---
name: "NetPacket::TCP"
section: "3pm"
platform: "linux"
package: "libnetpacket-perl"
version: "1.7.2-2"
release: "Debian 13 (Trixie)"
platforms: ["linux"]
url: "https://new.carta.tech/NetPacket::TCP"
---

# NetPacket::TCP(3pm)

Assemble and disassemble TCP (Transmission Control Protocol) packets.

## NAME

NetPacket::TCP - Assemble and disassemble TCP (Transmission Control Protocol) packets.

## VERSION

version 1.7.2

## SYNOPSIS

```
  use NetPacket::TCP;
  $tcp_obj = NetPacket::TCP->decode($raw_pkt);
  $tcp_pkt = $tcp_obj->encode($ip_pkt);
  $tcp_data = NetPacket::TCP::strip($raw_pkt);
```

## DESCRIPTION

`"NetPacket::TCP"` provides a set of routines for assembling and disassembling packets using TCP (Transmission Control Protocol).

### Methods

- "NetPacket::TCP-\>decode(\[RAW PACKET\])"

  Decode the raw packet data given and return an object containing instance data.  This method will quite happily decode garbage input. It is the responsibility of the programmer to ensure valid packet data is passed to this method.

- "NetPacket::TCP-\>encode($ip\_obj)"

  Return a TCP packet encoded with the instance data specified. Needs parts of the ip header contained in `$ip_obj` in order to calculate the TCP checksum.

- "$packet-\>parse\_tcp\_options"

  Returns a hash (or a hash ref in scalar context) containing the packet's options.

  For now the method only recognizes well-known and widely used options (MSS, noop, windows scale factor, SACK permitted, SACK, timestamp). If the packet contains options unknown to the method, it may fail.

### Functions

- "NetPacket::TCP::strip(\[RAW PACKET\])"

  Return the encapsulated data (or payload) contained in the TCP packet.  This data is suitable to be used as input for other `"NetPacket::*"` modules.

  This function is equivalent to creating an object using the `"decode()"` constructor and returning the `"data"` field of that object.

### Instance data

The instance data for the `"NetPacket::TCP"` object consists of the following fields.

- src\_port

  The source TCP port for the packet.

- dest\_port

  The destination TCP port for the packet.

- seqnum

  The TCP sequence number for this packet.

- acknum

  The TCP acknowledgement number for this packet.

- hlen

  The header length for this packet.

- reserved

  The 6-bit "reserved" space in the TCP header.

- flags

  Contains the urg, ack, psh, rst, syn, fin, ece and cwr flags for this packet.

- winsize

  The TCP window size for this packet.

- cksum

  The TCP checksum.

- urg

  The TCP urgent pointer.

- options

  Any TCP options for this packet in binary form.

- data

  The encapsulated data (payload) for this packet.

### Exports

- default

  FIN SYN RST PSH ACK URG ECE CWR Can be used to set the appropriate flag.

- exportable

  tcp\_strip

- tags

  The following tags group together related exportable items.

  - ":strip"

    Import the strip function `"tcp_strip"`.

  - ":ALL"

    All the above exportable items.

## EXAMPLE

The following script is a primitive pop3 sniffer.

```
  #!/usr/bin/perl -w
  use strict;
  use Net::PcapUtils;
  use NetPacket::Ethernet qw(:strip);
  use NetPacket::IP qw(:strip);
  use NetPacket::TCP;
  sub process_pkt {
      my($arg, $hdr, $pkt) = @_;
      my $tcp_obj = NetPacket::TCP->decode(ip_strip(eth_strip($pkt)));
      if (($tcp_obj->{src_port} == 110) or ($tcp_obj->{dest_port} == 110)) {
          print($tcp_obj->{data});
      }
  }
  Net::PcapUtils::loop(\&process_pkt, FILTER => 'tcp');
```

The following uses NetPacket together with Net::Divert to add a syn flag to all TCP packets passing through:

```
  #!/usr/bin/perl
  use Net::Divert;
  use NetPacket::IP qw(IP_PROTO_TCP);
  use NetPacket::TCP;
  $divobj = Net::Divert->new('yourhostname',9999);
  $divobj->getPackets(\&alterPacket);
  sub alterPacket {
      my($packet,$fwtag) = @_;
      # decode the IP header
      $ip_obj = NetPacket::IP->decode($packet);
      # check if this is a TCP packet
      if($ip_obj->{proto} == IP_PROTO_TCP) {
          # decode the TCP header
          $tcp_obj = NetPacket::TCP->decode($ip_obj->{data});
          # set the syn flag
          $tcp_obj->{flags} |= SYN;
          # construct the new ip packet
          $ip_obj->{data} = $tcp_obj->encode($ip_obj);
          $packet = $ip_obj->encode;
      }
      # write it back out
      $divobj->putPacket($packet,$fwtag);
   }
```

## TODO

- Assembly of TCP fragments into a data stream

  - Option processing

  - Nicer processing of TCP flags

  ## COPYRIGHT

Copyright (c) 2001 Tim Potter and Stephanie Wehner.

Copyright (c) 1995,1996,1997,1998,1999 ANU and CSIRO on behalf of the participants in the CRC for Advanced Computational Systems ('ACSys').

This module is free software.  You can redistribute it and/or modify it under the terms of the Artistic License 2.0.

This program is distributed in the hope that it will be useful, but without any warranty; without even the implied warranty of merchantability or fitness for a particular purpose.

## AUTHOR

Tim Potter \<tpot@samba.org\>

Stephanie Wehner \<atrak@itsx.com\>