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 shared state
31 required for DANE support.
32 Individual connections associated with the context can then enable
33 per-connection DANE support as appropriate.
34 DANE authentication is implemented in the L<X509_verify_cert(3)> function, and
35 applications that override L<X509_verify_cert(3)> via
36 L<SSL_CTX_set_cert_verify_callback(3)> are responsible to authenticate the peer
37 chain in whatever manner they see fit.
39 SSL_CTX_dane_mtype_set() may then be called zero or more times to to adjust the
40 supported digest algorithms.
41 This must be done before any SSL handles are created for the context.
43 The B<mtype> argument specifies a DANE TLSA matching type and the B<md>
44 argument specifies the associated digest algorithm handle.
45 The B<ord> argument specifies a strength ordinal.
46 Algorithms with a larger strength ordinal are considered more secure.
47 Strength 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.
50 Matching type Full(0) cannot be modified or disabled.
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() must be called before the SSL handshake is initiated with
58 L<SSL_connect(3)> if (and only if) you want 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
63 Additional server names can be specified via L<SSL_add1_host(3)>.
64 The B<basedomain> is used as the default SNI hint if none has yet been
65 specified via L<SSL_set_tlsext_host_name(3)>.
67 SSL_dane_tlsa_add() may then be called one or more times, to load each of the
68 TLSA records that apply to the remote TLS peer.
69 (This too must be done prior to the beginning of the SSL handshake).
70 The arguments specify the fields of the TLSA record.
71 The B<data> field is provided in binary (wire RDATA) form, not the hexadecimal
72 ASCII presentation form, with an explicit length passed via B<dlen>.
73 A return value of 0 indicates that "unusable" TLSA records (with invalid or
74 unsupported parameters) were provided, a negative return value indicates an
75 internal error in processing the records.
76 If DANE authentication is enabled, but no TLSA records are added successfully,
77 authentication will fail, and the handshake may not complete, depending on the
78 B<mode> argument of L<SSL_set_verify(3)> and any verification callback.
80 SSL_get0_dane_authority() can be used to get more detailed information about
81 the matched DANE trust-anchor after successful connection completion.
82 The return value is negative if DANE verification failed (or was not enabled),
83 0 if an EE TLSA record directly matched the leaf certificate, or a positive
84 number indicating the depth at which a TA record matched an issuer certificate.
86 If the B<mcert> argument is not B<NULL> and a TLSA record matched a chain
87 certificate, a pointer to the matching certificate is returned via B<mcert>.
88 The returned address is a short-term internal reference to the certificate and
89 must not be freed by the application.
90 Applications that want to retain access to the certificate can call
91 L<X509_up_ref(3)> to obtain a long-term reference which must then be freed via
92 L<X509_free(3)> once no longer needed.
94 If no TLSA records directly matched any elements of the certificate chain, but
95 a DANE-TA(2) SPKI(1) Full(0) record provided the public key that signed an
96 element of the chain, then that key is returned via B<mspki> argument (if not
98 In this case the return value is the depth of the top-most element of the
99 validated certificate chain.
100 As with B<mcert> this is a short-term internal reference, and
101 L<EVP_PKEY_up_ref(3)> and L<EVP_PKEY_free(3)> can be used to acquire and
102 release long-term references respectively.
104 SSL_get0_dane_tlsa() can be used to retrieve the fields of the TLSA record that
105 matched the peer certificate chain.
106 The return value indicates the match depth or failure to match just as with
107 SSL_get0_dane_authority().
108 When the return value is non-negative, the storage pointed to by the B<usage>,
109 B<selector>, B<mtype> and B<data> parameters is updated to the corresponding
111 The B<data> field is in binary wire form, and is therefore not NUL-terminated,
112 its length is returned via the B<dlen> parameter.
113 If any of these parameters is NULL, the corresponding field is not returned.
114 The B<data> parameter is set to a short-term internal-copy of the associated
115 data field and must not be freed by the application.
116 Applications that need long-term access to 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 on success.
122 Negative return values indicate resource problems (out of memory, etc.) in the
123 SSL library, while a return value of B<0> indicates incorrect usage or invalid
124 input, such as an unsupported TLSA record certificate usage, selector or
126 Invalid input also includes malformed data, either a digest length that does
127 not match the digest algorithm, or a C<Full(0)> (binary ASN.1 DER form)
128 certificate or a public key that fails to parse.
130 The functions SSL_get0_dane_authority() and SSL_get0_dane_tlsa() return a
131 negative value when DANE authentication failed or was not enabled, a
132 non-negative value indicates the chain depth at which the TLSA record matched a
133 chain certificate, or the depth of the top-most certificate, when the TLSA
134 record is a full public key that is its signer.
138 Suppose "smtp.example.com" is the MX host of the domain "example.com", and has
139 DNSSEC-validated TLSA records.
140 The calls below will perform DANE authentication and arrange to match either
141 the MX hostname or the destination domain name in the SMTP server certificate.
142 Wildcards are supported, but must match the entire label.
143 The actual name matched in the certificate (which might be a wildcard) is
144 retrieved, and must be copied by the application if it is to be retained beyond
145 the lifetime of the SSL connection.
150 const char *nexthop_domain = "example.com";
151 const char *dane_tlsa_domain = "smtp.example.com";
152 uint8_t usage, selector, mtype;
154 if ((ctx = SSL_CTX_new(TLS_client_method())) == NULL)
156 if (SSL_CTX_dane_enable(ctx) <= 0)
159 if ((ssl = SSL_new(ctx)) == NULL)
162 if (SSL_dane_enable(ssl, dane_tlsa_domain) <= 0)
164 if (!SSL_add1_host(ssl, nexthop_domain))
166 SSL_set_hostflags(ssl, X509_CHECK_FLAG_NO_PARTIAL_WILDCARDS);
168 for (... each TLSA record ...) {
173 /* set usage, selector, mtype, data, len */
175 /* Opportunistic DANE TLS clients treat usages 0, 1 as unusable. */
177 case 0: /* PKIX-TA(0) */
178 case 1: /* PKIX-EE(1) */
182 ret = SSL_dane_tlsa_add(ssl, usage, selector, mtype, data, len);
183 /* free data as appropriate */
186 /* handle SSL library internal error */
188 /* handle unusable TLSA record */
194 * Opportunistic DANE clients use unauthenticated TLS when all TLSA records
195 * are unusable, so continue the handshake even if authentication fails.
197 if (num_usable == 0) {
198 int (*cb)(int ok, X509_STORE_CTX *sctx) = NULL;
200 /* Log all records unusable? */
201 /* Set cb to a non-NULL callback of your choice? */
203 SSL_set_verify(ssl, SSL_VERIFY_NONE, cb);
206 /* Perform SSL_connect() handshake and handle errors here */
208 if (SSL_get_verify_result(ssl) == X509_V_OK) {
209 const char *peername = SSL_get0_peername(ssl);
210 EVP_PKEY *mspki = NULL;
212 int depth = SSL_get0_dane_authority(ssl, NULL, &mspki);
214 (void) SSL_get0_dane_tlsa(ssl, &usage, &selector, &mtype, NULL, NULL);
215 printf("DANE TLSA %d %d %d %s at depth %d\n", usage, selector, mtype,
216 (mspki != NULL) ? "TA public key verified certificate" :
217 depth ? "matched TA certificate" : "matched EE certificate",
220 if (peername != NULL) {
221 /* Name checks were in scope and matched the peername */
222 printf(bio, "Verified peername: %s\n", peername);
226 * Not authenticated, presumably all TLSA rrs unusable, but possibly a
227 * callback suppressed connection termination despite presence of TLSA
228 * usable RRs none of which matched. Do whatever is appropriate for
229 * unauthenticated connections.
235 It is expected that the majority of clients employing DANE TLS will be doing
236 "opportunistic DANE TLS" in the sense of RFC7672 and RFC7435.
237 That is, they will use DANE authentication when DNSSEC-validated TLSA records
238 are published for a given peer, and otherwise will use unauthenticated TLS or
241 Such applications should generally treat any TLSA records published by the peer
242 with usages PKIX-TA(0) and PKIX-EE(1) as "unusable", and should not include
243 them among the TLSA records used to authenticate peer connections.
244 In addition, some TLSA records with supported usages may be "unusable" as a
245 result of invalid or unsupported parameters.
247 When a peer has TLSA records, but none are "usable", an opportunistic
248 application must avoid cleartext, but cannot authenticate the peer,
249 and so should generally proceed with an unauthenticated connection.
250 Opportunistic applications need to note the return value of each
251 call to SSL_dane_tlsa_add(), and if all return 0 (due to invalid
252 or unsupported parameters) disable peer authentication by calling
253 L<SSL_set_verify(3)> with B<mode> equal to B<SSL_VERIFY_NONE>.
259 L<SSL_set_hostflags(3)>,
260 L<SSL_set_tlsext_host_name(3)>,
261 L<SSL_set_verify(3)>,
262 L<SSL_CTX_set_cert_verify_callback(3)>,
263 L<X509_verify_cert(3)>,
265 L<SSL_get0_peername(3)>,
266 L<EVP_get_digestbyname(3)>,
269 L<EVP_PKEY_up_ref(3)>,
274 These functions were first added to OpenSSL 1.1.0.