Packet Analysis. This section will focus on peaking into the packets to extract the information (which is what we wanted to begin with). First off we must arm. Programming with Libpcap: a PCAP Tutorial. by Tim Carstens (Email: timcarst at yahoo dot com). Ok, lets begin by defining who this document is written for. This tutorial will show how to use libpcap to transcribe packets from one data source to another (in a fashion similar to the effect of tcpreplay).

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Now we get to the meat of the tutorial. This is actually a very simple process. Therefore, clients of libvei should have at least two threads: The obvious advantage to this is that it provides more packets for sniffing, which may or may not be helpful depending on the reason you are sniffing the network.

Using libpcap in C

The IP header length is always stored in a 4 byte integer at byte offset 4 of the IP header. Live Capture The next example program will demonstrate how to open a network device for live capturing, and capture a single packet.

Here are those things:. Look at this psuedo-code.

Libpcap tutorial –

We won’t be able to do anything else if we can’t get a device to work with. Every time the user presses a key, my program will call the callback function. So before getting too far into packet dissection it would probably benefit us to regress a bit and talk about Tutlrial This client also calls the library’s shutdown function. The last argument is useful in some applications, but many times is simply set as NULL.

Third, on high traffic networks, the host can become quite taxed for system resources.

In this case, pcap just sets the device on its own. Both of them call a callback function every time a packet is sniffed that meets our filter requirements if any filter exists, of course. This is a function we declare and implement within libvei, but its signature must follow the contract declared by libpcap so that libpcap can safely deliver information about packets it sees to us. Promiscuous mode sniffing is detectable; a host can test with strong reliability to determine if another host is doing promiscuous sniffing.


But how do you make use of this variable named “packet” in our prototype? Or perhaps we want to highjack a file being sent over port 21 FTP.

Programming with pcap

In this case, we report a number of metrics and return to the calling thread. It is difficult to memorize all the function calls and what types you have to pass for each argument. It is mainly libpccap library for managing the reading and writing process of packets to and from a data source. So lets make a chart:. It prints out some diagnostic information and passes two parameters to the actual “work” function.

This page was last modified on 14 Mayat The third argument is the name of the callback function just it’s identifier, no parenthesizes. You pass it a raw pointer and a length and it will send whatever it finds in memory to the handle.

We first need another global variable to keep track of the “last” time of a saved packet. The most important element of libpccap ether header to us is the ether type. Lastly, the payload which isn’t really a structure, just a character string is located after all of them. For a more in depth discussion of their differences, see the pcap man page.

This function is described in the Miscellaneous section at the end of the document.

Monitor mode lets the card listen to wireless packets without being associated to an access point. Well, as luck would have it, pcap uses the exact same structures when sniffing packets. Lets start by looking at the datalink headers.

Promiscuous mode lets the card listen to all packets, even ones not intended for it. Additionally, some basic understanding of networking might help, given that this is a packet sniffer and all. The first thing to understand is the general layout of a pcap sniffer. The syntax is documented quite well in the man page for tcpdump; I leave you to read it on your own.


One subtle issue is that of timing; we want this transcription to be faithful to the timing of the original trace. Ok, lets begin by defining who this document is written for. This is where we do it. Since this program will continuously loop and process packets, you will have to use CTRL-C to end the program or use the kill command. For example, the trace may have been collected over an hour, but libpcap is yutorial back the PCAP dump file contents as fast as the OS will allow libpcap to read it which is potentially pretty fast in comparison.

View the discussion thread. Did you run the program? Below is a copy of the main program I intend on using nothing specialgo ahead and cut and paste it or download it here. However, there are regressions. Following that is a reference to the place we will store the compiled version of our filter. Getting Started Lbipcap there is an awful lot to cover.

The main data structures, however, are those that provide handles to the dump files we are working with. It is responsible for a few things, including opening the files involved via the C library, tutorlal the library initialization routine, and calling the library transcription routine.

Both of these programs are capable of analyzing all fields of a packet, plus the data.

Programming with pcap

We have to get the IP header length to figure out how much further we have to look to find the beginning of the TCP header. The program above will look up the device like the first program, but will go a step further and get information about the device as well.

We start with the pointer to the beginning lihpcap the packet.