Check if sufficient arguments were supplied Struct bpf_program fp /* to hold compiled program */ Printf("\nPacket number, length of this packet is: %d\n", count , pkthdr->len) Void callback(u_char *useless,const struct pcap_pkthdr* pkthdr,const u_char* After capturing the packets, inside the callback function, the length of each packet is printed on stdout. The code below makes use of the libpcap functions to achieve a basic packet capture. The above mentioned four steps are the basic steps to start a packet capture through libpcap. Once a packet is captures, a callback function is called in which whole of the packet is available to print its details or use it an any other way.Next the pcap library enters into its packet capturing loop where it captures number of packets as set by the program.This filter is compiled and then applied using a set of libpcap library functions. Next, we can apply filter options for cases like if we want to sniff only TCP/IP packets or if we want to specify that sniff packets only from a particular source or destination port etc.Once the device is chosen, initialize the pcap library with this device.For example ‘eth0’, ‘wlan0’ etc on Linux. Choose the network interface device on which the packet sniffing is to be done.To understand the use of this library, one requires basic understanding of C programming language. Libpcap is the underlying library used for packet sniffing by many of the popular network monitoring tools. For command line tool, refer to tcpdump, which also does packet sniffing but produces output on the command line. To understand packet capture and display filters, refer to our tutorial on wireshark. Choosing a filter option is not necessary. Once the interface is selected, there can be some options through which one can filter out the packets based on protocol, source port, destination port etc. A list of interfaces can be obtained by the command ifconfig. Packet sniffers work by sniffing on an interface device like eth0 etc. But, overall packet sniffers are handy for debugging network related problems and can be used by anyone who has the required privileges. Mostly packet sniffers are used by network administrators and developers working on network tools. The data travels on network in form of packets and a packet sniffing tool can easily capture these packets. Packet sniffing is a technique through which the network data to and from your computer can be monitored easily. What is Packet Sniffing and How it Works? This article provides a basic overview of the libpcap library which forms the base of packet sniffing for many network monitoring tools including wireshark, tcpdump, snort, etc. Network tools like wireshark, tcpdump, etc, are fairly popular for packet sniffing.
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