=pod

=head1 NAME

X509_VERIFY_PARAM_set_flags, X509_VERIFY_PARAM_clear_flags,
X509_VERIFY_PARAM_get_flags, X509_VERIFY_PARAM_set_purpose,
X509_VERIFY_PARAM_get_purpose,
X509_VERIFY_PARAM_get_inh_flags, X509_VERIFY_PARAM_set_inh_flags,
X509_VERIFY_PARAM_set_trust, X509_VERIFY_PARAM_set_depth,
X509_VERIFY_PARAM_get_depth, X509_VERIFY_PARAM_set_auth_level,
X509_VERIFY_PARAM_get_auth_level, X509_VERIFY_PARAM_set_time,
X509_VERIFY_PARAM_get_time,
X509_VERIFY_PARAM_add0_policy, X509_VERIFY_PARAM_set1_policies,
X509_VERIFY_PARAM_get0_host,
X509_VERIFY_PARAM_set1_host, X509_VERIFY_PARAM_add1_host,
X509_VERIFY_PARAM_set_hostflags,
X509_VERIFY_PARAM_get_hostflags,
X509_VERIFY_PARAM_get0_peername,
X509_VERIFY_PARAM_get0_email,
X509_VERIFY_PARAM_set1_email,
X509_VERIFY_PARAM_set1_rfc822, X509_VERIFY_PARAM_add1_rfc822,
X509_VERIFY_PARAM_set1_smtputf8, X509_VERIFY_PARAM_add1_smtputf8,
X509_VERIFY_PARAM_set1_ip, X509_VERIFY_PARAM_add1_ip,
X509_VERIFY_PARAM_set1_ip_asc, X509_VERIFY_PARAM_add1_ip_asc,
X509_VERIFY_PARAM_get1_ip_asc,
X509_VERIFY_PARAM_set1_host_input_validation,
X509_VERIFY_PARAM_set1_rfc822_input_validation,
X509_VERIFY_PARAM_set1_smtputf8_input_validation,
X509_VERIFY_PARAM_set1_ip_input_validation
- X509 verification parameters

=head1 SYNOPSIS

 #include <openssl/x509_vfy.h>

 int X509_VERIFY_PARAM_set_flags(X509_VERIFY_PARAM *param,
                                 unsigned long flags);
 int X509_VERIFY_PARAM_clear_flags(X509_VERIFY_PARAM *param,
                                   unsigned long flags);
 unsigned long X509_VERIFY_PARAM_get_flags(const X509_VERIFY_PARAM *param);

 int X509_VERIFY_PARAM_set_inh_flags(X509_VERIFY_PARAM *param,
                                     uint32_t flags);
 uint32_t X509_VERIFY_PARAM_get_inh_flags(const X509_VERIFY_PARAM *param);

 int X509_VERIFY_PARAM_set_purpose(X509_VERIFY_PARAM *param, int purpose);
 int X509_VERIFY_PARAM_get_purpose(X509_VERIFY_PARAM *param);
 int X509_VERIFY_PARAM_set_trust(X509_VERIFY_PARAM *param, int trust);

 void X509_VERIFY_PARAM_set_time(X509_VERIFY_PARAM *param, time_t t);
 time_t X509_VERIFY_PARAM_get_time(const X509_VERIFY_PARAM *param);

 int X509_VERIFY_PARAM_add0_policy(X509_VERIFY_PARAM *param,
                                   ASN1_OBJECT *policy);
 int X509_VERIFY_PARAM_set1_policies(X509_VERIFY_PARAM *param,
                                     STACK_OF(ASN1_OBJECT) *policies);

 void X509_VERIFY_PARAM_set_depth(X509_VERIFY_PARAM *param, int depth);
 int X509_VERIFY_PARAM_get_depth(const X509_VERIFY_PARAM *param);

 void X509_VERIFY_PARAM_set_auth_level(X509_VERIFY_PARAM *param,
                                       int auth_level);
 int X509_VERIFY_PARAM_get_auth_level(const X509_VERIFY_PARAM *param);

 char *X509_VERIFY_PARAM_get0_host(X509_VERIFY_PARAM *param, int n);
 int X509_VERIFY_PARAM_set1_host(X509_VERIFY_PARAM *param,
                                 const char *name, size_t namelen);
 int X509_VERIFY_PARAM_add1_host(X509_VERIFY_PARAM *param,
                                 const char *name, size_t namelen);
 void X509_VERIFY_PARAM_set_hostflags(X509_VERIFY_PARAM *param,
                                      unsigned int flags);
 unsigned int X509_VERIFY_PARAM_get_hostflags(const X509_VERIFY_PARAM *param);
 char *X509_VERIFY_PARAM_get0_peername(const X509_VERIFY_PARAM *param);
 char *X509_VERIFY_PARAM_get0_email(X509_VERIFY_PARAM *param);
 int X509_VERIFY_PARAM_set1_email(X509_VERIFY_PARAM *param,
                                  const char *email, size_t emaillen);
 int X509_VERIFY_PARAM_set1_rfc822(X509_VERIFY_PARAM *param,
                                  const char *email, size_t emaillen);
 int X509_VERIFY_PARAM_add1_rfc822(X509_VERIFY_PARAM *param,
                                 const char *email, size_t emaillen);
 int X509_VERIFY_PARAM_set1_smtputf8(X509_VERIFY_PARAM *param,
                                  const char *email, size_t emaillen);
 int X509_VERIFY_PARAM_add1_smtputf8(X509_VERIFY_PARAM *param,
                                 const char *email, size_t emaillen);
 char *X509_VERIFY_PARAM_get1_ip_asc(X509_VERIFY_PARAM *param);
 int X509_VERIFY_PARAM_set1_ip(X509_VERIFY_PARAM *param,
                               const unsigned char *ip, size_t iplen);
 int X509_VERIFY_PARAM_add1_ip(X509_VERIFY_PARAM *param,
                               const unsigned char *ip, size_t iplen);
 int X509_VERIFY_PARAM_set1_ip_asc(X509_VERIFY_PARAM *param, const char *ip_asc);
 int X509_VERIFY_PARAM_add1_ip_asc(X509_VERIFY_PARAM *param, const char *ip_asc);
 void X509_VERIFY_PARAM_set1_ip_input_validation(X509_VERIFY_PARAM *param,
    int (*validate_ip)(const uint8_t *name, size_t len));
 void X509_VERIFY_PARAM_set1_host_input_validation(X509_VERIFY_PARAM *param,
    int (*validate_host)(const char *name, size_t len));
 void X509_VERIFY_PARAM_set1_rfc822_input_validation(X509_VERIFY_PARAM *param,
    int (*validate_rfc822)(const char *name, size_t len));
 void X509_VERIFY_PARAM_set1_smtputf8_input_validation(X509_VERIFY_PARAM *param,
    int (*validate_smtputf8)(const char *name, size_t len));

=head1 DESCRIPTION

These functions manipulate the B<X509_VERIFY_PARAM> structure associated with
a certificate verification operation.

The X509_VERIFY_PARAM_set_flags() function sets the flags in I<param> by oring
it with I<flags>. See L</VERIFICATION FLAGS> for a complete
description of values the I<flags> parameter can take.

X509_VERIFY_PARAM_get_flags() returns the flags in I<param>.

X509_VERIFY_PARAM_get_inh_flags() returns the inheritance flags in I<param>
which specifies how verification flags are copied from one structure to
another. X509_VERIFY_PARAM_set_inh_flags() sets the inheritance flags.
See the L</INHERITANCE FLAGS> section for a description of these bits.

X509_VERIFY_PARAM_clear_flags() clears the flags I<flags> in I<param>.

X509_VERIFY_PARAM_set_purpose() sets the verification purpose in I<param>
to I<purpose>. This determines the acceptable purpose of the certificate
chain, for example B<X509_PURPOSE_SSL_CLIENT>.
The purpose requirement is cleared if I<purpose> is B<X509_PURPOSE_DEFAULT_ANY>.

X509_VERIFY_PARAM_get_purpose() returns the purpose in I<param>.

X509_VERIFY_PARAM_set_trust() sets the trust setting in I<param> to
I<trust>.

X509_VERIFY_PARAM_set_time() sets the verification time in I<param> to
I<t>. Normally the current time is used.

X509_VERIFY_PARAM_add0_policy() adds I<policy> to the acceptable policy set.
Contrary to preexisting documentation of this function it does not enable
policy checking.

X509_VERIFY_PARAM_set1_policies() enables policy checking (it is disabled
by default) and sets the acceptable policy set to I<policies>. Any existing
policy set is cleared. The I<policies> parameter can be NULL to clear
an existing policy set.

X509_VERIFY_PARAM_set_depth() sets the maximum verification depth to I<depth>.
That is the maximum number of intermediate CA certificates that can appear in a
chain.
A maximal depth chain contains 2 more certificates than the limit, since
neither the end-entity certificate nor the trust-anchor count against this
limit.
Thus a I<depth> limit of 0 only allows the end-entity certificate to be signed
directly by the trust anchor, while with a I<depth> limit of 1 there can be one
intermediate CA certificate between the trust anchor and the end-entity
certificate.

X509_VERIFY_PARAM_set_auth_level() sets the authentication security level to
I<auth_level>.
The authentication security level determines the acceptable signature and public
key strength when verifying certificate chains.
For a certificate chain to validate, the public keys of all the certificates
must meet the specified security level.
The signature algorithm security level is not enforced for the chain's I<trust
anchor> certificate, which is either directly trusted or validated by means other
than its signature.
See L<SSL_CTX_set_security_level(3)> for the definitions of the available
levels.
The default security level is -1, or "not set".
At security level 0 or lower all algorithms are acceptable.
Security level 1 requires at least 80-bit-equivalent security and is broadly
interoperable, though it will, for example, reject MD5 signatures or RSA keys
shorter than 1024 bits.

X509_VERIFY_PARAM_get0_host() returns the I<n>th expected DNS hostname that has
been set using X509_VERIFY_PARAM_set1_host() or X509_VERIFY_PARAM_add1_host().
To obtain all names start with I<n> = 0 and increment I<n> as long as no NULL
pointer is returned.

X509_VERIFY_PARAM_set1_host() sets in I<param> the expected
DNS hostname to I<name>, clearing any previously specified hostname.
If I<name> is NULL or the empty string, the list of hostnames is cleared
and hostname checks are not performed on the peer certificate.
If I<namelen> is zero, I<name> must be NUL-terminated,
otherwise I<namelen> must be set to the length of I<name>.

When a hostname is specified,
certificate verification automatically invokes L<X509_check_host(3)>
with flags equal to the I<flags> argument given to
X509_VERIFY_PARAM_set_hostflags() (default zero).  Applications
are strongly advised to use this interface in preference to explicitly
calling L<X509_check_host(3)>, hostname checks may be out of scope
with the DANE-EE(3) certificate usage, and the internal check will
be suppressed as appropriate when DANE verification is enabled.

When the subject CommonName will not be ignored, whether as a result of the
B<X509_CHECK_FLAG_ALWAYS_CHECK_SUBJECT> host flag, or because no DNS subject
alternative names are present in the certificate, any DNS name constraints in
issuer certificates apply to the subject CommonName as well as the subject
alternative name extension.

When the subject CommonName will be ignored, whether as a result of the
B<X509_CHECK_FLAG_NEVER_CHECK_SUBJECT> host flag, or because some DNS subject
alternative names are present in the certificate, DNS name constraints in
issuer certificates will not be applied to the subject DN.
As described in X509_check_host(3) the B<X509_CHECK_FLAG_NEVER_CHECK_SUBJECT>
flag takes precedence over the B<X509_CHECK_FLAG_ALWAYS_CHECK_SUBJECT> flag.

X509_VERIFY_PARAM_get_hostflags() returns any host flags previously set via a
call to X509_VERIFY_PARAM_set_hostflags().

X509_VERIFY_PARAM_add1_host() adds I<name> as an additional reference
identifier that can match the peer's certificate.  Any previous names
set via X509_VERIFY_PARAM_set1_host() or X509_VERIFY_PARAM_add1_host()
are retained, no change is made if I<name> is NULL or the empty string.  When
multiple names are configured, the peer is considered verified when
any name matches.

X509_VERIFY_PARAM_get0_peername() returns the DNS hostname or subject
CommonName from the peer certificate that matched one of the reference
identifiers.  When wildcard matching is not disabled, or when a
reference identifier specifies a parent domain (starts with ".")
rather than a hostname, the peer name may be a wildcard name or a
sub-domain of the reference identifier respectively.  The return
string is allocated by the library and is no longer valid once the
associated I<param> argument is freed.  Applications must not free
the return value.

X509_VERIFY_PARAM_get0_email() returns the expected RFC822 email address.

The _rfc822() family of functions is used for email names that have
ASCII localpart addresses, in which case the domain part of the
address must be represented in A-label form. They are used to
specify the list of values to match against the SAN Email names in
certificates.

X509_VERIFY_PARAM_set1_rfc822() clears all expected RFC822 email
addresses, and sets the expected RFC822 email address to I<email>.  If
I<email> is NULL no expected address is set. Otherwise, if
I<emaillen> is zero, I<email> must be NUL-terminated; if I<emaillen>
is nonzero, I<emaillen> must be set to the length of I<email>.  When
any email address is specified, certificate verification automatically
invokes L<X509_check_email(3)>.

X509_VERIFY_PARAM_add1_rfc822() adds I<email> as an additional
reference identifier that can match RFC822 email addresses in the
peer's certificate. Any previous names set via
X509_VERIFY_PARAM_set1_rfc822(), X509_VERIFY_PARAM_add1_rfc822(), or
X509_VERIFY_PARAM_set1_email() are
retained on success, no change is made on failure. It is a failure if
email is NULL or the empty string.
The peer is considered verified
when any one of the specified RFC822 or SMTPUTF8 names matches a corresponding email
address SAN in the certificate.

The _smtputf8() family of functions is used for email names that have
a non-ASCII localpart addresses, in which case the domain part of the
address must be represented in U-label form. They are used to
specify the list of values to match against the OTHERNAME SMTPUTF8 names in
certificates.

X509_VERIFY_PARAM_set1_smtputf8() sets the expected SMTPUTF8 email address to
I<email>.
If I<email> is NULL, SMTPUTF8 email checking is disabled. Otherwise,
if I<emaillen> is zero, I<email> must be NUL-terminated; if I<emaillen> is nonzero,
I<emaillen> must be set to the length of I<email>.  When any email address
is specified, certificate verification automatically invokes
L<X509_check_email(3)>.

X509_VERIFY_PARAM_add1_smtputf8() adds I<email> as an additional
reference identifier that can match SMTPUTF8 email addresses in the
peer's certificate.  Any previous names set via
X509_VERIFY_PARAM_set1_smtputf8(), X509_VERIFY_PARAM_add1_smtputf8(), or
X509_VERIFY_PARAM_set1_email() are
retained on success, no change is made on failure. It is a failure if
email is NULL or the empty string. The peer is considered verified
when any one of the specified RFC822 or SMTPUTF8 names matches a corresponding email
address SAN in the certificate.

X509_VERIFY_PARAM_set1_email() calls X509_VERIFY_PARAM_set_rfc822(), and
X509_VERIFY_PARAM_set_smtputf8() and succeeds if either call succeeds.

X509_VERIFY_PARAM_get1_ip_asc() returns the expected IP address as a string.
The caller is responsible for freeing it.

X509_VERIFY_PARAM_set1_ip() sets the expected IP address to I<ip>.
If I<ip> is NULL, IP address checking is disabled. Otherwise,
the I<ip> argument must be in binary format, in network byte-order and
I<iplen> must be set to 4 for IPv4 and 16 for IPv6.  When an IP
address is specified, certificate verification automatically invokes
L<X509_check_ip(3)>.

X509_VERIFY_PARAM_add1_ip() adds I<ip> as an additional reference
identifier that can match the peer's certificate on success.  Any
previous addresses set via X509_VERIFY_PARAM_set1_ip(),
X509_VERIFY_PARAM_add1_ip(), X509_VERIFY_PARAM_set1_ip_asc(), or
X509_VERIFY_PARAM_add1_ip_asc() are retained. No change is made on
failure.
It is a failure if I<ip> is NULL or the value I<iplen> is neither 4 nor 16 bytes.
When
multiple names are configured, the peer is considered verified when
any name matches.

X509_VERIFY_PARAM_set1_ip_asc() sets the expected IP address to
I<ip_asc>.  The I<ip_asc> argument must be a NUL-terminated ASCII string:
dotted decimal quad for IPv4 and colon-separated hexadecimal for
IPv6.  The condensed "::" notation is supported for IPv6 addresses.

X509_VERIFY_PARAM_add1_ip_asc() adds I<ip_asc> as an additional
reference identifier that can match the peer's certificate on success.
The I<ip_asc> argument must be a NUL-terminated ASCII string: dotted
decimal quad for IPv4 and colon-separated hexadecimal for IPv6.  The
condensed "::" notation is supported for IPv6 addresses.  Any previous
names set via X509_VERIFY_PARAM_set1_ip(),
X509_VERIFY_PARAM_add1_ip(), X509_VERIFY_PARAM_set1_ip_asc(), or
X509_VERIFY_PARAM_add1_ip_asc() are retained. No change is made on
failure.
It is a failure if I<ip_asc> is NULL or the empty string.
When multiple addresses are configured, the peer is considered verified
when any one of the specified addresses matches a corresponding IP address SAN in the certificate.

X509_VERIFY_PARAM_set1_host_input_validation(),
X509_VERIFY_PARAM_set1_rfc822_input_validation(),
X509_VERIFY_PARAM_set1_smtputf8_input_validation(), and
X509_VERIFY_PARAM_set1_ip_input_validation() set a verification
function to validate the input on setting the corresponding validation
parameter expected values.
These functions make it possible to override OpenSSL's built-in input validation of
the corresponding verification parameters.
If the provided function succeeds, the corresponding
input will be accepted for use in certificate verification.

=head1 RETURN VALUES

X509_VERIFY_PARAM_set_flags(), X509_VERIFY_PARAM_clear_flags(),
X509_VERIFY_PARAM_set_inh_flags(),
X509_VERIFY_PARAM_set_purpose(), X509_VERIFY_PARAM_set_trust(),
X509_VERIFY_PARAM_add0_policy() X509_VERIFY_PARAM_set1_policies(),
X509_VERIFY_PARAM_set1_host(), X509_VERIFY_PARAM_add1_host(),
X509_VERIFY_PARAM_set1_email(),
X509_VERIFY_PARAM_set1_ip(), X509_VERIFY_PARAM_add1_ip(),
X509_VERIFY_PARAM_set1_ip_asc(),
X509_VERIFY_PARAM_add1_ip_asc() return 1 for success and 0 for
failure.

X509_VERIFY_PARAM_get0_host(), X509_VERIFY_PARAM_get0_email(), and
X509_VERIFY_PARAM_get1_ip_asc(), return the string pointers specified above
or NULL if the respective value has not been set or on error.

X509_VERIFY_PARAM_get_flags() returns the current verification flags.

X509_VERIFY_PARAM_get_hostflags() returns any current host flags.

X509_VERIFY_PARAM_get_inh_flags() returns the current inheritance flags.

X509_VERIFY_PARAM_set_time() and X509_VERIFY_PARAM_set_depth() do not return
values.

X509_VERIFY_PARAM_get_depth() returns the current verification depth.

X509_VERIFY_PARAM_get_auth_level() returns the current authentication security
level.

X509_VERIFY_PARAM_get_purpose() returns the current purpose,
which may be B<X509_PURPOSE_DEFAULT_ANY> if unset.

=head1 VERIFICATION FLAGS

The verification flags consists of zero or more of the following flags
ored together.

B<X509_V_FLAG_CRL_CHECK> enables CRL checking for the certificate chain leaf
certificate. An error occurs if a suitable CRL cannot be found.

B<X509_V_FLAG_CRL_CHECK_ALL> expands CRL checking to the entire certificate
chain if B<X509_V_FLAG_CRL_CHECK> has also been enabled, and is otherwise ignored.

B<X509_V_FLAG_OCSP_RESP_CHECK> enables Online Certificate Status Protocol (OCSP)
checking for the certificate chain leaf certificate. An error occurs if a suitable
OCSP response cannot be found.

B<X509_V_FLAG_OCSP_RESP_CHECK_ALL> expands OCSP checking to the entire certificate
chain if B<X509_V_FLAG_OCSP_RESP_CHECK> has also been enabled, and is otherwise
ignored.

B<X509_V_FLAG_IGNORE_CRITICAL> disables critical extension checking. By default
any unhandled critical extensions in certificates or (if checked) CRLs result
in a fatal error. If this flag is set unhandled critical extensions are
ignored. B<WARNING> setting this option for anything other than debugging
purposes can be a security risk. Finer control over which extensions are
supported can be performed in the verification callback.

The B<X509_V_FLAG_X509_STRICT> flag disables workarounds for some broken
certificates and makes the verification strictly apply B<X509> rules.

B<X509_V_FLAG_ALLOW_PROXY_CERTS> enables proxy certificate verification.

B<X509_V_FLAG_POLICY_CHECK> enables certificate policy checking, by default
no policy checking is performed. Additional information is sent to the
verification callback relating to policy checking.

B<X509_V_FLAG_EXPLICIT_POLICY>, B<X509_V_FLAG_INHIBIT_ANY> and
B<X509_V_FLAG_INHIBIT_MAP> set the C<require explicit policy>, C<inhibit any
policy> and C<inhibit policy mapping> flags respectively as defined in
RFC 5280. Policy checking is automatically enabled if any of these flags
are set.

If B<X509_V_FLAG_NOTIFY_POLICY> is set and the policy checking is successful
a special status code is set to the verification callback. This permits it
to examine the valid policy tree and perform additional checks or simply
log it for debugging purposes.

By default some additional features such as indirect CRLs and CRLs signed by
different keys are disabled. If B<X509_V_FLAG_EXTENDED_CRL_SUPPORT> is set
they are enabled.

If B<X509_V_FLAG_USE_DELTAS> is set delta CRLs (if present) are used to
determine certificate status. If not set deltas are ignored.

B<X509_V_FLAG_CHECK_SS_SIGNATURE> requests checking the signature of
the last certificate in a chain if the certificate is supposedly self-signed.
This is prohibited and will result in an error if it is a non-conforming CA
certificate with key usage restrictions not including the I<keyCertSign> bit.
By default this check is disabled because it doesn't
add any additional security but in some cases applications might want to
check the signature anyway. A side effect of not checking the self-signature
of such a certificate is that disabled or unsupported message digests used for
the signature are not treated as fatal errors.

When B<X509_V_FLAG_TRUSTED_FIRST> is set, which is the default since
OpenSSL 1.1.0, construction of the certificate chain
in L<X509_verify_cert(3)> searches the trust store for issuer certificates
before searching the provided untrusted certificates.
Local issuer certificates are often more likely to satisfy local security
requirements and lead to a locally trusted root.
This is especially important when some certificates in the trust store have
explicit trust settings (see "TRUST SETTINGS" in L<openssl-x509(1)>).

The B<X509_V_FLAG_NO_ALT_CHAINS> flag suppresses checking for alternative chains.

The B<X509_V_FLAG_PARTIAL_CHAIN> flag causes non-self-signed certificates in the
trust store to be treated as trust anchors, in the same way as self-signed
root CA certificates.
This makes it possible to trust self-issued certificates as well as certificates
issued by an intermediate CA without having to trust their ancestor root CA.
With B<X509_V_FLAG_PARTIAL_CHAIN> set, chain
construction stops as soon as the first certificate contained in the trust store
is added to the chain, whether that certificate is a self-signed "root"
certificate or a not self-signed "intermediate" or self-issued certificate.
Thus, when an intermediate certificate is found in the trust store, the
verified chain passed to callbacks may be shorter than it otherwise would
be without the B<X509_V_FLAG_PARTIAL_CHAIN> flag.

The B<X509_V_FLAG_NO_CHECK_TIME> flag suppresses checking the validity period
of certificates and CRLs against the current time. If X509_VERIFY_PARAM_set_time()
is used to specify a verification time, the check is not suppressed.

=head1 INHERITANCE FLAGS

These flags specify how parameters are "inherited" from one structure to
another.

If B<X509_VP_FLAG_ONCE> is set then the current setting is zeroed
after the next call.

If B<X509_VP_FLAG_LOCKED> is set then no values are copied.  This overrides
all of the following flags.

If B<X509_VP_FLAG_DEFAULT> is set then anything set in the source is copied
to the destination. Effectively the values in "to" become default values
which will be used only if nothing new is set in "from".  This is the
default.

If B<X509_VP_FLAG_OVERWRITE> is set then all value are copied across whether
they are set or not. Flags is still Ored though.

If B<X509_VP_FLAG_RESET_FLAGS> is set then the flags value is copied instead
of ORed.

=head1 NOTES

The above functions should be used to manipulate verification parameters
instead of functions which work in specific structures such as
X509_STORE_CTX_set_flags() which are likely to be deprecated in a future
release.

TLS clients are recommended to set up validation of server hostname(s) and/or
IP address (directly using the above functions
or more conveniently using L<SSL_set1_host(3)> or L<SSL_add1_host(3)>)
and to use L<SSL_set_tlsext_host_name(3)> for Server Name Indication (SNI),
which may be crucial also for correct routing of the connection request.

=head1 BUGS

Delta CRL checking is currently primitive. Only a single delta can be used and
(partly due to limitations of B<X509_STORE>) constructed CRLs are not
maintained.

If CRLs checking is enable CRLs are expected to be available in the
corresponding B<X509_STORE> structure. No attempt is made to download
CRLs from the CRL distribution points extension.

=head1 EXAMPLES

Enable CRL checking when performing certificate verification during SSL
connections associated with an B<SSL_CTX> structure I<ctx>:

 X509_VERIFY_PARAM *param;

 param = X509_VERIFY_PARAM_new();
 X509_VERIFY_PARAM_set_flags(param, X509_V_FLAG_CRL_CHECK);
 SSL_CTX_set1_param(ctx, param);
 X509_VERIFY_PARAM_free(param);

=head1 SEE ALSO

L<SSL_CTX_set_security_level(3)>,
L<X509_verify_cert(3)>,
L<X509_check_host(3)>,
L<X509_check_email(3)>,
L<X509_check_ip(3)>,
L<SSL_set_tlsext_host_name(3)>,
L<openssl-x509(1)>

=head1 HISTORY

Until OpenSSL 1.1.0, by default (unless B<X509_V_FLAG_TRUSTED_FIRST> was set), when building a
certificate chain, if the first certificate chain found was not trusted,
OpenSSL would attempt to replace untrusted certificates supplied by the peer
with certificates from the trust store to see if an alternative chain can be
found that is trusted.
Since OpenSSL 1.1.0 version, the B<X509_V_FLAG_TRUSTED_FIRST> is set by default.

The B<X509_V_FLAG_NO_ALT_CHAINS> flag was added in OpenSSL 1.1.0.
The flag B<X509_V_FLAG_CB_ISSUER_CHECK> was deprecated in OpenSSL 1.1.0
and has no effect.

The X509_VERIFY_PARAM_get_hostflags() function was added in OpenSSL 1.1.0i.

The X509_VERIFY_PARAM_get0_host(), X509_VERIFY_PARAM_get0_email(),
and X509_VERIFY_PARAM_get1_ip_asc() functions were added in OpenSSL 3.0.

The function X509_VERIFY_PARAM_add0_policy() was historically documented as
enabling policy checking however the implementation has never done this.
The documentation was changed to align with the implementation.

The X509_VERIFY_PARAM_get_purpose() function was added in OpenSSL 3.5.

=head1 COPYRIGHT

Copyright 2009-2026 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
L<https://www.openssl.org/source/license.html>.

=cut
