Steganography - hiding data within JPG file

The data file must be a ZIP file ('data.zip') and image.jpeg is a image file.

# The data file must be a ZIP file ('data.zip') and 'image.jpeg' is a image file.

>> cat image.jpeg data.zip  > test.jpg

To see the content of jpg having image + data file

>> unzip -t test.jpg

To extract content from jpg having image + data file

>> unzip test.jpg

Defrag MYSQL database

Over a period of time, "mysql" tables usually get fragmented. It degrades the performance of "mysql-server" significantly.

You may also find it worth your while to defragment your database tables regularly if you are using VARCHAR fields: these variable-length columns are particularly prone to fragmentation.

To defragment tables manually from command-line using mysqlcheck

Run the following command by replacing USER & PASS values:

mysqlcheck -Aos -u <user> -p <password>

Parallelise your build using make

If you're running a multiprocessor system (SMP) with a moderate amount of RAM, you can usually see significant benefits by performing a parallel make when building code. Compared to doing serial builds when running make (as is the default), a parallel build is a vast improvement. To tell make to allow more than one child at a time while building, use the -j switch:

make -j4; make -j4 modules

'C' multi-buffer tokeniser (reentrant version)

char *strtok_r(char *str, const char *delim, char **saveptr);

The strtok_r() function is a reentrant version strtok(). The saveptr argument is a pointer to a char * variable that is used internally by strtok_r() in order to maintain context between successive calls that parse the same string.

strtok_r() is used almost exactly as strtok() except with additional last parameter char ** saveptr is passed. This is used internally, to save of buffer state.


#include <stdio.h>
#include <stdlib.h>
#include <strings.h>


int main() {
  char buffer1[50];
  char buffer2[50];
  char *save1, *save2, *token1, *token2;

  strcpy(buffer1, "A-B-C-D");
  strcpy(buffer2, "1:2:3:4");

  token1 = strtok_r(buffer1, "-", &save1);
  token2 = strtok_r(buffer2, ":", &save2);

  while(token1 != NULL ||
        token2 != NULL) {
     printf("[%s%s]\n", token1, token2);
     token1 = strtok_r(NULL, "-", &save1);
     token2 = strtok_r(NULL, ":", &save2);
  }
  return 0;

}

Writing Linux Kernel module

  lsmod – List all loaded kernel modules




Sample kernel module

hello_world.c module in 'C'
 


Compiling kernel module
 

make -C /lib/modules/<`uname -r`>/build M=<path_to_hello_world.c file> modules

#on successful compilation : hello_world.ko file will be generated

 Adding /inserting module 

 # loading the module
insmod hello_world.ko

# checking whether module is loaded.
dmesg |grep

# checking using lsmod
lsmod | grep hello

shell - Indirect variable references

KSH

       i = 10

       var_ref=variable_sequence _$i

       eval  pointer_to_val=\$$var_ref

BASH

      i = 10

       var_ref=variable_sequence _$i

       pointer_to_val=${!var_ref}

Radar slant and blind range

Slant range:

Slant range is line of sight distance between radar antenna and flying aircraft.

The distance is determined by measuring the time between propagating electromagnetic waves and receiving it back.

Electromagnetic waves velocity is 3 X 108 m / s (speed of light)

Slant range (in meter) = Velocity * time taken / 2

Radar Blind range:

This is minimal measuring distance for any given radar, this is because radar receiver is switch-off until is  transmission is completed.