cap.c

#include <pcap.h>
#include <stdio.h>
#include <stdlib.h>
#include <stdint.h>
#include <string.h>
#include <stdbool.h>
#include <arpa/inet.h>

#include "cap.h"

#define SIZE_ETHERNET 14
#define ETH_ADDR_LEN   6
#define IP_ADDR_LEN    4

struct arp_hdr {
  u_int16_t arp_htype;
  u_int16_t arp_ptype;
  u_char    arp_hlen;
  u_char    arp_plen;
  u_int16_t arp_oper;
  u_char    arp_sha[ETH_ADDR_LEN];
  u_char    arp_sip[IP_ADDR_LEN];
  u_char    arp_dha[ETH_ADDR_LEN];
  u_char    arp_dip[IP_ADDR_LEN];
};

/* We assume we can just receive ARP here */
void got_packet (u_char *args, const struct pcap_pkthdr *pkthdr, const u_char *packet) {
  struct host_list *tmp = (struct host_list *)args;
  struct host_list *p;
  bool repeated = false;
  char sourceip[16];
  char sourcemac[18];
  uint32_t ip;
  const struct arp_hdr *arp = (struct arp_hdr *)(packet + SIZE_ETHERNET);

  snprintf (sourceip,  16, "%d.%d.%d.%d", arp->arp_sip[0], arp->arp_sip[1], arp->arp_sip[2], arp->arp_sip[3]);
  snprintf (sourcemac, 18, "%02X:%02X:%02X:%02X:%02X:%02X", arp->arp_sha[0], arp->arp_sha[1], arp->arp_sha[2], arp->arp_sha[3], arp->arp_sha[4], arp->arp_sha[5]);
  ip = inet_addr(sourceip);

  while(!repeated && tmp->next != NULL && tmp->next->ip <= ip) {
    tmp = tmp->next;
    if(tmp->ip == ip)repeated = true;
  }

  if(!repeated) {
    if(tmp->next == NULL) {
      tmp->next = malloc(sizeof(struct host_list));
      tmp->next->next = NULL;
    }
    else {
      p = tmp->next;
      tmp->next = malloc(sizeof(struct host_list));
      tmp->next->next = p;
    }

    tmp->next->ip = ip;
    strncpy(tmp->next->ip_str, sourceip, 16);
    strncpy(tmp->next->mac_str, sourcemac, 18);
  }
}

void* cap(void* args) {
  struct cap_struct *s = (struct cap_struct*)args;

  pcap_t *handle;
  char errbuf[PCAP_ERRBUF_SIZE];
  bpf_u_int32 mask;
  bpf_u_int32 net;

  /* Filter */
  struct bpf_program fp;
  char filter_exp[] = "arp";

  /* Sniffing */
  const u_char *packet;
  struct pcap_pkthdr header;

  *s->ok = true;
  s->list = malloc(sizeof(struct host_list));
  s->list->next = NULL;

  if (pcap_lookupnet(s->dev, &net, &mask, errbuf) == -1) {
    fprintf(stderr, "%s\n", errbuf);
    *s->ok = false;

    if(sem_post(s->sem) == -1) {
      perror("client");
    }

    return NULL;
  }

  if ((handle = pcap_open_live(s->dev, BUFSIZ, 1, 100, errbuf)) == NULL) {
    fprintf(stderr, "%s\n", errbuf);
    *s->ok = false;

    if(sem_post(s->sem) == -1) {
      perror("client");
    }

    return NULL;
  }

  s->ctx = handle;

  if (pcap_compile(handle, &fp, filter_exp, 0, net) == -1) {
    fprintf(stderr, "%s\n", pcap_geterr(handle));
    *s->ok = false;
    if(sem_post(s->sem) == -1) {
      perror("client");
    }

    return NULL;
  }

  if (pcap_setfilter(handle, &fp) == -1) {
    fprintf(stderr, "%s\n", pcap_geterr(handle));
    *s->ok = false;

    if(sem_post(s->sem) == -1) {
      perror("client");
    }

    return NULL;
  }

  if(pcap_setnonblock(handle, 1, errbuf) == -1) {
    fprintf(stderr, "%s\n", errbuf);
    *s->ok = false;

    if(sem_post(s->sem) == -1) {
      perror("client");
      return NULL;
    }
  }

  if(sem_post(s->sem) == -1) {
    perror("client");
    *s->ok = false;
    return NULL;
  }

  if(pcap_loop(handle, -1, got_packet, (u_char *)s->list) == -1) {
    fprintf(stderr, "%s\n", pcap_geterr(handle));
    *s->ok = false;
    return NULL;
  }

  return NULL;
}