SSL_SET1_HOST(3ossl) OpenSSL SSL_SET1_HOST(3ossl)
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 *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);
DESCRIPTION
These functions configure server hostname checks in the SSL client.
SSL_set1_host(3) sets in the verification parameters of s the expected
DNS hostname or IP address to host, clearing any previously specified
IP address and hostnames. If host is NULL or the empty string, IP
address and hostname checks are not performed on the peer certificate.
When a nonempty host 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(3).
SSL_add1_host() adds host as an additional reference identifier that
can match the peer's certificate. Any previous hostnames set via
SSL_set1_host(3) 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 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(3) or SSL_add1_host()
for server hostname or IP address validation, as well as
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 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(3) 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 RFC 7671 DANE-EE(3) certificate usage, and the internal
check will be suppressed as appropriate when DANE is enabled.
RETURN VALUES
SSL_set1_host(3) 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 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_set_tlsext_host_name(3),
SSL_get_verify_result(3), SSL_dane_enable(3)
HISTORY
These functions were added in OpenSSL 1.1.0.
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
<https://www.openssl.org/source/license.html>.
3.5.1 2025-07-02 SSL_SET1_HOST(3ossl)
openssl 3.5.1 - Generated Sat Aug 2 05:16:40 CDT 2025