IPv4 in CIDR

cidr.cc

#ifdef __FreeBSD__
#include <sys/socket.h>
#endif
#include <arpa/inet.h>
#include <netinet/in.h>
#include <cstdlib>
#include <cstring>
#include <cerrno>
#include <cassert>
#include <iostream>
#include <cstdint>

int inet_cidrtoaddr(int cidr, struct in_addr *addr) {
  int ocets;

  if (cidr < 0 || cidr > 32) {
    errno = EINVAL;
    return -1;
  }
  ocets = (cidr + 7) / 8;

  addr->s_addr = 0;
  if (ocets > 0) {
    memset(&addr->s_addr, 255, (size_t)ocets - 1);
    memset((unsigned char *)&addr->s_addr + (ocets - 1),
           (256 - (1 << (32 - cidr) % 8)), 1);
  }

  return 0;
}

bool cidr_match(const in_addr &addr, const in_addr &net, uint8_t bits) {
  if (bits == 0) {
    // u32 << 32 is undefined behaviour
    return true;
  }
  return !((addr.s_addr ^ net.s_addr) & htonl(0xFFFFFFFFu << (32 - bits)));
}

int main() {
  in_addr ip, net, netmask;
  // Check if 192.168.0.17 is present in 192.168.0.0/24
  inet_aton("192.168.0.17", &ip);
  inet_aton("192.168.0.0", &net);
  inet_cidrtoaddr(24, &netmask);
  std::cout << "Got netmask: " << inet_ntoa(netmask) << std::endl;
  bool a = ((ip.s_addr & netmask.s_addr) == (net.s_addr & netmask.s_addr));
  std::cout << "Test A: " << a << std::endl;
  assert(a);

  // Another way
  bool b = cidr_match(ip, net, 24);
  std::cout << "Test B: " << b << std::endl;
  assert(b);

  // Check 0.0.0.0/0
  in_addr all;
  inet_aton("0.0.0.0", &all);
  bool c = cidr_match(ip, all, 0);
  std::cout << "Test C: " << c << std::endl;
  assert(c);

  // Check it does in fact return 0 if outside the range
  in_addr outside;
  inet_aton("192.168.1.0", &outside);
  bool d = cidr_match(outside, net, 24);
  std::cout << "Test D: " << d << std::endl;
  assert(!d);

  // Test specifying a wrong network address
  // 192.168.0.1/24 should be 192.168.0.0/24
  in_addr wrong_net;
  inet_aton("192.168.0.1", &wrong_net);

  bool e = ((ip.s_addr & netmask.s_addr) == (net.s_addr & netmask.s_addr));
  std::cout << "Test E: " << e << std::endl;
  assert(a);

  bool f = cidr_match(ip, wrong_net, 24);
  std::cout << "Test F: " << f << std::endl;
  assert(f);

  // Check that is in fact class less (not restricted to class C / 24)
  // Make sure 192.168.0.17 is outside 192.168.0.0/28, inside 192.168.0.16/28
  in_addr classless_net;
  inet_aton("192.168.0.16", &classless_net);
  bool g = cidr_match(ip, classless_net, 28);
  bool h = cidr_match(ip, net, 28);
  std::cout << "Test G: " << g << std::endl;
  std::cout << "Test H: " << h << std::endl;
  assert(g);
  assert(!h);
}