What is TCPdump in Cyber Security?

TCPdump is a network captive and protocol analysis tool. It is an essential element in the toolkit of cybersecurity professionals. Assembled on the foundation of the libpcap interface, which provides a portable system-independent interface for attaining network datagrams at the user level, TCPdump guide offers an adaptable result for capturing and determining the flow of network traffic. One of the crucial advantages of TCPdump is its wide accessibility, making it the de facto standard for capturing network traffic.

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What is TCPdump in Cyber Security? 

TCPdump is an extensively used open-source network packet analyzer that operates in a command-line interface (CLI) environment. It acts as an unresistant sniffer, interdicting and acquiring network packets flowing through a specific network interface. TCPdump is a necessary tool for network administrators, security analysts, and experimenters involved in cybersecurity operations.

• TCPdump supports filter expressions, permitting users to constrict their captures grounded on specific benchmarks, like source or destination IP addresses, port figures, or protocol types.
  • These filter expressions help concentrate the analysis on applicable network traffic, reducing noise and easing effective disquisition.
• The captured packets are presented in a standardized format, which includes timestamps, source, and destination IP addresses, ports, protocol types, packet lengths, and payload data.
  • This detailed information empowers analysts to inspect network exertion, identify implicit security breaches, discover anomalies, and gain perceptivity into the actions of network operations.
• When an incident occurs, security analysts can reckon on TCPdump to give critical substantiation and contextual information about the event.
• It helps in understanding the compass and impact of an incident, allowing for prompt and effective response measures. also, network administrators influence TCPdump to diagnose and resolve network-related issues.
• By nearly querying packet-level details, they can distinguish misconfigurations, identify interpretation backups, and troubleshoot connectivity challenges.

To use TCPdump effectively, the following command format is used:

• $ TCPdump options (filter expression)

Let's reckon an illustration example. Consider you have a capture file named"traffic.cap" and you want to analyze it using TCPdump while discarding the interpretation of port numbers and displaying the timestamp in the applicable format. To achieve this, you can run the following command.

• $ TCPdump -ntttt -r traffic.cap 

By default, TCPdump extracts only the original 68 bytes of each datagram. still, in situations where the complete content of network traffic is needed, it may be required to acclimate the snaplen value by using the- s option. This allows for acquiring a larger portion of the packet payload, permitting a more complete analysis.

• TCPdump furnishes assets for the "ether" qualifier, permitting users to establish Ethernet TCPdump mac addresses in the standard colon-separated format. For illustration, to capture any broadcast network traffic, you can employ the below command:
  • $ TCPdump  ether dst ff:ff:ff:ff:ff:ff
• To capture traffic sent to or from a distinctive TCPdump show MAC address, the subsequent command can be used:
  • $ TCPdump ether host e8:2a:ea:44:55:66

By employing the "ether" qualifier along with the appropriate MAC address, TCPdump can selectively capture and analyze network traffic based on Ethernet addresses.

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What does TCPdump do?

TCPdump is a command-line packet analyzer tool employed for network troubleshooting, monitoring, and packet capture. It allows you to capture and analyze network traffic on a distinct network interface or capture TCPdump to file. Here are some crucial features and functions of TCPdump 

Packet capture TCPdump captures packets transmitted over a network interface or reads packets from a preliminarily saved capture file. It captures packets grounded on various filtering criteria like source destination IP addresses, ports, protocols, or specific packet content. 

Protocol analysis TCPdump can interpret captured packets and give circumstantial data about diverse network protocols, including Ethernet, TCPdump IP, TCP, UDP, ICMP, DNS, HTTP, and more. It decodes the packet headers and payload, allowing you to analyze the network traffic at a granular position. Filtering and display options TCPdump provides significant filtering capabilities to widely capture or display precise packets of interest. You can apply filters grounded on various criteria, including source destination IP addresses, ports, protocols, packet length, and more. 

What is the Format of TCPdump? 

TCPdump the command line tool logs and caches the network traffic in the pcap file format which is short for packet capture. This file format is widely used for accumulating the network packets that have been documented earlier and is reinforced by various network analysis tools, that does include the TCPdump and applications like TCPdump Wireshark.

TCPdump provides a rich set of options and functionalities, permitting users to knit their network captures to precise conditions. Figure 1 showcases some of the most consequential options, demonstrating the versatility and inflexibility of the tool. 

How TCPdump Works in Cyber Security? 

TCPdump is a very useful tool and could be used to recon the advanced system administration and troubleshoot. The first step when performing a security test in Cyber Security is to recon all the parameters and attack surface available for the target. It is mainly used in performing DNS (Domain Name System) network security. 

• A passive DNS scan is a way to track the DNS queries made using an external or internal network. 
• A recursive DNS query keeps a log of all the transactions made including the source IP address, name server information, and other logs which could be important in the recon stage. 

To begin, TCPdump permits immediate packet capture from the network. By running TCPdump without any command line arguments, it will capture packets from the network interface with the most inferior number. Each captured packet will be displayed as a concise summary line in the existing terminal. To have more control over this process, we can use the "-i" argument to specify the interface for packet capture and the "-nn" reverse to undermine host and protocol name resolution.

• To list all the network interfaces during an active recon
  • $ tcpdump --list-interfaces
• To list one interface and analyse the GET and POST responses
  • $ tcpdump –i enp0s8 –s 0 –A 

What are the Characteristics of TCPdump? 

In the realm of networking, data is transferred in the form of packets, which are comprised of structured bits adhering to specific protocols. Packet sniffers, regardless, do not untangle the layers of encapsulation and decode the traffic based on factors such as destination and source computers, payload content, targeted port numbers, or information exchanged between computers. 

TCPdump functions by precluding and publishing a summary of the captured network data, while precise commands regulate other features like storage. It operates in the following manner:

1. Read/Write Captured TCPdump to Files: TCPdump permits the reading and writing of captured network data to Packet Capture (PCAP) files using command-line interface (CLI) commands. This facilitates the storage and recovery of captured packets for future reckoning or sharing with other tools.
2. Packet Filtering: TCPdump furnishes the ability to filter packets based on established parameters. You can restrict filters to capture only packets that meet certain benchmarks, such as TCPdump source/destination IP addresses, ports, protocols, packet length, or specific content patterns. Packet filtering allows to focus on specific network traffic of interest.

1. TCPdump Filtering Packets:

TCPdump will continually capture packets until it acquires an interrupt indication, which can be initiated by pressing Ctrl+C. In the provided example, TCPdump captured over 9,000 packets due to the persistent SSH connection with the server. To confine the number of captured packets and stop TCPdump, you can employ the -c option followed by an established count value.

As stated earlier, TCPdump holds the potential to capture a large number of packets, including those that may not be suitable for the characteristic troubleshooting task. For illustration, when troubleshooting a connectivity issue with a web server, SSH traffic might not be of interest. By filtering out SSH packets from the output, it does help better to focus on the authentic problem.

One of the unique strengths of TCPdump lies in its credentials to filter captured packets using diverse parameters. These parameters include source and destination IP addresses, ports, protocols, and more. This filtering feature is extremely powerful and reserves in isolating the desired packets for analysis.

2. TCPdump Protocol:

To selectively filter packets established on their protocol, you can select the preferred protocol directly in the command line. For illustration, if you want to capture only the ICMP (Internet Control Message Protocol) packets, you can employ the below command:

Further, try pinging the same interface from the different machine:

Upon inspecting the TCPdump capture, it is prominent that only ICMP-related packets are being captured and displayed. Yet, it is necessary to note that TCPdump is not displaying the name resolution packets that were affected during the process of resolving the name "opensource.com".

3. TCPdump Host

Using the following command TCPdump will only capture and display the network packets coming from host “54.204.39.132”

4. TCPdump Port

To filter packets established on a characteristic service or port, you can utilize the TCPdump filter port. For illustration, if you would want to capture packets associated with a web (HTTP) service, you can operate the following command:

5. Source IP or Hostname

If you would want to capture the network packets precisely generated from a specific host associated with the IP address 192.168.122.98, you can utilize the below command:

Benefits of TCPdump 

• TCPdump is a command-line tool developed for capturing and analyzing network traffic on any arbitrary user's system.
• It performs as a beneficial tool for troubleshooting network issues and serves as a security tool.
• TCPdump proposes a wide TCPdump IP range of alternatives and filters, assembling it into an adaptable and influential tool for diverse use cases.
• Due to its command-line character, TCPdump is especially beneficial in remote server environments or devices that lack a graphical user interface.
• It permits the collection of data that can be explored at a later time, presenting flexibility in data collection and analysis.
• TCPdump can be executed in the background or scheduled as a job using tools like cron, furnishing automation abilities for network monitoring tasks.

Use Cases of TCPdump in Cyber Security 

TCPdump is most widely used as a network analysis tool that aids you in helping with critical data getting straight to the crucial information without a bunch of unwanted data. TCPdump is bundled with numerous Unix-based distributions and can be conveniently installed using the operating system's package manager if it's not already included. Security Onion, for instance, comes pre-installed with TCPdump as part of its default package.

• Network Packet Analysis
• List all the network interfaces at time of recon
• Querying Passive DNS transactions
• View the Content of the Packets captured, monitor it for further use cases
• Network troubleshooting
• Attack and Défense Response
• Network Filter type flexibility

TCPdump Examples in Cyber Security 

TCPdump is used for extracting diverse network packets information and logs: Refer example Figure 1, Figure 2, Figure 3, Figure 4 and Figure 5.

• Extracting the HTTP user Agent data
• Can Capture GET and POST HTTP method request packets.
• Extract passwords transferring via HTTP POST methods.
• Can Capture all the ICMP packets independently.
• Can capture all the DNS active transactions.
• Can troubleshoot all network-related issues. 
• Ease operations when working with Wireshark.
• Capability of capturing the port scan from within the network.
• Capability of operating with the NSE scripts testing.

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Conclusion 

TCPdump provides valuable insights into network behavior, making it an excellent choice for quickly examining network traffic flowing through a computer's network interface. By directly capturing and displaying packets from the network interface in a human-friendly format within the terminal, TCPdump offers a convenient way to observe network activity.

In scenarios where graphical user interface (GUI) programs are unsuitable for displaying traffic between emulated hosts in ComNetsEmu, TCPdump proves to be a useful tool. It enables the capture and analysis of network traffic, even in situations where GUI-based tools are not available.