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1 .\" $OpenBSD: SSL_CTX_set_cert_verify_callback.3,v 1.2 2016/11/30 17:23:53 schwarze Exp $
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51 .Dd $Mdocdate: November 30 2016 $
52 .Dt SSL_CTX_SET_CERT_VERIFY_CALLBACK 3
53 .Os
54 .Sh NAME
55 .Nm SSL_CTX_set_cert_verify_callback
56 .Nd set peer certificate verification procedure
57 .Sh SYNOPSIS
58 .In openssl/ssl.h
59 .Ft void
60 .Fo SSL_CTX_set_cert_verify_callback
61 .Fa "SSL_CTX *ctx"
62 .Fa "int (*callback)(X509_STORE_CTX *, void *)"
63 .Fa "void *arg"
64 .Fc
65 .Sh DESCRIPTION
66 .Fn SSL_CTX_set_cert_verify_callback
67 sets the verification callback function for
68 .Fa ctx .
69 .Vt SSL
70 objects that are created from
71 .Fa ctx
72 inherit the setting valid at the time when
73 .Xr SSL_new 3
74 is called.
75 .Sh NOTES
76 Whenever a certificate is verified during a SSL/TLS handshake,
77 a verification function is called.
78 If the application does not explicitly specify a verification callback
79 function, the built-in verification function is used.
80 If a verification callback
81 .Fa callback
82 is specified via
83 .Fn SSL_CTX_set_cert_verify_callback ,
84 the supplied callback function is called instead.
85 By setting
86 .Fa callback
87 to
88 .Dv NULL ,
89 the default behaviour is restored.
90 .Pp
91 When the verification must be performed,
92 .Fa callback
93 will be called with the arguments
94 .Fn callback "X509_STORE_CTX *x509_store_ctx" "void *arg" .
95 The argument
96 .Fa arg
97 is specified by the application when setting
98 .Fa callback .
99 .Pp
100 .Fa callback
101 should return 1 to indicate verification success and 0 to indicate verification
102 failure.
103 If
104 .Dv SSL_VERIFY_PEER
105 is set and
106 .Fa callback
107 returns 0, the handshake will fail.
108 As the verification procedure may allow the connection to continue in case of
109 failure (by always returning 1) the verification result must be set in any case
110 using the
111 .Fa error
112 member of
113 .Fa x509_store_ctx
114 so that the calling application will be informed about the detailed result of
115 the verification procedure!
116 .Pp
117 Within
118 .Fa x509_store_ctx ,
119 .Fa callback
120 has access to the
121 .Fa verify_callback
122 function set using
123 .Xr SSL_CTX_set_verify 3 .
124 .Sh SEE ALSO
125 .Xr ssl 3 ,
126 .Xr SSL_CTX_load_verify_locations 3 ,
127 .Xr SSL_CTX_set_verify 3 ,
128 .Xr SSL_get_verify_result 3
129 .Sh HISTORY
130 Previous to OpenSSL 0.9.7, the
131 .Fa arg
132 argument to
133 .Fn SSL_CTX_set_cert_verify_callback
134 was ignored, and
135 .Fa callback
136 was called
137 simply as
138 .Ft int
139 .Fn (*callback) "X509_STORE_CTX *" .
140 To compile software written for previous versions of OpenSSL,
141 a dummy argument will have to be added to
142 .Fa callback .
143 .Sh CAVEATS
144 Do not mix the verification callback described in this function with the
145 .Fa verify_callback
146 function called during the verification process.
147 The latter is set using the
148 .Xr SSL_CTX_set_verify 3
149 family of functions.
150 .Pp
151 Providing a complete verification procedure including certificate purpose
152 settings, etc., is a complex task.
153 The built-in procedure is quite powerful and in most cases it should be
154 sufficient to modify its behaviour using the
155 .Fa verify_callback
156 function.
157 .Sh BUGS
158 .Fn SSL_CTX_set_cert_verify_callback
159 does not provide diagnostic information.