X509_CHECK_HOST(3SSL) OpenSSL X509_CHECK_HOST(3SSL)
NAME
X509_check_host, X509_check_email, X509_check_ip, X509_check_ip_asc -
X.509 certificate matching
SYNOPSIS
#include <openssl/x509v3.h>
int X509_check_host(X509 *, const char *name, size_t namelen,
unsigned int flags, char **peername);
int X509_check_email(X509 *, const char *address, size_t addresslen,
unsigned int flags);
int X509_check_ip(X509 *, const unsigned char *address, size_t addresslen,
unsigned int flags);
int X509_check_ip_asc(X509 *, const char *address, unsigned int flags);
DESCRIPTION
The certificate matching functions are used to check whether a
certificate matches a given hostname, email address, or IP address. The
validity of the certificate and its trust level has to be checked by
other means.
X509_check_host() checks if the certificate Subject Alternative Name
(SAN) or Subject CommonName (CN) matches the specified hostname, which
must be encoded in the preferred name syntax described in section 3.5 of
RFC 1034. By default, wildcards are supported and they match only in
the left-most label; but they may match part of that label with an
explicit prefix or suffix. For example, by default, the host name
"www.example.com" would match a certificate with a SAN or CN value of
"*.example.com", "w*.example.com" or "*w.example.com".
Per section 6.4.2 of RFC 6125, name values representing international
domain names must be given in A-label form. The namelen argument must
be the number of characters in the name string or zero in which case the
length is calculated with strlen(name). When name starts with a dot
(e.g. ".example.com"), it will be matched by a certificate valid for any
sub-domain of name, (see also X509_CHECK_FLAG_SINGLE_LABEL_SUBDOMAINS
below).
When the certificate is matched, and peername is not NULL, a pointer to
a copy of the matching SAN or CN from the peer certificate is stored at
the address passed in peername. The application is responsible for
freeing the peername via OPENSSL_free() when it is no longer needed.
X509_check_email() checks if the certificate matches the specified email
address. The mailbox syntax of RFC 822 is supported, comments are not
allowed, and no attempt is made to normalize quoted characters. The
mailbox syntax of RFC 6531 is supported for SmtpUTF8Mailbox address in
subjectAltName according to RFC 8398, with similar limitations as for
RFC 822 syntax, and no attempt is made to convert from A-label to
U-label before comparison. The addresslen argument must be the number
of characters in the address string or zero in which case the length is
calculated with strlen(address).
X509_check_ip() checks if the certificate matches a specified IPv4 or
IPv6 address. The address array is in binary format, in network byte
order. The length is either 4 (IPv4) or 16 (IPv6). Only explicitly
marked addresses in the certificates are considered; IP addresses stored
in DNS names and Common Names are ignored. There are currently no flags
that would affect the behavior of this call.
X509_check_ip_asc() is similar, except that the NUL-terminated string
address is first converted to the internal representation.
The flags argument is usually 0. It can be the bitwise OR of the flags:
X509_CHECK_FLAG_ALWAYS_CHECK_SUBJECT,
X509_CHECK_FLAG_NEVER_CHECK_SUBJECT,
X509_CHECK_FLAG_NO_WILDCARDS,
X509_CHECK_FLAG_NO_PARTIAL_WILDCARDS,
X509_CHECK_FLAG_MULTI_LABEL_WILDCARDS.
X509_CHECK_FLAG_SINGLE_LABEL_SUBDOMAINS.
The X509_CHECK_FLAG_ALWAYS_CHECK_SUBJECT flag causes the function to
consider the subject DN even if the certificate contains at least one
subject alternative name of the right type (DNS name or email address as
appropriate); the default is to ignore the subject DN when at least one
corresponding subject alternative names is present.
The X509_CHECK_FLAG_NEVER_CHECK_SUBJECT flag causes the function to
never consider the subject DN even if the certificate contains no
subject alternative names of the right type (DNS name or email address
as appropriate); the default is to use the subject DN when no
corresponding subject alternative names are present. If both
X509_CHECK_FLAG_ALWAYS_CHECK_SUBJECT and
X509_CHECK_FLAG_NEVER_CHECK_SUBJECT are specified, the latter takes
precedence and the subject DN is not checked for matching names.
If set, X509_CHECK_FLAG_NO_WILDCARDS disables wildcard expansion; this
only applies to X509_check_host.
If set, X509_CHECK_FLAG_NO_PARTIAL_WILDCARDS suppresses support for "*"
as wildcard pattern in labels that have a prefix or suffix, such as:
"www*" or "*www"; this only applies to X509_check_host.
If set, X509_CHECK_FLAG_MULTI_LABEL_WILDCARDS allows a "*" that
constitutes the complete label of a DNS name (e.g. "*.example.com") to
match more than one label in name; this flag only applies to
X509_check_host.
If set, X509_CHECK_FLAG_SINGLE_LABEL_SUBDOMAINS restricts name values
which start with ".", that would otherwise match any sub-domain in the
peer certificate, to only match direct child sub-domains. Thus, for
instance, with this flag set a name of ".example.com" would match a peer
certificate with a DNS name of "www.example.com", but would not match a
peer certificate with a DNS name of "www.sub.example.com"; this flag
only applies to X509_check_host.
RETURN VALUES
The functions return 1 for a successful match, 0 for a failed match and
-1 for an internal error: typically a memory allocation failure or an
ASN.1 decoding error.
All functions can also return -2 if the input is malformed. For example,
X509_check_host() returns -2 if the provided name contains embedded
NULs.
NOTES
Applications are encouraged to use X509_VERIFY_PARAM_set1_host() rather
than explicitly calling X509_check_host(3). Hostname checks may be out
of scope with the DANE-EE(3) certificate usage, and the internal checks
will be suppressed as appropriate when DANE support is enabled.
SEE ALSO
SSL_get_verify_result(3), X509_VERIFY_PARAM_set1_host(3),
X509_VERIFY_PARAM_add1_host(3), X509_VERIFY_PARAM_set1_email(3),
X509_VERIFY_PARAM_set1_ip(3)
HISTORY
These functions were added in OpenSSL 1.0.2.
COPYRIGHT
Copyright 2012-2022 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>.
3.5.7 2026-06-09 X509_CHECK_HOST(3SSL)
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