arper/main.c

#define _DEFAULT_SOURCE
#include <stdio.h>
#include <stdint.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/ioctl.h>
#include <sys/socket.h>
#include <net/if.h>
#include <netinet/in.h>
#include <net/ethernet.h>
#include <linux/if_packet.h>

#define MTU 1500

void parse_mac(const char* str, uint8_t* mac) {
    char* s = strdup(str);
    char* octet = strtok(s, ":");
    int i = 0;
    
    while (octet != NULL && i < 6) {
        mac[i++] = strtol(octet, NULL, 16);

        octet = strtok(NULL, ":");
    }

    free(s);
}

void print_mac(uint8_t* mac) {
    printf("%02x:%02x:%02x:%02x:%02x:%02x\n",
        mac[0], mac[1], mac[2], mac[3], mac[4], mac[5]);
}

struct linkinterface {
    char* if_name;
    socklen_t if_len;

    uint8_t host[ETH_ALEN];
    uint8_t gateway[ETH_ALEN];

    int fd; // raw socket
    int if_idx;
};

struct linkinterface* link_parse(const char* if_name, const char* gateway) {
    struct linkinterface* link = (struct linkinterface*)malloc(sizeof(struct linkinterface));
    link->if_name = strdup(if_name);
    link->if_len = strlen(link->if_name);

    parse_mac(gateway, link->gateway);

    return link;
}

int link_open(struct linkinterface* link) {
    link->fd = socket(PF_PACKET, SOCK_RAW, htons(ETH_P_ALL));
    if (link->fd < 0) {
        return 1;
    }
    setsockopt(link->fd, SOL_SOCKET, SO_BINDTODEVICE, link->if_name, link->if_len);

    // get interface index and MAC
    struct ifreq netlink;
    memset(&netlink, '\0', sizeof(netlink));
    strncpy(netlink.ifr_ifrn.ifrn_name, link->if_name, IFNAMSIZ-1);
    if (ioctl(link->fd, SIOCGIFINDEX, &netlink) < 0) {
        return 1;
    }
    link->if_idx = netlink.ifr_ifru.ifru_ivalue; // index

    memset(&netlink, '\0', sizeof(netlink));
    strncpy(netlink.ifr_ifrn.ifrn_name, link->if_name, IFNAMSIZ-1);
    if (ioctl(link->fd, SIOCGIFHWADDR, &netlink) < 0) {
        return 1;
    }
    memcpy(link->host, netlink.ifr_ifru.ifru_hwaddr.sa_data, ETH_ALEN); // MAC

    return 0;
}

void link_free(struct linkinterface* link) {
    close(link->fd);
    free(link->if_name);
    free(link);
}

ssize_t link_send(struct linkinterface* link, uint16_t type, uint8_t* packet, size_t len) {
    struct ether_header* ether = (struct ether_header*)packet;
    memcpy(ether->ether_shost, link->host, ETH_ALEN);
    memcpy(ether->ether_dhost, link->gateway, ETH_ALEN);
    ether->ether_type = htons(type);

    return send(link->fd, packet, len, 0);
}

ssize_t link_recv(struct linkinterface* link, uint16_t type, uint8_t* packet, size_t len) {
    char buffer[MTU];
    uint16_t want_type = htons(type);
    do {
        ssize_t rd = recv(link->fd, buffer, MTU, 0);
        if (rd < 0) return -1;

        struct ether_header* ether = (struct ether_header*)buffer;
        // check received packet's source MAC
        // if its someone else than gateway - skip
        if (memcmp(ether->ether_shost, link->gateway, ETH_ALEN)) {
            continue; // not our gateway
        }
        if (memcmp(ether->ether_dhost, link->host, ETH_ALEN)) {
            continue; // not our host
        }
        if (ether->ether_type != want_type) {
            continue; // not wanted type
        }

        // copy packet to recv buffer
        memcpy(packet, buffer, rd);
        return rd;
    } while (1);

    // TODO: timeout
}

#define ARP_HW_ETHER        0x000F
#define ARP_PT_IP           0x0800

#define ARP_OP_REQUEST      1
#define ARP_OP_REPLY        2

// Inverse ARP
#define INARP_OP_REQUEST    8
#define INART_OP_REPLY      9

typedef struct arp_packet_s {
    struct ether_header ether;
    
    uint16_t hrd;           // Hardware address space (e.g., Ethernet, Packet Radio Net.)
    uint16_t pro;           // Protocol address space.  For Ethernet hardware, this is from the set of type fields ether_typ$<protocol>.
    uint8_t hln;            // byte length of each hardware address
    uint8_t pln;            // byte length of each protocol address
    uint16_t op;            // opcode (ares_op$REQUEST | ares_op$REPLY)
    uint8_t sha[ETH_ALEN];  // Hardware address of sender of this packet
    uint8_t spa[4];         // Protocol address of sender of this packet
    uint8_t tha[ETH_ALEN];  // Hardware address of sender of this packet
    uint8_t tpa[4];         // Protocol address of sender of this packet
} __attribute__((packed)) arp_packet_t;

int main(int argc, char** argv) {
    // ifname hostMAC gatewayMAC targetMAC
    struct linkinterface* link = link_parse(argv[1], argv[2]);

    uint8_t target[ETH_ALEN];
    parse_mac(argv[3], target);

    if (link_open(link) < 0) {
        perror("link_open");
        return 1;
    }
    print_mac(link->host);

    uint8_t ip[4] = {192, 168, 100, 3};

    // test arp send
    arp_packet_t arp;
    arp.hrd = htons(ARP_HW_ETHER);
    arp.pro = htons(ARP_PT_IP);
    arp.hln = htons(ETH_ALEN);
    arp.pln = htons(4);
    arp.op = htons(INARP_OP_REQUEST);
    memcpy(arp.sha, link->host, ETH_ALEN);
    memcpy(arp.spa, ip, 4);
    memcpy(arp.tha, target, ETH_ALEN);
    memset(arp.tpa, '\0', 4);

    ssize_t sent = link_send(link, ETHERTYPE_ARP, (uint8_t*)&arp, sizeof(arp));
    printf("sent %ld\n", sent);

    link_free(link);
    return 0;
}