Get default gateway in C language

紀錄一下這件看起來很簡單的事

呼叫 SxNetDefaultGateway() 傳回 default gateway

Source code below



/* numeric: & 0x8000: default instead of *,
 *          & 0x4000: host instead of net,
 *          & 0x0fff: don't resolve
 */
char *INET_rresolve(struct sockaddr_in *s_in, int numeric, uint32_t netmask)
{
  /* addr-to-name cache */
  struct addr {
    struct addr *next;
    struct sockaddr_in addr;
    int host;
    char name[1];
  };
/* 
  static struct addr *cache = NULL;

  struct addr *pn;
  char *name;
  uint32_t ad, host_ad;
  int host = 0;
*/
  uint32_t ad;
 
  /* Grmpf. -FvK */
  if (s_in->sin_family != AF_INET) {
#ifdef DEBUG
    printf("rresolve: unsupported address family %d!",
          s_in->sin_family);
#endif
    errno = EAFNOSUPPORT;
    return NULL;
  }
  ad = s_in->sin_addr.s_addr;
#ifdef DEBUG
  printf("rresolve: %08x, mask %08x, num %08x", (unsigned)ad, netmask, numeric);
#endif
  if (ad == INADDR_ANY) {
    if ((numeric & 0x0FFF) == 0) {
      if (numeric & 0x8000)
        return strdup("default");
      return strdup("*");
    }
  }
  if (numeric & 0x0FFF)
    return strdup(inet_ntoa(s_in->sin_addr));
 
  return 0;
}

#ifndef RTF_UP
/* Keep this in sync with /usr/src/linux/include/linux/route.h */
#define RTF_UP          0x0001  /* route usable                 */
#define RTF_GATEWAY     0x0002  /* destination is a gateway     */
#define RTF_HOST        0x0004  /* host entry (net otherwise)   */
#define RTF_REINSTATE   0x0008  /* reinstate route after tmout  */
#define RTF_DYNAMIC     0x0010  /* created dyn. (by redirect)   */
#define RTF_MODIFIED    0x0020  /* modified dyn. (by redirect)  */
#define RTF_MTU         0x0040  /* specific MTU for this route  */
#ifndef RTF_MSS
#define RTF_MSS         RTF_MTU /* Compatibility :-(            */
#endif
#define RTF_WINDOW      0x0080  /* per route window clamping    */
#define RTF_IRTT        0x0100  /* Initial round trip time      */
#define RTF_REJECT      0x0200  /* Reject route                 */
#endif

static const unsigned flagvals[] = { /* Must agree with flagchars[]. */
  RTF_GATEWAY,
  RTF_HOST,
  RTF_REINSTATE,
  RTF_DYNAMIC,
  RTF_MODIFIED,
#if ENABLE_FEATURE_IPV6
  RTF_DEFAULT,
  RTF_ADDRCONF,
  RTF_CACHE
#endif
};

#define IPV4_MASK (RTF_GATEWAY|RTF_HOST|RTF_REINSTATE|RTF_DYNAMIC|RTF_MODIFIED)
#define IPV6_MASK (RTF_GATEWAY|RTF_HOST|RTF_DEFAULT|RTF_ADDRCONF|RTF_CACHE)

/* Must agree with flagvals[]. */
static const char flagchars[] =
  "GHRDM"
#if ENABLE_FEATURE_IPV6
  "DAC"
#endif
;

static void set_flags(char *flagstr, int flags)
{
  int i;

  *flagstr++ = 'U';

  for (i = 0; (*flagstr = flagchars[i]) != 0; i++) {
    if (flags & flagvals[i]) {
      ++flagstr;
    }
  }
}

const char *SxNetDefaultGateway()
{
  char devname[64], flags[16];
  static char *sdest = 0, *sgw = 0;
  unsigned long d, g, m;
  int flgs, ref, use, metric, mtu, win, ir;
  struct sockaddr_in s_addr;
  struct in_addr mask;

  FILE *fp = fopen("/proc/net/route", "r");
  if(!fp)
    return 0;
 
  if (fscanf(fp, "%*[^\n]\n") < 0) { /* Skip the first line. */
    goto ERROR;      /* Empty or missing line, or read error. */
  }
  while (1) {
    int r;
    r = fscanf(fp, "%63s%lx%lx%X%d%d%d%lx%d%d%d\n",
           devname, &d, &g, &flgs, &ref, &use, &metric, &m,
           &mtu, &win, &ir);
    if (r != 11) {
      if ((r < 0) && feof(fp)) { /* EOF with no (nonspace) chars read. */
        break;
      }
 ERROR:
      fclose(fp);
      perror("fscanf");
      return 0;
    }

    if (!(flgs & RTF_UP)) { /* Skip interfaces that are down. */
      continue;
    }

    set_flags(flags, (flgs & IPV4_MASK));
#ifdef RTF_REJECT
    if (flgs & RTF_REJECT) {
      flags[0] = '!';
    }
#endif
    if(sdest) free(sdest);
    if(sgw)  free(sgw);

    memset(&s_addr, 0, sizeof(struct sockaddr_in));
    s_addr.sin_family = AF_INET;
    s_addr.sin_addr.s_addr = d;
    sdest = INET_rresolve(&s_addr, (0x0fff | 0x8000), m); /* 'default' instead of '*' */
    s_addr.sin_addr.s_addr = g;
    sgw = INET_rresolve(&s_addr, (0x0fff | 0x4000), m); /* Host instead of net */
    mask.s_addr = m;  
    /* "%15.15s" truncates hostnames, do we really want that? */
    /*
    printf("%-15.15s %-15.15s %-16s%-6s", sdest, sgw, inet_ntoa(mask), flags);   
    if (netstatfmt) {
      printf("%5d %-5d %6d %s\n", mtu, win, ir, devname);
    } else {
      printf("%-6d %-2d %7d %s\n", metric, ref, use, devname);
    }
    */
    if(flgs & RTF_GATEWAY)  {
      fclose(fp);
      return sgw;
    }
  }
 
  fclose(fp);
  return 0;
}