xref: /freebsd/usr.sbin/arp/arp.c (revision 1e413cf93298b5b97441a21d9a50fdcd0ee9945e)
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  * 4. Neither the name of the University nor the names of its contributors
17  *    may be used to endorse or promote products derived from this software
18  *    without specific prior written permission.
19  *
20  * THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
21  * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
22  * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
23  * ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
24  * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
25  * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
26  * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
27  * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
28  * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
29  * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
30  * SUCH DAMAGE.
31  */
32 
33 #if 0
34 #ifndef lint
35 static char const copyright[] =
36 "@(#) Copyright (c) 1984, 1993\n\
37 	The Regents of the University of California.  All rights reserved.\n";
38 #endif /* not lint */
39 
40 #ifndef lint
41 static char const sccsid[] = "@(#)from: arp.c	8.2 (Berkeley) 1/2/94";
42 #endif /* not lint */
43 #endif
44 #include <sys/cdefs.h>
45 __FBSDID("$FreeBSD$");
46 
47 /*
48  * arp - display, set, and delete arp table entries
49  */
50 
51 
52 #include <sys/param.h>
53 #include <sys/file.h>
54 #include <sys/socket.h>
55 #include <sys/sockio.h>
56 #include <sys/sysctl.h>
57 #include <sys/ioctl.h>
58 #include <sys/time.h>
59 
60 #include <net/if.h>
61 #include <net/if_dl.h>
62 #include <net/if_types.h>
63 #include <net/route.h>
64 #include <net/iso88025.h>
65 
66 #include <netinet/in.h>
67 #include <netinet/if_ether.h>
68 
69 #include <arpa/inet.h>
70 
71 #include <ctype.h>
72 #include <err.h>
73 #include <errno.h>
74 #include <netdb.h>
75 #include <nlist.h>
76 #include <paths.h>
77 #include <stdio.h>
78 #include <stdlib.h>
79 #include <string.h>
80 #include <strings.h>
81 #include <unistd.h>
82 
83 typedef void (action_fn)(struct sockaddr_dl *sdl,
84 	struct sockaddr_inarp *s_in, struct rt_msghdr *rtm);
85 
86 static int search(u_long addr, action_fn *action);
87 static action_fn print_entry;
88 static action_fn nuke_entry;
89 
90 static int delete(char *host, int do_proxy);
91 static void usage(void);
92 static int set(int argc, char **argv);
93 static int get(char *host);
94 static int file(char *name);
95 static struct rt_msghdr *rtmsg(int cmd,
96     struct sockaddr_inarp *dst, struct sockaddr_dl *sdl);
97 static int get_ether_addr(in_addr_t ipaddr, struct ether_addr *hwaddr);
98 static struct sockaddr_inarp *getaddr(char *host);
99 static int valid_type(int type);
100 
101 static int nflag;	/* no reverse dns lookups */
102 static char *rifname;
103 
104 static int	expire_time, flags, doing_proxy, proxy_only;
105 
106 /* which function we're supposed to do */
107 #define F_GET		1
108 #define F_SET		2
109 #define F_FILESET	3
110 #define F_REPLACE	4
111 #define F_DELETE	5
112 
113 #define SETFUNC(f)	{ if (func) usage(); func = (f); }
114 
115 int
116 main(int argc, char *argv[])
117 {
118 	int ch, func = 0;
119 	int rtn = 0;
120 	int aflag = 0;	/* do it for all entries */
121 
122 	while ((ch = getopt(argc, argv, "andfsSi:")) != -1)
123 		switch((char)ch) {
124 		case 'a':
125 			aflag = 1;
126 			break;
127 		case 'd':
128 			SETFUNC(F_DELETE);
129 			break;
130 		case 'n':
131 			nflag = 1;
132 			break;
133 		case 'S':
134 			SETFUNC(F_REPLACE);
135 			break;
136 		case 's':
137 			SETFUNC(F_SET);
138 			break;
139 		case 'f' :
140 			SETFUNC(F_FILESET);
141 			break;
142 		case 'i':
143 			rifname = optarg;
144 			break;
145 		case '?':
146 		default:
147 			usage();
148 		}
149 	argc -= optind;
150 	argv += optind;
151 
152 	if (!func)
153 		func = F_GET;
154 	if (rifname) {
155 		if (func != F_GET && !(func == F_DELETE && aflag))
156 			errx(1, "-i not applicable to this operation");
157 		if (if_nametoindex(rifname) == 0) {
158 			if (errno == ENXIO)
159 				errx(1, "interface %s does not exist", rifname);
160 			else
161 				err(1, "if_nametoindex(%s)", rifname);
162 		}
163 	}
164 	switch (func) {
165 	case F_GET:
166 		if (aflag) {
167 			if (argc != 0)
168 				usage();
169 			search(0, print_entry);
170 		} else {
171 			if (argc != 1)
172 				usage();
173 			rtn = get(argv[0]);
174 		}
175 		break;
176 	case F_SET:
177 	case F_REPLACE:
178 		if (argc < 2 || argc > 6)
179 			usage();
180 		if (func == F_REPLACE)
181 			(void)delete(argv[0], 0);
182 		rtn = set(argc, argv) ? 1 : 0;
183 		break;
184 	case F_DELETE:
185 		if (aflag) {
186 			if (argc != 0)
187 				usage();
188 			search(0, nuke_entry);
189 		} else {
190 			if (argc == 2 && strncmp(argv[1], "pub", 3) == 0)
191 				ch = SIN_PROXY;
192 			else if (argc == 1)
193 				ch = 0;
194 			else
195 				usage();
196 			rtn = delete(argv[0], ch);
197 		}
198 		break;
199 	case F_FILESET:
200 		if (argc != 1)
201 			usage();
202 		rtn = file(argv[0]);
203 		break;
204 	}
205 
206 	return (rtn);
207 }
208 
209 /*
210  * Process a file to set standard arp entries
211  */
212 static int
213 file(char *name)
214 {
215 	FILE *fp;
216 	int i, retval;
217 	char line[100], arg[5][50], *args[5], *p;
218 
219 	if ((fp = fopen(name, "r")) == NULL)
220 		err(1, "cannot open %s", name);
221 	args[0] = &arg[0][0];
222 	args[1] = &arg[1][0];
223 	args[2] = &arg[2][0];
224 	args[3] = &arg[3][0];
225 	args[4] = &arg[4][0];
226 	retval = 0;
227 	while(fgets(line, sizeof(line), fp) != NULL) {
228 		if ((p = strchr(line, '#')) != NULL)
229 			*p = '\0';
230 		for (p = line; isblank(*p); p++);
231 		if (*p == '\n' || *p == '\0')
232 			continue;
233 		i = sscanf(p, "%49s %49s %49s %49s %49s", arg[0], arg[1],
234 		    arg[2], arg[3], arg[4]);
235 		if (i < 2) {
236 			warnx("bad line: %s", line);
237 			retval = 1;
238 			continue;
239 		}
240 		if (set(i, args))
241 			retval = 1;
242 	}
243 	fclose(fp);
244 	return (retval);
245 }
246 
247 /*
248  * Given a hostname, fills up a (static) struct sockaddr_inarp with
249  * the address of the host and returns a pointer to the
250  * structure.
251  */
252 static struct sockaddr_inarp *
253 getaddr(char *host)
254 {
255 	struct hostent *hp;
256 	static struct sockaddr_inarp reply;
257 
258 	bzero(&reply, sizeof(reply));
259 	reply.sin_len = sizeof(reply);
260 	reply.sin_family = AF_INET;
261 	reply.sin_addr.s_addr = inet_addr(host);
262 	if (reply.sin_addr.s_addr == INADDR_NONE) {
263 		if (!(hp = gethostbyname(host))) {
264 			warnx("%s: %s", host, hstrerror(h_errno));
265 			return (NULL);
266 		}
267 		bcopy((char *)hp->h_addr, (char *)&reply.sin_addr,
268 			sizeof reply.sin_addr);
269 	}
270 	return (&reply);
271 }
272 
273 /*
274  * Returns true if the type is a valid one for ARP.
275  */
276 static int
277 valid_type(int type)
278 {
279 
280 	switch (type) {
281 	case IFT_ETHER:
282 	case IFT_FDDI:
283 	case IFT_ISO88023:
284 	case IFT_ISO88024:
285 	case IFT_ISO88025:
286 	case IFT_L2VLAN:
287 	case IFT_BRIDGE:
288 		return (1);
289 	default:
290 		return (0);
291 	}
292 }
293 
294 /*
295  * Set an individual arp entry
296  */
297 static int
298 set(int argc, char **argv)
299 {
300 	struct sockaddr_inarp *addr;
301 	struct sockaddr_inarp *dst;	/* what are we looking for */
302 	struct sockaddr_dl *sdl;
303 	struct rt_msghdr *rtm;
304 	struct ether_addr *ea;
305 	char *host = argv[0], *eaddr = argv[1];
306 	struct sockaddr_dl sdl_m;
307 
308 	argc -= 2;
309 	argv += 2;
310 
311 	bzero(&sdl_m, sizeof(sdl_m));
312 	sdl_m.sdl_len = sizeof(sdl_m);
313 	sdl_m.sdl_family = AF_LINK;
314 
315 	dst = getaddr(host);
316 	if (dst == NULL)
317 		return (1);
318 	doing_proxy = flags = proxy_only = expire_time = 0;
319 	while (argc-- > 0) {
320 		if (strncmp(argv[0], "temp", 4) == 0) {
321 			struct timeval tv;
322 			gettimeofday(&tv, 0);
323 			expire_time = tv.tv_sec + 20 * 60;
324 		}
325 		else if (strncmp(argv[0], "pub", 3) == 0) {
326 			flags |= RTF_ANNOUNCE;
327 			doing_proxy = 1;
328 			if (argc && strncmp(argv[1], "only", 3) == 0) {
329 				proxy_only = 1;
330 				dst->sin_other = SIN_PROXY;
331 				argc--; argv++;
332 			}
333 		} else if (strncmp(argv[0], "trail", 5) == 0) {
334 			/* XXX deprecated and undocumented feature */
335 			printf("%s: Sending trailers is no longer supported\n",
336 				host);
337 		}
338 		argv++;
339 	}
340 	ea = (struct ether_addr *)LLADDR(&sdl_m);
341 	if (doing_proxy && !strcmp(eaddr, "auto")) {
342 		if (!get_ether_addr(dst->sin_addr.s_addr, ea)) {
343 			printf("no interface found for %s\n",
344 			       inet_ntoa(dst->sin_addr));
345 			return (1);
346 		}
347 		sdl_m.sdl_alen = ETHER_ADDR_LEN;
348 	} else {
349 		struct ether_addr *ea1 = ether_aton(eaddr);
350 
351 		if (ea1 == NULL) {
352 			warnx("invalid Ethernet address '%s'", eaddr);
353 			return (1);
354 		} else {
355 			*ea = *ea1;
356 			sdl_m.sdl_alen = ETHER_ADDR_LEN;
357 		}
358 	}
359 	for (;;) {	/* try at most twice */
360 		rtm = rtmsg(RTM_GET, dst, &sdl_m);
361 		if (rtm == NULL) {
362 			warn("%s", host);
363 			return (1);
364 		}
365 		addr = (struct sockaddr_inarp *)(rtm + 1);
366 		sdl = (struct sockaddr_dl *)(SA_SIZE(addr) + (char *)addr);
367 		if (addr->sin_addr.s_addr != dst->sin_addr.s_addr)
368 			break;
369 		if (sdl->sdl_family == AF_LINK &&
370 		    (rtm->rtm_flags & RTF_LLINFO) &&
371 		    !(rtm->rtm_flags & RTF_GATEWAY) &&
372 		    valid_type(sdl->sdl_type) )
373 			break;
374 		if (doing_proxy == 0) {
375 			printf("set: can only proxy for %s\n", host);
376 			return (1);
377 		}
378 		if (dst->sin_other & SIN_PROXY) {
379 			printf("set: proxy entry exists for non 802 device\n");
380 			return (1);
381 		}
382 		dst->sin_other = SIN_PROXY;
383 		proxy_only = 1;
384 	}
385 
386 	if (sdl->sdl_family != AF_LINK) {
387 		printf("cannot intuit interface index and type for %s\n", host);
388 		return (1);
389 	}
390 	sdl_m.sdl_type = sdl->sdl_type;
391 	sdl_m.sdl_index = sdl->sdl_index;
392 	return (rtmsg(RTM_ADD, dst, &sdl_m) == NULL);
393 }
394 
395 /*
396  * Display an individual arp entry
397  */
398 static int
399 get(char *host)
400 {
401 	struct sockaddr_inarp *addr;
402 
403 	addr = getaddr(host);
404 	if (addr == NULL)
405 		return (1);
406 	if (0 == search(addr->sin_addr.s_addr, print_entry)) {
407 		printf("%s (%s) -- no entry",
408 		    host, inet_ntoa(addr->sin_addr));
409 		if (rifname)
410 			printf(" on %s", rifname);
411 		printf("\n");
412 		return (1);
413 	}
414 	return (0);
415 }
416 
417 /*
418  * Delete an arp entry
419  */
420 static int
421 delete(char *host, int do_proxy)
422 {
423 	struct sockaddr_inarp *addr, *dst;
424 	struct rt_msghdr *rtm;
425 	struct sockaddr_dl *sdl;
426 
427 	dst = getaddr(host);
428 	if (dst == NULL)
429 		return (1);
430 	dst->sin_other = do_proxy;
431 	for (;;) {	/* try twice */
432 		rtm = rtmsg(RTM_GET, dst, NULL);
433 		if (rtm == NULL) {
434 			warn("%s", host);
435 			return (1);
436 		}
437 		addr = (struct sockaddr_inarp *)(rtm + 1);
438 		sdl = (struct sockaddr_dl *)(SA_SIZE(addr) + (char *)addr);
439 		if (addr->sin_addr.s_addr == dst->sin_addr.s_addr &&
440 		    sdl->sdl_family == AF_LINK &&
441 		    (rtm->rtm_flags & RTF_LLINFO) &&
442 		    !(rtm->rtm_flags & RTF_GATEWAY) &&
443 		    valid_type(sdl->sdl_type) )
444 			break;	/* found it */
445 		if (dst->sin_other & SIN_PROXY) {
446 			fprintf(stderr, "delete: cannot locate %s\n",host);
447 			return (1);
448 		}
449 		dst->sin_other = SIN_PROXY;
450 	}
451 	if (rtmsg(RTM_DELETE, dst, NULL) != NULL) {
452 		printf("%s (%s) deleted\n", host, inet_ntoa(addr->sin_addr));
453 		return (0);
454 	}
455 	return (1);
456 }
457 
458 /*
459  * Search the arp table and do some action on matching entries
460  */
461 static int
462 search(u_long addr, action_fn *action)
463 {
464 	int mib[6];
465 	size_t needed;
466 	char *lim, *buf, *newbuf, *next;
467 	struct rt_msghdr *rtm;
468 	struct sockaddr_inarp *sin2;
469 	struct sockaddr_dl *sdl;
470 	char ifname[IF_NAMESIZE];
471 	int st, found_entry = 0;
472 
473 	mib[0] = CTL_NET;
474 	mib[1] = PF_ROUTE;
475 	mib[2] = 0;
476 	mib[3] = AF_INET;
477 	mib[4] = NET_RT_FLAGS;
478 	mib[5] = RTF_LLINFO;
479 	if (sysctl(mib, 6, NULL, &needed, NULL, 0) < 0)
480 		err(1, "route-sysctl-estimate");
481 	if (needed == 0)	/* empty table */
482 		return 0;
483 	buf = NULL;
484 	for (;;) {
485 		newbuf = realloc(buf, needed);
486 		if (newbuf == NULL) {
487 			if (buf != NULL)
488 				free(buf);
489 			errx(1, "could not reallocate memory");
490 		}
491 		buf = newbuf;
492 		st = sysctl(mib, 6, buf, &needed, NULL, 0);
493 		if (st == 0 || errno != ENOMEM)
494 			break;
495 		needed += needed / 8;
496 	}
497 	if (st == -1)
498 		err(1, "actual retrieval of routing table");
499 	lim = buf + needed;
500 	for (next = buf; next < lim; next += rtm->rtm_msglen) {
501 		rtm = (struct rt_msghdr *)next;
502 		sin2 = (struct sockaddr_inarp *)(rtm + 1);
503 		sdl = (struct sockaddr_dl *)((char *)sin2 + SA_SIZE(sin2));
504 		if (rifname && if_indextoname(sdl->sdl_index, ifname) &&
505 		    strcmp(ifname, rifname))
506 			continue;
507 		if (addr) {
508 			if (addr != sin2->sin_addr.s_addr)
509 				continue;
510 			found_entry = 1;
511 		}
512 		(*action)(sdl, sin2, rtm);
513 	}
514 	free(buf);
515 	return (found_entry);
516 }
517 
518 /*
519  * Display an arp entry
520  */
521 static void
522 print_entry(struct sockaddr_dl *sdl,
523 	struct sockaddr_inarp *addr, struct rt_msghdr *rtm)
524 {
525 	const char *host;
526 	struct hostent *hp;
527 	struct iso88025_sockaddr_dl_data *trld;
528 	char ifname[IF_NAMESIZE];
529 	int seg;
530 
531 	if (nflag == 0)
532 		hp = gethostbyaddr((caddr_t)&(addr->sin_addr),
533 		    sizeof addr->sin_addr, AF_INET);
534 	else
535 		hp = 0;
536 	if (hp)
537 		host = hp->h_name;
538 	else {
539 		host = "?";
540 		if (h_errno == TRY_AGAIN)
541 			nflag = 1;
542 	}
543 	printf("%s (%s) at ", host, inet_ntoa(addr->sin_addr));
544 	if (sdl->sdl_alen) {
545 		if ((sdl->sdl_type == IFT_ETHER ||
546 		    sdl->sdl_type == IFT_L2VLAN ||
547 		    sdl->sdl_type == IFT_BRIDGE) &&
548 		    sdl->sdl_alen == ETHER_ADDR_LEN)
549 			printf("%s", ether_ntoa((struct ether_addr *)LLADDR(sdl)));
550 		else {
551 			int n = sdl->sdl_nlen > 0 ? sdl->sdl_nlen + 1 : 0;
552 
553 			printf("%s", link_ntoa(sdl) + n);
554 		}
555 	} else
556 		printf("(incomplete)");
557 	if (if_indextoname(sdl->sdl_index, ifname) != NULL)
558 		printf(" on %s", ifname);
559 	if (rtm->rtm_rmx.rmx_expire == 0)
560 		printf(" permanent");
561 	if (addr->sin_other & SIN_PROXY)
562 		printf(" published (proxy only)");
563 	if (rtm->rtm_addrs & RTA_NETMASK) {
564 		addr = (struct sockaddr_inarp *)
565 			(SA_SIZE(sdl) + (char *)sdl);
566 		if (addr->sin_addr.s_addr == 0xffffffff)
567 			printf(" published");
568 		if (addr->sin_len != 8)
569 			printf("(weird)");
570 	}
571         switch(sdl->sdl_type) {
572 	case IFT_ETHER:
573                 printf(" [ethernet]");
574                 break;
575 	case IFT_ISO88025:
576                 printf(" [token-ring]");
577 		trld = SDL_ISO88025(sdl);
578 		if (trld->trld_rcf != 0) {
579 			printf(" rt=%x", ntohs(trld->trld_rcf));
580 			for (seg = 0;
581 			     seg < ((TR_RCF_RIFLEN(trld->trld_rcf) - 2 ) / 2);
582 			     seg++)
583 				printf(":%x", ntohs(*(trld->trld_route[seg])));
584 		}
585                 break;
586 	case IFT_FDDI:
587                 printf(" [fddi]");
588                 break;
589 	case IFT_ATM:
590                 printf(" [atm]");
591                 break;
592 	case IFT_L2VLAN:
593 		printf(" [vlan]");
594 		break;
595 	case IFT_IEEE1394:
596                 printf(" [firewire]");
597                 break;
598 	case IFT_BRIDGE:
599 		printf(" [bridge]");
600 		break;
601 	default:
602 		break;
603         }
604 
605 	printf("\n");
606 
607 }
608 
609 /*
610  * Nuke an arp entry
611  */
612 static void
613 nuke_entry(struct sockaddr_dl *sdl __unused,
614 	struct sockaddr_inarp *addr, struct rt_msghdr *rtm __unused)
615 {
616 	char ip[20];
617 
618 	snprintf(ip, sizeof(ip), "%s", inet_ntoa(addr->sin_addr));
619 	(void)delete(ip, 0);
620 }
621 
622 static void
623 usage(void)
624 {
625 	fprintf(stderr, "%s\n%s\n%s\n%s\n%s\n%s\n%s\n",
626 		"usage: arp [-n] [-i interface] hostname",
627 		"       arp [-n] [-i interface] -a",
628 		"       arp -d hostname [pub]",
629 		"       arp -d [-i interface] -a",
630 		"       arp -s hostname ether_addr [temp] [pub [only]]",
631 		"       arp -S hostname ether_addr [temp] [pub [only]]",
632 		"       arp -f filename");
633 	exit(1);
634 }
635 
636 static struct rt_msghdr *
637 rtmsg(int cmd, struct sockaddr_inarp *dst, struct sockaddr_dl *sdl)
638 {
639 	static int seq;
640 	int rlen;
641 	int l;
642 	struct sockaddr_in so_mask, *som = &so_mask;
643 	static int s = -1;
644 	static pid_t pid;
645 
646 	static struct	{
647 		struct	rt_msghdr m_rtm;
648 		char	m_space[512];
649 	}	m_rtmsg;
650 
651 	struct rt_msghdr *rtm = &m_rtmsg.m_rtm;
652 	char *cp = m_rtmsg.m_space;
653 
654 	if (s < 0) {	/* first time: open socket, get pid */
655 		s = socket(PF_ROUTE, SOCK_RAW, 0);
656 		if (s < 0)
657 			err(1, "socket");
658 		pid = getpid();
659 	}
660 	bzero(&so_mask, sizeof(so_mask));
661 	so_mask.sin_len = 8;
662 	so_mask.sin_addr.s_addr = 0xffffffff;
663 
664 	errno = 0;
665 	/*
666 	 * XXX RTM_DELETE relies on a previous RTM_GET to fill the buffer
667 	 * appropriately.
668 	 */
669 	if (cmd == RTM_DELETE)
670 		goto doit;
671 	bzero((char *)&m_rtmsg, sizeof(m_rtmsg));
672 	rtm->rtm_flags = flags;
673 	rtm->rtm_version = RTM_VERSION;
674 
675 	switch (cmd) {
676 	default:
677 		errx(1, "internal wrong cmd");
678 	case RTM_ADD:
679 		rtm->rtm_addrs |= RTA_GATEWAY;
680 		rtm->rtm_rmx.rmx_expire = expire_time;
681 		rtm->rtm_inits = RTV_EXPIRE;
682 		rtm->rtm_flags |= (RTF_HOST | RTF_STATIC);
683 		dst->sin_other = 0;
684 		if (doing_proxy) {
685 			if (proxy_only)
686 				dst->sin_other = SIN_PROXY;
687 			else {
688 				rtm->rtm_addrs |= RTA_NETMASK;
689 				rtm->rtm_flags &= ~RTF_HOST;
690 			}
691 		}
692 		/* FALLTHROUGH */
693 	case RTM_GET:
694 		rtm->rtm_addrs |= RTA_DST;
695 	}
696 #define NEXTADDR(w, s) \
697 	if ((s) != NULL && rtm->rtm_addrs & (w)) { \
698 		bcopy((s), cp, sizeof(*(s))); cp += SA_SIZE(s);}
699 
700 	NEXTADDR(RTA_DST, dst);
701 	NEXTADDR(RTA_GATEWAY, sdl);
702 	NEXTADDR(RTA_NETMASK, som);
703 
704 	rtm->rtm_msglen = cp - (char *)&m_rtmsg;
705 doit:
706 	l = rtm->rtm_msglen;
707 	rtm->rtm_seq = ++seq;
708 	rtm->rtm_type = cmd;
709 	if ((rlen = write(s, (char *)&m_rtmsg, l)) < 0) {
710 		if (errno != ESRCH || cmd != RTM_DELETE) {
711 			warn("writing to routing socket");
712 			return (NULL);
713 		}
714 	}
715 	do {
716 		l = read(s, (char *)&m_rtmsg, sizeof(m_rtmsg));
717 	} while (l > 0 && (rtm->rtm_seq != seq || rtm->rtm_pid != pid));
718 	if (l < 0)
719 		warn("read from routing socket");
720 	return (rtm);
721 }
722 
723 /*
724  * get_ether_addr - get the hardware address of an interface on the
725  * the same subnet as ipaddr.
726  */
727 #define MAX_IFS		32
728 
729 static int
730 get_ether_addr(in_addr_t ipaddr, struct ether_addr *hwaddr)
731 {
732 	struct ifreq *ifr, *ifend, *ifp;
733 	in_addr_t ina, mask;
734 	struct sockaddr_dl *dla;
735 	struct ifreq ifreq;
736 	struct ifconf ifc;
737 	struct ifreq ifs[MAX_IFS];
738 	int sock;
739 	int retval = 0;
740 
741 	sock = socket(AF_INET, SOCK_DGRAM, 0);
742 	if (sock < 0)
743 		err(1, "socket");
744 
745 	ifc.ifc_len = sizeof(ifs);
746 	ifc.ifc_req = ifs;
747 	if (ioctl(sock, SIOCGIFCONF, &ifc) < 0) {
748 		warnx("ioctl(SIOCGIFCONF)");
749 		goto done;
750 	}
751 
752 #define NEXTIFR(i)						\
753     ((struct ifreq *)((char *)&(i)->ifr_addr			\
754 	+ MAX((i)->ifr_addr.sa_len, sizeof((i)->ifr_addr))) )
755 
756 	/*
757 	 * Scan through looking for an interface with an Internet
758 	 * address on the same subnet as `ipaddr'.
759 	 */
760 	ifend = (struct ifreq *)(ifc.ifc_buf + ifc.ifc_len);
761 	for (ifr = ifc.ifc_req; ifr < ifend; ifr = NEXTIFR(ifr) ) {
762 		if (ifr->ifr_addr.sa_family != AF_INET)
763 			continue;
764 		strncpy(ifreq.ifr_name, ifr->ifr_name,
765 			sizeof(ifreq.ifr_name));
766 		ifreq.ifr_addr = ifr->ifr_addr;
767 		/*
768 		 * Check that the interface is up,
769 		 * and not point-to-point or loopback.
770 		 */
771 		if (ioctl(sock, SIOCGIFFLAGS, &ifreq) < 0)
772 			continue;
773 		if ((ifreq.ifr_flags &
774 		     (IFF_UP|IFF_BROADCAST|IFF_POINTOPOINT|
775 				IFF_LOOPBACK|IFF_NOARP))
776 		     != (IFF_UP|IFF_BROADCAST))
777 			continue;
778 		/*
779 		 * Get its netmask and check that it's on
780 		 * the right subnet.
781 		 */
782 		if (ioctl(sock, SIOCGIFNETMASK, &ifreq) < 0)
783 			continue;
784 		mask = ((struct sockaddr_in *)
785 			&ifreq.ifr_addr)->sin_addr.s_addr;
786 		ina = ((struct sockaddr_in *)
787 			&ifr->ifr_addr)->sin_addr.s_addr;
788 		if ((ipaddr & mask) == (ina & mask))
789 			break; /* ok, we got it! */
790 	}
791 
792 	if (ifr >= ifend)
793 		goto done;
794 
795 	/*
796 	 * Now scan through again looking for a link-level address
797 	 * for this interface.
798 	 */
799 	ifp = ifr;
800 	for (ifr = ifc.ifc_req; ifr < ifend; ifr = NEXTIFR(ifr))
801 		if (strcmp(ifp->ifr_name, ifr->ifr_name) == 0 &&
802 		    ifr->ifr_addr.sa_family == AF_LINK)
803 			break;
804 	if (ifr >= ifend)
805 		goto done;
806 	/*
807 	 * Found the link-level address - copy it out
808 	 */
809 	dla = (struct sockaddr_dl *) &ifr->ifr_addr;
810 	memcpy(hwaddr,  LLADDR(dla), dla->sdl_alen);
811 	printf("using interface %s for proxy with address ",
812 		ifp->ifr_name);
813 	printf("%s\n", ether_ntoa(hwaddr));
814 	retval = dla->sdl_alen;
815 done:
816 	close(sock);
817 	return (retval);
818 }
819