Add suport for TLS key logging

dnscrypt-proxy/config.go

@@ -92,6 +92,7 @@ type Config struct {
 	LogMaxBackups            int                         `toml:"log_files_max_backups"`
 	TLSDisableSessionTickets bool                        `toml:"tls_disable_session_tickets"`
 	TLSCipherSuite           []uint16                    `toml:"tls_cipher_suite"`
+	TLSKeyLogFile            string                      `toml:"tls_key_log_file"`
 	NetprobeAddress          string                      `toml:"netprobe_address"`
 	NetprobeTimeout          int                         `toml:"netprobe_timeout"`
 	OfflineMode              bool                        `toml:"offline_mode"`
@@ -143,6 +144,7 @@ func newConfig() Config {
 		LogMaxBackups:            1,
 		TLSDisableSessionTickets: false,
 		TLSCipherSuite:           nil,
+		TLSKeyLogFile:            "",
 		NetprobeTimeout:          60,
 		OfflineMode:              false,
 		RefusedCodeInResponses:   false,
@@ -628,6 +630,16 @@ func ConfigLoad(proxy *Proxy, flags *ConfigFlags) error {
 	proxy.skipAnonIncompatibleResolvers = config.AnonymizedDNS.SkipIncompatible
 	proxy.anonDirectCertFallback = config.AnonymizedDNS.DirectCertFallback
 
+	if len(config.TLSKeyLogFile) > 0 {
+		f, err := os.OpenFile(config.TLSKeyLogFile, os.O_APPEND|os.O_CREATE|os.O_WRONLY, 0600)
+		if err != nil {
+			dlog.Fatalf("Unable to create key log file [%s]: [%s]", config.TLSKeyLogFile, err)
+		}
+		dlog.Warnf("TLS key log file [%s] enabled", config.TLSKeyLogFile)
+		proxy.xTransport.keyLogWriter = f
+		proxy.xTransport.rebuildTransport()
+	}
+
 	if config.DoHClientX509AuthLegacy.Creds != nil {
 		return errors.New("[tls_client_auth] has been renamed to [doh_client_x509_auth] - Update your config file")
 	}

dnscrypt-proxy/example-dnscrypt-proxy.toml

@@ -223,6 +223,14 @@ cert_refresh_delay = 240
 # tls_cipher_suite = [52392, 49199]
 
 
+## Log TLS key material to a file, for debugging purposes only.
+## This file will contain the TLS master key, which can be used to decrypt
+## all TLS traffic to/from DoH servers.
+## Never ever enable except for debugging purposes with a tool such as mitmproxy.
+
+# tls_key_log_file = '/tmp/keylog.txt'
+
+
 ## Bootstrap resolvers
 ##
 ## These are normal, non-encrypted DNS resolvers, that will be only used

dnscrypt-proxy/xtransport.go

@@ -75,6 +75,7 @@ type XTransport struct {
 	proxyDialer              *netproxy.Dialer
 	httpProxyFunction        func(*http.Request) (*url.URL, error)
 	tlsClientCreds           DOHClientCreds
+	keyLogWriter             io.Writer
 }
 
 func NewXTransport() *XTransport {
@@ -93,6 +94,7 @@ func NewXTransport() *XTransport {
 		useIPv6:                  false,
 		tlsDisableSessionTickets: false,
 		tlsCipherSuite:           nil,
+		keyLogWriter:             nil,
 	}
 	return &xTransport
 }
@@ -187,6 +189,10 @@ func (xTransport *XTransport) rebuildTransport() {
 	tlsClientConfig := tls.Config{}
 	certPool, certPoolErr := x509.SystemCertPool()
 
+	if xTransport.keyLogWriter != nil {
+		tlsClientConfig.KeyLogWriter = xTransport.keyLogWriter
+	}
+
 	if clientCreds.rootCA != "" {
 		if certPool == nil {
 			dlog.Fatalf("Additional CAs not supported on this platform: %v", certPoolErr)