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How to Use tcpdump to Capture and Analyze Network Traffic

How to Use tcpdump to Capture and Analyze Network Traffic
Hostman Team
Technical writer
Network
05.11.2024
Reading time: 7 min

Sometimes, troubleshooting network issues requires capturing network traffic. tcpdump is a network traffic analyzer, or "sniffer," that allows you to intercept and analyze network traffic passing through the utility. This tool provides a rich set of options and various filters, making it versatile for different purposes. tcpdump is entirely console-based, meaning it doesn’t have a graphical interface, so it can be run on servers without GUI support. The first version of tcpdump was released back in 1988. Since then, it has been actively maintained, with new versions released every year.

This article will cover various scenarios for using tcpdump.

Prerequisites

To follow this tutorial, you will need: 

  • A cloud server or virtual machine with a Linux OS installed. Any Linux distribution will work.
  • Access to the root user or a user with sudo privileges.

Installing tcpdump

We will install tcpdump on Ubuntu 22.04. The tcpdump package is available in the OS’s official repository. First, update the package index:

sudo apt update

Next, install the utility:

sudo apt -y install tcpdump

Confirm that the installation was successful by checking the tcpdump version:

tcpdump --version

Note that further use of the utility requires running it as the root user or a user with sudo privileges.

Running tcpdump Without Parameters

If you run tcpdump without any parameters, it will start capturing all traffic on all available interfaces in the system and display the data on the screen (stdout):

tcpdump

Image15

To stop the program, press Ctrl + C.

After each run, tcpdump provides the following information:

  • packets captured — shows the number of packets captured (packets that were received and processed by tcpdump).

  • packets received by filter — shows the number of packets captured using filters.

  • packets dropped by kernel — shows the number of packets dropped by the OS kernel.

Image17

By default, tcpdump does not save its output. We will discuss saving the output to a file later in the article.

tcpdump Output Format

Let's analyze the output of a captured packet using the TCP protocol as an example. By default, tcpdump displays the following data for each capture:

09:33:57.063196 IP nexus-test.com.ssh > 192.168.111.1.50653: Flags [P.], seq 27376:27440, ack 321, win 521, length 64

The parameter descriptions are provided in the table below.

Parameter

Description

09:33:57.063196

Timestamp. Uses the format hours:minutes:seconds.fraction, where "fraction" represents seconds from midnight. In this example, the packet was captured at 9:33:57.063196.

IP

Protocol used.

nexus-test.com.ssh

Domain name (or IP address) and port of the source host. Here, ssh is shown instead of port number 22. To display addresses and protocols as numbers, run tcpdump with the -n option.

192.168.111.1.50653

Domain name (or IP address) and port of the destination host.

Flags [P.]

ACK flag(s) used to indicate the connection state. Multiple values are possible. In this example, P is used, indicating the PUSH flag for processing packets immediately rather than buffering them.

seq 27376:27440

Sequence number of data in the packet. Shows the data range as bytes 27376 through 27440 in the packet.

ack 321

Acknowledgment of the received packet.

win 521

Window size in bytes, showing the available buffer space for receiving data.

length 64

Packet length in bytes, indicating the payload size as the difference between the first and last sequence bytes.

Practical Use of tcpdump

Let’s move on to practical applications of tcpdump with examples.

Displaying a List of Network Interfaces

To list all network interfaces available in the system for traffic capture, use:

tcpdump -D

Image2

Capturing Traffic from a Specific Network Interface

By default, tcpdump captures traffic from all available interfaces. To capture traffic from a specific network interface (e.g., ens33), use:

tcpdump -i ens33

Image5

Disabling IP Address to Hostname Resolution

By default, tcpdump converts IP addresses to hostnames and replaces port numbers with service names. To prevent tcpdump from converting IP addresses to hostnames, add the -n option:

tcpdump -n

3a51a06a 252e 4368 B887 B11a871e5d9f

To disable both IP-to-hostname and port-to-service name conversions, use the -nn option:

tcpdump -nn

Capturing a Specific Number of Packets

By default, tcpdump captures an unlimited number of packets. To capture a specified number of packets, for example, 4 packets, use the -c option:

tcpdump -c 4

Image19

Adding Date Information

tcpdump does not display the date of packet capture by default. To include the date in the output, use the -tttt option. The date will appear at the beginning of each line in the format year:month:day:

tcpdump -tttt

1055939d 0924 4655 Bd08 3607a36e7af5

Packet Filtering in tcpdump

tcpdump has extensive filters that allow capturing only the desired packets. Here are some key filters.

Filtering by Port

To capture traffic on a specific port, use the port option. For example, to capture traffic on port 80 directed towards the destination, you can specify dst:

tcpdump dst -n port 80

Image3

You can also specify a range of ports:

tcpdump -n portrange 80-443

Image23

Filtering by Protocol

tcpdump supports filtering by protocols. Supported protocol values include: ether, fddi, tr, wlan, ppp, slip, link, ip, arp, rarp, tcp, udp, icmp, and ipv6. Examples for capturing specific protocols are:

tcpdump icmp

Image20

tcpdump tcp

486dfb79 Ea7c 4e90 B3d5 38f6775f39e7

tcpdump arp

Image21

tcpdump udp

D85a6b9d 32a8 4ea2 Ae5c Ebae57600e97

Filtering by Packet Size

tcpdump allows capturing packets of a specified size using two options:

  • less — captures packets smaller than the specified number of bytes.
  • greater — captures packets larger than the specified number of bytes.

Here are some examples:

Capture traffic with packets that are no more than 43 bytes in size:

tcpdump less 43

Image14

Capture traffic with packets larger than 43 bytes:

tcpdump greater 43

Image25

Note that the packet size includes header size: an Ethernet header without CRC occupies 14 bytes, an IPv4 header occupies 20 bytes, and an ICMP header occupies 8 bytes.

Filtering by MAC Address

To filter by MAC address, use the ether host option. For example, to capture any traffic sent to or from a specified MAC address (e.g., 00:0c:29:c7:00:3f), use:

tcpdump ether host 00:0c:29:c7:00:3f

Image26

Filtering by Source or Destination Address

You can filter traffic using the IP address or hostname of the source or destination.

To capture traffic originating from a specific host, use the src option:

tcpdump -nn src 192.168.36.132

Image18

To capture traffic directed to a specific host, use the dst option:

tcpdump -nn dst 192.168.36.132

Image11

Using Logical Operators in tcpdump

tcpdump supports various logical operators, allowing you to combine options. The following operators are supported:

  • and or && — logical "AND." Combines multiple conditions and shows results matching all conditions.
  • or or || — logical "OR." Combines multiple conditions and shows results matching at least one condition.
  • not or ! — logical "NOT." Excludes specified conditions, showing results that do not match the given condition.

Here are examples of using logical operators:

Capture packets sent from the host 192.168.36.132 and only those listening on port 22:

tcpdump -nn src 192.168.36.132 and port 22

Image12

Capture packets on all available interfaces that are listening on either port 22 or port 80:

tcpdump -nn port 22 or port 80

45ba8aec 5798 4002 B6a6 4933d37a3a9f

Capture all packets except ICMP packets:

tcpdump -nn not icmp

C0e7ddc6 9c61 43f9 9bbf 7a1a6945001b

Saving Output to a File

As previously mentioned, tcpdump does not save its output to a file by default. To save captured data to a file, use the -w option, specifying the filename with a .pcap extension:

tcpdump -nn src 192.168.36.132 -w results.pcap

Image4

While saving to a file, results will not display in the terminal. To stop capturing packets, press CTRL + C.

To read the data saved in the file, use the -r option, followed by the filename where the tcpdump results were saved:

tcpdump -r results.pcap

973be19d 3392 4e9c 8496 77b622acf941

Conclusion

tcpdump is a powerful command-line tool for analyzing networks and identifying issues. The utility supports a wide array of options, enabling users to filter for specific packet information.

Network
05.11.2024
Reading time: 7 min

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How to Create an SSH Tunnel for Secure Connections over VNC

One of the major drawbacks of the VNC (Virtual Network Computing) protocol for remote access to computers is the complete lack of session encryption.  Image source: FAQ on the TightVNC website One way to address this issue is by creating an SSH tunnel over which the VNC session will run, ensuring full encryption of the VNC session. An SSH tunnel creates an encrypted data channel between the client device and the server. In addition to establishing a secure connection to the remote device, an SSH tunnel can also be used to transfer data. In this article, we will explore several methods for creating an SSH tunnel, including using the standard ssh utility, as well as third-party client applications such as PuTTY and MobaXterm. Prerequisites A server or virtual machine with VNC installed. You can use TightVNC for this. We explain how to install it in another article. A second server or virtual machine with a pre-installed Linux OS with a graphical interface. You can use any modern Linux distribution or a home computer or laptop running Windows. Both Home and Professional editions, as well as Windows Server versions, are suitable. Creating an SSH Tunnel  Method 1. The ssh Utility Let’s start by setting up an SSH tunnel using the standard OpenSSH client, which comes pre-installed by default on almost all modern Linux distributions, as well as on Windows operating systems starting from Windows 10 version 1709 and above. Windows Server 2019 and Windows Server 2022 would also work.  On Windows systems, you can also use any WSL distribution (Windows Subsystem for Linux). The following command for setting up an SSH tunnel works the same on both Linux and Windows: ssh -L 5901:localhost:5901 root@<server-IP-address> Where: -L — the flag for local port forwarding. In local forwarding, a port from the client device is forwarded to the server. All subsequent connections to this local port will pass through the SSH tunnel. 5901:localhost:5901 — syntax for forwarding the remote port. In this example, we inform SSH that we want to forward port 5901 (the port of the VNC server) located on the remote server to gain access to the VNC server. At the same time, we also open port 5901 on our local device (localhost). root@<server-IP-address> — the standard syntax for SSH connection. After entering the command, the system will prompt for the user’s password, and upon successful entry, you will log into the server. After this, the SSH tunnel will be established. It's important to remain connected to the server; otherwise, the SSH session (and the tunnel) will be interrupted. If you need to launch the SSH tunnel in "daemon" mode (in the background), use the -fNT options, for example: ssh -fNT -L 5901:localhost:5901 root@<server-IP-address> Where: -f — after the password is entered, instead of launching a shell, the ssh process will switch to the background; -N — do not execute any command on the remote server after starting the tunnel; -T — disables the use of a terminal. Once the SSH tunnel is successfully established, you can connect using any VNC client utility, for example, TightVNC Connection. Launch the utility and enter the address localhost::5901 in the “Remote Host” field: Note: The use of two colons after localhost applies only to the TightVNC Connection program. After entering the address, click the “Connect” button. The program will request the password for the VNC session, which is set during the VNC server configuration: After entering the password, a window with the graphical interface of the server will open: All traffic between your device and the VNC server is now fully protected and encrypted. Method 2. PuTTY In addition to using the standard ssh utility, a tunnel can also be set up using the popular client utility for connecting to remote servers — PuTTY. To do this, follow these steps: Launch PuTTY and in the main menu fill in the following fields: Host Name (or IP address): enter the IP address of the VNC server; Port: specify the port used by SSH; Saved Sessions: enter any name for the session so that it can be saved and launched quickly in the future. Click the “Save” button to save the current session. In the left menu, find the “Connection” section, expand it, and go to “Tunnels”: In the opened section, fill in the following details: Source port: specify the port to be opened on the client device, e.g., 5901; Destination: enter the IP address of the VNC server and the VNC server’s port. After entering the data, click the “Add” button: Return to the PuTTY main menu (the “Session” section) and connect to the server by clicking the “Open” button. During the first login, you will need to accept the host key by clicking the “Accept” button. After entering the user account password, the server terminal will open: Without closing the PuTTY session window, open your VNC client application (e.g., TightVNC Connection) and enter the address localhost:5901: After entering the VNC session password, the server’s graphical interface will be displayed. Method 3. MobaXterm Another popular program for Windows OS used to connect to remote servers is MobaXterm. It can also be used to create an SSH tunnel. To do so, follow these steps: Launch the program and click on the “Tunneling” tab at the top: In the tunnel settings window, make sure the option “Local port forwarding” is selected and fill in the following information: In the “My computer with MobaXterm” section, enter the local port (5901) to be opened on the device; In the “SSH server” section, enter the address of the remote VNC server, along with the login and password to connect to the server; In the “Remote server” section, enter localhost as the address and 5901 as the port. Click the “Save” button to save the settings. In the opened window, click the start button in the “Start/stop” section: Once the SSH tunnel is launched, go to the “Session” section: In the “Remote hostname or IP address” field, enter localhost, and in the “Port” field, enter 5901: Click the “OK” button to connect. After entering the VNC session password, the server’s graphical interface will appear: Conclusion Although the VNC protocol does not encrypt its traffic by default, this issue can be resolved by using an SSH tunnel. In this article, we reviewed several methods for setting up an SSH tunnel on your device.
16 June 2025 · 6 min to read

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