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.\" ========================================================================
.\"
.IX Title "SSL_SET1_HOST 3"
.TH SSL_SET1_HOST 3 "2023-09-11" "1.1.1w" "OpenSSL"
.\" For nroff, turn off justification.  Always turn off hyphenation; it makes
.\" way too many mistakes in technical documents.
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.nh
.SH "NAME"
SSL_set1_host, SSL_add1_host, SSL_set_hostflags, SSL_get0_peername \- SSL server verification parameters
.SH "SYNOPSIS"
.IX Header "SYNOPSIS"
.Vb 1
\& #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);
.Ve
.SH "DESCRIPTION"
.IX Header "DESCRIPTION"
These functions configure server hostname checks in the \s-1SSL\s0 client.
.PP
\&\fBSSL_set1_host()\fR sets the expected \s-1DNS\s0 hostname to \fBname\fR clearing
any previously specified hostname or names.  If \fBname\fR is \s-1NULL,\s0
or the empty string the list of hostnames is cleared, and name
checks are not performed on the peer certificate.  When a nonempty
\&\fBname\fR is specified, certificate verification automatically checks
the peer hostname via \fBX509_check_host\fR\|(3) with \fBflags\fR as specified
via \fBSSL_set_hostflags()\fR.  Clients that enable \s-1DANE TLSA\s0 authentication
via \fBSSL_dane_enable\fR\|(3) should leave it to that function to set
the primary reference identifier of the peer, and should not call
\&\fBSSL_set1_host()\fR.
.PP
\&\fBSSL_add1_host()\fR adds \fBname\fR as an additional reference identifier
that can match the peer's certificate.  Any previous names set via
\&\fBSSL_set1_host()\fR or \fBSSL_add1_host()\fR are retained, no change is made
if \fBname\fR is \s-1NULL\s0 or empty.  When multiple names are configured,
the peer is considered verified when any name matches.  This function
is required for \s-1DANE TLSA\s0 in the presence of service name indirection
via \s-1CNAME, MX\s0 or \s-1SRV\s0 records as specified in \s-1RFC7671, RFC7672\s0 or
\&\s-1RFC7673.\s0
.PP
\&\fBSSL_set_hostflags()\fR sets the \fBflags\fR that will be passed to
\&\fBX509_check_host\fR\|(3) when name checks are applicable, by default
the \fBflags\fR value is 0.  See \fBX509_check_host\fR\|(3) for the list
of available flags and their meaning.
.PP
\&\fBSSL_get0_peername()\fR returns the \s-1DNS\s0 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 \fBSSL_set1_host()\fR or
\&\fBSSL_add1_host()\fR starts with \*(L".\*(R", 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 \fBssl\fR handle
is cleared or freed, or a renegotiation takes place.  Applications
must not free the return value.
.PP
\&\s-1SSL\s0 clients are advised to use these functions in preference to
explicitly calling \fBX509_check_host\fR\|(3).  Hostname checks may be out
of scope with the \s-1RFC7671 \fBDANE\-EE\s0\fR\|(3) certificate usage, and the
internal check will be suppressed as appropriate when \s-1DANE\s0 is
enabled.
.SH "RETURN VALUES"
.IX Header "RETURN VALUES"
\&\fBSSL_set1_host()\fR and \fBSSL_add1_host()\fR return 1 for success and 0 for
failure.
.PP
\&\fBSSL_get0_peername()\fR returns \s-1NULL\s0 if peername verification is not
applicable (as with \s-1RFC7671 \fBDANE\-EE\s0\fR\|(3)), or no trusted peername was
matched.  Otherwise, it returns the matched peername.  To determine
whether verification succeeded call \fBSSL_get_verify_result\fR\|(3).
.SH "EXAMPLES"
.IX Header "EXAMPLES"
Suppose \*(L"smtp.example.com\*(R" is the \s-1MX\s0 host of the domain \*(L"example.com\*(R".
The calls below will arrange to match either the \s-1MX\s0 hostname or the
destination domain name in the \s-1SMTP\s0 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 \s-1SSL\s0 connection.
.PP
.Vb 5
\& 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 */
\& }
.Ve
.SH "SEE ALSO"
.IX Header "SEE ALSO"
\&\fBX509_check_host\fR\|(3),
\&\fBSSL_get_verify_result\fR\|(3).
\&\fBSSL_dane_enable\fR\|(3).
.SH "HISTORY"
.IX Header "HISTORY"
These functions were added in OpenSSL 1.1.0.
.SH "COPYRIGHT"
.IX Header "COPYRIGHT"
Copyright 2016\-2020 The OpenSSL Project Authors. All Rights Reserved.
.PP
Licensed under the OpenSSL license (the \*(L"License\*(R").  You may not use
this file except in compliance with the License.  You can obtain a copy
in the file \s-1LICENSE\s0 in the source distribution or at
<https://www.openssl.org/source/license.html>.