Usefull Commands and Instructions

• To check using the command line if a server is running on a certain port:

nc -zv localhost 80

• To send a packet of information to the server using the netcat command use:

echo -n “Ground Control For Major Tom” | nc localhost 8080

• Send curl command to mimic websocket handshake request (notice the -v command to see the header responses):

curl --include -v --no-buffer --header "Connection: Upgrade" --header "Upgrade: websocket" --header "Host: example.com:80" --header "Origin: http://example.com:80"   --header "Sec-WebSocket-Key: dGhlIHNhbXBsZSBub25jZQ==" --header "Sec-WebSocket-Version: 13" http://localhost:8080

Basic terminology

• byte is the basic unit of data used to store and represent information. A byte is a group of 8 bits, which can represent 256 different values (from 0 to 255). When dealing with network communication, data is typically read and written as raw bytes, regardless of the type of data it represents (text, numbers, etc.).

• bit is the most basic unit of data in computing and can hold one of two possible values: 0 or 1. 8 bits you can represent 256 (2^8) unique numbers which correspond to characters in the ASCII character set. Ex: the byte 01001000 represents H in ASCII.

Converting Number to Binary

• Example convert 72 to binary

1. Binary place values are:

   128 64 32 16 8 4 2 1
   

2. Start with the highest place value 128. Since 72 is less than 128, put a 0.

3. Compare 72 with 64. Since 72 is greater than 64, put a 1 and and subtract 64 from 72, leaving 8.

4. Continue until the end.

Therefore 72 in binary is 01001000

ASCII Encoding

Converting from byte array to string

• Suppose we have a byte array like the following:

  [72, 101, 108, 108, 111]

• Do a look up in the ASCII table to determine which character the number coresponds to.