Rede (Networking)

Exemplo em Ruby para construir uma mensagem version (com funções auxiliares para formatar os dados):

require 'digest' # necessário para criar checksums

# Funções auxiliares para deixar os dados no formato certo para as mensagens
def hexadecimal(number)
    return number.to_s(16)
end

def size(data, size)
    return data.rjust(size*2, '0') # preenche a esquerda com zeros até um nº de bytes (2 chars hex = 1 byte)
end

def reversebytes(bytes)
    return bytes.scan(/../).reverse.join() # pega cada 2 chars (1 byte), inverte e junta de novo
end

def ascii2hex(string)
    # Converte cada caractere da string para sua representação em bytes hexadecimais
    bytes = string.each_byte.map {|c| c.to_s(16) }.join()

    # Preenche até 12 bytes (mantendo os bytes da string ascii à esquerda)
    return bytes.ljust(24, '0')
end

def checksum(bytes)
    # Faz o hash dos dados duas vezes
    hash = Digest::SHA256.digest(Digest::SHA256.digest([bytes].pack("H*"))).unpack("H*")[0]

    # Retorna os primeiros 4 bytes (8 caracteres)
    return hash[0...8]
end

# Cria o payload de uma mensagem version
payload  = reversebytes(size(hexadecimal(70014), 4))         # versão do protocolo
payload += reversebytes(size(hexadecimal(0), 8))             # serviços (ex.: 1<<3 | 1<<2 | 1<<0)
payload += reversebytes(size(hexadecimal(1640961477), 8))    # tempo
payload += reversebytes(size(hexadecimal(0), 8))             # serviços do nó remoto
payload += "00000000000000000000ffff2e13894a"                # ipv6 do nó remoto
payload += size(hexadecimal(8333), 2)                        # porta do nó remoto
payload += reversebytes(size(hexadecimal(0), 8))             # serviços do nó local
payload += "00000000000000000000ffff7f000001"                # ipv6 do nó local
payload += size(hexadecimal(8333), 2)                        # porta do nó local
payload += reversebytes(size(hexadecimal(0), 8))             # nonce
payload += "00"                                              # user agent (compact_size + bytes ascii)
payload += reversebytes(size(hexadecimal(0), 4))             # último bloco

# Cria o cabeçalho da mensagem
magic_bytes = 'f9beb4d9'
command     = ascii2hex('version')                                 # 76 65 72 73 69 6F 6E 00 00 00 00 00
size        = reversebytes(size(hexadecimal(payload.length/2), 4)) # 55 00 00 00
checksum    = checksum(payload)
header      = magic_bytes + command + size + checksum

# Junta cabeçalho e payload
message = header + payload

O programa completo deste tutorial: conecta a um nó, faz o handshake e fica lendo mensagens (respondendo a ping e inv). Os comentários do código estão em inglês (como no original).

# Sockets are in the standard library in Ruby
require 'socket'

# Open a TCP connection to an IP and port
socket = TCPSocket.open("127.0.0.1", 8333) # local computer = 127.0.0.1

require 'digest' # needed for creating checksums

# Handy functions for getting data in the right format for messages
def hexadecimal(number)
    return number.to_s(16)
end

def size(data, size)
    return data.rjust(size*2, '0') # pad the left of the data out with zeros up to a specific number of bytes (2 hexadecimal chars = 1 byte)
end

def reversebytes(bytes)
    return bytes.scan(/../).reverse.join() # grab each 2 characters (1 byte) as an array, reverse the array, then join back together
end

def ascii2hex(string)
    # Convert each character in the string to its hexadecimal byte representation
    bytes = string.each_byte.map {|c| c.to_s(16) }.join()

    # Pad up to 12 bytes (keeping the bytes for the ascii string on the left)
    return bytes.ljust(24, '0')
end

def checksum(bytes)
    # Hash the data twice
    hash = Digest::SHA256.digest(Digest::SHA256.digest([bytes].pack("H*"))).unpack("H*")[0]

    # Return the first 4 bytes (8 characters)
    return hash[0...8]
end

# Create the payload for a version message
payload  = reversebytes(size(hexadecimal(70014), 4))         # protocol version
payload += reversebytes(size(hexadecimal(0), 8))             # services e.g. (1<<3 | 1<<2 | 1<<0)
payload += reversebytes(size(hexadecimal(1640961477), 8))    # time
payload += reversebytes(size(hexadecimal(0), 8))             # remote node services
payload += "00000000000000000000ffff2e13894a"                # remote node ipv6 (https://dnschecker.org/ipv4-to-ipv6.php)
payload += size(hexadecimal(8333), 2)                        # remote node port
payload += reversebytes(size(hexadecimal(0), 8))             # local node services
payload += "00000000000000000000ffff7f000001"                # local node ipv6
payload += size(hexadecimal(8333), 2)                        # local node port
payload += reversebytes(size(hexadecimal(0), 8))             # nonce
payload += "00"                                              # user agent (compact_size, followed by ascii bytes)
payload += reversebytes(size(hexadecimal(0), 4))             # last block

# Create the message header
magic_bytes = 'f9beb4d9'
command     = ascii2hex('version')                                 # 76 65 72 73 69 6F 6E 00 00 00 00 00
size        = reversebytes(size(hexadecimal(payload.length/2), 4)) # 55 00 00 00
checksum    = checksum(payload)
header      = magic_bytes + command + size + checksum

# Combine the header and payload
message = header + payload

# 1. Send Version Message

# Prepare version message
version = message

# Write the message to the socket (the protocol sends and receives messages in raw bytes)
socket.write [version].pack("H*")
puts "version->"
puts version
puts


# 2. Receive Version Message

# Read the message header response from the socket
magic_bytes = socket.read(4)
command     = socket.read(12)
size        = socket.read(4)
checksum    = socket.read(4)

# View the message header
puts "<-version"
puts "magic_bytes: " + magic_bytes.unpack("H*").join  # convert raw bytes to hexadecimal characters
puts "command:     " + command.to_s                   # to_s automatically converts raw bytes to ASCII characters
puts "size:        " + size.unpack("V").join          # V = 32-byte unsigned, little-endian
puts "checksum:    " + checksum.unpack("H*").join

# Read the message payload
size = size.unpack("V").join.to_i
payload = socket.read(size)

# View the message payload
puts "payload:     " + payload.unpack("H*").join
puts


# 3. Receive Verack Message (verack = version acknowledged)

# Read the message header response from the socket
magic_bytes = socket.read(4)
command     = socket.read(12)
size        = socket.read(4)
checksum    = socket.read(4)

# View the message header
puts "<-verack"
puts "magic_bytes: " + magic_bytes.unpack("H*").join  # convert raw bytes to hexadecimal characters
puts "command:     " + command.to_s                   # to_s automatically converts raw bytes to ASCII characters
puts "size:        " + size.unpack("V").join          # V = 32-byte unsigned, little-endian
puts "checksum:    " + checksum.unpack("H*").join

# Read the message payload (there shouldn't be any)
size = size.unpack("V").join.to_i
payload = socket.read(size)

# View the message payload (there shouldn't be any)
puts "payload:     " + payload.unpack("H*").join
puts

# 4. Send Verack Message

# Create verack message
payload     = '' # verack has no payload, it's just a message header
magic_bytes = 'f9beb4d9'
command     = ascii2hex('verack')
size        = reversebytes(size(hexadecimal(payload.size/2), 4))
checksum    = checksum(payload)
verack      = magic_bytes + command + size + checksum + payload

# Write the message to the socket
socket.write [verack].pack("H*")
puts "verack->"
puts "magic_bytes: " + magic_bytes
puts "command:     " + 'verack'
puts "size:        " + size.to_i(16).to_s
puts "checksum:    " + checksum
puts "payload:     " + payload
puts

# Keep reading messages
loop do

    # Create an empty buffer to help us find the next stream of magic bytes (the start of a new message)
    buffer = ''

    # Keep looping to read bytes from the socket
    loop do

        # Read one byte at the time
        byte = socket.read(1)

        # Check that we haven't been disconnected from the node.
        if byte.nil?
            puts "Read a nil byte from the socket. Looks like the remote node has disconnected from us. We probably failed the handshake too many times, or didn't respond to enough pings. No worries, try connecting to another node for the time being instead."
            exit
        end

        # Add each byte to the temporary buffer
        buffer += byte.unpack("H*").join unless byte.nil? # do not do anything if we got a nil byte for some reason

        # Check the buffer when it reaches 4 bytes
        if (buffer.size == 8) # 8 hexadecimal characters = 4 bytes

            # See if the buffer matches the magic bytes
            if (buffer == 'f9beb4d9')

                # If we've got the magic bytes we're looking for, go ahead and read the full message from the socket
                command     = socket.read(12).to_s.delete("\x00")  # convert to ascii and remove any empty bytes
                size        = socket.read(4).unpack("V").join.to_i # convert to an integer
                checksum    = socket.read(4).unpack("H*").join     # convert to hexadecimal string of bytes
                payload     = socket.read(size).unpack("H*").join  # use the size from the header to read the payload, then convert to a hexadecimal string

                # Print the message
                puts "<-#{command}"
                puts "magic_bytes: " + buffer
                puts "command:     " + command
                puts "size:        " + size.to_s
                puts "checksum:    " + checksum
                puts "payload:     " + payload
                puts

                # Respond to all inv messages with getdata messages
                if command == "inv"

                    # Set new command name
                    command = "getdata"

                    # Use the same payload as the one we got from the inv message
                    payload = payload

                    # Create message
                    magic_bytes = 'f9beb4d9'
                    command_hex = ascii2hex(command)
                    size        = reversebytes(size(hexadecimal(payload.size/2), 4))
                    checksum    = checksum(payload)
                    message     = magic_bytes + command_hex + size + checksum + payload

                    # Print the message header and payload
                    puts "#{command}->"
                    puts "magic_bytes: " + magic_bytes
                    puts "command:     " + command
                    puts "size:        " + (payload.size/2).to_s
                    puts "checksum:    " + checksum
                    puts "payload:     " + payload
                    puts

                    # Send the message (convert from hexadecimal string to raw bytes first)
                    socket.write [message].pack("H*")

                end

                # Respond to all ping messages with pong messages
                if command == "ping"

                    # Set new command name
                    command = "pong"

                    # Use the same payload as the one we got from the ping message
                    payload = payload

                    # Create message
                    magic_bytes = 'f9beb4d9'
                    command_hex = ascii2hex(command)
                    size        = reversebytes(size(hexadecimal(payload.size/2), 4))
                    checksum    = checksum(payload)
                    message     = magic_bytes + command_hex + size + checksum + payload

                    # Print the message header and payload
                    puts "#{command}->"
                    puts "magic_bytes: " + magic_bytes
                    puts "command:     " + command
                    puts "size:        " + (payload.size/2).to_s
                    puts "checksum:    " + checksum
                    puts "payload:     " + payload
                    puts

                    # Send the message (convert from hexadecimal string to raw bytes first)
                    socket.write [message].pack("H*")

                end

                # Break out of the loop for reading a single message
                break

            end

            # Reset the buffer and keep looking for a stream of magic bytes
            buffer = ''

        end

    end

end