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How to Program in C


Guide to (mostly) Harmless Hacking

Vol. 5 Programmers' Series

No. 4: How to Program in C, part 1

New hackers have been going by the droves to the top two places to get
computer break-in programs: http://www.rootshell.com and http://www.securityfocus.com. Then they try to stick these things into the windows of their Web browsers, throw them at their would-be victim computers via telnet and ftp, print them out and burn them at altars. OK, I'm exaggerating, but only a tiny bit. Their problem is that they don't know how to use these exploit programs.

The reason for this cluelessness is that they don't know how to program. To be specific, they don't know how to program in C on a Unix type computer! The problem is that there are so many super easy ways to break into computers, and so many hackers who never learned to program, that lots of people assume it is all easy. But if you are serious about breaking into well-defended computers, and especially if you are serious about learning how to defend Unix type computers, you must learn how to write, patch and compile C code into working programs.

If you aspire to become an Uberhacker against Unix systems, you absolutely
MUST become a C programmer. One heck of a good C programmer! If you want to escape us playing practical jokes on you when you play Hacker Wargames, you absolutely MUST become a C programmer.

In this Chapter you will:

In order to do this lesson, you must have a shell account, whether at an ISP or by running some kind of Unix (for example, Linux) on your home computer. DO NOT email us asking how to get a shell account! The answer is in the chapter "How to Get a Good Shell Account."

Why is C the single most important programming language for a hacker to learn?

C is the language in which the Unix class of operating systems is for the most part written. It also is the language of almost all applications that run on Unix.

As one of my hacker friends who insists on anonymity explains, both Unix and C "were developed by the same team at Bell labs, and compliment each other nicely. Unix was not originally written in C, but was re-written in C to make it easier to understand/maintain/debug and a lot of other reasons. This was not without controversy, but it was Ritchie's decision (the inventor of C), and I believe he was proven right."

Not surprisingly, then, most exploits are also written in C. True, you don't even need to learn to compile a C program to use these exploits -- if you can get someone else to give you a version of that exploit compiled to run on the type of operating system and shell you are planning to use for the break-in.

Newbie note: Wonder why there are all those "Free Kevin Mitnick" Web sites in the haxor scene? Many people think it is ludicrous to keep that man behind bars because he was such a lame hacker that he apparently didn't even know how to program. In fact, there is evidence that he didn't even compile his own C programs! There are transcripts of him on IRC begging his friends to compile programs for him. Note: Mr. Mitnick disputes this statement, threatens a lawsuit, and we dispute him right back. Click here to read all about it.

C is especially important for White Hat hackers because you don't have a prayer of a chance to patch security holes in your computer unless you can compile and run C programs to fix the problem parts of your operating system.

Meino Christian Cramer adds, "And: using precompiled binaries/executables is always a risk. Think of the ... viruses introduced to computers by simply executing 'I-believe-it-has-no-virus'-programs. Using the source code instead... you can check the code for "back doors" and 'traps' -- which means you have to learn 'the one and only' C..."

C also is good for hacking because it is able to run "low level" code as part of its language. In programming, "low level" means that you can play with the most basic things on your computer, for example opening and closing sockets or linking to the information on what sockets are open at any given instant.

Newbie note: A socket is a round-trip or two-way network connection. For example, when you telnet into another computer's login sequence, you connect to port 23 on that computer. It completes a round-trip connection by assigning some high number port, for example port 3587, to complete the socket. If you have a shell account on a good ISP, you can see everyone's sockets by giving the "last" or "netstat" commands.

Another important thing about C being a high level language that easily incorporates low level (assembly language) commands is that you can write it to run super fast.

More C programming --->>

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