NetfilterQueue

NetfilterQueue provides access to packets matched by an iptables rule in Linux. Packets so matched can be accepted, dropped, altered, or given a mark.

Libnetfilter_queue (the netfilter library, not this module) is part of the Netfilter project.

Example

The following script prints a short description of each packet before accepting it.

from netfilterqueue import QueueHandler

class PacketPrinter(QueueHandler):
    def handle(self, packet):
        print packet
        packet.accept()

p = PacketPrinter()
p.bind(1)
try:
    p.run()
except KeyboardInterrupt:
    print

To send packets destined for your LAN to the script, type something like:

iptables -I INPUT -d 192.168.0.0/24 -j NFQUEUE --queue-num 1

Installation

NetfilterQueue is a C extention module that links against libnetfilter_queue. Before installing, ensure you have:

1. A C compiler
2. Python development files
3. Libnetfilter_queue development files and associated dependencies

On Debian or Ubuntu, install these files with:

sudo apt-get install build-essential python-dev libnetfilter-queue-dev

From PyPI

To install from PyPI by pip:

pip install NetfilterQueue

From source

To install from source:

wget http://pypi.python.org/packages/source/N/NetfilterQueue/NetfilterQueue-0.2.tar.gz
tar -xvzf NetfilterQueue-0.2.tar.gz
cd NetfilterQueue-0.2
python setup.py install

Setup will use Cython if it is installed, regenerating the .c source from the .pyx before compiling the .so.

API

NetfilterQueue.COPY_NONE

NetfilterQueue.COPY_META

NetfilterQueue.COPY_PACKET

These constants specify how much of the packet should be given to the script- nothing, metadata, or the whole packet.

QueueHandler objects

You should define a class that inherits from QueueHandler and implenents the handle() method. Handle() is called for each packet that appears in the queue.

QueueHandler.bind(queue_num[, max_len[, mode[, range]]])

Create and bind to the queue. queue_num must match the number in your iptables rule. max_len sets the largest number of packets that can be in the queue; new packets are dropped if the size of the queue reaches this number. mode determines how much of the packet data is provided to your script. Use the constants above. range defines how many bytes of the packet you want to get. For example, if you only want the source and destination IPs of a IPv4 packet, range could be 20.

QueueHandler.unbind()

Remove the queue. Packets matched by your iptables rule will be dropped.

QueueHandler.run()

Begin accepting packets.

QueueHandler.handle(packet)

Handle a single packet from the queue. You must call either packet.accept() or packet.drop().

Packet objects

Objects of this type are passed to your handle() method.

Packet.get_payload()

Return the packet's payload as a string.

Packet.get_payload_len()

Return the size of the payload.

Packet.set_mark(mark)

Give the packet a kernel mark. mark is a 32-bit number.

Packet.accept()

Accept the packet.

Packet.drop()

Drop the packet.

Usage

To route packets to the queue:

iptables -I <table or chain> <match specification> -j NFQUEUE --queue-num <queue number>

For example:

iptables -I INPUT -d 192.168.0.0/24 -j NFQUEUE --queue-num 1

The only special part of the rule is the target. Rules can have any match and can be added to any table or chain.

Valid queue numbers are integers from 0 to 65,536 inclusive.

To view libnetfilter_queue stats, refer to /proc/net/netfilter/nfnetlink_queue:

cat /proc/net/netfilter/nfnetlink_queue
1  31621     0 2  4016     0     0        2  1

The fields are:

1. Queue ID
2. Bound process ID
3. Number of currently queued packets
4. Copy mode
5. Copy size
6. Number of packets dropped due to reaching max queue size
7. Number of packets dropped due to netlink socket failure
8. Total number of packets sent to queue
9. Libnetfilter_queue internal use

Limitations

More details coming soon...

• Compiled with a 4096-byte buffer for packets, so it probably won't work on loopback or Ethernet with jumbo packets. If this is a problem, either lower MTU on your loopback, disable jumbo packets, or get Cython, change DEF BufferSize = 4096 in netfilterqueue.pyx, and rebuild.

• Full libnetfilter_queue API is not yet implemented:

  • Omits packet.set_payload() for altering packet data
  • Omits methods for getting information about the interface a packet has arrived on or is leaving on
  • Probably other stuff is omitted too

• When a packet has been marked, we use nfq_set_verdict_mark rather than nfq_set_verdict2. Apparently nfq_set_verdict_mark is broken, although it works for me.

Source

https://github.com/kti/python-netfilterqueue

License

Copyright (c) 2011, Kerkhoff Technologies, Inc. All rights reserved.

Licensed under BSD