[tripe] / pathmtu / pathmtu.c

/* -*-c-*-
 *
 * Report MTU on path to specified host
 *
 * (c) 2008 Straylight/Edgeware
 */

/*----- Licensing notice --------------------------------------------------*
 *
 * This file is part of Trivial IP Encryption (TrIPE).
 *
 * TrIPE is free software; you can redistribute it and/or modify
 * it 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.
 *
 * TrIPE is 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 TrIPE; if not, write to the Free Software Foundation,
 * Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
 */

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

#if defined(linux)
#  define _BSD_SOURCE
#endif

#include "config.h"

#include <errno.h>
#include <stddef.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <time.h>

#include <sys/types.h>
#include <sys/time.h>
#include <unistd.h>

#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <netdb.h>

#include <netinet/in_systm.h>
#include <netinet/ip.h>
#include <netinet/ip_icmp.h>
#include <netinet/udp.h>

#include <net/if.h>
#include <ifaddrs.h>
#include <sys/ioctl.h>

#include <mLib/alloc.h>
#include <mLib/bits.h>
#include <mLib/dstr.h>
#include <mLib/hex.h>
#include <mLib/mdwopt.h>
#include <mLib/quis.h>
#include <mLib/report.h>
#include <mLib/tv.h>

/*----- Static variables --------------------------------------------------*/

static unsigned char buf[65536];

#define POLY 0x1002d

/*----- Utility functions -------------------------------------------------*/

/* Step a value according to a simple LFSR. */
#define STEP(q)								\
  do (q) = ((q) & 0x8000) ? ((q) << 1) ^ POLY : ((q) << 1); while (0)

/* Fill buffer with a constant but pseudorandom string.  Uses a simple
 * LFSR.
 */
static void fillbuffer(unsigned char *p, size_t sz)
{
  unsigned int y = 0xbc20;
  const unsigned char *l = p + sz;
  int i;

  while (p < l) {
    *p++ = y & 0xff;
    for (i = 0; i < 8; i++) STEP(y);
  }
}

/* Convert a string to floating point. */
static double s2f(const char *s, const char *what)
{
  double f;
  char *q;

  errno = 0;
  f = strtod(s, &q);
  if (errno || *q) die(EXIT_FAILURE, "bad %s", what);
  return (f);
}

/* Convert a floating-point value into a struct timeval. */
static void f2tv(struct timeval *tv, double t)
  { tv->tv_sec = t; tv->tv_usec = (t - tv->tv_sec)*MILLION; }

/*----- Main algorithm skeleton -------------------------------------------*/

struct param {
  unsigned f;				/* Various flags */
#define F_VERBOSE 1u			/*   Give a running commentary */
  double retx;				/* Initial retransmit interval */
  double regr;				/* Retransmit growth factor */
  double timeout;			/* Retransmission timeout */
  int seqoff;				/* Offset to write sequence number */
  const struct probe_ops *pops;		/* Probe algorithm description */
  struct sockaddr_in sin;		/* Destination address */
};

struct probestate {
  const struct param *pp;
  unsigned q;
};

struct probe_ops {
  const char *name;
  const struct probe_ops *next;
  size_t statesz;
  int (*setup)(void *, int, const struct param *);
  void (*finish)(void *);
  void (*selprep)(void *, int *, fd_set *);
  int (*xmit)(void *, int);
  int (*selproc)(void *, fd_set *, struct probestate *);
};

#define OPS_CHAIN 0

enum {
  RC_FAIL = -99,
  RC_OK = 0,
  RC_LOWER = -1,
  RC_HIGHER = -2,
  RC_NOREPLY = -3
  /* or a positive MTU upper-bound */
};

/* Add a file descriptor FD to the set `fd_in', updating `*maxfd'. */
#define ADDFD(fd) \
  do { FD_SET(fd, fd_in); if (*maxfd < fd) *maxfd = fd; } while (0)

/* Check whether a buffer contains a packet from our current probe. */
static int mypacketp(struct probestate *ps,
		     const unsigned char *p, size_t sz)
{
  const struct param *pp = ps->pp;

  return (sz >= pp->seqoff + 2 && LOAD16(p + pp->seqoff) == ps->q);
}

/* See whether MTU is an acceptable MTU value.  Return an appropriate
 * RC_... code or a new suggested MTU.
 */
static int probe(struct probestate *ps, void *st, int mtu)
{
  const struct param *pp = ps->pp;
  fd_set fd_in;
  struct timeval tv, now, when, done;
  double timer = pp->retx;
  int rc, maxfd;

  /* Set up the first retransmit and give-up timers. */
  gettimeofday(&now, 0);
  f2tv(&tv, pp->timeout); TV_ADD(&done, &now, &tv);
  f2tv(&tv, timer); TV_ADD(&when, &now, &tv);
  if (TV_CMP(&when, >, &done)) when = done;

  /* Send the initial probe. */
  if (pp->f & F_VERBOSE)
    moan("sending probe of size %d (seq = %04x)", mtu, ps->q);
  STEP(ps->q);
  STORE16(buf + pp->seqoff, ps->q);
  if ((rc = pp->pops->xmit(st, mtu)) != RC_OK) return (rc);

  for (;;) {

    /* Wait for something interesting to happen. */
    maxfd = 0; FD_ZERO(&fd_in);
    pp->pops->selprep(st, &maxfd, &fd_in);
    TV_SUB(&tv, &when, &now);
    if (select(maxfd + 1, &fd_in, 0, 0, &tv) < 0) return (RC_FAIL);
    gettimeofday(&now, 0);

    /* See whether the probe method has any answers for us. */
    if ((rc = pp->pops->selproc(st, &fd_in, ps)) != RC_OK) return (rc);

    /* If we've waited too long, give up.  If we should retransmit, do
     * that.
     */
    if (TV_CMP(&now, >, &done))
      return (RC_NOREPLY);
    else if (TV_CMP(&now, >, &when)) {
      if (pp->f & F_VERBOSE) moan("re-sending probe of size %d", mtu);
      if ((rc = pp->pops->xmit(st, mtu)) != RC_OK) return (rc);
      do {
	timer *= pp->regr; f2tv(&tv, timer); TV_ADD(&when, &when, &tv);
      } while (TV_CMP(&when, <, &now));
      if (TV_CMP(&when, >, &done)) when = done;
    }
  }
}

/* Discover the path MTU to the destination address. */
static int pathmtu(const struct param *pp)
{
  int sk;
  int mtu, lo, hi;
  int rc, droppy = -1;
  void *st;
  struct probestate ps;

  /* Build and connect a UDP socket.  We'll need this to know the local port
   * number to use if nothing else.  Set other stuff up.
   */
  if ((sk = socket(PF_INET, SOCK_DGRAM, 0)) < 0) goto fail_0;
  if (connect(sk, (struct sockaddr *)&pp->sin, sizeof(pp->sin))) goto fail_1;
  st = xmalloc(pp->pops->statesz);
  if ((mtu = pp->pops->setup(st, sk, pp)) < 0) goto fail_2;
  ps.pp = pp; ps.q = rand() & 0xffff;
  lo = 576; hi = mtu;

  /* And now we do a thing which is sort of like a binary search, except that
   * we also take explicit clues as establishing a new upper bound, and we
   * try to hug that initially.
   */
  for (;;) {
    rc = probe(&ps, st, mtu);
    switch (rc) {

      case RC_FAIL:
	if (pp->f & F_VERBOSE) moan("probe failed");
	goto fail_3;

      case RC_NOREPLY:
	/* If we've not seen a dropped packet before then we don't know what
	 * this means yet -- in particular, we don't know which bit of the
	 * network is swallowing packets.  Send a minimum-size probe.  If
	 * that doesn't come back then assume that the remote host is
	 * swallowing our packets.  If it does, then we assume that dropped
	 * packets are a result of ICMP fragmentation-needed reports being
	 * lost or suppressed.
	 */
	if (pp->f & F_VERBOSE) moan("gave up: black hole detected");
	if (droppy == -1) {
	  if (pp->f & F_VERBOSE) moan("sending minimum-size probe");
	  switch (probe(&ps, st, lo)) {
	    case RC_FAIL:
	      goto fail_3;
	    case RC_NOREPLY:
	      if (pp->f & F_VERBOSE) {
		moan("no reply from min-size probe: "
		     "assume black hole at target");
	      }
	      droppy = 1;
	      break;
	    case RC_HIGHER:
	      if (pp->f & F_VERBOSE) {
		moan("reply from min-size probe OK: "
		     "assume black hole in network");
	      }
	      droppy = 0;
	      break;
	    default:
	      if (pp->f & F_VERBOSE)
		moan("unexpected return code from probe");
	      errno = ENOTCONN;
	      goto fail_3;
	  }
	}

	if (droppy) goto higher; else goto lower;

      case RC_HIGHER:
      higher:
	if (droppy == -1) {
	  if (pp->f & F_VERBOSE)
	    moan("probe returned: remote host is not a black hole");
	  droppy = 0;
	}
	if (mtu == hi) {
	  if (pp->f & F_VERBOSE) moan("probe returned: found correct MTU");
	  goto done;
	}
	if (pp->f & F_VERBOSE) moan("probe returned: guessing higher");
	lo = mtu;
	mtu += (hi - lo + 1)/2;
	break;

      case RC_LOWER:
      lower:
	if (mtu == lo) {
	  if (pp->f & F_VERBOSE) moan("error returned: found correct MTU");
	  goto done;
	}
	if (pp->f & F_VERBOSE) moan("error returned: guessing lower");
	hi = mtu - 1;
	mtu -= (hi - lo + 1)/2;
	break;

      default:
	if (pp->f & F_VERBOSE) moan("error returned with new MTU estimate");
	mtu = hi = rc;
	break;
    }
  }

done:
  /* Clean up and return our result. */
  pp->pops->finish(st);
  xfree(st);
  close(sk);
  return (mtu);

fail_3:
  pp->pops->finish(st);
fail_2:
  xfree(st);
fail_1:
  close(sk);
fail_0:
  return (-1);
}

/*----- Doing it the hard way ---------------------------------------------*/

#if defined(linux) || defined(__OpenBSD__)
#define IPHDR_SANE
#endif

#ifdef IPHDR_SANE
#  define sane_htons htons
#  define sane_htonl htonl
#else
#  define sane_htons
#  define sane_htonl
#endif

static int rawicmp = -1, rawudp = -1, rawerr = 0;

#define IPCK_INIT 0xffff

/* Compute an IP checksum over some data.  This is a restartable interface:
 * initialize A to `IPCK_INIT' for the first call.
 */
static unsigned ipcksum(const void *buf, size_t n, unsigned a)
{
  unsigned long aa = a ^ 0xffff;
  const unsigned char *p = buf, *l = p + n;

  while (p < l - 1) { aa += LOAD16_B(p); p += 2; }
  if (p < l) { aa += (unsigned)(*p) << 8; }
  do aa = (aa & 0xffff) + (aa >> 16); while (aa >= 0x10000);
  return (aa == 0xffff ? aa : aa ^ 0xffff);
}

/* TCP/UDP pseudoheader structure. */
struct phdr {
  struct in_addr ph_src, ph_dst;
  u_char ph_z, ph_p;
  u_short ph_len;
};

struct raw_state {
  struct sockaddr_in me, sin;
  int sk, rawicmp, rawudp;
  unsigned q;
};

static int raw_setup(void *stv, int sk, const struct param *pp)
{
  struct raw_state *st = stv;
  size_t sz;
  int i, mtu = -1;
  struct ifaddrs *ifa, *ifaa, *ifap;
  struct ifreq ifr;

  /* If we couldn't acquire raw sockets, we fail here. */
  if (rawerr) { errno = rawerr; goto fail_0; }
  st->rawicmp = rawicmp; st->rawudp = rawudp; st->sk = sk;

  /* Initialize the sequence number. */
  st->q = rand() & 0xffff;

  /* Snaffle the local and remote address and port number. */
  st->sin = pp->sin;
  sz = sizeof(st->me);
  if (getsockname(sk, (struct sockaddr *)&st->me, &sz))
    goto fail_0;

  /* There isn't a portable way to force the DF flag onto a packet through
   * UDP, or even through raw IP, unless we write the entire IP header
   * ourselves.  This is somewhat annoying, especially since we have an
   * uphill struggle keeping track of which systems randomly expect which
   * header fields to be presented in host byte order.  Oh, well.
   */
  i = 1;
  if (setsockopt(rawudp, IPPROTO_IP, IP_HDRINCL, &i, sizeof(i))) goto fail_0;

  /* Find an upper bound on the MTU.  Do two passes over the interface
   * list.  If we can find matches for our local address then use the
   * highest one of those; otherwise do a second pass and simply take the
   * highest MTU of any network interface.
   */
  if (getifaddrs(&ifaa)) goto fail_0;
  for (i = 0; i < 2; i++) {
    for (ifap = 0, ifa = ifaa; ifa; ifa = ifa->ifa_next) {
      if (!(ifa->ifa_flags & IFF_UP) || !ifa->ifa_addr ||
	  ifa->ifa_addr->sa_family != AF_INET ||
	  (i == 0 &&
	   ((struct sockaddr_in *)ifa->ifa_addr)->sin_addr.s_addr !=
		st->me.sin_addr.s_addr) ||
	  (i == 1 && ifap && strcmp(ifap->ifa_name, ifa->ifa_name) == 0) ||
	  strlen(ifa->ifa_name) >= sizeof(ifr.ifr_name))
	continue;
      ifap = ifa;
      strcpy(ifr.ifr_name, ifa->ifa_name);
      if (ioctl(sk, SIOCGIFMTU, &ifr)) goto fail_1;
      if (mtu < ifr.ifr_mtu) mtu = ifr.ifr_mtu;
    }
    if (mtu > 0) break;
  }
  if (mtu < 0) { errno = ENOTCONN; goto fail_1; }
  freeifaddrs(ifaa);

  /* Done. */
  return (mtu);

fail_1:
  freeifaddrs(ifaa);
fail_0:
  return (-1);
}

static void raw_finish(void *stv) { ; }

static void raw_selprep(void *stv, int *maxfd, fd_set *fd_in)
  { struct raw_state *st = stv; ADDFD(st->sk); ADDFD(st->rawicmp); }

static int raw_xmit(void *stv, int mtu)
{
  struct raw_state *st = stv;
  unsigned char b[65536], *p;
  struct ip *ip;
  struct udphdr *udp;
  struct phdr ph;
  unsigned ck;

  /* Build the IP header. */
  ip = (struct ip *)b;
  ip->ip_v = 4;
  ip->ip_hl = sizeof(*ip)/4;
  ip->ip_tos = IPTOS_RELIABILITY;
  ip->ip_len = sane_htons(mtu);
  STEP(st->q); ip->ip_id = htons(st->q);
  ip->ip_off = sane_htons(0 | IP_DF);
  ip->ip_ttl = 64;
  ip->ip_p = IPPROTO_UDP;
  ip->ip_sum = 0;
  ip->ip_src = st->me.sin_addr;
  ip->ip_dst = st->sin.sin_addr;

  /* Build a UDP packet in the output buffer. */
  udp = (struct udphdr *)(ip + 1);
  udp->uh_sport = st->me.sin_port;
  udp->uh_dport = st->sin.sin_port;
  udp->uh_ulen = htons(mtu - sizeof(*ip));
  udp->uh_sum = 0;

  /* Copy the payload. */
  p = (unsigned char *)(udp + 1);
  memcpy(p, buf, mtu - (p - b));

  /* Calculate the UDP checksum. */
  ph.ph_src = ip->ip_src;
  ph.ph_dst = ip->ip_dst;
  ph.ph_z = 0;
  ph.ph_p = IPPROTO_UDP;
  ph.ph_len = udp->uh_ulen;
  ck = IPCK_INIT;
  ck = ipcksum(&ph, sizeof(ph), ck);
  ck = ipcksum(udp, mtu - sizeof(*ip), ck);
  udp->uh_sum = htons(ck);

  /* Send the whole thing off.  If we're too big for the interface then we
   * might need to trim immediately.
   */
  if (sendto(st->rawudp, b, mtu, 0,
	     (struct sockaddr *)&st->sin, sizeof(st->sin)) < 0) {
    if (errno == EMSGSIZE) return (RC_LOWER);
    else goto fail_0;
  }

  /* Done. */
  return (RC_OK);

fail_0:
  return (RC_FAIL);
}

static int raw_selproc(void *stv, fd_set *fd_in, struct probestate *ps)
{
  struct raw_state *st = stv;
  unsigned char b[65536];
  struct ip *ip;
  struct icmp *icmp;
  struct udphdr *udp;
  ssize_t n;

  /* An ICMP packet: see what's inside. */
  if (FD_ISSET(st->rawicmp, fd_in)) {
    if ((n = read(st->rawicmp, b, sizeof(b))) < 0) goto fail_0;

    ip = (struct ip *)b;
    if (n < sizeof(*ip) || n < sizeof(4*ip->ip_hl) ||
	ip->ip_v != 4 || ip->ip_p != IPPROTO_ICMP)
      goto skip_icmp;
    n -= sizeof(4*ip->ip_hl);

    icmp = (struct icmp *)(b + 4*ip->ip_hl);
    if (n < sizeof(*icmp) || icmp->icmp_type != ICMP_UNREACH)
      goto skip_icmp;
    n -= offsetof(struct icmp, icmp_ip);

    ip = &icmp->icmp_ip;
    if (n < sizeof(*ip) ||
	ip->ip_p != IPPROTO_UDP || ip->ip_hl != sizeof(*ip)/4 ||
	ip->ip_id != htons(st->q) ||
	ip->ip_src.s_addr != st->me.sin_addr.s_addr ||
	ip->ip_dst.s_addr != st->sin.sin_addr.s_addr)
      goto skip_icmp;
    n -= sizeof(*ip);

    udp = (struct udphdr *)(ip + 1);
    if (n < sizeof(udp) || udp->uh_sport != st->me.sin_port ||
	udp->uh_dport != st->sin.sin_port)
      goto skip_icmp;
    n -= sizeof(*udp);

    if (icmp->icmp_code == ICMP_UNREACH_PORT) return (RC_HIGHER);
    else if (icmp->icmp_code != ICMP_UNREACH_NEEDFRAG) goto skip_icmp;
    else if (icmp->icmp_nextmtu) return (htons(icmp->icmp_nextmtu));
    else return (RC_LOWER);
  }
skip_icmp:;

  /* If we got a reply to the current probe then we're good.  If we got an
   * error, or the packet's sequence number is wrong, then ignore it.
   */
  if (FD_ISSET(st->sk, fd_in)) {
    if ((n = read(st->sk, b, sizeof(b))) < 0) return (RC_OK);
    else if (mypacketp(ps, b, n)) return (RC_HIGHER);
    else return (RC_OK);
  }

  return (RC_OK);

fail_0:
  return (RC_FAIL);
}

static const struct probe_ops raw_ops = {
  "raw", OPS_CHAIN, sizeof(struct raw_state),
  raw_setup, raw_finish,
  raw_selprep, raw_xmit, raw_selproc
};

#undef OPS_CHAIN
#define OPS_CHAIN &raw_ops

/*----- Doing the job on Linux --------------------------------------------*/

#if defined(linux)

#ifndef IP_MTU
#  define IP_MTU 14			/* Blech! */
#endif

struct linux_state {
  int sk;
};

static int linux_setup(void *stv, int sk, const struct param *pp)
{
  struct linux_state *st = stv;
  int i, mtu;
  size_t sz;

  /* Snaffle the UDP socket. */
  st->sk = sk;

  /* Turn on kernel path-MTU discovery and force DF on. */
  i = IP_PMTUDISC_DO;
  if (setsockopt(st->sk, IPPROTO_IP, IP_MTU_DISCOVER, &i, sizeof(i)))
    return (-1);

  /* Read the initial MTU guess back and report it. */
  sz = sizeof(mtu);
  if (getsockopt(st->sk, IPPROTO_IP, IP_MTU, &mtu, &sz))
    return (-1);

  /* Done. */
  return (mtu);
}

static void linux_finish(void *stv) { ; }

static void linux_selprep(void *stv, int *maxfd, fd_set *fd_in)
  { struct linux_state *st = stv; ADDFD(st->sk); }

static int linux_xmit(void *stv, int mtu)
{
  struct linux_state *st = stv;

  /* Write the packet. */
  if (write(st->sk, buf, mtu - 28) >= 0) return (RC_OK);
  else if (errno == EMSGSIZE) return (RC_LOWER);
  else return (RC_FAIL);
}

static int linux_selproc(void *stv, fd_set *fd_in, struct probestate *ps)
{
  struct linux_state *st = stv;
  int mtu;
  size_t sz;
  ssize_t n;
  unsigned char b[65536];

  /* Read an answer.  If it looks like the right kind of error then report a
   * success.  This is potentially wrong, since we can't tell whether an
   * error was delayed from an earlier probe.  However, we never return
   * RC_LOWER from this method, so the packet sizes ought to be monotonically
   * decreasing and this won't cause trouble.  Otherwise update from the
   * kernel's idea of the right MTU.
   */
  if (FD_ISSET(st->sk, fd_in)) {
    n = read(st->sk, &buf, sizeof(buf));
    if (n >= 0 ?
	mypacketp(ps, b, n) :
	errno == ECONNREFUSED || errno == EHOSTUNREACH)
      return (RC_HIGHER);
    sz = sizeof(mtu);
    if (getsockopt(st->sk, IPPROTO_IP, IP_MTU, &mtu, &sz))
      return (RC_FAIL);
    return (mtu);
  }
  return (RC_OK);
}

static const struct probe_ops linux_ops = {
  "linux", OPS_CHAIN, sizeof(struct linux_state),
  linux_setup, linux_finish,
  linux_selprep, linux_xmit, linux_selproc
};

#undef OPS_CHAIN
#define OPS_CHAIN &linux_ops

#endif

/*----- Help options ------------------------------------------------------*/

static const struct probe_ops *probe_ops = OPS_CHAIN;

static void version(FILE *fp)
  { pquis(fp, "$, TrIPE version " VERSION "\n"); }

static void usage(FILE *fp)
{
  pquis(fp, "Usage: $ [-H HEADER] [-m METHOD]\n\
	 [-r SECS] [-g FACTOR] [-t SECS] HOST [PORT]\n");
}

static void help(FILE *fp)
{
  const struct probe_ops *ops;

  version(fp);
  fputc('\n', fp);
  usage(fp);
  fputs("\
\n\
Options in full:\n\
\n\
-h, --help		Show this help text.\n\
-v, --version		Show version number.\n\
-u, --usage		Show brief usage message.\n\
\n\
-g, --growth=FACTOR	Growth factor for retransmit interval.\n\
-m, --method=METHOD	Use METHOD to probe for MTU.\n\
-r, --retransmit=SECS	Retransmit if no reply after SEC.\n\
-t, --timeout=SECS	Give up expecting a reply after SECS.\n\
-H, --header=HEX	Packet header, in hexadecimal.\n\
\n\
Probe methods:\n\
", fp);
  for (ops = probe_ops; ops; ops = ops->next)
    printf("\t%s\n", ops->name);
}

/*----- Main code ---------------------------------------------------------*/

int main(int argc, char *argv[])
{
  struct param pp = { 0, 0.333, 3.0, 8.0, 0, OPS_CHAIN };
  hex_ctx hc;
  dstr d = DSTR_INIT;
  size_t sz;
  int i;
  unsigned long u;
  char *q;
  struct hostent *h;
  struct servent *s;
  unsigned f = 0;

#define f_bogus 1u

  if ((rawicmp = socket(PF_INET, SOCK_RAW, IPPROTO_ICMP)) < 0 ||
      (rawudp = socket(PF_INET, SOCK_RAW, IPPROTO_UDP)) < 0)
    rawerr = errno;
  if (setuid(getuid()))
    abort();

  ego(argv[0]);
  fillbuffer(buf, sizeof(buf));
  pp.sin.sin_port = htons(7);

  for (;;) {
    static const struct option opts[] = {
      { "help",		0,		0,	'h' },
      { "version",	0,		0,	'V' },
      { "usage",	0,		0,	'u' },
      { "header",	OPTF_ARGREQ,	0,	'H' },
      { "growth",	OPTF_ARGREQ,	0,	'g' },
      { "method",	OPTF_ARGREQ,	0,	'm' },
      { "retransmit",	OPTF_ARGREQ,	0,	'r' },
      { "timeout",	OPTF_ARGREQ,	0,	't' },
      { "verbose",	0,		0,	'v' },
      { 0,		0,		0,	0 }
    };

    i = mdwopt(argc, argv, "hVu" "H:g:m:r:t:v", opts, 0, 0, 0);
    if (i < 0) break;
    switch (i) {
      case 'h': help(stdout); exit(0);
      case 'V': version(stdout); exit(0);
      case 'u': usage(stdout); exit(0);

      case 'H':
	DRESET(&d);
	hex_init(&hc);
	hex_decode(&hc, optarg, strlen(optarg), &d);
	hex_decode(&hc, 0, 0, &d);
	sz = d.len < 532 ? d.len : 532;
	memcpy(buf, d.buf, sz);
	pp.seqoff = sz;
	break;

      case 'g': pp.regr = s2f(optarg, "retransmit growth factor"); break;
      case 'r': pp.retx = s2f(optarg, "retransmit interval"); break;
      case 't': pp.timeout = s2f(optarg, "timeout"); break;

      case 'm':
	for (pp.pops = OPS_CHAIN; pp.pops; pp.pops = pp.pops->next)
	  if (strcmp(pp.pops->name, optarg) == 0) goto found_alg;
	die(EXIT_FAILURE, "unknown probe algorithm `%s'", optarg);
      found_alg:
	break;

      case 'v': pp.f |= F_VERBOSE; break;

      default:
	f |= f_bogus;
	break;
    }
  }
  argv += optind; argc -= optind;
  if ((f & f_bogus) || 1 > argc || argc > 2) {
    usage(stderr);
    exit(EXIT_FAILURE);
  }

  if ((h = gethostbyname(*argv)) == 0)
    die(EXIT_FAILURE, "unknown host `%s': %s", *argv, hstrerror(h_errno));
  if (h->h_addrtype != AF_INET)
    die(EXIT_FAILURE, "unsupported address family for host `%s'", *argv);
  memcpy(&pp.sin.sin_addr, h->h_addr, sizeof(struct in_addr));
  argv++; argc--;

  if (*argv) {
    errno = 0;
    u = strtoul(*argv, &q, 0);
    if (!errno && !*q)
      pp.sin.sin_port = htons(u);
    else if ((s = getservbyname(*argv, "udp")) == 0)
      die(EXIT_FAILURE, "unknown UDP service `%s'", *argv);
    else
      pp.sin.sin_port = s->s_port;
  }

  pp.sin.sin_family = AF_INET;
  i = pathmtu(&pp);
  if (i < 0)
    die(EXIT_FAILURE, "failed to discover MTU: %s", strerror(errno));
  printf("%d\n", i);
  if (ferror(stdout) || fflush(stdout) || fclose(stdout))
    die(EXIT_FAILURE, "failed to write result: %s", strerror(errno));
  return (0);
}

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