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That's because 1) libpcap-dev (probably) depends on the libpcap package, and will bring it in and 2) if you have source code that needs libpcap, you not only will need the libpcap package (whatever it's called - its name might be 'libpcap0.8', for various Debian reasons), which provides shared libraries for already-built programs that use libpcap, but you will also need the libpcap-dev package. Note that on some older versions of Mac OS X, you may have to replace the command./configure with./configure CPP=/usr/bin/cpp. Also, on some newer Mac OS X versions, the libpcap version of the library provided by Apple may be too old. WireEdit allows WYSIWYG editing of Pcap data in situ for any network stack at any stack layer while preserving the binary integrity of the data. Data editing is done in a break-proof manner with the lengths, checksums, offsets, and other inter and intra-packet dependencies recalculated on-the-fly for all affected packets and protocol layers. Libpcap uses the underlying libpcap C shared library as specified in libpcap.cfg (system’s libpcap shared library is the default), but there is also ability to specify it programmatically by one of the following ways. Import libpcap libpcap. Config (LIBPCAP = None) # system's libpcap library will be use # or libpcap. Config (LIBPCAP = 'npcap') # or libpcap. Config (LIBPCAP = 'wpcap.

Packet Analysis

This section will focus on peaking into the packets toextract the information (which is what we wanted to begin with).First off we must arm ourselves! Go ahead and get all the releventRFC's. Lets start off with RFC 791 (IP)RFC 768 (UDP)RFC 826 (ARP)RFC 792 (ICMPv4) and of courseRFC 793 (TCPv4) The truth is, once you have these files youdont really need me *sigh* but then again.. why right your own codewhen you can just copy mine! hehe

I would highly recommend you use another packet sniffer to double checkyour programs.. tcpdump will do just fine, and ethereal just kicksass, you can get either (and more!!) at Both of these programs arecapable of analyzing all fields of a packet, plus the data. Sure wecould use them instead of creating our own.. but what fun would that be?

I would prefer not to have to rewrite the main body of the program foreach new example like I have done previously. Instead I am going to usethe same main program and only post the callback function which getspassed to the pcap_loop() or pcap_dispatch() function. Below is a copyof the main program I intend on using (nothing special), go ahead andcut and paste it or download it here.

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I will be using the above program and merely replacing the callbackfunction my_callback for demo programs in this section.


Lets start by looking at the datalink headers. 'Didn't we already dothis', you ask. Sure.. sort of, but we didn't spend much time on it solets just get this out of the way. Looking at the datalink header isn'tall too exciting, but it certainly is something we want to stick in ourtoolkit so we will gloss over the important stuff and continue on. Themost important element of the ether header to us is the ether type.Remember struct ether_header from net/ethernet.h? just soyou don't have to click back, here it is again whith the definition ofan ether_addr.Fortunatly (at least in Linux) netinet/ether.h provides uswith some fuzzy routines to convert ethernet headers to readableascii and well as ethernet address to HOSTNAME resolution (that should ring a bell.:-)Previously I pasted some code shamelessly stolen from Steven's UnixNetwork PRogramming to print out the ethernet header, from now on wetake the easy route. Here is a straightforward callback function tohandle ethernet headers, print out the source and destination addressesand handle the type.

You can download the full code here.

Whew! Ok got that out of the way, currently we have a relatively simpleframework to print out an ethernet header (if we want) and then handle thetype. Lets start by looking at the IP header.

We'll need to wip out our handy dandy RFC's (791 in this case) andtake a look at what it has to say about IP headers.. here is a copyof the section which decsribes the header.Now lets peak at netinet/ip.hCool, they seem to match up perfectly.. this of course wouldbe fine to use, but I prefer to follow the tcpdump methodof handling the version and header length.Lets take a first stab at peaking into the IP header.. Rukmini and radha same. considerthe following function (full source here).Given a clean arp cache this is what the output looks like on my machine,when I try to telnet to try and reconstruct the conversation shall we?

  • my computer: Who has the gateways IP (
    ETH: 0:10:a4:8b:d3:b4 ff:ff:ff:ff:ff:ff (ARP) 42
  • gateway: I do!!
    ETH: 0:20:78:d1:e8:1 0:10:a4:8b:d3:b4 (ARP) 60
  • my computer(through gateway): Hello Mr. can we talk?
    ETH: 0:10:a4:8b:d3:b4 0:20:78:d1:e8:1 (IP) 74IP: 5 4 60 16384
  • Nope, I'm not listening
    ETH: 0:20:78:d1:e8:1 0:10:a4:8b:d3:b4 (IP) 60IP: 5 4 40 0

I have admittedly skipped TONS of information in a rush to provide youwith code to display the IP header (thats all you really wanted anywayswasn't it :-). That said, if you are lost don'tworry, I will slow down and attempt to describe what exactly is going on.All that you really need to know up to this point is.

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  • All packets are sent via ethernet
  • The ethernet header defines the protocol type of the packetit is carrying
  • IP is one of these types (as well as ARP and RARP)
  • The IP header is confusing ..
So before getting too far into packet dissection it would probablybenefit us to regress a bit and talk about IP..
'awww but.. that sounds boring!',you say. Well if you arereally anxious I would suggest you grab the tcpdump source andtake a look at the following methods .. :-)
  • ether_if_print (print-ether.c)
  • ip_print (print-ip.c)
  • tcp_print (print-tcp.c)
  • udp_print (print-udp.c)

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I've also foundthe sniffit source to be a great read.

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