X509_CHECK_HOST(3) | Library Functions Manual | X509_CHECK_HOST(3)

# NAME

`X509_check_host`, `X509_check_email`, `X509_check_ip`, `X509_check_ip_asc` — X.509 certificate matching

# SYNOPSIS

library “libcrypto”
`#include <openssl/x509v3.h>`

int
`X509_check_host`(X509 *x, const char *name, size_t namelen, unsigned int flags, char **peername);

int
`X509_check_email`(X509 *x, const char *address, size_t addresslen, unsigned int flags);

int
`X509_check_ip`(X509 *x, const unsigned char *address, size_t addresslen, unsigned int flags);

int
`X509_check_ip_asc`(X509 *x, 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; 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 free(3) when it is no longer needed.

`X509_check_email`() checks if the certificate matches the specified email address. Only the mailbox syntax of RFC 822 is supported. Comments are not allowed, and no attempt is made to normalize quoted characters. 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.

`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, but it can be the bitwise OR of the following flags.

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 remaining flags are only meaningful for `X509_check_host`().

The `X509_CHECK_FLAG_NO_WILDCARDS` flag disables wildcard expansion.

The `X509_CHECK_FLAG_NO_PARTIAL_WILDCARDS` flag suppresses support for "*" as a wildcard pattern in labels that have a prefix or suffix, such as "www*" or "*www".

The `X509_CHECK_FLAG_MULTI_LABEL_WILDCARDS` flag allows a "*" that constitutes the complete label of a DNS name (e.g. "*.example.com") to match more than one label in name.

The `X509_CHECK_FLAG_SINGLE_LABEL_SUBDOMAINS` flag 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".

# 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 NUL bytes.

# SEE ALSO

SSL_set1_host(3), X509_EXTENSION_new(3), X509_get1_email(3), X509_new(3), X509_VERIFY_PARAM_set1_host(3)

# HISTORY

These functions first appeared in OpenSSL 1.0.2 and have been available since OpenBSD 6.1.

June 8, 2025 | Debian