I learned the hard way that modern Windows machines do not permit sending TCP data over raw sockets after trying to perform the TCP handshake myself in Python. And yet Scapy, a Python library, is able to do it seemingly just fine. Other libraries, like Npcap and WinPcap, also seem to be able to send raw TCP data just fine on Windows. How is this possible? What are these libraries doing under the hood that enables them to bypass this limitation?
WinPcap (the windows implementation of libpcap) authors say in their website:
WinPcap consists of a driver that extends the operating system to provide low-level network access and a library that is used to easily access low-level network layers.
So the answer to your question would be: in windows, the implementation of libpcap (which is what Scapy uses according to their site) uses a driver to get access to the low-level networking stuff
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Currently I'm developing a data acquisition program for my experiment in C++ from a Linux based machine (Ubuntu), I also have many VIs in Labview who is programmed in Windows to control the instruments of the experiment (motors, Signal Generator..). The purpose is to have a 2-way communication between 2 pc, the Linux will ask which VIs to be executed, and when it's finished, send back a signal to Linux machine.
My questions are:
Can I send a signal or a command to Labview in Windows from Linux (Terminal, and it can be implemented into my C code) and vice versa? How?
TCP Labview could be a solution? Or should I try to set the inter-PC "talking" through serial communication (which is easy to setup physically)?
The best (also the easiest) way is to implement TCP-based client-server communication (TCP will ensure data is lossless. When using other mechanisms like UDP or serial you should always make sure your commands are received correctly).
At LabVIEW site, you will have TCP listener (server) which will listen to commands from the Linux machine at your specified port.
Upon command reception, LabVIEW code can do the work and reply by the same TCP connection.
This is very good article about your question: https://decibel.ni.com/content/docs/DOC-9131
Their are several choices for communicating between C++ and LabVIEW. (As well as Linux / Windows).
If you are willing to run LabVIEW on your linux machine you can make use of several of the LabVIEW communication architectures. Here is NI's white paper.
http://www.ni.com/white-paper/12079/en/
Provides choices such as Shared Variable, Network Streams, Web Services, TCP/IP.
You can also take your LabVIEW code and compile it to a DLL and call it from C++ to make use of some of the above features. If not you are likely going to have to go to the TCP/IP route or web service.
I would recommend using TCP/IP, its pretty simple to implement on both sides.
If you are more familiar with serial protocols you can also use them to communicate.
libpcap is used for package capturing. As I understand, it can capture the network packages from all ports. And it can capture the package data in link layer (such as ethernet frame).
This looks a little confusing to me, because it seems impossible to intercept all network traffic (from all ports) by just using the socket API in Unix-like system. Moreover, socket API seems unable to get the information in link layer (such as the header of Ethernet frame).
Is it true that libpcap is implemented by socket API? If not, which OS-level API is used to implement it?
libpcap is not part of the sockets API. On Linux PF_PACKET is used, which is an evolution of the BSD mechanism. On other operating systems there are other mechanisms (DLPI, Windows requires a DLL).
The capture on any interface mechanism is a Linux specific mechanism, and the capture mechanism occurs above the layer of the network interface.
The capture mechanism inside the kernel either has an explicit call out to a kernel packet filter, or is inserted by adjusting the plumbing (SVR4).
Is it true that libpcap is implemented by socket API?
If you're on Linux or IRIX, it is true. If you're on another flavor of UN*X, it is not true.
If not, which OS-level API is used to implement it?
On *BSD, OS X, AIX, and Solaris 11 and later: BPF.
On earlier versions of Solaris, and on HP-UX: STREAMS+DLPI.
it seems impossible to intercept all network traffic (from all ports) by just using the socket API in Unix-like system
On Linux, if you open a PF_PACKET socket, and don't bind it to a particular interface, packets from all interfaces are delivered to the socket.
socket API seems unable to get the information in link layer
You have to use the right type of socket, namely a PF_PACKET socket on Linux or a PF_RAW socket with a protocol of RAWPROTO_SNOOP on IRIX. Other UN*Xes don't have socket types for packet capture, and use other mechanisms.
On Linux, access to the raw packets needed by libpcap is done using a PF_PACKET socket.
See http://man7.org/linux/man-pages/man7/packet.7.html
It's implemented by inserting a driver into the network stack.
Normally, applications use kernel-level TCP stack. Instead of using default kernel-level implementation, by using your own implementation of TCP/IP stack processing in user-space, you can be bypass the kernel.
more readings
"zero copy networking" vs "kernel bypass"?
according to that StackOverflow post pcap is also doing kernel Bypass
I'll have to do packet inspection, mangling, dropping and injection of packets on a Linux system. Ideally, this would be in user space and on IP packets and Ethernet frames, too.
Unfortunately, I cannot go OpenSource for this which basically rules out any approach based on NFQUEUE and libnetfilter_queue, since all of netfilter (and their dog) is GPL only.
I thought about using TAP/TUN devices in parallel to controlling netfilter by simply calling iptables, but this seems to be messy at best...
So, are there any alternatives to netfilter?
I believe your issue is that libnetfilter is subject to the GPLv2 licence (not LGPL) and any project building on these would thus be subject to the GPLv2 licence too; this is what you want to avoid (I think).
An alternative would be to use a language binding which is not subject to the GPLv2 licence. One candidate would appear to be the Go bindings - see here for example, which appears to be under the Apache licence. I have obviously not checked the provenance of every file therein. Another way would be to divide your application into two - a small layer that communicates with Netfilter, communicating via (e.g.) an RPC interface with the rest of your application.
However, the last time I faced this, I used libpcap instead, which is BSD licensed. It's a little known fact that libpcap can send raw packets as well as receive them. However, it is much lower level than netfilter - you get raw packets and that's about it.
The license does not apply to your userspace application.
Sorry for the rather long post.
I need some input regarding a project that I am going to undertake.
I am trying to make an application that collects kernel debugging information from a guest Linux OS, located inside a VmWare Virtual Machine, and send them to a host OS efficiently.
So far, I have found a similar project, but written for Windows[1].
The author of the project wrote a DLL that is loaded into memory, and replaces the implementation of the KdSendPacket and KdReceivePacket functions, to use the VmWare GuestRpc[2] mechanism, instead of the slow serial port.
The data are then send to a debugging application on the host(Kd or WinDbg) trough a named pipe.
The author claims that there is a speed-up up to 45%, by avoiding the serial port transmission.
I am trying to achieve something similar ,but for Linux, and try to make the debugging process a little faster, than using the serial port.
My concrete questions are :
Do any similar applications exist?
I didn't manage to find any.
Would such an application be worth it ,comparing its functionality to netconsole[3], for example?
What method of intercepting printk messages would you suggest ?
Is there an equivalent of KdSendPacket/KdReceivePacket on Linux ?
[1]. http://virtualkd.sysprogs.org/dox/operation.html
[2]. http://articles.sysprogs.org/kdvmware/guestrpc.shtml
[3]. http://www.kernel.org/doc/Documentation/networking/netconsole.txt
Using the serial port is really suboptimal.. even the (virtual) network would be preferable to that, but getting back to host-guest IPC channels, VMware's VMCI comes to mind.
many approaches can use to achieve your goal, below methods can be applied if network is connected:
use syslog service and transfer log though network to your server:
syslogd, syslogng seems support sending log to a log server with some filter critiera.
directly call tcp/udp socket functions in your kernel module to sends your collected data back to server.
other approaches, you may write application on host machine that calls hypervisor's share memory access function to read the memory buffer of your kernel module. However, the xen/kvm hypervisor both support these apis and i am not sure about weather vmware have this kind of library.
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.
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.
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
Probably you want something like libpcap.
Libpcap allows you to read/inject raw packets from/into a network interface.
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:
Support monitor mode and frame injection
Support monitor mode and not frame injection
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
Python code is much quicker to write than C
scapy provides a significant amount of classes that you can use to intuitively create a frame layer by layer
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
It sounds like you are getting the media and transport layers mixed up.
802.11 is what's commonly referred to as a "link", "physical", or "media" layer, meaning it only deals with the transmission of raw datagrams.
Concepts like ACKs, retransmissions, backoff (flow control) apply to the "transport" layer, and those particular terms are strongly associated with TCP/IP.
Implementing your own complete transport layer from scratch is very difficult and almost certainly not what you want to do. If instead you want to use the existing TCP/IP stack on top of your own custom interpretation of 802.11, then you probably want to create a virtual network interface. This would act as an intermediary between TCP/IP and the media layer.
Hopefully this gives you some better context and keywords to look for.