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=head1 NAME

SSL_set1_host, SSL_add1_host, SSL_set_hostflags, SSL_get0_peername -

SSL server verification parameters

=head1 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);

=head1 DESCRIPTION

These functions configure server hostname checks in the SSL client.

SSL_set1_host() sets the expected DNS hostname to B<name> clearing

any previously specified host name or names.  If B<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

B<name> is specified, certificate verification automatically checks

the peer hostname via L<X509_check_host(3)> with B<flags> as specified

via SSL_set_hostflags().  Clients that enable DANE TLSA authentication

via L<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 B<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 B<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 B<flags> that will be passed to

L<X509_check_host(3)> when name checks are applicable, by default

the B<flags> value is 0.  See L<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the library and is no longer valid once the associated B<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 L<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.

=head1 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 L<SSL_get_verify_result(3)>.

=head1 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 */

 }

=head1 SEE ALSO

L<X509_check_host(3)>,

L<SSL_get_verify_result(3)>.

L<SSL_dane_enable(3)>.

=head1 HISTORY

These functions were added in OpenSSL 1.1.0.

=head1 COPYRIGHT

Copyright 2016-2019 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

L<https://www.openssl.org/source/license.html>.

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