Ubuntu Manpage: X509_check_host, X509_check_email, X509_check_ip, X509_check_ip_asc

Provided by: libssl-doc_3.0.8-1ubuntu1_all

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.

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>.