1/*-
2 * SPDX-License-Identifier: BSD-2-Clause
3 *
4 * Copyright (c) 2010 Bjoern A. Zeeb <bz@FreeBSD.org>
5 * All rights reserved.
6 *
7 * Redistribution and use in source and binary forms, with or without
8 * modification, are permitted provided that the following conditions
9 * are met:
10 * 1. Redistributions of source code must retain the above copyright
11 * notice, this list of conditions and the following disclaimer.
12 * 2. Redistributions in binary form must reproduce the above copyright
13 * notice, this list of conditions and the following disclaimer in the
14 * documentation and/or other materials provided with the distribution.
15 *
16 * THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
17 * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
18 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
19 * ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
20 * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
21 * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
22 * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
23 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
24 * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
25 * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
26 * SUCH DAMAGE.
27 */
28
29#include <string.h>
30#include <unistd.h>
31
32#include <sys/types.h>
33#include <sys/socket.h>
34
35#include <netinet/in.h>
36
37#include "findsaddr.h"
38#include "traceroute.h"
39
40/*
41 * Return the source address for the given destination address.
42 *
43 * This makes use of proper source address selection in the FreeBSD kernel
44 * even taking jails into account (sys/netinet/in_pcb.c:in_pcbladdr()).
45 * We open a UDP socket, and connect to the destination, letting the kernel
46 * do the bind and then read the source IPv4 address using getsockname(2).
47 * This has multiple advantages: no need to do PF_ROUTE operations possibly
48 * needing special privileges, jails properly taken into account and most
49 * important - getting the result the kernel would give us rather than
50 * best-guessing ourselves.
51 */
52const char *
53findsaddr(register const struct sockaddr_in *to,
54    register struct sockaddr_in *from)
55{
56	const char *errstr;
57	struct sockaddr_in cto, cfrom;
58	int s;
59	socklen_t len;
60
61	s = socket(AF_INET, SOCK_DGRAM, 0);
62	if (s == -1)
63		return ("failed to open DGRAM socket for src addr selection.");
64
65	errstr = NULL;
66	len = sizeof(struct sockaddr_in);
67	memcpy(&cto, to, len);
68	cto.sin_port = htons(65535);	/* Dummy port for connect(2). */
69	if (connect(s, (struct sockaddr *)&cto, len) == -1) {
70		errstr = "failed to connect to peer for src addr selection.";
71		goto err;
72	}
73
74	if (getsockname(s, (struct sockaddr *)&cfrom, &len) == -1) {
75		errstr = "failed to get socket name for src addr selection.";
76		goto err;
77	}
78
79	if (len != sizeof(struct sockaddr_in) || cfrom.sin_family != AF_INET) {
80		errstr = "unexpected address family in src addr selection.";
81		goto err;
82	}
83
84	/* Update source address for traceroute. */
85	setsin(from, cfrom.sin_addr.s_addr);
86
87err:
88	(void) close(s);
89
90	/* No error (string) to return. */
91	return (errstr);
92}
93
94/* end */
95