X509_CHECK_HOST(3)

✨ NAME ✨

okay bestie, this is the X509_CHECK_HOST(3) page from the Library Functions Manual, and it’s all about X.509 certificate matching 💅

the four icons in question are: X509_check_host, X509_check_email, X509_check_ip, and X509_check_ip_asc 🎀

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🌸 SYNOPSIS 🌸

you’re pulling from the library “libcrypto”, and you need this include:

#include <openssl/x509v3.h>

then the function signatures are exactly these, no cap:

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);

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🦋 DESCRIPTION 🦋

so these certificate matching functions are here to check whether a certificate matches a given hostname, email address, or IP address. that’s the vibe. but bestie, listen: the validity of the certificate and its trust level has to be checked by other means ⚠️

🫶 important: matching is not the same as “this cert is valid and trustworthy.” these functions do the matching part. other means have to do the validity and trust-level part.

X509_check_host() checks if the certificate Subject Alternative Name (SAN) or Subject CommonName (CN) matches the specified hostname, and that hostname 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 🔁

🚩 flags, bestie

the flags argument is usually 0, but it can be the bitwise OR of the following flags:

• X509_CHECK_FLAG_ALWAYS_CHECK_SUBJECT — 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 name is present 👀

the remaining flags are only meaningful for X509_check_host():

• X509_CHECK_FLAG_NO_WILDCARDS — disables wildcard expansion 🙅‍♀️
• X509_CHECK_FLAG_NO_PARTIAL_WILDCARDS — suppresses support for * as a wildcard pattern in labels that have a prefix or suffix, such as www* or *www ✂️
• 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 🌈
• 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 🎯

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💀 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 🙈

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🔥 SEE ALSO 🔥

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

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🧚 HISTORY 🧚

these functions first appeared in OpenSSL 1.0.2 and have been available since OpenBSD 6.1 ⭐

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sign-off bestie: this page is Debian’s, dated June 8, 2025, and it’s the X509_CHECK_HOST(3) manual — mwah, slay, bye 💅🫶