xref: /src/sys/netinet/ip_output.c

/*	$NetBSD: ip_output.c,v 1.83.2.2 2001/03/13 20:29:49 nathanw Exp $	*/

/*
 * Copyright (C) 1995, 1996, 1997, and 1998 WIDE Project.
 * 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 project 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 PROJECT 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 PROJECT 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.
 */

/*-
 * Copyright (c) 1998 The NetBSD Foundation, Inc.
 * All rights reserved.
 *
 * This code is derived from software contributed to The NetBSD Foundation
 * by Public Access Networks Corporation ("Panix").  It was developed under
 * contract to Panix by Eric Haszlakiewicz and Thor Lancelot Simon.
 *
 * 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. All advertising materials mentioning features or use of this software
 *    must display the following acknowledgement:
 *	This product includes software developed by the NetBSD
 *	Foundation, Inc. and its contributors.
 * 4. Neither the name of The NetBSD Foundation 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 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.
 */

/*
 * Copyright (c) 1982, 1986, 1988, 1990, 1993
 *	The Regents of the University of California.  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. All advertising materials mentioning features or use of this software
 *    must display the following acknowledgement:
 *	This product includes software developed by the University of
 *	California, Berkeley and its contributors.
 * 4. Neither the name of the University 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 REGENTS 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 REGENTS 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.
 *
 *	@(#)ip_output.c	8.3 (Berkeley) 1/21/94
 */

#include "opt_pfil_hooks.h"
#include "opt_ipsec.h"
#include "opt_mrouting.h"

#include <sys/param.h>
#include <sys/malloc.h>
#include <sys/mbuf.h>
#include <sys/errno.h>
#include <sys/protosw.h>
#include <sys/socket.h>
#include <sys/socketvar.h>
#include <sys/systm.h>
#include <sys/lwp.h>
#include <sys/proc.h>

#include <net/if.h>
#include <net/route.h>
#include <net/pfil.h>

#include <netinet/in.h>
#include <netinet/in_systm.h>
#include <netinet/ip.h>
#include <netinet/in_pcb.h>
#include <netinet/in_var.h>
#include <netinet/ip_var.h>

#ifdef MROUTING
#include <netinet/ip_mroute.h>
#endif

#ifdef __vax__
#include <machine/mtpr.h>
#endif

#include <machine/stdarg.h>

#ifdef IPSEC
#include <netinet6/ipsec.h>
#include <netkey/key.h>
#include <netkey/key_debug.h>
#endif /*IPSEC*/

static struct mbuf *ip_insertoptions __P((struct mbuf *, struct mbuf *, int *));
static struct ifnet *ip_multicast_if __P((struct in_addr *, int *));
static void ip_mloopback
	__P((struct ifnet *, struct mbuf *, struct sockaddr_in *));

#ifdef PFIL_HOOKS
extern struct pfil_head inet_pfil_hook;			/* XXX */
#endif

/*
 * IP output.  The packet in mbuf chain m contains a skeletal IP
 * header (with len, off, ttl, proto, tos, src, dst).
 * The mbuf chain containing the packet will be freed.
 * The mbuf opt, if present, will not be freed.
 */
int
#if __STDC__
ip_output(struct mbuf *m0, ...)
#else
ip_output(m0, va_alist)
	struct mbuf *m0;
	va_dcl
#endif
{
	struct ip *ip, *mhip;
	struct ifnet *ifp;
	struct mbuf *m = m0;
	int hlen = sizeof (struct ip);
	int len, off, error = 0;
	struct route iproute;
	struct sockaddr_in *dst;
	struct in_ifaddr *ia;
	struct mbuf *opt;
	struct route *ro;
	int flags;
	int *mtu_p;
	int mtu;
	struct ip_moptions *imo;
	va_list ap;
#ifdef IPSEC
	struct socket *so;
	struct secpolicy *sp = NULL;
#endif /*IPSEC*/
	u_int16_t ip_len;

	va_start(ap, m0);
	opt = va_arg(ap, struct mbuf *);
	ro = va_arg(ap, struct route *);
	flags = va_arg(ap, int);
	imo = va_arg(ap, struct ip_moptions *);
	if (flags & IP_RETURNMTU)
		mtu_p = va_arg(ap, int *);
	else
		mtu_p = NULL;
	va_end(ap);

#ifdef IPSEC
	so = ipsec_getsocket(m);
	(void)ipsec_setsocket(m, NULL);
#endif /*IPSEC*/

#ifdef	DIAGNOSTIC
	if ((m->m_flags & M_PKTHDR) == 0)
		panic("ip_output no HDR");
#endif
	if (opt) {
		m = ip_insertoptions(m, opt, &len);
		hlen = len;
	}
	ip = mtod(m, struct ip *);
	/*
	 * Fill in IP header.
	 */
	if ((flags & (IP_FORWARDING|IP_RAWOUTPUT)) == 0) {
		ip->ip_v = IPVERSION;
		ip->ip_off &= IP_DF;
		ip->ip_id = htons(ip_id++);
		ip->ip_hl = hlen >> 2;
		ipstat.ips_localout++;
	} else {
		hlen = ip->ip_hl << 2;
	}
	/*
	 * Route packet.
	 */
	if (ro == 0) {
		ro = &iproute;
		bzero((caddr_t)ro, sizeof (*ro));
	}
	dst = satosin(&ro->ro_dst);
	/*
	 * If there is a cached route,
	 * check that it is to the same destination
	 * and is still up.  If not, free it and try again.
	 */
	if (ro->ro_rt && ((ro->ro_rt->rt_flags & RTF_UP) == 0 ||
	    !in_hosteq(dst->sin_addr, ip->ip_dst))) {
		RTFREE(ro->ro_rt);
		ro->ro_rt = (struct rtentry *)0;
	}
	if (ro->ro_rt == 0) {
		dst->sin_family = AF_INET;
		dst->sin_len = sizeof(*dst);
		dst->sin_addr = ip->ip_dst;
	}
	/*
	 * If routing to interface only,
	 * short circuit routing lookup.
	 */
	if (flags & IP_ROUTETOIF) {
		if ((ia = ifatoia(ifa_ifwithladdr(sintosa(dst)))) == 0) {
			ipstat.ips_noroute++;
			error = ENETUNREACH;
			goto bad;
		}
		ifp = ia->ia_ifp;
		mtu = ifp->if_mtu;
		ip->ip_ttl = 1;
	} else {
		if (ro->ro_rt == 0)
			rtalloc(ro);
		if (ro->ro_rt == 0) {
			ipstat.ips_noroute++;
			error = EHOSTUNREACH;
			goto bad;
		}
		ia = ifatoia(ro->ro_rt->rt_ifa);
		ifp = ro->ro_rt->rt_ifp;
		if ((mtu = ro->ro_rt->rt_rmx.rmx_mtu) == 0)
			mtu = ifp->if_mtu;
		ro->ro_rt->rt_use++;
		if (ro->ro_rt->rt_flags & RTF_GATEWAY)
			dst = satosin(ro->ro_rt->rt_gateway);
	}
	if (IN_MULTICAST(ip->ip_dst.s_addr) ||
	    (ip->ip_dst.s_addr == INADDR_BROADCAST)) {
		struct in_multi *inm;

		m->m_flags |= (ip->ip_dst.s_addr == INADDR_BROADCAST) ?
			M_BCAST : M_MCAST;
		/*
		 * IP destination address is multicast.  Make sure "dst"
		 * still points to the address in "ro".  (It may have been
		 * changed to point to a gateway address, above.)
		 */
		dst = satosin(&ro->ro_dst);
		/*
		 * See if the caller provided any multicast options
		 */
		if (imo != NULL) {
			ip->ip_ttl = imo->imo_multicast_ttl;
			if (imo->imo_multicast_ifp != NULL) {
				ifp = imo->imo_multicast_ifp;
				mtu = ifp->if_mtu;
			}
		} else
			ip->ip_ttl = IP_DEFAULT_MULTICAST_TTL;
		/*
		 * Confirm that the outgoing interface supports multicast.
		 */
		if (((m->m_flags & M_MCAST) &&
		     (ifp->if_flags & IFF_MULTICAST) == 0) ||
		    ((m->m_flags & M_BCAST) &&
		     (ifp->if_flags & IFF_BROADCAST) == 0))  {
			ipstat.ips_noroute++;
			error = ENETUNREACH;
			goto bad;
		}
		/*
		 * If source address not specified yet, use an address
		 * of outgoing interface.
		 */
		if (in_nullhost(ip->ip_src)) {
			struct in_ifaddr *ia;

			IFP_TO_IA(ifp, ia);
			ip->ip_src = ia->ia_addr.sin_addr;
		}

		IN_LOOKUP_MULTI(ip->ip_dst, ifp, inm);
		if (inm != NULL &&
		   (imo == NULL || imo->imo_multicast_loop)) {
			/*
			 * If we belong to the destination multicast group
			 * on the outgoing interface, and the caller did not
			 * forbid loopback, loop back a copy.
			 */
			ip_mloopback(ifp, m, dst);
		}
#ifdef MROUTING
		else {
			/*
			 * If we are acting as a multicast router, perform
			 * multicast forwarding as if the packet had just
			 * arrived on the interface to which we are about
			 * to send.  The multicast forwarding function
			 * recursively calls this function, using the
			 * IP_FORWARDING flag to prevent infinite recursion.
			 *
			 * Multicasts that are looped back by ip_mloopback(),
			 * above, will be forwarded by the ip_input() routine,
			 * if necessary.
			 */
			extern struct socket *ip_mrouter;

			if (ip_mrouter && (flags & IP_FORWARDING) == 0) {
				if (ip_mforward(m, ifp) != 0) {
					m_freem(m);
					goto done;
				}
			}
		}
#endif
		/*
		 * Multicasts with a time-to-live of zero may be looped-
		 * back, above, but must not be transmitted on a network.
		 * Also, multicasts addressed to the loopback interface
		 * are not sent -- the above call to ip_mloopback() will
		 * loop back a copy if this host actually belongs to the
		 * destination group on the loopback interface.
		 */
		if (ip->ip_ttl == 0 || (ifp->if_flags & IFF_LOOPBACK) != 0) {
			m_freem(m);
			goto done;
		}

		goto sendit;
	}
#ifndef notdef
	/*
	 * If source address not specified yet, use address
	 * of outgoing interface.
	 */
	if (in_nullhost(ip->ip_src))
		ip->ip_src = ia->ia_addr.sin_addr;
#endif

	/*
	 * packets with Class-D address as source are not valid per
	 * RFC 1112
	 */
	if (IN_MULTICAST(ip->ip_src.s_addr)) {
		ipstat.ips_odropped++;
		error = EADDRNOTAVAIL;
		goto bad;
	}

	/*
	 * Look for broadcast address and
	 * and verify user is allowed to send
	 * such a packet.
	 */
	if (in_broadcast(dst->sin_addr, ifp)) {
		if ((ifp->if_flags & IFF_BROADCAST) == 0) {
			error = EADDRNOTAVAIL;
			goto bad;
		}
		if ((flags & IP_ALLOWBROADCAST) == 0) {
			error = EACCES;
			goto bad;
		}
		/* don't allow broadcast messages to be fragmented */
		if ((u_int16_t)ip->ip_len > ifp->if_mtu) {
			error = EMSGSIZE;
			goto bad;
		}
		m->m_flags |= M_BCAST;
	} else
		m->m_flags &= ~M_BCAST;

sendit:
	/*
	 * If we're doing Path MTU Discovery, we need to set DF unless
	 * the route's MTU is locked.
	 */
	if ((flags & IP_MTUDISC) != 0 && ro->ro_rt != NULL &&
	    (ro->ro_rt->rt_rmx.rmx_locks & RTV_MTU) == 0)
		ip->ip_off |= IP_DF;

	/*
	 * Remember the current ip_len and ip_off, and swap them into
	 * network order.
	 */
	ip_len = ip->ip_len;

	HTONS(ip->ip_len);
	HTONS(ip->ip_off);

#ifdef IPSEC
	/* get SP for this packet */
	if (so == NULL)
		sp = ipsec4_getpolicybyaddr(m, IPSEC_DIR_OUTBOUND, flags, &error);
	else
		sp = ipsec4_getpolicybysock(m, IPSEC_DIR_OUTBOUND, so, &error);

	if (sp == NULL) {
		ipsecstat.out_inval++;
		goto bad;
	}

	error = 0;

	/* check policy */
	switch (sp->policy) {
	case IPSEC_POLICY_DISCARD:
		/*
		 * This packet is just discarded.
		 */
		ipsecstat.out_polvio++;
		goto bad;

	case IPSEC_POLICY_BYPASS:
	case IPSEC_POLICY_NONE:
		/* no need to do IPsec. */
		goto skip_ipsec;

	case IPSEC_POLICY_IPSEC:
		if (sp->req == NULL) {
			/* XXX should be panic ? */
			printf("ip_output: No IPsec request specified.\n");
			error = EINVAL;
			goto bad;
		}
		break;

	case IPSEC_POLICY_ENTRUST:
	default:
		printf("ip_output: Invalid policy found. %d\n", sp->policy);
	}

	/*
	 * ipsec4_output() expects ip_len and ip_off in network
	 * order.  They have been set to network order above.
	 */

    {
	struct ipsec_output_state state;
	bzero(&state, sizeof(state));
	state.m = m;
	if (flags & IP_ROUTETOIF) {
		state.ro = &iproute;
		bzero(&iproute, sizeof(iproute));
	} else
		state.ro = ro;
	state.dst = (struct sockaddr *)dst;

	error = ipsec4_output(&state, sp, flags);

	m = state.m;
	if (flags & IP_ROUTETOIF) {
		/*
		 * if we have tunnel mode SA, we may need to ignore
		 * IP_ROUTETOIF.
		 */
		if (state.ro != &iproute || state.ro->ro_rt != NULL) {
			flags &= ~IP_ROUTETOIF;
			ro = state.ro;
		}
	} else
		ro = state.ro;
	dst = (struct sockaddr_in *)state.dst;
	if (error) {
		/* mbuf is already reclaimed in ipsec4_output. */
		m0 = NULL;
		switch (error) {
		case EHOSTUNREACH:
		case ENETUNREACH:
		case EMSGSIZE:
		case ENOBUFS:
		case ENOMEM:
			break;
		default:
			printf("ip4_output (ipsec): error code %d\n", error);
			/*fall through*/
		case ENOENT:
			/* don't show these error codes to the user */
			error = 0;
			break;
		}
		goto bad;
	}
    }

	/* be sure to update variables that are affected by ipsec4_output() */
	ip = mtod(m, struct ip *);
#ifdef _IP_VHL
	hlen = IP_VHL_HL(ip->ip_vhl) << 2;
#else
	hlen = ip->ip_hl << 2;
#endif
	ip_len = ntohs(ip->ip_len);

	if (ro->ro_rt == NULL) {
		if ((flags & IP_ROUTETOIF) == 0) {
			printf("ip_output: "
				"can't update route after IPsec processing\n");
			error = EHOSTUNREACH;	/*XXX*/
			goto bad;
		}
	} else {
		/* nobody uses ia beyond here */
		ifp = ro->ro_rt->rt_ifp;
	}

skip_ipsec:
#endif /*IPSEC*/

#ifdef PFIL_HOOKS
	/*
	 * Run through list of hooks for output packets.
	 */
	if ((error = pfil_run_hooks(&inet_pfil_hook, &m, ifp,
				    PFIL_OUT)) != 0)
		goto done;
	if (m == NULL)
		goto done;

	ip = mtod(m, struct ip *);
#endif /* PFIL_HOOKS */

	/*
	 * If small enough for mtu of path, can just send directly.
	 */
	if (ip_len <= mtu) {
#if IFA_STATS
		/*
		 * search for the source address structure to
		 * maintain output statistics.
		 */
		INADDR_TO_IA(ip->ip_src, ia);
		if (ia)
			ia->ia_ifa.ifa_data.ifad_outbytes += ip_len;
#endif
		ip->ip_sum = 0;
		ip->ip_sum = in_cksum(m, hlen);
#ifdef IPSEC
		/* clean ipsec history once it goes out of the node */
		ipsec_delaux(m);
#endif
		error = (*ifp->if_output)(ifp, m, sintosa(dst), ro->ro_rt);
		goto done;
	}

	/*
	 * Too large for interface; fragment if possible.
	 * Must be able to put at least 8 bytes per fragment.
	 *
	 * Note we swap ip_len and ip_off into host order to make
	 * the logic below a little simpler.
	 */

	NTOHS(ip->ip_len);
	NTOHS(ip->ip_off);

	if (ip->ip_off & IP_DF) {
		if (flags & IP_RETURNMTU)
			*mtu_p = mtu;
		error = EMSGSIZE;
		ipstat.ips_cantfrag++;
		goto bad;
	}
	len = (mtu - hlen) &~ 7;
	if (len < 8) {
		error = EMSGSIZE;
		goto bad;
	}

    {
	int mhlen, firstlen = len;
	struct mbuf **mnext = &m->m_nextpkt;
	int fragments = 0;
	int s;

	/*
	 * Loop through length of segment after first fragment,
	 * make new header and copy data of each part and link onto chain.
	 */
	m0 = m;
	mhlen = sizeof (struct ip);
	for (off = hlen + len; off < (u_int16_t)ip->ip_len; off += len) {
		MGETHDR(m, M_DONTWAIT, MT_HEADER);
		if (m == 0) {
			error = ENOBUFS;
			ipstat.ips_odropped++;
			goto sendorfree;
		}
		*mnext = m;
		mnext = &m->m_nextpkt;
		m->m_data += max_linkhdr;
		mhip = mtod(m, struct ip *);
		*mhip = *ip;
		/* we must inherit MCAST and BCAST flags */
		m->m_flags |= m0->m_flags & (M_MCAST|M_BCAST);
		if (hlen > sizeof (struct ip)) {
			mhlen = ip_optcopy(ip, mhip) + sizeof (struct ip);
			mhip->ip_hl = mhlen >> 2;
		}
		m->m_len = mhlen;
		mhip->ip_off = ((off - hlen) >> 3) + (ip->ip_off & ~IP_MF);
		if (ip->ip_off & IP_MF)
			mhip->ip_off |= IP_MF;
		if (off + len >= (u_int16_t)ip->ip_len)
			len = (u_int16_t)ip->ip_len - off;
		else
			mhip->ip_off |= IP_MF;
		mhip->ip_len = htons((u_int16_t)(len + mhlen));
		m->m_next = m_copy(m0, off, len);
		if (m->m_next == 0) {
			error = ENOBUFS;	/* ??? */
			ipstat.ips_odropped++;
			goto sendorfree;
		}
		m->m_pkthdr.len = mhlen + len;
		m->m_pkthdr.rcvif = (struct ifnet *)0;
		HTONS(mhip->ip_off);
		mhip->ip_sum = 0;
		mhip->ip_sum = in_cksum(m, mhlen);
		ipstat.ips_ofragments++;
		fragments++;
	}
	/*
	 * Update first fragment by trimming what's been copied out
	 * and updating header, then send each fragment (in order).
	 */
	m = m0;
	m_adj(m, hlen + firstlen - (u_int16_t)ip->ip_len);
	m->m_pkthdr.len = hlen + firstlen;
	ip->ip_len = htons((u_int16_t)m->m_pkthdr.len);
	ip->ip_off |= IP_MF;
	HTONS(ip->ip_off);
	ip->ip_sum = 0;
	ip->ip_sum = in_cksum(m, hlen);
sendorfree:
	/*
	 * If there is no room for all the fragments, don't queue
	 * any of them.
	 */
	s = splimp();
	if (ifp->if_snd.ifq_maxlen - ifp->if_snd.ifq_len < fragments)
		error = ENOBUFS;
	splx(s);
	for (m = m0; m; m = m0) {
		m0 = m->m_nextpkt;
		m->m_nextpkt = 0;
		if (error == 0) {
#if IFA_STATS
			/*
			 * search for the source address structure to
			 * maintain output statistics.
			 */
			INADDR_TO_IA(ip->ip_src, ia);
			if (ia) {
				ia->ia_ifa.ifa_data.ifad_outbytes +=
					ntohs(ip->ip_len);
			}
#endif
#ifdef IPSEC
			/* clean ipsec history once it goes out of the node */
			ipsec_delaux(m);
#endif
			error = (*ifp->if_output)(ifp, m, sintosa(dst),
			    ro->ro_rt);
		} else
			m_freem(m);
	}

	if (error == 0)
		ipstat.ips_fragmented++;
    }
done:
	if (ro == &iproute && (flags & IP_ROUTETOIF) == 0 && ro->ro_rt) {
		RTFREE(ro->ro_rt);
		ro->ro_rt = 0;
	}

#ifdef IPSEC
	if (sp != NULL) {
		KEYDEBUG(KEYDEBUG_IPSEC_STAMP,
			printf("DP ip_output call free SP:%p\n", sp));
		key_freesp(sp);
	}
#endif /* IPSEC */

	return (error);
bad:
	m_freem(m);
	goto done;
}

/*
 * Determine the maximum length of the options to be inserted;
 * we would far rather allocate too much space rather than too little.
 */

u_int
ip_optlen(inp)
	struct inpcb *inp;
{
	struct mbuf *m = inp->inp_options;

	if (m && m->m_len > offsetof(struct ipoption, ipopt_dst))
		return(m->m_len - offsetof(struct ipoption, ipopt_dst));
	else
		return 0;
}


/*
 * Insert IP options into preformed packet.
 * Adjust IP destination as required for IP source routing,
 * as indicated by a non-zero in_addr at the start of the options.
 */
static struct mbuf *
ip_insertoptions(m, opt, phlen)
	struct mbuf *m;
	struct mbuf *opt;
	int *phlen;
{
	struct ipoption *p = mtod(opt, struct ipoption *);
	struct mbuf *n;
	struct ip *ip = mtod(m, struct ip *);
	unsigned optlen;

	optlen = opt->m_len - sizeof(p->ipopt_dst);
	if (optlen + (u_int16_t)ip->ip_len > IP_MAXPACKET)
		return (m);		/* XXX should fail */
	if (!in_nullhost(p->ipopt_dst))
		ip->ip_dst = p->ipopt_dst;
	if (m->m_flags & M_EXT || m->m_data - optlen < m->m_pktdat) {
		MGETHDR(n, M_DONTWAIT, MT_HEADER);
		if (n == 0)
			return (m);
		n->m_pkthdr.len = m->m_pkthdr.len + optlen;
		m->m_len -= sizeof(struct ip);
		m->m_data += sizeof(struct ip);
		n->m_next = m;
		m = n;
		m->m_len = optlen + sizeof(struct ip);
		m->m_data += max_linkhdr;
		bcopy((caddr_t)ip, mtod(m, caddr_t), sizeof(struct ip));
	} else {
		m->m_data -= optlen;
		m->m_len += optlen;
		m->m_pkthdr.len += optlen;
		memmove(mtod(m, caddr_t), ip, sizeof(struct ip));
	}
	ip = mtod(m, struct ip *);
	bcopy((caddr_t)p->ipopt_list, (caddr_t)(ip + 1), (unsigned)optlen);
	*phlen = sizeof(struct ip) + optlen;
	ip->ip_len += optlen;
	return (m);
}

/*
 * Copy options from ip to jp,
 * omitting those not copied during fragmentation.
 */
int
ip_optcopy(ip, jp)
	struct ip *ip, *jp;
{
	u_char *cp, *dp;
	int opt, optlen, cnt;

	cp = (u_char *)(ip + 1);
	dp = (u_char *)(jp + 1);
	cnt = (ip->ip_hl << 2) - sizeof (struct ip);
	for (; cnt > 0; cnt -= optlen, cp += optlen) {
		opt = cp[0];
		if (opt == IPOPT_EOL)
			break;
		if (opt == IPOPT_NOP) {
			/* Preserve for IP mcast tunnel's LSRR alignment. */
			*dp++ = IPOPT_NOP;
			optlen = 1;
			continue;
		}
#ifdef DIAGNOSTIC
		if (cnt < IPOPT_OLEN + sizeof(*cp))
			panic("malformed IPv4 option passed to ip_optcopy");
#endif
		optlen = cp[IPOPT_OLEN];
#ifdef DIAGNOSTIC
		if (optlen < IPOPT_OLEN + sizeof(*cp) || optlen > cnt)
			panic("malformed IPv4 option passed to ip_optcopy");
#endif
		/* bogus lengths should have been caught by ip_dooptions */
		if (optlen > cnt)
			optlen = cnt;
		if (IPOPT_COPIED(opt)) {
			bcopy((caddr_t)cp, (caddr_t)dp, (unsigned)optlen);
			dp += optlen;
		}
	}
	for (optlen = dp - (u_char *)(jp+1); optlen & 0x3; optlen++)
		*dp++ = IPOPT_EOL;
	return (optlen);
}

/*
 * IP socket option processing.
 */
int
ip_ctloutput(op, so, level, optname, mp)
	int op;
	struct socket *so;
	int level, optname;
	struct mbuf **mp;
{
	struct inpcb *inp = sotoinpcb(so);
	struct mbuf *m = *mp;
	int optval = 0;
	int error = 0;
#ifdef IPSEC
#ifdef __NetBSD__
	struct proc *p = (curproc ? curproc->l_proc : 0);	/*XXX*/
#endif
#endif

	if (level != IPPROTO_IP) {
		error = EINVAL;
		if (op == PRCO_SETOPT && *mp)
			(void) m_free(*mp);
	} else switch (op) {

	case PRCO_SETOPT:
		switch (optname) {
		case IP_OPTIONS:
#ifdef notyet
		case IP_RETOPTS:
			return (ip_pcbopts(optname, &inp->inp_options, m));
#else
			return (ip_pcbopts(&inp->inp_options, m));
#endif

		case IP_TOS:
		case IP_TTL:
		case IP_RECVOPTS:
		case IP_RECVRETOPTS:
		case IP_RECVDSTADDR:
		case IP_RECVIF:
			if (m == NULL || m->m_len != sizeof(int))
				error = EINVAL;
			else {
				optval = *mtod(m, int *);
				switch (optname) {

				case IP_TOS:
					inp->inp_ip.ip_tos = optval;
					break;

				case IP_TTL:
					inp->inp_ip.ip_ttl = optval;
					break;
#define	OPTSET(bit) \
	if (optval) \
		inp->inp_flags |= bit; \
	else \
		inp->inp_flags &= ~bit;

				case IP_RECVOPTS:
					OPTSET(INP_RECVOPTS);
					break;

				case IP_RECVRETOPTS:
					OPTSET(INP_RECVRETOPTS);
					break;

				case IP_RECVDSTADDR:
					OPTSET(INP_RECVDSTADDR);
					break;

				case IP_RECVIF:
					OPTSET(INP_RECVIF);
					break;
				}
			}
			break;
#undef OPTSET

		case IP_MULTICAST_IF:
		case IP_MULTICAST_TTL:
		case IP_MULTICAST_LOOP:
		case IP_ADD_MEMBERSHIP:
		case IP_DROP_MEMBERSHIP:
			error = ip_setmoptions(optname, &inp->inp_moptions, m);
			break;

		case IP_PORTRANGE:
			if (m == 0 || m->m_len != sizeof(int))
				error = EINVAL;
			else {
				optval = *mtod(m, int *);

				switch (optval) {

				case IP_PORTRANGE_DEFAULT:
				case IP_PORTRANGE_HIGH:
					inp->inp_flags &= ~(INP_LOWPORT);
					break;

				case IP_PORTRANGE_LOW:
					inp->inp_flags |= INP_LOWPORT;
					break;

				default:
					error = EINVAL;
					break;
				}
			}
			break;

#ifdef IPSEC
		case IP_IPSEC_POLICY:
		{
			caddr_t req = NULL;
			size_t len = 0;
			int priv = 0;

#ifdef __NetBSD__
			if (p == 0 || suser(p->p_ucred, &p->p_acflag))
				priv = 0;
			else
				priv = 1;
#else
			priv = (in6p->in6p_socket->so_state & SS_PRIV);
#endif
			if (m) {
				req = mtod(m, caddr_t);
				len = m->m_len;
			}
			error = ipsec4_set_policy(inp, optname, req, len, priv);
			break;
		    }
#endif /*IPSEC*/

		default:
			error = ENOPROTOOPT;
			break;
		}
		if (m)
			(void)m_free(m);
		break;

	case PRCO_GETOPT:
		switch (optname) {
		case IP_OPTIONS:
		case IP_RETOPTS:
			*mp = m = m_get(M_WAIT, MT_SOOPTS);
			if (inp->inp_options) {
				m->m_len = inp->inp_options->m_len;
				bcopy(mtod(inp->inp_options, caddr_t),
				    mtod(m, caddr_t), (unsigned)m->m_len);
			} else
				m->m_len = 0;
			break;

		case IP_TOS:
		case IP_TTL:
		case IP_RECVOPTS:
		case IP_RECVRETOPTS:
		case IP_RECVDSTADDR:
		case IP_RECVIF:
		case IP_ERRORMTU:
			*mp = m = m_get(M_WAIT, MT_SOOPTS);
			m->m_len = sizeof(int);
			switch (optname) {

			case IP_TOS:
				optval = inp->inp_ip.ip_tos;
				break;

			case IP_TTL:
				optval = inp->inp_ip.ip_ttl;
				break;

			case IP_ERRORMTU:
				optval = inp->inp_errormtu;
				break;

#define	OPTBIT(bit)	(inp->inp_flags & bit ? 1 : 0)

			case IP_RECVOPTS:
				optval = OPTBIT(INP_RECVOPTS);
				break;

			case IP_RECVRETOPTS:
				optval = OPTBIT(INP_RECVRETOPTS);
				break;

			case IP_RECVDSTADDR:
				optval = OPTBIT(INP_RECVDSTADDR);
				break;

			case IP_RECVIF:
				optval = OPTBIT(INP_RECVIF);
				break;
			}
			*mtod(m, int *) = optval;
			break;

#ifdef IPSEC
		case IP_IPSEC_POLICY:
		{
			caddr_t req = NULL;
			size_t len = 0;

			if (m) {
				req = mtod(m, caddr_t);
				len = m->m_len;
			}
			error = ipsec4_get_policy(inp, req, len, mp);
			break;
		}
#endif /*IPSEC*/

		case IP_MULTICAST_IF:
		case IP_MULTICAST_TTL:
		case IP_MULTICAST_LOOP:
		case IP_ADD_MEMBERSHIP:
		case IP_DROP_MEMBERSHIP:
			error = ip_getmoptions(optname, inp->inp_moptions, mp);
			break;

		case IP_PORTRANGE:
			*mp = m = m_get(M_WAIT, MT_SOOPTS);
			m->m_len = sizeof(int);

			if (inp->inp_flags & INP_LOWPORT)
				optval = IP_PORTRANGE_LOW;
			else
				optval = IP_PORTRANGE_DEFAULT;

			*mtod(m, int *) = optval;
			break;

		default:
			error = ENOPROTOOPT;
			break;
		}
		break;
	}
	return (error);
}

/*
 * Set up IP options in pcb for insertion in output packets.
 * Store in mbuf with pointer in pcbopt, adding pseudo-option
 * with destination address if source routed.
 */
int
#ifdef notyet
ip_pcbopts(optname, pcbopt, m)
	int optname;
#else
ip_pcbopts(pcbopt, m)
#endif
	struct mbuf **pcbopt;
	struct mbuf *m;
{
	int cnt, optlen;
	u_char *cp;
	u_char opt;

	/* turn off any old options */
	if (*pcbopt)
		(void)m_free(*pcbopt);
	*pcbopt = 0;
	if (m == (struct mbuf *)0 || m->m_len == 0) {
		/*
		 * Only turning off any previous options.
		 */
		if (m)
			(void)m_free(m);
		return (0);
	}

#ifndef	vax
	if (m->m_len % sizeof(int32_t))
		goto bad;
#endif
	/*
	 * IP first-hop destination address will be stored before
	 * actual options; move other options back
	 * and clear it when none present.
	 */
	if (m->m_data + m->m_len + sizeof(struct in_addr) >= &m->m_dat[MLEN])
		goto bad;
	cnt = m->m_len;
	m->m_len += sizeof(struct in_addr);
	cp = mtod(m, u_char *) + sizeof(struct in_addr);
	memmove(cp, mtod(m, caddr_t), (unsigned)cnt);
	bzero(mtod(m, caddr_t), sizeof(struct in_addr));

	for (; cnt > 0; cnt -= optlen, cp += optlen) {
		opt = cp[IPOPT_OPTVAL];
		if (opt == IPOPT_EOL)
			break;
		if (opt == IPOPT_NOP)
			optlen = 1;
		else {
			if (cnt < IPOPT_OLEN + sizeof(*cp))
				goto bad;
			optlen = cp[IPOPT_OLEN];
			if (optlen < IPOPT_OLEN  + sizeof(*cp) || optlen > cnt)
				goto bad;
		}
		switch (opt) {

		default:
			break;

		case IPOPT_LSRR:
		case IPOPT_SSRR:
			/*
			 * user process specifies route as:
			 *	->A->B->C->D
			 * D must be our final destination (but we can't
			 * check that since we may not have connected yet).
			 * A is first hop destination, which doesn't appear in
			 * actual IP option, but is stored before the options.
			 */
			if (optlen < IPOPT_MINOFF - 1 + sizeof(struct in_addr))
				goto bad;
			m->m_len -= sizeof(struct in_addr);
			cnt -= sizeof(struct in_addr);
			optlen -= sizeof(struct in_addr);
			cp[IPOPT_OLEN] = optlen;
			/*
			 * Move first hop before start of options.
			 */
			bcopy((caddr_t)&cp[IPOPT_OFFSET+1], mtod(m, caddr_t),
			    sizeof(struct in_addr));
			/*
			 * Then copy rest of options back
			 * to close up the deleted entry.
			 */
			memmove(&cp[IPOPT_OFFSET+1],
                            (caddr_t)(&cp[IPOPT_OFFSET+1] + sizeof(struct in_addr)),
			    (unsigned)cnt + sizeof(struct in_addr));
			break;
		}
	}
	if (m->m_len > MAX_IPOPTLEN + sizeof(struct in_addr))
		goto bad;
	*pcbopt = m;
	return (0);

bad:
	(void)m_free(m);
	return (EINVAL);
}

/*
 * following RFC1724 section 3.3, 0.0.0.0/8 is interpreted as interface index.
 */
static struct ifnet *
ip_multicast_if(a, ifindexp)
	struct in_addr *a;
	int *ifindexp;
{
	int ifindex;
	struct ifnet *ifp;

	if (ifindexp)
		*ifindexp = 0;
	if (ntohl(a->s_addr) >> 24 == 0) {
		ifindex = ntohl(a->s_addr) & 0xffffff;
		if (ifindex < 0 || if_index < ifindex)
			return NULL;
		ifp = ifindex2ifnet[ifindex];
		if (ifindexp)
			*ifindexp = ifindex;
	} else {
		INADDR_TO_IFP(*a, ifp);
	}
	return ifp;
}

/*
 * Set the IP multicast options in response to user setsockopt().
 */
int
ip_setmoptions(optname, imop, m)
	int optname;
	struct ip_moptions **imop;
	struct mbuf *m;
{
	int error = 0;
	u_char loop;
	int i;
	struct in_addr addr;
	struct ip_mreq *mreq;
	struct ifnet *ifp;
	struct ip_moptions *imo = *imop;
	struct route ro;
	struct sockaddr_in *dst;
	int ifindex;

	if (imo == NULL) {
		/*
		 * No multicast option buffer attached to the pcb;
		 * allocate one and initialize to default values.
		 */
		imo = (struct ip_moptions *)malloc(sizeof(*imo), M_IPMOPTS,
		    M_WAITOK);

		if (imo == NULL)
			return (ENOBUFS);
		*imop = imo;
		imo->imo_multicast_ifp = NULL;
		imo->imo_multicast_addr.s_addr = INADDR_ANY;
		imo->imo_multicast_ttl = IP_DEFAULT_MULTICAST_TTL;
		imo->imo_multicast_loop = IP_DEFAULT_MULTICAST_LOOP;
		imo->imo_num_memberships = 0;
	}

	switch (optname) {

	case IP_MULTICAST_IF:
		/*
		 * Select the interface for outgoing multicast packets.
		 */
		if (m == NULL || m->m_len != sizeof(struct in_addr)) {
			error = EINVAL;
			break;
		}
		addr = *(mtod(m, struct in_addr *));
		/*
		 * INADDR_ANY is used to remove a previous selection.
		 * When no interface is selected, a default one is
		 * chosen every time a multicast packet is sent.
		 */
		if (in_nullhost(addr)) {
			imo->imo_multicast_ifp = NULL;
			break;
		}
		/*
		 * The selected interface is identified by its local
		 * IP address.  Find the interface and confirm that
		 * it supports multicasting.
		 */
		ifp = ip_multicast_if(&addr, &ifindex);
		if (ifp == NULL || (ifp->if_flags & IFF_MULTICAST) == 0) {
			error = EADDRNOTAVAIL;
			break;
		}
		imo->imo_multicast_ifp = ifp;
		if (ifindex)
			imo->imo_multicast_addr = addr;
		else
			imo->imo_multicast_addr.s_addr = INADDR_ANY;
		break;

	case IP_MULTICAST_TTL:
		/*
		 * Set the IP time-to-live for outgoing multicast packets.
		 */
		if (m == NULL || m->m_len != 1) {
			error = EINVAL;
			break;
		}
		imo->imo_multicast_ttl = *(mtod(m, u_char *));
		break;

	case IP_MULTICAST_LOOP:
		/*
		 * Set the loopback flag for outgoing multicast packets.
		 * Must be zero or one.
		 */
		if (m == NULL || m->m_len != 1 ||
		   (loop = *(mtod(m, u_char *))) > 1) {
			error = EINVAL;
			break;
		}
		imo->imo_multicast_loop = loop;
		break;

	case IP_ADD_MEMBERSHIP:
		/*
		 * Add a multicast group membership.
		 * Group must be a valid IP multicast address.
		 */
		if (m == NULL || m->m_len != sizeof(struct ip_mreq)) {
			error = EINVAL;
			break;
		}
		mreq = mtod(m, struct ip_mreq *);
		if (!IN_MULTICAST(mreq->imr_multiaddr.s_addr)) {
			error = EINVAL;
			break;
		}
		/*
		 * If no interface address was provided, use the interface of
		 * the route to the given multicast address.
		 */
		if (in_nullhost(mreq->imr_interface)) {
			bzero((caddr_t)&ro, sizeof(ro));
			ro.ro_rt = NULL;
			dst = satosin(&ro.ro_dst);
			dst->sin_len = sizeof(*dst);
			dst->sin_family = AF_INET;
			dst->sin_addr = mreq->imr_multiaddr;
			rtalloc(&ro);
			if (ro.ro_rt == NULL) {
				error = EADDRNOTAVAIL;
				break;
			}
			ifp = ro.ro_rt->rt_ifp;
			rtfree(ro.ro_rt);
		} else {
			ifp = ip_multicast_if(&mreq->imr_interface, NULL);
		}
		/*
		 * See if we found an interface, and confirm that it
		 * supports multicast.
		 */
		if (ifp == NULL || (ifp->if_flags & IFF_MULTICAST) == 0) {
			error = EADDRNOTAVAIL;
			break;
		}
		/*
		 * See if the membership already exists or if all the
		 * membership slots are full.
		 */
		for (i = 0; i < imo->imo_num_memberships; ++i) {
			if (imo->imo_membership[i]->inm_ifp == ifp &&
			    in_hosteq(imo->imo_membership[i]->inm_addr,
				      mreq->imr_multiaddr))
				break;
		}
		if (i < imo->imo_num_memberships) {
			error = EADDRINUSE;
			break;
		}
		if (i == IP_MAX_MEMBERSHIPS) {
			error = ETOOMANYREFS;
			break;
		}
		/*
		 * Everything looks good; add a new record to the multicast
		 * address list for the given interface.
		 */
		if ((imo->imo_membership[i] =
		    in_addmulti(&mreq->imr_multiaddr, ifp)) == NULL) {
			error = ENOBUFS;
			break;
		}
		++imo->imo_num_memberships;
		break;

	case IP_DROP_MEMBERSHIP:
		/*
		 * Drop a multicast group membership.
		 * Group must be a valid IP multicast address.
		 */
		if (m == NULL || m->m_len != sizeof(struct ip_mreq)) {
			error = EINVAL;
			break;
		}
		mreq = mtod(m, struct ip_mreq *);
		if (!IN_MULTICAST(mreq->imr_multiaddr.s_addr)) {
			error = EINVAL;
			break;
		}
		/*
		 * If an interface address was specified, get a pointer
		 * to its ifnet structure.
		 */
		if (in_nullhost(mreq->imr_interface))
			ifp = NULL;
		else {
			ifp = ip_multicast_if(&mreq->imr_interface, NULL);
			if (ifp == NULL) {
				error = EADDRNOTAVAIL;
				break;
			}
		}
		/*
		 * Find the membership in the membership array.
		 */
		for (i = 0; i < imo->imo_num_memberships; ++i) {
			if ((ifp == NULL ||
			     imo->imo_membership[i]->inm_ifp == ifp) &&
			     in_hosteq(imo->imo_membership[i]->inm_addr,
				       mreq->imr_multiaddr))
				break;
		}
		if (i == imo->imo_num_memberships) {
			error = EADDRNOTAVAIL;
			break;
		}
		/*
		 * Give up the multicast address record to which the
		 * membership points.
		 */
		in_delmulti(imo->imo_membership[i]);
		/*
		 * Remove the gap in the membership array.
		 */
		for (++i; i < imo->imo_num_memberships; ++i)
			imo->imo_membership[i-1] = imo->imo_membership[i];
		--imo->imo_num_memberships;
		break;

	default:
		error = EOPNOTSUPP;
		break;
	}

	/*
	 * If all options have default values, no need to keep the mbuf.
	 */
	if (imo->imo_multicast_ifp == NULL &&
	    imo->imo_multicast_ttl == IP_DEFAULT_MULTICAST_TTL &&
	    imo->imo_multicast_loop == IP_DEFAULT_MULTICAST_LOOP &&
	    imo->imo_num_memberships == 0) {
		free(*imop, M_IPMOPTS);
		*imop = NULL;
	}

	return (error);
}

/*
 * Return the IP multicast options in response to user getsockopt().
 */
int
ip_getmoptions(optname, imo, mp)
	int optname;
	struct ip_moptions *imo;
	struct mbuf **mp;
{
	u_char *ttl;
	u_char *loop;
	struct in_addr *addr;
	struct in_ifaddr *ia;

	*mp = m_get(M_WAIT, MT_SOOPTS);

	switch (optname) {

	case IP_MULTICAST_IF:
		addr = mtod(*mp, struct in_addr *);
		(*mp)->m_len = sizeof(struct in_addr);
		if (imo == NULL || imo->imo_multicast_ifp == NULL)
			*addr = zeroin_addr;
		else if (imo->imo_multicast_addr.s_addr) {
			/* return the value user has set */
			*addr = imo->imo_multicast_addr;
		} else {
			IFP_TO_IA(imo->imo_multicast_ifp, ia);
			*addr = ia ? ia->ia_addr.sin_addr : zeroin_addr;
		}
		return (0);

	case IP_MULTICAST_TTL:
		ttl = mtod(*mp, u_char *);
		(*mp)->m_len = 1;
		*ttl = imo ? imo->imo_multicast_ttl
			   : IP_DEFAULT_MULTICAST_TTL;
		return (0);

	case IP_MULTICAST_LOOP:
		loop = mtod(*mp, u_char *);
		(*mp)->m_len = 1;
		*loop = imo ? imo->imo_multicast_loop
			    : IP_DEFAULT_MULTICAST_LOOP;
		return (0);

	default:
		return (EOPNOTSUPP);
	}
}

/*
 * Discard the IP multicast options.
 */
void
ip_freemoptions(imo)
	struct ip_moptions *imo;
{
	int i;

	if (imo != NULL) {
		for (i = 0; i < imo->imo_num_memberships; ++i)
			in_delmulti(imo->imo_membership[i]);
		free(imo, M_IPMOPTS);
	}
}

/*
 * Routine called from ip_output() to loop back a copy of an IP multicast
 * packet to the input queue of a specified interface.  Note that this
 * calls the output routine of the loopback "driver", but with an interface
 * pointer that might NOT be &loif -- easier than replicating that code here.
 */
static void
ip_mloopback(ifp, m, dst)
	struct ifnet *ifp;
	struct mbuf *m;
	struct sockaddr_in *dst;
{
	struct ip *ip;
	struct mbuf *copym;

	copym = m_copy(m, 0, M_COPYALL);
	if (copym != NULL
	 && (copym->m_flags & M_EXT || copym->m_len < sizeof(struct ip)))
		copym = m_pullup(copym, sizeof(struct ip));
	if (copym != NULL) {
		/*
		 * We don't bother to fragment if the IP length is greater
		 * than the interface's MTU.  Can this possibly matter?
		 */
		ip = mtod(copym, struct ip *);
		HTONS(ip->ip_len);
		HTONS(ip->ip_off);
		ip->ip_sum = 0;
		ip->ip_sum = in_cksum(copym, ip->ip_hl << 2);
		(void) looutput(ifp, copym, sintosa(dst), NULL);
	}
}