Mikhail Flionov - Linux through the eyes of a hacker. Mikhail Flionov: Linux through the eyes of a hacker Open source is safe

Foreword This edition of the book is not just a supplement, but a completely revised version of the previous one. Everything has changed in iOS 7: the look and feel of the operating system, the way we use our iOS devices and, most importantly, the principles

Foreword I'm sure that there is something extraordinary in each of us, but it takes a lot of time to understand what is really important, to pull it out of ourselves. You cannot become outstanding if you don’t love your environment, your tools, your area.

Considered the issues of tuning Linux OS for maximum performance and security. Basic administration and access control, Firewall configuration, file sharing server, WEB-, FTP- and Proxy-servers, delivery programs are described. email, dNS servicesas well as system monitoring policy and data archiving. Potential vulnerabilities are presented, recommendations are given to prevent possible attacks and it is shown how to act in case of an attack or hacking of the system in order to restore its operability as quickly as possible and prevent data loss. In the third edition, the material has been revised and supplemented with new information in accordance with modern realities. The CD contains additional documentation and source programs.

What is Linux?

Linux OS is free operating system, the source codes of which are open for public viewing and even for making changes. Most do not look at the source code, but perceive their presence as an additional pleasant freebie. But even a freebie should be useful, and if it is useless, then only litter.

No, having the source code is good, and I'm not arguing here. This is an advantage for such a product as an OS, because you can recompile the kernel and optimize its operation as much as possible for your hardware. If for custom utilities recompilation is not needed and, say, a text editor can be optimized only slightly, then for the OS the ability to compile source codes is a big and bold plus, which

allows you to squeeze the maximum out of your computer. But Linux has many other benefits, far more important.

Why am I saying this? Besides, I love Linux OS not for freebies and not for the availability of source codes, but for the quality. I will love it even if the source codes are closed or charged.

Is open source safe?

…
In addition, no matter how many people look at the source code, the security will not increase if the viewers do not have the necessary qualifications. Let's say that a billion people who don't understand programming look at codes, will security change? No. The number of real security specialists who have looked at the code is not so high, and they are able to quickly and efficiently find errors. This is a separate skill that requires special training.

…
Costly support can end up costing more Linux ownership than Windows. In North America, where IT pay is high, Linux often outperforms Windows in terms of cost of ownership.

Distributions

For a long time I could not decide which distribution to use, but then I decided to choose Ubuntu. I took a look at a couple of Linux sites and looked at the statistics of the operating systems from which users entered the site. Ubuntu turned out to be the most popular. In principle, knowing one distribution, it is very easy to switch to another, because each of them is still Linux.
…
But in reality, in 99% of cases in distributions everything is identical, so the problem is more exaggerated than it has real grounds.

I hope this book helps you learn more about security in general and Linux security in particular. We've talked a lot about both security and various attack methods, and it might seem that every administrator is fighting attackers. Personally, I believe that there are no hackers and crackers. This is a myth that administrators are trying to scare with.

There are strong and weak people in any world. Hackers, for the most part, are young guys who just know more than others and know how to apply their knowledge in practice.

For some reason the sphere information technologies many see it as something supernatural. But this is no longer the case. Computers have become part of our lives and have become the same household items as radio or television. So let's treat them as well.

It is normal for an automotive technician to interfere with the engine, bore it, or simply decorate the car. The manufacturer does not prohibit doing this, but does not guarantee stable operation after such actions. However, only the guarantee is lost, and it never occurs to anyone to pursue the auto repairman as an “auto hacker”.

All this suggests that hackers need to be dealt with intelligently. If your server has been hacked, this does not mean that a hacker should be jailed. No, you just need to pay much more attention to safety. If we can improve the overall level of knowledge and the quality of the services provided on the network, then there will be much less hacks and hackers.

Learn, develop and improve your own, rather than relying on ready-made solutions that should protect you. If you leave all the doors open, then no one will protect you, including law enforcement agencies.

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The issues of tuning Linux OS for maximum performance and security are considered. Potential vulnerabilities and recommendations for preventing possible attacks are described. Given detailed description setting up access rights and configuring the firewall. It is shown how to act in case of an attack or breach of the system in order to restore its operability as quickly as possible and prevent data loss.

For users, administrators and security professionals

Foreword

This book is devoted to the consideration of one of the most popular operating systems (OS) installed on servers - Linux OS. For home use, this system is not yet as popular as among professional administrators, but recently there have been prerequisites for capturing this market.

Installing the OS is getting easier, and the graphical interface and usability are in some cases on par with the most famous Windows OS in the small business environment.

This book will be useful for Linux administrators and those users who want to get to know this system better. The configuration and security issues discussed will be useful for specialists using various operating systems, because most of the information is not tied to a specific system.

You will be able to find out how hackers infiltrate the servers and defend against intrusion. Since some of the examples in this book can be used not only for defense, but also for attack, I would like to warn young crackers. Healthy curiosity is good, but remember that law enforcement does not sleep and always gets its way. If once you were lucky with a break-in and no one paid attention to it, then next time you may find yourself in the hands of justice.

Part of the book is written from a hacker's point of view and demonstrates how they infiltrate the system. In the hope that this information will not be used to hack servers, I tried to focus specifically on protection and left some things outside of the presentation or simply did not finish, so that there would be no temptation to use the methods of hackers and break the law. Of course, to implement my ideas (to bring them to their logical conclusion), you need to spend a few minutes programming or searching the Internet. But while the book can serve as a starting point for a hacker, I hope it doesn't. Remember the legality of your actions.

Chapter 1

Introduction

I once showed a Windows administrator how to install and use Linux. He liked the installation process itself, because in the latest versions it is quite simple. But when we installed and decided to configure the Samba server, a bunch of questions followed: "Why configure Samba?", "Why can't you get access automatically?" Windows administrators are lazy and used to the OS doing everything for them. what is needed, but when their system is hacked, they start asking questions: "Why didn't Microsoft give us the right toolsto prohibit certain actions? ".

If you look at Linux from the user's point of view, then after installing the system, nothing needs to be configured. You can immediately start working with any office applications and user utilities. But if we are talking about network and server programs, then more complex settings are already required here, and nothing will automatically work. By default, almost all actions on the system that could lead to an unwanted result or network intrusion are prohibited. To change restrictions, you need to set up configuration files, which are extremely inconvenient to edit, or use specialized utilities, most of which have a command line interface.

Because of these inconveniences, my friend Windows administrator said: "Linux was invented by administrators who have nothing to do at work in order to play with configuration files." A week later, the same person was setting up the IIS (Internet Information Services) service on a new server running Windows 2003. He swore the same words, because this service is not installed by default from the OS, and you need it before you start working connect and clearly state what should be used and what should not.

Microsoft began to make the OS on the "if only it was convenient" principle, so it was enough to connect the required components. But now Windows is getting more complex every year, most of the convenient security features are simply disabled and you have to open them when necessary. In Linux, everything was the other way around, this OS was created from the point of view "if only it was safer", and now they are moving towards increasing services.

Convenience and safety are two sides of the same coin, so the manufacturer has to sacrifice something.

1.1. Hacker attacks

Before we get acquainted with Linux and its security principles, we need to know how hackers can get into the system. In order to protect the system, you need to be aware of the possible actions of an attacker. Let's take a look at the process of hacking a computer. We need to know what hackers think about, what they breathe and what they eat ☺. This is the only way we can build an impenetrable information wall for a server or network.

It is impossible to give specific recipes for hacking. In each case, this is a process that depends on the system itself and its security settings. Most often, hacking occurs through errors in some programs, and each administrator can use different software.

Why is the number of attacks only increasing every year? Previously, all information about vulnerabilities was stored on closed BBS (Bulletin Board System, electronic bulletin board) and was available only to a select few. Hackers also belonged to this category. It was they who carried out attacks with impunity, because the level of enlightenment and experience of such people was quite high.

On the other hand, the elite of the hacker world consisted mainly of respectable people for whom security research was not the goal of destruction.

Currently, information about vulnerabilities lies on every corner and is in the public domain. Hacking can now be done by anyone. I would immediately like to ask the fighters for freedom of information: "How did it happen?" It's just that excessive freedom will ultimately lead to destruction. There is a certain category of people who do not feed with bread, let me spoil it somewhere. If a person, using publicly available information, succumbs to this weakness, then he will turn into a burglar.

1.1.1. Research

Let's say you have a server that needs to be hacked or tested for intrusion resistance. Where do you start? What to do first? Many questions immediately arise and not a single answer.

There is no clear sequence of actions. Hacking is a creative process, which means that it must be approached from this point of view. There are no definite rules and you can't fit everything under one template. But I can give several recommendations that it is advisable to adhere to.

The very first thing to start hacking or testing a system for vulnerability is port scanning. For what? And in order to find out what services (in Linux these are daemons) are installed on the system. Each open port is a service program installed on a server to which you can connect and perform certain actions. For example, an FTP service is hanging on port 21. If you can connect to it, you will be able to download and upload files to the server. But this is only if you have the appropriate rights.

First, you need to scan the first 1024 ports. Among them there are a lot of standard services such as FTP, HTTP, Telnet, etc. Each open port is a door with a lock to enter the server. The more such doors there are, the more likely it is that some kind of bolt will not withstand the onslaught and will open.

1.1.2. Hacking a WWW server

Hacking a WWW server has its own peculiarities. If CGI / PHP or other scripts are executed on it, the hacking is carried out in a completely different way. First, you need to scan the server for vulnerable CGI scripts. Believe it or not, again, according to the research of various companies, a large number of "leaky" scripts are working on the Internet. This is due to the fact that when developing sites, errors are initially introduced. Novice programmers very rarely check the input parameters in the hope that the user will not change the code of the page or the URL, where the server receives the necessary data to perform any action.

One of the famous content management systems, PHP-nuke, had an error with parameters. This is a set of scripts that allow you to create a forum, chat, news feed and manage the content of the site. All parameters in scripts are passed through the browser URL string, and the rendering was contained in the ID parameter. The developers assumed that a number would be transmitted in it, but did not check it. A hacker who knows the structure of the database (which is not difficult, because the PHP-nuke source codes are available) could easily put an SQL query to the server's database in the ID parameter and get the passwords of all users registered on the site. Of course, the clients will be encrypted, but it doesn't take much effort to decrypt, and we'll look at this a little later (

see sect. 14.10

The problem is compounded by the fact that some languages \u200b\u200b(for example, Perl) were not originally designed for use on the Internet. Because of this, they have dangerous functions for manipulating the system, and if a programmer inadvertently used them in his modules, then an attacker can take advantage of such imprudence.

Potentially dangerous functions are found almost everywhere, only in different proportions. The only more or less protected language is Java, but it slows down the system very much and requires a lot of resources, which is why Webmasters are not willing to use it. But even this language in clumsy hands can turn into a big gate for hackers with the words: "Welcome!"

But the biggest vulnerability is an illiterate programmer. Due to the lack of specialists in this area, programming began to be done by all and sundry. Many self-taught people do not even try to think about security, and hackers only benefit from it.

1.1.3. Hammer and sickle

Where it was not possible to hack the server with the help of skill and knowledge, you can always use the purely Russian method of "Hammer and Sickle". This does not mean that the sickle should be put to the administrator's throat and knock on his head with a hammer. It's just that dumb password guessing is always left in stock.

Let's go back to statistics. All research firms have come to the same conclusion that most beginners choose the names of their favorite dogs, cats, dates of birth or phone numbers as their password. A well-chosen dictionary can break almost any system. there are always inexperienced users with such passwords. The worst thing is if these "dummies" have big enough rights.

Don't you believe me yet? Let's remember the famous "Morris worm" that infiltrated the system by hacking it using a dictionary. The worm's own vocabulary was small enough, less than a hundred words. In addition, the search used terms from the dictionary installed in the system. There weren't that many of them either. And thanks to such a primitive algorithm, the worm was able to infect a huge number of computers and servers. It was one of the most massive hacks !!! Yes, the case is old, but the average professionalism of users is not growing, because many of them are experienced, but there are enough beginners.

1.1.4. The local network

Hack in local network may be easier for many reasons:

□ computers are connected via a high-speed connection of 10 Mbps and above;

□ it is possible to listen to the traffic of other computers on the network;

□ you can create dummy servers;

□ firewalls are used very rarely, because they are installed mainly in front of Internet access.

1.2. What is Linux?

Linux is a free operating system, the source code of which is open to public viewing and even modification.

The core OS was created in 1991 by a student at the University of Helsinki named Linus Torvalds. He wrote the backbone, functionally similar to Unix-systems, and posted it for public viewing, asking for help in improving and increasing the capabilities of the new OS. Quite a lot of people responded, and the work began to boil.

Hackers from various countries joined this project on a voluntary basis and began to create the most scandalous OS. And the booze around Linux appears almost every day, because the OS has become widespread and is absolutely free. Some software manufacturers consider this project promising, others (for example, Microsoft) - periodically turn it into an enemy.

Official version OS kernel 1.0 was released in 1994, i.e. three years after the first "rumors" about Linux. This speed of development was achieved thanks to a large number of professionals who agreed to develop Linus's interesting idea.

Linux OS is a multi-user and multitasking system that allows several users to work with a computer at once and perform different tasks at the same time.

1.3. Is open source safe?

There is an opinion that open source software is more reliable and safer than commercial software.

Proponents of this claim believe that such a system is being researched by many people. different ways and thus reveal all possible errors. The Windows XP operating system shows a fairly high reliability and security, although it is a commercial product. Most importantly, errors are fixed in a timely manner, are available for free download and are easy to install.

Yes, it is much easier and more effective to find errors at the code level together with testing the finished product, but the result is far from ideal. Despite mass testing, lapses are often found in Linux. And if you look at which army of users surveyed latest versions Windows, one would think that it would become flawless. Testing is one thing, and application in "combat conditions" shows completely unpredictable results.

Openness to Linux has one advantage - excellent value for money. The ability to install an OS for free saves you a lot of money. But the costs will come in terms of support, which is quite expensive for Linux, so there may be problems with timely updates. In addition, Linux administration requires more skills and abilities than Windows. There are no masters to make life easier. Need to know linux commands and be able to use them without prompting. That is why this OS has not yet settled in our homes.

Why is Linux so hard? The answer is simple. Performance and usability are incompatible things. In Windows everything is very clear, but it can take a lot of mouse clicks and several views to complete an operation. dialog boxeswhich takes up valuable time. On Linux, you need to start the console and execute the required directive. The only problem is that you have to remember a lot of commands.

1.4. Core

The kernel is the heart of the OS, which manages the physical and software resources of the computer. In addition, it allows you to access various hardware. For example, early versions the kernels only supported two USB devices: a keyboard and a mouse. Starting from version 2.4, built-in support for USB camcorders, printers and other devices.

The Linux kernel version number consists of three numbers:

□ first (major number) - indicates significant changes in the kernel;

□ the second (minor number) - characterizes the appearance of small changes. It can be used to determine whether the kernel is verified or intended for testing and is not sure that it does not contain errors. If the number is even, then the kernel has been rigorously tested. Otherwise, installing this version does not guarantee stable operation;

□ third - the number of the next working release (build). In some cases, this number is omitted. For example, we have already talked about the 2.4 branch in this chapter, and in this case, the assembly is not specified.