5 SSL_CTX_dane_enable, SSL_CTX_dane_mtype_set, SSL_dane_enable,
6 SSL_dane_tlsa_add, SSL_get0_dane_authority, SSL_get0_dane_tlsa -
7 enable DANE TLS authentication of the remote TLS server in the local
12 #include <openssl/ssl.h>
14 int SSL_CTX_dane_enable(SSL_CTX *ctx);
15 int SSL_CTX_dane_mtype_set(SSL_CTX *ctx, const EVP_MD *md,
16 uint8_t mtype, uint8_t ord);
17 int SSL_dane_enable(SSL *s, const char *basedomain);
18 int SSL_dane_tlsa_add(SSL *s, uint8_t usage, uint8_t selector,
19 uint8_t mtype, unsigned char *data, size_t dlen);
20 int SSL_get0_dane_authority(SSL *s, X509 **mcert, EVP_PKEY **mspki);
21 int SSL_get0_dane_tlsa(SSL *s, uint8_t *usage, uint8_t *selector,
22 uint8_t *mtype, unsigned const char **data,
27 These functions implement support for DANE TLSA (RFC6698 and RFC7671)
30 SSL_CTX_dane_enable() must be called first to initialize the
31 shared state required for DANE support. Individual connections
32 associated with the context can then enable per-connection DANE
33 support as appropriate. DANE authentication is implemented in the
34 L<X509_verify_cert(3)> function, and applications that override
35 L<X509_verify_cert(3)> via L<SSL_CTX_set_cert_verify_callback(3)>
36 are responsible to authenticate the peer chain in whatever manner
39 SSL_CTX_dane_mtype_set() may then be called zero or more times to
40 to adjust the supported digest algorithms. This must be done before
41 any SSL handles are created for the context.
43 The B<mtype> argument specifies a DANE TLSA matching type and the
44 B<md> argument specifies the associated digest algorithm handle.
45 The B<ord> argument specifies a strength ordinal. Algorithms with
46 a larger strength ordinal are considered more secure. Strength
47 ordinals are used to implement RFC7671 digest algorithm agility.
48 Specifying a B<NULL> digest algorithm for a matching type disables
49 support for that matching type. Matching type Full(0) cannot be
52 By default, matching type C<SHA2-256(1)> (see RFC7218 for definitions
53 of the DANE TLSA parameter acronyms) is mapped to C<EVP_sha256()>
54 with a strength ordinal of C<1> and matching type C<SHA2-512(2)>
55 is mapped to C<EVP_sha512()> with a strength ordinal of C<2>.
57 SSL_dane_enable() may be called before the SSL handshake is
58 initiated with L<SSL_connect(3)> to enable DANE for that connection.
59 (The connection must be associated with a DANE-enabled SSL context).
60 The B<basedomain> argument specifies the RFC7671 TLSA base domain,
61 which will be the primary peer reference identifier for certificate
62 name checks. Additional server names can be specified via
63 L<SSL_add1_host(3)>. The B<basedomain> is used as the default SNI
64 hint if none has yet been specified via L<SSL_set_tlsext_host_name(3)>.
66 SSL_dane_tlsa_add() may then be called one or more times, to
67 load each of the TLSA records that apply to the remote TLS peer.
68 (This too must be done prior to the beginning of the SSL handshake).
69 The arguments specify the fields of the TLSA record. The B<data>
70 field is provided in binary (wire RDATA) form, not the hexadecimal ASCII
71 presentation form, with an explicit length passed via B<dlen>.
72 A return value of 0 indicates that "unusable" TLSA records
73 (with invalid or unsupported parameters) were provided, a negative
74 return value indicates an internal error in processing the records.
75 If DANE authentication is enabled, but no TLSA records are added
76 successfully, authentication will fail, and the handshake may not
77 complete, depending on the B<mode> argument of L<SSL_set_verify(3)>
78 and any verification callback.
80 SSL_get0_dane_authority() can be used to get more detailed information
81 about the matched DANE trust-anchor after successful connection
82 completion. The return value is negative if DANE verification
83 failed (or was not enabled), 0 if an EE TLSA record directly matched
84 the leaf certificate, or a positive number indicating the depth at
85 which a TA record matched an issuer certificate.
87 If the B<mcert> argument is not B<NULL> and a TLSA record matched
88 a chain certificate, a pointer to the matching certificate is
89 returned via B<mcert>. The returned address is a short-term internal
90 reference to the certificate and must not be freed by the application.
91 Applications that want to retain access to the certificate can call
92 L<X509_up_ref(3)> to obtain a long-term reference which must then
93 be freed via L<X509_free(3)> once no longer needed.
95 If no TLSA records directly matched any elements of the certificate
96 chain, but a DANE-TA(2) SPKI(1) Full(0) record provided the public
97 key that signed an element of the chain, then that key is returned
98 via B<mspki> argument (if not NULL). In this case the return value
99 is the depth of the top-most element of the validated certificate
100 chain. As with B<mcert> this is a short-term internal reference,
101 and L<EVP_PKEY_up_ref(3)> and L<EVP_PKEY_free(3)> can be used to
102 acquire and release long-term references respectively.
104 SSL_get0_dane_tlsa() can be used to retrieve the fields of the
105 TLSA record that matched the peer certificate chain. The return
106 value indicates the match depth or failure to match just as with
107 SSL_get0_dane_authority(). When the return value is non-negative,
108 the storage pointed to by the B<usage>, B<selector>, B<mtype> and
109 B<data> parameters is updated to the corresponding TLSA record
110 fields. The B<data> field is in binary wire form, and is therefore
111 not NUL-terminated, its length is returned via the B<dlen> parameter.
112 If any of these parameters is NULL, the corresponding field
113 is not returned. The B<data> parameter is set to a short-term
114 internal-copy of the associated data field and must not be freed
115 by the application. Applications that need long-term access to
116 this field need to copy the content.
120 The functions SSL_CTX_dane_enable(), SSL_CTX_dane_mtype_set(),
121 SSL_dane_enable() and SSL_dane_tlsa_add() return a positive value
122 on success. Negative return values indicate resource problems (out
123 of memory, etc.) in the SSL library, while a return value of B<0>
124 indicates incorrect usage or invalid input, such as an unsupported
125 TLSA record certificate usage, selector or matching type. Invalid
126 input also includes malformed data, either a digest length that
127 does not match the digest algorithm, or a C<Full(0)> (binary ASN.1
128 DER form) certificate or a public key that fails to parse.
130 The functions SSL_get0_dane_authority() and SSL_get0_dane_tlsa()
131 return a negative value when DANE authentication failed or was not
132 enabled, a non-negative value indicates the chain depth at which
133 the TLSA record matched a chain certificate, or the depth of the
134 top-most certificate, when the TLSA record is a full public key
139 Suppose "smtp.example.com" is the MX host of the domain "example.com",
140 and has DNSSEC-validated TLSA records. The calls below will perform
141 DANE authentication and arrange to match either the MX hostname or
142 the destination domain name in the SMTP server certificate. Wildcards
143 are supported, but must match the entire label. The actual name
144 matched in the certificate (which might be a wildcard) is retrieved,
145 and must be copied by the application if it is to be retained beyond
146 the lifetime of the SSL connection.
151 const char *nexthop_domain = "example.com";
152 const char *dane_tlsa_domain = "smtp.example.com";
153 uint8_t usage, selector, mtype;
155 if ((ctx = SSL_CTX_new(TLS_client_method())) == NULL)
157 if (SSL_CTX_dane_enable(ctx) <= 0)
160 if ((ssl = SSL_new(ctx)) == NULL)
163 if (SSL_dane_enable(ssl, dane_tlsa_domain) <= 0)
165 if (!SSL_add1_host(ssl, nexthop_domain))
167 SSL_set_hostflags(ssl, X509_CHECK_FLAG_NO_PARTIAL_WILDCARDS);
169 for (... each TLSA record ...) {
174 /* set usage, selector, mtype, data, len */
176 /* Opportunistic DANE TLS clients treat usages 0, 1 as unusable. */
178 case 0: /* PKIX-TA(0) */
179 case 1: /* PKIX-EE(1) */
183 ret = SSL_dane_tlsa_add(ssl, usage, selector, mtype, data, len);
184 /* free data as appropriate */
187 /* handle SSL library internal error */
189 /* handle unusable TLSA record */
195 * Opportunistic DANE clients use unauthenticated TLS when all TLSA records
196 * are unusable, so continue the handshake even if authentication fails.
198 if (num_usable == 0) {
199 int (*cb)(int ok, X509_STORE_CTX *sctx) = NULL;
201 /* Log all records unusable? */
202 /* Set cb to a non-NULL callback of your choice? */
204 SSL_set_verify(ssl, SSL_VERIFY_NONE, cb);
207 /* Perform SSL_connect() handshake and handle errors here */
209 if (SSL_get_verify_result(ssl) == X509_V_OK) {
210 const char *peername = SSL_get0_peername(ssl);
211 EVP_PKEY *mspki = NULL;
213 int depth = SSL_get0_dane_authority(s, NULL, &mspki);
215 (void) SSL_get0_dane_tlsa(s, &usage, &selector, &mtype, NULL, NULL);
216 printf("DANE TLSA %d %d %d %s at depth %d\n", usage, selector, mtype,
217 (mspki != NULL) ? "TA public key verified certificate" :
218 depth ? "matched TA certificate" : "matched EE certificate",
221 if (peername != NULL) {
222 /* Name checks were in scope and matched the peername */
223 printf(bio, "Verified peername: %s\n", peername);
227 * Not authenticated, presumably all TLSA rrs unusable, but possibly a
228 * callback suppressed connection termination despite presence of TLSA
229 * usable RRs none of which matched. Do whatever is appropriate for
230 * unauthenticated connections.
236 It is expected that the majority of clients employing DANE TLS will
237 be doing "opportunistic DANE TLS" in the sense of RFC7672 and
238 RFC7435. That is, they will use DANE authentication when
239 DNSSEC-validated TLSA records are published for a given peer, and
240 otherwise will use unauthenticated TLS or even cleartext.
242 Such applications should generally treat any TLSA records published
243 by the peer with usages PKIX-TA(0) and PKIX-EE(1) as "unusable",
244 and should not include them among the TLSA records used to authenticate
245 peer connections. In addition, some TLSA records with supported
246 usages may be "unusable" as a result of invalid or unsupported
249 When a peer has TLSA records, but none are "usable", an opportunistic
250 application must avoid cleartext, but cannot authenticate the peer,
251 and so should generally proceed with an unauthenticated connection.
252 Opportunistic applications need to note the return value of each
253 call to SSL_dane_tlsa_add(), and if all return 0 (due to invalid
254 or unsupported parameters) disable peer authentication by calling
255 L<SSL_set_verify(3)> with B<mode> equal to B<SSL_VERIFY_NONE>.
261 L<SSL_set_hostflags(3)>,
262 L<SSL_set_tlsext_host_name(3)>,
263 L<SSL_set_verify(3)>,
264 L<SSL_CTX_set_cert_verify_callback(3)>,
265 L<X509_verify_cert(3)>,
267 L<SSL_get0_peername(3)>,
268 L<EVP_get_digestbyname(3)>,
271 L<EVP_PKEY_up_ref(3)>,
276 These functions were first added to OpenSSL 1.1.0.