Home > Articles

Introduction to Troubleshooting Linux Firewalls

  • Print
  • + Share This
The importance of a firewall to your security plan cannot be stressed enough, but what happens when your firewall fails? Find out in this introduction to firewall troubleshooting.
This chapter is from the book

This chapter is from the book

The Methodical Approach and the Need for a Methodology

Oh no you say—not more management speak! Please, I get enough of that already! Fear not; we promise that we won't waste your time with YAUM (Yet Another Useless Methodology). We want you to find your problem and fix it quickly. So you can call this a process, a method, a way, or if you like, call it a methodology—whatever works for you. What we don't want to do is fill your head with some useless babble. This methodology is hard won from years of solving problems.

It all started many years ago. Painted on the wall of our parents' garage was a slogan: "1st law of wrenchin': always check the spark plugs!" Scott painted this on the wall as a reminder of a valuable lesson we learned as teenagers. You need a plan for fixing things, and you need some "laws" to make sure you don't waste a lot of time figuring out what your problem is when it's staring you in the face.

As it was, we had cars, as many young men do in high school, which were the primary focus of our budding engineer minds. They were complex, they were fun, they took us places to have fun, and they were essential parts of teenagers' social lives. Without a car, life was a little less fun, and things were much farther away. However, our cars were used and needed work—sometimes a lot of work—to keep running. So we learned to take care of our cars, to fix them when they broke down, and to modify them to make them better. After all, what kind of engineer is happy with something as it is out of the box? Everything could be a little better, couldn't it? All of this meant that we had to figure out what was wrong with them while not wasting too much time doing so. We paid for those muscle cars, and we wanted to drive them! Enter the first law of wrenchin'.

The first law of wrenchin' was the result of a marathon session of trying to figure out what was wrong with one of our cars (Mike had a 1972 Pontiac GTO; Scott still has a 1970 Oldsmobile 442). For days we struggled to figure out why the motor wasn't performing as it should. We changed the timing, retuned the carburetor, drained the gas tank, uncorked the headers, and so on. We did everything we could think of...except check the spark plugs. That was too easy! It couldn't be the spark plugs, we told ourselves. Five minutes after we stopped convincing ourselves it wasn't the spark plugs, we pulled them out. They were covered with soot. The result of all our other twiddling had made already fouled plugs an absolute mess. In short, the problem was absolute—there was no way we could have gotten that GTO to run correctly, and the only solution was possibly the simplest and easiest to execute. After we had that figured out and had replaced the spark plugs, a five-minute job, we had that 1972 GTO running perfectly. What was really galling about this experience was that we knew better! Of course it could be the spark plugs; we just didn't check them, even though all the signs that this was the root cause were staring us in the face, as it were. The idea that the spark plugs were the cause of the problem just seemed too simple. We thought, wrongly, that it had to be something more complex.

Back then, our problem was caused by the lack of a clear system for problem solving or a mechanism for working through the problem to rule things out before leaping to what we might have thought was the root cause. We needed to work our way up the motor, ruling things out before moving on to the next component, and we had to learn to start with the simple things first. From that moment on, we decided that we wanted to spend more time using our cars as opposed to fixing them. So the journey started toward putting a plan in place to help prevent those forays into endlessly searching for the root cause of a problem. Over the years, we learned to work through the problem carefully, and we discovered that other people had come up with well thought out means of troubleshooting as well. It is that hard won experience and knowledge that we present here to help you to troubleshoot more effectively.

With Linux firewalls, this need for specific steps and a clearly defined approach is no different. There are so many variables in a modern firewall that it's easy to overlook the real source of any problem. For instance, we once had a big high-end SMP DEC Alpha Linux box that we used as a high-end packet switch and firewall. It worked perfectly in our lab in Virginia. We shipped it to our ISP, and all the network cards simply stopped working. We were puzzled, so we got on the box from a console and found that the network cards were up, that they had IPs, but that they simply could not talk to anything.

The NIC driver module was loaded, and the machine had worked without any problem for weeks on our lab's network. The co-location facility's network should have been no different, except for one small change: The ISP's network was Ethernet, and our lab was FastEthernet. We didn't expect that this would create any problems. After all, the network cards were designed to support both types of network, Ethernet and FastEthernet, and we had worked with these drivers before on both mediums. Our network was faster, and all we did was put the box on a slower network. Stepping down, as it were, shouldn't have made any difference. We were flummoxed. Why would that seemingly insignificant difference cause the cards to stop working?

The simple reason was that we were running the wrong driver. This might seem obvious now, but the cards we were using were DEC cards, and the wrong drivers would load and work with the wrong card, and they would work on a FastEthernet network! In our case, the system thought the card was Tulip FastEthernet card, when the cards were really DE500s. To make matters worse, they even look the same.

So back in our lab, the Tulip driver worked fine at FastEthernet speeds and never once complained about the fact that the card was not a Tulip. Couple this with the fact that we only stocked DEC NIC cards, Tulips, and DE500s, and you can see how this mistake was made. They look the same, and even the OS thought they were the same when it detected them. Fortunately, we had a simple process that we will illustrate here to work through this problem quickly, and we had the system up in short order. Total time to debug this problem and fix it was under five minutes.

The point here is that the best and quickest way to arrive at the root cause of your firewall problem is through the application of a well thought out process of elimination. It's very tempting sometimes to leap to a conclusion about what's causing the problem with your firewall, but unless you're lucky, you can actually make the problem worse. Take our word for it...relax, sit down, and work through the problem step by step. Stick with the approach we have outlined in this book, and you'll soon be tackling the thorniest firewall problems like a guru!

  • + Share This
  • 🔖 Save To Your Account

InformIT Promotional Mailings & Special Offers

I would like to receive exclusive offers and hear about products from InformIT and its family of brands. I can unsubscribe at any time.

Overview


Pearson Education, Inc., 221 River Street, Hoboken, New Jersey 07030, (Pearson) presents this site to provide information about products and services that can be purchased through this site.

This privacy notice provides an overview of our commitment to privacy and describes how we collect, protect, use and share personal information collected through this site. Please note that other Pearson websites and online products and services have their own separate privacy policies.

Collection and Use of Information


To conduct business and deliver products and services, Pearson collects and uses personal information in several ways in connection with this site, including:

Questions and Inquiries

For inquiries and questions, we collect the inquiry or question, together with name, contact details (email address, phone number and mailing address) and any other additional information voluntarily submitted to us through a Contact Us form or an email. We use this information to address the inquiry and respond to the question.

Online Store

For orders and purchases placed through our online store on this site, we collect order details, name, institution name and address (if applicable), email address, phone number, shipping and billing addresses, credit/debit card information, shipping options and any instructions. We use this information to complete transactions, fulfill orders, communicate with individuals placing orders or visiting the online store, and for related purposes.

Surveys

Pearson may offer opportunities to provide feedback or participate in surveys, including surveys evaluating Pearson products, services or sites. Participation is voluntary. Pearson collects information requested in the survey questions and uses the information to evaluate, support, maintain and improve products, services or sites, develop new products and services, conduct educational research and for other purposes specified in the survey.

Contests and Drawings

Occasionally, we may sponsor a contest or drawing. Participation is optional. Pearson collects name, contact information and other information specified on the entry form for the contest or drawing to conduct the contest or drawing. Pearson may collect additional personal information from the winners of a contest or drawing in order to award the prize and for tax reporting purposes, as required by law.

Newsletters

If you have elected to receive email newsletters or promotional mailings and special offers but want to unsubscribe, simply email information@informit.com.

Service Announcements

On rare occasions it is necessary to send out a strictly service related announcement. For instance, if our service is temporarily suspended for maintenance we might send users an email. Generally, users may not opt-out of these communications, though they can deactivate their account information. However, these communications are not promotional in nature.

Customer Service

We communicate with users on a regular basis to provide requested services and in regard to issues relating to their account we reply via email or phone in accordance with the users' wishes when a user submits their information through our Contact Us form.

Other Collection and Use of Information


Application and System Logs

Pearson automatically collects log data to help ensure the delivery, availability and security of this site. Log data may include technical information about how a user or visitor connected to this site, such as browser type, type of computer/device, operating system, internet service provider and IP address. We use this information for support purposes and to monitor the health of the site, identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents and appropriately scale computing resources.

Web Analytics

Pearson may use third party web trend analytical services, including Google Analytics, to collect visitor information, such as IP addresses, browser types, referring pages, pages visited and time spent on a particular site. While these analytical services collect and report information on an anonymous basis, they may use cookies to gather web trend information. The information gathered may enable Pearson (but not the third party web trend services) to link information with application and system log data. Pearson uses this information for system administration and to identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents, appropriately scale computing resources and otherwise support and deliver this site and its services.

Cookies and Related Technologies

This site uses cookies and similar technologies to personalize content, measure traffic patterns, control security, track use and access of information on this site, and provide interest-based messages and advertising. Users can manage and block the use of cookies through their browser. Disabling or blocking certain cookies may limit the functionality of this site.

Do Not Track

This site currently does not respond to Do Not Track signals.

Security


Pearson uses appropriate physical, administrative and technical security measures to protect personal information from unauthorized access, use and disclosure.

Children


This site is not directed to children under the age of 13.

Marketing


Pearson may send or direct marketing communications to users, provided that

  • Pearson will not use personal information collected or processed as a K-12 school service provider for the purpose of directed or targeted advertising.
  • Such marketing is consistent with applicable law and Pearson's legal obligations.
  • Pearson will not knowingly direct or send marketing communications to an individual who has expressed a preference not to receive marketing.
  • Where required by applicable law, express or implied consent to marketing exists and has not been withdrawn.

Pearson may provide personal information to a third party service provider on a restricted basis to provide marketing solely on behalf of Pearson or an affiliate or customer for whom Pearson is a service provider. Marketing preferences may be changed at any time.

Correcting/Updating Personal Information


If a user's personally identifiable information changes (such as your postal address or email address), we provide a way to correct or update that user's personal data provided to us. This can be done on the Account page. If a user no longer desires our service and desires to delete his or her account, please contact us at customer-service@informit.com and we will process the deletion of a user's account.

Choice/Opt-out


Users can always make an informed choice as to whether they should proceed with certain services offered by InformIT. If you choose to remove yourself from our mailing list(s) simply visit the following page and uncheck any communication you no longer want to receive: www.informit.com/u.aspx.

Sale of Personal Information


Pearson does not rent or sell personal information in exchange for any payment of money.

While Pearson does not sell personal information, as defined in Nevada law, Nevada residents may email a request for no sale of their personal information to NevadaDesignatedRequest@pearson.com.

Supplemental Privacy Statement for California Residents


California residents should read our Supplemental privacy statement for California residents in conjunction with this Privacy Notice. The Supplemental privacy statement for California residents explains Pearson's commitment to comply with California law and applies to personal information of California residents collected in connection with this site and the Services.

Sharing and Disclosure


Pearson may disclose personal information, as follows:

  • As required by law.
  • With the consent of the individual (or their parent, if the individual is a minor)
  • In response to a subpoena, court order or legal process, to the extent permitted or required by law
  • To protect the security and safety of individuals, data, assets and systems, consistent with applicable law
  • In connection the sale, joint venture or other transfer of some or all of its company or assets, subject to the provisions of this Privacy Notice
  • To investigate or address actual or suspected fraud or other illegal activities
  • To exercise its legal rights, including enforcement of the Terms of Use for this site or another contract
  • To affiliated Pearson companies and other companies and organizations who perform work for Pearson and are obligated to protect the privacy of personal information consistent with this Privacy Notice
  • To a school, organization, company or government agency, where Pearson collects or processes the personal information in a school setting or on behalf of such organization, company or government agency.

Links


This web site contains links to other sites. Please be aware that we are not responsible for the privacy practices of such other sites. We encourage our users to be aware when they leave our site and to read the privacy statements of each and every web site that collects Personal Information. This privacy statement applies solely to information collected by this web site.

Requests and Contact


Please contact us about this Privacy Notice or if you have any requests or questions relating to the privacy of your personal information.

Changes to this Privacy Notice


We may revise this Privacy Notice through an updated posting. We will identify the effective date of the revision in the posting. Often, updates are made to provide greater clarity or to comply with changes in regulatory requirements. If the updates involve material changes to the collection, protection, use or disclosure of Personal Information, Pearson will provide notice of the change through a conspicuous notice on this site or other appropriate way. Continued use of the site after the effective date of a posted revision evidences acceptance. Please contact us if you have questions or concerns about the Privacy Notice or any objection to any revisions.

Last Update: November 17, 2020