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.）
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：
- 修改openssl.conf
sudo vim /etc/ssl/openssl.cnf
- 在 oid_section = new_oids 下增加：
openssl_conf = default_conf
- 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
- 保存文件并使之生效：
source /etc/profile
不生效则重启
Original: https://www.cnblogs.com/EminemJK/p/14500864.html
Author: 山治先生
Title: .Net Core部署Linux的坑
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