cdp-frame.c

cdp-frame.c

#include <errno.h>
#include <string.h>
#include <arpa/inet.h>
#include <net/ethernet.h>
#include <net/if.h>
#include <netinet/if_ether.h>
#include <netpacket/packet.h>
#include <sys/ioctl.h>
#include <sys/socket.h>
#include <stdio.h>

union ethframe
{
    struct
    {
        struct ethhdr       header;
        unsigned char       data[ETH_DATA_LEN];
    } field;

    unsigned char buffer[ETH_FRAME_LEN];
};

int main() 
{
    char *iface = "eth0";

    // CDP Broadcast address
    unsigned char dest[] = { 
        0x01, 0x00, 0x0c, 0xcc, 0xcc, 0xcc 
    };

    // Logical-Link Control Fields
    unsigned char logical_link_control[] = {
        0xaa,   // DSAP
        0xaa,   // SSAP
        0x03,   // Control field
        0x00,   // Organization Code
        0x00,   // ...
        0x0c,   //
        0x20,   // PID
        0x00,   // ...
    };

    unsigned char data[] = "";
    unsigned short data_len = strlen(data);

    // Create the socket
    int s;
    if ((s = socket(AF_PACKET, SOCK_RAW, htons(logical_link_control))) < 0) {
        printf("Error: could not open socket\n");
        return -1;
    }

    // Get the interface index
    struct ifreq buffer;
    int ifindex;
    memset(&buffer, 0x00, sizeof(buffer));
    strncpy(buffer.ifr_name, iface, IFNAMSIZ);
    if (ioctl(s, SIOCGIFINDEX, &buffer) < 0) {
        printf("Error: could not get interface index\n");
        close(s);
        return -1;
    }
    ifindex = buffer.ifr_ifindex; 

    printf("%d\n", ifindex);

    // Get the source mac address
    
    unsigned char source[ETH_ALEN];
    if (ioctl(s, SIOCGIFHWADDR, &buffer) < 0) {
        printf("Error: could not get interface address\n");
        close(s);
        return -1;
    }
    memcpy((void*)source, (void*)(buffer.ifr_hwaddr.sa_data),
            ETH_ALEN);
 

    // Create the Ethernet frame
    union ethframe frame;

    // Set frame source and destination MAC addresses
    memcpy(frame.field.header.h_dest, dest, ETH_ALEN);
    memcpy(frame.field.header.h_source, source, ETH_ALEN);
    
    // Set the frame protocol
    frame.field.header.h_proto = htons(logical_link_control);

    // Set the frame payload
    memcpy(frame.field.data, data, data_len);

    // Fill in the sockaddr_ll
    unsigned int frame_len = data_len + ETH_HLEN;

    struct sockaddr_ll saddrll;
    memset((void *) &saddrll, 0, sizeof(saddrll));
    saddrll.sll_family = PF_PACKET;
    saddrll.sll_ifindex = ifindex;
    saddrll.sll_halen = ETH_ALEN;
    memcpy((void *) (saddrll.sll_addr), (void *) dest, ETH_ALEN);

    // Send the packet
    sendto(
        s, 
        frame.buffer, 
        frame_len,
        0,
        (struct sockaddr *)&saddrll,
        sizeof(saddrll));
}