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SSL_set1_host(3)                    OpenSSL                   SSL_set1_host(3)

       SSL_set1_host, SSL_add1_host, SSL_set_hostflags, SSL_get0_peername -
       SSL server verification parameters

       libcrypto, -lcrypto

        #include <openssl/ssl.h>

        int SSL_set1_host(SSL *s, const char *hostname);
        int SSL_add1_host(SSL *s, const char *hostname);
        void SSL_set_hostflags(SSL *s, unsigned int flags);
        const char *SSL_get0_peername(SSL *s);

       These functions configure server hostname checks in the SSL client.

       SSL_set1_host() sets the expected DNS hostname to name clearing any
       previously specified host name or names.  If name is NULL, or the empty
       string the list of hostnames is cleared, and name checks are not
       performed on the peer certificate.  When a non-empty name is specified,
       certificate verification automatically checks the peer hostname via
       X509_check_host(3) with flags as specified via SSL_set_hostflags().
       Clients that enable DANE TLSA authentication via SSL_dane_enable(3)
       should leave it to that function to set the primary reference
       identifier of the peer, and should not call SSL_set1_host().

       SSL_add1_host() adds name as an additional reference identifier that
       can match the peer's certificate.  Any previous names set via
       SSL_set1_host() or SSL_add1_host() are retained, no change is made if
       name is NULL or empty.  When multiple names are configured, the peer is
       considered verified when any name matches.  This function is required
       for DANE TLSA in the presence of service name indirection via CNAME, MX
       or SRV records as specified in RFC7671, RFC7672 or RFC7673.

       SSL_set_hostflags() sets the flags that will be passed to
       X509_check_host(3) when name checks are applicable, by default the
       flags value is 0.  See X509_check_host(3) for the list of available
       flags and their meaning.

       SSL_get0_peername() returns the DNS hostname or subject CommonName from
       the peer certificate that matched one of the reference identifiers.
       When wildcard matching is not disabled, the name matched in the peer
       certificate may be a wildcard name.  When one of the reference
       identifiers configured via SSL_set1_host() or SSL_add1_host() starts
       with ".", which indicates a parent domain prefix rather than a fixed
       name, the matched peer name may be a sub-domain of the reference
       identifier.  The returned string is allocated by the library and is no
       longer valid once the associated ssl handle is cleared or freed, or a
       renegotiation takes place.  Applications must not free the return

       SSL clients are advised to use these functions in preference to
       explicitly calling X509_check_host(3).  Hostname checks may be out of
       scope with the RFC7671 DANE-EE(3) certificate usage, and the internal
       check will be suppressed as appropriate when DANE is enabled.

       SSL_set1_host() and SSL_add1_host() return 1 for success and 0 for

       SSL_get0_peername() returns NULL if peername verification is not
       applicable (as with RFC7671 DANE-EE(3)), or no trusted peername was
       matched.  Otherwise, it returns the matched peername.  To determine
       whether verification succeeded call SSL_get_verify_result(3).

       Suppose "smtp.example.com" is the MX host of the domain "example.com".
       The calls below will arrange to match either the MX hostname or the
       destination domain name in the SMTP server certificate.  Wildcards are
       supported, but must match the entire label.  The actual name matched in
       the certificate (which might be a wildcard) is retrieved, and must be
       copied by the application if it is to be retained beyond the lifetime
       of the SSL connection.

        SSL_set_hostflags(ssl, X509_CHECK_FLAG_NO_PARTIAL_WILDCARDS);
        if (!SSL_set1_host(ssl, "smtp.example.com"))
            /* error */
        if (!SSL_add1_host(ssl, "example.com"))
            /* error */

        /* XXX: Perform SSL_connect() handshake and handle errors here */

        if (SSL_get_verify_result(ssl) == X509_V_OK) {
            const char *peername = SSL_get0_peername(ssl);

            if (peername != NULL)
                /* Name checks were in scope and matched the peername */

       X509_check_host(3), SSL_get_verify_result(3).  SSL_dane_enable(3).

       These functions were first added to OpenSSL 1.1.0.

       Copyright 2016-2018 The OpenSSL Project Authors. All Rights Reserved.

       Licensed under the OpenSSL license (the "License").  You may not use
       this file except in compliance with the License.  You can obtain a copy
       in the file LICENSE in the source distribution or at

1.1.1                             2018-09-17                  SSL_set1_host(3)