Native messaging

Extensions can exchange messages with native applications using an API that is similar to the other message passing APIs. Native applications that support this feature must register a native messaging host that can communicate with the extension. Chrome starts the host in a separate process and communicates with it using standard input and standard output streams.

Native messaging host

To register a native messaging host, the application must save a file that defines the native messaging host configuration.

An example of the file is as follows:

{
  "name": "com.my_company.my_application",
  "description": "My Application",
  "path": "C:\\Program Files\\My Application\\chrome_native_messaging_host.exe",
  "type": "stdio",
  "allowed_origins": ["chrome-extension://knldjmfmopnpolahpmmgbagdohdnhkik/"]
}

The native messaging host manifest file must be valid JSON and contain the following fields:

name

Name of the native messaging host. Clients pass this string to runtime.connectNative() or runtime.sendNativeMessage(). This name can only contain lowercase alphanumeric characters, underscores and dots. The name can't start or end with a dot, and a dot can't be followed by another dot.

description

Short application description.

path

Path to the native messaging host binary. On Linux and macOS the path must be absolute. On Windows it can be relative to the directory containing the manifest file. The host process is started with the current directory set to the directory that contains the host binary. For example if this parameter is set to C:\Application\nm_host.exe then it will be started with current directory `C:\Application`.

type

Type of the interface used to communicate with the native messaging host. This parameter has one possible value: stdio. It indicates that Chrome should use stdin and stdout to communicate with the host.

allowed_origins

List of extensions that should have access to the native messaging host. allowed_origins values can't contain wildcards.

Native messaging host location

The location of the manifest file depends on the platform.

On Windows, the manifest file can be located anywhere in the file system. The application installer must create a registry key, either HKEY_LOCAL_MACHINE\SOFTWARE\Google\Chrome\NativeMessagingHosts\com.my_company.my_application or HKEY_CURRENT_USER\SOFTWARE\Google\Chrome\NativeMessagingHosts\com.my_company.my_application, and set the default value of that key to the full path to the manifest file. For example, using the following command:

REG ADD "HKCU\Software\Google\Chrome\NativeMessagingHosts\com.my_company.my_application" /ve /t REG_SZ /d "C:\path\to\nmh-manifest.json" /f

or using the following .reg file:

Windows Registry Editor Version 5.00
[HKEY_CURRENT_USER\Software\Google\Chrome\NativeMessagingHosts\com.my_company.my_application]
@="C:\\path\\to\\nmh-manifest.json"

When Chrome looks for native messaging hosts, first the 32-bit registry is queried, then the 64-bit registry.

On macOS and Linux, the location of the native messaging host's manifest file varies by the browser (Google Chrome, Google Chrome for Testing or Chromium). The system-wide native messaging hosts are looked up at a fixed location, while the user-level native messaging hosts are looked up in the NativeMessagingHosts/ subdirectory of the user profile directory.

Note: In versions of Chrome earlier than Chrome 146, Google Chrome for Testing used the same locations as Google Chrome.

macOS (system-wide)

Google Chrome: /Library/Google/Chrome/NativeMessagingHosts/com.my_company.my_application.json

Google Chrome for Testing: /Library/Google/ChromeForTesting/NativeMessagingHosts/com.my_company.my_application.json

Chromium: /Library/Application Support/Chromium/NativeMessagingHosts/com.my_company.my_application.json

macOS (user-specific, default path)

Google Chrome: ~/Library/Application Support/Google/Chrome/NativeMessagingHosts/com.my_company.my_application.json

Google Chrome for Testing: ~/Library/Application Support/Google/ChromeForTesting/NativeMessagingHosts/com.my_company.my_application.json

Chromium: ~/Library/Application Support/Chromium/NativeMessagingHosts/com.my_company.my_application.json

Linux (system-wide)

Google Chrome: /etc/opt/chrome/native-messaging-hosts/com.my_company.my_application.json

Google Chrome for Testing: /etc/opt/chrome_for_testing/native-messaging-hosts/com.my_company.my_application.json

Chromium: /etc/chromium/native-messaging-hosts/com.my_company.my_application.json

Linux (user-specific, default path)

Google Chrome: ~/.config/google-chrome/NativeMessagingHosts/com.my_company.my_application.json

Google Chrome for Testing: ~/.config/google-chrome-for-testing/NativeMessagingHosts/com.my_company.my_application.json

Chromium: ~/.config/chromium/NativeMessagingHosts/com.my_company.my_application.json

Native messaging protocol

Chrome starts each native messaging host in a separate process and communicates with it using standard input (stdin) and standard output (stdout). The same format is used to send messages in both directions; each message is serialized using JSON, UTF-8 encoded and is preceded with 32-bit message length in native byte order. The maximum size of a single message from the native messaging host is 1 MB, mainly to protect Chrome from misbehaving native applications. The maximum size of the message sent to the native messaging host is 64 MiB.

The first argument to the native messaging host is the origin of the caller, usually chrome-extension://[ID of allowed extension]. This allows native messaging hosts to identify the source of the message when multiple extensions are specified in the allowed_origins key in the native messaging host manifest.

On Windows, the native messaging host is also passed a command line argument with a handle to the calling Chrome native window: --parent-window=<decimal handle value>. This lets the native messaging host create native UI windows that are correctly parented. Note that this value will be 0 if the calling context is a service worker.

When a messaging port is created using runtime.connectNative() Chrome starts a native messaging host process and keeps it running until the port is destroyed. On the other hand, when a message is sent using runtime.sendNativeMessage(), without creating a messaging port, Chrome starts a new native messaging host process for each message. In that case the first message generated by the host process is handled as a response to the original request, and Chrome will pass it to the response callback specified when runtime.sendNativeMessage() is called. All other messages generated by the native messaging host in that case are ignored.

Connecting to a native application

Sending and receiving messages to and from a native application is very similar to cross-extension messaging. The main difference is that runtime.connectNative() is used instead of runtime.connect(), and runtime.sendNativeMessage() is used instead of runtime.sendMessage().

To use these methods, the "nativeMessaging" permission must be declared in your extension's manifest file.

These methods are not available inside content scripts, only inside your extension's pages and service worker. To communicate from a content script to the native application, send the message to your service worker to pass it along to the native application.

Warning: Because content scripts share a renderer process with untrusted web pages, your service worker must validate sender.origin (or sender.url) and sanitize the message payload before forwarding any data to the native host.

The following example creates a runtime.Port object that's connected to native messaging host com.my_company.my_application, starts listening for messages from that port and sends one outgoing message:

const port = chrome.runtime.connectNative('com.my_company.my_application');
port.onMessage.addListener((msg) => {
  console.log('Received', msg);
});
port.onDisconnect.addListener(() => {
  if (chrome.runtime.lastError) {
    console.error(
      'Disconnected due to error:',
      chrome.runtime.lastError.message
    );
  } else {
    console.log('Disconnected');
  }
});
port.postMessage({text: 'Hello, my_application'});

Use runtime.sendNativeMessage to send a message to the native application without creating a port, e.g.:

chrome.runtime.sendNativeMessage(
  'com.my_company.my_application',
  {text: 'Hello'},
  (response) => {
    if (chrome.runtime.lastError) {
      console.error(
        'Error sending native message:',
        chrome.runtime.lastError.message
      );
      return;
    }
    console.log('Received', response);
  }
);

Debug native messaging

When native messaging failures occur, diagnostic output is written to Chrome's error log.

Linux and macOS

# Linux
google-chrome --enable-logging=stderr --log-level=1 2>&1 | \
  grep -E "native_messag|launch_context"

# macOS
/Applications/Google\ Chrome.app/Contents/MacOS/Google\ Chrome \
  --enable-logging=stderr --log-level=1 2>&1 | \
  grep -E "native_messag|launch_context"

Windows

Launch Chrome with logging enabled:

chrome.exe --enable-logging --log-level=1

Pass --user-data-dir="%TEMP%\nm-debug" to launch a separate instance without attaching to an existing Chrome process.

To view the output, stream chrome_debug.log using PowerShell:

$log = "$env:LOCALAPPDATA\Google\Chrome\User Data\chrome_debug.log"
Get-Content -Wait $log | Select-String "native_messag|launch_context"

You can also open chrome_debug.log in a text editor and search for launch_context.cc or native_message_process_host.cc.

Key logging details

• Manifest lookup and parse failures in launch_context.cc are logged as warnings. Use --log-level=1 rather than 2 (ERROR), which suppresses these startup diagnostics.
• Search for launch_context to find manifest and binary launch errors, and native_messag to find payload size and pipe communication errors.

Common errors

Here are some common errors and tips for solving them:

Failed to start native messaging host.

Check whether you have sufficient permissions to execute the native messaging host file.

Invalid native messaging host name specified.

Check whether the name contains invalid characters. Only lowercase alphanumeric characters, underscores, and dots are allowed. A name cannot start or end with a dot, and a dot cannot be followed by another dot.

Native host has exited.

The pipe to the native messaging host was broken before the message was read by Chrome. This is most likely initiated from your native messaging host.

Specified native messaging host not found.

Check the following:

• Is the name spelled correctly in the extension and in the manifest file?
• On Windows, does the registry key exist under HKEY_CURRENT_USER or HKEY_LOCAL_MACHINE, and does its default value point to the full manifest path? Chrome queries the 32-bit registry view first, then the 64-bit view. Use regedit to verify the key. See native messaging host location.
• On macOS and Linux, is the manifest file located in the expected directory and named after the host (such as com.my_company.my_application.json)? See native messaging host location.
• Is the manifest file in the correct format? In particular, is the JSON valid and well-formed and do the values match the definition of a native messaging host manifest?
• Does the file specified in path exist? On Windows, paths may be relative, but on macOS and Linux, the paths must be absolute.

Access to the specified native messaging host is forbidden.

Is the extension's origin listed in allowed_origins?

Error when communicating with the native messaging host.

This indicates an incorrect implementation of the communication protocol in the native messaging host.

• Make sure that all output in stdout adheres to the native messaging protocol. If you want to print some data for debugging purposes, write to stderr.
• Make sure that the 32-bit message length is in the platform's native integer format (little-endian / big-endian).
• The message length must not exceed 1024*1024.
• The message size must be equal to the number of bytes in the message. This may differ from the "length" of a string, because characters may be represented by multiple bytes.
• Windows-only: Make sure that the program's I/O mode is set to O_BINARY. By default, the I/O mode is O_TEXT, which corrupts the message format as line breaks (\n = 0A) are replaced with Windows-style line endings (\r\n = 0D 0A). The I/O mode can be set using __setmode.