arp.c revision 128054 
1/*
2 * Copyright (c) 1984, 1993
3 *	The Regents of the University of California.  All rights reserved.
4 *
5 * This code is derived from software contributed to Berkeley by
6 * Sun Microsystems, Inc.
7 *
8 * Redistribution and use in source and binary forms, with or without
9 * modification, are permitted provided that the following conditions
10 * are met:
11 * 1. Redistributions of source code must retain the above copyright
12 *    notice, this list of conditions and the following disclaimer.
13 * 2. Redistributions in binary form must reproduce the above copyright
14 *    notice, this list of conditions and the following disclaimer in the
15 *    documentation and/or other materials provided with the distribution.
16 * 3. All advertising materials mentioning features or use of this software
17 *    must display the following acknowledgement:
18 *	This product includes software developed by the University of
19 *	California, Berkeley and its contributors.
20 * 4. Neither the name of the University nor the names of its contributors
21 *    may be used to endorse or promote products derived from this software
22 *    without specific prior written permission.
23 *
24 * THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
25 * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
26 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
27 * ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
28 * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
29 * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
30 * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
31 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
32 * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
33 * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
34 * SUCH DAMAGE.
35 */
36
37#if 0
38#ifndef lint
39static char const copyright[] =
40"@(#) Copyright (c) 1984, 1993\n\
41	The Regents of the University of California.  All rights reserved.\n";
42#endif /* not lint */
43
44#ifndef lint
45static char const sccsid[] = "@(#)from: arp.c	8.2 (Berkeley) 1/2/94";
46#endif /* not lint */
47#endif
48#include <sys/cdefs.h>
49__FBSDID("$FreeBSD: head/usr.sbin/arp/arp.c 128054 2004-04-09 14:27:28Z mux $");
50
51/*
52 * arp - display, set, and delete arp table entries
53 */
54
55
56#include <sys/param.h>
57#include <sys/file.h>
58#include <sys/socket.h>
59#include <sys/sockio.h>
60#include <sys/sysctl.h>
61#include <sys/ioctl.h>
62#include <sys/time.h>
63
64#include <net/if.h>
65#include <net/if_dl.h>
66#include <net/if_types.h>
67#include <net/route.h>
68#include <net/iso88025.h>
69
70#include <netinet/in.h>
71#include <netinet/if_ether.h>
72
73#include <arpa/inet.h>
74
75#include <ctype.h>
76#include <err.h>
77#include <errno.h>
78#include <netdb.h>
79#include <nlist.h>
80#include <paths.h>
81#include <stdio.h>
82#include <stdlib.h>
83#include <string.h>
84#include <strings.h>
85#include <unistd.h>
86
87void search(u_long addr, void (*action)(struct sockaddr_dl *sdl,
88	struct sockaddr_inarp *sin, struct rt_msghdr *rtm));
89void print_entry(struct sockaddr_dl *sdl,
90	struct sockaddr_inarp *addr, struct rt_msghdr *rtm);
91void nuke_entry(struct sockaddr_dl *sdl,
92	struct sockaddr_inarp *addr, struct rt_msghdr *rtm);
93int delete(char *host, char *info);
94void usage(void);
95int set(int argc, char **argv);
96int get(char *host);
97int file(char *name);
98void getsocket(void);
99int my_ether_aton(char *a, struct ether_addr *n);
100int rtmsg(int cmd);
101int get_ether_addr(u_int32_t ipaddr, struct ether_addr *hwaddr);
102
103static pid_t pid;
104static int nflag;	/* no reverse dns lookups */
105static int aflag;	/* do it for all entries */
106static int s = -1;
107static char *rifname;
108
109struct	sockaddr_in so_mask;
110struct	sockaddr_inarp blank_sin, sin_m;
111struct	sockaddr_dl blank_sdl, sdl_m;
112int	expire_time, flags, doing_proxy, proxy_only, found_entry;
113struct	{
114	struct	rt_msghdr m_rtm;
115	char	m_space[512];
116}	m_rtmsg;
117
118/* which function we're supposed to do */
119#define F_GET		1
120#define F_SET		2
121#define F_FILESET	3
122#define F_REPLACE	4
123#define F_DELETE	5
124
125#define ROUNDUP(a) \
126	((a) > 0 ? (1 + (((a) - 1) | (sizeof(long) - 1))) : sizeof(long))
127#define SETFUNC(f)	{ if (func) usage(); func = (f); }
128
129int
130main(int argc, char *argv[])
131{
132	int ch, func = 0;
133	int rtn = 0;
134
135	pid = getpid();
136	while ((ch = getopt(argc, argv, "andfsSi:")) != -1)
137		switch((char)ch) {
138		case 'a':
139			aflag = 1;
140			break;
141		case 'd':
142			SETFUNC(F_DELETE);
143			break;
144		case 'n':
145			nflag = 1;
146			break;
147		case 'S':
148			SETFUNC(F_REPLACE);
149			break;
150		case 's':
151			SETFUNC(F_SET);
152			break;
153		case 'f' :
154			SETFUNC(F_FILESET);
155			break;
156		case 'i':
157			rifname = optarg;
158			break;
159		case '?':
160		default:
161			usage();
162		}
163	argc -= optind;
164	argv += optind;
165
166	bzero(&so_mask, sizeof(so_mask));
167	so_mask.sin_len = 8;
168	so_mask.sin_addr.s_addr = 0xffffffff;
169	bzero(&blank_sin, sizeof(blank_sin));
170	blank_sin.sin_len = sizeof(blank_sin);
171	blank_sin.sin_family = AF_INET;
172	bzero(&blank_sdl, sizeof(blank_sdl));
173	blank_sdl.sdl_len = sizeof(blank_sdl);
174	blank_sdl.sdl_family = AF_LINK;
175
176	if (!func)
177		func = F_GET;
178	if (rifname) {
179		if (func != F_GET)
180			errx(1, "-i not applicable to this operation");
181		if (if_nametoindex(rifname) == 0) {
182			if (errno == ENXIO)
183				errx(1, "interface %s does not exist", rifname);
184			else
185				err(1, "if_nametoindex(%s)", rifname);
186		}
187	}
188	switch (func) {
189	case F_GET:
190		if (aflag) {
191			if (argc != 0)
192				usage();
193			search(0, print_entry);
194		} else {
195			if (argc != 1)
196				usage();
197			get(argv[0]);
198		}
199		break;
200	case F_SET:
201	case F_REPLACE:
202		if (argc < 2 || argc > 6)
203			usage();
204		if (func == F_REPLACE)
205			(void) delete(argv[0], NULL);
206		rtn = set(argc, argv) ? 1 : 0;
207		break;
208	case F_DELETE:
209		if (aflag) {
210			if (argc != 0)
211				usage();
212			search(0, nuke_entry);
213		} else {
214			if (argc < 1 || argc > 2)
215				usage();
216			rtn = delete(argv[0], argv[1]);
217		}
218		break;
219	case F_FILESET:
220		if (argc != 1)
221			usage();
222		rtn = file(argv[0]);
223		break;
224	}
225
226	return(rtn);
227}
228
229/*
230 * Process a file to set standard arp entries
231 */
232int
233file(char *name)
234{
235	FILE *fp;
236	int i, retval;
237	char line[100], arg[5][50], *args[5], *p;
238
239	if ((fp = fopen(name, "r")) == NULL)
240		err(1, "cannot open %s", name);
241	args[0] = &arg[0][0];
242	args[1] = &arg[1][0];
243	args[2] = &arg[2][0];
244	args[3] = &arg[3][0];
245	args[4] = &arg[4][0];
246	retval = 0;
247	while(fgets(line, 100, fp) != NULL) {
248		if ((p = strchr(line, '#')) != NULL)
249			*p = '\0';
250		for (p = line; isblank(*p); p++);
251		if (*p == '\n' || *p == '\0')
252			continue;
253		i = sscanf(p, "%49s %49s %49s %49s %49s", arg[0], arg[1],
254		    arg[2], arg[3], arg[4]);
255		if (i < 2) {
256			warnx("bad line: %s", line);
257			retval = 1;
258			continue;
259		}
260		if (set(i, args))
261			retval = 1;
262	}
263	fclose(fp);
264	return (retval);
265}
266
267void
268getsocket(void)
269{
270	if (s < 0) {
271		s = socket(PF_ROUTE, SOCK_RAW, 0);
272		if (s < 0)
273			err(1, "socket");
274	}
275}
276
277/*
278 * Set an individual arp entry
279 */
280int
281set(int argc, char **argv)
282{
283	struct hostent *hp;
284	struct sockaddr_inarp *addr = &sin_m;
285	struct sockaddr_dl *sdl;
286	struct rt_msghdr *rtm = &(m_rtmsg.m_rtm);
287	struct ether_addr *ea;
288	char *host = argv[0], *eaddr = argv[1];
289
290	getsocket();
291	argc -= 2;
292	argv += 2;
293	sdl_m = blank_sdl;
294	sin_m = blank_sin;
295	addr->sin_addr.s_addr = inet_addr(host);
296	if (addr->sin_addr.s_addr == INADDR_NONE) {
297		if (!(hp = gethostbyname(host))) {
298			warnx("%s: %s", host, hstrerror(h_errno));
299			return (1);
300		}
301		bcopy((char *)hp->h_addr, (char *)&addr->sin_addr,
302		    sizeof addr->sin_addr);
303	}
304	doing_proxy = flags = proxy_only = expire_time = 0;
305	while (argc-- > 0) {
306		if (strncmp(argv[0], "temp", 4) == 0) {
307			struct timeval tv;
308			gettimeofday(&tv, 0);
309			expire_time = tv.tv_sec + 20 * 60;
310		}
311		else if (strncmp(argv[0], "pub", 3) == 0) {
312			flags |= RTF_ANNOUNCE;
313			doing_proxy = 1;
314			if (argc && strncmp(argv[1], "only", 3) == 0) {
315				proxy_only = 1;
316				sin_m.sin_other = SIN_PROXY;
317				argc--; argv++;
318			}
319		} else if (strncmp(argv[0], "trail", 5) == 0) {
320			printf("%s: Sending trailers is no longer supported\n",
321				host);
322		}
323		argv++;
324	}
325	ea = (struct ether_addr *)LLADDR(&sdl_m);
326	if (doing_proxy && !strcmp(eaddr, "auto")) {
327		if (!get_ether_addr(addr->sin_addr.s_addr, ea)) {
328			printf("no interface found for %s\n",
329			       inet_ntoa(addr->sin_addr));
330			return (1);
331		}
332		sdl_m.sdl_alen = ETHER_ADDR_LEN;
333	} else {
334		if (my_ether_aton(eaddr, ea) == 0)
335			sdl_m.sdl_alen = ETHER_ADDR_LEN;
336	}
337tryagain:
338	if (rtmsg(RTM_GET) < 0) {
339		warn("%s", host);
340		return (1);
341	}
342	addr = (struct sockaddr_inarp *)(rtm + 1);
343	sdl = (struct sockaddr_dl *)(ROUNDUP(addr->sin_len) + (char *)addr);
344	if (addr->sin_addr.s_addr == sin_m.sin_addr.s_addr) {
345		if (sdl->sdl_family == AF_LINK &&
346		    (rtm->rtm_flags & RTF_LLINFO) &&
347		    !(rtm->rtm_flags & RTF_GATEWAY)) switch (sdl->sdl_type) {
348		case IFT_ETHER: case IFT_FDDI: case IFT_ISO88023:
349		case IFT_ISO88024: case IFT_ISO88025: case IFT_L2VLAN:
350			goto overwrite;
351		}
352		if (doing_proxy == 0) {
353			printf("set: can only proxy for %s\n", host);
354			return (1);
355		}
356		if (sin_m.sin_other & SIN_PROXY) {
357			printf("set: proxy entry exists for non 802 device\n");
358			return(1);
359		}
360		sin_m.sin_other = SIN_PROXY;
361		proxy_only = 1;
362		goto tryagain;
363	}
364overwrite:
365	if (sdl->sdl_family != AF_LINK) {
366		printf("cannot intuit interface index and type for %s\n", host);
367		return (1);
368	}
369	sdl_m.sdl_type = sdl->sdl_type;
370	sdl_m.sdl_index = sdl->sdl_index;
371	return (rtmsg(RTM_ADD));
372}
373
374/*
375 * Display an individual arp entry
376 */
377int
378get(char *host)
379{
380	struct hostent *hp;
381	struct sockaddr_inarp *addr = &sin_m;
382
383	sin_m = blank_sin;
384	addr->sin_addr.s_addr = inet_addr(host);
385	if (addr->sin_addr.s_addr == INADDR_NONE) {
386		if (!(hp = gethostbyname(host)))
387			errx(1, "%s: %s", host, hstrerror(h_errno));
388		bcopy((char *)hp->h_addr, (char *)&addr->sin_addr,
389		    sizeof addr->sin_addr);
390	}
391	search(addr->sin_addr.s_addr, print_entry);
392	if (found_entry == 0) {
393		printf("%s (%s) -- no entry",
394		    host, inet_ntoa(addr->sin_addr));
395		if (rifname)
396			printf(" on %s", rifname);
397		printf("\n");
398		return(1);
399	}
400	return(0);
401}
402
403/*
404 * Delete an arp entry
405 */
406int
407delete(char *host, char *info)
408{
409	struct hostent *hp;
410	struct sockaddr_inarp *addr = &sin_m;
411	struct rt_msghdr *rtm = &m_rtmsg.m_rtm;
412	struct sockaddr_dl *sdl;
413
414	getsocket();
415	sin_m = blank_sin;
416	if (info) {
417		if (strncmp(info, "pub", 3) == 0)
418			sin_m.sin_other = SIN_PROXY;
419		else
420			usage();
421	}
422	addr->sin_addr.s_addr = inet_addr(host);
423	if (addr->sin_addr.s_addr == INADDR_NONE) {
424		if (!(hp = gethostbyname(host))) {
425			warnx("%s: %s", host, hstrerror(h_errno));
426			return (1);
427		}
428		bcopy((char *)hp->h_addr, (char *)&addr->sin_addr,
429		    sizeof addr->sin_addr);
430	}
431tryagain:
432	if (rtmsg(RTM_GET) < 0) {
433		warn("%s", host);
434		return (1);
435	}
436	addr = (struct sockaddr_inarp *)(rtm + 1);
437	sdl = (struct sockaddr_dl *)(ROUNDUP(addr->sin_len) + (char *)addr);
438	if (addr->sin_addr.s_addr == sin_m.sin_addr.s_addr) {
439		if (sdl->sdl_family == AF_LINK &&
440		    (rtm->rtm_flags & RTF_LLINFO) &&
441		    !(rtm->rtm_flags & RTF_GATEWAY)) switch (sdl->sdl_type) {
442		case IFT_ETHER: case IFT_FDDI: case IFT_ISO88023:
443		case IFT_ISO88024: case IFT_ISO88025: case IFT_L2VLAN:
444			goto delete;
445		}
446	}
447	if (sin_m.sin_other & SIN_PROXY) {
448		fprintf(stderr, "delete: can't locate %s\n",host);
449		return (1);
450	} else {
451		sin_m.sin_other = SIN_PROXY;
452		goto tryagain;
453	}
454delete:
455	if (sdl->sdl_family != AF_LINK) {
456		printf("cannot locate %s\n", host);
457		return (1);
458	}
459	if (rtmsg(RTM_DELETE) == 0) {
460		printf("%s (%s) deleted\n", host, inet_ntoa(addr->sin_addr));
461		return (0);
462	}
463	return (1);
464}
465
466/*
467 * Search the arp table and do some action on matching entries
468 */
469void
470search(u_long addr, void (*action)(struct sockaddr_dl *sdl,
471	struct sockaddr_inarp *sin, struct rt_msghdr *rtm))
472{
473	int mib[6];
474	size_t needed;
475	char *lim, *buf, *next;
476	struct rt_msghdr *rtm;
477	struct sockaddr_inarp *sin2;
478	struct sockaddr_dl *sdl;
479	char ifname[IF_NAMESIZE];
480
481	mib[0] = CTL_NET;
482	mib[1] = PF_ROUTE;
483	mib[2] = 0;
484	mib[3] = AF_INET;
485	mib[4] = NET_RT_FLAGS;
486	mib[5] = RTF_LLINFO;
487	if (sysctl(mib, 6, NULL, &needed, NULL, 0) < 0)
488		err(1, "route-sysctl-estimate");
489	if (needed == 0)
490		return;
491	if ((buf = malloc(needed)) == NULL)
492		err(1, "malloc");
493	if (sysctl(mib, 6, buf, &needed, NULL, 0) < 0)
494		err(1, "actual retrieval of routing table");
495	lim = buf + needed;
496	for (next = buf; next < lim; next += rtm->rtm_msglen) {
497		rtm = (struct rt_msghdr *)next;
498		sin2 = (struct sockaddr_inarp *)(rtm + 1);
499		(char *)sdl = (char *)sin2 + ROUNDUP(sin2->sin_len);
500		if (rifname && if_indextoname(sdl->sdl_index, ifname) &&
501		    strcmp(ifname, rifname))
502			continue;
503		if (addr) {
504			if (addr != sin2->sin_addr.s_addr)
505				continue;
506			found_entry = 1;
507		}
508		(*action)(sdl, sin2, rtm);
509	}
510	free(buf);
511}
512
513/*
514 * Display an arp entry
515 */
516void
517print_entry(struct sockaddr_dl *sdl,
518	struct sockaddr_inarp *addr, struct rt_msghdr *rtm)
519{
520	const char *host;
521	struct hostent *hp;
522	struct iso88025_sockaddr_dl_data *trld;
523	char ifname[IF_NAMESIZE];
524	int seg;
525
526	if (nflag == 0)
527		hp = gethostbyaddr((caddr_t)&(addr->sin_addr),
528		    sizeof addr->sin_addr, AF_INET);
529	else
530		hp = 0;
531	if (hp)
532		host = hp->h_name;
533	else {
534		host = "?";
535		if (h_errno == TRY_AGAIN)
536			nflag = 1;
537	}
538	printf("%s (%s) at ", host, inet_ntoa(addr->sin_addr));
539	if (sdl->sdl_alen)
540		printf("%s", ether_ntoa((struct ether_addr *)LLADDR(sdl)));
541	else
542		printf("(incomplete)");
543	if (if_indextoname(sdl->sdl_index, ifname) != NULL)
544		printf(" on %s", ifname);
545	if (rtm->rtm_rmx.rmx_expire == 0)
546		printf(" permanent");
547	if (addr->sin_other & SIN_PROXY)
548		printf(" published (proxy only)");
549	if (rtm->rtm_addrs & RTA_NETMASK) {
550		addr = (struct sockaddr_inarp *)
551			(ROUNDUP(sdl->sdl_len) + (char *)sdl);
552		if (addr->sin_addr.s_addr == 0xffffffff)
553			printf(" published");
554		if (addr->sin_len != 8)
555			printf("(weird)");
556	}
557        switch(sdl->sdl_type) {
558            case IFT_ETHER:
559                printf(" [ethernet]");
560                break;
561            case IFT_ISO88025:
562                printf(" [token-ring]");
563		trld = SDL_ISO88025(sdl);
564		if (trld->trld_rcf != 0) {
565			printf(" rt=%x", ntohs(trld->trld_rcf));
566			for (seg = 0;
567			     seg < ((TR_RCF_RIFLEN(trld->trld_rcf) - 2 ) / 2);
568			     seg++)
569				printf(":%x", ntohs(*(trld->trld_route[seg])));
570		}
571                break;
572            case IFT_FDDI:
573                printf(" [fddi]");
574                break;
575            case IFT_ATM:
576                printf(" [atm]");
577                break;
578	    case IFT_L2VLAN:
579		printf(" [vlan]");
580		break;
581            default:
582		break;
583        }
584
585	printf("\n");
586
587}
588
589/*
590 * Nuke an arp entry
591 */
592void
593nuke_entry(struct sockaddr_dl *sdl __unused,
594	struct sockaddr_inarp *addr, struct rt_msghdr *rtm __unused)
595{
596	char ip[20];
597
598	snprintf(ip, sizeof(ip), "%s", inet_ntoa(addr->sin_addr));
599	delete(ip, NULL);
600}
601
602int
603my_ether_aton(char *a, struct ether_addr *n)
604{
605	struct ether_addr *ea;
606
607	if ((ea = ether_aton(a)) == NULL) {
608		warnx("invalid Ethernet address '%s'", a);
609		return (1);
610	}
611	*n = *ea;
612	return (0);
613}
614
615void
616usage(void)
617{
618	fprintf(stderr, "%s\n%s\n%s\n%s\n%s\n%s\n%s\n",
619		"usage: arp [-n] [-i interface] hostname",
620		"       arp [-n] [-i interface] -a",
621		"       arp -d hostname [pub]",
622		"       arp -d -a",
623		"       arp -s hostname ether_addr [temp] [pub]",
624		"       arp -S hostname ether_addr [temp] [pub]",
625		"       arp -f filename");
626	exit(1);
627}
628
629int
630rtmsg(int cmd)
631{
632	static int seq;
633	int rlen;
634	struct rt_msghdr *rtm = &m_rtmsg.m_rtm;
635	char *cp = m_rtmsg.m_space;
636	int l;
637
638	errno = 0;
639	if (cmd == RTM_DELETE)
640		goto doit;
641	bzero((char *)&m_rtmsg, sizeof(m_rtmsg));
642	rtm->rtm_flags = flags;
643	rtm->rtm_version = RTM_VERSION;
644
645	switch (cmd) {
646	default:
647		errx(1, "internal wrong cmd");
648	case RTM_ADD:
649		rtm->rtm_addrs |= RTA_GATEWAY;
650		rtm->rtm_rmx.rmx_expire = expire_time;
651		rtm->rtm_inits = RTV_EXPIRE;
652		rtm->rtm_flags |= (RTF_HOST | RTF_STATIC);
653		sin_m.sin_other = 0;
654		if (doing_proxy) {
655			if (proxy_only)
656				sin_m.sin_other = SIN_PROXY;
657			else {
658				rtm->rtm_addrs |= RTA_NETMASK;
659				rtm->rtm_flags &= ~RTF_HOST;
660			}
661		}
662		/* FALLTHROUGH */
663	case RTM_GET:
664		rtm->rtm_addrs |= RTA_DST;
665	}
666#define NEXTADDR(w, s) \
667	if (rtm->rtm_addrs & (w)) { \
668		bcopy((char *)&s, cp, sizeof(s)); cp += ROUNDUP(sizeof(s));}
669
670	NEXTADDR(RTA_DST, sin_m);
671	NEXTADDR(RTA_GATEWAY, sdl_m);
672	NEXTADDR(RTA_NETMASK, so_mask);
673
674	rtm->rtm_msglen = cp - (char *)&m_rtmsg;
675doit:
676	l = rtm->rtm_msglen;
677	rtm->rtm_seq = ++seq;
678	rtm->rtm_type = cmd;
679	if ((rlen = write(s, (char *)&m_rtmsg, l)) < 0) {
680		if (errno != ESRCH || cmd != RTM_DELETE) {
681			warn("writing to routing socket");
682			return (-1);
683		}
684	}
685	do {
686		l = read(s, (char *)&m_rtmsg, sizeof(m_rtmsg));
687	} while (l > 0 && (rtm->rtm_seq != seq || rtm->rtm_pid != pid));
688	if (l < 0)
689		warn("read from routing socket");
690	return (0);
691}
692
693/*
694 * get_ether_addr - get the hardware address of an interface on the
695 * the same subnet as ipaddr.
696 */
697#define MAX_IFS		32
698
699int
700get_ether_addr(u_int32_t ipaddr, struct ether_addr *hwaddr)
701{
702	struct ifreq *ifr, *ifend, *ifp;
703	u_int32_t ina, mask;
704	struct sockaddr_dl *dla;
705	struct ifreq ifreq;
706	struct ifconf ifc;
707	struct ifreq ifs[MAX_IFS];
708	int sock;
709
710	sock = socket(AF_INET, SOCK_DGRAM, 0);
711	if (sock < 0)
712		err(1, "socket");
713
714	ifc.ifc_len = sizeof(ifs);
715	ifc.ifc_req = ifs;
716	if (ioctl(sock, SIOCGIFCONF, &ifc) < 0) {
717		warnx("ioctl(SIOCGIFCONF)");
718		close(sock);
719		return 0;
720	}
721
722	/*
723	* Scan through looking for an interface with an Internet
724	* address on the same subnet as `ipaddr'.
725	*/
726	ifend = (struct ifreq *) (ifc.ifc_buf + ifc.ifc_len);
727	for (ifr = ifc.ifc_req; ifr < ifend; ) {
728		if (ifr->ifr_addr.sa_family == AF_INET) {
729			ina = ((struct sockaddr_in *)
730				&ifr->ifr_addr)->sin_addr.s_addr;
731			strncpy(ifreq.ifr_name, ifr->ifr_name,
732				sizeof(ifreq.ifr_name));
733			/*
734			 * Check that the interface is up,
735			 * and not point-to-point or loopback.
736			 */
737			if (ioctl(sock, SIOCGIFFLAGS, &ifreq) < 0)
738				continue;
739			if ((ifreq.ifr_flags &
740			     (IFF_UP|IFF_BROADCAST|IFF_POINTOPOINT|
741					IFF_LOOPBACK|IFF_NOARP))
742			     != (IFF_UP|IFF_BROADCAST))
743				goto nextif;
744			/*
745			 * Get its netmask and check that it's on
746			 * the right subnet.
747			 */
748			if (ioctl(sock, SIOCGIFNETMASK, &ifreq) < 0)
749				continue;
750			mask = ((struct sockaddr_in *)
751				&ifreq.ifr_addr)->sin_addr.s_addr;
752			if ((ipaddr & mask) != (ina & mask))
753				goto nextif;
754			break;
755		}
756nextif:
757		ifr = (struct ifreq *) ((char *)&ifr->ifr_addr
758		    + MAX(ifr->ifr_addr.sa_len, sizeof(ifr->ifr_addr)));
759	}
760
761	if (ifr >= ifend) {
762		close(sock);
763		return 0;
764	}
765
766	/*
767	* Now scan through again looking for a link-level address
768	* for this interface.
769	*/
770	ifp = ifr;
771	for (ifr = ifc.ifc_req; ifr < ifend; ) {
772		if (strcmp(ifp->ifr_name, ifr->ifr_name) == 0
773		    && ifr->ifr_addr.sa_family == AF_LINK) {
774			/*
775			 * Found the link-level address - copy it out
776			 */
777		 	dla = (struct sockaddr_dl *) &ifr->ifr_addr;
778			memcpy(hwaddr,  LLADDR(dla), dla->sdl_alen);
779			close (sock);
780			printf("using interface %s for proxy with address ",
781				ifp->ifr_name);
782			printf("%s\n", ether_ntoa(hwaddr));
783			return dla->sdl_alen;
784		}
785		ifr = (struct ifreq *) ((char *)&ifr->ifr_addr
786		    + MAX(ifr->ifr_addr.sa_len, sizeof(ifr->ifr_addr)));
787	}
788	return 0;
789}
790