[openssl-users] How to send alert in handshake?

Angus Robertson - Magenta Systems Ltd angus at magsys.co.uk
Wed Jun 27 16:55:00 UTC 2018

> In order to implement SNI you need an SNI callback
> The callback should return:
> SSL_TLSEXT_ERR_OK, if it successfully processed the SNI
> SSL_TLSEXT_ERR_ALERT_WARNING, to send a warning alert back
> SSL_TLSEXT_ERR_ALERT_FATAL, to send a fatal alert back
> SSL_TLSEXT_ERR_NOACK, to continue without acknowledging the SNI 
> at all

Our SSL library used to return SSL_TLSEXT_ERR_ALERT_WARNING if no name
match was found, until we discovered that Java SSL treats this as a
fatal error when we changed to OK.  

But it did not seem to cause a problem with any browsers or OpenSSL
clients, which I assume ignore it.  

What would SSL_alert_type_string return in the client?  


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