This aligns with OpenSSL's behavior. RFC7301 says servers should return
no_application_protocol if the client supported ALPN but no common
protocol was found. We currently interpret all values as
SSL_TLSEXT_ERR_NOACK. Instead, implement both modes and give guidance on
whne to use each. (NOACK is still useful because the callback may be
shared across multiple configurations, some of which don't support ALPN
at all. Those would want to return NOACK to ignore the list.)
To match upstream, I've also switched SSL_R_MISSING_ALPN, added for
QUIC, to SSL_R_NO_APPLICATION_PROTOCOL.
Update-Note: Callers that return SSL_TLSEXT_ERR_ALERT_FATAL from the
ALPN callback will change behavior. The old behavior may be restored by
returning SSL_TLSEXT_ERR_NOACK, though see the documentation for new
recommendations on return values.
Change-Id: Ib7917b5f8a098571bed764c79aa7a4ce0f728297
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/45504
Reviewed-by: Adam Langley <agl@google.com>
Commit-Queue: David Benjamin <davidben@google.com>
David Benjamin
c02c19e0d8