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node_pcap

This is a set of bindings from libpcap to node as well as some useful libraries to decode, print, and analyze packets. libpcap is a packet capture library used by programs like tcpdump and wireshark. It has been tested on OSX and Linux.

Sadly, node_pcap is not done yet. While it is incomplete, it is still already useful for capturing and manipulating packets in JavaScript.

Why?

There are already many tools for capturing, decoding, and analyzing packets. Many of them are thoroughly tested and very fast. Why would anybody want to capture and manipulate packets in JavaScript? A few reasons:

  • JavaScript makes writing event-based programs very natural. Each packet that is captured generates an event, and as higher level protocols are decoded, they might generate events as well. Writing code to handle these events is much easier and more readable with anonymous functions and closures.

  • node makes handling binary data in JavaScript fast and efficient with its Buffer class. Decoding packets involves a lot of binary slicing and dicing which can be awkward with JavaScript strings.

  • Writing servers that capture packets, process them somehow, and then serve the processed data up in some way is very straightforward in node.

Help Wanted

I want to build up decoders and printers for all popular protocols. I'm already working on HTTP, DNS, and 802.11 "monitor" mode. If you want to write a decoder or printer for another protocol, let me know, or just send me a patch.

Installation

You will need libpcap installed. Most OSX machines seem to have it. All major Linux distributions have it available either by default or with a package like libpcap-dev.

The easiest way to get node_pcap and its tools is with npm:

npm install pcap

If you want to hack on the source code, you can get it from github. Clone the repo like this:

git clone git://github.com/mranney/node_pcap.git

To compile the native code bindings, do this:

cd node_pcap
node-waf configure build

Assuming it built without errors, you should be able to run the examples and then write your own packet capture programs.

Usage

There are several example programs that show how to use node_pcap. These examples are best documentation. Try them out and see what they do.

To start a pcap session:

var pcap = require('./pcap'),
    pcap_session = pcap.createSession(interface, filter);

interface is the name of the interface on which to capture packets. If passed an empty string, libpcap will try to pick a "default" interface, which is often just the first one in some list.

fitler is a pcap filter expression, see pcap-filter(7) for more information. An empty string will capture all packets visible on the interface.

pcap_session is an EventEmitter that emits a packet event. The only argument to the callback will be a Buffer object with the raw bytes returned by libpcap.

Listening for packets:

pcap_session.addListener('packet', function (raw_packet) {
    // do some stuff with a raw packet
});

To convert the raw packet into a JavaScript object that is easy to work with, decode it:

var packet = pcap.decode.packet(raw_packet);

The protocol stack is exposed as a nested set of objects. For example, the TCP destination port is part of TCP which is encapsulated within IP, which is encapsulated within a link layer. Access it like this:

packet.link.ip.tcp.dport

This structure is easy to explore with sys.inspect.

Note that node_pcap always opens the interface in promiscuous mode, which generally requires running as root. Unless you are root already, you'll probably want to start your node program like this:

sudo node test.js

examples/http_trace

This is a handy standalone program that can help diagnose HTTP traffic.

The TCP tracker looks for HTTP at the beginning of every TCP connection. If found, all captured on this connection will be fed to node's HTTP parser and events will be generated. http_trace has listeners for these events and will print out some helpful information.

http_trace screenshot

examples/simple_capture

This program captures packets and prints them as best it can. Here's a sample of it's output. In another window I ran curl nodejs.org.

mjr:~/work/node_pcap$ sudo node examples/simple_capture.js en1 ""
libpcap version 1.0.0
en0 no address
* en1 10.240.0.133/255.255.255.0
lo0 127.0.0.1/255.0.0.0
00:18:39:ff:f9:1c -> 00:1f:5b:ce:3e:29 10.240.0.1 ARP request 10.240.0.133
00:1f:5b:ce:3e:29 -> 00:18:39:ff:f9:1c 10.240.0.133 ARP reply 10.240.0.1 hwaddr 00:18:39:ff:f9:1c
00:1f:5b:ce:3e:29 -> 00:18:39:ff:f9:1c 10.240.0.133:53808 -> 97.107.132.72:80 TCP len 64
00:1f:5b:ce:3e:29 -> 00:18:39:ff:f9:1c 10.240.0.133:57052 -> 10.240.0.1:53 DNS question 133.0.240.10.in-addr.arpa PTR
00:1f:5b:ce:3e:29 -> 00:18:39:ff:f9:1c 10.240.0.133:57052 -> 10.240.0.1:53 DNS question 72.132.107.97.in-addr.arpa PTR
00:1f:5b:ce:3e:29 -> 00:18:39:ff:f9:1c 10.240.0.133:57052 -> 10.240.0.1:53 DNS question 1.0.240.10.in-addr.arpa PTR
00:18:39:ff:f9:1c -> 00:1f:5b:ce:3e:29 10.240.0.1:53 -> 10.240.0.133:57052 DNS answer 133.0.240.10.in-addr.arpa PTR
00:18:39:ff:f9:1c -> 00:1f:5b:ce:3e:29 10.240.0.1:53 -> rv-mjr2.ranney.com:57052 DNS answer 72.132.107.97.in-addr.arpa PTR
00:18:39:ff:f9:1c -> 00:1f:5b:ce:3e:29 10.240.0.1:53 -> rv-mjr2.ranney.com:57052 DNS answer 1.0.240.10.in-addr.arpa PTR
00:18:39:ff:f9:1c -> 00:1f:5b:ce:3e:29 tinyclouds.org:80 -> rv-mjr2.ranney.com:53808 TCP len 60
00:1f:5b:ce:3e:29 -> 00:18:39:ff:f9:1c rv-mjr2.ranney.com:53808 -> tinyclouds.org:80 TCP len 52
00:1f:5b:ce:3e:29 -> 00:18:39:ff:f9:1c rv-mjr2.ranney.com:53808 -> tinyclouds.org:80 TCP len 196
00:18:39:ff:f9:1c -> 00:1f:5b:ce:3e:29 tinyclouds.org:80 -> rv-mjr2.ranney.com:53808 TCP len 52
00:18:39:ff:f9:1c -> 00:1f:5b:ce:3e:29 tinyclouds.org:80 -> rv-mjr2.ranney.com:53808 TCP len 1500
00:18:39:ff:f9:1c -> 00:1f:5b:ce:3e:29 tinyclouds.org:80 -> rv-mjr2.ranney.com:53808 TCP len 1500
00:1f:5b:ce:3e:29 -> 00:18:39:ff:f9:1c rv-mjr2.ranney.com:53808 -> tinyclouds.org:80 TCP len 52
00:18:39:ff:f9:1c -> 00:1f:5b:ce:3e:29 tinyclouds.org:80 -> rv-mjr2.ranney.com:53808 TCP len 1500
00:1f:5b:ce:3e:29 -> 00:18:39:ff:f9:1c rv-mjr2.ranney.com:53808 -> tinyclouds.org:80 TCP len 52
00:18:39:ff:f9:1c -> 00:1f:5b:ce:3e:29 tinyclouds.org:80 -> rv-mjr2.ranney.com:53808 TCP len 1500
00:18:39:ff:f9:1c -> 00:1f:5b:ce:3e:29 tinyclouds.org:80 -> rv-mjr2.ranney.com:53808 TCP len 1500
00:1f:5b:ce:3e:29 -> 00:18:39:ff:f9:1c rv-mjr2.ranney.com:53808 -> tinyclouds.org:80 TCP len 52
00:18:39:ff:f9:1c -> 00:1f:5b:ce:3e:29 tinyclouds.org:80 -> rv-mjr2.ranney.com:53808 TCP len 1500
00:1f:5b:ce:3e:29 -> 00:18:39:ff:f9:1c rv-mjr2.ranney.com:53808 -> tinyclouds.org:80 TCP len 52
00:18:39:ff:f9:1c -> 00:1f:5b:ce:3e:29 tinyclouds.org:80 -> rv-mjr2.ranney.com:53808 TCP len 1500
00:18:39:ff:f9:1c -> 00:1f:5b:ce:3e:29 tinyclouds.org:80 -> rv-mjr2.ranney.com:53808 TCP len 337
00:1f:5b:ce:3e:29 -> 00:18:39:ff:f9:1c rv-mjr2.ranney.com:53808 -> tinyclouds.org:80 TCP len 52
00:1f:5b:ce:3e:29 -> 00:18:39:ff:f9:1c rv-mjr2.ranney.com:53808 -> tinyclouds.org:80 TCP len 52
00:1f:5b:ce:3e:29 -> 00:18:39:ff:f9:1c rv-mjr2.ranney.com:53808 -> tinyclouds.org:80 TCP len 52
00:18:39:ff:f9:1c -> 00:1f:5b:ce:3e:29 tinyclouds.org:80 -> rv-mjr2.ranney.com:53808 TCP len 52
00:1f:5b:ce:3e:29 -> 00:18:39:ff:f9:1c rv-mjr2.ranney.com:53808 -> tinyclouds.org:80 TCP len 52

Output from session.findalldevs:

[ { name: 'en0'
  , addresses: 
     [ { addr: '10.51.2.183'
       , netmask: '255.255.255.0'
       , broadaddr: '10.51.2.255'
       }
     ]
  }
, { name: 'fw0', addresses: [] }
, { name: 'en1', addresses: [] }
, { name: 'lo0'
  , addresses: [ { addr: '127.0.0.1', netmask: '255.0.0.0' } ]
  , flags: 'PCAP_IF_LOOPBACK'
  }
]

Deep decode of curl nodejs.org:

Running sys.inspect on the first three decoded packets of this TCP session.

First packet, TCP SYN:

{ ethernet: 
   { dhost: '00:18:39:ff:f9:1c'
   , shost: '00:1f:5b:ce:3e:29'
   , ethertype: 2048
   , ip: 
      { version: 4
      , header_length: 5
      , diffserv: 0
      , total_length: 64
      , identification: 49042
      , flags: { reserved: 0, df: 1, mf: 0 }
      , fragment_offset: 0
      , ttl: 64
      , protocol: 6
      , header_checksum: 35325
      , saddr: '10.240.0.133'
      , daddr: '97.107.132.72'
      , protocol_name: 'TCP'
      , tcp: 
         { sport: 57230
         , dport: 80
         , seqno: 4179361823
         , ackno: 1540242985
         , data_offset: 11
         , reserved: 0
         , flags: 
            { cwr: 0
            , ece: 0
            , urg: 0
            , ack: 0
            , psh: 0
            , rst: 0
            , syn: 1
            , fin: 0
            }
         , window_size: 65535
         , checksum: 2601
         , urgent_pointer: 0
         , payload_offset: 78
         , payload: { length: 0 }
         }
      }
   }
, pcap_header: 
   { time: Sat, 22 May 2010 07:48:40 GMT
   , tv_sec: 1274514520
   , tv_usec: 820479
   , caplen: 78
   , len: 78
   , link_type: 'LINKTYPE_ETHERNET'
   }
}

Second packet, TCP SYN+ACK:

{ ethernet: 
   { dhost: '00:1f:5b:ce:3e:29'
   , shost: '00:18:39:ff:f9:1c'
   , ethertype: 2048
   , ip: 
      { version: 4
      , header_length: 5
      , diffserv: 32
      , total_length: 60
      , identification: 0
      , flags: { reserved: 0, df: 1, mf: 0 }
      , fragment_offset: 0
      , ttl: 48
      , protocol: 6
      , header_checksum: 22900
      , saddr: '97.107.132.72'
      , daddr: '10.240.0.133'
      , protocol_name: 'TCP'
      , tcp: 
         { sport: 80
         , dport: 57230
         , seqno: 1042874392
         , ackno: 973076764
         , data_offset: 10
         , reserved: 0
         , flags: 
            { cwr: 0
            , ece: 0
            , urg: 0
            , ack: 1
            , psh: 0
            , rst: 0
            , syn: 1
            , fin: 0
            }
         , window_size: 5792
         , checksum: 35930
         , urgent_pointer: 0
         , payload_offset: 74
         , payload: { length: 0 }
         }
      }
   }
, pcap_header: 
   { time: Sat, 22 May 2010 07:48:40 GMT
   , tv_sec: 1274514520
   , tv_usec: 915980
   , caplen: 74
   , len: 74
   , link_type: 'LINKTYPE_ETHERNET'
   }
}

Third packet, TCP ACK, 3-way handshake is now complete:

{ ethernet: 
   { dhost: '00:18:39:ff:f9:1c'
   , shost: '00:1f:5b:ce:3e:29'
   , ethertype: 2048
   , ip: 
      { version: 4
      , header_length: 5
      , diffserv: 0
      , total_length: 52
      , identification: 39874
      , flags: { reserved: 0, df: 1, mf: 0 }
      , fragment_offset: 0
      , ttl: 64
      , protocol: 6
      , header_checksum: 44505
      , saddr: '10.240.0.133'
      , daddr: '97.107.132.72'
      , protocol_name: 'TCP'
      , tcp: 
         { sport: 57230
         , dport: 80
         , seqno: 4179361823
         , ackno: 1540242985
         , data_offset: 8
         , reserved: 0
         , flags: 
            { cwr: 0
            , ece: 0
            , urg: 0
            , ack: 1
            , psh: 0
            , rst: 0
            , syn: 0
            , fin: 0
            }
         , window_size: 65535
         , checksum: 53698
         , urgent_pointer: 0
         , payload_offset: 66
         , payload: { length: 0 }
         }
      }
   }
, pcap_header: 
   { time: Sat, 22 May 2010 07:48:40 GMT
   , tv_sec: 1274514520
   , tv_usec: 916054
   , caplen: 66
   , len: 66
   , link_type: 'LINKTYPE_ETHERNET'
   }
}

LICENSE - "MIT License"

Copyright (c) 2010 Matthew Ranney, http://ranney.com/

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.

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