cap

A cross-platform binding for performing packet capturing with node.js

npm install cap
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Description

A cross-platform binding for performing packet capturing with node.js.

This binding is tested on Windows and Linux.

Requirements

  • node.js -- v0.8.0 or newer

  • For Windows: WinPcap

  • For *nix: libpcap and libpcap-dev packages

Install

npm install cap

Examples

  • Capture and decode all outgoing TCP data packets destined for port 80 on the interface for 192.168.0.10:
var Cap = require('cap').Cap,
    decoders = require('cap').decoders,
    PROTOCOL = decoders.PROTOCOL;

var c = new Cap(),
    device = Cap.findDevice('192.168.0.10'),
    filter = 'tcp and dst port 80',
    bufSize = 10 * 1024 * 1024,
    buffer = new Buffer(65535);

var linkType = c.open(device, filter, bufSize, buffer);

c.setMinBytes && c.setMinBytes(0);

c.on('packet', function(nbytes, trunc) {
  console.log('packet: length ' + nbytes + ' bytes, truncated? '
              + (trunc ? 'yes' : 'no'));

  // raw packet data === buffer.slice(0, nbytes)

  if (linkType === 'ETHERNET') {
    var ret = decoders.Ethernet(buffer);

    if (ret.info.type === PROTOCOL.ETHERNET.IPV4) {
      console.log('Decoding IPv4 ...');

      ret = decoders.IPV4(buffer, ret.offset);
      console.log('from: ' + ret.info.srcaddr + ' to ' + ret.info.dstaddr);

      if (ret.info.protocol === PROTOCOL.IP.TCP) {
        var datalen = ret.info.totallen - ret.hdrlen;

        console.log('Decoding TCP ...');

        ret = decoders.TCP(buffer, ret.offset);
        console.log(' from port: ' + ret.info.srcport + ' to port: ' + ret.info.dstport);
        datalen -= ret.hdrlen;
        console.log(buffer.toString('binary', ret.offset, ret.offset + datalen));
      } else if (ret.info.protocol === PROTOCOL.IP.UDP) {
        console.log('Decoding UDP ...');

        ret = decoders.UDP(buffer, ret.offset);
        console.log(' from port: ' + ret.info.srcport + ' to port: ' + ret.info.dstport);
        console.log(buffer.toString('binary', ret.offset, ret.offset + ret.info.length));
      } else
        console.log('Unsupported IPv4 protocol: ' + PROTOCOL.IP[ret.info.protocol]);
    } else
      console.log('Unsupported Ethertype: ' + PROTOCOL.ETHERNET[ret.info.type]);
  }
});
  • List all network devices:
var Cap = require('cap').Cap;

console.dir(Cap.deviceList());

// example output on Linux:
// [ { name: 'eth0',
//     addresses:
//      [ { addr: '192.168.0.10',
//          netmask: '255.255.255.0',
//          broadaddr: '192.168.0.255' } ] },
//   { name: 'nflog',
//     description: 'Linux netfilter log (NFLOG) interface',
//     addresses: [] },
//   { name: 'any',
//     description: 'Pseudo-device that captures on all interfaces',
//     addresses: [] },
//   { name: 'lo',
//     addresses:
//      [ { addr: '127.0.0.1', netmask: '255.0.0.0' },
//        { addr: '::1',
//          netmask: 'ffff:ffff:ffff:ffff:ffff:ffff:ffff:ffff' } ],
//     flags: 'PCAP_IF_LOOPBACK' } ]

API

Cap events

  • packet(< integer >nbytes, < boolean >truncated) - A packet nbytes in size was captured. truncated indicates if the entire packet did not fit inside the Buffer supplied to open().

Cap methods

  • (constructor)() - Creates and returns a new Cap instance.

  • open(< string >device, < string >filter, < integer >bufSize, < Buffer >buffer) - (void) - Opens device and starts capturing packets using filter. bufSize is the size of the internal buffer that libpcap uses to temporarily store packets until they are emitted. buffer is a Buffer large enough to store one packet. If open() is called again without a previous call to close(), an implicit close() will occur first.

  • close() - (void) - Stops capturing.

  • setMinBytes(< integer >nBytes) - _(void) - (Windows ONLY) This sets the minimum number of packet bytes that must be captured before the full packet data is made available. If this value is set too high, you may not receive any packets until WinPCap's internal buffer fills up. Therefore it's generally best to pass in 0 to this function after calling open(), despite it resulting in more syscalls.

Cap static methods

  • findDevice([< string >ip]) - mixed - If ip is given, the (first) device name associated with ip, or undefined is returned if not found. If ip is not given, the device name of the first non-loopback device is returned.

  • deviceList() - array - Returns a list of available devices and related information.

Decoders static methods

The following methods are available off of require('cap').decoders. They parse the relevant protocol header and return an object containing the parsed information:

  • Link Layer Protocols

    • Ethernet(< Buffer buf[, < integer >bufOffset=0])
  • Internet Layer Protocols

    • IPV4(< Buffer buf[, < integer >bufOffset=0])

    • IPV6(< Buffer buf[, < integer >bufOffset=0])

    • ICMPV4(< Buffer buf, < integer >nbytes[, < integer >bufOffset=0])

  • Transport Layer Protocols

    • TCP(< Buffer buf[, < integer >bufOffset=0])

    • UDP(< Buffer buf[, < integer >bufOffset=0])

    • SCTP(< Buffer buf, < integer >nbytes[, < integer >bufOffset=0])

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