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