tls: cli option to enable TLS key logging to file

Debugging HTTPS or TLS connections from a Node.js app with (for example)
Wireshark is unreasonably difficult without the ability to get the TLS
key log. In theory, the application can be modified to use the
`'keylog'` event directly, but for complex apps, or apps that define
there own HTTPS Agent (like npm), this is unreasonably difficult.

Use of the option triggers a warning to be emitted so the user is
clearly notified of what is happening and its effect.

doc/api/cli.md

@@ -679,6 +679,15 @@ added: v4.0.0
 Specify an alternative default TLS cipher list. Requires Node.js to be built
 with crypto support (default).
 
+### `--tls-keylog=file`
+<!-- YAML
+added: REPLACEME
+-->
+
+Log TLS key material to a file. The key material is in NSS `SSLKEYLOGFILE`
+format and can be used by software (such as Wireshark) to decrypt the TLS
+traffic.
+
 ### `--tls-max-v1.2`
 <!-- YAML
 added: v12.0.0
@@ -1073,6 +1082,7 @@ Node.js options that are allowed are:
 * `--throw-deprecation`
 * `--title`
 * `--tls-cipher-list`
+* `--tls-keylog`
 * `--tls-max-v1.2`
 * `--tls-max-v1.3`
 * `--tls-min-v1.0`

doc/node.1

@@ -302,6 +302,11 @@ Specify process.title on startup.
 Specify an alternative default TLS cipher list.
 Requires Node.js to be built with crypto support. (Default)
 .
+.It Fl -tls-keylog Ns = Ns Ar file
+Log TLS key material to a file. The key material is in NSS SSLKEYLOGFILE
+format and can be used by software (such as Wireshark) to decrypt the TLS
+traffic.
+.
 .It Fl -tls-max-v1.2
 Set default  maxVersion to 'TLSv1.2'. Use to disable support for TLSv1.3.
 .

lib/_tls_wrap.js

@@ -60,6 +60,8 @@ const {
 const { getOptionValue } = require('internal/options');
 const { validateString } = require('internal/validators');
 const traceTls = getOptionValue('--trace-tls');
+const tlsKeylog = getOptionValue('--tls-keylog');
+const { appendFile } = require('fs');
 const kConnectOptions = Symbol('connect-options');
 const kDisableRenegotiation = Symbol('disable-renegotiation');
 const kErrorEmitted = Symbol('error-emitted');
@@ -560,6 +562,8 @@ TLSSocket.prototype._destroySSL = function _destroySSL() {
 };
 
 // Constructor guts, arbitrarily factored out.
+let warnOnTlsKeylog = true;
+let warnOnTlsKeylogError = true;
 TLSSocket.prototype._init = function(socket, wrap) {
   const options = this._tlsOptions;
   const ssl = this._handle;
@@ -643,6 +647,24 @@ TLSSocket.prototype._init = function(socket, wrap) {
     }
   }
 
+  if (tlsKeylog) {
+    if (warnOnTlsKeylog) {
+      warnOnTlsKeylog = false;
+      process.emitWarning('Using --tls-keylog makes TLS connections insecure ' +
+        'by writing secret key material to file ' + tlsKeylog);
+      ssl.enableKeylogCallback();
+      this.on('keylog', (line) => {
+        appendFile(tlsKeylog, line, { mode: 0o600 }, (err) => {
+          if (err && warnOnTlsKeylogError) {
+            warnOnTlsKeylogError = false;
+            process.emitWarning('Failed to write TLS keylog (this warning ' +
+              'will not be repeated): ' + err);
+          }
+        });
+      });
+    }
+  }
+
   ssl.onerror = onerror;
 
   // If custom SNICallback was given, or if

src/node_options.cc

@@ -506,6 +506,10 @@ EnvironmentOptionsParser::EnvironmentOptionsParser() {
 
   AddOption("--napi-modules", "", NoOp{}, kAllowedInEnvironment);
 
+  AddOption("--tls-keylog",
+            "log TLS decryption keys to named file for traffic analysis",
+            &EnvironmentOptions::tls_keylog, kAllowedInEnvironment);
+
   AddOption("--tls-min-v1.0",
             "set default TLS minimum to TLSv1.0 (default: TLSv1.2)",
             &EnvironmentOptions::tls_min_v1_0,

src/node_options.h

@@ -161,6 +161,7 @@ class EnvironmentOptions : public Options {
   bool tls_min_v1_3 = false;
   bool tls_max_v1_2 = false;
   bool tls_max_v1_3 = false;
+  std::string tls_keylog;
 
   std::vector<std::string> preload_modules;
 

test/parallel/test-tls-enable-keylog-cli.js

@@ -0,0 +1,57 @@
+'use strict';
+const common = require('../common');
+if (!common.hasCrypto) common.skip('missing crypto');
+const fixtures = require('../common/fixtures');
+
+// Test --tls-keylog CLI flag.
+
+const assert = require('assert');
+const path = require('path');
+const fs = require('fs');
+const { fork } = require('child_process');
+
+if (process.argv[2] === 'test')
+  return test();
+
+const tmpdir = require('../common/tmpdir');
+tmpdir.refresh();
+const file = path.resolve(tmpdir.path, 'keylog.log');
+
+const child = fork(__filename, ['test'], {
+  execArgv: ['--tls-keylog=' + file]
+});
+
+child.on('close', common.mustCall((code, signal) => {
+  assert.strictEqual(code, 0);
+  assert.strictEqual(signal, null);
+  const log = fs.readFileSync(file, 'utf8');
+  assert(/SECRET/.test(log));
+}));
+
+function test() {
+  const {
+    connect, keys
+  } = require(fixtures.path('tls-connect'));
+
+  connect({
+    client: {
+      checkServerIdentity: (servername, cert) => { },
+      ca: `${keys.agent1.cert}\n${keys.agent6.ca}`,
+    },
+    server: {
+      cert: keys.agent6.cert,
+      key: keys.agent6.key
+    },
+  }, common.mustCall((err, pair, cleanup) => {
+    if (pair.server.err) {
+      console.trace('server', pair.server.err);
+    }
+    if (pair.client.err) {
+      console.trace('client', pair.client.err);
+    }
+    assert.ifError(pair.server.err);
+    assert.ifError(pair.client.err);
+
+    return cleanup();
+  }));
+}