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NetfilterQueue

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

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

The current version of NetfilterQueue requires Python 3.6 or later. The last version with support for Python 2.7 was 0.9.0.

Example

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

from netfilterqueue import NetfilterQueue

def print_and_accept(pkt):
    print(pkt)
    pkt.accept()

nfqueue = NetfilterQueue()
nfqueue.bind(1, print_and_accept)
try:
    nfqueue.run()
except KeyboardInterrupt:
    print('')

nfqueue.unbind()

You can also make your own socket so that it can be used with gevent, for example.

from netfilterqueue import NetfilterQueue
import socket

def print_and_accept(pkt):
    print(pkt)
    pkt.accept()

nfqueue = NetfilterQueue()
nfqueue.bind(1, print_and_accept)
s = socket.fromfd(nfqueue.get_fd(), socket.AF_UNIX, socket.SOCK_STREAM)
try:
    nfqueue.run_socket(s)
except KeyboardInterrupt:
    print('')

s.close()
nfqueue.unbind()

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:

A C compiler
Python development files
Libnetfilter_queue development files and associated dependencies

On Debian or Ubuntu, install these files with:

apt-get install build-essential python3-dev libnetfilter-queue-dev

From PyPI

To install from PyPI by pip:

pip install NetfilterQueue

From source

To install from source:

pip install cython
git clone https://github.com/oremanj/python-netfilterqueue
cd python-netfilterqueue
pip install .

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.

NetfilterQueue objects

A NetfilterQueue object represents a single queue. Configure your queue with a call to bind, then start receiving packets with a call to run.

NetfilterQueue.bind(queue_num, callback, max_len=1024, mode=COPY_PACKET, range=65535, sock_len=...): Create and bind to the queue. queue_num uniquely identifies this queue for the kernel. It must match the --queue-num in your iptables rule, but there is no ordering requirement: it's fine to either bind() first or set up the iptables rule first. callback is a function or method that takes one argument, a Packet object (see below). 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. sock_len sets the receive socket buffer size.
NetfilterQueue.unbind(): Remove the queue. Packets matched by your iptables rule will be dropped.
NetfilterQueue.get_fd(): Get the file descriptor of the socket used to receive queued packets and send verdicts. If you're using an async event loop, you can poll this FD for readability and call run(False) every time data appears on it.
NetfilterQueue.run(block=True): Send packets to your callback. By default, this method blocks, running until an exception is raised (such as by Ctrl+C). Set block=False to process the pending messages without waiting for more; in conjunction with the get_fd method, you can use this to integrate with async event loops.
NetfilterQueue.run_socket(socket): Send packets to your callback, but use the supplied socket instead of recv, so that, for example, gevent can monkeypatch it. You can make a socket with socket.fromfd(nfqueue.get_fd(), socket.AF_NETLINK, socket.SOCK_RAW) and optionally make it non-blocking with socket.setblocking(False).

Packet objects

Objects of this type are passed to your callback.

Packet.get_payload(): Return the packet's payload as a bytes object. The returned value starts with the IP header. You must call retain() if you want to be able to get_payload() after your callback has returned. If you have already called set_payload(), then get_payload() returns what you passed to set_payload().
Packet.set_payload(payload): Set the packet payload. Call this before accept() if you want to change the contents of the packet before allowing it to be released. Don't forget to update the transport-layer checksum (or clear it, if you're using UDP), or else the recipient is likely to drop the packet. If you're changing the length of the packet, you'll also need to update the IP length, IP header checksum, and probably some transport-level fields (such as UDP length for UDP).
Packet.get_payload_len(): Return the size of the payload.
Packet.set_mark(mark): Give the packet a kernel mark, which can be used in future iptables rules. mark is a 32-bit number.
Packet.get_mark(): Get the mark on the packet (either the one you set using set_mark(), or the one it arrived with if you haven't called set_mark()).
Packet.get_hw(): Return the source hardware address of the packet as a Python bytestring, or None if the source hardware address was not captured (packets captured by the OUTPUT or PREROUTING hooks). For example, on Ethernet the result will be a six-byte MAC address. The destination hardware address is not available because it is determined in the kernel only after packet filtering is complete.
Packet.get_timestamp(): Return the time at which this packet was received by the kernel, as a floating-point Unix timestamp with microsecond precision (comparable to the result of time.time(), for example). Packets captured by the OUTPUT or POSTROUTING hooks do not have a timestamp, and get_timestamp() will return 0.0 for them.
Packet.id: The identifier assigned to this packet by the kernel. Typically the first packet received by your queue starts at 1 and later ones count up from there.
Packet.hw_protocol: The link-layer protocol for this packet. For example, IPv4 packets on Ethernet would have this set to the EtherType for IPv4, which is 0x0800.
Packet.mark: The mark that had been assigned to this packet when it was enqueued. Unlike the result of get_mark(), this does not change if you call set_mark().
Packet.hook: The netfilter hook (iptables chain, roughly) that diverted this packet into our queue. Values 0 through 4 correspond to PREROUTING, INPUT, FORWARD, OUTPUT, and POSTROUTING respectively.
Packet.indev, Packet.outdev, Packet.physindev, Packet.physoutdev: The interface indices on which the packet arrived (indev) or is slated to depart (outdev). These are integers, which can be converted to names like "eth0" by using socket.if_indextoname(). Zero means no interface is applicable, either because the packet was locally generated or locally received, or because the interface information wasn't available when the packet was queued (for example, PREROUTING rules don't yet know the outdev). If the indev or outdev refers to a bridge device, then the corresponding physindev or physoutdev will name the bridge member on which the actual traffic occurred; otherwise physindev and physoutdev will be zero.
Packet.retain(): Allocate a copy of the packet payload for use after the callback has returned. get_payload() will raise an exception at that point if you didn't call retain().
Packet.accept(): Accept the packet. You can reorder packets by accepting them in a different order than the order in which they were passed to your callback.
Packet.drop(): Drop the packet.
Packet.repeat(): Restart processing of this packet from the beginning of its Netfilter hook (iptables chain, roughly). Any changes made using set_payload() or set_mark() are preserved; in the absence of such changes, the packet will probably come right back to the same queue.

Callback objects

Your callback can be any one-argument callable and will be invoked with a Packet object as argument. You must call retain() within the callback if you want to be able to get_payload() after the callback has returned. You can hang onto Packet objects and resolve them later, but note that packets continue to count against the queue size limit until they've been given a verdict (accept, drop, or repeat). Also, the kernel stores the enqueued packets in a linked list, so keeping lots of packets outstanding is likely to adversely impact performance.

Monitoring a different network namespace

If you are using Linux network namespaces (man 7 network_namespaces) in some kind of containerization system, all of the Netfilter queue state is kept per-namespace; queue 1 in namespace X is not the same as queue 1 in namespace Y. NetfilterQueue will ordinarily pass you the traffic for the network namespace you're a part of. If you want to monitor a different one, you can do so with a bit of trickery and cooperation from a process in that namespace; this section describes how.

You'll need to arrange for a process in the network namespace you want to monitor to call socket(AF_NETLINK, SOCK_RAW, 12) and pass you the resulting file descriptor using something like socket.send_fds() over a Unix domain socket. (12 is NETLINK_NETFILTER, a constant which is not exposed by the Python socket module.) Once you've received that file descriptor in your process, you can create a NetfilterQueue object using the special constructor NetfilterQueue(sockfd=N) where N is the file descriptor you received. Because the socket was originally created in the other network namespace, the kernel treats it as part of that namespace, and you can use it to access that namespace even though it's not the namespace you're in yourself.

Usage

To send 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 65,535 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:

Queue ID
Bound process ID
Number of currently queued packets
Copy mode
Copy size
Number of packets dropped due to reaching max queue size
Number of packets dropped due to netlink socket failure
Total number of packets sent to queue
Something for libnetfilter_queue's internal use

Limitations

We use a fixed-size 4096-byte buffer for packets, so you are likely to see truncation on loopback and on Ethernet with jumbo packets. If this is a problem, either lower the MTU on your loopback, disable jumbo packets, or get Cython, change DEF BufferSize = 4096 in netfilterqueue.pyx, and rebuild.
Not all information available from libnetfilter_queue is exposed: missing pieces include packet input/output network interface names, checksum offload flags, UID/GID and security context data associated with the packet (if any).
Not all information available from the kernel is even processed by libnetfilter_queue: missing pieces include additional link-layer header data for some packets (including VLAN tags), connection-tracking state, and incoming packet length (if truncated for queueing).
We do not expose the libnetfilter_queue interface for changing queue flags. Most of these pertain to other features we don't support (listed above), but there's one that could set the queue to accept (rather than dropping) packets received when it's full.

Source

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

Authorship

python-netfilterqueue was originally written by Matthew Fox of Kerkhoff Technologies, Inc. Since 2022 it has been maintained by Joshua Oreman of Hudson River Trading LLC. Both authors wish to thank their employers for their support of open source.

License

MIT licensed

oremanj/python-netfilterqueue

oremanj

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