dns.c revision 295367 
1/* $OpenBSD: dns.c,v 1.35 2015/08/20 22:32:42 deraadt Exp $ */
2
3/*
4 * Copyright (c) 2003 Wesley Griffin. All rights reserved.
5 * Copyright (c) 2003 Jakob Schlyter. 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 ``AS IS'' AND ANY EXPRESS OR
17 * IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
18 * OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
19 * IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
20 * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
21 * NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
22 * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
23 * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
24 * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
25 * THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
26 */
27
28#include "includes.h"
29
30#include <sys/types.h>
31#include <sys/socket.h>
32
33#include <netdb.h>
34#include <stdarg.h>
35#include <stdio.h>
36#include <string.h>
37#include <stdarg.h>
38#include <stdlib.h>
39
40#include "xmalloc.h"
41#include "sshkey.h"
42#include "ssherr.h"
43#include "dns.h"
44#include "log.h"
45#include "digest.h"
46
47static const char *errset_text[] = {
48	"success",		/* 0 ERRSET_SUCCESS */
49	"out of memory",	/* 1 ERRSET_NOMEMORY */
50	"general failure",	/* 2 ERRSET_FAIL */
51	"invalid parameter",	/* 3 ERRSET_INVAL */
52	"name does not exist",	/* 4 ERRSET_NONAME */
53	"data does not exist",	/* 5 ERRSET_NODATA */
54};
55
56static const char *
57dns_result_totext(unsigned int res)
58{
59	switch (res) {
60	case ERRSET_SUCCESS:
61		return errset_text[ERRSET_SUCCESS];
62	case ERRSET_NOMEMORY:
63		return errset_text[ERRSET_NOMEMORY];
64	case ERRSET_FAIL:
65		return errset_text[ERRSET_FAIL];
66	case ERRSET_INVAL:
67		return errset_text[ERRSET_INVAL];
68	case ERRSET_NONAME:
69		return errset_text[ERRSET_NONAME];
70	case ERRSET_NODATA:
71		return errset_text[ERRSET_NODATA];
72	default:
73		return "unknown error";
74	}
75}
76
77/*
78 * Read SSHFP parameters from key buffer.
79 */
80static int
81dns_read_key(u_int8_t *algorithm, u_int8_t *digest_type,
82    u_char **digest, size_t *digest_len, struct sshkey *key)
83{
84	int r, success = 0;
85	int fp_alg = -1;
86
87	switch (key->type) {
88	case KEY_RSA:
89		*algorithm = SSHFP_KEY_RSA;
90		if (!*digest_type)
91			*digest_type = SSHFP_HASH_SHA1;
92		break;
93	case KEY_DSA:
94		*algorithm = SSHFP_KEY_DSA;
95		if (!*digest_type)
96			*digest_type = SSHFP_HASH_SHA1;
97		break;
98	case KEY_ECDSA:
99		*algorithm = SSHFP_KEY_ECDSA;
100		if (!*digest_type)
101			*digest_type = SSHFP_HASH_SHA256;
102		break;
103	case KEY_ED25519:
104		*algorithm = SSHFP_KEY_ED25519;
105		if (!*digest_type)
106			*digest_type = SSHFP_HASH_SHA256;
107		break;
108	default:
109		*algorithm = SSHFP_KEY_RESERVED; /* 0 */
110		*digest_type = SSHFP_HASH_RESERVED; /* 0 */
111	}
112
113	switch (*digest_type) {
114	case SSHFP_HASH_SHA1:
115		fp_alg = SSH_DIGEST_SHA1;
116		break;
117	case SSHFP_HASH_SHA256:
118		fp_alg = SSH_DIGEST_SHA256;
119		break;
120	default:
121		*digest_type = SSHFP_HASH_RESERVED; /* 0 */
122	}
123
124	if (*algorithm && *digest_type) {
125		if ((r = sshkey_fingerprint_raw(key, fp_alg, digest,
126		    digest_len)) != 0)
127			fatal("%s: sshkey_fingerprint_raw: %s", __func__,
128			   ssh_err(r));
129		success = 1;
130	} else {
131		*digest = NULL;
132		*digest_len = 0;
133		success = 0;
134	}
135
136	return success;
137}
138
139/*
140 * Read SSHFP parameters from rdata buffer.
141 */
142static int
143dns_read_rdata(u_int8_t *algorithm, u_int8_t *digest_type,
144    u_char **digest, size_t *digest_len, u_char *rdata, int rdata_len)
145{
146	int success = 0;
147
148	*algorithm = SSHFP_KEY_RESERVED;
149	*digest_type = SSHFP_HASH_RESERVED;
150
151	if (rdata_len >= 2) {
152		*algorithm = rdata[0];
153		*digest_type = rdata[1];
154		*digest_len = rdata_len - 2;
155
156		if (*digest_len > 0) {
157			*digest = xmalloc(*digest_len);
158			memcpy(*digest, rdata + 2, *digest_len);
159		} else {
160			*digest = (u_char *)xstrdup("");
161		}
162
163		success = 1;
164	}
165
166	return success;
167}
168
169/*
170 * Check if hostname is numerical.
171 * Returns -1 if hostname is numeric, 0 otherwise
172 */
173static int
174is_numeric_hostname(const char *hostname)
175{
176	struct addrinfo hints, *ai;
177
178	/*
179	 * We shouldn't ever get a null host but if we do then log an error
180	 * and return -1 which stops DNS key fingerprint processing.
181	 */
182	if (hostname == NULL) {
183		error("is_numeric_hostname called with NULL hostname");
184		return -1;
185	}
186
187	memset(&hints, 0, sizeof(hints));
188	hints.ai_socktype = SOCK_DGRAM;
189	hints.ai_flags = AI_NUMERICHOST;
190
191	if (getaddrinfo(hostname, NULL, &hints, &ai) == 0) {
192		freeaddrinfo(ai);
193		return -1;
194	}
195
196	return 0;
197}
198
199/*
200 * Verify the given hostname, address and host key using DNS.
201 * Returns 0 if lookup succeeds, -1 otherwise
202 */
203int
204verify_host_key_dns(const char *hostname, struct sockaddr *address,
205    struct sshkey *hostkey, int *flags)
206{
207	u_int counter;
208	int result;
209	struct rrsetinfo *fingerprints = NULL;
210
211	u_int8_t hostkey_algorithm;
212	u_int8_t hostkey_digest_type = SSHFP_HASH_RESERVED;
213	u_char *hostkey_digest;
214	size_t hostkey_digest_len;
215
216	u_int8_t dnskey_algorithm;
217	u_int8_t dnskey_digest_type;
218	u_char *dnskey_digest;
219	size_t dnskey_digest_len;
220
221	*flags = 0;
222
223	debug3("verify_host_key_dns");
224	if (hostkey == NULL)
225		fatal("No key to look up!");
226
227	if (is_numeric_hostname(hostname)) {
228		debug("skipped DNS lookup for numerical hostname");
229		return -1;
230	}
231
232	result = getrrsetbyname(hostname, DNS_RDATACLASS_IN,
233	    DNS_RDATATYPE_SSHFP, 0, &fingerprints);
234	if (result) {
235		verbose("DNS lookup error: %s", dns_result_totext(result));
236		return -1;
237	}
238
239	if (fingerprints->rri_flags & RRSET_VALIDATED) {
240		*flags |= DNS_VERIFY_SECURE;
241		debug("found %d secure fingerprints in DNS",
242		    fingerprints->rri_nrdatas);
243	} else {
244		debug("found %d insecure fingerprints in DNS",
245		    fingerprints->rri_nrdatas);
246	}
247
248	/* Initialize default host key parameters */
249	if (!dns_read_key(&hostkey_algorithm, &hostkey_digest_type,
250	    &hostkey_digest, &hostkey_digest_len, hostkey)) {
251		error("Error calculating host key fingerprint.");
252		freerrset(fingerprints);
253		return -1;
254	}
255
256	if (fingerprints->rri_nrdatas)
257		*flags |= DNS_VERIFY_FOUND;
258
259	for (counter = 0; counter < fingerprints->rri_nrdatas; counter++) {
260		/*
261		 * Extract the key from the answer. Ignore any badly
262		 * formatted fingerprints.
263		 */
264		if (!dns_read_rdata(&dnskey_algorithm, &dnskey_digest_type,
265		    &dnskey_digest, &dnskey_digest_len,
266		    fingerprints->rri_rdatas[counter].rdi_data,
267		    fingerprints->rri_rdatas[counter].rdi_length)) {
268			verbose("Error parsing fingerprint from DNS.");
269			continue;
270		}
271
272		if (hostkey_digest_type != dnskey_digest_type) {
273			hostkey_digest_type = dnskey_digest_type;
274			free(hostkey_digest);
275
276			/* Initialize host key parameters */
277			if (!dns_read_key(&hostkey_algorithm,
278			    &hostkey_digest_type, &hostkey_digest,
279			    &hostkey_digest_len, hostkey)) {
280				error("Error calculating key fingerprint.");
281				freerrset(fingerprints);
282				return -1;
283			}
284		}
285
286		/* Check if the current key is the same as the given key */
287		if (hostkey_algorithm == dnskey_algorithm &&
288		    hostkey_digest_type == dnskey_digest_type) {
289			if (hostkey_digest_len == dnskey_digest_len &&
290			    timingsafe_bcmp(hostkey_digest, dnskey_digest,
291			    hostkey_digest_len) == 0)
292				*flags |= DNS_VERIFY_MATCH;
293		}
294		free(dnskey_digest);
295	}
296
297	free(hostkey_digest); /* from sshkey_fingerprint_raw() */
298	freerrset(fingerprints);
299
300	if (*flags & DNS_VERIFY_FOUND)
301		if (*flags & DNS_VERIFY_MATCH)
302			debug("matching host key fingerprint found in DNS");
303		else
304			debug("mismatching host key fingerprint found in DNS");
305	else
306		debug("no host key fingerprint found in DNS");
307
308	return 0;
309}
310
311/*
312 * Export the fingerprint of a key as a DNS resource record
313 */
314int
315export_dns_rr(const char *hostname, struct sshkey *key, FILE *f, int generic)
316{
317	u_int8_t rdata_pubkey_algorithm = 0;
318	u_int8_t rdata_digest_type = SSHFP_HASH_RESERVED;
319	u_int8_t dtype;
320	u_char *rdata_digest;
321	size_t i, rdata_digest_len;
322	int success = 0;
323
324	for (dtype = SSHFP_HASH_SHA1; dtype < SSHFP_HASH_MAX; dtype++) {
325		rdata_digest_type = dtype;
326		if (dns_read_key(&rdata_pubkey_algorithm, &rdata_digest_type,
327		    &rdata_digest, &rdata_digest_len, key)) {
328			if (generic) {
329				fprintf(f, "%s IN TYPE%d \\# %zu %02x %02x ",
330				    hostname, DNS_RDATATYPE_SSHFP,
331				    2 + rdata_digest_len,
332				    rdata_pubkey_algorithm, rdata_digest_type);
333			} else {
334				fprintf(f, "%s IN SSHFP %d %d ", hostname,
335				    rdata_pubkey_algorithm, rdata_digest_type);
336			}
337			for (i = 0; i < rdata_digest_len; i++)
338				fprintf(f, "%02x", rdata_digest[i]);
339			fprintf(f, "\n");
340			free(rdata_digest); /* from sshkey_fingerprint_raw() */
341			success = 1;
342		}
343	}
344
345	/* No SSHFP record was generated at all */
346	if (success == 0) {
347		error("%s: unsupported algorithm and/or digest_type", __func__);
348	}
349
350	return success;
351}
352