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

/*
 * Copyright (c) 1982, 1986, 1988 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.
 *
 *	from: @(#)if_ether.c	7.13 (Berkeley) 10/31/90
 *	$Id: if_arp.c,v 1.9 1994/04/18 06:18:16 glass Exp $
 */

/*
 * Ethernet address resolution protocol.
 * TODO:
 *	run at splnet (add ARP protocol intr.)
 *	link entries onto hash chains, keep free list
 *	add "inuse/lock" bit (or ref. count) along with valid bit
 */

#include <sys/param.h>
#include <sys/systm.h>
#include <sys/malloc.h>
#include <sys/mbuf.h>
#include <sys/socket.h>
#include <sys/time.h>
#include <sys/kernel.h>
#include <sys/errno.h>
#include <sys/ioctl.h>
#include <sys/syslog.h>

#include <net/if.h>

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

#ifdef GATEWAY
#define	ARPTAB_BSIZ	16		/* bucket size */
#define	ARPTAB_NB	37		/* number of buckets */
#else
#define	ARPTAB_BSIZ	9		/* bucket size */
#define	ARPTAB_NB	19		/* number of buckets */
#endif
#define	ARPTAB_SIZE	(ARPTAB_BSIZ * ARPTAB_NB)
struct	arptab arptab[ARPTAB_SIZE];
int	arptab_size = ARPTAB_SIZE;	/* for arp command */

/*
 * ARP trailer negotiation.  Trailer protocol is not IP specific,
 * but ARP request/response use IP addresses.
 */
#define	ETHERTYPE_IPTRAILERS ETHERTYPE_TRAIL

#define	ARPTAB_HASH(a) \
	((u_long)(a) % ARPTAB_NB)

#define	ARPTAB_LOOK(at,addr) { \
	register n; \
	at = &arptab[ARPTAB_HASH(addr) * ARPTAB_BSIZ]; \
	for (n = 0 ; n < ARPTAB_BSIZ ; n++,at++) \
		if (at->at_iaddr.s_addr == addr) \
			break; \
	if (n >= ARPTAB_BSIZ) \
		at = 0; \
}

/* timer values */
#define	ARPT_AGE	(60*1)	/* aging timer, 1 min. */
#define	ARPT_KILLC	20	/* kill completed entry in 20 mins. */
#define	ARPT_KILLI	3	/* kill incomplete entry in 3 minutes */

/* revarp state*/
struct in_addr myip;
int myip_initialized = 0;
int revarp_in_progress = 0;
struct ifnet *myip_ifp = NULL;

extern struct ifnet loif;

/*
 * Timeout routine.  Age arp_tab entries once a minute.
 */
/* ARGSUSED */
void
arptimer(caddr_t arg)
{
	register struct arptab *at;
	register i;

	timeout(arptimer, (caddr_t)0, ARPT_AGE * hz);
	at = &arptab[0];
	for (i = 0; i < ARPTAB_SIZE; i++, at++) {
		if (at->at_flags == 0 || (at->at_flags & ATF_PERM))
			continue;
		if (++at->at_timer < ((at->at_flags&ATF_COM) ?
		    ARPT_KILLC : ARPT_KILLI))
			continue;
		/* timer has expired, clear entry */
		arptfree(at);
	}
}

/*
 * Broadcast an ARP packet, asking who has addr on interface ac.
 */
arpwhohas(ac, addr)
	register struct arpcom *ac;
	struct in_addr *addr;
{
	register struct mbuf *m;
	register struct ether_header *eh;
	register struct ether_arp *ea;
	struct sockaddr sa;

	if ((m = m_gethdr(M_DONTWAIT, MT_DATA)) == NULL)
		return;
	m->m_len = sizeof(*ea);
	m->m_pkthdr.len = sizeof(*ea);
	MH_ALIGN(m, sizeof(*ea));
	ea = mtod(m, struct ether_arp *);
	eh = (struct ether_header *)sa.sa_data;
	bzero((caddr_t)ea, sizeof (*ea));
	bcopy((caddr_t)etherbroadcastaddr, (caddr_t)eh->ether_dhost,
	    sizeof(eh->ether_dhost));
	eh->ether_type = htons(ETHERTYPE_ARP);
	ea->arp_hrd = htons(ARPHRD_ETHER);
	ea->arp_pro = htons(ETHERTYPE_IP);
	ea->arp_hln = sizeof(ea->arp_sha);	/* hardware address length */
	ea->arp_pln = sizeof(ea->arp_spa);	/* protocol address length */
	ea->arp_op = htons(ARPOP_REQUEST);
	bcopy((caddr_t)ac->ac_enaddr, (caddr_t)ea->arp_sha,
	   sizeof(ea->arp_sha));
	bcopy((caddr_t)&ac->ac_ipaddr, (caddr_t)ea->arp_spa,
	   sizeof(ea->arp_spa));
	bcopy((caddr_t)addr, (caddr_t)ea->arp_tpa, sizeof(ea->arp_tpa));
	sa.sa_family = AF_UNSPEC;
	sa.sa_len = sizeof(sa);
	(*ac->ac_if.if_output)(&ac->ac_if, m, &sa, (struct rtentry *)0);
}

int	useloopback = 1;	/* use loopback interface for local traffic */

/*
 * Resolve an IP address into an ethernet address.  If success,
 * desten is filled in.  If there is no entry in arptab,
 * set one up and broadcast a request for the IP address.
 * Hold onto this mbuf and resend it once the address
 * is finally resolved.  A return value of 1 indicates
 * that desten has been filled in and the packet should be sent
 * normally; a 0 return indicates that the packet has been
 * taken over here, either now or for later transmission.
 *
 * We do some (conservative) locking here at splimp, since
 * arptab is also altered from input interrupt service (ecintr/ilintr
 * calls arpinput when ETHERTYPE_ARP packets come in).
 */
arpresolve(ac, m, destip, desten, usetrailers)
	register struct arpcom *ac;
	struct mbuf *m;
	register struct in_addr *destip;
	register u_char *desten;
	int *usetrailers;
{
	register struct arptab *at;
	struct sockaddr_in sin;
	register struct in_ifaddr *ia;
	u_long lna;
	int s;

	*usetrailers = 0;
	if (m->m_flags & M_BCAST) {	/* broadcast */
		bcopy((caddr_t)etherbroadcastaddr, (caddr_t)desten,
		    sizeof(etherbroadcastaddr));
		return (1);
	}
	if (m->m_flags & M_MCAST) {	/* multicast */
		ETHER_MAP_IP_MULTICAST(destip, desten);
		return(1);
	}
	lna = in_lnaof(*destip);
	/* if for us, use software loopback driver if up */
	for (ia = in_ifaddr; ia; ia = ia->ia_next)
	    if ((ia->ia_ifp == &ac->ac_if) &&
		(destip->s_addr == ia->ia_addr.sin_addr.s_addr)) {
		/*
		 * This test used to be
		 *	if (loif.if_flags & IFF_UP)
		 * It allowed local traffic to be forced
		 * through the hardware by configuring the loopback down.
		 * However, it causes problems during network configuration
		 * for boards that can't receive packets they send.
		 * It is now necessary to clear "useloopback"
		 * to force traffic out to the hardware.
		 */
		if (useloopback) {
			sin.sin_family = AF_INET;
			sin.sin_addr = *destip;
			(void) looutput(&loif, m, (struct sockaddr *)&sin, 0);
			/*
			 * The packet has already been sent and freed.
			 */
			return (0);
		} else {
			bcopy((caddr_t)ac->ac_enaddr, (caddr_t)desten,
			    sizeof(ac->ac_enaddr));
			return (1);
		}
	}
	s = splimp();
	ARPTAB_LOOK(at, destip->s_addr);
	if (at == 0) {			/* not found */
		if (ac->ac_if.if_flags & IFF_NOARP) {
			bcopy((caddr_t)ac->ac_enaddr, (caddr_t)desten, 3);
			desten[3] = (lna >> 16) & 0x7f;
			desten[4] = (lna >> 8) & 0xff;
			desten[5] = lna & 0xff;
			splx(s);
			return (1);
		} else {
			at = arptnew(destip);
			if (at == 0)
				panic("arpresolve: no free entry");
			at->at_hold = m;
			arpwhohas(ac, destip);
			splx(s);
			return (0);
		}
	}
	at->at_timer = 0;		/* restart the timer */
	if (at->at_flags & ATF_COM) {	/* entry IS complete */
		bcopy((caddr_t)at->at_enaddr, (caddr_t)desten,
		    sizeof(at->at_enaddr));
		if (at->at_flags & ATF_USETRAILERS)
			*usetrailers = 1;
		splx(s);
		return (1);
	}
	/*
	 * There is an arptab entry, but no ethernet address
	 * response yet.  Replace the held mbuf with this
	 * latest one.
	 */
	if (at->at_hold)
		m_freem(at->at_hold);
	at->at_hold = m;
	arpwhohas(ac, destip);		/* ask again */
	splx(s);
	return (0);
}

/*
 * Called from 10 Mb/s Ethernet interrupt handlers
 * when ether packet type ETHERTYPE_ARP
 * is received.  Common length and type checks are done here,
 * then the protocol-specific routine is called.
 */
arpinput(ac, m)
	struct arpcom *ac;
	struct mbuf *m;
{
	register struct arphdr *ar;

	if (ac->ac_if.if_flags & IFF_NOARP)
		goto out;
	if (m->m_len < sizeof(struct arphdr))
		goto out;
	ar = mtod(m, struct arphdr *);
	if (ntohs(ar->ar_hrd) != ARPHRD_ETHER)
		goto out;
	if (m->m_len < sizeof(struct arphdr) + 2 * ar->ar_hln + 2 * ar->ar_pln)
		goto out;

	switch (ntohs(ar->ar_pro)) {

	case ETHERTYPE_IP:
	case ETHERTYPE_IPTRAILERS:
		in_arpinput(ac, m);
		return;

	default:
		break;
	}
out:
	m_freem(m);
}

/*
 * ARP for Internet protocols on 10 Mb/s Ethernet.
 * Algorithm is that given in RFC 826.
 * In addition, a sanity check is performed on the sender
 * protocol address, to catch impersonators.
 * We also handle negotiations for use of trailer protocol:
 * ARP replies for protocol type ETHERTYPE_TRAIL are sent
 * along with IP replies if we want trailers sent to us,
 * and also send them in response to IP replies.
 * This allows either end to announce the desire to receive
 * trailer packets.
 * We reply to requests for ETHERTYPE_TRAIL protocol as well,
 * but don't normally send requests.
 */
in_arpinput(ac, m)
	register struct arpcom *ac;
	struct mbuf *m;
{
	register struct ether_arp *ea;
	struct ether_header *eh;
	register struct arptab *at;  /* same as "merge" flag */
	register struct in_ifaddr *ia;
	struct in_ifaddr *maybe_ia = 0;
	struct mbuf *mcopy = 0;
	struct sockaddr_in sin;
	struct sockaddr sa;
	struct in_addr isaddr, itaddr, myaddr;
	int proto, op, s, completed = 0;

	ea = mtod(m, struct ether_arp *);
	proto = ntohs(ea->arp_pro);
	op = ntohs(ea->arp_op);
	bcopy((caddr_t)ea->arp_spa, (caddr_t)&isaddr, sizeof (isaddr));
	bcopy((caddr_t)ea->arp_tpa, (caddr_t)&itaddr, sizeof (itaddr));
	for (ia = in_ifaddr; ia; ia = ia->ia_next)
		if (ia->ia_ifp == &ac->ac_if) {
			maybe_ia = ia;
			if ((itaddr.s_addr == ia->ia_addr.sin_addr.s_addr) ||
			     (isaddr.s_addr == ia->ia_addr.sin_addr.s_addr))
				break;
		}
	if (maybe_ia == 0)
		goto out;
	myaddr = ia ? ia->ia_addr.sin_addr : maybe_ia->ia_addr.sin_addr;
	if (!bcmp((caddr_t)ea->arp_sha, (caddr_t)ac->ac_enaddr,
	    sizeof (ea->arp_sha)))
		goto out;	/* it's from me, ignore it. */
	if (!bcmp((caddr_t)ea->arp_sha, (caddr_t)etherbroadcastaddr,
	    sizeof (ea->arp_sha))) {
		log(LOG_ERR,
		    "arp: ether address is broadcast for IP address %x!\n",
		    ntohl(isaddr.s_addr));
		goto out;
	}
	if (isaddr.s_addr == myaddr.s_addr) {
		log(LOG_ERR,
		   "duplicate IP address %x!! sent from ethernet address: %s\n",
		   ntohl(isaddr.s_addr), ether_sprintf(ea->arp_sha));
		itaddr = myaddr;
		if (op == ARPOP_REQUEST)
			goto reply;
		goto out;
	}
	s = splimp();
	ARPTAB_LOOK(at, isaddr.s_addr);
	if (at) {
		bcopy((caddr_t)ea->arp_sha, (caddr_t)at->at_enaddr,
		    sizeof(ea->arp_sha));
		if ((at->at_flags & ATF_COM) == 0)
			completed = 1;
		at->at_flags |= ATF_COM;
		if (at->at_hold) {
			sin.sin_family = AF_INET;
			sin.sin_addr = isaddr;
			(*ac->ac_if.if_output)(&ac->ac_if, at->at_hold,
				(struct sockaddr *)&sin, (struct rtentry *)0);
			at->at_hold = 0;
		}
	}
	if (at == 0 && itaddr.s_addr == myaddr.s_addr) {
		/* ensure we have a table entry */
		if (at = arptnew(&isaddr)) {
			bcopy((caddr_t)ea->arp_sha, (caddr_t)at->at_enaddr,
			    sizeof(ea->arp_sha));
			completed = 1;
			at->at_flags |= ATF_COM;
		}
	}
	splx(s);
reply:
	switch (proto) {

	case ETHERTYPE_IPTRAILERS:
		/* partner says trailers are OK */
		if (at)
			at->at_flags |= ATF_USETRAILERS;
		/*
		 * Reply to request iff we want trailers.
		 */
		if (op != ARPOP_REQUEST || ac->ac_if.if_flags & IFF_NOTRAILERS)
			goto out;
		break;

	case ETHERTYPE_IP:
		/*
		 * Reply if this is an IP request,
		 * or if we want to send a trailer response.
		 * Send the latter only to the IP response
		 * that completes the current ARP entry.
		 */
		if (op != ARPOP_REQUEST &&
		    (completed == 0 || ac->ac_if.if_flags & IFF_NOTRAILERS))
			goto out;
	}
	if (itaddr.s_addr == myaddr.s_addr) {
		/* I am the target */
		bcopy((caddr_t)ea->arp_sha, (caddr_t)ea->arp_tha,
		    sizeof(ea->arp_sha));
		bcopy((caddr_t)ac->ac_enaddr, (caddr_t)ea->arp_sha,
		    sizeof(ea->arp_sha));
	} else {
		ARPTAB_LOOK(at, itaddr.s_addr);
		if (at == NULL || (at->at_flags & ATF_PUBL) == 0)
			goto out;
		bcopy((caddr_t)ea->arp_sha, (caddr_t)ea->arp_tha,
		    sizeof(ea->arp_sha));
		bcopy((caddr_t)at->at_enaddr, (caddr_t)ea->arp_sha,
		    sizeof(ea->arp_sha));
	}

	bcopy((caddr_t)ea->arp_spa, (caddr_t)ea->arp_tpa,
	    sizeof(ea->arp_spa));
	bcopy((caddr_t)&itaddr, (caddr_t)ea->arp_spa,
	    sizeof(ea->arp_spa));
	ea->arp_op = htons(ARPOP_REPLY);
	/*
	 * If incoming packet was an IP reply,
	 * we are sending a reply for type IPTRAILERS.
	 * If we are sending a reply for type IP
	 * and we want to receive trailers,
	 * send a trailer reply as well.
	 */
	if (op == ARPOP_REPLY)
		ea->arp_pro = htons(ETHERTYPE_IPTRAILERS);
	else if (proto == ETHERTYPE_IP &&
	    (ac->ac_if.if_flags & IFF_NOTRAILERS) == 0)
		mcopy = m_copy(m, 0, (int)M_COPYALL);
	eh = (struct ether_header *)sa.sa_data;
	bcopy((caddr_t)ea->arp_tha, (caddr_t)eh->ether_dhost,
	    sizeof(eh->ether_dhost));
	eh->ether_type = htons(ETHERTYPE_ARP);
	sa.sa_family = AF_UNSPEC;
	sa.sa_len = sizeof(sa);
	(*ac->ac_if.if_output)(&ac->ac_if, m, &sa, (struct rtentry *)0);
	if (mcopy) {
		ea = mtod(mcopy, struct ether_arp *);
		ea->arp_pro = htons(ETHERTYPE_IPTRAILERS);
		(*ac->ac_if.if_output)(&ac->ac_if,
					mcopy, &sa, (struct rtentry *)0);
	}
	return;
out:
	m_freem(m);
	return;
}

/*
 * Free an arptab entry.
 */
arptfree(at)
	register struct arptab *at;
{
	int s = splimp();

	if (at->at_hold)
		m_freem(at->at_hold);
	at->at_hold = 0;
	at->at_timer = at->at_flags = 0;
	at->at_iaddr.s_addr = 0;
	splx(s);
}

/*
 * Enter a new address in arptab, pushing out the oldest entry
 * from the bucket if there is no room.
 * This always succeeds since no bucket can be completely filled
 * with permanent entries (except from arpioctl when testing whether
 * another permanent entry will fit).
 * MUST BE CALLED AT SPLIMP.
 */
struct arptab *
arptnew(addr)
	struct in_addr *addr;
{
	register n;
	int oldest = -1;
	register struct arptab *at, *ato = NULL;
	static int first = 1;

	if (first) {
		first = 0;
		timeout(arptimer, (caddr_t)0, hz);
	}
	at = &arptab[ARPTAB_HASH(addr->s_addr) * ARPTAB_BSIZ];
	for (n = 0; n < ARPTAB_BSIZ; n++,at++) {
		if (at->at_flags == 0)
			goto out;	 /* found an empty entry */
		if (at->at_flags & ATF_PERM)
			continue;
		if ((int) at->at_timer > oldest) {
			oldest = at->at_timer;
			ato = at;
		}
	}
	if (ato == NULL)
		return (NULL);
	at = ato;
	arptfree(at);
out:
	at->at_iaddr = *addr;
	at->at_flags = ATF_INUSE;
	return (at);
}

arpioctl(cmd, data)
	int cmd;
	caddr_t data;
{
	register struct arpreq *ar = (struct arpreq *)data;
	register struct arptab *at;
	register struct sockaddr_in *sin;
	int s;

	sin = (struct sockaddr_in *)&ar->arp_ha;
#if defined(COMPAT_43) && BYTE_ORDER != BIG_ENDIAN
	if (sin->sin_family == 0 && sin->sin_len < 16)
		sin->sin_family = sin->sin_len;
#endif
	sin->sin_len = sizeof(ar->arp_ha);
	sin = (struct sockaddr_in *)&ar->arp_pa;
#if defined(COMPAT_43) && BYTE_ORDER != BIG_ENDIAN
	if (sin->sin_family == 0 && sin->sin_len < 16)
		sin->sin_family = sin->sin_len;
#endif
	sin->sin_len = sizeof(ar->arp_pa);
	if (ar->arp_pa.sa_family != AF_INET ||
	    ar->arp_ha.sa_family != AF_UNSPEC)
		return (EAFNOSUPPORT);
	s = splimp();
	ARPTAB_LOOK(at, sin->sin_addr.s_addr);
	if (at == NULL) {		/* not found */
		if (cmd != SIOCSARP) {
			splx(s);
			return (ENXIO);
		}
		if (ifa_ifwithnet(&ar->arp_pa) == NULL) {
			splx(s);
			return (ENETUNREACH);
		}
	}
	switch (cmd) {

	case SIOCSARP:		/* set entry */
		if (at == NULL) {
			at = arptnew(&sin->sin_addr);
			if (at == NULL) {
				splx(s);
				return (EADDRNOTAVAIL);
			}
			if (ar->arp_flags & ATF_PERM) {
			/* never make all entries in a bucket permanent */
				register struct arptab *tat;

				/* try to re-allocate */
				tat = arptnew(&sin->sin_addr);
				if (tat == NULL) {
					arptfree(at);
					splx(s);
					return (EADDRNOTAVAIL);
				}
				arptfree(tat);
			}
		}
		bcopy((caddr_t)ar->arp_ha.sa_data, (caddr_t)at->at_enaddr,
		    sizeof(at->at_enaddr));
		at->at_flags = ATF_COM | ATF_INUSE |
			(ar->arp_flags & (ATF_PERM|ATF_PUBL|ATF_USETRAILERS));
		at->at_timer = 0;
		break;

	case SIOCDARP:		/* delete entry */
		arptfree(at);
		break;

	case SIOCGARP:		/* get entry */
	case OSIOCGARP:
		bcopy((caddr_t)at->at_enaddr, (caddr_t)ar->arp_ha.sa_data,
		    sizeof(at->at_enaddr));
#ifdef COMPAT_43
		if (cmd == OSIOCGARP)
			*(u_short *)&ar->arp_ha = ar->arp_ha.sa_family;
#endif
		ar->arp_flags = at->at_flags;
		break;
	}
	splx(s);
	return (0);
}

/*
 * Called from 10 Mb/s Ethernet interrupt handlers
 * when ether packet type ETHERTYPE_REVARP
 * is received.  Common length and type checks are done here,
 * then the protocol-specific routine is called.
 */
void revarpinput(ac, m)
	struct arpcom *ac;
	struct mbuf *m;
{
	struct arphdr *ar;
	int op, s;

	if (m->m_len < sizeof(struct arphdr))
		goto out;
	ar = mtod(m, struct arphdr *);
	if (ntohs(ar->ar_hrd) != ARPHRD_ETHER)
		goto out;
	if (m->m_len < sizeof(struct arphdr) + 2 * ar->ar_hln +
	    2 * ar->ar_pln)
		goto out;
	switch (ntohs(ar->ar_pro)) {

	case ETHERTYPE_IP:
	case ETHERTYPE_IPTRAILERS:
		in_revarpinput(ac, m);
		return;

	default:
		break;
	}
out:
	m_freem(m);
}

/*
 * RARP for Internet protocols on 10 Mb/s Ethernet.
 * Algorithm is that given in RFC 903.
 * We are only using for bootstrap purposes to get an ip address for one of
 * our interfaces.  Thus we support no user-interface.
 *
 * Since the contents of the RARP reply are specific to the interface that
 * sent the request, this code must ensure that they are properly associated.
 *
 * Note: also supports ARP via RARP packets, per the RFC.
 */
in_revarpinput(ac, m)
	struct arpcom *ac;
	struct mbuf *m;
{
	struct ether_arp *ar;
	int op, s;

	ar = mtod(m, struct ether_arp *);
	op = ntohs(ar->arp_op);
	switch (op) {
	case ARPOP_REQUEST:
	case ARPOP_REPLY:	/* per RFC */
		if (ac->ac_if.if_flags & IFF_NOARP)
			goto out;
		in_arpinput(ac, m);
		return;
		break;
	case REVARP_REPLY:
		break;
	case REVARP_REQUEST:	/* handled by rarpd(8) */
	default:
		goto out;
	}
	if (!revarp_in_progress)
		goto out;
	if ((struct ifnet *) ac != myip_ifp) /* !same interface */
		goto out;
	if (myip_initialized != 0)
		goto out;
	if (bcmp(ar->arp_tha, ac->ac_enaddr, sizeof(ar->arp_tha)))
		goto out;
	bcopy((caddr_t) ar->arp_tpa, (caddr_t) &myip, sizeof(myip));
	myip_initialized = 1;
	wakeup((caddr_t) &myip);
 out:
	m_freem(m);
}

/*
 * Send a RARP request for the ip address of the specified interface.
 * The request should be RFC 903-compliant.
 */
void revarp_request(ifp)
	struct ifnet *ifp;
{
	struct sockaddr sa;
	struct mbuf *m;
	struct ether_header *eh;
	struct ether_arp *ea;
	struct arpcom *ac = (struct arpcom *) ifp;

	if ((m = m_gethdr(M_DONTWAIT, MT_DATA)) == NULL)
		return;
	m->m_len = sizeof(*ea);
	m->m_pkthdr.len = sizeof(*ea);
	MH_ALIGN(m, sizeof(*ea));
	ea = mtod(m, struct ether_arp *);
	eh = (struct ether_header *) sa.sa_data;
	bzero((caddr_t)ea, sizeof (*ea));
	bcopy((caddr_t)etherbroadcastaddr, (caddr_t)eh->ether_dhost,
	      sizeof(eh->ether_dhost));
	eh->ether_type = htons(ETHERTYPE_REVARP);
	ea->arp_hrd = htons(ARPHRD_ETHER);
	ea->arp_pro = htons(ETHERTYPE_IP);
	ea->arp_hln = sizeof(ea->arp_sha);	/* hardware address length */
	ea->arp_pln = sizeof(ea->arp_spa);	/* protocol address length */
	ea->arp_op = htons(REVARP_REQUEST);
	bcopy((caddr_t)ac->ac_enaddr, (caddr_t)ea->arp_sha,
	   sizeof(ea->arp_sha));
	bcopy((caddr_t)ac->ac_enaddr, (caddr_t)ea->arp_tha,
	   sizeof(ea->arp_tha));
	sa.sa_family = AF_UNSPEC;
	sa.sa_len = sizeof(sa);
	ifp->if_output(ifp, m, &sa, (struct rtentry *)0);
}

/*
 * RARP for the ip address of the specified interface.  Timeout if
 * no response is received.
 */
int revarp_whoami(in, ifp)
	struct in_addr *in;
	struct ifnet *ifp;
{
	int result, count = 20;

	if (myip_initialized)
		return EIO;

	myip_ifp = ifp;
	revarp_in_progress = 1;
	while (count--) {
		revarp_request(ifp);
		result = tsleep((caddr_t) &myip, PSOCK, "revarp", hz/2);
		if (result != EWOULDBLOCK) break;
	}
	revarp_in_progress = 0;
	if (!myip_initialized)
		return ENETUNREACH;

	bcopy((caddr_t) &myip, in, sizeof(*in));
	return 0;
}