noip.c (encode_inet_addr): Avoid collisions with wildcard addresses.

[preload-hacks] / noip.c

/* -*-c-*-
 *
 * Make programs use Unix-domain sockets instead of IP
 *
 * (c) 2008 Straylight/Edgeware
 */

/*----- Licensing notice --------------------------------------------------*
 *
 * This file is part of the preload-hacks package.
 *
 * Preload-hacks are free software; you can redistribute it and/or modify
 * them under the terms of the GNU General Public License as published by
 * the Free Software Foundation; either version 2 of the License, or (at
 * your option) any later version.
 *
 * Preload-hacks are distributed in the hope that it will be useful, but
 * WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
 * or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
 * for more details.
 *
 * You should have received a copy of the GNU General Public License along
 * with preload-hacks; if not, write to the Free Software Foundation, Inc.,
 * 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
 */

#define _GNU_SOURCE
#undef sun
#undef SUN
#define DEBUG

/*----- Header files ------------------------------------------------------*/

#include <assert.h>
#include <ctype.h>
#include <errno.h>
#include <stdarg.h>
#include <stddef.h>
#include <stdio.h>
#include <stdlib.h>

#include <unistd.h>
#include <dirent.h>
#include <dlfcn.h>
#include <fcntl.h>
#include <pwd.h>

#include <sys/ioctl.h>
#include <sys/socket.h>
#include <sys/stat.h>
#include <sys/un.h>

#include <netinet/in.h>
#include <arpa/inet.h>
#include <netinet/tcp.h>
#include <netinet/udp.h>
#include <ifaddrs.h>
#include <netdb.h>

/*----- Data structures ---------------------------------------------------*/

enum { UNUSED, STALE, USED };           /* Unix socket status values */
enum { WANT_FRESH, WANT_EXISTING };     /* Socket address dispositions */
enum { DENY, ALLOW };                   /* ACL verdicts */

static int address_families[] = { AF_INET, AF_INET6, -1 };

#define ADDRBUFSZ 64

/* Address representations. */
typedef union ipaddr {
  struct in_addr v4;
  struct in6_addr v6;
} ipaddr;

/* Convenient socket address hacking. */
typedef union address {
  struct sockaddr sa;
  struct sockaddr_in sin;
  struct sockaddr_in6 sin6;
} address;

/* Access control list nodes */
typedef struct aclnode {
  struct aclnode *next;
  int act;
  int af;
  ipaddr minaddr, maxaddr;
  unsigned short minport, maxport;
} aclnode;

/* Local address records */
typedef struct full_ipaddr {
  int af;
  ipaddr addr;
} full_ipaddr;
#define MAX_LOCAL_IPADDRS 64
static full_ipaddr local_ipaddrs[MAX_LOCAL_IPADDRS];
static int n_local_ipaddrs;

/* General configuration */
static uid_t uid;
static char *sockdir = 0;
static int debug = 0;
static unsigned minautoport = 16384, maxautoport = 65536;

/* Access control lists */
static aclnode *bind_real, **bind_tail = &bind_real;
static aclnode *connect_real,  **connect_tail = &connect_real;

/*----- Import the real versions of functions -----------------------------*/

/* The list of functions to immport. */
#define IMPORTS(_)                                                      \
  _(socket, int, (int, int, int))                                       \
  _(socketpair, int, (int, int, int, int *))                            \
  _(connect, int, (int, const struct sockaddr *, socklen_t))            \
  _(bind, int, (int, const struct sockaddr *, socklen_t))               \
  _(accept, int, (int, struct sockaddr *, socklen_t *))                 \
  _(getsockname, int, (int, struct sockaddr *, socklen_t *))            \
  _(getpeername, int, (int, struct sockaddr *, socklen_t *))            \
  _(getsockopt, int, (int, int, int, void *, socklen_t *))              \
  _(setsockopt, int, (int, int, int, const void *, socklen_t))          \
  _(sendto, ssize_t, (int, const void *buf, size_t, int,                \
                      const struct sockaddr *to, socklen_t tolen))      \
  _(recvfrom, ssize_t, (int, void *buf, size_t, int,                    \
                        struct sockaddr *from, socklen_t *fromlen))     \
  _(sendmsg, ssize_t, (int, const struct msghdr *, int))                \
  _(recvmsg, ssize_t, (int, struct msghdr *, int))                      \
  _(ioctl, int, (int, unsigned long, ...))

/* Function pointers to set up. */
#define DECL(imp, ret, args) static ret (*real_##imp) args;
IMPORTS(DECL)
#undef DECL

/* Import the system calls. */
static void import(void)
{
#define IMPORT(imp, ret, args)                                          \
    real_##imp = (ret (*)args)dlsym(RTLD_NEXT, #imp);
  IMPORTS(IMPORT)
#undef IMPORT
}

/*----- Utilities ---------------------------------------------------------*/

/* Socket address casts */
#define SA(sa) ((struct sockaddr *)(sa))
#define SIN(sa) ((struct sockaddr_in *)(sa))
#define SIN6(sa) ((struct sockaddr_in6 *)(sa))
#define SUN(sa) ((struct sockaddr_un *)(sa))

/* Raw bytes */
#define UC(ch) ((unsigned char)(ch))

/* Memory allocation */
#define NEW(x) ((x) = xmalloc(sizeof(*x)))
#define NEWV(x, n) ((x) = xmalloc(sizeof(*x) * (n)))

/* Debugging */
#ifdef DEBUG
#  define D(body) { if (debug) { body } }
#  define Dpid pid_t pid = debug ? getpid() : -1
#else
#  define D(body) ;
#  define Dpid
#endif

/* Preservation of error status */
#define PRESERVING_ERRNO(body) do {                                     \
  int _err = errno; { body } errno = _err;                              \
} while (0)

/* Allocate N bytes of memory; abort on failure. */
static void *xmalloc(size_t n)
{
  void *p;
  if (!n) return (0);
  if ((p = malloc(n)) == 0) { perror("malloc"); exit(127); }
  return (p);
}

/* Allocate a copy of the null-terminated string P; abort on failure. */
static char *xstrdup(const char *p)
{
  size_t n = strlen(p) + 1;
  char *q = xmalloc(n);
  memcpy(q, p, n);
  return (q);
}

/*----- Address-type hacking ----------------------------------------------*/

/* If M is a simple mask, i.e., consists of a sequence of zero bits followed
 * by a sequence of one bits, then return the length of the latter sequence
 * (which may be zero); otherwise return -1.
 */
static int simple_mask_length(unsigned long m)
{
  int n = 0;

  while (m & 1) { n++; m >>= 1; }
  return (m ? -1 : n);
}

/* Answer whether AF is an interesting address family. */
static int family_known_p(int af)
{
  switch (af) {
    case AF_INET:
    case AF_INET6:
      return (1);
    default:
      return (0);
  }
}

/* Return the socket address length for address family AF. */
static socklen_t family_socklen(int af)
{
  switch (af) {
    case AF_INET: return (sizeof(struct sockaddr_in));
    case AF_INET6: return (sizeof(struct sockaddr_in6));
    default: abort();
  }
}

/* Return the width of addresses of kind AF. */
static int address_width(int af)
{
  switch (af) {
    case AF_INET: return 32;
    case AF_INET6: return 128;
    default: abort();
  }
}

/* If addresses A and B share a common prefix then return its length;
 * otherwise return -1.
 */
static int common_prefix_length(int af, const ipaddr *a, const ipaddr *b)
{
  switch (af) {
    case AF_INET: {
      unsigned long aa = ntohl(a->v4.s_addr), bb = ntohl(b->v4.s_addr);
      unsigned long m = aa^bb;
      if ((aa&m) == 0 && (bb&m) == m) return (32 - simple_mask_length(m));
      else return (-1);
    } break;
    case AF_INET6: {
      const uint8_t *aa = a->v6.s6_addr, *bb = b->v6.s6_addr;
      unsigned m;
      unsigned n;
      int i;

      for (i = 0; i < 16 && aa[i] == bb[i]; i++);
      n = 8*i;
      if (i < 16) {
        m = aa[i]^bb[i];
        if ((aa[i]&m) != 0 || (bb[i]&m) != m) return (-1);
        n += 8 - simple_mask_length(m);
        for (i++; i < 16; i++)
          if (aa[i] || bb[i] != 0xff) return (-1);
      }
      return (n);
    } break;
    default:
      abort();
  }
}

/* Extract the port number (in host byte-order) from SA. */
static int port_from_sockaddr(const struct sockaddr *sa)
{
  switch (sa->sa_family) {
    case AF_INET: return (ntohs(SIN(sa)->sin_port));
    case AF_INET6: return (ntohs(SIN6(sa)->sin6_port));
    default: abort();
  }
}

/* Store the port number PORT (in host byte-order) in SA. */
static void port_to_sockaddr(struct sockaddr *sa, int port)
{
  switch (sa->sa_family) {
    case AF_INET: SIN(sa)->sin_port = htons(port); break;
    case AF_INET6: SIN6(sa)->sin6_port = htons(port); break;
    default: abort();
  }
}

/* Extract the address part from SA and store it in A. */
static void ipaddr_from_sockaddr(ipaddr *a, const struct sockaddr *sa)
{
  switch (sa->sa_family) {
    case AF_INET: a->v4 = SIN(sa)->sin_addr; break;
    case AF_INET6: a->v6 = SIN6(sa)->sin6_addr; break;
    default: abort();
  }
}

/* Copy a whole socket address about. */
static void copy_sockaddr(struct sockaddr *sa_dst,
                          const struct sockaddr *sa_src)
  { memcpy(sa_dst, sa_src, family_socklen(sa_src->sa_family)); }

/* Answer whether two addresses are equal. */
static int ipaddr_equal_p(int af, const ipaddr *a, const ipaddr *b)
{
  switch (af) {
    case AF_INET: return (a->v4.s_addr == b->v4.s_addr);
    case AF_INET6: return (memcmp(a->v6.s6_addr, b->v6.s6_addr, 16) == 0);
    default: abort();
  }
}

/* Answer whether the address part of SA is between A and B (inclusive).  We
 * assume that SA has the correct address family.
 */
static int sockaddr_in_range_p(const struct sockaddr *sa,
                               const ipaddr *a, const ipaddr *b)
{
  switch (sa->sa_family) {
    case AF_INET: {
      unsigned long addr = ntohl(SIN(sa)->sin_addr.s_addr);
      return (ntohl(a->v4.s_addr) <= addr &&
              addr <= ntohl(b->v4.s_addr));
    } break;
    case AF_INET6: {
      const uint8_t *ss = SIN6(sa)->sin6_addr.s6_addr;
      const uint8_t *aa = a->v6.s6_addr, *bb = b->v6.s6_addr;
      int h = 1, l = 1;
      int i;

      for (i = 0; h && l && i < 16; i++, ss++, aa++, bb++) {
        if (*ss < *aa || *bb < *ss) return (0);
        if (*aa < *ss) l = 0;
        if (*ss < *bb) h = 0;
      }
      return (1);
    } break;
    default:
      abort();
  }
}

/* Fill in SA with the appropriate wildcard address. */
static void wildcard_address(int af, struct sockaddr *sa)
{
  switch (af) {
    case AF_INET: {
      struct sockaddr_in *sin = SIN(sa);
      memset(sin, 0, sizeof(*sin));
      sin->sin_family = AF_INET;
      sin->sin_port = 0;
      sin->sin_addr.s_addr = INADDR_ANY;
    } break;
    case AF_INET6: {
      struct sockaddr_in6 *sin6 = SIN6(sa);
      memset(sin6, 0, sizeof(*sin6));
      sin6->sin6_family = AF_INET6;
      sin6->sin6_port = 0;
      sin6->sin6_addr = in6addr_any;
      sin6->sin6_scope_id = 0;
      sin6->sin6_flowinfo = 0;
    } break;
    default:
      abort();
  }
}

/* Mask the address A, forcing all but the top PLEN bits to zero or one
 * according to HIGHP.
 */
static void mask_address(int af, ipaddr *a, int plen, int highp)
{
  switch (af) {
    case AF_INET: {
      unsigned long addr = ntohl(a->v4.s_addr);
      unsigned long mask = plen ? ~0ul << (32 - plen) : 0;
      addr &= mask;
      if (highp) addr |= ~mask;
      a->v4.s_addr = htonl(addr & 0xffffffff);
    } break;
    case AF_INET6: {
      int i = plen/8;
      unsigned m = (0xff << (8 - plen%8)) & 0xff;
      unsigned s = highp ? 0xff : 0;
      if (m) {
        a->v6.s6_addr[i] = (a->v6.s6_addr[i] & m) | (s & ~m);
        i++;
      }
      for (; i < 16; i++) a->v6.s6_addr[i] = s;
    } break;
    default:
      abort();
  }
}

/* Write a presentation form of SA to BUF, a buffer of length SZ.  LEN is the
 * address length; if it's zero, look it up based on the address family.
 * Return a pointer to the string (which might, in an emergency, be a static
 * string rather than your buffer).
 */
static char *present_sockaddr(const struct sockaddr *sa, socklen_t len,
                              char *buf, size_t sz)
{
#define WANT(n_) do { if (sz < (n_)) goto nospace; } while (0)
#define PUTC(c_) do { *buf++ = (c_); sz--; } while (0)

  if (!sa) return "<null-address>";
  if (!sz) return "<no-space-in-buffer>";
  if (!len) len = family_socklen(sa->sa_family);

  switch (sa->sa_family) {
    case AF_UNIX: {
      struct sockaddr_un *sun = SUN(sa);
      char *p = sun->sun_path;
      size_t n = len - offsetof(struct sockaddr_un, sun_path);

      assert(n);
      if (*p == 0) {
        WANT(1); PUTC('@');
        p++; n--;
        while (n) {
          switch (*p) {
            case 0: WANT(2); PUTC('\\'); PUTC('0'); break;
            case '\a': WANT(2); PUTC('\\'); PUTC('a'); break;
            case '\n': WANT(2); PUTC('\\'); PUTC('n'); break;
            case '\r': WANT(2); PUTC('\\'); PUTC('r'); break;
            case '\t': WANT(2); PUTC('\\'); PUTC('t'); break;
            case '\v': WANT(2); PUTC('\\'); PUTC('v'); break;
            case '\\': WANT(2); PUTC('\\'); PUTC('\\'); break;
            default:
              if (*p > ' ' && *p <= '~')
                { WANT(1); PUTC(*p); }
              else {
                WANT(4); PUTC('\\'); PUTC('x');
                PUTC((*p >> 4)&0xf); PUTC((*p >> 0)&0xf);
              }
              break;
          }
          p++; n--;
        }
      } else {
        if (*p != '/') { WANT(2); PUTC('.'); PUTC('/'); }
        while (n && *p) { WANT(1); PUTC(*p); p++; n--; }
      }
      WANT(1); PUTC(0);
    } break;
    case AF_INET: case AF_INET6: {
      char addrbuf[NI_MAXHOST], portbuf[NI_MAXSERV];
      int err = getnameinfo(sa, len,
                            addrbuf, sizeof(addrbuf),
                            portbuf, sizeof(portbuf),
                            NI_NUMERICHOST | NI_NUMERICSERV);
      assert(!err);
      snprintf(buf, sz, strchr(addrbuf, ':') ? "[%s]:%s" : "%s:%s",
               addrbuf, portbuf);
    } break;
    default:
      snprintf(buf, sz, "<unknown-address-family %d>", sa->sa_family);
      break;
  }
  return (buf);

nospace:
  buf[sz - 1] = 0;
  return (buf);
}

/* Guess the family of a textual socket address. */
static int guess_address_family(const char *p)
  { return (strchr(p, ':') ? AF_INET6 : AF_INET); }

/* Parse a socket address P and write the result to SA. */
static int parse_sockaddr(struct sockaddr *sa, const char *p)
{
  char buf[ADDRBUFSZ];
  char *q;
  struct addrinfo *ai, ai_hint = { 0 };

  if (strlen(p) >= sizeof(buf) - 1) return (-1);
  strcpy(buf, p); p = buf;
  if (*p != '[') {
    if ((q = strchr(p, ':')) == 0) return (-1);
    *q++ = 0;
  } else {
    p++;
    if ((q = strchr(p, ']')) == 0) return (-1);
    *q++ = 0;
    if (*q != ':') return (-1);
    q++;
  }

  ai_hint.ai_family = AF_UNSPEC;
  ai_hint.ai_socktype = SOCK_DGRAM;
  ai_hint.ai_flags = AI_NUMERICHOST | AI_NUMERICSERV;
  if (getaddrinfo(p, q, &ai_hint, &ai)) return (-1);
  memcpy(sa, ai->ai_addr, ai->ai_addrlen);
  freeaddrinfo(ai);
  return (0);
}

/*----- Access control lists ----------------------------------------------*/

#ifdef DEBUG

/* Write to standard error a description of the ACL node A. */
static void dump_aclnode(const aclnode *a)
{
  char buf[ADDRBUFSZ];
  const char *p;
  int plen;

  fprintf(stderr, "noip(%d):   %c ", getpid(), a->act ? '+' : '-');
  plen = common_prefix_length(a->af, &a->minaddr, &a->maxaddr);
  p = inet_ntop(a->af, &a->minaddr, buf, sizeof(buf));
  fprintf(stderr, strchr(p, ':') ? "[%s]" : "%s", p);
  if (plen < 0) {
    p = inet_ntop(a->af, &a->maxaddr, buf, sizeof(buf));
    fprintf(stderr, strchr(p, ':') ? "-[%s]" : "-%s", p);
  } else if (plen < address_width(a->af))
    fprintf(stderr, "/%d", plen);
  if (a->minport != 0 || a->maxport != 0xffff) {
    fprintf(stderr, ":%u", (unsigned)a->minport);
    if (a->minport != a->maxport)
      fprintf(stderr, "-%u", (unsigned)a->maxport);
  }
  fputc('\n', stderr);
}

static void dump_acl(const aclnode *a)
{
  int act = ALLOW;

  for (; a; a = a->next) {
    dump_aclnode(a);
    act = a->act;
  }
  fprintf(stderr, "noip(%d):   [default policy: %s]\n", getpid(),
          act == ALLOW ? "DENY" : "ALLOW");
}

#endif

/* Returns nonzero if the ACL A allows the socket address SA. */
static int acl_allows_p(const aclnode *a, const struct sockaddr *sa)
{
  unsigned short port = port_from_sockaddr(sa);
  int act = ALLOW;
  Dpid;

  D({ char buf[ADDRBUFSZ];
      fprintf(stderr, "noip(%d): check %s\n", pid,
              present_sockaddr(sa, 0, buf, sizeof(buf))); })
  for (; a; a = a->next) {
    D( dump_aclnode(a); )
    if (sockaddr_in_range_p(sa, &a->minaddr, &a->maxaddr) &&
        a->minport <= port && port <= a->maxport) {
      D( fprintf(stderr, "noip(%d): aha!  %s\n", pid,
                 a->act ? "ALLOW" : "DENY"); )
      return (a->act);
    }
    act = a->act;
  }
  D( fprintf(stderr, "noip(%d): nothing found: %s\n", pid,
             act ? "DENY" : "ALLOW"); )
  return (!act);
}

/*----- Socket address conversion -----------------------------------------*/

/* Return a uniformly distributed integer between MIN and MAX inclusive. */
static unsigned randrange(unsigned min, unsigned max)
{
  unsigned mask, i;

  /* It's so nice not to have to care about the quality of the generator
   * much!
   */
  max -= min;
  for (mask = 1; mask < max; mask = (mask << 1) | 1)
    ;
  do i = rand() & mask; while (i > max);
  return (i + min);
}

/* Return the status of Unix-domain socket address SUN.  Returns: UNUSED if
 * the socket doesn't exist; USED if the path refers to an active socket, or
 * isn't really a socket at all, or we can't tell without a careful search
 * and QUICKP is set; or STALE if the file refers to a socket which isn't
 * being used any more.
 */
static int unix_socket_status(struct sockaddr_un *sun, int quickp)
{
  struct stat st;
  FILE *fp = 0;
  size_t len, n;
  int rc;
  char buf[256];

  if (stat(sun->sun_path, &st))
    return (errno == ENOENT ? UNUSED : USED);
  if (!S_ISSOCK(st.st_mode) || quickp)
    return (USED);
  rc = USED;
  if ((fp = fopen("/proc/net/unix", "r")) == 0)
    goto done;
  if (!fgets(buf, sizeof(buf), fp)) goto done; /* skip header */
  len = strlen(sun->sun_path);
  while (fgets(buf, sizeof(buf), fp)) {
    n = strlen(buf);
    if (n >= len + 2 && buf[n - len - 2] == ' ' && buf[n - 1] == '\n' &&
        memcmp(buf + n - len - 1, sun->sun_path, len) == 0)
      goto done;
  }
  if (ferror(fp))
    goto done;
  rc = STALE;
done:
  if (fp) fclose(fp);
  return (rc);
}

/* Convert the IP address SA to a Unix-domain address SUN.  Fail if the
 * address seems already taken.  If DESPARATEP then try cleaning up stale old
 * sockets.
 */
static int encode_unused_inet_addr(struct sockaddr *sa,
                                   struct sockaddr_un *sun,
                                   int desperatep)
{
  address waddr;
  struct sockaddr_un wsun;
  int rc;
  char buf[ADDRBUFSZ];

  snprintf(sun->sun_path, sizeof(sun->sun_path), "%s/%s", sockdir,
           present_sockaddr(sa, 0, buf, sizeof(buf)));
  if ((rc = unix_socket_status(sun, !desperatep)) == USED) return (-1);
  else if (rc == STALE) unlink(sun->sun_path);

  wildcard_address(sa->sa_family, &waddr.sa);
  port_to_sockaddr(&waddr.sa, port_from_sockaddr(sa));
  snprintf(wsun.sun_path, sizeof(wsun.sun_path), "%s/%s", sockdir,
           present_sockaddr(&waddr.sa, 0, buf, sizeof(buf)));
  if ((rc = unix_socket_status(&wsun, !desperatep)) == USED) return (-1);
  else if (rc == STALE) unlink(wsun.sun_path);

  return (0);
}

/* Encode the Internet address SA as a Unix-domain address SUN.  If WANT is
 * WANT_FRESH, and SA's port number is zero, then we pick an arbitrary local
 * port.  Otherwise we pick the port given.  There's an unpleasant hack to
 * find servers bound to local wildcard addresses.  Returns zero on success;
 * -1 on failure.
 */
static int encode_inet_addr(struct sockaddr_un *sun,
                            const struct sockaddr *sa,
                            int want)
{
  int i;
  int desperatep = 0;
  address addr;
  char buf[ADDRBUFSZ];
  int rc;

  D( fprintf(stderr, "noip(%d): encode %s (%s)", getpid(),
             present_sockaddr(sa, 0, buf, sizeof(buf)),
             want == WANT_EXISTING ? "EXISTING" : "FRESH"); )
  sun->sun_family = AF_UNIX;
  if (port_from_sockaddr(sa) || want == WANT_EXISTING) {
    snprintf(sun->sun_path, sizeof(sun->sun_path), "%s/%s", sockdir,
             present_sockaddr(sa, 0, buf, sizeof(buf)));
    rc = unix_socket_status(sun, 0);
    if (rc == STALE) unlink(sun->sun_path);
    if (rc != USED && want == WANT_EXISTING) {
      wildcard_address(sa->sa_family, &addr.sa);
      port_to_sockaddr(&addr.sa, port_from_sockaddr(sa));
      snprintf(sun->sun_path, sizeof(sun->sun_path), "%s/%s", sockdir,
               present_sockaddr(&addr.sa, 0, buf, sizeof(buf)));
      if (unix_socket_status(sun, 0) == STALE) unlink(sun->sun_path);
    }
  } else {
    copy_sockaddr(&addr.sa, sa);
    for (i = 0; i < 10; i++) {
      port_to_sockaddr(&addr.sa, randrange(minautoport, maxautoport));
      if (!encode_unused_inet_addr(&addr.sa, sun, 0)) goto found;
    }
    for (desperatep = 0; desperatep < 2; desperatep++) {
      for (i = minautoport; i <= maxautoport; i++) {
        port_to_sockaddr(&addr.sa, i);
        if (!encode_unused_inet_addr(&addr.sa, sun, 0)) goto found;
      }
    }
    errno = EADDRINUSE;
    D( fprintf(stderr, " -- can't resolve\n"); )
    return (-1);
  found:;
  }
  D( fprintf(stderr, " -> `%s'\n", sun->sun_path); )
  return (0);
}

/* Decode the Unix address SUN to an Internet address SIN.  If AF_HINT is
 * nonzero, an empty address (indicative of an unbound Unix-domain socket) is
 * translated to a wildcard Internet address of the appropriate family.
 * Returns zero on success; -1 on failure (e.g., it wasn't one of our
 * addresses).
 */
static int decode_inet_addr(struct sockaddr *sa, int af_hint,
                            const struct sockaddr_un *sun,
                            socklen_t len)
{
  char buf[ADDRBUFSZ];
  size_t n = strlen(sockdir), nn;
  address addr;

  if (!sa) sa = &addr.sa;
  if (sun->sun_family != AF_UNIX) return (-1);
  if (len > sizeof(*sun)) return (-1);
  ((char *)sun)[len] = 0;
  nn = strlen(sun->sun_path);
  D( fprintf(stderr, "noip(%d): decode `%s'", getpid(), sun->sun_path); )
  if (af_hint && !sun->sun_path[0]) {
    wildcard_address(af_hint, sa);
    D( fprintf(stderr, " -- unbound socket\n"); )
    return (0);
  }
  if (nn < n + 1 || nn - n >= sizeof(buf) || sun->sun_path[n] != '/' ||
      memcmp(sun->sun_path, sockdir, n) != 0) {
    D( fprintf(stderr, " -- not one of ours\n"); )
    return (-1);
  }
  if (parse_sockaddr(sa, sun->sun_path + n + 1)) return (-1);
  D( fprintf(stderr, " -> %s\n",
             present_sockaddr(sa, 0, buf, sizeof(buf))); )
  return (0);
}

/* SK is (or at least might be) a Unix-domain socket we created when an
 * Internet socket was asked for.  We've decided it should be an Internet
 * socket after all, with family AF_HINT, so convert it.  If TMP is not null,
 * then don't replace the existing descriptor: store the new socket in *TMP
 * and return zero.
 */
static int fixup_real_ip_socket(int sk, int af_hint, int *tmp)
{
  int nsk;
  int type;
  int f, fd;
  struct sockaddr_un sun;
  address addr;
  socklen_t len;

#define OPTS(_)                                                         \
  _(DEBUG, int)                                                         \
  _(REUSEADDR, int)                                                     \
  _(DONTROUTE, int)                                                     \
  _(BROADCAST, int)                                                     \
  _(SNDBUF, int)                                                        \
  _(RCVBUF, int)                                                        \
  _(OOBINLINE, int)                                                     \
  _(NO_CHECK, int)                                                      \
  _(LINGER, struct linger)                                              \
  _(BSDCOMPAT, int)                                                     \
  _(RCVLOWAT, int)                                                      \
  _(RCVTIMEO, struct timeval)                                           \
  _(SNDTIMEO, struct timeval)

  len = sizeof(sun);
  if (real_getsockname(sk, SA(&sun), &len))
    return (-1);
  if (decode_inet_addr(&addr.sa, af_hint, &sun, len))
    return (0); /* Not one of ours */
  len = sizeof(type);
  if (real_getsockopt(sk, SOL_SOCKET, SO_TYPE, &type, &len) < 0 ||
      (nsk = real_socket(addr.sa.sa_family, type, 0)) < 0)
    return (-1);
#define FIX(opt, ty) do {                                               \
  ty ov_;                                                               \
  len = sizeof(ov_);                                                    \
  if (real_getsockopt(sk, SOL_SOCKET, SO_##opt, &ov_, &len) < 0 ||      \
      real_setsockopt(nsk, SOL_SOCKET, SO_##opt, &ov_, len)) {          \
    close(nsk);                                                         \
    return (-1);                                                        \
  }                                                                     \
} while (0);
  OPTS(FIX)
#undef FIX
  if (tmp)
    *tmp = nsk;
  else {
    if ((f = fcntl(sk, F_GETFL)) < 0 ||
        (fd = fcntl(sk, F_GETFD)) < 0 ||
        fcntl(nsk, F_SETFL, f) < 0 ||
        dup2(nsk, sk) < 0) {
      close(nsk);
      return (-1);
    }
    unlink(sun.sun_path);
    close(nsk);
    if (fcntl(sk, F_SETFD, fd) < 0) {
      perror("noip: fixup_real_ip_socket F_SETFD");
      abort();
    }
  }
  return (0);
}

/* The socket SK is about to be used to communicate with the remote address
 * SA.  Assign it a local address so that getpeername(2) does something
 * useful.
 */
static int do_implicit_bind(int sk, const struct sockaddr **sa,
                            socklen_t *len, struct sockaddr_un *sun)
{
  address addr;
  socklen_t mylen = sizeof(*sun);

  if (acl_allows_p(connect_real, *sa)) {
    if (fixup_real_ip_socket(sk, (*sa)->sa_family, 0)) return (-1);
  } else {
    if (real_getsockname(sk, SA(sun), &mylen) < 0) return (-1);
    if (sun->sun_family == AF_UNIX) {
      if (mylen < sizeof(*sun)) ((char *)sun)[mylen] = 0;
      if (!sun->sun_path[0]) {
        wildcard_address((*sa)->sa_family, &addr.sa);
        encode_inet_addr(sun, &addr.sa, WANT_FRESH);
        if (real_bind(sk, SA(sun), SUN_LEN(sun))) return (-1);
      }
      encode_inet_addr(sun, *sa, WANT_EXISTING);
      *sa = SA(sun);
      *len = SUN_LEN(sun);
    }
  }
  return (0);
}

/* We found the real address SA, with length LEN; if it's a Unix-domain
 * address corresponding to a fake socket, convert it to cover up the
 * deception.  Whatever happens, put the result at FAKE and store its length
 * at FAKELEN.
 */
static void return_fake_name(struct sockaddr *sa, socklen_t len,
                             struct sockaddr *fake, socklen_t *fakelen)
{
  address addr;
  socklen_t alen;

  if (sa->sa_family == AF_UNIX &&
      !decode_inet_addr(&addr.sa, 0, SUN(sa), len)) {
    sa = &addr.sa;
    len = family_socklen(addr.sa.sa_family);
  }
  alen = len;
  if (len > *fakelen) len = *fakelen;
  if (len > 0) memcpy(fake, sa, len);
  *fakelen = alen;
}

/*----- Configuration -----------------------------------------------------*/

/* Return the process owner's home directory. */
static char *home(void)
{
  char *p;
  struct passwd *pw;

  if (getuid() == uid &&
      (p = getenv("HOME")) != 0)
    return (p);
  else if ((pw = getpwuid(uid)) != 0)
    return (pw->pw_dir);
  else
    return "/notexist";
}

/* Return a good temporary directory to use. */
static char *tmpdir(void)
{
  char *p;

  if ((p = getenv("TMPDIR")) != 0) return (p);
  else if ((p = getenv("TMP")) != 0) return (p);
  else return ("/tmp");
}

/* Return the user's name, or at least something distinctive. */
static char *user(void)
{
  static char buf[16];
  char *p;
  struct passwd *pw;

  if ((p = getenv("USER")) != 0) return (p);
  else if ((p = getenv("LOGNAME")) != 0) return (p);
  else if ((pw = getpwuid(uid)) != 0) return (pw->pw_name);
  else {
    snprintf(buf, sizeof(buf), "uid-%lu", (unsigned long)uid);
    return (buf);
  }
}

/* Skip P over space characters. */
#define SKIPSPC do { while (*p && isspace(UC(*p))) p++; } while (0)

/* Set Q to point to the next word following P, null-terminate it, and step P
 * past it. */
#define NEXTWORD(q) do {                                                \
  SKIPSPC;                                                              \
  q = p;                                                                \
  while (*p && !isspace(UC(*p))) p++;                                   \
  if (*p) *p++ = 0;                                                     \
} while (0)

/* Set Q to point to the next dotted-quad address, store the ending delimiter
 * in DEL, null-terminate it, and step P past it. */
static void parse_nextaddr(char **pp, char **qq, int *del)
{
  char *p = *pp;

  SKIPSPC;
  if (*p == '[') {
    p++; SKIPSPC;
    *qq = p;
    p += strcspn(p, "]");
    if (*p) *p++ = 0;
    *del = 0;
  } else {
    *qq = p;
    while (*p && (*p == '.' || isdigit(UC(*p)))) p++;
    *del = *p;
    if (*p) *p++ = 0;
  }
  *pp = p;
}

/* Set Q to point to the next decimal number, store the ending delimiter in
 * DEL, null-terminate it, and step P past it. */
#define NEXTNUMBER(q, del) do {                                         \
  SKIPSPC;                                                              \
  q = p;                                                                \
  while (*p && isdigit(UC(*p))) p++;                                    \
  del = *p;                                                             \
  if (*p) *p++ = 0;                                                     \
} while (0)

/* Push the character DEL back so we scan it again, unless it's zero
 * (end-of-file). */
#define RESCAN(del) do { if (del) *--p = del; } while (0)

/* Evaluate true if P is pointing to the word KW (and not some longer string
 * of which KW is a prefix). */

#define KWMATCHP(kw) (strncmp(p, kw, sizeof(kw) - 1) == 0 &&            \
                      !isalnum(UC(p[sizeof(kw) - 1])) &&                \
                      (p += sizeof(kw) - 1))

/* Parse a port list, starting at *PP.  Port lists have the form
 * [:LOW[-HIGH]]: if omitted, all ports are included; if HIGH is omitted,
 * it's as if HIGH = LOW.  Store LOW in *MIN, HIGH in *MAX and set *PP to the
 * rest of the string.
 */
static void parse_ports(char **pp, unsigned short *min, unsigned short *max)
{
  char *p = *pp, *q;
  int del;

  SKIPSPC;
  if (*p != ':')
    { *min = 0; *max = 0xffff; }
  else {
    p++;
    NEXTNUMBER(q, del); *min = strtoul(q, 0, 0); RESCAN(del);
    SKIPSPC;
    if (*p == '-')
      { p++; NEXTNUMBER(q, del); *max = strtoul(q, 0, 0); RESCAN(del); }
    else
      *max = *min;
  }
  *pp = p;
}

/* Make a new ACL node.  ACT is the verdict; AF is the address family;
 * MINADDR and MAXADDR are the ranges on IP addresses; MINPORT and MAXPORT
 * are the ranges on port numbers; TAIL is the list tail to attach the new
 * node to.
 */
#define ACLNODE(tail_, act_,                                            \
                af_, minaddr_, maxaddr_, minport_, maxport_) do {       \
  aclnode *a_;                                                          \
  NEW(a_);                                                              \
  a_->act = (act_);                                                     \
  a_->af = (af_);                                                       \
  a_->minaddr = (minaddr_); a_->maxaddr = (maxaddr_);                   \
  a_->minport = (minport_); a_->maxport = (maxport_);                   \
  *tail_ = a_; tail_ = &a_->next;                                       \
} while (0)

/* Parse an ACL line.  *PP points to the end of the line; *TAIL points to
 * the list tail (i.e., the final link in the list).  An ACL entry has the
 * form +|- [any | local | ADDR | ADDR - ADDR | ADDR/ADDR | ADDR/INT] PORTS
 * where PORTS is parsed by parse_ports above; an ACL line consists of a
 * comma-separated sequence of entries..
 */
static void parse_acl_line(char **pp, aclnode ***tail)
{
  ipaddr minaddr, maxaddr;
  unsigned short minport, maxport;
  int i, af, n;
  int act;
  int del;
  char *p = *pp;
  char *q;

  for (;;) {
    SKIPSPC;
    if (*p == '+') act = ALLOW;
    else if (*p == '-') act = DENY;
    else goto bad;

    p++;
    SKIPSPC;
    if (KWMATCHP("any")) {
      parse_ports(&p, &minport, &maxport);
      for (i = 0; address_families[i] >= 0; i++) {
        af = address_families[i];
        memset(&minaddr, 0, sizeof(minaddr));
        maxaddr = minaddr; mask_address(af, &maxaddr, 0, 1);
        ACLNODE(*tail, act, af, minaddr, maxaddr, minport, maxport);
      }
    } else if (KWMATCHP("local")) {
      parse_ports(&p, &minport, &maxport);
      for (i = 0; address_families[i] >= 0; i++) {
        af = address_families[i];
        memset(&minaddr, 0, sizeof(minaddr));
        maxaddr = minaddr; mask_address(af, &maxaddr, 0, 1);
        ACLNODE(*tail, act, af, minaddr, minaddr, minport, maxport);
        ACLNODE(*tail, act, af, maxaddr, maxaddr, minport, maxport);
      }
      for (i = 0; i < n_local_ipaddrs; i++) {
        ACLNODE(*tail, act, local_ipaddrs[i].af,
                local_ipaddrs[i].addr, local_ipaddrs[i].addr,
                minport, maxport);
      }
    } else {
      parse_nextaddr(&p, &q, &del);
      af = guess_address_family(q);
      if (inet_pton(af, q, &minaddr) <= 0) goto bad;
      RESCAN(del);
      SKIPSPC;
      if (*p == '-') {
        p++;
        parse_nextaddr(&p, &q, &del);
        if (inet_pton(af, q, &maxaddr) <= 0) goto bad;
        RESCAN(del);
      } else if (*p == '/') {
        p++;
        NEXTNUMBER(q, del);
        n = strtoul(q, 0, 0);
        maxaddr = minaddr;
        mask_address(af, &minaddr, n, 0);
        mask_address(af, &maxaddr, n, 1);
        RESCAN(del);
      } else
        maxaddr = minaddr;
      parse_ports(&p, &minport, &maxport);
      ACLNODE(*tail, act, af, minaddr, maxaddr, minport, maxport);
    }
    SKIPSPC;
    if (*p != ',') break;
    if (*p) p++;
  }
  if (*p) goto bad;
  *pp = p;
  return;

bad:
  D( fprintf(stderr, "noip(%d): bad acl spec (ignored)\n", getpid()); )
  return;
}

/* Parse the autoports configuration directive.  Syntax is MIN - MAX. */
static void parse_autoports(char **pp)
{
  char *p = *pp, *q;
  unsigned x, y;
  int del;

  SKIPSPC;
  NEXTNUMBER(q, del); x = strtoul(q, 0, 0); RESCAN(del);
  SKIPSPC;
  if (*p != '-') goto bad; p++;
  NEXTNUMBER(q, del); y = strtoul(q, 0, 0); RESCAN(del);
  minautoport = x; maxautoport = y;
  SKIPSPC; if (*p) goto bad;
  *pp = p;
  return;

bad:
  D( fprintf(stderr, "noip(%d): bad port range (ignored)\n", getpid()); )
  return;
}

/* Parse an ACL from an environment variable VAR, attaching it to the list
 * TAIL. */
static void parse_acl_env(const char *var, aclnode ***tail)
{
  char *p, *q;

  if ((p = getenv(var)) != 0) {
    p = q = xstrdup(p);
    parse_acl_line(&q, tail);
    free(p);
  }
}

/* Read the configuration from the config file and environment. */
static void readconfig(void)
{
  FILE *fp;
  char buf[1024];
  size_t n;
  char *p, *q, *cmd;
  Dpid;

  parse_acl_env("NOIP_REALBIND_BEFORE", &bind_tail);
  parse_acl_env("NOIP_REALCONNECT_BEFORE", &connect_tail);
  if ((p = getenv("NOIP_AUTOPORTS")) != 0) {
    p = q = xstrdup(p);
    parse_autoports(&q);
    free(p);
  }
  if ((p = getenv("NOIP_CONFIG")) == 0)
    snprintf(p = buf, sizeof(buf), "%s/.noip", home());
  D( fprintf(stderr, "noip(%d): config file: %s\n", pid, p); )

  if ((fp = fopen(p, "r")) == 0) {
    D( fprintf(stderr, "noip(%d): couldn't read config: %s\n",
               pid, strerror(errno)); )
    goto done;
  }
  while (fgets(buf, sizeof(buf), fp)) {
    n = strlen(buf);
    p = buf;

    SKIPSPC;
    if (!*p || *p == '#') continue;
    while (n && isspace(UC(buf[n - 1]))) n--;
    buf[n] = 0;
    NEXTWORD(cmd);
    SKIPSPC;

    if (strcmp(cmd, "socketdir") == 0)
      sockdir = xstrdup(p);
    else if (strcmp(cmd, "realbind") == 0)
      parse_acl_line(&p, &bind_tail);
    else if (strcmp(cmd, "realconnect") == 0)
      parse_acl_line(&p, &connect_tail);
    else if (strcmp(cmd, "autoports") == 0)
      parse_autoports(&p);
    else if (strcmp(cmd, "debug") == 0)
      debug = *p ? atoi(p) : 1;
    else
      D( fprintf(stderr, "noip: bad config command %s\n", cmd); )
  }
  fclose(fp);

done:
  parse_acl_env("NOIP_REALBIND", &bind_tail);
  parse_acl_env("NOIP_REALCONNECT", &connect_tail);
  parse_acl_env("NOIP_REALBIND_AFTER", &bind_tail);
  parse_acl_env("NOIP_REALCONNECT_AFTER", &connect_tail);
  *bind_tail = 0;
  *connect_tail = 0;
  if (!sockdir) sockdir = getenv("NOIP_SOCKETDIR");
  if (!sockdir) {
    snprintf(buf, sizeof(buf), "%s/noip-%s", tmpdir(), user());
    sockdir = xstrdup(buf);
  }
  D( fprintf(stderr, "noip(%d): socketdir: %s\n", pid, sockdir);
     fprintf(stderr, "noip(%d): autoports: %u-%u\n",
             pid, minautoport, maxautoport);
     fprintf(stderr, "noip(%d): realbind acl:\n", pid);
     dump_acl(bind_real);
     fprintf(stderr, "noip(%d): realconnect acl:\n", pid);
     dump_acl(connect_real); )
}

/*----- Overridden system calls -------------------------------------------*/

static void dump_syserr(long rc)
  { fprintf(stderr, " => %ld (E%d)\n", rc, errno); }

static void dump_sysresult(long rc)
{
  if (rc < 0) dump_syserr(rc);
  else fprintf(stderr, " => %ld\n", rc);
}

static void dump_addrresult(long rc, const struct sockaddr *sa,
                            socklen_t len)
{
  char addrbuf[ADDRBUFSZ];

  if (rc < 0) dump_syserr(rc);
  else {
    fprintf(stderr, " => %ld [%s]\n", rc,
            present_sockaddr(sa, len, addrbuf, sizeof(addrbuf)));
  }
}

int socket(int pf, int ty, int proto)
{
  int sk;

  D( fprintf(stderr, "noip(%d): SOCKET pf=%d, type=%d, proto=%d",
             getpid(), pf, ty, proto); )

  switch (pf) {
    default:
      if (!family_known_p(pf)) {
        D( fprintf(stderr, " -> unknown; refuse\n"); )
        errno = EAFNOSUPPORT;
        sk = -1;
      }
      D( fprintf(stderr, " -> inet; substitute"); )
      pf = PF_UNIX;
      proto = 0;
      break;
    case PF_UNIX:
#ifdef PF_NETLINK
    case PF_NETLINK:
#endif
      D( fprintf(stderr, " -> safe; permit"); )
      break;
  }
  sk = real_socket(pf, ty, proto);
  D( dump_sysresult(sk); )
  return (sk);
}

int socketpair(int pf, int ty, int proto, int *sk)
{
  int rc;

  D( fprintf(stderr, "noip(%d): SOCKETPAIR pf=%d, type=%d, proto=%d",
             getpid(), pf, ty, proto); )
  if (!family_known_p(pf))
    D( fprintf(stderr, " -> unknown; permit"); )
  else {
    D( fprintf(stderr, " -> inet; substitute"); )
    pf = PF_UNIX;
    proto = 0;
  }
  rc = real_socketpair(pf, ty, proto, sk);
  D( if (rc < 0) dump_syserr(rc);
     else fprintf(stderr, " => %d (%d, %d)\n", rc, sk[0], sk[1]); )
  return (rc);
}

int bind(int sk, const struct sockaddr *sa, socklen_t len)
{
  struct sockaddr_un sun;
  int rc;
  Dpid;

  D({ char buf[ADDRBUFSZ];
      fprintf(stderr, "noip(%d): BIND sk=%d, sa[%d]=%s", pid,
              sk, len, present_sockaddr(sa, len, buf, sizeof(buf))); })

  if (!family_known_p(sa->sa_family))
    D( fprintf(stderr, " -> unknown af; pass through"); )
  else {
    D( fprintf(stderr, " -> checking...\n"); )
    PRESERVING_ERRNO({
      if (acl_allows_p(bind_real, sa)) {
        if (fixup_real_ip_socket(sk, sa->sa_family, 0))
          return (-1);
      } else {
        encode_inet_addr(&sun, sa, WANT_FRESH);
        sa = SA(&sun);
        len = SUN_LEN(&sun);
      }
    });
    D( fprintf(stderr, "noip(%d): BIND ...", pid); )
  }
  rc = real_bind(sk, sa, len);
  D( dump_sysresult(rc); )
  return (rc);
}

int connect(int sk, const struct sockaddr *sa, socklen_t len)
{
  struct sockaddr_un sun;
  int rc;
  Dpid;

  D({ char buf[ADDRBUFSZ];
      fprintf(stderr, "noip(%d): CONNECT sk=%d, sa[%d]=%s", pid,
              sk, len, present_sockaddr(sa, len, buf, sizeof(buf))); })

  if (!family_known_p(sa->sa_family)) {
    D( fprintf(stderr, " -> unknown af; pass through"); )
    rc = real_connect(sk, sa, len);
  } else {
    D( fprintf(stderr, " -> checking...\n"); )
    PRESERVING_ERRNO({
      do_implicit_bind(sk, &sa, &len, &sun);
    });
    D( fprintf(stderr, "noip(%d): CONNECT ...", pid); )
    rc = real_connect(sk, sa, len);
    if (rc < 0) {
      switch (errno) {
        case ENOENT: errno = ECONNREFUSED; break;
      }
    }
  }
  D( dump_sysresult(rc); )
  return (rc);
}

ssize_t sendto(int sk, const void *buf, size_t len, int flags,
               const struct sockaddr *to, socklen_t tolen)
{
  struct sockaddr_un sun;
  ssize_t n;
  Dpid;

  D({ char addrbuf[ADDRBUFSZ];
      fprintf(stderr, "noip(%d): SENDTO sk=%d, len=%lu, flags=%d, to[%d]=%s",
              pid, sk, (unsigned long)len, flags, tolen,
              present_sockaddr(to, tolen, addrbuf, sizeof(addrbuf))); })

  if (!to)
    D( fprintf(stderr, " -> null address; leaving"); )
  else if (!family_known_p(to->sa_family))
    D( fprintf(stderr, " -> unknown af; pass through"); )
  else {
    D( fprintf(stderr, " -> checking...\n"); )
    PRESERVING_ERRNO({
      do_implicit_bind(sk, &to, &tolen, &sun);
    });
    D( fprintf(stderr, "noip(%d): SENDTO ...", pid); )
  }
  n = real_sendto(sk, buf, len, flags, to, tolen);
  D( dump_sysresult(n); )
  return (n);
}

ssize_t recvfrom(int sk, void *buf, size_t len, int flags,
                 struct sockaddr *from, socklen_t *fromlen)
{
  char sabuf[1024];
  socklen_t mylen = sizeof(sabuf);
  ssize_t n;
  Dpid;

  D( fprintf(stderr, "noip(%d): RECVFROM sk=%d, len=%lu, flags=%d",
             pid, sk, (unsigned long)len, flags); )

  if (!from) {
    D( fprintf(stderr, " -> null addr; pass through"); )
    n = real_recvfrom(sk, buf, len, flags, 0, 0);
  } else {
    PRESERVING_ERRNO({
      n = real_recvfrom(sk, buf, len, flags, SA(sabuf), &mylen);
      if (n >= 0) {
        D( fprintf(stderr, " -> converting...\n"); )
        return_fake_name(SA(sabuf), mylen, from, fromlen);
        D( fprintf(stderr, "noip(%d): ... RECVFROM", pid); )
      }
    });
  }
  D( dump_addrresult(n, from, fromlen ? *fromlen : 0); )
  return (n);
}

ssize_t sendmsg(int sk, const struct msghdr *msg, int flags)
{
  struct sockaddr_un sun;
  const struct sockaddr *sa = SA(msg->msg_name);
  struct msghdr mymsg;
  ssize_t n;
  Dpid;

  D({ char addrbuf[ADDRBUFSZ];
      fprintf(stderr, "noip(%d): SENDMSG sk=%d, "
                      "msg_flags=%d, msg_name[%d]=%s, ...",
              pid, sk, msg->msg_flags, msg->msg_namelen,
              present_sockaddr(sa, msg->msg_namelen,
                               addrbuf, sizeof(addrbuf))); })

  if (!sa)
    D( fprintf(stderr, " -> null address; leaving"); )
  else if (!family_known_p(sa->sa_family))
    D( fprintf(stderr, " -> unknown af; pass through"); )
  else {
    D( fprintf(stderr, " -> checking...\n"); )
    PRESERVING_ERRNO({
      mymsg = *msg;
      do_implicit_bind(sk, &sa, &mymsg.msg_namelen, &sun);
      mymsg.msg_name = SA(sa);
      msg = &mymsg;
    });
    D( fprintf(stderr, "noip(%d): SENDMSG ...", pid); )
  }
  n = real_sendmsg(sk, msg, flags);
  D( dump_sysresult(n); )
  return (n);
}

ssize_t recvmsg(int sk, struct msghdr *msg, int flags)
{
  char sabuf[1024];
  struct sockaddr *sa = SA(msg->msg_name);
  socklen_t len = msg->msg_namelen;
  ssize_t n;
  Dpid;

  D( fprintf(stderr, "noip(%d): RECVMSG sk=%d msg_flags=%d, ...",
             pid, sk, msg->msg_flags); )

  if (!msg->msg_name) {
    D( fprintf(stderr, " -> null addr; pass through"); )
    return (real_recvmsg(sk, msg, flags));
  } else {
    PRESERVING_ERRNO({
      msg->msg_name = sabuf;
      msg->msg_namelen = sizeof(sabuf);
      n = real_recvmsg(sk, msg, flags);
      if (n >= 0) {
        D( fprintf(stderr, " -> converting...\n"); )
        return_fake_name(SA(sabuf), msg->msg_namelen, sa, &len);
        D( fprintf(stderr, "noip(%d): ... RECVMSG", pid); )
      }
      msg->msg_name = sa;
      msg->msg_namelen = len;
    });
  }
  D( dump_addrresult(n, sa, len); )
  return (n);
}

int accept(int sk, struct sockaddr *sa, socklen_t *len)
{
  char sabuf[1024];
  socklen_t mylen = sizeof(sabuf);
  int nsk;
  Dpid;

  D( fprintf(stderr, "noip(%d): ACCEPT sk=%d", pid, sk); )

  nsk = real_accept(sk, SA(sabuf), &mylen);
  if (nsk < 0) /* failed */;
  else if (!sa) D( fprintf(stderr, " -> address not wanted"); )
  else {
    D( fprintf(stderr, " -> converting...\n"); )
    return_fake_name(SA(sabuf), mylen, sa, len);
    D( fprintf(stderr, "noip(%d): ... ACCEPT", pid); )
  }
  D( dump_addrresult(nsk, sa, len ? *len : 0); )
  return (nsk);
}

int getsockname(int sk, struct sockaddr *sa, socklen_t *len)
{
  int rc;
  Dpid;

  D( fprintf(stderr, "noip(%d): GETSOCKNAME sk=%d", pid, sk); )
  PRESERVING_ERRNO({
    char sabuf[1024];
    socklen_t mylen = sizeof(sabuf);
    rc = real_getsockname(sk, SA(sabuf), &mylen);
    if (rc >= 0) {
      D( fprintf(stderr, " -> converting...\n"); )
      return_fake_name(SA(sabuf), mylen, sa, len);
      D( fprintf(stderr, "noip(%d): ... GETSOCKNAME", pid); )
    }
  });
  D( dump_addrresult(rc, sa, *len); )
  return (rc);
}

int getpeername(int sk, struct sockaddr *sa, socklen_t *len)
{
  int rc;
  Dpid;

  D( fprintf(stderr, "noip(%d): GETPEERNAME sk=%d", pid, sk); )
  PRESERVING_ERRNO({
    char sabuf[1024];
    socklen_t mylen = sizeof(sabuf);
    rc = real_getpeername(sk, SA(sabuf), &mylen);
    if (rc >= 0) {
      D( fprintf(stderr, " -> converting...\n"); )
      return_fake_name(SA(sabuf), mylen, sa, len);
      D( fprintf(stderr, "noip(%d): ... GETPEERNAME", pid); )
    }
  });
  D( dump_addrresult(rc, sa, *len); )
  return (0);
}

int getsockopt(int sk, int lev, int opt, void *p, socklen_t *len)
{
  switch (lev) {
    case SOL_IP:
    case SOL_TCP:
    case SOL_UDP:
      if (*len > 0)
        memset(p, 0, *len);
      return (0);
  }
  return (real_getsockopt(sk, lev, opt, p, len));
}

int setsockopt(int sk, int lev, int opt, const void *p, socklen_t len)
{
  switch (lev) {
    case SOL_IP:
    case SOL_TCP:
    case SOL_UDP:
      return (0);
  }
  switch (opt) {
    case SO_BINDTODEVICE:
    case SO_ATTACH_FILTER:
    case SO_DETACH_FILTER:
      return (0);
  }
  return (real_setsockopt(sk, lev, opt, p, len));
}

int ioctl(int fd, unsigned long op, ...)
{
  va_list ap;
  void *arg;
  int sk;
  int rc;

  va_start(ap, op);
  arg = va_arg(ap, void *);

  switch (op) {
    case SIOCGIFADDR:
    case SIOCGIFBRDADDR:
    case SIOCGIFDSTADDR:
    case SIOCGIFNETMASK:
      PRESERVING_ERRNO({
        if (fixup_real_ip_socket(fd, AF_INET, &sk)) goto real;
      });
      rc = real_ioctl(sk, op, arg);
      PRESERVING_ERRNO({ close(sk); });
      break;
    default:
    real:
      rc = real_ioctl(fd, op, arg);
      break;
  }
  va_end(ap);
  return (rc);
}

/*----- Initialization ----------------------------------------------------*/

/* Clean up the socket directory, deleting stale sockets. */
static void cleanup_sockdir(void)
{
  DIR *dir;
  struct dirent *d;
  address addr;
  struct sockaddr_un sun;
  struct stat st;
  Dpid;

  if ((dir = opendir(sockdir)) == 0) return;
  sun.sun_family = AF_UNIX;
  while ((d = readdir(dir)) != 0) {
    if (d->d_name[0] == '.') continue;
    snprintf(sun.sun_path, sizeof(sun.sun_path),
             "%s/%s", sockdir, d->d_name);
    if (decode_inet_addr(&addr.sa, 0, &sun, SUN_LEN(&sun)) ||
        stat(sun.sun_path, &st) ||
        !S_ISSOCK(st.st_mode)) {
      D( fprintf(stderr, "noip(%d): ignoring unknown socketdir entry `%s'\n",
                 pid, sun.sun_path); )
      continue;
    }
    if (unix_socket_status(&sun, 0) == STALE) {
      D( fprintf(stderr, "noip(%d): clearing away stale socket %s\n",
                 pid, d->d_name); )
      unlink(sun.sun_path);
    }
  }
  closedir(dir);
}

/* Find the addresses attached to local network interfaces, and remember them
 * in a table.
 */
static void get_local_ipaddrs(void)
{
  struct ifaddrs *ifa_head, *ifa;
  ipaddr a;
  int i;
  Dpid;

  D( fprintf(stderr, "noip(%d): fetching local addresses...\n", pid); )
  if (getifaddrs(&ifa_head)) { perror("getifaddrs"); return; }
  for (n_local_ipaddrs = 0, ifa = ifa_head;
       n_local_ipaddrs < MAX_LOCAL_IPADDRS && ifa;
       ifa = ifa->ifa_next) {
    if (!ifa->ifa_addr || !family_known_p(ifa->ifa_addr->sa_family))
      continue;
    ipaddr_from_sockaddr(&a, ifa->ifa_addr);
    D({ char buf[ADDRBUFSZ];
        fprintf(stderr, "noip(%d):   local addr %s = %s", pid,
                ifa->ifa_name,
                inet_ntop(ifa->ifa_addr->sa_family, &a,
                          buf, sizeof(buf))); })
    for (i = 0; i < n_local_ipaddrs; i++) {
      if (ifa->ifa_addr->sa_family == local_ipaddrs[i].af &&
          ipaddr_equal_p(local_ipaddrs[i].af, &a, &local_ipaddrs[i].addr)) {
        D( fprintf(stderr, " (duplicate)\n"); )
        goto skip;
      }
    }
    D( fprintf(stderr, "\n"); )
    local_ipaddrs[n_local_ipaddrs].af = ifa->ifa_addr->sa_family;
    local_ipaddrs[n_local_ipaddrs].addr = a;
    n_local_ipaddrs++;
  skip:;
  }
  freeifaddrs(ifa_head);
}

/* Print the given message to standard error.  Avoids stdio. */
static void printerr(const char *p)
  { if (write(STDERR_FILENO, p, strlen(p))) ; }

/* Create the socket directory, being careful about permissions. */
static void create_sockdir(void)
{
  struct stat st;

  if (lstat(sockdir, &st)) {
    if (errno == ENOENT) {
      if (mkdir(sockdir, 0700)) {
        perror("noip: creating socketdir");
        exit(127);
      }
      if (!lstat(sockdir, &st))
        goto check;
    }
    perror("noip: checking socketdir");
    exit(127);
  }
check:
  if (!S_ISDIR(st.st_mode)) {
    printerr("noip: bad socketdir: not a directory\n");
    exit(127);
  }
  if (st.st_uid != uid) {
    printerr("noip: bad socketdir: not owner\n");
    exit(127);
  }
  if (st.st_mode & 077) {
    printerr("noip: bad socketdir: not private\n");
    exit(127);
  }
}

/* Initialization function. */
static void setup(void) __attribute__((constructor));
static void setup(void)
{
  PRESERVING_ERRNO({
    char *p;

    import();
    uid = geteuid();
    if ((p = getenv("NOIP_DEBUG")) && atoi(p))
      debug = 1;
    get_local_ipaddrs();
    readconfig();
    create_sockdir();
    cleanup_sockdir();
  });
}

/*----- That's all, folks -------------------------------------------------*/