Click here to close now.

Welcome!

Linux Authors: Pat Romanski, Roger Strukhoff, Carmen Gonzalez, Ian Khan, VictorOps Blog

Related Topics: Linux

Linux: Article

10 minutes to an iptables-based Linux firewall

You can harden your Linux 2.4 machines in no time at all.

(LinuxWorld) -- As I write this column, the world suffers infestation by yet another Internet worm, and again the worms are intended to attack Microsoft-based machines. Fortunately for my company, we don't run any Microsoft-based machines, but the Nimda worm still affects us.

 

Nimda eats our bandwidth since other companies sharing our subnet are running infectious Windows 98, NT, and 2000 machines. These diseased, ill-maintained beasts from the land of insufficient light are now trying to infest our stable, secure, long running, easy-to-use, well-behaved Linux machines. (Can you tell I'm a bit irritated?)

 

This has led me to start editing some of our firewall rules, and in the process, consider that, as we have recently migrated to the 2.4 kernel, some people may be interested in how to create a Linux 2.4 kernel-based firewall quickly.

 

The following examples do not create the perfect firewall. What I describe is designed to give a system administrator a little piece of mind by fortifying a machine from obvious attacks.

 

Why 2.4/iptables?

 

The 2.2 version of the Linux kernel used the ipchains application to control the firewall. For standard firewalling, ipchains is a decent solution. We still use it on some of our machines, and there is still a positive argument for 2.2 kernel-based firewalls, because the 2.4 kernel still has some stability issues under heavy load.

 

Those heavy load issues aside, the 2.4 kernel provides a wealth of networking capabilities 2.2 lacks. These include stateful firewalling and solid quality-of-service options. One could argue that the 2.4 kernel, and its iptables firewall code, enables a person to build intricate firewalls capable of competing with the likes of CheckPoint.

 

Using iptables

 

The command to execute iptables is simple: as root type iptables. The execution of the previous command should display output similar to the following:

 

[root@jd root]# /sbin/iptables
iptables v1.2.1: no command specified
Try `iptables -h' or 'iptables --help' for more information.
[root@jd root]#

 

If you would like an output of the available options when using the iptables you can pass the -h flag during program execution. The -h command will result in output similar to the following:

 

[root@jd root]# /sbin/iptables -h
iptables v1.2.1
Usage: iptables -[ADC] chain rule-specification [options]
       iptables -[RI] chain rulenum rule-specification [options]
       iptables -D chain rulenum [options]
       iptables -[LFZ] [chain] [options]
       iptables -[NX] chain
       iptables -E old-chain-name new-chain-name
       iptables -P chain target [options]
       iptables -h (print this help information)

Commands: Either long or short options are allowed. --append -A chain Append to chain --delete -D chain Delete matching rule from chain --delete -D chain rulenum [...]

 

For this article, I am not going to go into exhaustive explanation of all the iptables options. If you want an advanced introduction, to iptables I suggest the Linux 2.4 Packet Filtering HOWTO (see resources below).

 

When developing a personal or desktop firewall I practice a very simple philosophy. If you aren't going to use it, don't open it. For example, if you are not hosting a Web site, do not open port 80. If you are not using telnet (and there is not a good reason on this Earth to use telnet), do not open port 23!

 

In an effort to follow my philosophy, the quickest way to port protection nirvana is the following iptables chain:

 

/sbin/iptables -A INPUT -p tcp --syn -j DROP

 

The previous statement will allow you to, as the user of the computer, performed all your normal Internet activities. You will be able to browse the Web, ssh out, or chat with a colleague on ICQ. On the other hand, the outside world, when trying to connect to your Linux box via TCP/IP, will simply be ignored. This is a reasonable solution for most Linux computers.

 

However, one of the benefits of Linux is its remote management capabilities. One of the more popular ways that people remotely manage Linux machines is via the SSH (see resources) suite. SSH typically operates on port 22 and thus, we would need to enable connections to port 22, while keeping the rest of the connections closed. This can be done with the following iptable chains:

 

/sbin/iptables -A INPUT -p tcp --syn --destination-port 22 -j ACCEPT
/sbin/iptables -A INPUT -p tcp --syn -j DROP

 

It is probably not a good idea to let the world connect to your machine on port 22 unless you run a public server. Therefore, we can limit which machines can connect to port 22 by modifying the iptable chain, and adding the -s option. The -s in this example specifies what source address is allowed to connect to the server.

 

/sbin/iptables -A INPUT -p tcp --syn -s 192.168.1.110/32 --destination-port 22 -j ACCEPT
/sbin/iptables -A INPUT -p tcp --syn -j DROP

 

The addition of the -s 192.168.1.110/32 will enable only the remote machine with the IP address of 192.168.1.110 to connect to your protected host.

 

When you create an iptables-based firewall, each chain (for simplicity's sake, each line) will be read sequentially. Thus, it is possible to have the previous configuration of only one machine having rights to connect via SSH, and to run a public Web server. This could be done with the following commands:

 

/sbin/iptables -A INPUT -p tcp --syn -s 192.168.1.110/32 --destination-port 22 -j ACCEPT
/sbin/iptables -A INPUT -p tcp --syn --destination-port 80 -j ACCEPT
/sbin/iptables -A INPUT -p tcp --syn -j DROP

 

This is just an introduction to iptables, but it does give a reasonable representation of the bare essentials for a firewall. If you are currently running Linux with kernel 2.4, it may be a good time to review what your firewall looks like. On a closing note, if you are still running kernel 2.2 and you are looking for a good firewall configuration utility, take a look at Guard Dog (see resources).

More Stories By Joshua Drake

Joshua Drake is the co-founder of Command Prompt, Inc., a PostgreSQL and Linux custom development company. He is also the current author of the Linux Networking HOWTO, Linux PPP HOWTO, and Linux Consultants HOWTO. His most demanding project at this time is a new PostgreSQL book for O'Reilly, 'Practical PostgreSQL'

Comments (3) View Comments

Share your thoughts on this story.

Add your comment
You must be signed in to add a comment. Sign-in | Register

In accordance with our Comment Policy, we encourage comments that are on topic, relevant and to-the-point. We will remove comments that include profanity, personal attacks, racial slurs, threats of violence, or other inappropriate material that violates our Terms and Conditions, and will block users who make repeated violations. We ask all readers to expect diversity of opinion and to treat one another with dignity and respect.


Most Recent Comments
Peter 10/04/04 12:38:21 PM EDT

Thank you... this is the info I have been looking for, a quick and simple way to secure a basic web server.

Jules 07/24/04 02:19:10 PM EDT

Thankyou! At last an article that shows me how to create a simple set of rules.

Pankaj Kumar 09/27/03 12:17:26 AM EDT

Good introductory article on iptables for newbies.

@ThingsExpo Stories
The true value of the Internet of Things (IoT) lies not just in the data, but through the services that protect the data, perform the analysis and present findings in a usable way. With many IoT elements rooted in traditional IT components, Big Data and IoT isn’t just a play for enterprise. In fact, the IoT presents SMBs with the prospect of launching entirely new activities and exploring innovative areas. CompTIA research identifies several areas where IoT is expected to have the greatest impact.
Wearable devices have come of age. The primary applications of wearables so far have been "the Quantified Self" or the tracking of one's fitness and health status. We propose the evolution of wearables into social and emotional communication devices. Our BE(tm) sensor uses light to visualize the skin conductance response. Our sensors are very inexpensive and can be massively distributed to audiences or groups of any size, in order to gauge reactions to performances, video, or any kind of presentation. In her session at @ThingsExpo, Jocelyn Scheirer, CEO & Founder of Bionolux, will discuss ho...
Cloud data governance was previously an avoided function when cloud deployments were relatively small. With the rapid adoption in public cloud – both rogue and sanctioned, it’s not uncommon to find regulated data dumped into public cloud and unprotected. This is why enterprises and cloud providers alike need to embrace a cloud data governance function and map policies, processes and technology controls accordingly. In her session at 15th Cloud Expo, Evelyn de Souza, Data Privacy and Compliance Strategy Leader at Cisco Systems, will focus on how to set up a cloud data governance program and s...
The Workspace-as-a-Service (WaaS) market will grow to $6.4B by 2018. In his session at 16th Cloud Expo, Seth Bostock, CEO of IndependenceIT, will begin by walking the audience through the evolution of Workspace as-a-Service, where it is now vs. where it going. To look beyond the desktop we must understand exactly what WaaS is, who the users are, and where it is going in the future. IT departments, ISVs and service providers must look to workflow and automation capabilities to adapt to growing demand and the rapidly changing workspace model.
As organizations shift toward IT-as-a-service models, the need for managing and protecting data residing across physical, virtual, and now cloud environments grows with it. CommVault can ensure protection &E-Discovery of your data – whether in a private cloud, a Service Provider delivered public cloud, or a hybrid cloud environment – across the heterogeneous enterprise. In his session at 16th Cloud Expo, Randy De Meno, Chief Technologist - Windows Products and Microsoft Partnerships, will discuss how to cut costs, scale easily, and unleash insight with CommVault Simpana software, the only si...
Hadoop as a Service (as offered by handful of niche vendors now) is a cloud computing solution that makes medium and large-scale data processing accessible, easy, fast and inexpensive. In his session at Big Data Expo, Kumar Ramamurthy, Vice President and Chief Technologist, EIM & Big Data, at Virtusa, will discuss how this is achieved by eliminating the operational challenges of running Hadoop, so one can focus on business growth. The fragmented Hadoop distribution world and various PaaS solutions that provide a Hadoop flavor either make choices for customers very flexible in the name of opti...
Containers and microservices have become topics of intense interest throughout the cloud developer and enterprise IT communities. Accordingly, attendees at the upcoming 16th Cloud Expo at the Javits Center in New York June 9-11 will find fresh new content in a new track called PaaS | Containers & Microservices Containers are not being considered for the first time by the cloud community, but a current era of re-consideration has pushed them to the top of the cloud agenda. With the launch of Docker's initial release in March of 2013, interest was revved up several notches. Then late last...
HP and Aruba Networks on Monday announced a definitive agreement for HP to acquire Aruba, a provider of next-generation network access solutions for the mobile enterprise, for $24.67 per share in cash. The equity value of the transaction is approximately $3.0 billion, and net of cash and debt approximately $2.7 billion. Both companies' boards of directors have approved the deal. "Enterprises are facing a mobile-first world and are looking for solutions that help them transition legacy investments to the new style of IT," said Meg Whitman, Chairman, President and Chief Executive Officer of HP...
Roberto Medrano, Executive Vice President at SOA Software, had reached 30,000 page views on his home page - http://RobertoMedrano.SYS-CON.com/ - on the SYS-CON family of online magazines, which includes Cloud Computing Journal, Internet of Things Journal, Big Data Journal, and SOA World Magazine. He is a recognized executive in the information technology fields of SOA, internet security, governance, and compliance. He has extensive experience with both start-ups and large companies, having been involved at the beginning of four IT industries: EDA, Open Systems, Computer Security and now SOA.
The industrial software market has treated data with the mentality of “collect everything now, worry about how to use it later.” We now find ourselves buried in data, with the pervasive connectivity of the (Industrial) Internet of Things only piling on more numbers. There’s too much data and not enough information. In his session at @ThingsExpo, Bob Gates, Global Marketing Director, GE’s Intelligent Platforms business, to discuss how realizing the power of IoT, software developers are now focused on understanding how industrial data can create intelligence for industrial operations. Imagine ...
Operational Hadoop and the Lambda Architecture for Streaming Data Apache Hadoop is emerging as a distributed platform for handling large and fast incoming streams of data. Predictive maintenance, supply chain optimization, and Internet-of-Things analysis are examples where Hadoop provides the scalable storage, processing, and analytics platform to gain meaningful insights from granular data that is typically only valuable from a large-scale, aggregate view. One architecture useful for capturing and analyzing streaming data is the Lambda Architecture, representing a model of how to analyze rea...
SYS-CON Events announced today that Vitria Technology, Inc. will exhibit at SYS-CON’s @ThingsExpo, which will take place on June 9-11, 2015, at the Javits Center in New York City, NY. Vitria will showcase the company’s new IoT Analytics Platform through live demonstrations at booth #330. Vitria’s IoT Analytics Platform, fully integrated and powered by an operational intelligence engine, enables customers to rapidly build and operationalize advanced analytics to deliver timely business outcomes for use cases across the industrial, enterprise, and consumer segments.
The explosion of connected devices / sensors is creating an ever-expanding set of new and valuable data. In parallel the emerging capability of Big Data technologies to store, access, analyze, and react to this data is producing changes in business models under the umbrella of the Internet of Things (IoT). In particular within the Insurance industry, IoT appears positioned to enable deep changes by altering relationships between insurers, distributors, and the insured. In his session at @ThingsExpo, Michael Sick, a Senior Manager and Big Data Architect within Ernst and Young's Financial Servi...
SYS-CON Events announced today that Open Data Centers (ODC), a carrier-neutral colocation provider, will exhibit at SYS-CON's 16th International Cloud Expo®, which will take place June 9-11, 2015, at the Javits Center in New York City, NY. Open Data Centers is a carrier-neutral data center operator in New Jersey and New York City offering alternative connectivity options for carriers, service providers and enterprise customers.
The explosion of connected devices / sensors is creating an ever-expanding set of new and valuable data. In parallel the emerging capability of Big Data technologies to store, access, analyze, and react to this data is producing changes in business models under the umbrella of the Internet of Things (IoT). In particular within the Insurance industry, IoT appears positioned to enable deep changes by altering relationships between insurers, distributors, and the insured. In his session at @ThingsExpo, Michael Sick, a Senior Manager and Big Data Architect within Ernst and Young's Financial Servi...
PubNub on Monday has announced that it is partnering with IBM to bring its sophisticated real-time data streaming and messaging capabilities to Bluemix, IBM’s cloud development platform. “Today’s app and connected devices require an always-on connection, but building a secure, scalable solution from the ground up is time consuming, resource intensive, and error-prone,” said Todd Greene, CEO of PubNub. “PubNub enables web, mobile and IoT developers building apps on IBM Bluemix to quickly add scalable realtime functionality with minimal effort and cost.”
Sensor-enabled things are becoming more commonplace, precursors to a larger and more complex framework that most consider the ultimate promise of the IoT: things connecting, interacting, sharing, storing, and over time perhaps learning and predicting based on habits, behaviors, location, preferences, purchases and more. In his session at @ThingsExpo, Tom Wesselman, Director of Communications Ecosystem Architecture at Plantronics, will examine the still nascent IoT as it is coalescing, including what it is today, what it might ultimately be, the role of wearable tech, and technology gaps stil...
With several hundred implementations of IoT-enabled solutions in the past 12 months alone, this session will focus on experience over the art of the possible. Many can only imagine the most advanced telematics platform ever deployed, supporting millions of customers, producing tens of thousands events or GBs per trip, and hundreds of TBs per month. With the ability to support a billion sensor events per second, over 30PB of warm data for analytics, and hundreds of PBs for an data analytics archive, in his session at @ThingsExpo, Jim Kaskade, Vice President and General Manager, Big Data & Ana...
In the consumer IoT, everything is new, and the IT world of bits and bytes holds sway. But industrial and commercial realms encompass operational technology (OT) that has been around for 25 or 50 years. This grittier, pre-IP, more hands-on world has much to gain from Industrial IoT (IIoT) applications and principles. But adding sensors and wireless connectivity won’t work in environments that demand unwavering reliability and performance. In his session at @ThingsExpo, Ron Sege, CEO of Echelon, will discuss how as enterprise IT embraces other IoT-related technology trends, enterprises with i...
When it comes to the Internet of Things, hooking up will get you only so far. If you want customers to commit, you need to go beyond simply connecting products. You need to use the devices themselves to transform how you engage with every customer and how you manage the entire product lifecycle. In his session at @ThingsExpo, Sean Lorenz, Technical Product Manager for Xively at LogMeIn, will show how “product relationship management” can help you leverage your connected devices and the data they generate about customer usage and product performance to deliver extremely compelling and reliabl...