computer science

Packet crafting

Packet crafting is a computer science topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Packet crafting rather than just read about it. In short: Packet crafting is a technique that allows network administrators to probe firewall rule-sets and find entry points into a targeted system or network. This is done by manually generating packets to test network devices and behaviour, instead of using existing network traffic.

Key takeaways

Packet crafting belongs to computer science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Packet crafting to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Packet crafting from memory before moving on to harder problems.

Reference excerpt

Packet crafting is a technique that allows network administrators to probe firewall rule-sets and find entry points into a targeted system or network. This is done by manually generating packets to test network devices and behaviour, instead of using existing network traffic. Testing may target the firewall, IDS, TCP/IP stack, router or any other component of the network. Packets are usually created by using a packet generator or packet analyzer which allows for specific options and flags to be set on the created packets. The act of packet crafting can be broken into four stages: Packet Assembly, Packet Editing, Packet Play and Packet Decoding. Tools exist for each of the stages - some tools are focused only on one stage while others such as Ostinato try to encompass all stages.

Packet assembly Packet Assembly is the creation of the packets to be sent. Some popular programs used for packet assembly are Hping, Nemesis, Ostinato, Cat Karat packet builder, Libcrafter, libtins, PcapPlusPlus, Scapy, Wirefloss and Yersinia. Packets may be of any protocol and are designed to test specific rules or situations. For example, a TCP packet may be created with a set of erroneous flags to ensure that the target machine sends a RESET command or that the firewall blocks any response.

Packet editing Packet Editing is the modification of created or captured packets. This involves modifying packets in manners which are difficult or impossible to do in the Packet Assembly stage, such as modifying the payload of a packet. Programs such as Scapy, Ostinato, Netdude allow a user to modify recorded packets' fields, checksums and payloads quite easily. These modified packets can be saved in packet streams which may be stored in pcap files to be replayed later.

Packet play Packet Play or Packet Replay is the act of sending a pre-generated or captured series of packets. Packets may come from Packet Assembly and Editing or from captured network attacks. This allows for testing of a given usage or attack scenario for the targeted network. Tcpreplay is the most common program for this task since it is capable of taking a stored packet stream in the pcap format and sending those packets at the original rate or a user-defined rate. Scapy also supports send functions to replay any saved packets/pcap. Some packet analyzers are also capable of packet replay.

Packet decoding Packet Decoding is the capture and analysis of the network traffic generated during Packet Play. In order to determine the targeted network's response to the scenario created by Packet Play, the response must be captured by a packet analyzer and decoded according to the appropriate specifications. Depending on the packets sent, a desired response may be no packets were returned or that a connection was successfully established, among others. The most famous tools for that task are Wireshark and Scapy.

See also Comparison of packet analyzers Replay attack Packet Sender

References

External links Packet Crafting for Firewall & IDS Audits (Part 1 of 2) by Don Parker [1] Wikiformat article detailing Packet crafting Bit-Twist - Libpcap-based Ethernet packet generator [2] Packet Sender - open source packet generator focused on ease-of-use

Worked examples

Example 1 — a first encounter with Packet crafting

Start with the simplest possible case. Write down what Packet crafting claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In computer science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Packet crafting before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Packet crafting ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Packet crafting

In research: Packet crafting appears in computer science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Packet crafting in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Packet crafting is common in secondary-school and first-year university syllabi. It links to neighbouring topics Network analyzers, so understanding it makes those chapters shorter.
In everyday life: Look for Packet crafting outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Packet crafting in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Packet crafting means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Packet crafting out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Packet crafting in simple terms?

Why does Packet crafting matter?

Because it connects several computer science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Packet crafting?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Packet crafting.