Get SNI for OpenSSL (crystal-lang#7291)

spec/std/openssl/ssl/server_spec.cr

@@ -76,4 +76,21 @@ describe OpenSSL::SSL::Server do
       end
     end
   end
+
+  it "detects SNI hostname" do
+    tcp_server = TCPServer.new(0)
+    server_context, client_context = ssl_context_pair
+
+    OpenSSL::SSL::Server.open tcp_server, server_context do |server|
+      spawn do
+        sleep 1
+        OpenSSL::SSL::Socket::Client.open(TCPSocket.new(tcp_server.local_address.address, tcp_server.local_address.port), client_context, hostname: "example.com") do |socket|
+        end
+      end
+
+      client = server.accept
+      client.hostname.should eq("example.com")
+      client.close
+    end
+  end
 end

src/openssl/lib_ssl.cr

@@ -155,6 +155,7 @@ lib LibSSL
   fun tlsv1_2_method = TLSv1_2_method : SSLMethod
 
   fun ssl_get_error = SSL_get_error(handle : SSL, ret : Int) : SSLError
+  fun ssl_get_servername = SSL_get_servername(ssl : SSL, host_type : TLSExt) : UInt8*
   fun ssl_set_bio = SSL_set_bio(handle : SSL, rbio : LibCrypto::Bio*, wbio : LibCrypto::Bio*)
   fun ssl_select_next_proto = SSL_select_next_proto(output : Char**, output_len : Char*, input : Char*, input_len : Int, client : Char*, client_len : Int) : Int
   fun ssl_ctrl = SSL_ctrl(handle : SSL, cmd : Int, larg : Long, parg : Void*) : Long

src/openssl/ssl/socket.cr

@@ -181,4 +181,11 @@ abstract class OpenSSL::SSL::Socket < IO
   def unbuffered_rewind
     raise IO::Error.new("Can't rewind OpenSSL::SSL::Socket::Client")
   end
+
+  # Returns the hostname provided through Server Name Indication (SNI)
+  def hostname : String?
+    if host_name = LibSSL.ssl_get_servername(@ssl, LibSSL::TLSExt::NAMETYPE_host_name)
+      String.new(host_name)
+    end
+  end
 end