Accessing wireless interface (802.11) at MAC layer (Linux)

Question

I spent the last days reading through man pages, documentations and anything else google brought up, but I suppose I'm even more confused now than I was at the beginning.

Here is what I want to do: I want to send and receive data packets with my own layer 3-x protocol(s) via a wireless interface (802.11) on Linux systems with C/C++. So far, so good. I do not require beacons, association or any AP/SSID related stuff. However, for data transmissions I'd like the MAC layer to behave "as usual", meaning unicast packets are ACK'd, retransmissions, backoff etc. I'd also like to enjoy the extended QoS capabilites (802.11e with 4 queues and different access categories). Promiscuous mode on the other hand is not a concern, I require only broadcast packets and packets sent to the specific station.

What would be the right way to go about it? Most of the documentation out there on raw socket access seems to be focused on network sniffing and that does not help. I've been playing around with the monitor mode for some time now, but from what I've read so far, received packets are not ACK'd in monitor mode etc. Without monitor mode, what would be the alternative? Using ad hoc mode and unix raw sockets? Or do I have to fiddle around with the drivers?

I'm not looking for a complete solution, just some good ideas, where to start. I read through the man pages for socket(2), socket(7) and packet(7) but that did not help concerning the behaviour of the MAC layer in different modes.

Thanks in advance.

Answer 1

802.11 is layer 2 (and 1) protocol specification. It was designed in a way, which allows higher-layer protocols to treat it as Ethernet network. Addressing and behaviour is generally the same. So for a layer 3 protocol you should not be concerned about 802.11 at all and write your code as if you were expecting it to run on Ethernet network.

To make it work you should first connect to a wireless network of some sort (which is conceptually equal to plugging a wire into a Ethernet card). Here you may choose ad-hoc (aka IBSS) or infrastructural (aka BSS) network (or PBSS once 802.11ad is approved ;).

Operating cards without any sort of association with network (just spitting out packets on air) is not a good idea for a couple of reasons. Most importantly it's very hardware dependent and unreliable. You can still do it using RF mon (AKA monitor mode) interface on one side and packet injection (using radiotap header) on the other but I don't recommend that. Even if you have a set of identical cards you'll most likely encounter hard to explain and random behaviour at some point. 802.11 NICs are just not designed for this kind of operation and keep different mount of state inside firmware (read about FullMAC vs. SoftMAC cards). Even SoftMAC cards differ significantly. For example theoretically in monitor mode, as you said, card should not ACK received packets. There are cards though that will ACK received frame anyway, because they base their decision exclusively on the fact that said frame is addressed to them. Some cards may even try to ACK all frames they see. Similar thing will happen with retransmissions: some cards will send injected packet only once (that's how it should work). In other NICs, retransmissions are handled by hardware (and firmware) and driver cannot turn it off, so you will get automatic retransmission even with injected data.

Sticking with layer 3 and using existing modes (like ad hoc), will give you all capabilities you want and more (QoS etc.). Ethernet frame that you send to interface will be "translated" by the kernel to 802.11 format with QoS mapping etc.

If you want to find out about MAC behaviour in various modes you'll have to either read the mac80211 code or 802.11 standard itself. http://linuxwireless.org wiki my help you with a few things, but kernel hackers are usually to busy to write documentation other than comments in the code ;)

L3 protocol implementation itself can be also done either in kernel or user mode (using raw sockets). As usual kernel-side will be harder to do, but more powerful.

Answer 2

Because you want to create own network layer protocol (replacement for IP), the keyword is: "raw ethernet socket". So ignore "Raw IP socket" stuff.

This is where to start:

int sockfd = socket( PF_PACKET, SOCK_RAW, htons(XXX) ); 

Correct man page is: packet(7).

Find more information by googling with the keyword. One quite complete example here.

Edit: The link to the example seems to be currently broken: another examples

Answer 3

Probably you want something like libpcap.

Libpcap allows you to read/inject raw packets from/into a network interface.

Answer 4

First, there’s something you should be aware of when trying to transmit raw 802.12 frames- the device driver must support packet injection.

You mentioned monitor mode, which is at a high level the rx equivalent of the injection capability- which is not a “mode”, jist a capability/feature. I say this because some 892.11 device drivers on Linux either:

1. Support monitor mode and frame injection
2. Support monitor mode and not frame injection
3. Support neither

I don’t know any straightforward way to check if the driver supports frame injection aside from attempting frame injection and sniffing the air on another device to confirm it was seen.

Monitor mode is usually easy to check by using sudo wlan0 set monitor and seeing what the return code and/or output is.

It’s been a few years since I’ve worked on this but at the time, very few devices supported monitor mode and frame injection “out of the box”. Many only supported monitor mode with a modified version of the vendor or kernel driver

You’ll want to make sure your device has a driver available that fully supports both. This sort of task (frame monitoring and injection) is common for Penetration Testers who tend to use Kali Linux, which is really just an Ubuntu distribution with a bunch of “hacking” tools and (modified) 802.11 device drivers preloaded and in its repositories. You can often save time finding a well supported card by using a search engine to find the device and driver recommended for Kali users

I’m bringing this monitor/injection capability up explicitly because when I first worked on a similar project a few years ago, I needed to use a patched version of the official kernel driver to support monitor mode- it was an rtl8812au chipset. At that time, I made an incorrect assumption that monitor mode support in the driver implied full injection support. I spent 2 days banging my head against the wall, convinced my frames weren’t built correctly in my application, causing no frames to leave the card. Turned out I needed a more recent branch of the driver I was using to get the full injection support. This driver in particular supports both monitor mode and frame injection now. The most frustrating thing about diagnosing that problem was that I did not receive any errors from system calls or in kernel messages while trying to transmit the frames- they were just being silently discarded somewhere, presumably in the driver

To your main question about how to do this- the answer is almost certainly libpcap if you’re writing your application in C/C++ as libpcap provides not only packet capture APIs but also packet injection APIs

If you do it in Python, scapy is an excellent option. The benefit of Python/scapy is that

1. Python code is much quicker to write than C
2. scapy provides a significant amount of classes that you can use to intuitively create a frame layer by layer
3. Because the layers are implemented as classes, you can also extend and “register” existing classes to make certain frames easier to create (or parse when received)

You can do this in straight C using the UNIX sockets API with raw sockets directly- but you’ll have to deal with things that libpcap exists to abstract from you- like underlying system calls that may be required when doing raw frame transmission, aside from the standard socket(), send(), recv() calls. I’m speculating that there are a handful of ioctl calls you may need at the least, specific to the kernel 802.11x subsystem/framework- and these ioctl() calls and their values may not be completely portable across different major kernel versions. I’ll admit I ended up not using the pure C (without libpcap) approach, so I’m not 100% sure about this potential problem. It’s something you should look more into if you plan to do it without libpcap. I don’t recommend it unless you have a really good reason to