SSL/TLS Client Authentication
Go/src/crypto/tls/common.go:289
// ClientAuth determines the server's policy for
// TLS Client Authentication. The default is NoClientCert.
ClientAuth ClientAuthType
// ClientAuthType declares the policy the server will follow for
// TLS Client Authentication.
type ClientAuthType int
const (
// NoClientCert indicates that no client certificate should be requested
// during the handshake, and if any certificates are sent they will not
// be verified.
NoClientCert ClientAuthType = iota
// RequestClientCert indicates that a client certificate should be requested
// during the handshake, but does not require that the client send any
// certificates.
RequestClientCert
// RequireAnyClientCert indicates that a client certificate should be requested
// during the handshake, and that at least one certificate is required to be
// sent by the client, but that certificate is not required to be valid.
RequireAnyClientCert
// VerifyClientCertIfGiven indicates that a client certificate should be requested
// during the handshake, but does not require that the client sends a
// certificate. If the client does send a certificate it is required to be
// valid.
VerifyClientCertIfGiven
// RequireAndVerifyClientCert indicates that a client certificate should be requested
// during the handshake, and that at least one valid certificate is required
// to be sent by the client.
RequireAndVerifyClientCert
)
The Ultimate Guide to SSL/TLS Client Authentication: Know How it Works https://aboutssl.org/ssl-tls-client-authentication-how-does-it-works/