NAME

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

SYNOPSIS

        #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);

DESCRIPTION

       These functions configure server hostname checks in the SSL client.

       SSL_set1_host() sets the expected DNS hostname to name clearing any previously specified
       hostname.  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 nonempty 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       the library and is no longer valid once the associated ssl handle is cleared or freed, or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 value.

       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.

RETURN VALUES

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

       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).

EXAMPLES

       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 */
        }

SEE ALSO

       ssl(7), X509_check_host(3), SSL_get_verify_result(3).  SSL_dane_enable(3).

HISTORY

       These functions were added in OpenSSL 1.1.0.

COPYRIGHT

       Copyright 2016-2020 The OpenSSL Project Authors. All Rights Reserved.

       Licensed under the Apache License 2.0 (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 <https://www.openssl.org/source/license.html>.