/*      $OpenBSD: packet.c,v 1.15 2025/02/07 23:08:48 bluhm Exp $       */

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

/*
 * 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/types.h>
#include <sys/socket.h>
#include <sys/mbuf.h>

#include <arpa/inet.h>

#include <net/if.h>
#include <net/if_enc.h>

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

#include <string.h>

#include "dhcp.h"
#include "dhcpd.h"
#include "log.h"


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

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

ssize_t
assemble_hw_header(unsigned char *buf, size_t buflen,
    size_t offset, struct packet_ctx *pc, unsigned int intfhtype)
{
        struct ether_header eh;

        switch (intfhtype) {
        case HTYPE_ETHER:
                if (buflen < offset + ETHER_HDR_LEN)
                        return (-1);

                /* Use the supplied address or let the kernel fill it. */
                memcpy(eh.ether_shost, pc->pc_smac, ETHER_ADDR_LEN);
                memcpy(eh.ether_dhost, pc->pc_dmac, ETHER_ADDR_LEN);

                eh.ether_type = htons(ETHERTYPE_IP);

                memcpy(&buf[offset], &eh, ETHER_HDR_LEN);
                offset += ETHER_HDR_LEN;
                break;
        default:
                return (-1);
        }

        return (offset);
}

ssize_t
assemble_udp_ip_header(unsigned char *buf, size_t buflen, size_t offset,
    struct packet_ctx *pc, unsigned char *data, size_t datalen)
{
        struct ip ip;
        struct udphdr udp;

        if (buflen < offset + sizeof(ip) + sizeof(udp))
                return (-1);

        ip.ip_v = 4;
        ip.ip_hl = 5;
        ip.ip_tos = IPTOS_LOWDELAY;
        ip.ip_len = htons(sizeof(ip) + sizeof(udp) + datalen);
        ip.ip_id = 0;
        ip.ip_off = 0;
        ip.ip_ttl = 16;
        ip.ip_p = IPPROTO_UDP;
        ip.ip_sum = 0;
        ip.ip_src.s_addr = ss2sin(&pc->pc_src)->sin_addr.s_addr;
        ip.ip_dst.s_addr = ss2sin(&pc->pc_dst)->sin_addr.s_addr;

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

        udp.uh_sport = ss2sin(&pc->pc_src)->sin_port;
        udp.uh_dport = ss2sin(&pc->pc_dst)->sin_port;
        udp.uh_ulen = htons(sizeof(udp) + datalen);
        memset(&udp.uh_sum, 0, sizeof(udp.uh_sum));

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

        memcpy(&buf[offset], &udp, sizeof(udp));
        offset += sizeof(udp);

        return (offset);
}

ssize_t
decode_hw_header(unsigned char *buf, size_t buflen,
    size_t offset, struct packet_ctx *pc, unsigned int intfhtype)
{
        u_int32_t ip_len;
        struct ip *ip;

        switch (intfhtype) {
        case HTYPE_IPSEC_TUNNEL:
                if (buflen < offset + ENC_HDRLEN + sizeof(*ip))
                        return (-1);
                offset += ENC_HDRLEN;
                ip_len = (buf[offset] & 0xf) << 2;
                if (buflen < offset + ip_len)
                        return (-1);

                ip = (struct ip *)(buf + offset);

                /* Encapsulated IP */
                if (ip->ip_p != IPPROTO_IPIP)
                        return (-1);

                memset(pc->pc_dmac, 0xff, ETHER_ADDR_LEN);
                offset += ip_len;

                pc->pc_htype = ARPHRD_ETHER;
                pc->pc_hlen = ETHER_ADDR_LEN;
                break;
        case HTYPE_ETHER:
                if (buflen < offset + ETHER_HDR_LEN)
                        return (-1);

                memcpy(pc->pc_dmac, buf + offset, ETHER_ADDR_LEN);
                memcpy(pc->pc_smac, buf + offset + ETHER_ADDR_LEN,
                    ETHER_ADDR_LEN);
                offset += ETHER_HDR_LEN;

                pc->pc_htype = ARPHRD_ETHER;
                pc->pc_hlen = ETHER_ADDR_LEN;
                break;
        default:
                return (-1);
        }

        return (offset);
}

ssize_t
decode_udp_ip_header(unsigned char *buf, size_t buflen,
    size_t offset, struct packet_ctx *pc, u_int16_t csumflags)
{
        struct ip *ip;
        struct udphdr *udp;
        unsigned char *data;
        u_int32_t ip_len;
        static unsigned int ip_packets_seen;
        static unsigned int ip_packets_bad_checksum;
        static unsigned int udp_packets_seen;
        static unsigned int udp_packets_bad_checksum;
        static unsigned int udp_packets_length_checked;
        static unsigned int udp_packets_length_overflow;
        int len;

        /* Assure that an entire IP header is within the buffer. */
        if (buflen < offset + sizeof(*ip))
                return (-1);
        ip_len = (buf[offset] & 0xf) << 2;
        if (buflen < offset + ip_len)
                return (-1);

        ip = (struct ip *)(buf + offset);
        ip_packets_seen++;

        /* Check the IP header checksum - it should be zero. */
        if ((csumflags & M_IPV4_CSUM_IN_OK) == 0 &&
            wrapsum(checksum(buf + offset, 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) {
                        log_info("%u bad IP checksums seen in %u packets",
                            ip_packets_bad_checksum, ip_packets_seen);
                        ip_packets_seen = ip_packets_bad_checksum = 0;
                }
                return (-1);
        }

        pc->pc_src.ss_len = sizeof(struct sockaddr_in);
        pc->pc_src.ss_family = AF_INET;
        memcpy(&ss2sin(&pc->pc_src)->sin_addr, &ip->ip_src,
            sizeof(ss2sin(&pc->pc_src)->sin_addr));

        pc->pc_dst.ss_len = sizeof(struct sockaddr_in);
        pc->pc_dst.ss_family = AF_INET;
        memcpy(&ss2sin(&pc->pc_dst)->sin_addr, &ip->ip_dst,
            sizeof(ss2sin(&pc->pc_dst)->sin_addr));

#ifdef DEBUG
        if (buflen != offset + ntohs(ip->ip_len))
                log_debug("ip length %d disagrees with bytes received %zd.",
                    ntohs(ip->ip_len), buflen - offset);
#endif

        /* Assure that the entire IP packet is within the buffer. */
        if (buflen < offset + ntohs(ip->ip_len))
                return (-1);

        /* Assure that the UDP header is within the buffer. */
        if (buflen < offset + ip_len + sizeof(*udp))
                return (-1);
        udp = (struct udphdr *)(buf + offset + ip_len);
        udp_packets_seen++;

        /* Assure that the entire UDP packet is within the buffer. */
        if (buflen < offset + ip_len + ntohs(udp->uh_ulen))
                return (-1);
        data = buf + offset + ip_len + sizeof(*udp);

        /*
         * 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.
         */
        udp_packets_length_checked++;
        len = ntohs(udp->uh_ulen) - sizeof(*udp);
        if ((len < 0) || (len + data > buf + buflen)) {
                udp_packets_length_overflow++;
                if (udp_packets_length_checked > 4 &&
                    udp_packets_length_overflow != 0 &&
                    (udp_packets_length_checked /
                    udp_packets_length_overflow) < 2) {
                        log_info("%u udp packets in %u 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 + buflen)
                log_debug("accepting packet with data after udp payload.");

        udp_packets_seen++;

        if ((csumflags & M_UDP_CSUM_IN_OK) == 0 &&
            udp->uh_sum != 0) {
                udp->uh_sum = wrapsum(checksum((uint8_t *)udp, sizeof(*udp),
                    checksum(data, len,
                    checksum((uint8_t *)&ip->ip_src, 2 * sizeof(ip->ip_src),
                    IPPROTO_UDP + ntohs(udp->uh_ulen)))));

                if (udp->uh_sum != 0) {
                        udp_packets_bad_checksum++;
                        if (udp_packets_seen > 4 &&
                            udp_packets_bad_checksum != 0 &&
                            (udp_packets_seen / udp_packets_bad_checksum) < 2) {
                                log_info("%u bad udp checksums in %u packets",
                                    udp_packets_bad_checksum, udp_packets_seen);
                                udp_packets_seen = udp_packets_bad_checksum = 0;
                        }
                        return (-1);
                }
        }

        ss2sin(&pc->pc_src)->sin_port = udp->uh_sport;
        ss2sin(&pc->pc_dst)->sin_port = udp->uh_dport;

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