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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If not, consult your local C reference text, as an explanation of tutoral is beyond the scope of this document. We first declare two global references to the file handles of the source and destination files:. Whatever the case, rarely do we just want to blindly sniff all network traffic. The IP header length is always stored in a 4 byte integer at byte offset 4 of the IP header.

Contact information has changed, please send your hate-mail to casado at cs. Since we’re just dumbly replaying here, we don’t peer inside the packet, although in many lkbpcap, depending on the type of replay you want to control or the type of network interface you are replaying to, you would want to update various fields in the layer 2, layer 3, and layer 4 headers and possibly payload. This tutorlal is described in the Miscellaneous section at the end of the document.

Learn to use the man pages efficiently. There are different pages. We have finished handling the packet that libpcap gave us, and we will wait for the next delivery. The next example program will demonstrate how to open a network device for live capturing, and capture tutoriak single packet. And that’s how we set our device. The filter expression is kept in a regular string char array.


This is because the members of each of these structures can have different sizes on different platforms. Your mileage may vary. Page 3 is the C library functions and 7 is miscellaneous.

If tutoriall get a device called “any” bound to 0. The following steps describe a set of tasks, building off how to set up the development environment to writing simple packet replay code to adding in some advanced features.

See the manual page for time for more information on its structure. Normally Libppcap would have simply just used the definitions in those libraries, but it has been my experience that the libraries vary slightly from platform to platform, making it complicated to implement them quickly.

The second argument is a pointer oibpcap a structure that holds general information about the packet, specifically the time in which it was sniffed, the length of this packet, and the tutoeial of his specific portion incase it is fragmented, for example. So for demonstration purposes we will just avoid that mess and simply copy the relevant structures. For the sake of simplicity, we’ll say that the address this pointer is set to is the value X. We let the injecter thread take care of that.

To make any use of it, we must do some interesting typecasting. That said, if you are lost don’t worry, I will slow down and attempt to describe what exactly is going on.

Using libpcap in C

Well, we just asked libpcap to give us some specs on an interface to listen on. Ok, lets begin by defining who this document is written for.

For instance, there may be times when all we want is to sniff on port 23 telnet in search of passwords. We will begin by looking at how to capture a single packet, then look at methods of using loops. If it was not defined, then I had to use a different structure definition for the TCP header.


Note also how we need to obtain both the lock on the internal buffer and the lock on the output PCAP stream. This code fragment opens the device stored in the strong “somedev”, tells it to read however many bytes are specified in BUFSIZ which is defined in pcap.

Note how we actually don’t set the timestamp values.

Programming with Libpcap: a PCAP Tutorial

The library will have an API containing three functions: This section will focus on peaking into the packets to extract the information which is what we wanted to begin with. This tutorial assumes a cursory knowledge in networks; what a packet is, Libpvap vs. Who has the gateways IP We need to get the actual header length for both IP and TCP layers in order to calculate the offset for the payload. The syntax is documented quite well in the man page for tcpdump; I leave you to tuhorial it on your own.

For the purpose of this example, lets pretend that my program wants a user to press a key on the keyboard. For many situations, the easiest approach is to use tcpdump to write to a file and then write programs to analyze the file offline. Both of these programs are capable of yutorial all fields of a packet, plus the data. It is responsible for a few things, including opening the files involved via the C library, calling the library initialization routine, and calling the library transcription routine.

Or perhaps we want to highjack a file being sent over port 21 FTP. All source in this section lipbcap written and tested on linux, kernel 2.