libcnet - A C-based library for TCP/IP Networking

Warning: This project is still under development. It might contains some bugs. Use it carefully

Welcome to libcnet a simple lightweight high-level C library (for GNU Linux) for interacting with Sockets and IP Networking. The main goal developing this library was to keep an high-level OOP-like approach, implementing several functionalities like: crafting your own IP packets from scratch, sending IP Packets through sockets, receiving packets from the network, syncrhonizing the sender and the receiver and more on.

I would like you to notice that this is my first C project, there might be some bugs that will be fixed and memory leaks, altough I check for memory safetiness with valgrind every time a new functionality is implemented. Notice also, that some memory leaks might be caused by the thread(s) used with the Receiver.

As I already said, the main goal of this library, despite IP and socket programming, is to keep things as high-level as possible, which means no deeply technical knowledge is required. However, also more low-level functionalities are provided. In other words, you can let us decide or do it in your own.

Please, take a look at Notes On Development for the state of the project and future development.

A simple example

Here is a simple example on how, you can create and sent an ICMP Echo message

/*
    FILENAME: icmp.c
*/

#include <sender.h>
#include <ip.h>

int main(int argc, char **argv)
{
    char raddr[INET_ADDRSTRLEN];
    getHostnameIP(argv[2], raddr);

    char payload[18];
    generateRandomData(payload, 18);
    
    bool verbose = true;

    Sender *sender = Sender_new(argv[1], raddr, NULL, INADDR_ANY, "icmp", verbose);
    IpPacket* pckt = Sender_craftIcmp(sender, ICMP_ECHO_TYPE, ICMP_ECHO_CODE, payload, 18);

    if (verbose)
    {
        IpHeader_printInfo(&pckt->_iphdr);
        IcmpHeader_printInfo(&pckt->_payload._icmp->_hdr);
    }

    Sender_send(sender, pckt, 0.0);

    Sender_delete(sender);
    IpPacket_delete(pckt);

    return 0;
}

Now, with the following command you can compile the program

gcc -Wall -O3 -I<include-path> -o icmp icmp.c -L<lib-folder> -lcnet

Finally, you can run it

$ sudo ./icmp eth0 google.com

[*] Printing Header Information Fields
Ip Version: 4
Internet Header Length: 5
Distributed Service Code Point: 0
Explicit Congestion Network: 0
Total Length: 46
Identification: 28472
IP Flags (X D M): 010
Fragment Offset: 0
Time To Live: 128
IP Protocol: 1
Header Checksum: 0
Source Address: 172.19.11.1
Destination Address: 142.251.209.14

[*] Printing Information of the ICMP Header
ICMP Message Type: 8
ICMP Message Code: 0
ICMP Header Checksum: 38223
ICMP Header Identifier: 1
ICMP Header Sequence Number: 0

[*] Sent 46 bytes of IP Packet

Notice, this output is quite verbose since we have set the verbosity of the sender to True.

The following image is wireshark confirming that the message has being sent and also received.

Notice that, the verbose mode of the sender also generates a new file in the current folder. This new file contains the entire IP packet that has been sent.

$ xxd sent_28473_1_icmp.bin

00000000: 4500 002e 6f38 4000 8001 0000 ac13 0b01  E...o8@.........
00000010: 8efb d10e 0800 954f 0001 0000 d170 b548  .......O.....p.H
00000020: c152 b3cb d009 2e71 b1eb e7ea ce85       .R.....q......

The most important thing you need to remember when using this library is, when creating "objects" using the corresponding new function, at the bottom of the program must always be the corresponding delete function in order to freed the memory previously allocated.

One last important note is the sudo prefix when running an executable using this library. Under the hood, since we are crafting from zero IP packets, meaning the header and payload, the sockets of the sender and the receiver are created using the SOCK_RAW option, which requires to the calling user the CAP_NET_ADMIN or CAP_NET_RAW capabilities. The fastest way to obtain this capility is being root, here it comes the sudo command. However, there are two more possibilities:

1. Set the SUID bit to the udp executable, using sudo chmod u+s udp
2. Set either the CAP_NET_ADMIN or CAT_NET_RAW capability for the current user when running.

In general, setting the SUID bit is not a good idea in terms of security, since it can leads to easy lateral movement, once an attacker has gained access to your machine as a non-privileged user.

This was just a simple example, you can find more complex ones into the example folder.

Installation

The installation is quite easy since no third-party libraries are used, just the default ones. These are the requirements

• git to download this repository with the source code
• gcc to compile the source and create the libraries
• make to run Makefiles

Before installing, please run the following commands:

sudo apt-get update && apt-get install -y build-essential
sudo apt-get install -y git make gcc

Once you have all the pre-requisites installed, then you can proceed with the installation

$ git clone https://github.com/lmriccardo/libcnet.git
$ cd libcnet
$ sudo ./scripts/build.sh

The previous command will populate the build folder with

• include folder with all the header files
• bin with all the object files
• static containing the static library libcnet.a
• shared containing the shared library libcnet.so

References

[1] RFC 792 for ICMP Packets

[2] RFC 768 for UDP Packets

[3] RFC 791 and RFC 2474 for IP Packets

[4] RFC 1191 for vanilla Path MTU Discovery Implementation

[5] RFC 9293 for TCP Packets

[6] Linux Man Page for various functionality with sockets, threads and so on.

Conclusions

I hope that you like this library. Please free to test it and giving me feedback. If you like this library, please leave a star.