xref: /src/sys/net/npf/npf_inet.c

/*	$NetBSD: npf_inet.c,v 1.37.12.4 2018/04/07 04:12:19 pgoyette Exp $	*/

/*-
 * Copyright (c) 2009-2014 The NetBSD Foundation, Inc.
 * All rights reserved.
 *
 * This material is based upon work partially supported by The
 * NetBSD Foundation under a contract with Mindaugas Rasiukevicius.
 *
 * 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.
 *
 * THIS SOFTWARE IS PROVIDED BY THE NETBSD FOUNDATION, INC. 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 FOUNDATION 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.
 */

/*
 * Various protocol related helper routines.
 *
 * This layer manipulates npf_cache_t structure i.e. caches requested headers
 * and stores which information was cached in the information bit field.
 * It is also responsibility of this layer to update or invalidate the cache
 * on rewrites (e.g. by translation routines).
 */

#ifdef _KERNEL
#include <sys/cdefs.h>
__KERNEL_RCSID(0, "$NetBSD: npf_inet.c,v 1.37.12.4 2018/04/07 04:12:19 pgoyette Exp $");

#include <sys/param.h>
#include <sys/types.h>

#include <net/pfil.h>
#include <net/if.h>
#include <net/ethertypes.h>
#include <net/if_ether.h>

#include <netinet/in_systm.h>
#include <netinet/in.h>
#include <netinet6/in6_var.h>
#include <netinet/ip.h>
#include <netinet/ip6.h>
#include <netinet/tcp.h>
#include <netinet/udp.h>
#include <netinet/ip_icmp.h>
#endif

#include "npf_impl.h"

/*
 * npf_fixup{16,32}_cksum: incremental update of the Internet checksum.
 */

uint16_t
npf_fixup16_cksum(uint16_t cksum, uint16_t odatum, uint16_t ndatum)
{
	uint32_t sum;

	/*
	 * RFC 1624:
	 *	HC' = ~(~HC + ~m + m')
	 *
	 * Note: 1's complement sum is endian-independent (RFC 1071, page 2).
	 */
	sum = ~cksum & 0xffff;
	sum += (~odatum & 0xffff) + ndatum;
	sum = (sum >> 16) + (sum & 0xffff);
	sum += (sum >> 16);

	return ~sum & 0xffff;
}

uint16_t
npf_fixup32_cksum(uint16_t cksum, uint32_t odatum, uint32_t ndatum)
{
	uint32_t sum;

	/*
	 * Checksum 32-bit datum as as two 16-bit.  Note, the first
	 * 32->16 bit reduction is not necessary.
	 */
	sum = ~cksum & 0xffff;
	sum += (~odatum & 0xffff) + (ndatum & 0xffff);

	sum += (~odatum >> 16) + (ndatum >> 16);
	sum = (sum >> 16) + (sum & 0xffff);
	sum += (sum >> 16);
	return ~sum & 0xffff;
}

/*
 * npf_addr_cksum: calculate checksum of the address, either IPv4 or IPv6.
 */
uint16_t
npf_addr_cksum(uint16_t cksum, int sz, const npf_addr_t *oaddr,
    const npf_addr_t *naddr)
{
	const uint32_t *oip32 = (const uint32_t *)oaddr;
	const uint32_t *nip32 = (const uint32_t *)naddr;

	KASSERT(sz % sizeof(uint32_t) == 0);
	do {
		cksum = npf_fixup32_cksum(cksum, *oip32++, *nip32++);
		sz -= sizeof(uint32_t);
	} while (sz);

	return cksum;
}

/*
 * npf_addr_sum: provide IP addresses as a XORed 32-bit integer.
 * Note: used for hash function.
 */
uint32_t
npf_addr_mix(const int sz, const npf_addr_t *a1, const npf_addr_t *a2)
{
	uint32_t mix = 0;

	KASSERT(sz > 0 && a1 != NULL && a2 != NULL);

	for (int i = 0; i < (sz >> 2); i++) {
		mix ^= a1->word32[i];
		mix ^= a2->word32[i];
	}
	return mix;
}

/*
 * npf_addr_mask: apply the mask to a given address and store the result.
 */
void
npf_addr_mask(const npf_addr_t *addr, const npf_netmask_t mask,
    const int alen, npf_addr_t *out)
{
	const int nwords = alen >> 2;
	uint_fast8_t length = mask;

	/* Note: maximum length is 32 for IPv4 and 128 for IPv6. */
	KASSERT(length <= NPF_MAX_NETMASK);

	for (int i = 0; i < nwords; i++) {
		uint32_t wordmask;

		if (length >= 32) {
			wordmask = htonl(0xffffffff);
			length -= 32;
		} else if (length) {
			wordmask = htonl(0xffffffff << (32 - length));
			length = 0;
		} else {
			wordmask = 0;
		}
		out->word32[i] = addr->word32[i] & wordmask;
	}
}

/*
 * npf_addr_cmp: compare two addresses, either IPv4 or IPv6.
 *
 * => Return 0 if equal and negative/positive if less/greater accordingly.
 * => Ignore the mask, if NPF_NO_NETMASK is specified.
 */
int
npf_addr_cmp(const npf_addr_t *addr1, const npf_netmask_t mask1,
    const npf_addr_t *addr2, const npf_netmask_t mask2, const int alen)
{
	npf_addr_t realaddr1, realaddr2;

	if (mask1 != NPF_NO_NETMASK) {
		npf_addr_mask(addr1, mask1, alen, &realaddr1);
		addr1 = &realaddr1;
	}
	if (mask2 != NPF_NO_NETMASK) {
		npf_addr_mask(addr2, mask2, alen, &realaddr2);
		addr2 = &realaddr2;
	}
	return memcmp(addr1, addr2, alen);
}

/*
 * npf_tcpsaw: helper to fetch SEQ, ACK, WIN and return TCP data length.
 *
 * => Returns all values in host byte-order.
 */
int
npf_tcpsaw(const npf_cache_t *npc, tcp_seq *seq, tcp_seq *ack, uint32_t *win)
{
	const struct tcphdr *th = npc->npc_l4.tcp;
	u_int thlen;

	KASSERT(npf_iscached(npc, NPC_TCP));

	*seq = ntohl(th->th_seq);
	*ack = ntohl(th->th_ack);
	*win = (uint32_t)ntohs(th->th_win);
	thlen = th->th_off << 2;

	if (npf_iscached(npc, NPC_IP4)) {
		const struct ip *ip = npc->npc_ip.v4;
		return ntohs(ip->ip_len) - npc->npc_hlen - thlen;
	} else if (npf_iscached(npc, NPC_IP6)) {
		const struct ip6_hdr *ip6 = npc->npc_ip.v6;
		return ntohs(ip6->ip6_plen) -
		    (npc->npc_hlen - sizeof(*ip6)) - thlen;
	}
	return 0;
}

/*
 * npf_fetch_tcpopts: parse and return TCP options.
 */
bool
npf_fetch_tcpopts(npf_cache_t *npc, uint16_t *mss, int *wscale)
{
	nbuf_t *nbuf = npc->npc_nbuf;
	const struct tcphdr *th = npc->npc_l4.tcp;
	int topts_len, step;
	bool setmss = false;
	uint8_t *nptr;
	uint8_t val;
	bool ok;

	KASSERT(npf_iscached(npc, NPC_IP46));
	KASSERT(npf_iscached(npc, NPC_TCP));

	/* Determine if there are any TCP options, get their length. */
	topts_len = (th->th_off << 2) - sizeof(struct tcphdr);
	if (topts_len <= 0) {
		/* No options. */
		return false;
	}
	KASSERT(topts_len <= MAX_TCPOPTLEN);

	/* Determine if we want to set or get the mss. */
	if (mss) {
		setmss = (*mss != 0);
	}

	/* First step: IP and TCP header up to options. */
	step = npc->npc_hlen + sizeof(struct tcphdr);
	nbuf_reset(nbuf);
next:
	if ((nptr = nbuf_advance(nbuf, step, 1)) == NULL) {
		ok = false;
		goto done;
	}
	val = *nptr;

	switch (val) {
	case TCPOPT_EOL:
		/* Done. */
		ok = true;
		goto done;
	case TCPOPT_NOP:
		topts_len--;
		step = 1;
		break;
	case TCPOPT_MAXSEG:
		if ((nptr = nbuf_ensure_contig(nbuf, TCPOLEN_MAXSEG)) == NULL) {
			ok = false;
			goto done;
		}
		if (mss) {
			if (setmss) {
				memcpy(nptr + 2, mss, sizeof(uint16_t));
			} else {
				memcpy(mss, nptr + 2, sizeof(uint16_t));
			}
		}
		topts_len -= TCPOLEN_MAXSEG;
		step = TCPOLEN_MAXSEG;
		break;
	case TCPOPT_WINDOW:
		/* TCP Window Scaling (RFC 1323). */
		if ((nptr = nbuf_ensure_contig(nbuf, TCPOLEN_WINDOW)) == NULL) {
			ok = false;
			goto done;
		}
		val = *(nptr + 2);
		*wscale = (val > TCP_MAX_WINSHIFT) ? TCP_MAX_WINSHIFT : val;
		topts_len -= TCPOLEN_WINDOW;
		step = TCPOLEN_WINDOW;
		break;
	default:
		if ((nptr = nbuf_ensure_contig(nbuf, 2)) == NULL) {
			ok = false;
			goto done;
		}
		val = *(nptr + 1);
		if (val < 2 || val > topts_len) {
			ok = false;
			goto done;
		}
		topts_len -= val;
		step = val;
	}

	/* Any options left? */
	if (__predict_true(topts_len > 0)) {
		goto next;
	}
	ok = true;
done:
	if (nbuf_flag_p(nbuf, NBUF_DATAREF_RESET)) {
		npf_recache(npc);
	}
	return ok;
}

static int
npf_cache_ip(npf_cache_t *npc, nbuf_t *nbuf)
{
	const void *nptr = nbuf_dataptr(nbuf);
	const uint8_t ver = *(const uint8_t *)nptr;
	int flags = 0;

	/*
	 * We intentionally don't read the L4 payload after IPPROTO_AH.
	 */

	switch (ver >> 4) {
	case IPVERSION: {
		struct ip *ip;

		ip = nbuf_ensure_contig(nbuf, sizeof(struct ip));
		if (ip == NULL) {
			return NPC_FMTERR;
		}

		/* Retrieve the complete header. */
		if ((u_int)(ip->ip_hl << 2) < sizeof(struct ip)) {
			return NPC_FMTERR;
		}
		ip = nbuf_ensure_contig(nbuf, (u_int)(ip->ip_hl << 2));
		if (ip == NULL) {
			return NPC_FMTERR;
		}

		if (ip->ip_off & ~htons(IP_DF | IP_RF)) {
			/* Note fragmentation. */
			flags |= NPC_IPFRAG;
		}

		/* Cache: layer 3 - IPv4. */
		npc->npc_alen = sizeof(struct in_addr);
		npc->npc_ips[NPF_SRC] = (npf_addr_t *)&ip->ip_src;
		npc->npc_ips[NPF_DST] = (npf_addr_t *)&ip->ip_dst;
		npc->npc_hlen = ip->ip_hl << 2;
		npc->npc_proto = ip->ip_p;

		npc->npc_ip.v4 = ip;
		flags |= NPC_IP4;
		break;
	}

	case (IPV6_VERSION >> 4): {
		struct ip6_hdr *ip6;
		struct ip6_ext *ip6e;
		struct ip6_frag *ip6f;
		size_t off, hlen;
		int frag_present;

		ip6 = nbuf_ensure_contig(nbuf, sizeof(struct ip6_hdr));
		if (ip6 == NULL) {
			return NPC_FMTERR;
		}

		/*
		 * XXX: We don't handle IPv6 Jumbograms.
		 */

		/* Set initial next-protocol value. */
		hlen = sizeof(struct ip6_hdr);
		npc->npc_proto = ip6->ip6_nxt;
		npc->npc_hlen = hlen;

		frag_present = 0;

		/*
		 * Advance by the length of the current header.
		 */
		off = nbuf_offset(nbuf);
		while ((ip6e = nbuf_advance(nbuf, hlen, sizeof(*ip6e))) != NULL) {
			/*
			 * Determine whether we are going to continue.
			 */
			switch (npc->npc_proto) {
			case IPPROTO_HOPOPTS:
			case IPPROTO_DSTOPTS:
			case IPPROTO_ROUTING:
				hlen = (ip6e->ip6e_len + 1) << 3;
				break;
			case IPPROTO_FRAGMENT:
				if (frag_present++)
					return NPC_FMTERR;
				ip6f = nbuf_ensure_contig(nbuf, sizeof(*ip6f));
				if (ip6f == NULL)
					return NPC_FMTERR;

				/* RFC6946: Skip dummy fragments. */
				if (!ntohs(ip6f->ip6f_offlg & IP6F_OFF_MASK) &&
				    !(ip6f->ip6f_offlg & IP6F_MORE_FRAG)) {
					hlen = sizeof(struct ip6_frag);
					break;
				}

				hlen = 0;
				flags |= NPC_IPFRAG;

				break;
			default:
				hlen = 0;
				break;
			}

			if (!hlen) {
				break;
			}
			npc->npc_proto = ip6e->ip6e_nxt;
			npc->npc_hlen += hlen;
		}

		if (ip6e == NULL) {
			return NPC_FMTERR;
		}

		/*
		 * Re-fetch the header pointers (nbufs might have been
		 * reallocated).  Restore the original offset (if any).
		 */
		nbuf_reset(nbuf);
		ip6 = nbuf_dataptr(nbuf);
		if (off) {
			nbuf_advance(nbuf, off, 0);
		}

		/* Cache: layer 3 - IPv6. */
		npc->npc_alen = sizeof(struct in6_addr);
		npc->npc_ips[NPF_SRC] = (npf_addr_t *)&ip6->ip6_src;
		npc->npc_ips[NPF_DST] = (npf_addr_t *)&ip6->ip6_dst;

		npc->npc_ip.v6 = ip6;
		flags |= NPC_IP6;
		break;
	}
	default:
		break;
	}
	return flags;
}

/*
 * npf_cache_all: general routine to cache all relevant IP (v4 or v6)
 * and TCP, UDP or ICMP headers.
 *
 * => nbuf offset shall be set accordingly.
 */
int
npf_cache_all(npf_cache_t *npc)
{
	nbuf_t *nbuf = npc->npc_nbuf;
	int flags, l4flags;
	u_int hlen;

	/*
	 * This routine is a main point where the references are cached,
	 * therefore clear the flag as we reset.
	 */
again:
	nbuf_unset_flag(nbuf, NBUF_DATAREF_RESET);

	/*
	 * First, cache the L3 header (IPv4 or IPv6).  If IP packet is
	 * fragmented, then we cannot look into L4.
	 */
	flags = npf_cache_ip(npc, nbuf);
	if ((flags & NPC_IP46) == 0 || (flags & NPC_IPFRAG) != 0 ||
	    (flags & NPC_FMTERR) != 0) {
		goto out;
	}
	hlen = npc->npc_hlen;

	/*
	 * Note: we guarantee that the potential "Query Id" field of the
	 * ICMPv4/ICMPv6 packets is in the nbuf. This field is used in the
	 * ICMP ALG.
	 */
	switch (npc->npc_proto) {
	case IPPROTO_TCP:
		/* Cache: layer 4 - TCP. */
		npc->npc_l4.tcp = nbuf_advance(nbuf, hlen,
		    sizeof(struct tcphdr));
		l4flags = NPC_LAYER4 | NPC_TCP;
		break;
	case IPPROTO_UDP:
		/* Cache: layer 4 - UDP. */
		npc->npc_l4.udp = nbuf_advance(nbuf, hlen,
		    sizeof(struct udphdr));
		l4flags = NPC_LAYER4 | NPC_UDP;
		break;
	case IPPROTO_ICMP:
		/* Cache: layer 4 - ICMPv4. */
		npc->npc_l4.icmp = nbuf_advance(nbuf, hlen,
		    ICMP_MINLEN);
		l4flags = NPC_LAYER4 | NPC_ICMP;
		break;
	case IPPROTO_ICMPV6:
		/* Cache: layer 4 - ICMPv6. */
		npc->npc_l4.icmp6 = nbuf_advance(nbuf, hlen,
		    sizeof(struct icmp6_hdr));
		l4flags = NPC_LAYER4 | NPC_ICMP;
		break;
	default:
		l4flags = 0;
		break;
	}

	/* Error out if nbuf_advance failed. */
	if (l4flags && npc->npc_l4.hdr == NULL) {
		goto err;
	}

	if (nbuf_flag_p(nbuf, NBUF_DATAREF_RESET)) {
		goto again;
	}

	flags |= l4flags;
	npc->npc_info |= flags;
	return flags;

err:
	flags = NPC_FMTERR;
out:
	nbuf_unset_flag(nbuf, NBUF_DATAREF_RESET);
	npc->npc_info |= flags;
	return flags;
}

void
npf_recache(npf_cache_t *npc)
{
	nbuf_t *nbuf = npc->npc_nbuf;
	const int mflags __diagused = npc->npc_info & (NPC_IP46 | NPC_LAYER4);
	int flags __diagused;

	nbuf_reset(nbuf);
	npc->npc_info = 0;
	flags = npf_cache_all(npc);

	KASSERT((flags & mflags) == mflags);
	KASSERT(nbuf_flag_p(nbuf, NBUF_DATAREF_RESET) == 0);
}

/*
 * npf_rwrip: rewrite required IP address.
 */
bool
npf_rwrip(const npf_cache_t *npc, u_int which, const npf_addr_t *addr)
{
	KASSERT(npf_iscached(npc, NPC_IP46));
	KASSERT(which == NPF_SRC || which == NPF_DST);

	memcpy(npc->npc_ips[which], addr, npc->npc_alen);
	return true;
}

/*
 * npf_rwrport: rewrite required TCP/UDP port.
 */
bool
npf_rwrport(const npf_cache_t *npc, u_int which, const in_port_t port)
{
	const int proto = npc->npc_proto;
	in_port_t *oport;

	KASSERT(npf_iscached(npc, NPC_TCP) || npf_iscached(npc, NPC_UDP));
	KASSERT(proto == IPPROTO_TCP || proto == IPPROTO_UDP);
	KASSERT(which == NPF_SRC || which == NPF_DST);

	/* Get the offset and store the port in it. */
	if (proto == IPPROTO_TCP) {
		struct tcphdr *th = npc->npc_l4.tcp;
		oport = (which == NPF_SRC) ? &th->th_sport : &th->th_dport;
	} else {
		struct udphdr *uh = npc->npc_l4.udp;
		oport = (which == NPF_SRC) ? &uh->uh_sport : &uh->uh_dport;
	}
	memcpy(oport, &port, sizeof(in_port_t));
	return true;
}

/*
 * npf_rwrcksum: rewrite IPv4 and/or TCP/UDP checksum.
 */
bool
npf_rwrcksum(const npf_cache_t *npc, u_int which,
    const npf_addr_t *addr, const in_port_t port)
{
	const npf_addr_t *oaddr = npc->npc_ips[which];
	const int proto = npc->npc_proto;
	const int alen = npc->npc_alen;
	uint16_t *ocksum;
	in_port_t oport;

	KASSERT(npf_iscached(npc, NPC_LAYER4));
	KASSERT(which == NPF_SRC || which == NPF_DST);

	if (npf_iscached(npc, NPC_IP4)) {
		struct ip *ip = npc->npc_ip.v4;
		uint16_t ipsum = ip->ip_sum;

		/* Recalculate IPv4 checksum and rewrite. */
		ip->ip_sum = npf_addr_cksum(ipsum, alen, oaddr, addr);
	} else {
		/* No checksum for IPv6. */
		KASSERT(npf_iscached(npc, NPC_IP6));
	}

	/* Nothing else to do for ICMP. */
	if (proto == IPPROTO_ICMP || proto == IPPROTO_ICMPV6) {
		return true;
	}
	KASSERT(npf_iscached(npc, NPC_TCP) || npf_iscached(npc, NPC_UDP));

	/*
	 * Calculate TCP/UDP checksum:
	 * - Skip if UDP and the current checksum is zero.
	 * - Fixup the IP address change.
	 * - Fixup the port change, if required (non-zero).
	 */
	if (proto == IPPROTO_TCP) {
		struct tcphdr *th = npc->npc_l4.tcp;

		ocksum = &th->th_sum;
		oport = (which == NPF_SRC) ? th->th_sport : th->th_dport;
	} else {
		struct udphdr *uh = npc->npc_l4.udp;

		KASSERT(proto == IPPROTO_UDP);
		ocksum = &uh->uh_sum;
		if (*ocksum == 0) {
			/* No need to update. */
			return true;
		}
		oport = (which == NPF_SRC) ? uh->uh_sport : uh->uh_dport;
	}

	uint16_t cksum = npf_addr_cksum(*ocksum, alen, oaddr, addr);
	if (port) {
		cksum = npf_fixup16_cksum(cksum, oport, port);
	}

	/* Rewrite TCP/UDP checksum. */
	memcpy(ocksum, &cksum, sizeof(uint16_t));
	return true;
}

/*
 * npf_napt_rwr: perform address and/or port translation.
 */
int
npf_napt_rwr(const npf_cache_t *npc, u_int which,
    const npf_addr_t *addr, const in_addr_t port)
{
	const unsigned proto = npc->npc_proto;

	/*
	 * Rewrite IP and/or TCP/UDP checksums first, since we need the
	 * current (old) address/port for the calculations.  Then perform
	 * the address translation i.e. rewrite source or destination.
	 */
	if (!npf_rwrcksum(npc, which, addr, port)) {
		return EINVAL;
	}
	if (!npf_rwrip(npc, which, addr)) {
		return EINVAL;
	}
	if (port == 0) {
		/* Done. */
		return 0;
	}

	switch (proto) {
	case IPPROTO_TCP:
	case IPPROTO_UDP:
		/* Rewrite source/destination port. */
		if (!npf_rwrport(npc, which, port)) {
			return EINVAL;
		}
		break;
	case IPPROTO_ICMP:
	case IPPROTO_ICMPV6:
		KASSERT(npf_iscached(npc, NPC_ICMP));
		/* Nothing. */
		break;
	default:
		return ENOTSUP;
	}
	return 0;
}

/*
 * IPv6-to-IPv6 Network Prefix Translation (NPTv6), as per RFC 6296.
 */

int
npf_npt66_rwr(const npf_cache_t *npc, u_int which, const npf_addr_t *pref,
    npf_netmask_t len, uint16_t adj)
{
	npf_addr_t *addr = npc->npc_ips[which];
	unsigned remnant, word, preflen = len >> 4;
	uint32_t sum;

	KASSERT(which == NPF_SRC || which == NPF_DST);

	if (!npf_iscached(npc, NPC_IP6)) {
		return EINVAL;
	}
	if (len <= 48) {
		/*
		 * The word to adjust.  Cannot translate the 0xffff
		 * subnet if /48 or shorter.
		 */
		word = 3;
		if (addr->word16[word] == 0xffff) {
			return EINVAL;
		}
	} else {
		/*
		 * Also, all 0s or 1s in the host part are disallowed for
		 * longer than /48 prefixes.
		 */
		if ((addr->word32[2] == 0 && addr->word32[3] == 0) ||
		    (addr->word32[2] == ~0U && addr->word32[3] == ~0U))
			return EINVAL;

		/* Determine the 16-bit word to adjust. */
		for (word = 4; word < 8; word++)
			if (addr->word16[word] != 0xffff)
				break;
	}

	/* Rewrite the prefix. */
	for (unsigned i = 0; i < preflen; i++) {
		addr->word16[i] = pref->word16[i];
	}

	/*
	 * If prefix length is within a 16-bit word (not dividable by 16),
	 * then prepare a mask, determine the word and adjust it.
	 */
	if ((remnant = len - (preflen << 4)) != 0) {
		const uint16_t wordmask = (1U << remnant) - 1;
		const unsigned i = preflen;

		addr->word16[i] = (pref->word16[i] & wordmask) |
		    (addr->word16[i] & ~wordmask);
	}

	/*
	 * Performing 1's complement sum/difference.
	 */
	sum = addr->word16[word] + adj;
	while (sum >> 16) {
		sum = (sum >> 16) + (sum & 0xffff);
	}
	if (sum == 0xffff) {
		/* RFC 1071. */
		sum = 0x0000;
	}
	addr->word16[word] = sum;
	return 0;
}

#if defined(DDB) || defined(_NPF_TESTING)

const char *
npf_addr_dump(const npf_addr_t *addr, int alen)
{
	if (alen == sizeof(struct in_addr)) {
		struct in_addr ip;
		memcpy(&ip, addr, alen);
		return inet_ntoa(ip);
	}
	return "[IPv6]";
}

#endif