300 lines
12 KiB
Text
300 lines
12 KiB
Text
=pod
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=head1 NAME
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SSL_CTX_dane_enable, SSL_CTX_dane_mtype_set, SSL_dane_enable,
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SSL_dane_tlsa_add, SSL_get0_dane_authority, SSL_get0_dane_tlsa -
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enable DANE TLS authentication of the remote TLS server in the local
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TLS client
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=head1 SYNOPSIS
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#include <openssl/ssl.h>
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int SSL_CTX_dane_enable(SSL_CTX *ctx);
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int SSL_CTX_dane_mtype_set(SSL_CTX *ctx, const EVP_MD *md,
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uint8_t mtype, uint8_t ord);
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int SSL_dane_enable(SSL *s, const char *basedomain);
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int SSL_dane_tlsa_add(SSL *s, uint8_t usage, uint8_t selector,
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uint8_t mtype, unsigned char *data, size_t dlen);
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int SSL_get0_dane_authority(SSL *s, X509 **mcert, EVP_PKEY **mspki);
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int SSL_get0_dane_tlsa(SSL *s, uint8_t *usage, uint8_t *selector,
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uint8_t *mtype, unsigned const char **data,
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size_t *dlen);
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=head1 DESCRIPTION
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These functions implement support for DANE TLSA (RFC6698 and RFC7671)
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peer authentication.
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SSL_CTX_dane_enable() must be called first to initialize the shared state
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required for DANE support.
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Individual connections associated with the context can then enable
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per-connection DANE support as appropriate.
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DANE authentication is implemented in the L<X509_verify_cert(3)> function, and
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applications that override L<X509_verify_cert(3)> via
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L<SSL_CTX_set_cert_verify_callback(3)> are responsible to authenticate the peer
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chain in whatever manner they see fit.
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SSL_CTX_dane_mtype_set() may then be called zero or more times to adjust the
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supported digest algorithms.
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This must be done before any SSL handles are created for the context.
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The B<mtype> argument specifies a DANE TLSA matching type and the B<md>
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argument specifies the associated digest algorithm handle.
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The B<ord> argument specifies a strength ordinal.
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Algorithms with a larger strength ordinal are considered more secure.
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Strength ordinals are used to implement RFC7671 digest algorithm agility.
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Specifying a B<NULL> digest algorithm for a matching type disables
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support for that matching type.
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Matching type Full(0) cannot be modified or disabled.
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By default, matching type C<SHA2-256(1)> (see RFC7218 for definitions
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of the DANE TLSA parameter acronyms) is mapped to C<EVP_sha256()>
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with a strength ordinal of C<1> and matching type C<SHA2-512(2)>
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is mapped to C<EVP_sha512()> with a strength ordinal of C<2>.
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SSL_dane_enable() must be called before the SSL handshake is initiated with
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L<SSL_connect(3)> if (and only if) you want to enable DANE for that connection.
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(The connection must be associated with a DANE-enabled SSL context).
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The B<basedomain> argument specifies the RFC7671 TLSA base domain,
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which will be the primary peer reference identifier for certificate
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name checks.
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Additional server names can be specified via L<SSL_add1_host(3)>.
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The B<basedomain> is used as the default SNI hint if none has yet been
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specified via L<SSL_set_tlsext_host_name(3)>.
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SSL_dane_tlsa_add() may then be called one or more times, to load each of the
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TLSA records that apply to the remote TLS peer.
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(This too must be done prior to the beginning of the SSL handshake).
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The arguments specify the fields of the TLSA record.
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The B<data> field is provided in binary (wire RDATA) form, not the hexadecimal
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ASCII presentation form, with an explicit length passed via B<dlen>.
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A return value of 0 indicates that "unusable" TLSA records (with invalid or
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unsupported parameters) were provided, a negative return value indicates an
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internal error in processing the records.
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If DANE authentication is enabled, but no TLSA records are added successfully,
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authentication will fail, and the handshake may not complete, depending on the
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B<mode> argument of L<SSL_set_verify(3)> and any verification callback.
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SSL_get0_dane_authority() can be used to get more detailed information about
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the matched DANE trust-anchor after successful connection completion.
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The return value is negative if DANE verification failed (or was not enabled),
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0 if an EE TLSA record directly matched the leaf certificate, or a positive
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number indicating the depth at which a TA record matched an issuer certificate.
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The complete verified chain can be retrieved via L<SSL_get0_verified_chain(3)>.
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The return value is an index into this verified chain, rather than the list of
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certificates sent by the peer as returned by L<SSL_get_peer_cert_chain(3)>.
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If the B<mcert> argument is not B<NULL> and a TLSA record matched a chain
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certificate, a pointer to the matching certificate is returned via B<mcert>.
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The returned address is a short-term internal reference to the certificate and
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must not be freed by the application.
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Applications that want to retain access to the certificate can call
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L<X509_up_ref(3)> to obtain a long-term reference which must then be freed via
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L<X509_free(3)> once no longer needed.
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If no TLSA records directly matched any elements of the certificate chain, but
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a DANE-TA(2) SPKI(1) Full(0) record provided the public key that signed an
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element of the chain, then that key is returned via B<mspki> argument (if not
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NULL).
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In this case the return value is the depth of the top-most element of the
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validated certificate chain.
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As with B<mcert> this is a short-term internal reference, and
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L<EVP_PKEY_up_ref(3)> and L<EVP_PKEY_free(3)> can be used to acquire and
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release long-term references respectively.
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SSL_get0_dane_tlsa() can be used to retrieve the fields of the TLSA record that
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matched the peer certificate chain.
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The return value indicates the match depth or failure to match just as with
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SSL_get0_dane_authority().
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When the return value is non-negative, the storage pointed to by the B<usage>,
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B<selector>, B<mtype> and B<data> parameters is updated to the corresponding
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TLSA record fields.
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The B<data> field is in binary wire form, and is therefore not NUL-terminated,
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its length is returned via the B<dlen> parameter.
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If any of these parameters is NULL, the corresponding field is not returned.
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The B<data> parameter is set to a short-term internal-copy of the associated
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data field and must not be freed by the application.
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Applications that need long-term access to this field need to copy the content.
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=head1 RETURN VALUES
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The functions SSL_CTX_dane_enable(), SSL_CTX_dane_mtype_set(),
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SSL_dane_enable() and SSL_dane_tlsa_add() return a positive value on success.
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Negative return values indicate resource problems (out of memory, etc.) in the
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SSL library, while a return value of B<0> indicates incorrect usage or invalid
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input, such as an unsupported TLSA record certificate usage, selector or
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matching type.
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Invalid input also includes malformed data, either a digest length that does
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not match the digest algorithm, or a C<Full(0)> (binary ASN.1 DER form)
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certificate or a public key that fails to parse.
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The functions SSL_get0_dane_authority() and SSL_get0_dane_tlsa() return a
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negative value when DANE authentication failed or was not enabled, a
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non-negative value indicates the chain depth at which the TLSA record matched a
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chain certificate, or the depth of the top-most certificate, when the TLSA
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record is a full public key that is its signer.
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=head1 EXAMPLE
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Suppose "smtp.example.com" is the MX host of the domain "example.com", and has
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DNSSEC-validated TLSA records.
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The calls below will perform DANE authentication and arrange to match either
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the MX hostname or the destination domain name in the SMTP server certificate.
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Wildcards are supported, but must match the entire label.
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The actual name matched in the certificate (which might be a wildcard) is
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retrieved, and must be copied by the application if it is to be retained beyond
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the lifetime of the SSL connection.
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SSL_CTX *ctx;
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SSL *ssl;
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int num_usable = 0;
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const char *nexthop_domain = "example.com";
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const char *dane_tlsa_domain = "smtp.example.com";
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uint8_t usage, selector, mtype;
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if ((ctx = SSL_CTX_new(TLS_client_method())) == NULL)
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/* handle error */
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if (SSL_CTX_dane_enable(ctx) <= 0)
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/* handle error */
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if ((ssl = SSL_new(ctx)) == NULL)
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/* handle error */
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if (SSL_dane_enable(ssl, dane_tlsa_domain) <= 0)
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/* handle error */
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if (!SSL_add1_host(ssl, nexthop_domain))
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/* handle error */
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SSL_set_hostflags(ssl, X509_CHECK_FLAG_NO_PARTIAL_WILDCARDS);
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for (... each TLSA record ...) {
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unsigned char *data;
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size_t len;
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int ret;
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/* set usage, selector, mtype, data, len */
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/* Opportunistic DANE TLS clients treat usages 0, 1 as unusable. */
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switch (usage) {
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case 0: /* PKIX-TA(0) */
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case 1: /* PKIX-EE(1) */
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continue;
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}
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ret = SSL_dane_tlsa_add(ssl, usage, selector, mtype, data, len);
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/* free data as appropriate */
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if (ret < 0)
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/* handle SSL library internal error */
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else if (ret == 0)
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/* handle unusable TLSA record */
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else
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++num_usable;
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}
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/*
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* Opportunistic DANE clients use unauthenticated TLS when all TLSA records
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* are unusable, so continue the handshake even if authentication fails.
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*/
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if (num_usable == 0) {
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int (*cb)(int ok, X509_STORE_CTX *sctx) = NULL;
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/* Log all records unusable? */
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/* Set cb to a non-NULL callback of your choice? */
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SSL_set_verify(ssl, SSL_VERIFY_NONE, cb);
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}
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/*
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* Load any saved session for resumption, making sure that the previous
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* session applied the same security and authentication requirements that
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* would be expected of a fresh connection.
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*/
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/* Perform SSL_connect() handshake and handle errors here */
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if (SSL_session_reused(ssl)) {
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if (SSL_get_verify_result(ssl) == X509_V_OK) {
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/*
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* Resumed session was originally verified, this connection is
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* authenticated.
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*/
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} else {
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/*
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* Resumed session was not originally verified, this connection is not
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* authenticated.
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*/
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}
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} else if (SSL_get_verify_result(ssl) == X509_V_OK) {
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const char *peername = SSL_get0_peername(ssl);
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EVP_PKEY *mspki = NULL;
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int depth = SSL_get0_dane_authority(ssl, NULL, &mspki);
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if (depth >= 0) {
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(void) SSL_get0_dane_tlsa(ssl, &usage, &selector, &mtype, NULL, NULL);
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printf("DANE TLSA %d %d %d %s at depth %d\n", usage, selector, mtype,
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(mspki != NULL) ? "TA public key verified certificate" :
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depth ? "matched TA certificate" : "matched EE certificate",
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depth);
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}
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if (peername != NULL) {
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/* Name checks were in scope and matched the peername */
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printf(bio, "Verified peername: %s\n", peername);
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}
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} else {
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/*
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* Not authenticated, presumably all TLSA rrs unusable, but possibly a
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* callback suppressed connection termination despite presence of TLSA
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* usable RRs none of which matched. Do whatever is appropriate for
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* unauthenticated connections.
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*/
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}
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=head1 NOTES
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It is expected that the majority of clients employing DANE TLS will be doing
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"opportunistic DANE TLS" in the sense of RFC7672 and RFC7435.
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That is, they will use DANE authentication when DNSSEC-validated TLSA records
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are published for a given peer, and otherwise will use unauthenticated TLS or
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even cleartext.
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Such applications should generally treat any TLSA records published by the peer
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with usages PKIX-TA(0) and PKIX-EE(1) as "unusable", and should not include
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them among the TLSA records used to authenticate peer connections.
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In addition, some TLSA records with supported usages may be "unusable" as a
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result of invalid or unsupported parameters.
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When a peer has TLSA records, but none are "usable", an opportunistic
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application must avoid cleartext, but cannot authenticate the peer,
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and so should generally proceed with an unauthenticated connection.
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Opportunistic applications need to note the return value of each
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call to SSL_dane_tlsa_add(), and if all return 0 (due to invalid
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or unsupported parameters) disable peer authentication by calling
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L<SSL_set_verify(3)> with B<mode> equal to B<SSL_VERIFY_NONE>.
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=head1 SEE ALSO
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L<SSL_new(3)>,
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L<SSL_add1_host(3)>,
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L<SSL_set_hostflags(3)>,
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L<SSL_set_tlsext_host_name(3)>,
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L<SSL_set_verify(3)>,
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L<SSL_CTX_set_cert_verify_callback(3)>,
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L<SSL_get0_verified_chain(3)>,
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L<SSL_get_peer_cert_chain(3)>,
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L<SSL_get_verify_result(3)>,
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L<SSL_connect(3)>,
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L<SSL_get0_peername(3)>,
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L<X509_verify_cert(3)>,
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L<X509_up_ref(3)>,
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L<X509_free(3)>,
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L<EVP_get_digestbyname(3)>,
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L<EVP_PKEY_up_ref(3)>,
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L<EVP_PKEY_free(3)>
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=head1 HISTORY
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These functions were first added to OpenSSL 1.1.0.
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=cut
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