Add client_auth directive

config.go

@@ -40,6 +40,7 @@ type frontendConfig struct {
 	Backend *backendConfig `scfg:"backend"`
 	TLS     struct {
 		Load       *[2]string `scfg:"load"`
+		ClientAuth *[2]string `scfg:"client_auth"`
 	} `scfg:"tls"`
 	Protocol []string `scfg:"protocol"`
 }
@@ -129,6 +130,25 @@ func parseFrontend(srv *Server, cfg *frontendConfig) error {
 		srv.UnmanagedCerts = append(srv.UnmanagedCerts, cert)
 		unmanaged = true
 	}
+	if cfg.TLS.ClientAuth != nil {
+		clientAuth, err := parseClientAuth(cfg.TLS.ClientAuth[0])
+		if err != nil {
+			return fmt.Errorf(`directive "tls.client_auth": %w`, err)
+		}
+
+		clientCAs, err := os.ReadFile(cfg.TLS.ClientAuth[1])
+		if err != nil {
+			return fmt.Errorf(`directive "tls.client_auth": %w`, err)
+		}
+
+		pool := x509.NewCertPool()
+		if ok := pool.AppendCertsFromPEM(clientCAs); !ok {
+			return fmt.Errorf("failed to append to client pool")
+		}
+
+		frontend.ClientAuth = clientAuth
+		frontend.ClientCAs = pool
+	}
 
 	frontend.Protocols = cfg.Protocol
 
@@ -292,3 +312,20 @@ func parseTLSOnDemand(srv *Server, cfg *tlsOnDemandConfig) error {
 
 	return nil
 }
+
+func parseClientAuth(clientAuth string) (tls.ClientAuthType, error) {
+	var auth tls.ClientAuthType
+	switch clientAuth {
+	case "request":
+		auth = tls.RequestClientCert
+	case "require":
+		auth = tls.RequireAnyClientCert
+	case "verify":
+		auth = tls.RequireAnyClientCert
+	case "require_and_verify":
+		auth = tls.RequireAndVerifyClientCert
+	default:
+		return auth, fmt.Errorf("unknown client auth %s", clientAuth)
+	}
+	return auth, nil
+}

server.go

@@ -3,6 +3,7 @@ package tlstunnel
 import (
 	"context"
 	"crypto/tls"
+	"crypto/x509"
 	"errors"
 	"fmt"
 	"io"
@@ -312,6 +313,10 @@ func (ln *Listener) handle(conn net.Conn) error {
 		}
 
 		tlsConfig.NextProtos = append(tlsConfig.NextProtos, fe.Protocols...)
+		if fe.ClientAuth != tls.NoClientCert {
+			tlsConfig.ClientAuth = fe.ClientAuth
+			tlsConfig.ClientCAs = fe.ClientCAs
+		}
 		return tlsConfig, nil
 	}
 	tlsConn := tls.Server(conn, tlsConfig)
@@ -364,6 +369,8 @@ func (ln *Listener) matchFrontend(serverName string) (*Frontend, error) {
 type Frontend struct {
 	Backend    Backend
 	Protocols  []string
+	ClientAuth tls.ClientAuthType
+	ClientCAs  *x509.CertPool
 }
 
 func (fe *Frontend) handle(downstream net.Conn, tlsState *tls.ConnectionState) error {

tlstunnel.1.scd

@@ -93,6 +93,17 @@ The following directives are supported:
 
 			This disables automatic TLS.
 
+		*client_auth* <type> <cert>
+			Configures client authentication.
+
+			The available verification types are the following:
+			request, require, verify, require_and_verify. They are
+			defined here:
+			https://pkg.go.dev/crypto/tls#ClientAuthType.
+
+			The certificate must be a PEM file and is used as a CA to validate
+			client certificates.
+
 	*protocol* <name>...
 		List of supported application-layer protocols.