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C Programming on Linux

In: Computers and Technology

Submitted By jamest2013
Words 1293
Pages 6
C Programming on Linux
What You Need for This Project * A Kali Linux virtual machine. You could use other operating systems too, if they have a C compiler.
Writing the hello.c Source Code
In a Terminal window, execute this command: nano hello.c
The nano editor opens. Type in the program shown below. #include <stdio.h> main() { printf("Hello World!\n"); }
Save your file with Ctrl+X, Y, Enter.

Compiling hello.c to Create the hello File
In a Terminal window, execute these commands: gcc hello.c -o hello
These commands compile the hello.c program, creating an executable machine language file named hello, and run the hello executable.
You should see "Hello World!", as shown below.

This program works, but it would be nicer if it greeted you by name, and if it put a couple of newline characters after the greeting to make it cleaner-looking. The next version, hello2, will add these features.
Writing the hello2.c Source Code
In a Terminal window, execute this command: nano hello2.c
The nano editor opens. Type in the program shown below. #include <stdio.h> main() { char name[10]; printf("What is your name?"); scanf("%s", name); printf("Hi, %s\n\n", name); } >>>Take a screen capture for to include in your report
Save your file with Ctrl+X, Y, Enter.
>>>Take a screen capture for to include in your report
Compiling and running hello2
In a Terminal window, execute these commands: gcc hello2.c -o hello2
These commands compile and run hello2.
It should ask you for your name. When you type in your name (no longer than 10 characters), you should be greeted by name, as shown below:

Crashing the hello2 Program With a Long Name--Buffer Overflow
>>>Take a screen capture for to include in your report

The hello2 program is poorly written, and exposes your machine to being exploited by hackers. That's because it takes the name from typed input and puts it in the name string, but the name string has a size limit--it only has enough room for 10 characters.

Special Note: Names longer than 10 characters will cause user-input data to overwrite parts of memory that were not intended to store data, making the program crash. This is a Buffer Overflow.

In a Terminal window, execute this command:
When you see the "What is your name?" prompt, type in this name, followed by the Enter key:
You see a "Segmentation fault" error, as shown below.

Although this just crashes the machine, which could result in a denial of service, with carefully crafted false data it is often possible to use such errors to open a shell on the host, giving you complete control over it. That's how many of the Metasploit exploits work.
Saving a Screen Image
Make sure the "Segmentation fault" error message is visible, as shown above.
Click the taskbar at the bottom of your host Windows 7 desktop, to make the host machine listen to the keyboard, instead of the virtual machine.
>>>Take a screen capture for to include in your report
Writing the hello3.c Source Code
We need to patch this code. So we'll make another version.
In a Terminal window, execute these commands: cp hello2.c hello3.c nano hello3.c
This makes a copy of hello2.c named hello3.c, and opens it in the nano editor.
The nano editor opens. Modify the scanf call to match the program shown below: #include <stdio.h> main() { char name[10]; printf("What is your name?"); scanf("%10s", name); printf("Hi, %s\n\n", name); }
Save your file with Ctrl+X, Y, Enter.

>>>Take a screen capture for to include in your report

Compiling and running hello3
In a Terminal window, execute these commands: gcc hello3.c -o hello3
When you see the "What is your name?" prompt, type in this name, followed by the Enter key:
The program now runs without an error, as shown below:

The program now just ignores any characters after the first ten. There is no error message, and no stack overflow. The program is patched. This is what many Microsoft security patches do--correct code to remove buffer overflow vulnerabilities.
By the way, this is a sloppy patch, because it leaves some keyboard characters in an input buffer which could lead to unexpected results later in the program. For a more thorough way of patching scanf, see Activity 7-2
>>>Take a screen capture for to include in your report
Using ping
In a Terminal window, execute this command: ping
That number is the default gateway in S214. If you are not in S214, use your default gateway instead of that address.
You should see a series of lines starting "64 bytes from", as shown below:

Press Ctrl+C to stop the pings. In a Terminal window, execute this command: ping -w1
Note that the last two characters are the letter w and the numeral 1.
This makes ping end faster--it stops after one second.
>>>Take a screen capture for to include in your report
Writing the pingscan.c Source Code
We will make a simple ping scanner, like one of the Nmap functions. It will ping each of 100 IP addresses for one second to see if there is any response. This works, although it is a lot slower and clumsier than Nmap.
In a Terminal window, execute this command: nano pingscan.c
The nano editor opens. Type in the program shown below. #include <stdio.h> main() { int i; for (i=0; i<100; i++) printf("ping 192.168.1.%d -w1\n", i); }
Note that the last two characters in the printf format string are the letter w and the numeral 1. (If you are on a different subnet, replace 192.168.1 with the first 3 numbers in your LAN's IP address.)
Save your file with Ctrl+X, Y, Enter.

>>>Take a screen capture for to include in your report

Compiling and running pingscan
In a Terminal window, execute these commands: gcc pingscan.c -o pingscan
The program prints 100 ping command lines on the terminal, as shown below. However, it doesn't execute the PINGs, it just prints out the commands.

Making a Script File with Output Redirection
To make the commands execute, we need to put them into a file and make the file executable.
In a Terminal window, execute this command:
./pingscan > ping100
You see another # prompt with no message, which is what Linux does when there is no problem. The > sign is the output redirection operator, and it took the lines of text that were going to the screen and put them into a file named ping100 instead.
In a Terminal window, execute this command: nano ping100
The ping100 file opens in the nano editor, as shown below. It has 100 ping commands.
Press Ctrl+X to close nano.

>>>Take a screen capture for to include in your report

Making ping100 Executable
In a Terminal window, execute this command: chmod a+x ping100
This command changes the mode of the ping100 file to make it executable by all users.
In a Terminal window, execute this command:
The ping scan should run, with results like those shown below. It will take about 100 seconds to finish.

Saving a Screen Image
Make sure the Terminal window is visible, showing some of the pings.
>>>Take a screen capture for to include in your report

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