

# cp sitebackup.tar.gz /backup

d) You can use /backup directory to dump backup using mysql script or backup shell script.

A note for FreeBSD user

If you would like to access NAS server from FreeBSD use following command (NOTE: you must type following command on a single line):

# mkdir /backup
# mount_smbfs -I [email protected]/sharename /backup



Related previous articles

Updated for accuracy.

Linux: Iptables Allow MYSQL server incoming request on port 3306

Posted on in Categories Howto, Iptables, Linux, MySQL, Security, Tips last updated July 28, 2005

MySQL database is a popular for web applications and acts as the database component of the LAMP, MAMP, and WAMP platforms. Its popularity as a web application is closely tied to the popularity of PHP, which is often combined with MySQL. MySQL is open source database server and by default it listen on TCP port 3306. In this tutorial you will learn how to open TCP port # 3306 using iptables command line tool on Linux operating system.

Linux Iptables block or open DNS / bind service port 53

Posted on in Categories BIND Dns, Debian Linux, Gentoo Linux, Howto, Iptables, Linux, Networking, RedHat/Fedora Linux, Security, Suse Linux, Tips, Troubleshooting last updated July 13, 2005

The domain name service provided by BIND (named) software. It uses both UDP and TCP protocol and listen on port 53. DNS queries less than 512 bytes are transferred using UDP protocol and large queries are handled by TCP protocol such as zone transfer.

i) named/bind server – TCP/UDP port 53

ii)Client (browser, dig etc) – port > 1023

Allow outgoing DNS client request:

Following iptables rules can be added to your shell script.

SERVER_IP is your server ip address

DNS_SERVER stores the nameserver (DNS) IP address provided by ISP or your own name servers.

Following rules are useful when you run single web/smtp server or even DSL/LL/dialup Internet connections:

for ip in $DNS_SERVER
iptables -A OUTPUT -p udp -s $SERVER_IP --sport 1024:65535 -d $ip --dport 53 -m state --state NEW,ESTABLISHED -j ACCEPT
iptables -A INPUT -p udp -s $ip --sport 53 -d $SERVER_IP --dport 1024:65535 -m state --state ESTABLISHED -j ACCEPT
iptables -A OUTPUT-p tcp -s $SERVER_IP --sport 1024:65535 -d $ip --dport 53 -m state --state NEW,ESTABLISHED -j ACCEPT
iptables -A INPUT -p tcp -s $ip --sport 53 -d $SERVER_IP --dport 1024:65535 -m state --state ESTABLISHED -j ACCEPT

(B) Allow incoming DNS request at port 53:

Use following rules only if you are protecting dedicated DNS server.

SERVER_IP is IP address where BIND(named) is listing on port 53 for incoming DNS queries.

Please note that here I’m not allowing TCP protocol as I don’t have secondary DNS server to do zone transfer.

iptables -A INPUT -p udp -s 0/0 --sport 1024:65535 -d $SERVER_IP --dport 53 -m state --state NEW,ESTABLISHED -j ACCEPT
iptables -A OUTPUT -p udp -s $SERVER_IP --sport 53 -d 0/0 --dport 1024:65535 -m state --state ESTABLISHED -j ACCEPT
iptables -A INPUT -p udp -s 0/0 --sport 53 -d $SERVER_IP --dport 53 -m state --state NEW,ESTABLISHED -j ACCEPT
iptables -A OUTPUT -p udp -s $SERVER_IP --sport 53 -d 0/0 --dport 53 -m state --state ESTABLISHED -j ACCEPT

Please note if you have secondary server, add following rules to above rules so that secondary server can do zone transfer from primary DNS server:

iptables -A INPUT -p tcp -s $DNS2_IP --sport 1024:65535 -d $SERVER_IP --dport 53 -m state --state NEW,ESTABLISHED -j ACCEPT
iptables -A OUTPUT -p tcp -s $SERVER_IP --sport 53 -d $DNS2_IP --dport 1024:65535 -m state --state ESTABLISHED -j ACCEPT

How to: Linux Iptables block common attacks

Posted on in Categories Iptables, Linux, RedHat/Fedora Linux, Security, Ubuntu Linux last updated July 6, 2005

Following list summaries the common attack on any type of Linux computer:

Syn-flood protection

In this attack system is floods with a series of SYN packets. Each packets causes system to issue a SYN-ACK responses. Then system waits for ACK that follows the SYN+ACK (3 way handshake). Since attack never sends back ACK again entire system resources get fulled aka backlog queue. Once the queue is full system will ignored incoming request from legitimate users for services (http/mail etc). Hence it is necessary to stop this attack with iptables.

Force SYN packets check

Make sure NEW incoming tcp connections are SYN packets; otherwise we need to drop them:

iptables -A INPUT -p tcp ! --syn -m state --state NEW -j DROP

Force Fragments packets check

Packets with incoming fragments drop them. This attack result into Linux server panic such data loss.

iptables -A INPUT -f -j DROP

XMAS packets

Incoming malformed XMAS packets drop them:

iptables -A INPUT -p tcp --tcp-flags ALL ALL -j DROP

Drop all NULL packets

Incoming malformed NULL packets:

iptables -A INPUT -p tcp --tcp-flags ALL NONE -j DROP

Block Spoofing and bad addresses

Using iptables you can filter to drop suspicious source address. Network server should not accept packets claiming from the Internet that claim to originate from inside your network. Spoofing can be classified as:
a) IP spoofing – Disable the source address of authentication, for example rhosts based authentication. Filter RPC based services such as portmap and NFS,
b) DNS spoofing
Please see Iptables: How to avoid Spoofing and bad addresses attack tip for more information.

Also use NAT for your internal network. This makes difficult for attacker to spoof IP address from outside.

Filter incoming ICMP, PING traffic

It includes the ping of death attack and ICMP floods. You should block all ICMP and PING traffic for outside except for your own internal network (so that you can ping to see status of your own server) . See Linux : Iptables Allow or block ICMP ping request article.

Once system is secured, test your firewall with nmap or hping2 command:
# nmap -v -f FIREWALL-IP
# nmap -v -sX FIREWALL-IP
# nmap -v -sN FIREWALL-IP
# hping2 -X FIREWALL-IP

Further readings

  • Man page : hping2(8), nmap(1), iptables(8)