连接外部数据库问题

A connection was successfully established with the server, but then an error occurred during the pre-login handshake. (provider: SSL Provider, error: 31 - Encryption(ssl/tls) handshake failed)

具体错误：

System.Data.SqlClient.SqlException (0x80131904): A connection was successfully established with the server, but then an error occurred during the pre-login handshake. (provider: SSL Provider, error: 31 - Encryption(ssl/tls) handshake failed)
 ---> System.IO.EndOfStreamException: End of stream reached
   at System.Data.SqlClient.SNI.SslOverTdsStream.ReadInternal(Byte[] buffer, Int32 offset, Int32 count, CancellationToken token, Boolean async)
   at System.Data.SqlClient.SNI.SslOverTdsStream.Read(Byte[] buffer, Int32 offset, Int32 count)
   at System.IO.Stream.Read(Span`1 buffer)
   at System.Net.Security.SslStream.FillHandshakeBufferAsync[TIOAdapter](TIOAdapter adapter, Int32 minSize)
   at System.Net.Security.SslStream.ReceiveBlobAsync[TIOAdapter](TIOAdapter adapter)
   at System.Net.Security.SslStream.ForceAuthenticationAsync[TIOAdapter](TIOAdapter adapter, Boolean receiveFirst, Byte[] reAuthenticationData, Boolean isApm)
   at System.Net.Security.SslStream.ProcessAuthentication(Boolean isAsync, Boolean isApm, CancellationToken cancellationToken)
   at System.Net.Security.SslStream.AuthenticateAsClient(SslClientAuthenticationOptions sslClientAuthenticationOptions)
   at System.Net.Security.SslStream.AuthenticateAsClient(String targetHost, X509CertificateCollection clientCertificates, SslProtocols enabledSslProtocols, Boolean checkCertificateRevocation)
   at System.Net.Security.SslStream.AuthenticateAsClient(String targetHost)
   at System.Data.SqlClient.SNI.SNITCPHandle.EnableSsl(UInt32 options)
   at System.Data.SqlClient.SNI.SNIProxy.EnableSsl(SNIHandle handle, UInt32 options)
   at System.Data.SqlClient.SqlInternalConnectionTds..ctor(DbConnectionPoolIdentity identity, SqlConnectionString connectionOptions,

　原因是Ubuntu 20.04默认是最低的TLS是1.2版本，所以降级处理：

1. 修改openssl.conf

sudo vim /etc/ssl/openssl.conf

2. 在 oid_section = new_oids 下增加：

openssl_conf = default_conf  

3. 文件末尾追加：

[default_conf]
ssl_conf = ssl_sect
[ssl_sect]
system_default = system_default_sect
[system_default_sect]
MinProtocol = TLSv1
CipherString = DEFAULT@SECLEVEL=1

4. 保存文件并使之生效：

source /etc/profile