
/***************************************************************************
 * tcpip.cc -- Various functions relating to low level TCP/IP handling,    *
 * including sending raw packets, routing, printing packets, reading from  *
 * libpcap, etc.                                                           *
 *                                                                         *
 ***********************IMPORTANT NMAP LICENSE TERMS************************
 *
 * The Nmap Security Scanner is (C) 1996-2026 Nmap Software LLC ("The Nmap
 * Project"). Nmap is also a registered trademark of the Nmap Project.
 *
 * This program is distributed under the terms of the Nmap Public Source
 * License (NPSL). The exact license text applying to a particular Nmap
 * release or source code control revision is contained in the LICENSE
 * file distributed with that version of Nmap or source code control
 * revision. More Nmap copyright/legal information is available from
 * https://nmap.org/book/man-legal.html, and further information on the
 * NPSL license itself can be found at https://nmap.org/npsl/ . This
 * header summarizes some key points from the Nmap license, but is no
 * substitute for the actual license text.
 *
 * Nmap is generally free for end users to download and use themselves,
 * including commercial use. It is available from https://nmap.org.
 *
 * The Nmap license generally prohibits companies from using and
 * redistributing Nmap in commercial products, but we sell a special Nmap
 * OEM Edition with a more permissive license and special features for
 * this purpose. See https://nmap.org/oem/
 *
 * If you have received a written Nmap license agreement or contract
 * stating terms other than these (such as an Nmap OEM license), you may
 * choose to use and redistribute Nmap under those terms instead.
 *
 * The official Nmap Windows builds include the Npcap software
 * (https://npcap.com) for packet capture and transmission. It is under
 * separate license terms which forbid redistribution without special
 * permission. So the official Nmap Windows builds may not be redistributed
 * without special permission (such as an Nmap OEM license).
 *
 * Source is provided to this software because we believe users have a
 * right to know exactly what a program is going to do before they run it.
 * This also allows you to audit the software for security holes.
 *
 * Source code also allows you to port Nmap to new platforms, fix bugs, and
 * add new features. You are highly encouraged to submit your changes as a
 * Github PR or by email to the dev@nmap.org mailing list for possible
 * incorporation into the main distribution. Unless you specify otherwise, it
 * is understood that you are offering us very broad rights to use your
 * submissions as described in the Nmap Public Source License Contributor
 * Agreement. This is important because we fund the project by selling licenses
 * with various terms, and also because the inability to relicense code has
 * caused devastating problems for other Free Software projects (such as KDE
 * and NASM).
 *
 * The free version of Nmap 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. Warranties,
 * indemnification and commercial support are all available through the
 * Npcap OEM program--see https://nmap.org/oem/
 *
 ***************************************************************************/

/* $Id: tcpip.cc 39394 2026-03-24 21:47:15Z dmiller $ */

#include "nmap.h"

#include <locale.h>
#include "nbase.h"
#include <dnet.h>
#include "tcpip.h"
#include "NmapOps.h"
#include "Target.h"
#include "utils.h"
#include "nmap_error.h"
#include "libnetutil/netutil.h"

#include "struct_ip.h"

#if HAVE_NETINET_IF_ETHER_H
#ifndef NETINET_IF_ETHER_H
#include <netinet/if_ether.h>
#define NETINET_IF_ETHER_H
#endif /* NETINET_IF_ETHER_H */
#endif /* HAVE_NETINET_IF_ETHER_H */

extern NmapOps o;

static PacketCounter PktCt;

/* Fill buf (up to buflen -- truncate if necessary but always
   terminate) with a short representation of the packet stats.
   Returns buf.  Aborts if there is a problem. */
char *getFinalPacketStats(char *buf, int buflen) {
  char sendbytesasc[16], recvbytesasc[16];

  if (buflen <= 10 || !buf)
    fatal("%s called with woefully inadequate parameters", __func__);

  Snprintf(buf, buflen,
#if WIN32
           "Raw packets sent: %I64u (%s) | Rcvd: %I64u (%s)",
#else
           "Raw packets sent: %llu (%s) | Rcvd: %llu (%s)",
#endif
           PktCt.sendPackets,
           format_bytecount(PktCt.sendBytes, sendbytesasc,
                            sizeof(sendbytesasc)), PktCt.recvPackets,
           format_bytecount(PktCt.recvBytes, recvbytesasc,
                            sizeof(recvbytesasc)));
  return buf;
}

/* Takes an ARP PACKET (not including ethernet header) and
   prints it if packet tracing is enabled. The
   direction must be PacketTrace::SENT or PacketTrace::RCVD .
   Optional 'now' argument makes this function slightly more
   efficient by avoiding a gettimeofday() call. */
void PacketTrace::traceArp(pdirection pdir, const u8 *frame, u32 len,
                           struct timeval *now) {
  struct timeval tv;
  char arpdesc[128];
  char who_has[INET_ADDRSTRLEN], tell[INET_ADDRSTRLEN];

  if (pdir == SENT) {
    PktCt.sendPackets++;
    PktCt.sendBytes += len;
  } else {
    PktCt.recvPackets++;
    PktCt.recvBytes += len;
  }

  if (!o.packetTrace())
    return;

  if (now)
    tv = *now;
  else
    gettimeofday(&tv, NULL);

  if (len < 28) {
    error("Packet tracer: Arp packets must be at least 28 bytes long.  Should be exactly that length excl. ethernet padding.");
    return;
  }

  if (frame[7] == 1) { /* arp REQUEST */
    inet_ntop(AF_INET, (void *)(frame + 24), who_has, sizeof(who_has));
    inet_ntop(AF_INET, (void *)(frame + 14), tell, sizeof(tell));
    Snprintf(arpdesc, sizeof(arpdesc), "who-has %s tell %s", who_has, tell);
  } else { /* ARP REPLY */
    inet_ntop(AF_INET, (void *)(frame + 14), who_has, sizeof(who_has));
    Snprintf(arpdesc, sizeof(arpdesc),
             "reply %s is-at %02X:%02X:%02X:%02X:%02X:%02X", who_has,
             frame[8], frame[9], frame[10], frame[11], frame[12],
             frame[13]);
  }

  log_write(LOG_STDOUT | LOG_NORMAL, "%s (%.4fs) ARP %s\n",
            (pdir == SENT) ? "SENT" : "RCVD",
            o.TimeSinceStart(&tv), arpdesc);

  return;
}

/* Takes a Neighbor Discovery packet and prints it if packet tracing is
   enabled. frame must point to the IPv6 header. */
void PacketTrace::traceND(pdirection pdir, const u8 *frame, u32 len,
                          struct timeval *now) {
  struct timeval tv;
  const struct ip6_hdr *ip6;
  const struct icmpv6_hdr *icmpv6;
  const union icmpv6_msg *msg;
  size_t msg_len;
  const char *label;
  char src[INET6_ADDRSTRLEN], dst[INET6_ADDRSTRLEN];
  char who_has[INET6_ADDRSTRLEN], tgt_is[INET6_ADDRSTRLEN];
  char desc[128];

  if (pdir == SENT) {
    PktCt.sendPackets++;
    PktCt.sendBytes += len;
  } else {
    PktCt.recvPackets++;
    PktCt.recvBytes += len;
  }

  if (!o.packetTrace())
    return;

  if (now)
    tv = *now;
  else
    gettimeofday(&tv, NULL);

  if (len < sizeof(*ip6) + sizeof(*icmpv6)) {
    error("Packet tracer: ND packets must be at least %lu bytes long (is %lu).",
          (unsigned long) (sizeof(*ip6) + sizeof(*icmpv6)),
          (unsigned long) len);
    return;
  }
  ip6 = (struct ip6_hdr *) frame;
  icmpv6 = (struct icmpv6_hdr *) (frame + sizeof(*ip6));
  msg = (union icmpv6_msg *) (frame + sizeof(*ip6) + sizeof(*icmpv6));
  msg_len = frame + len - (u8 *) msg;

  if (icmpv6->icmpv6_type == ICMPV6_NEIGHBOR_SOLICITATION) {
    label = "neighbor solicitation";
    if (msg_len < 20) {
      Snprintf(desc, sizeof(desc), "packet too short");
    } else {
      inet_ntop(AF_INET6, (void *)&msg->nd.icmpv6_target, who_has, sizeof(who_has));
      Snprintf(desc, sizeof(desc), "who has %s", who_has);
    }
  } else if (icmpv6->icmpv6_type == ICMPV6_NEIGHBOR_ADVERTISEMENT) {
    label = "neighbor advertisement";
    if (msg_len < 28) {
      Snprintf(desc, sizeof(desc), "packet too short");
    } else if (msg->nd.icmpv6_option_length == 0 || msg->nd.icmpv6_option_type != 2) {
      /* We only handle target link-layer address in the first option. */
      Snprintf(desc, sizeof(desc), "no link-layer address");
    } else {
      inet_ntop(AF_INET6, (void *)&msg->nd.icmpv6_target, tgt_is, sizeof(tgt_is));
      Snprintf(desc, sizeof(desc), "%s is at %s",
               tgt_is, eth_ntoa(&msg->nd.icmpv6_mac));
    }
  } else {
    error("Unknown ICMPV6 type in %s.", __func__);
    return;
  }

  inet_ntop(AF_INET6, (void *)&ip6->ip6_src, src, sizeof(src));
  inet_ntop(AF_INET6, (void *)&ip6->ip6_dst, dst, sizeof(dst));
  log_write(LOG_STDOUT | LOG_NORMAL, "%s (%.4fs) %s %s > %s %s\n",
            (pdir == SENT) ? "SENT" : "RCVD",
            o.TimeSinceStart(&tv), label, src, dst, desc);

  return;
}


/* Returns a buffer of ASCII information about a packet that may look
   like "TCP 127.0.0.1:50923 > 127.0.0.1:3 S ttl=61 id=39516 iplen=40
   seq=625950769" or "ICMP PING (0/1) ttl=61 id=39516 iplen=40".
   IMPORTANT: This is a wrapper for function ippackethdrinfo(). Check
   nbase/nbase_net.c for details on the returned buffer. */
static const char *nmap_format_ippacket(const u8 *packet, u32 len) {
  int detail = LOW_DETAIL;
  if (o.debugging == 2) {
    detail = MEDIUM_DETAIL;
  } else if (o.debugging >= 3) {
    detail = HIGH_DETAIL;
  }
  return ippackethdrinfo(packet, len, detail);
}




/* Takes an IP PACKET and prints it if packet tracing is enabled.
   'packet' must point to the IPv4 header. The direction must be
   PacketTrace::SENT or PacketTrace::RCVD .  Optional 'now' argument
   makes this function slightly more efficient by avoiding a gettimeofday()
   call. */
void PacketTrace::trace(pdirection pdir, const u8 *packet, u32 len,
                        struct timeval *now) {
  struct timeval tv;

  if (pdir == SENT) {
    PktCt.sendPackets++;
    PktCt.sendBytes += len;
  } else {
    PktCt.recvPackets++;
    PktCt.recvBytes += len;
  }

  if (!o.packetTrace())
    return;

  if (now)
    tv = *now;
  else
    gettimeofday(&tv, NULL);

  if (len < 20) {
    error("Packet tracer: tiny packet encountered");
    return;
  }

  log_write(LOG_STDOUT | LOG_NORMAL, "%s (%.4fs) %s\n",
            (pdir == SENT) ? "SENT" : "RCVD",
            o.TimeSinceStart(&tv), nmap_format_ippacket(packet, len));

  return;
}

/* Adds a trace entry when a connect() is attempted if packet tracing
   is enabled.  Pass IPPROTO_TCP or IPPROTO_UDP as the protocol.  The
   sock may be a sockaddr_in or sockaddr_in6.  The return code of
   connect is passed in connectrc.  If the return code is -1, get the
   errno and pass that as connect_errno. */
void PacketTrace::traceConnect(u8 proto, const struct sockaddr *sock,
                               int socklen, int connectrc,
                               int connect_errno,
                               const struct timeval *now) {
  const struct sockaddr_in *sin = (struct sockaddr_in *) sock;
#if HAVE_IPV6
  const struct sockaddr_in6 *sin6 = (struct sockaddr_in6 *) sock;
#endif
  struct timeval tv;
  char errbuf[64] = "";
  char targetipstr[INET6_ADDRSTRLEN] = "";
  u16 targetport = 0;

  if (!o.packetTrace())
    return;

  if (now)
    tv = *now;
  else
    gettimeofday(&tv, NULL);

  assert(proto == IPPROTO_TCP || proto == IPPROTO_UDP);

  if (connectrc == 0) {
    Strncpy(errbuf, "Connected", sizeof(errbuf));
  }
#if WIN32
  else if (connect_errno == WSAEWOULDBLOCK) {
    /* Special case for WSAEWOULDBLOCK. socket_strerror returns the unwieldy
       "A non-blocking socket operation could not be completed immediately." */
    Strncpy(errbuf, "Operation now in progress", sizeof(errbuf));
  }
#endif
  else {
    Snprintf(errbuf, sizeof(errbuf), "%s", socket_strerror(connect_errno));
  }

  if (sin->sin_family == AF_INET) {
    if (inet_ntop(sin->sin_family, (char *) &sin->sin_addr, targetipstr,
                  sizeof(targetipstr)) == NULL)
      fatal("Failed to convert target IPv4 address to presentation format!?!");
    targetport = ntohs(sin->sin_port);
  } else {
#if HAVE_IPV6
    assert(sin->sin_family == AF_INET6);
    if (inet_ntop(sin->sin_family, (char *) &sin6->sin6_addr, targetipstr,
                  sizeof(targetipstr)) == NULL)
      fatal("Failed to convert target IPv6 address to presentation format!?!");
    targetport = ntohs(sin6->sin6_port);
#else
    assert(0);
#endif
  }

  log_write(LOG_STDOUT | LOG_NORMAL,
            "CONN (%.4fs) %s localhost > %s:%d => %s\n",
            o.TimeSinceStart(&tv),
            (proto == IPPROTO_TCP) ? "TCP" : "UDP", targetipstr,
            targetport, errbuf);
}

/* Converts an IP address given in a sockaddr_storage to an IPv4 or
   IPv6 IP address string.  Since a static buffer is returned, this is
   not thread-safe and can only be used once in calls like printf() */
const char *inet_socktop(const struct sockaddr_storage *ss) {
  static char buf[INET6_ADDRSTRLEN];
  const struct sockaddr_in *sin = (struct sockaddr_in *) ss;
#if HAVE_IPV6
  const struct sockaddr_in6 *sin6 = (struct sockaddr_in6 *) ss;
#endif

  if (inet_ntop(sin->sin_family, (sin->sin_family == AF_INET) ?
                (char *) &sin->sin_addr :
#if HAVE_IPV6
                (char *) &sin6->sin6_addr,
#else
                (char *) NULL,
#endif /* HAVE_IPV6 */
                buf, sizeof(buf)) == NULL) {
    fatal("Failed to convert target address to presentation format in %s!?!  Error: %s", __func__, strerror(socket_errno()));
  }
  return buf;
}

/* Tries to resolve the given name (or literal IP) into a sockaddr structure.
   This function calls getaddrinfo and returns the same addrinfo linked list
   that getaddrinfo produces. Returns NULL for any error or failure to resolve.
   You need to call freeaddrinfo on the result if non-NULL. */
struct addrinfo *resolve_all(const char *hostname, int pf) {
  struct addrinfo hints;
  struct addrinfo *result;
  int rc;

  memset(&hints, 0, sizeof(hints));
  hints.ai_family = pf;
  /* Otherwise we get multiple identical addresses with different socktypes. */
  hints.ai_socktype = SOCK_DGRAM;
#ifdef AI_IDN
  /* Try resolving internationalized domain names */
  hints.ai_flags = AI_IDN;
  setlocale(LC_CTYPE, "");
#endif
  rc = getaddrinfo(hostname, NULL, &hints, &result);
#ifdef AI_IDN
  setlocale(LC_CTYPE, o.locale);
#endif
  if (rc != 0){
    if (o.debugging > 1)
      error("Error resolving %s: %s", hostname, gai_strerror(rc));
    return NULL;
  }

  return result;
}


/* Send a pre-built IPv4 packet. Handles fragmentation and whether to send with
   an ethernet handle or a socket. */
static int send_ipv4_packet(int sd, const struct eth_nfo *eth,
                            const struct sockaddr_in *dst,
                            const u8 *packet, unsigned int packetlen) {
  const struct ip *ip = (struct ip *) packet;
  int res;

  assert(packet);
  assert((int) packetlen > 0);

  /* Fragmentation requested && packet is bigger than MTU */
  if (o.fragscan && !(ntohs(ip->ip_off) & IP_DF) &&
      (packetlen - ip->ip_hl * 4 > (unsigned int) o.fragscan)) {
    res = send_frag_ip_packet(sd, eth, dst, packet, packetlen, o.fragscan);
  } else {
    res = send_ip_packet_eth_or_sd(sd, eth, dst, packet, packetlen);
  }
  if (res != -1)
    PacketTrace::trace(PacketTrace::SENT, packet, packetlen);

  return res;
}

static int send_ipv6_packet(int sd, const struct eth_nfo *eth,
                            const struct sockaddr_in6 *dst,
                            const u8 *packet, unsigned int packetlen) {
  int res;

  res = send_ipv6_packet_eth_or_sd(sd, eth, dst, packet, packetlen);
  if (res != -1)
    PacketTrace::trace(PacketTrace::SENT, packet, packetlen);

  return res;
}

int send_ip_packet(int sd, const struct eth_nfo *eth,
                   const struct sockaddr_storage *dst,
                   const u8 *packet, unsigned int packetlen) {
  const struct ip *ip = (struct ip *) packet;

  /* Ensure there's enough to read ip->ip_v at least. */
  if (packetlen < 1)
    return -1;

  if (ip->ip_v == 4) {
    assert(dst->ss_family == AF_INET);
    return send_ipv4_packet(sd, eth, (struct sockaddr_in *) dst, packet, packetlen);
  } else if (ip->ip_v == 6) {
    assert(dst->ss_family == AF_INET6);
    return send_ipv6_packet(sd, eth, (struct sockaddr_in6 *) dst, packet, packetlen);
  }

  fatal("%s only understands IP versions 4 and 6 (got %u)", __func__, ip->ip_v);
}


/* Return an IPv4 pseudoheader checksum for the given protocol and data. Unlike
   ipv4_pseudoheader_cksum, this knows about STUPID_SOLARIS_CHECKSUM_BUG and
   takes care of o.badsum. */
static u16 ipv4_cksum(const struct in_addr *src, const struct in_addr *dst,
                      u8 proto, const void *data, u16 len) {
  u16 sum;

#if STUPID_SOLARIS_CHECKSUM_BUG
  sum = len;
#else
  sum = ipv4_pseudoheader_cksum(src, dst, proto, len, data);
#endif

  if (o.badsum) {
    --sum;
    if (proto == IPPROTO_UDP && sum == 0)
      sum = 0xffff; // UDP checksum=0 means no checksum
  }

  return sum;
}

/* Return an IPv6 pseudoheader checksum for the given protocol and data. Unlike
   ipv6_pseudoheader_cksum, this takes care of o.badsum. */
static u16 ipv6_cksum(const struct in6_addr *src, const struct in6_addr *dst,
                      u8 nxt, const void *data, u16 len) {
  u16 sum;

  sum = ipv6_pseudoheader_cksum(src, dst, nxt, len, data);

  if (o.badsum) {
    --sum;
    if (nxt == IPPROTO_UDP && sum == 0)
      sum = 0xffff; // UDP checksum=0 means no checksum
  }

  return sum;
}

// fill ip header. no error check.
// This function is also changing what's needed from host to network order.
static inline int fill_ip_raw(struct ip *ip, int packetlen, const u8 *ipopt,
                              int ipoptlen, int tos, int id,
                              int off, int ttl, int p,
                              const struct in_addr *ip_src,
                              const struct in_addr *ip_dst) {
  ip->ip_v = 4;
  ip->ip_hl = 5 + (ipoptlen / 4);
  ip->ip_tos = tos;
  ip->ip_len = htons(packetlen);
  ip->ip_id = htons(id);
  ip->ip_off = htons(off);
  ip->ip_ttl = ttl;
  ip->ip_p = p;
  ip->ip_src.s_addr = ip_src->s_addr;
  ip->ip_dst.s_addr = ip_dst->s_addr;

  if (ipoptlen)
    memcpy((u8 *) ip + sizeof(struct ip), ipopt, ipoptlen);

  // ip options source routing hack:
  if (ipoptlen && o.ipopt_firsthop && o.ipopt_lasthop) {
    u8 *ipo = (u8 *) ip + sizeof(struct ip);
    struct in_addr *newdst = (struct in_addr *) &ipo[o.ipopt_firsthop];
    struct in_addr *olddst = (struct in_addr *) &ipo[o.ipopt_lasthop];
    // our destination is somewhere else :)
    ip->ip_dst.s_addr = newdst->s_addr;

    // and last hop should be destination
    olddst->s_addr = ip_dst->s_addr;
  }

#if HAVE_IP_IP_SUM
  ip->ip_sum = 0;
  ip->ip_sum = in_cksum((unsigned short *) ip, sizeof(struct ip) + ipoptlen);
#endif
  return (sizeof(struct ip) + ipoptlen);
}

/* Builds an IP packet (including an IP header) by packing the fields
   with the given information.  It allocates a new buffer to store the
   packet contents, and then returns that buffer.  The packet is not
   actually sent by this function.  Caller must delete the buffer when
   finished with the packet.  The packet length is returned in
   packetlen, which must be a valid int pointer. */
u8 *build_ip_raw(const struct in_addr *source,
                 const struct in_addr *victim, u8 proto, int ttl,
                 u16 ipid, u8 tos, bool df, const u8 *ipopt, int ipoptlen,
                 const char *data, u16 datalen, u32 *outpacketlen) {
  int packetlen = sizeof(struct ip) + ipoptlen + datalen;
  u8 *packet = (u8 *) safe_malloc(packetlen);
  struct ip *ip = (struct ip *) packet;
  static int myttl = 0;

  /* check that required fields are there and not too silly */
  assert(source);
  assert(victim);
  assert(ipoptlen % 4 == 0);

  /* Time to live */
  if (ttl == -1) {
    myttl = (get_random_uint() % 23) + 37;
  } else {
    myttl = ttl;
  }

  fill_ip_raw(ip, packetlen, ipopt, ipoptlen,
              tos, ipid, df ? IP_DF : 0, myttl, proto, source, victim);

  /* We should probably copy the data over too */
  if (data && datalen)
    memcpy((u8 *) ip + sizeof(struct ip) + ipoptlen, data, datalen);

  *outpacketlen = packetlen;
  return packet;
}

u8 *build_ipv6_raw(const struct in6_addr *source,
                   const struct in6_addr *victim, u8 tc, u32 flowlabel,
                   u8 nextheader, int hoplimit,
                   const char *data, u16 datalen, u32 *outpacketlen) {
  u8 *packet;

  assert(source != NULL);
  assert(victim != NULL);

  if (hoplimit == -1)
    hoplimit = (get_random_uint() % 23) + 37;

  *outpacketlen = sizeof(struct ip6_hdr) + datalen;
  packet = (u8 *) safe_malloc(*outpacketlen);

  ip6_pack_hdr(packet, tc, flowlabel, datalen, nextheader, hoplimit, *source, *victim);
  memcpy(packet + sizeof(struct ip6_hdr), data, datalen);

  return packet;
}


/* Build a TCP packet (no IP header). Sets tcp->th_sum to 0 so it can be filled
   in by a function with knowledge of the higher-level pseudoheader. */
static u8 *build_tcp(u16 sport, u16 dport, u32 seq, u32 ack, u8 reserved,
                     u8 flags, u16 window, u16 urp,
                     const u8 *tcpopt, int tcpoptlen,
                     const char *data, u16 datalen, u32 *packetlen) {
  struct tcp_hdr *tcp;
  u8 *packet;

  if (tcpoptlen % 4 != 0)
    fatal("%s called with an option length argument of %d which is illegal because it is not divisible by 4. Just add \\0 padding to the end.", __func__, tcpoptlen);

  *packetlen = sizeof(*tcp) + tcpoptlen + datalen;
  packet = (u8 *) safe_malloc(*packetlen);
  tcp = (struct tcp_hdr *) packet;

  memset(tcp, 0, sizeof(*tcp));
  tcp->th_sport = htons(sport);
  tcp->th_dport = htons(dport);

  if (seq)
    tcp->th_seq = htonl(seq);
  else if (flags & TH_SYN)
    get_random_bytes(&(tcp->th_seq), 4);

  if (ack)
    tcp->th_ack = htonl(ack);

  if (reserved)
    tcp->th_x2 = reserved & 0x0F;
  tcp->th_off = 5 + (tcpoptlen / 4); /* words */
  tcp->th_flags = flags;

  if (window)
    tcp->th_win = htons(window);
  else
    tcp->th_win = htons(1024); /* Who cares */

  if (urp)
    tcp->th_urp = htons(urp);

  /* And the options */
  if (tcpoptlen)
    memcpy(packet + sizeof(*tcp), tcpopt, tcpoptlen);

  /* We should probably copy the data over too */
  if (data && datalen)
    memcpy(packet + sizeof(*tcp) + tcpoptlen, data, datalen);

  tcp->th_sum = 0;

  return packet;
}

/* Builds a TCP packet (including an IP header) by packing the fields
   with the given information.  It allocates a new buffer to store the
   packet contents, and then returns that buffer.  The packet is not
   actually sent by this function.  Caller must delete the buffer when
   finished with the packet.  The packet length is returned in
   packetlen, which must be a valid int pointer. */
u8 *build_tcp_raw(const struct in_addr *source,
                  const struct in_addr *victim, int ttl, u16 ipid, u8 tos,
                  bool df, const u8 *ipopt, int ipoptlen, u16 sport, u16 dport,
                  u32 seq, u32 ack, u8 reserved, u8 flags, u16 window,
                  u16 urp, const u8 *tcpopt, int tcpoptlen, const char *data,
                  u16 datalen, u32 *packetlen) {
  struct tcp_hdr *tcp;
  u32 tcplen;
  u8 *ip;

  tcp = (struct tcp_hdr *) build_tcp(sport, dport, seq, ack, reserved, flags,
                                     window, urp, tcpopt, tcpoptlen, data, datalen, &tcplen);
  tcp->th_sum = ipv4_cksum(source, victim, IPPROTO_TCP, tcp, tcplen);
  ip = build_ip_raw(source, victim, IPPROTO_TCP, ttl, ipid, tos, df,
                    ipopt, ipoptlen, (char *) tcp, tcplen, packetlen);
  free(tcp);

  return ip;
}

/* Builds a TCP packet (including an IPv6 header) by packing the fields
   with the given information.  It allocates a new buffer to store the
   packet contents, and then returns that buffer.  The packet is not
   actually sent by this function.  Caller must delete the buffer when
   finished with the packet.  The packet length is returned in
   packetlen, which must be a valid int pointer. */
u8 *build_tcp_raw_ipv6(const struct in6_addr *source,
                       const struct in6_addr *victim, u8 tc, u32 flowlabel,
                       u8 hoplimit, u16 sport, u16 dport, u32 seq, u32 ack,
                       u8 reserved, u8 flags, u16 window, u16 urp,
                       const u8 *tcpopt, int tcpoptlen, const char *data,
                       u16 datalen, u32 *packetlen) {
  struct tcp_hdr *tcp;
  u32 tcplen;
  u8 *ipv6;

  tcp = (struct tcp_hdr *) build_tcp(sport, dport, seq, ack, reserved, flags,
                                     window, urp, tcpopt, tcpoptlen, data, datalen, &tcplen);
  tcp->th_sum = ipv6_cksum(source, victim, IPPROTO_TCP, tcp, tcplen);
  ipv6 = build_ipv6_raw(source, victim, tc, flowlabel, IPPROTO_TCP, hoplimit,
                        (char *) tcp, tcplen, packetlen);
  free(tcp);

  return ipv6;
}

/* You need to call sethdrinclude(sd) on the sending sd before calling this */
int send_tcp_raw(int sd, const struct eth_nfo *eth,
                 const struct in_addr *source,
                 const struct in_addr *victim, int ttl, bool df,
                 u8 *ipops, int ipoptlen, u16 sport, u16 dport, u32 seq,
                 u32 ack, u8 reserved, u8 flags, u16 window, u16 urp,
                 u8 *options, int optlen, const char *data, u16 datalen) {
  struct sockaddr_storage dst;
  struct sockaddr_in *dst_in;
  unsigned int packetlen;
  int res = -1;

  u8 *packet = build_tcp_raw(source, victim,
                             ttl, get_random_u16(), IP_TOS_DEFAULT, df,
                             ipops, ipoptlen,
                             sport, dport,
                             seq, ack, reserved, flags, window, urp,
                             options, optlen,
                             data, datalen, &packetlen);
  if (!packet)
    return -1;
  memset(&dst, 0, sizeof(dst));
  dst_in = (struct sockaddr_in *) &dst;
  dst_in->sin_family = AF_INET;
  dst_in->sin_addr = *victim;
  res = send_ip_packet(sd, eth, &dst, packet, packetlen);

  free(packet);
  return res;
}

int send_tcp_raw_decoys(int sd, const struct eth_nfo *eth,
                        const struct in_addr *victim,
                        int ttl, bool df,
                        u8 *ipopt, int ipoptlen,
                        u16 sport, u16 dport,
                        u32 seq, u32 ack, u8 reserved, u8 flags,
                        u16 window, u16 urp, u8 *options, int optlen,
                        const char *data, u16 datalen) {
  int decoy;

  for (decoy = 0; decoy < o.numdecoys; decoy++)
    if (send_tcp_raw(sd, eth,
                     &((struct sockaddr_in *)&o.decoys[decoy])->sin_addr, victim,
                     ttl, df,
                     ipopt, ipoptlen,
                     sport, dport,
                     seq, ack, reserved, flags, window, urp,
                     options, optlen, data, datalen) == -1)
      return -1;

  return 0;
}


/* Build a UDP packet (no IP header). Sets udp->uh_sum to 0 so it can be filled
   in by a function with knowledge of the higher-level pseudoheader. */
static u8 *build_udp(u16 sport, u16 dport, const char *data, u16 datalen,
                     u32 *packetlen) {
  struct udp_hdr *udp;
  u8 *packet;

  *packetlen = sizeof(*udp) + datalen;
  packet = (u8 *) safe_malloc(*packetlen);
  udp = (struct udp_hdr *) packet;

  memset(udp, 0, sizeof(*udp));
  udp->uh_sport = htons(sport);
  udp->uh_dport = htons(dport);

  udp->uh_ulen = htons(*packetlen);
  if (data && datalen)
    memcpy(packet + sizeof(*udp), data, datalen);

  udp->uh_sum = 0;

  return packet;
}

/* Builds a UDP packet (including an IP header) by packing the fields
   with the given information.  It allocates a new buffer to store the
   packet contents, and then returns that buffer.  The packet is not
   actually sent by this function.  Caller must delete the buffer when
   finished with the packet.  The packet length is returned in
   packetlen, which must be a valid int pointer. */
u8 *build_udp_raw(const struct in_addr *source, const struct in_addr *victim,
                  int ttl, u16 ipid, u8 tos, bool df,
                  u8 *ipopt, int ipoptlen,
                  u16 sport, u16 dport,
                  const char *data, u16 datalen, u32 *packetlen) {
  struct udp_hdr *udp;
  u32 udplen;
  u8 *ip;

  udp = (struct udp_hdr *) build_udp(sport, dport, data, datalen, &udplen);
  udp->uh_sum = ipv4_cksum(source, victim, IPPROTO_UDP, udp, udplen);
  ip = build_ip_raw(source, victim, IPPROTO_UDP, ttl, ipid, tos, df,
                    ipopt, ipoptlen, (char *) udp, udplen, packetlen);
  free(udp);

  return ip;
}

/* Builds a UDP packet (including an IPv6 header) by packing the fields
   with the given information.  It allocates a new buffer to store the
   packet contents, and then returns that buffer.  The packet is not
   actually sent by this function.  Caller must delete the buffer when
   finished with the packet.  The packet length is returned in
   packetlen, which must be a valid int pointer. */
u8 *build_udp_raw_ipv6(const struct in6_addr *source,
                       const struct in6_addr *victim, u8 tc, u32 flowlabel,
                       u8 hoplimit, u16 sport, u16 dport,
                       const char *data, u16 datalen, u32 *packetlen) {
  struct udp_hdr *udp;
  u32 udplen;
  u8 *ipv6;

  udp = (struct udp_hdr *) build_udp(sport, dport, data, datalen, &udplen);
  udp->uh_sum = ipv6_cksum(source, victim, IPPROTO_UDP, udp, udplen);
  ipv6 = build_ipv6_raw(source, victim, tc, flowlabel, IPPROTO_UDP, hoplimit,
                        (char *) udp, udplen, packetlen);
  free(udp);

  return ipv6;
}

int send_udp_raw(int sd, const struct eth_nfo *eth,
                 struct in_addr *source, const struct in_addr *victim,
                 int ttl, u16 ipid,
                 u8 *ipopt, int ipoptlen,
                 u16 sport, u16 dport, const char *data, u16 datalen) {
  struct sockaddr_storage dst;
  struct sockaddr_in *dst_in;
  unsigned int packetlen;
  int res = -1;
  u8 *packet = build_udp_raw(source, victim,
                             ttl, ipid, IP_TOS_DEFAULT, false,
                             ipopt, ipoptlen,
                             sport, dport,
                             data, datalen, &packetlen);
  if (!packet)
    return -1;
  memset(&dst, 0, sizeof(dst));
  dst_in = (struct sockaddr_in *) &dst;
  dst_in->sin_family = AF_INET;
  dst_in->sin_addr = *victim;
  res = send_ip_packet(sd, eth, &dst, packet, packetlen);

  free(packet);
  return res;
}

int send_udp_raw_decoys(int sd, const struct eth_nfo *eth,
                        const struct in_addr *victim,
                        int ttl, u16 ipid,
                        u8 *ipops, int ipoptlen,
                        u16 sport, u16 dport, const char *data, u16 datalen) {
  int decoy;

  for (decoy = 0; decoy < o.numdecoys; decoy++)
    if (send_udp_raw(sd, eth, &((struct sockaddr_in *)&o.decoys[decoy])->sin_addr, victim,
                     ttl, ipid, ipops, ipoptlen,
                     sport, dport, data, datalen) == -1)
      return -1;

  return 0;
}


/* Build an SCTP packet (no IP header). */
static u8 *build_sctp(u16 sport, u16 dport, u32 vtag,
                      const char *chunks, int chunkslen,
                      const char *data, u16 datalen,
                      u32 *packetlen) {
  struct sctp_hdr *sctp;
  u8 *packet;

  *packetlen = sizeof(*sctp) + chunkslen + datalen;
  packet = (u8 *) safe_malloc(*packetlen);
  sctp = (struct sctp_hdr *) packet;

  sctp->sh_sport = htons(sport);
  sctp->sh_dport = htons(dport);
  sctp->sh_sum = 0;
  sctp->sh_vtag = htonl(vtag);

  if (chunks)
    memcpy(packet + sizeof(*sctp), chunks, chunkslen);

  if (data)
    memcpy(packet + sizeof(*sctp) + chunkslen, data, datalen);

  /* RFC 2960 originally defined Adler32 checksums, which was later
   * revised to CRC32C in RFC 3309 and RFC 4960 respectively.
   * Nmap uses CRC32C by default, unless --adler32 is given. */
  if (o.adler32)
    sctp->sh_sum = htonl(nbase_adler32(packet, *packetlen));
  else
    sctp->sh_sum = htonl(nbase_crc32c(packet, *packetlen));

  if (o.badsum)
    --sctp->sh_sum;

  return packet;
}

/* Builds an SCTP packet (including an IP header) by packing the fields
   with the given information.  It allocates a new buffer to store the
   packet contents, and then returns that buffer.  The packet is not
   actually sent by this function.  Caller must delete the buffer when
   finished with the packet.  The packet length is returned in
   packetlen, which must be a valid int pointer. */
u8 *build_sctp_raw(const struct in_addr *source,
                   const struct in_addr *victim, int ttl, u16 ipid,
                   u8 tos, bool df, u8 *ipopt, int ipoptlen, u16 sport,
                   u16 dport, u32 vtag, char *chunks, int chunkslen,
                   const char *data, u16 datalen, u32 *packetlen) {
  u8 *ip, *sctp;
  u32 sctplen;

  sctp = build_sctp(sport, dport, vtag, chunks, chunkslen, data, datalen, &sctplen);
  ip = build_ip_raw(source, victim, IPPROTO_SCTP, ttl, ipid, tos, df,
                    ipopt, ipoptlen, (char *) sctp, sctplen, packetlen);
  free(sctp);

  return ip;
}

u8 *build_sctp_raw_ipv6(const struct in6_addr *source,
                        const struct in6_addr *victim, u8 tc, u32 flowlabel,
                        u8 hoplimit, u16 sport, u16 dport, u32 vtag,
                        char *chunks, int chunkslen, const char *data, u16 datalen,
                        u32 *packetlen) {
  u8 *ipv6, *sctp;
  u32 sctplen;

  sctp = build_sctp(sport, dport, vtag, chunks, chunkslen, data, datalen, &sctplen);
  ipv6 = build_ipv6_raw(source, victim, tc, flowlabel, IPPROTO_SCTP, hoplimit,
                        (char *) sctp, sctplen, packetlen);
  free(sctp);

  return ipv6;
}


/* Builds an ICMP packet (including an IP header) by packing the fields
   with the given information.  It allocates a new buffer to store the
   packet contents, and then returns that buffer.  The packet is not
   actually sent by this function.  Caller must delete the buffer when
   finished with the packet.  The packet length is returned in
   packetlen, which must be a valid int pointer.  The id/seq will be converted
   to network byte order (if it differs from HBO) */
u8 *build_icmp_raw(const struct in_addr *source,
                   const struct in_addr *victim, int ttl, u16 ipid,
                   u8 tos, bool df, u8 *ipopt, int ipoptlen, u16 seq,
                   unsigned short id, u8 ptype, u8 pcode, const char *data,
                   u16 datalen, u32 *packetlen) {
  struct ppkt {
    u8 type;
    u8 code;
    u16 checksum;
    u16 id;
    u16 seq;
    u8 data[1500]; /* Note -- first 4-12 bytes can be used for ICMP header */
  } pingpkt;
  u8 *datastart = pingpkt.data;
  /* dlen is the amount of space remaining in the data buffer; it may be reduced
     depending on type. */
  int dlen = sizeof(pingpkt.data);
  int icmplen = 0;
  char *ping = (char *) &pingpkt;

  pingpkt.type = ptype;
  pingpkt.code = pcode;

  if (ptype == 8) {
    /* echo request */
    icmplen = 8;
  } else if (ptype == 13 && pcode == 0) {
    /* ICMP timestamp req */
    icmplen = 20;
    memset(datastart, 0, 12);
    datastart += 12;
    dlen -= 12;
  } else if (ptype == 17 && pcode == 0) {
    /* icmp netmask req */
    icmplen = 12;
    memset(datastart, 0, 4);
    datastart += 4;
    dlen -= 4;
  } else {
    fatal("Unknown icmp type/code (%d/%d) in %s", ptype, pcode, __func__);
  }

  /* Copy the data over too */
  if (datalen > 0) {
    icmplen += MIN(dlen, datalen);
    if (data == NULL)
      memset(datastart, 0, MIN(dlen, datalen));
    else
      memcpy(datastart, data, MIN(dlen, datalen));
  }

  /* Fill out the ping packet. All the ICMP types handled by this function have
     the id and seq fields. */
  pingpkt.id = htons(id);
  pingpkt.seq = htons(seq);
  pingpkt.checksum = 0;
  pingpkt.checksum = in_cksum((unsigned short *) ping, icmplen);

  if (o.badsum)
    --pingpkt.checksum;

  return build_ip_raw(source, victim, IPPROTO_ICMP, ttl, ipid, tos, df,
                      ipopt, ipoptlen, ping, icmplen, packetlen);
}


/* Builds an ICMPv6 packet (including an IPv6 header). */
u8 *build_icmpv6_raw(const struct in6_addr *source,
                     const struct in6_addr *victim, u8 tc, u32 flowlabel,
                     u8 hoplimit, u16 seq, u16 id, u8 ptype, u8 pcode,
                     const char *data, u16 datalen, u32 *packetlen) {
  char *packet;
  struct icmpv6_hdr *icmpv6;
  union icmpv6_msg *msg;
  unsigned int icmplen;
  u8 *ipv6;

  packet = (char *) safe_malloc(sizeof(*icmpv6) + sizeof(*msg) + datalen);
  icmpv6 = (struct icmpv6_hdr *) packet;
  msg = (union icmpv6_msg *) (packet + sizeof(*icmpv6));

  icmplen = sizeof(*icmpv6);
  icmpv6->icmpv6_type = ptype;
  icmpv6->icmpv6_code = pcode;

  if (ptype == ICMPV6_ECHO) {
    msg->echo.icmpv6_seq = htons(seq);
    msg->echo.icmpv6_id = htons(id);
    icmplen += sizeof(msg->echo);
  }

  /* At this point icmplen <= sizeof(*icmpv6) + sizeof(*msg). */
  memcpy(packet + icmplen, data, datalen);
  icmplen += datalen;

  icmpv6->icmpv6_cksum = 0;
  icmpv6->icmpv6_cksum = ipv6_pseudoheader_cksum(source, victim,
                         IPPROTO_ICMPV6, icmplen, icmpv6);
  if (o.badsum)
    icmpv6->icmpv6_cksum--;

  ipv6 = build_ipv6_raw(source, victim, tc, flowlabel, IPPROTO_ICMPV6, hoplimit,
                        packet, icmplen, packetlen);

  free(packet);
  return ipv6;
}

/* Builds an IGMP packet (including an IP header) by packing the fields
   with the given information.  It allocates a new buffer to store the
   packet contents, and then returns that buffer.  The packet is not
   actually sent by this function.  Caller must delete the buffer when
   finished with the packet.  The packet length is returned in packetlen,
   which must be a valid int pointer. */
u8 *build_igmp_raw(const struct in_addr *source,
                   const struct in_addr *victim, int ttl, u16 ipid,
                   u8 tos, bool df, u8 *ipopt, int ipoptlen, u8 ptype,
                   u8 pcode, const char *data, u16 datalen, u32 *packetlen) {
  struct {
    u8 igmp_type;
    u8 igmp_code;
    u16 igmp_cksum;
    u32 var; /* changes between types, unused. usually group address. */
    u8 data[1500];
  } igmp;
  u32 *datastart = (u32 *) igmp.data;
  int dlen = sizeof(igmp.data);
  int igmplen = 0;
  char *pkt = (char *) &igmp;

  igmp.igmp_type = ptype;
  igmp.igmp_code = pcode;

  if (ptype == 0x11) {
    /* Membership Query */
    igmplen = 8;
  } else if (ptype == 0x12) {
    /* v1 Membership Report */
    igmplen = 8;
  } else if (ptype == 0x16) {
    /* v2 Membership Report */
    igmplen = 8;
  } else if (ptype == 0x17) {
    /* v2 Leave Group */
    igmplen = 8;
  } else if (ptype == 0x22) {
    /* v3 Membership Report */
    igmplen = 8;
  } else {
    fatal("Unknown igmp type (%d) in %s", ptype, __func__);
  }

  if (datalen > 0) {
    igmplen += MIN(dlen, datalen);
    if (data == NULL)
      memset(datastart, 0, MIN(dlen, datalen));
    else
      memcpy(datastart, data, MIN(dlen, datalen));
  }

  igmp.igmp_cksum = 0;
  igmp.igmp_cksum = in_cksum((unsigned short *) pkt, igmplen);

  if (o.badsum)
    --igmp.igmp_cksum;

  return build_ip_raw(source, victim, IPPROTO_IGMP, ttl, ipid, tos, df,
                      ipopt, ipoptlen, pkt, igmplen, packetlen);
}


/* A simple function I wrote to help in debugging, shows the important fields
   of a TCP packet*/
int readtcppacket(const u8 *packet, int readdata) {

  const struct ip *ip = (struct ip *) packet;
  const struct tcp_hdr *tcp = (struct tcp_hdr *) (packet + sizeof(struct ip));
  const unsigned char *data = packet + sizeof(struct ip) + sizeof(struct tcp_hdr);
  int tot_len;
  struct in_addr bullshit, bullshit2;
  char sourcehost[16];
  int i;
  int realfrag = 0;

  if (!packet) {
    error("%s: packet is NULL!", __func__);
    return -1;
  }

  bullshit.s_addr = ip->ip_src.s_addr;
  bullshit2.s_addr = ip->ip_dst.s_addr;
  realfrag = htons(ntohs(ip->ip_off) & IP_OFFMASK);
  tot_len = htons(ip->ip_len);
  strncpy(sourcehost, inet_ntoa(bullshit), 16);
  i = 4 * (ntohs(ip->ip_hl) + ntohs(tcp->th_off));
  if (ip->ip_p == IPPROTO_TCP) {
    if (realfrag)
      log_write(LOG_PLAIN, "Packet is fragmented, offset field: %u\n",
                realfrag);
    else {
      log_write(LOG_PLAIN,
                "TCP packet: %s:%d -> %s:%d (total: %d bytes)\n",
                sourcehost, ntohs(tcp->th_sport), inet_ntoa(bullshit2),
                ntohs(tcp->th_dport), tot_len);
      log_write(LOG_PLAIN, "Flags: ");
      if (!tcp->th_flags)
        log_write(LOG_PLAIN, "(none)");
      if (tcp->th_flags & TH_RST)
        log_write(LOG_PLAIN, "RST ");
      if (tcp->th_flags & TH_SYN)
        log_write(LOG_PLAIN, "SYN ");
      if (tcp->th_flags & TH_ACK)
        log_write(LOG_PLAIN, "ACK ");
      if (tcp->th_flags & TH_PUSH)
        log_write(LOG_PLAIN, "PSH ");
      if (tcp->th_flags & TH_FIN)
        log_write(LOG_PLAIN, "FIN ");
      if (tcp->th_flags & TH_URG)
        log_write(LOG_PLAIN, "URG ");
      log_write(LOG_PLAIN, "\n");

      log_write(LOG_PLAIN, "ipid: %hu ttl: %hhu ", ntohs(ip->ip_id),
                ip->ip_ttl);

      if (tcp->th_flags & (TH_SYN | TH_ACK))
        log_write(LOG_PLAIN, "Seq: %u\tAck: %u\n",
                  (unsigned int) ntohl(tcp->th_seq),
                  (unsigned int) ntohl(tcp->th_ack));
      else if (tcp->th_flags & TH_SYN)
        log_write(LOG_PLAIN, "Seq: %u\n",
                  (unsigned int) ntohl(tcp->th_seq));
      else if (tcp->th_flags & TH_ACK)
        log_write(LOG_PLAIN, "Ack: %u\n",
                  (unsigned int) ntohl(tcp->th_ack));
    }
  }
  if (readdata && i < tot_len) {
    log_write(LOG_PLAIN, "Data portion:\n");
    while (i < tot_len) {
      log_write(LOG_PLAIN, "%2X%c", data[i], ((i + 1) % 16) ? ' ' : '\n');
      i++;
    }
    log_write(LOG_PLAIN, "\n");
  }

  return 0;
}

/* A simple function I wrote to help in debugging, shows the important fields
   of a UDP packet*/
int readudppacket(const u8 *packet, int readdata) {
  const struct ip *ip = (struct ip *) packet;
  const struct udp_hdr *udp = (struct udp_hdr *) (packet + sizeof(struct ip));
  const unsigned char *data = packet + sizeof(struct ip) + sizeof(struct udp_hdr);
  int tot_len;
  struct in_addr bullshit, bullshit2;
  char sourcehost[16];
  int i;
  int realfrag = 0;

  if (!packet) {
    error("%s: packet is NULL!", __func__);
    return -1;
  }

  bullshit.s_addr = ip->ip_src.s_addr;
  bullshit2.s_addr = ip->ip_dst.s_addr;
  realfrag = htons(ntohs(ip->ip_off) & IP_OFFMASK);
  tot_len = htons(ip->ip_len);
  strncpy(sourcehost, inet_ntoa(bullshit), 16);
  i = 4 * (ntohs(ip->ip_hl)) + 8;
  if (ip->ip_p == IPPROTO_UDP) {
    if (realfrag)
      log_write(LOG_PLAIN, "Packet is fragmented, offset field: %u\n",
                realfrag);
    else {
      log_write(LOG_PLAIN,
                "UDP packet: %s:%d -> %s:%d (total: %d bytes)\n",
                sourcehost, ntohs(udp->uh_sport), inet_ntoa(bullshit2),
                ntohs(udp->uh_dport), tot_len);

      log_write(LOG_PLAIN, "ttl: %hhu ", ip->ip_ttl);
    }
  }
  if (readdata && i < tot_len) {
    log_write(LOG_PLAIN, "Data portion:\n");
    while (i < tot_len) {
      log_write(LOG_PLAIN, "%2X%c", data[i], ((i + 1) % 16) ? ' ' : '\n');
      i++;
    }
    log_write(LOG_PLAIN, "\n");
  }
  return 0;
}


/* Used by validatepkt() to validate the TCP header (including option lengths).
   The options checked are MSS, WScale, SackOK, Sack, and Timestamp. */
static bool validateTCPhdr(const u8 *tcpc, unsigned len) {
  const struct tcp_hdr *tcp = (struct tcp_hdr *) tcpc;
  unsigned hdrlen, optlen;

  hdrlen = tcp->th_off * 4;

  /* Check header length */
  if (hdrlen > len || hdrlen < sizeof(struct tcp_hdr))
    return false;

  /* Get to the options */
  tcpc += sizeof(struct tcp_hdr);
  optlen = hdrlen - sizeof(struct tcp_hdr);

// This macro guarantees optlen does not underflow by returning if optlen < expected
#define OPTLEN_IS(expected) do { \
  if ((expected) == 0 || optlen < (expected) || hdrlen != (expected)) \
    return false; \
  optlen -= (expected); \
  tcpc += (expected); \
} while(0);

  while (optlen > 1) {
    hdrlen = *(tcpc + 1);
    switch (*tcpc) {
    case 0: // EOL
      /* Options processing is over. */
      return true;
    case 1: // NOP
      /* 1 byte, no length. All other options have a length. */
      optlen--;
      tcpc++;
      break;
    case 2: /* MSS */
      OPTLEN_IS(4);
      break;
    case 3: /* Window Scale */
      OPTLEN_IS(3);
      break;
    case 4: /* SACK Permitted */
      OPTLEN_IS(2);
      break;
    case 5: /* SACK */
      if (!(hdrlen - 2) || ((hdrlen - 2) % 8))
        return false;
      OPTLEN_IS(hdrlen);
      break;
    case 8: /* Timestamp */
      OPTLEN_IS(10);
      break;
    case 14: /* Alternate checksum */
      /* Sometimes used for hardware checksum offloading
       * ftp://ftp.ucsd.edu/pub/csl/fastnet/faq.txt
       */
      OPTLEN_IS(3);
      break;
    default:
      OPTLEN_IS(hdrlen);
      break;
    }
  }

  if (optlen == 1) {
    // Only 1 byte left in options, this has to be NOP or EOL
    return (*tcpc == 0 || *tcpc == 1);
  }
  // There is no way out of the previous loop that does not satisfy optlen == 0 or optlen == 1
  assert(optlen == 0);

  return true;
}

/* Used by readip_pcap() to validate an IP packet.  It checks to make sure:
 *
 * 1) there is enough room for an IP header in the amount of bytes read
 * 2) the IP version number is correct
 * 3) the IP length fields are at least as big as the standard header
 * 4) the IP packet received isn't a fragment, or is the initial fragment
 * 5) that next level headers seem reasonable (e.g. validateTCPhdr())
 *
 * Checking the IP total length (iplen) to see if its at least as large as the
 * number of bytes read (len) does not work because things like the Ethernet
 * CRC also get captured and are counted in len.  However, since the IP total
 * length field can't be trusted, we use len instead of iplen when doing any
 * further checks on lengths.  readip_pcap fixes the length on it's end if we
 * read more than the IP header says we should have so as to not pass garbage
 * data to the caller.
 */
static bool validatepkt(const u8 *ipc, unsigned *len) {
  const struct ip *ip = (struct ip *) ipc;
  const void *data;
  unsigned int datalen, iplen;
  u8 hdr;

  if (*len < 1) {
    if (o.debugging >= 3)
      error("Rejecting tiny, supposed IP packet (size %u)", *len);
    return false;
  }

  if (ip->ip_v == 4) {
    unsigned fragoff, iplen;

    datalen = *len;
    data = ipv4_get_data(ip, &datalen);
    if (data == NULL) {
      if (o.debugging >= 3)
        error("Rejecting IP packet because of invalid length");
      return false;
    }

    iplen = ntohs(ip->ip_len);

    fragoff = 8 * (ntohs(ip->ip_off) & IP_OFFMASK);
    if (fragoff) {
      if (o.debugging >= 3)
        error("Rejecting IP fragment (offset %u)", fragoff);
      return false;
    }

    /* OK, since the IP header has been validated, we don't want to tell
     * the caller they have more packet than they really have.  This can
     * be caused by the Ethernet CRC trailer being counted, for example. */
    if (*len > iplen)
      *len = iplen;

    hdr = ip->ip_p;
  } else if (ip->ip_v == 6) {
    const struct ip6_hdr *ip6 = (struct ip6_hdr *) ipc;

    datalen = *len;
    data = ipv6_get_data(ip6, &datalen, &hdr);
    if (data == NULL) {
      if (o.debugging >= 3)
        error("Rejecting IP packet because of invalid length");
      return false;
    }

    iplen = ntohs(ip6->ip6_plen);
    if (datalen > iplen)
      *len -= datalen - iplen;
  } else {
    if (o.debugging >= 3)
      error("Rejecting IP packet because of invalid version number %u", ip->ip_v);
    return false;
  }

  switch (hdr) {
  case IPPROTO_TCP:
    if (datalen < sizeof(struct tcp_hdr)) {
      if (o.debugging >= 3)
        error("Rejecting TCP packet because of incomplete header");
      return false;
    }
    if (!validateTCPhdr((u8 *) data, datalen)) {
      if (o.debugging >= 3)
        error("Rejecting TCP packet because of bad TCP header");
      return false;
    }
    break;
  case IPPROTO_UDP:
    if (datalen < sizeof(struct udp_hdr)) {
      if (o.debugging >= 3)
        error("Rejecting UDP packet because of incomplete header");
      return false;
    }
    break;
  default:
    break;
  }

  return true;
}

/* Read an IP packet using libpcap .  We return the packet and take
   a pcap descriptor and a pointer to the packet length (which we set
   in the function. If you want a maximum length returned, you
   should specify that in pcap_open_live() */
/* to_usec is the timeout period in microseconds -- use 0 to skip the
   test and -1 to block forever.  Note that we don't interrupt pcap, so
   low values (and 0) degenerate to the timeout specified
   in pcap_open_live() */
/* If rcvdtime is non-null and a packet is returned, rcvd will be
   filled with the time that packet was captured from the wire by
   pcap.  If linknfo is not NULL, linknfo->headerlen and
   linknfo->header will be filled with the appropriate values. */
/* Specifying true for validate will enable validity checks against the
   received IP packet.  See validatepkt() for a list of checks. */
const u8 *readipv4_pcap(pcap_t *pd, unsigned int *len, long to_usec,
                    struct timeval *rcvdtime, struct link_header *linknfo,
                    bool validate) {
  const u8 *buf;

  buf = readip_pcap(pd, len, to_usec, rcvdtime, linknfo, validate);
  if (buf != NULL) {
    const struct ip *ip;

    ip = (struct ip *) buf;
    if (*len < 1 || ip->ip_v != 4)
      return NULL;
  }

  return buf;
}

static bool accept_any (const unsigned char *p, const struct pcap_pkthdr *h, int datalink, size_t offset) {
  return true;
}

static bool accept_ip (const unsigned char *p, const struct pcap_pkthdr *h, int datalink, size_t offset) {
  const struct ip *ip = NULL;

  if (h->caplen < offset + sizeof(struct ip)) {
    return false;
  }
  ip = (struct ip *) (p + offset);
  switch (ip->ip_v) {
    case 4:
    case 6:
      break;
    default:
      return false;
      break;
  }

  return true;
}

const u8 *readip_pcap(pcap_t *pd, unsigned int *len, long to_usec,
                  struct timeval *rcvdtime, struct link_header *linknfo, bool validate) {
  int datalink;
  size_t offset = 0;
  struct pcap_pkthdr *head;
  const u8 *p;
  int got_one = 0;

  if (linknfo) {
    memset(linknfo, 0, sizeof(*linknfo));
  }

  if (validate) {
    got_one = read_reply_pcap(pd, to_usec, accept_ip, &p, &head, rcvdtime, &datalink, &offset);
  }
  else {
    got_one = read_reply_pcap(pd, to_usec, accept_any, &p, &head, rcvdtime, &datalink, &offset);
  }

  if (!got_one) {
    *len = 0;
    return NULL;
  }

  *len = head->caplen - offset;
  p += offset;

  if (validate) {
    if (!validatepkt(p, len)) {
      *len = 0;
      return NULL;
    }
  }
  if (offset && linknfo) {
    linknfo->datalinktype = datalink;
    linknfo->headerlen = offset;
    linknfo->header = p - offset;
  }
  if (rcvdtime)
    PacketTrace::trace(PacketTrace::RCVD, (u8 *) p, *len,
        rcvdtime);
  else
    PacketTrace::trace(PacketTrace::RCVD, (u8 *) p, *len);

  *len = head->caplen - offset;
  return p;
}

// Returns whether the packet receive time value obtained from libpcap
// (and thus by readip_pcap()) should be considered valid.  When
// invalid (Windows and Amiga), readip_pcap returns the time you called it.
bool pcap_recv_timeval_valid() {
#if defined(WIN32) || defined(__amigaos__)
  return false;
#else
  return true;
#endif
}

/* Prints stats from a pcap descriptor (number of received and dropped
   packets). */
void pcap_print_stats(int logt, pcap_t *pd) {
  struct pcap_stat stat;

  assert(pd != NULL);

  if (pcap_stats(pd, &stat) < 0) {
    error("%s: %s", __func__, pcap_geterr(pd));
    return;
  }

  log_write(logt, "pcap stats: %u packets received by filter, %u dropped by kernel.\n", stat.ps_recv, stat.ps_drop);
}




/* This function tries to determine the target's ethernet MAC address
   from a received packet as follows:
   1) If linkhdr is an ethernet header, grab the src mac (otherwise give up)
   2) If overwrite is 0 and a MAC is already set for this target, give up.
   3) If the packet source address is not the target, give up.
   4) Use the routing table to try to determine rather target is
      directly connected to the src host running Nmap.  If it is, set the MAC.

   This function returns 0 if it ends up setting the MAC, nonzero otherwise. */
int setTargetMACIfAvailable(Target *target, struct link_header *linkhdr,
                            const struct sockaddr_storage *src, int overwrite) {
  if (!linkhdr || !target || !src)
    return 1;

  if (linkhdr->datalinktype != DLT_EN10MB || linkhdr->headerlen < 14)
    return 2;

  if (!overwrite && target->MACAddress())
    return 3;

  if (sockaddr_storage_cmp(src, target->TargetSockAddr()) != 0)
    return 4;

  /* Sometimes bogus MAC address still gets through, like during some localhost scans */
  if (memcmp(linkhdr->header + 6, "\0\0\0\0\0\0", 6) == 0)
    return 5;

  if (target->ifType() == devt_ethernet && target->directlyConnected()) {
    /* Yay!  This MAC address seems valid */
    target->setMACAddress(linkhdr->header + 6);
    return 0;
  }

  return 5;
}


/* This function ensures that the next hop MAC address for a target is
   filled in.  This address is the target's own MAC if it is directly
   connected, and the next hop mac otherwise.  Returns true if the
   address is set when the function ends, false if not.  This function
   firt checks if it is already set, if not it tries the arp cache,
   and if that fails it sends an ARP request itself.  This should be
   called after an ARP scan if many directly connected machines are
   involved. setDirectlyConnected() (whether true or false) should
   have already been called on target before this.  The target device
   and src mac address should also already be set.  */
bool setTargetNextHopMAC(Target *target) {
  struct sockaddr_storage targetss;
  size_t sslen;
  u8 mac[6];

  if (target->ifType() != devt_ethernet)
    return false; /* Duh. */

  /* First check if we already have it, duh. */
  if (target->NextHopMACAddress())
    return true;

  /* For connected machines, it is the same as the target addy */
  if (target->directlyConnected() && target->MACAddress()) {
    target->setNextHopMACAddress(target->MACAddress());
    return true;
  }

  if (target->directlyConnected()) {
    target->TargetSockAddr(&targetss, &sslen);
  } else {
    if (!target->nextHop(&targetss, &sslen))
      fatal("%s: Failed to determine nextHop to target", __func__);
  }

  if (getNextHopMAC(target->deviceFullName(), target->SrcMACAddress(), target->SourceSockAddr(), &targetss, mac)) {
    target->setNextHopMACAddress(mac);
    return true;
  }

  /* I'm afraid that we couldn't find it!  Maybe it doesn't exist? */
  return false;
}

/* Like to getTargetNextHopMAC(), but for arbitrary hosts (not Targets) */
bool getNextHopMAC(const char *iface, const u8 *srcmac, const struct sockaddr_storage *srcss,
                   const struct sockaddr_storage *dstss, u8 *dstmac) {
  arp_t *a;
  struct arp_entry ae;

  /* First, let us check the Nmap arp cache ... */
  if (mac_cache_get(dstss, dstmac))
    return true;

  /* Maybe the system ARP cache will be more helpful */
  a = arp_open();
  if (a) {
    addr_ston((sockaddr *) dstss, &ae.arp_pa);
    if (arp_get(a, &ae) == 0) {
      mac_cache_set(dstss, ae.arp_ha.addr_eth.data);
      memcpy(dstmac, ae.arp_ha.addr_eth.data, 6);
      arp_close(a);
      return true;
    }
    arp_close(a);
  }

  /* OK, the last choice is to send our own damn ARP request (and
     retransmissions if necessary) to determine the MAC */
  if (dstss->ss_family == AF_INET) {
    if (doArp(iface, srcmac, srcss, dstss, dstmac, PacketTrace::traceArp)) {
      mac_cache_set(dstss, dstmac);
      return true;
    }
  } else if (dstss->ss_family == AF_INET6) {
    if (doND(iface, srcmac, srcss, dstss, dstmac, PacketTrace::traceND)) {
      mac_cache_set(dstss, dstmac);
      return true;
    }
  }

  return false;
}


int nmap_route_dst(const struct sockaddr_storage *dst, struct route_nfo *rnfo) {
  struct sockaddr_storage spoofss;
  size_t spoofsslen;

  if (o.spoofsource) {
    o.SourceSockAddr(&spoofss, &spoofsslen);
    return route_dst(dst, rnfo, o.device, &spoofss);
  } else {
    return route_dst(dst, rnfo, o.device, NULL);
  }
}


/* Maximize the receive buffer of a socket descriptor (up to 500K) */
void max_rcvbuf(int sd) {
  int optval = 524288; /* 2^19 */
  recvfrom6_t optlen = sizeof(int);

#ifndef WIN32
  if (setsockopt (sd, SOL_SOCKET, SO_RCVBUF, (const char *) &optval, optlen))
    if (o.debugging)
      perror("Problem setting large socket receive buffer");
  if (o.debugging) {
    getsockopt(sd, SOL_SOCKET, SO_RCVBUF, (char *) &optval, &optlen);
    log_write(LOG_STDOUT, "Our buffer size is now %d\n", optval);
  }
#endif /* WIN32 */
}


/* Do a receive (recv()) on a socket and stick the results (up to
   len) into buf .  Give up after 'seconds'.  Returns the number of
   bytes read (or -1 in the case of an error.  It only does one recv
   (it will not keep going until len bytes are read).  If timedout is
   not NULL, it will be set to zero (no timeout occurred) or 1 (it
   did). */
int recvtime(int sd, char *buf, int len, int seconds, int *timedout) {

  int res;
  struct timeval timeout;
  fd_set readfd;

  timeout.tv_sec = seconds;
  timeout.tv_usec = 0;
  FD_ZERO(&readfd);
  checked_fd_set(sd, &readfd);
  if (timedout)
    *timedout = 0;
  res = select(sd + 1, &readfd, NULL, NULL, &timeout);
  if (res > 0) {
    res = recv(sd, buf, len, 0);
    if (res >= 0)
      return res;
    gh_perror("recv in %s", __func__);
    return 0;
  } else if (!res) {
    if (timedout)
      *timedout = 1;
    return 0;
  }
  gh_perror("select() in %s", __func__);
  return -1;
}

/* Examines the given tcp packet and obtains the TCP timestamp option
   information if available.  Note that the CALLER must ensure that
   "tcp" contains a valid header (in particular the th_off must be the
   true packet length and tcp must contain it).  If a valid timestamp
   option is found in the header, nonzero is returned and the
   'timestamp' and 'echots' parameters are filled in with the
   appropriate value (if non-null).  Otherwise 0 is returned and the
   parameters (if non-null) are filled with 0.  Remember that the
   correct way to check for errors is to look at the return value
   since a zero ts or echots could possibly be valid. */
int gettcpopt_ts(const struct tcp_hdr *tcp, u32 *timestamp, u32 *echots) {

  unsigned char *p;
  int len = 0;
  int op;
  int oplen;

  /* first we find where the tcp options start ... */
  p = ((unsigned char *) tcp) + 20;
  len = 4 * tcp->th_off - 20;
  while (len > 0 && *p != 0 /* TCPOPT_EOL */ ) {
    op = *p++;
    if (op == 0 /* TCPOPT_EOL */ )
      break;
    if (op == 1 /* TCPOPT_NOP */ ) {
      len--;
      continue;
    }
    oplen = *p++;
    if (oplen < 2)
      break; /* No infinite loops, please */
    if (oplen > len)
      break; /* Not enough space */
    if (op == 8 /* TCPOPT_TIMESTAMP */  && oplen == 10) {
      /* Legitimate ts option */
      if (timestamp) {
        memcpy((char *) timestamp, p, 4);
        *timestamp = ntohl(*timestamp);
      }
      p += 4;
      if (echots) {
        memcpy((char *) echots, p, 4);
        *echots = ntohl(*echots);
      }
      return 1;
    }
    len -= oplen;
    p += oplen - 2;
  }

  /* Didn't find anything */
  if (timestamp)
    *timestamp = 0;
  if (echots)
    *echots = 0;
  return 0;
}
