/*      $OpenBSD: packet.c,v 1.9 2004/05/04 18:58:50 deraadt Exp $      */

/* Packet assembly code, originally contributed by Archie Cobbs. */

/*-
 * SPDX-License-Identifier: BSD-3-Clause
 *
 * Copyright (c) 1995, 1996, 1999 The Internet Software Consortium.
 * All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 *
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in the
 *    documentation and/or other materials provided with the distribution.
 * 3. Neither the name of The Internet Software Consortium nor the names
 *    of its contributors may be used to endorse or promote products derived
 *    from this software without specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE INTERNET SOFTWARE CONSORTIUM AND
 * CONTRIBUTORS ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES,
 * INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF
 * MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
 * DISCLAIMED.  IN NO EVENT SHALL THE INTERNET SOFTWARE CONSORTIUM OR
 * CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
 * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
 * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF
 * USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND
 * ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
 * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT
 * OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 * SUCH DAMAGE.
 *
 * This software has been written for the Internet Software Consortium
 * by Ted Lemon <mellon@fugue.com> in cooperation with Vixie
 * Enterprises.  To learn more about the Internet Software Consortium,
 * see ``http://www.vix.com/isc''.  To learn more about Vixie
 * Enterprises, see ``http://www.vix.com''.
 */

#include <sys/cdefs.h>
#include "dhcpd.h"

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

#define ETHER_HEADER_SIZE (ETHER_ADDR_LEN * 2 + sizeof(u_int16_t))

u_int32_t       checksum(unsigned char *, unsigned, u_int32_t);
u_int32_t       wrapsum(u_int32_t);

u_int32_t
checksum(unsigned char *buf, unsigned nbytes, u_int32_t sum)
{
        unsigned i;

        /* Checksum all the pairs of bytes first... */
        for (i = 0; i < (nbytes & ~1U); i += 2) {
                sum += (u_int16_t)ntohs(*((u_int16_t *)(buf + i)));
                if (sum > 0xFFFF)
                        sum -= 0xFFFF;
        }

        /*
         * If there's a single byte left over, checksum it, too.
         * Network byte order is big-endian, so the remaining byte is
         * the high byte.
         */
        if (i < nbytes) {
                sum += buf[i] << 8;
                if (sum > 0xFFFF)
                        sum -= 0xFFFF;
        }

        return (sum);
}

u_int32_t
wrapsum(u_int32_t sum)
{
        sum = ~sum & 0xFFFF;
        return (htons(sum));
}

void
assemble_hw_header(struct interface_info *interface, unsigned char *buf,
    int *bufix)
{
        struct ether_header eh;

        memset(eh.ether_dhost, 0xff, sizeof(eh.ether_dhost));
        if (interface->hw_address.hlen == sizeof(eh.ether_shost))
                memcpy(eh.ether_shost, interface->hw_address.haddr,
                    sizeof(eh.ether_shost));
        else
                memset(eh.ether_shost, 0x00, sizeof(eh.ether_shost));

        eh.ether_type = htons(ETHERTYPE_IP);

        memcpy(&buf[*bufix], &eh, ETHER_HEADER_SIZE);
        *bufix += ETHER_HEADER_SIZE;
}

void
assemble_udp_ip_header(unsigned char *buf, int *bufix, u_int32_t from,
    u_int32_t to, unsigned int port, unsigned char *data, int len)
{
        struct ip ip;
        struct udphdr udp;

        ip.ip_v = 4;
        ip.ip_hl = 5;
        ip.ip_tos = IPTOS_LOWDELAY;
        ip.ip_len = htons(sizeof(ip) + sizeof(udp) + len);
        ip.ip_id = 0;
        ip.ip_off = 0;
        ip.ip_ttl = 128;
        ip.ip_p = IPPROTO_UDP;
        ip.ip_sum = 0;
        ip.ip_src.s_addr = from;
        ip.ip_dst.s_addr = to;

        ip.ip_sum = wrapsum(checksum((unsigned char *)&ip, sizeof(ip), 0));
        memcpy(&buf[*bufix], &ip, sizeof(ip));
        *bufix += sizeof(ip);

        udp.uh_sport = htons(LOCAL_PORT);       /* XXX */
        udp.uh_dport = port;                    /* XXX */
        udp.uh_ulen = htons(sizeof(udp) + len);
        memset(&udp.uh_sum, 0, sizeof(udp.uh_sum));

        udp.uh_sum = wrapsum(checksum(data, len, checksum((unsigned char *)&udp,
            sizeof(udp), checksum((unsigned char *)&ip.ip_src,
            2 * sizeof(ip.ip_src),
            IPPROTO_UDP + (u_int32_t)ntohs(udp.uh_ulen)))));

        if (udp.uh_sum == htons(0))
                udp.uh_sum = htons(0xffff);

        memcpy(&buf[*bufix], &udp, sizeof(udp));
        *bufix += sizeof(udp);
}

ssize_t
decode_hw_header(unsigned char *buf, int bufix, struct hardware *from)
{
        struct ether_header eh;

        memcpy(&eh, buf + bufix, ETHER_HEADER_SIZE);

        memcpy(from->haddr, eh.ether_shost, sizeof(eh.ether_shost));
        from->htype = ARPHRD_ETHER;
        from->hlen = sizeof(eh.ether_shost);

        return (sizeof(eh) + (ntohs(eh.ether_type) == ETHERTYPE_VLAN ?
            ETHER_VLAN_ENCAP_LEN : 0));
}

ssize_t
decode_udp_ip_header(unsigned char *buf, int bufix, struct sockaddr_in *from,
    unsigned char *data, int buflen)
{
        struct ip *ip;
        struct udphdr *udp;
        u_int32_t ip_len = (buf[bufix] & 0xf) << 2;
        u_int32_t sum, usum, pseudo_sum;
        static int ip_packets_seen;
        static int ip_packets_bad_checksum;
        static int udp_packets_seen;
        static int udp_packets_bad_checksum;
        static int udp_packets_length_checked;
        static int udp_packets_length_overflow;
        int len = 0;

        ip = (struct ip *)(buf + bufix);
        udp = (struct udphdr *)(buf + bufix + ip_len);

        /* Check the IP header checksum - it should be zero. */
        ip_packets_seen++;
        if (wrapsum(checksum(buf + bufix, ip_len, 0)) != 0) {
                ip_packets_bad_checksum++;
                if (ip_packets_seen > 4 && ip_packets_bad_checksum != 0 &&
                    (ip_packets_seen / ip_packets_bad_checksum) < 2) {
                        note("%d bad IP checksums seen in %d packets",
                            ip_packets_bad_checksum, ip_packets_seen);
                        ip_packets_seen = ip_packets_bad_checksum = 0;
                }
                return (-1);
        }

        if (ntohs(ip->ip_len) != buflen)
                debug("ip length %d disagrees with bytes received %d.",
                    ntohs(ip->ip_len), buflen);

        memcpy(&from->sin_addr, &ip->ip_src, 4);

        /*
         * Compute UDP checksums, including the ``pseudo-header'', the
         * UDP header and the data.   If the UDP checksum field is zero,
         * we're not supposed to do a checksum.
         */
        if (!data) {
                data = buf + bufix + ip_len + sizeof(*udp);
                len = ntohs(udp->uh_ulen) - sizeof(*udp);
                udp_packets_length_checked++;
                if (len + data > buf + bufix + buflen) {
                        udp_packets_length_overflow++;
                        if (udp_packets_length_checked > 4 &&
                            (udp_packets_length_checked /
                            udp_packets_length_overflow) < 2) {
                                note("%d udp packets in %d too long - dropped",
                                    udp_packets_length_overflow,
                                    udp_packets_length_checked);
                                udp_packets_length_overflow =
                                    udp_packets_length_checked = 0;
                        }
                        return (-1);
                }
                if (len + data != buf + bufix + buflen)
                        debug("accepting packet with data after udp payload.");
        }

        usum = udp->uh_sum;
        udp_packets_seen++;

        if (usum != htons(0)) {
                udp->uh_sum = 0;

                pseudo_sum = checksum((unsigned char *)&ip->ip_src,
                    2 * sizeof(ip->ip_src),
                    IPPROTO_UDP + (u_int32_t)ntohs(udp->uh_ulen));
                sum = wrapsum(checksum(data, len,
                    checksum((unsigned char *)udp, sizeof(*udp), pseudo_sum)));
                if (sum == htons(0))
                        sum = htons(0xffff);

                /*
                 * In addition to accepting UDP packets with the correct
                 * checksum in the checksum field, accept also the ones which
                 * have the correct pseudo header checksum in the checksum
                 * field. This allows to process UDP packets, which have been
                 * marked for transmit checksum offloading by the sender side.
                 */
                if (usum != sum && usum != htons(pseudo_sum & 0x0000ffff)) {
                        udp_packets_bad_checksum++;
                        if (udp_packets_seen > 4 &&
                            udp_packets_bad_checksum != 0 &&
                            (udp_packets_seen / udp_packets_bad_checksum) < 2) {
                                note("%d bad udp checksums in %d packets",
                                    udp_packets_bad_checksum, udp_packets_seen);
                                udp_packets_seen = udp_packets_bad_checksum = 0;
                        }
                        return (-1);
                }
        }

        memcpy(&from->sin_port, &udp->uh_sport, sizeof(udp->uh_sport));

        return (ip_len + sizeof(*udp));
}