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SSL_CTX_add_server_custom_ext.3
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The \&\fBext_type\fR parameter is set to the extension type which will be added and \&\fBadd_arg\fR to the value set when the extension handler was added. .PP If the application wishes to include the extension \fBext_type\fR it should set \fB*out\fR to the extension data, set \fB*outlen\fR to the length of the extension data and return 1. .PP If the \fBadd_cb\fR does not wish to include the extension it must return 0. .PP If \fBadd_cb\fR returns \-1 a fatal handshake error occurs using the \s-1TLS\s0 alert value specified in \fB*al\fR. .PP For clients (but not servers) if \fBadd_cb\fR is set to \s-1NULL\s0 a zero length extension is added for \fBext_type\fR. .PP For clients every registered \fBadd_cb\fR is always called to see if the application wishes to add an extension to ClientHello. .PP For servers every registered \fBadd_cb\fR is called once if and only if the corresponding extension was received in ClientHello to see if the application wishes to add the extension to ServerHello. That is, if no corresponding extension was received in ClientHello then \fBadd_cb\fR will not be called. .PP If an extension is added (that is \fBadd_cb\fR returns 1) \fBfree_cb\fR is called (if it is set) with the value of \fBout\fR set by the add callback. It can be used to free up any dynamic extension data set by \fBadd_cb\fR. Since \fBout\fR is constant (to permit use of constant data in \fBadd_cb\fR) applications may need to cast away const to free the data. .PP The callback \fBparse_cb\fR receives data for \s-1TLS\s0 extensions. For \s-1TLS\s0 clients the extension data will come from ServerHello and for \s-1TLS\s0 servers it will come from ClientHello. .PP The extension data consists of \fBinlen\fR bytes in the buffer \fBin\fR for the extension \fBextension_type\fR. .PP If the \fBparse_cb\fR considers the extension data acceptable it must return 1. If it returns 0 or a negative value a fatal handshake error occurs using the \s-1TLS\s0 alert value specified in \fB*al\fR. .PP The buffer \fBin\fR is a temporary internal buffer which will not be valid after the callback returns. .SH "NOTES" .IX Header "NOTES" The \fBadd_arg\fR and \fBparse_arg\fR parameters can be set to arbitrary values which will be passed to the corresponding callbacks. They can, for example, be used to store the extension data received in a convenient structure or pass the extension data to be added or freed when adding extensions. .PP The \fBext_type\fR parameter corresponds to the \fBextension_type\fR field of \&\s-1RFC5246\s0 et al. It is \fBnot\fR a \s-1NID\s0. .PP If the same custom extension type is received multiple times a fatal \&\fBdecode_error\fR alert is sent and the handshake aborts. If a custom extension is received in ServerHello which was not sent in ClientHello a fatal \&\fBunsupported_extension\fR alert is sent and the handshake is aborted. The ServerHello \fBadd_cb\fR callback is only called if the corresponding extension was received in ClientHello. This is compliant with the \s-1TLS\s0 specifications. This behaviour ensures that each callback is called at most once and that an application can never send unsolicited extensions. .SH "RETURN VALUES" .IX Header "RETURN VALUES" \&\fISSL_CTX_add_client_custom_ext()\fR and \fISSL_CTX_add_server_custom_ext()\fR return 1 for success and 0 for failure. A failure can occur if an attempt is made to add the same \fBext_type\fR more than once, if an attempt is made to use an extension type handled internally by OpenSSL or if an internal error occurs (for example a memory allocation failure). .PP \&\fISSL_extension_supported()\fR returns 1 if the extension \fBext_type\fR is handled internally by OpenSSL and 0 otherwise.