X509_CHECK_HOST(3ossl)

✨ X509_CHECK_HOST — the OpenSSL certificate matching glow-up ✨

okay bestie, pull up a chair 💅 this is the OpenSSL manual for section 3ossl, and we're talking about the little squad of functions that go "does this certificate actually belong to the name you think it does?" 🎀

---

🌸 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 💖

so these certificate matching functions are the vibe-check squad 💅 they check whether a certificate matches a given hostname, email address, or IP address. and listen — that's literally all they do. the validity of the certificate and its trust level has to be checked by other means, no cap. don't come for them about stuff that isn't their job 🙈

🦋 X509_check_host()

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". iconic behaviour fr fr.

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. and heads up: the application is responsible for freeing the peername via OPENSSL_free() when it is no longer needed. don't forget or it's a memory leak moment 😭

🍓 X509_check_email()

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()

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()

X509_check_ip_asc() is similar, except that the NUL-terminated string address is first converted to the internal representation. ✨

🌟 the flags argument

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 FLAGS, EXPLAINED 💅

• X509_CHECK_FLAG_ALWAYS_CHECK_SUBJECT — this one makes the function 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. 👑
• X509_CHECK_FLAG_NEVER_CHECK_SUBJECT — this one makes the function 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. and if you set both X509_CHECK_FLAG_ALWAYS_CHECK_SUBJECT and X509_CHECK_FLAG_NEVER_CHECK_SUBJECT, the latter takes precedence and the subject DN is not checked for matching names. periodt. 💅
• X509_CHECK_FLAG_NO_WILDCARDS — if set, disables wildcard expansion. this only applies to X509_check_host. 🙅♀️
• X509_CHECK_FLAG_NO_PARTIAL_WILDCARDS — if set, 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. 🌸
• X509_CHECK_FLAG_MULTI_LABEL_WILDCARDS — if set, 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. 🔥
• X509_CHECK_FLAG_SINGLE_LABEL_SUBDOMAINS — if set, 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 😭
• -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. iykyk 💜

---

👑 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. throwback fr. ✨

---

🎀 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>.

---

and that's the whole page bestie 💖 this was OpenSSL 3.5.9, dated 2026-09-29, manual page X509_CHECK_HOST(3ossl) — go forth and match those certs responsibly, mwah 🫶🌈⭐