![]() These bytes are the ethernet layer, the layer below the internet and transport layer: Destination MAC Address Having again a closer look at the first packet we sent, we will see that there are 14 more bytes in front of our IP header (highlighted in blue). To verify them in Wireshark, go: Right click (on any packet) > Protocol Preferences > “Validate the TCP checksum if possible”. In case you don’t get the upper result, check whether your calculated checksums are correct. Therefore, a reset packet is sent, and the connection establishment is canceled. Despite providing 12345 as source port, there is no application on our side listening on that port to accept the incoming. This happened because of the value we set as source port of our packet. The third packet however is a reset sent from our client to the server. This server replies with a, the second step of a typical TCP three-way handshake. Utilizing Wireshark, we observe what happens when we send the packet:Īs expected, our Python script sends a packet to port 80 of our webserver. root user) to run them: python3 send_first_packet.py When working with raw sockets in scripts, most operating system require advanced privileges (e.g. For further details on the Python socket module, I recommend the Python documentation on sockets. With the setsockopt() we tell the kernel not to generate an IP header, since we are providing it ourselves. We are creating a socket s out of the Internet Protocol family AF_INET, in “raw” mode SOCK_RAW which will be sending TCP packets IPPROTO_TCP. For this example we are going to use Python’s built-in socket module: import socket s = socket.socket(socket.AF_INET, socket.SOCK_RAW, socket.IPPROTO_TCP) s.setsockopt(socket.IPPROTO_IP, socket.IP_HDRINCL, 1) ip_header = b'\x45\x00\x00\x28' # Version, IHL, Type of Service | Total Length ip_header += b'\xab\xcd\x00\x00' # Identification | Flags, Fragment Offset ip_header += b'\x40\x06\xa6\xec' # TTL, Protocol | Header Checksum ip_header += b'\x0a\x0a\x0a\x02' # Source Address ip_header += b'\x0a\x0a\x0a\x01' # Destination Address tcp_header = b'\x30\x39\x00\x50' # Source Port | Destination Port tcp_header += b'\x00\x00\x00\x00' # Sequence Number tcp_header += b'\x00\x00\x00\x00' # Acknowledgement Number tcp_header += b'\x50\x02\x71\x10' # Data Offset, Reserved, Flags | Window Size tcp_header += b'\xe6\x32\x00\x00' # Checksum | Urgent Pointer packet = ip_header + tcp_header s.sendto(packet, ( '10.10.10.1', 0)) As this is a packet we are sending to our webserver, we expect him to respond with a if everything works as planned. For better readability, they are arranged like this: 45Īfter we have now manually created our first TCP/IP packet, let’s put it on the wire. Now let’s put the calculated checksums in our blueprint. TTL, Protocol + Header Checksum (0x0000 in calculation)Īs you could see, both checksums follow the same algorithm, just their input values are different. Version, IHL, Type of Service + Total Length ![]() It consists out of all values in the IP header, again added in 16 bit words and prepended with zeros in case the value is too short: Description The IP header checksum is easy to calculate. (no actual header field, has to be counted!)ĭata Offset, Reserved, Flags, Window SizeĬhecksum (set to 0x0000 in calculation), Urgent Pointer TCP length (including the data part) in byte In case the value isn’t 16 bit long, it will be prepended with zeros. For the calculations, all necessary values are used in 16 bit words and added together as shown below. Remembering from the first part of this series we know, that the checksum consists of values of the TCP Header itself, as well as a pseudo-header. TCP Checksum & IP Header Checksum TCP Checksum You might have seen, that two values are missing for now, the Header Checksum for the IP segment and the Checksum from the TCP segment. As a small exercise, try to understand the meaning behind the values in Total Length and Data Offset. We will use our packet blueprints from the previous part of this series to aid us in creating the packet:īased on our knowledge from the previous parts of this series we filled our blueprint with the relevant values (Identification and Window Size are just random in this case). fill up the other headers with the necessary data.Let’s start by identifying the properties our packet should have: We now want to craft a packet and send it through the network. In this chapter we are going to to use our knowledge on packets to manually craft and put them on the wire. Manually create and send raw TCP/IP packets
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