Patrice Auffret > Net-Packet-3.27 > Net::Packet::Dump



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Net::Packet::Dump - a tcpdump-like object providing frame capturing and more


   require Net::Packet::Dump;
   use Net::Packet::Consts qw(:dump);

   # Example live capture (sniffer like)

   # Instanciate object
   my $dump = Net::Packet::Dump->new(
      mode          => NP_DUMP_MODE_ONLINE,
      file          => 'live.pcap',
      filter        => 'tcp',
      promisc       => 1,
      snaplen       => 1514,
      noStore       => 1,
      keepTimestamp => 1,
      unlinkOnClean => 0,
      overwrite     => 1,
   # Start capture

   while (1) {
      if (my $frame = $dump->next) {
         print $frame->l2->print, "\n" if $frame->l2;
         print $frame->l3->print, "\n" if $frame->l3;
         print $frame->l4->print, "\n" if $frame->l4;
         print $frame->l7->print, "\n" if $frame->l7;

   # Cleanup

   # Example offline analysis

   my $dump2 = Net::Packet::Dump->new(
      mode          => NP_DUMP_MODE_OFFLINE,
      file          => 'existant-file.pcap',
      unlinkOnClean => 0,

   # Analyze the .pcap file, build an array of Net::Packet::Frame's

   # Browses captured frames
   for ($dump2->frames) {
      # Do what you want
      print $_->l2->print, "\n" if $_->l2;
      print $_->l3->print, "\n" if $_->l3;
      print $_->l4->print, "\n" if $_->l4;
      print $_->l7->print, "\n" if $_->l7;

   # Cleanup

   # Example writing mode

   my $dump3 = Net::Packet::Dump->new(
      mode      => NP_DUMP_MODE_WRITER,
      file      => 'write.pcap',
      overwrite => 1,


   # Build or capture some frames here
   my $frame = Net::Packet::Frame->new;

   # Write them

   # Cleanup


This module is the capturing part of Net::Packet framework. It is basically a tcpdump process. When a capture starts, the tcpdump process is forked, and saves all traffic to a .pcap file. The parent process can call next or nextAll to convert captured frames from .pcap file to Net::Packet::Frames.

Then, you can call recv method on your sent frames to see if a corresponding reply is waiting in the frames array attribute of Net::Packet::Dump.

By default, if you use this module to analyze frames you've sent (very likely ;)), and you've sent those frames at layer 4 (using Net::Packet::DescL4) (for example), lower layers will be wiped on storing in frames array. This behaviour can be disabled by using noLayerWipe attribute.

Since Net::Packet 3.00, it is also possible to create complete .pcap files, thanks to the writer mode (see SYNOPSIS).



By default, this attribute is set to dev found in default $Env object. You can overwrite it by specifying another one in new constructor.


Stores a Net::Packet::Env object. It is used in start method, for example. The default is to use the global $Env object created when using Net::Packet::Env.


Where to save captured frames. By default, a random name file is chosen, named like `netpacket-tmp-$$.@{[getRandom32bitsInt()]}.pcap'.


A pcap filter to restrain what to capture. It also works in offline mode, to analyze only what you want, and not all traffic. Default to capture all traffic. WARNING: every time a packet passes this filter, and the next method is called, the internal counter used by b<timeoutOnNext> is reset. So the timeout attribute can only be used if you know exactly that the filter will only catch what you want and not perturbating traffic.


If the file exists, setting this to 1 will overwrite it. Default to not overwrite it.


Each time next method is called, an internal counter is incremented if no frame has been captured. When a frame is captured (that is, a frame passed the pcap filter), the timeout attribute is reset to 0. When the counter reaches the value of timeoutOnNext, the timeout attribute is set to 1, meaning no frames have been captured during the specified amount of time. Default to 3 seconds.


Is auto set to 1 when a timeout has occured. It is not reset to 0 automatically, you need to do it yourself.


If you want to capture in promiscuous mode, set it to 1. Default to 0.


If you want to capture a different snaplen, set it a number. Default to 1514.


This attribute tells which datalink type is used for .pcap files.


This one stores a pointer to the latest received frame after a call to next method. If a next call is done, and no frame is received, this attribute is set to undef.


When the capturing process is running (start has been called), this is set to 1. So, when start method has been called, it is set to 1, and when stop method is called, set to 0.


When the clean method is called, and this attribute is set to 1, the file is deleted from disk. Set it to 0 to avoid this behaviour. BEWARE: default to 1.


If you set this attribute to 1, frames will not be stored in frames array. It is used in sniffer-like programs, in order to avoid memory exhaustion by keeping all captured Net::Packet::Frame into memory. Default is to store frames.


As explained in DESCRIPTION, if you send packets at layer 4, layer 2 and 3 are not keeped when stored in frames. The same is true when sending at layer 3 (layer 2 is not kept). Default to wipe those layers. WARNING: if you set it to 1, and you need the recv method from Net::Packet::Frame, it will fail. In fact, this is a speed improvements, that is in order to find matching frame for your request, they are stored in a hash, using layer as keys (getKey and getKeyReverse are used to get keys from each layer. So, if you do not wipe layers, a key will be used to store the frame, but another will be used to search for it, and no match will be found. This is a current limitation I'm working on to remove.


When you crate a Net::Packet::Dump, you have 3 possible modes : online, offline and writer. You need to load constants from Net::Packet::Consts to have access to that (see SYNOPSIS). The three constants are:




Default behaviour is to use online mode.


Sometimes, when frames are captured and saved to a .pcap file, timestamps sucks. That is, you send a frame, and receive the reply, but your request appear to have been sent after the reply. So, to correct that, you can use Net::Packet framework own timestamping system. The default is 0. Set it manually to 1 if you need original .pcap frames timestamps.

frames [is an arrayref]

Stores all analyzed frames found in a pcap file in this arrayref.

framesSorted [is an hashref]

Stores all analyzed frames found in a pcap file in this hashref, using keys to store and search related frames request/replies.



Object contructor. Default values for attributes:

dev: $Env->dev

env: $Env

file: "netpacket-tmp-$$.@{[getRandom32bitsInt()]}.pcap"

filter: ''

overwrite: 0

timeout: 0

promisc: 0

snaplen: 1514

timeoutOnNext: 3

isRunning: 0

unlinkOnClean: 1

noStore: 0

noLayerWipe: 0


keepTimestamp: 0


Returns 1 if Net::Packet::Dump object is respectively set to online, offline or writer mode. 0 otherwise.


You MUST manually call this method to start frame capture, whatever mode you are in. In online mode, it will fork a tcpdump-like process to save captured frames to a .pcap file. It will not overwrite an existing file by default, use overwrite attribute for that. In offline mode, it will only provide analyzing methods. In writer mode, it will only provide writing methods for frames. It will set isRunning attribute to 1 when called.


You MUST manually call this method to stop the process. In online mode, it will not remove the generated .pcap file, you MUST call clean method. In offline mode, it will to nothing. In writer mode, it will call Net::Pcap::dump_close method. Then, it will set isRunning attribute to 0.


These methods will tell you if your current process is respectively the father, or son process of Net::Packet::Dump object.


You MUST call this method manually. It will never be called by Net::Packet framework. This method will remove the generated .pcap file in online mode if the unlinkOnClean attribute is set to 1. In other modes, it will do nothing.


Tries to get packet statistics on an open descriptor. It returns a reference to a hash that has to following fields: ps_recv, ps_drop, ps_ifdrop.


Will removed all analyzed frames from frames array and framesSorted hash. Use it with caution, because recv from Net::Packet::Frame relies on those.


Returns the next captured frame; undef if none found in .pcap file. In all cases, nextFrame attribute is set (either to the captured frame or undef). Each time this method is run, a comparison is done to see if no frame has been captured during timeoutOnNext amount of seconds. If so, timeout attribute is set to 1 to reflect the pending timeout. When a frame is received, it is stored in frames arrayref, and in framesSorted hashref, used to quickly recv it (see Net::Packet::Frame), and internal counter for time elapsed since last received packet is reset.


Calls next method until it returns undef (meaning no new frame waiting to be analyzed from pcap file).

write (scalar)

In writer mode, this method takes a Net::Packet::Frame as a parameter, and writes it to the .pcap file. Works only in writer mode.


Used to reset manually the timeout attribute. This is a helper method.

framesFor (scalar)

You pass a Net::Packet::Frame has parameter, and it returns an array of all frames relating to the connection. For example, when you send a TCP SYN packet, this method will return TCP packets relating to the used source/destination IP, source/destination port, and also related ICMP packets.

framesSorted (scalar)

Method mostly used internally to store in a hashref a captured frame. This is used to retrieve it quickly on recv call.



Constants for first layers within the pcap file.


Constants to set the dump mode.


Patrice <GomoR> Auffret


Copyright (c) 2004-2009, Patrice <GomoR> Auffret

You may distribute this module under the terms of the Artistic license. See LICENSE.Artistic file in the source distribution archive.


NetPacket, Net::RawIP, Net::RawSock

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