A value of 6 means TCP a value of 17 means UDP. IPv4 is described by RFC 791 the header of an IPv4 packet has a 1- byte protocol field, which contains an Internet Protocol Number value. If the type/length field has the value 0x0800, then the packet is an IPv4 packet. If the field has the value 1, then the packet begins with an Ethernet header, which begins with a 6-byte Ethernet destination address, followed by a 6-byte Ethernet source address, followed by a 2-byte type/length field. The list of link-layer header types shows what different values in that field mean. Now that tcpdump has been installed on your system, let's explore the different options and functionalities it offers.When it is UDP, byte 23 in the packet is set to 17, however in non-UDP, byte 23 doesn't have the same meaningĪ pcap file has, in the file header, a field that indicates the type of link-layer header that the packets in the file have. This should install the tcpdump utility and solve the "command not found" error. Note that if you're asked to install libcap, type in Yes or Y as it is a core dependency, without which tcpdump will refuse to start up. To install the tcpdump utility on Fedora, CentOS, and RHEL, issue the following command: sudo dnf install tcpdump On Arch-based systems, run: sudo pacman -S tcpdump On Debian/Ubuntu derivatives, run: sudo apt- get install tcpdump To install tcpdump, fire up the terminal and run the command corresponding to the Linux distro that you're currently using: In case you are unable to run the tcpdump command and are stuck at the " tcpdump: command not found" error, let's learn how to install tcpdump on your Linux machine. So you should be able to use it right away by typing in tcpdump with a sudo prefix. Tcpdump usually comes pre-installed with all mainstream Linux distributions and security-based alternatives.
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