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alpn_select_cb 

        # OpenSSL::SSL::SSLContext.alpn_select_cb

(from ruby core)
---

A callback invoked on the server side when the server needs to select a
protocol from the list sent by the client. Supported in OpenSSL 1.0.2
and higher. The callback must return a protocol of those advertised by
the client. If none is acceptable, raising an error in the callback will
cause the handshake to fail. Not setting this callback explicitly means
not supporting the ALPN extension on the server - any protocols
advertised by the client will be ignored.

### Example

    ctx.alpn_select_cb = lambda do |protocols|
      # inspect the protocols and select one
      protocols.first
    end

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