410 lines
16 KiB
Groff
410 lines
16 KiB
Groff
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.\" ========================================================================
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.\"
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.IX Title "SSL_CTX_set_verify 3"
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.TH SSL_CTX_set_verify 3 "2012-05-10" "1.0.1c" "OpenSSL"
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.\" For nroff, turn off justification. Always turn off hyphenation; it makes
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.\" way too many mistakes in technical documents.
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.if n .ad l
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.nh
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.SH "NAME"
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SSL_CTX_set_verify, SSL_set_verify, SSL_CTX_set_verify_depth, SSL_set_verify_depth \- set peer certificate verification parameters
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.SH "SYNOPSIS"
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.IX Header "SYNOPSIS"
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.Vb 1
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\& #include <openssl/ssl.h>
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\&
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\& void SSL_CTX_set_verify(SSL_CTX *ctx, int mode,
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\& int (*verify_callback)(int, X509_STORE_CTX *));
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\& void SSL_set_verify(SSL *s, int mode,
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\& int (*verify_callback)(int, X509_STORE_CTX *));
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\& void SSL_CTX_set_verify_depth(SSL_CTX *ctx,int depth);
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\& void SSL_set_verify_depth(SSL *s, int depth);
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\&
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\& int verify_callback(int preverify_ok, X509_STORE_CTX *x509_ctx);
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.Ve
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.SH "DESCRIPTION"
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.IX Header "DESCRIPTION"
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\&\fISSL_CTX_set_verify()\fR sets the verification flags for \fBctx\fR to be \fBmode\fR and
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specifies the \fBverify_callback\fR function to be used. If no callback function
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shall be specified, the \s-1NULL\s0 pointer can be used for \fBverify_callback\fR.
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.PP
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\&\fISSL_set_verify()\fR sets the verification flags for \fBssl\fR to be \fBmode\fR and
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specifies the \fBverify_callback\fR function to be used. If no callback function
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shall be specified, the \s-1NULL\s0 pointer can be used for \fBverify_callback\fR. In
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this case last \fBverify_callback\fR set specifically for this \fBssl\fR remains. If
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no special \fBcallback\fR was set before, the default callback for the underlying
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\&\fBctx\fR is used, that was valid at the time \fBssl\fR was created with
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\&\fISSL_new\fR\|(3).
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.PP
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\&\fISSL_CTX_set_verify_depth()\fR sets the maximum \fBdepth\fR for the certificate chain
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verification that shall be allowed for \fBctx\fR. (See the \s-1BUGS\s0 section.)
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.PP
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\&\fISSL_set_verify_depth()\fR sets the maximum \fBdepth\fR for the certificate chain
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verification that shall be allowed for \fBssl\fR. (See the \s-1BUGS\s0 section.)
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.SH "NOTES"
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.IX Header "NOTES"
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The verification of certificates can be controlled by a set of logically
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or'ed \fBmode\fR flags:
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.IP "\s-1SSL_VERIFY_NONE\s0" 4
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.IX Item "SSL_VERIFY_NONE"
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\&\fBServer mode:\fR the server will not send a client certificate request to the
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client, so the client will not send a certificate.
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.Sp
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\&\fBClient mode:\fR if not using an anonymous cipher (by default disabled), the
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server will send a certificate which will be checked. The result of the
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certificate verification process can be checked after the \s-1TLS/SSL\s0 handshake
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using the \fISSL_get_verify_result\fR\|(3) function.
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The handshake will be continued regardless of the verification result.
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.IP "\s-1SSL_VERIFY_PEER\s0" 4
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.IX Item "SSL_VERIFY_PEER"
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\&\fBServer mode:\fR the server sends a client certificate request to the client.
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The certificate returned (if any) is checked. If the verification process
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fails, the \s-1TLS/SSL\s0 handshake is
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immediately terminated with an alert message containing the reason for
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the verification failure.
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The behaviour can be controlled by the additional
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\&\s-1SSL_VERIFY_FAIL_IF_NO_PEER_CERT\s0 and \s-1SSL_VERIFY_CLIENT_ONCE\s0 flags.
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.Sp
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\&\fBClient mode:\fR the server certificate is verified. If the verification process
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fails, the \s-1TLS/SSL\s0 handshake is
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immediately terminated with an alert message containing the reason for
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the verification failure. If no server certificate is sent, because an
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anonymous cipher is used, \s-1SSL_VERIFY_PEER\s0 is ignored.
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.IP "\s-1SSL_VERIFY_FAIL_IF_NO_PEER_CERT\s0" 4
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.IX Item "SSL_VERIFY_FAIL_IF_NO_PEER_CERT"
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\&\fBServer mode:\fR if the client did not return a certificate, the \s-1TLS/SSL\s0
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handshake is immediately terminated with a \*(L"handshake failure\*(R" alert.
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This flag must be used together with \s-1SSL_VERIFY_PEER\s0.
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.Sp
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\&\fBClient mode:\fR ignored
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.IP "\s-1SSL_VERIFY_CLIENT_ONCE\s0" 4
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.IX Item "SSL_VERIFY_CLIENT_ONCE"
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\&\fBServer mode:\fR only request a client certificate on the initial \s-1TLS/SSL\s0
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handshake. Do not ask for a client certificate again in case of a
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renegotiation. This flag must be used together with \s-1SSL_VERIFY_PEER\s0.
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.Sp
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\&\fBClient mode:\fR ignored
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.PP
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Exactly one of the \fBmode\fR flags \s-1SSL_VERIFY_NONE\s0 and \s-1SSL_VERIFY_PEER\s0 must be
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set at any time.
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.PP
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The actual verification procedure is performed either using the built-in
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verification procedure or using another application provided verification
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function set with
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\&\fISSL_CTX_set_cert_verify_callback\fR\|(3).
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The following descriptions apply in the case of the built-in procedure. An
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application provided procedure also has access to the verify depth information
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and the \fIverify_callback()\fR function, but the way this information is used
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may be different.
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.PP
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\&\fISSL_CTX_set_verify_depth()\fR and \fISSL_set_verify_depth()\fR set the limit up
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to which depth certificates in a chain are used during the verification
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procedure. If the certificate chain is longer than allowed, the certificates
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above the limit are ignored. Error messages are generated as if these
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certificates would not be present, most likely a
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X509_V_ERR_UNABLE_TO_GET_ISSUER_CERT_LOCALLY will be issued.
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The depth count is \*(L"level 0:peer certificate\*(R", \*(L"level 1: \s-1CA\s0 certificate\*(R",
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\&\*(L"level 2: higher level \s-1CA\s0 certificate\*(R", and so on. Setting the maximum
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depth to 2 allows the levels 0, 1, and 2. The default depth limit is 9,
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allowing for the peer certificate and additional 9 \s-1CA\s0 certificates.
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.PP
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The \fBverify_callback\fR function is used to control the behaviour when the
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\&\s-1SSL_VERIFY_PEER\s0 flag is set. It must be supplied by the application and
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receives two arguments: \fBpreverify_ok\fR indicates, whether the verification of
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the certificate in question was passed (preverify_ok=1) or not
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(preverify_ok=0). \fBx509_ctx\fR is a pointer to the complete context used
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for the certificate chain verification.
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.PP
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The certificate chain is checked starting with the deepest nesting level
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(the root \s-1CA\s0 certificate) and worked upward to the peer's certificate.
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At each level signatures and issuer attributes are checked. Whenever
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a verification error is found, the error number is stored in \fBx509_ctx\fR
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and \fBverify_callback\fR is called with \fBpreverify_ok\fR=0. By applying
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X509_CTX_store_* functions \fBverify_callback\fR can locate the certificate
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in question and perform additional steps (see \s-1EXAMPLES\s0). If no error is
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found for a certificate, \fBverify_callback\fR is called with \fBpreverify_ok\fR=1
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before advancing to the next level.
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.PP
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The return value of \fBverify_callback\fR controls the strategy of the further
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verification process. If \fBverify_callback\fR returns 0, the verification
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process is immediately stopped with \*(L"verification failed\*(R" state. If
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\&\s-1SSL_VERIFY_PEER\s0 is set, a verification failure alert is sent to the peer and
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the \s-1TLS/SSL\s0 handshake is terminated. If \fBverify_callback\fR returns 1,
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the verification process is continued. If \fBverify_callback\fR always returns
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1, the \s-1TLS/SSL\s0 handshake will not be terminated with respect to verification
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failures and the connection will be established. The calling process can
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however retrieve the error code of the last verification error using
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\&\fISSL_get_verify_result\fR\|(3) or by maintaining its
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own error storage managed by \fBverify_callback\fR.
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.PP
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If no \fBverify_callback\fR is specified, the default callback will be used.
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Its return value is identical to \fBpreverify_ok\fR, so that any verification
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failure will lead to a termination of the \s-1TLS/SSL\s0 handshake with an
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alert message, if \s-1SSL_VERIFY_PEER\s0 is set.
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.SH "BUGS"
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.IX Header "BUGS"
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In client mode, it is not checked whether the \s-1SSL_VERIFY_PEER\s0 flag
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is set, but whether \s-1SSL_VERIFY_NONE\s0 is not set. This can lead to
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unexpected behaviour, if the \s-1SSL_VERIFY_PEER\s0 and \s-1SSL_VERIFY_NONE\s0 are not
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used as required (exactly one must be set at any time).
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.PP
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The certificate verification depth set with SSL[_CTX]\fI_verify_depth()\fR
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stops the verification at a certain depth. The error message produced
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will be that of an incomplete certificate chain and not
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X509_V_ERR_CERT_CHAIN_TOO_LONG as may be expected.
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.SH "RETURN VALUES"
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.IX Header "RETURN VALUES"
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The SSL*_set_verify*() functions do not provide diagnostic information.
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.SH "EXAMPLES"
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.IX Header "EXAMPLES"
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The following code sequence realizes an example \fBverify_callback\fR function
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that will always continue the \s-1TLS/SSL\s0 handshake regardless of verification
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failure, if wished. The callback realizes a verification depth limit with
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more informational output.
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.PP
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All verification errors are printed, informations about the certificate chain
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are printed on request.
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The example is realized for a server that does allow but not require client
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certificates.
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.PP
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The example makes use of the ex_data technique to store application data
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into/retrieve application data from the \s-1SSL\s0 structure
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(see \fISSL_get_ex_new_index\fR\|(3),
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\&\fISSL_get_ex_data_X509_STORE_CTX_idx\fR\|(3)).
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.PP
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.Vb 10
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\& ...
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\& typedef struct {
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\& int verbose_mode;
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\& int verify_depth;
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\& int always_continue;
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\& } mydata_t;
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\& int mydata_index;
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\& ...
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\& static int verify_callback(int preverify_ok, X509_STORE_CTX *ctx)
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\& {
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\& char buf[256];
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\& X509 *err_cert;
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\& int err, depth;
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\& SSL *ssl;
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\& mydata_t *mydata;
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\&
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\& err_cert = X509_STORE_CTX_get_current_cert(ctx);
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\& err = X509_STORE_CTX_get_error(ctx);
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\& depth = X509_STORE_CTX_get_error_depth(ctx);
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\&
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\& /*
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\& * Retrieve the pointer to the SSL of the connection currently treated
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\& * and the application specific data stored into the SSL object.
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\& */
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\& ssl = X509_STORE_CTX_get_ex_data(ctx, SSL_get_ex_data_X509_STORE_CTX_idx());
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\& mydata = SSL_get_ex_data(ssl, mydata_index);
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\&
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\& X509_NAME_oneline(X509_get_subject_name(err_cert), buf, 256);
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\&
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\& /*
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\& * Catch a too long certificate chain. The depth limit set using
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\& * SSL_CTX_set_verify_depth() is by purpose set to "limit+1" so
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\& * that whenever the "depth>verify_depth" condition is met, we
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\& * have violated the limit and want to log this error condition.
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\& * We must do it here, because the CHAIN_TOO_LONG error would not
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\& * be found explicitly; only errors introduced by cutting off the
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\& * additional certificates would be logged.
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\& */
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\& if (depth > mydata\->verify_depth) {
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\& preverify_ok = 0;
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\& err = X509_V_ERR_CERT_CHAIN_TOO_LONG;
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\& X509_STORE_CTX_set_error(ctx, err);
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\& }
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\& if (!preverify_ok) {
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\& printf("verify error:num=%d:%s:depth=%d:%s\en", err,
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\& X509_verify_cert_error_string(err), depth, buf);
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\& }
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\& else if (mydata\->verbose_mode)
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\& {
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\& printf("depth=%d:%s\en", depth, buf);
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\& }
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\&
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\& /*
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\& * At this point, err contains the last verification error. We can use
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\& * it for something special
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\& */
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\& if (!preverify_ok && (err == X509_V_ERR_UNABLE_TO_GET_ISSUER_CERT))
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\& {
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\& X509_NAME_oneline(X509_get_issuer_name(ctx\->current_cert), buf, 256);
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\& printf("issuer= %s\en", buf);
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\& }
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\&
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\& if (mydata\->always_continue)
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\& return 1;
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\& else
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\& return preverify_ok;
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\& }
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\& ...
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\&
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\& mydata_t mydata;
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\&
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\& ...
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\& mydata_index = SSL_get_ex_new_index(0, "mydata index", NULL, NULL, NULL);
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\&
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\& ...
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\& SSL_CTX_set_verify(ctx, SSL_VERIFY_PEER|SSL_VERIFY_CLIENT_ONCE,
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\& verify_callback);
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\&
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\& /*
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\& * Let the verify_callback catch the verify_depth error so that we get
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\& * an appropriate error in the logfile.
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\& */
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\& SSL_CTX_set_verify_depth(verify_depth + 1);
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\&
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\& /*
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\& * Set up the SSL specific data into "mydata" and store it into th SSL
|
|
\& * structure.
|
|
\& */
|
|
\& mydata.verify_depth = verify_depth; ...
|
|
\& SSL_set_ex_data(ssl, mydata_index, &mydata);
|
|
\&
|
|
\& ...
|
|
\& SSL_accept(ssl); /* check of success left out for clarity */
|
|
\& if (peer = SSL_get_peer_certificate(ssl))
|
|
\& {
|
|
\& if (SSL_get_verify_result(ssl) == X509_V_OK)
|
|
\& {
|
|
\& /* The client sent a certificate which verified OK */
|
|
\& }
|
|
\& }
|
|
.Ve
|
|
.SH "SEE ALSO"
|
|
.IX Header "SEE ALSO"
|
|
\&\fIssl\fR\|(3), \fISSL_new\fR\|(3),
|
|
\&\fISSL_CTX_get_verify_mode\fR\|(3),
|
|
\&\fISSL_get_verify_result\fR\|(3),
|
|
\&\fISSL_CTX_load_verify_locations\fR\|(3),
|
|
\&\fISSL_get_peer_certificate\fR\|(3),
|
|
\&\fISSL_CTX_set_cert_verify_callback\fR\|(3),
|
|
\&\fISSL_get_ex_data_X509_STORE_CTX_idx\fR\|(3),
|
|
\&\fISSL_get_ex_new_index\fR\|(3)
|