=pod

=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 *host);
 int SSL_add1_host(SSL *s, const char *host);
 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 in the verification parameters of I<s>
the expected DNS hostname or IP address to I<host>,
clearing any previously specified IP address and hostnames.
If I<host> is NULL or the empty string, IP address
and hostname checks are not performed on the peer certificate.
When a nonempty I<host> is specified, certificate verification automatically
checks the peer hostname via L<X509_check_host(3)> with I<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 I<host> as an additional reference identifier
that can match the peer's certificate.  Any previous hostnames
set via SSL_set1_host() or SSL_add1_host() are retained.
Adding an IP address is allowed only if no IP address has been set before.
No change is made if I<host> is NULL or empty.
When an IP address and/or multiple hostnames are configured,
the peer is considered verified when any of these matches.
This function is required for DANE TLSA in the presence of service name indirection
via CNAME, MX or SRV records as specified in RFCs 7671, 7672, and 7673.

TLS clients are recommended to use SSL_set1_host() or SSL_add1_host()
for server hostname or IP address validation,
as well as L<SSL_set_tlsext_host_name(3)> for Server Name Indication (SNI),
which may be crucial also for correct routing of the connection request.

SSL_set_hostflags() sets the I<flags> that will be passed to
L<X509_check_host(3)> when name checks are applicable, by default
the I<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 I<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 RFC 7671 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_set_hostflags() returns nothing at all.

SSL_get0_peername() returns NULL if peername verification is not
applicable (as with RFC 7671 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<ssl(7)>,
L<X509_check_host(3)>, L<SSL_set_tlsext_host_name(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 2016-2025 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
L<https://www.openssl.org/source/license.html>.

=cut
