Inspecting web sockets

Since Firefox 71, the Network Monitor has had the ability to inspect web socket connections. This article explores what functionality the Web Socket Inspector makes available.

Accessing the inspector

When you are inspecting a web app that utilizes a web socket connection, the web socket requests are listed in the list of requests in the Network Monitor along with all other requests.

WS filter in the network inspector

You can use the WS button to filter the list for just web socket connections. Only requests with the 101 status code (WebSocket Protocol Handshake) are visible, which indicates that the server is switching to a web socket connection.

Clicking on a web socket request opens the usual sidebar to reveal additional details, with the addition of a new Messages panel, which can be used to inspect web socket frames sent and received through the selected connection.

Messages panel in the web socket inspector

The live-updated table shows data for sent (green arrow) and received (red arrow) frames. Each frame expands on click, so you can inspect the formatted data.

Pausing web socket traffic

You can use the pause/resume button in the Network Monitor toolbar to stop intercepting web socket traffic. This allows you to capture only the frames that you are interested in.

Pausing the web socket inspector

Filtering web socket frames

To focus on specific messages, frames can be filtered using the filter at the top of the Messages panel.

web socket frame filter

There are also predefined filters, available in the tool bar of the message pane. Starting in Firefox 76, these are grouped into a selection list.

Screenshot showing the filter menu for WebSocket messages

The following filters are available:

All
Displays all messages (by default, except control messages).
Sent
Displays only messages sent by the browser (by default, except control messages).
Received
Displays only messages received from the server (by default, except control messages).
Control
(Available starting in Firefox 76). Displays messages for control frames (Ping, Pong, or Close). This filter can be combined with the others, to display, for example, only messages for control frames sent by the browser.

Columns in the Messages panel

In the Messages panel, you can choose to show the following information about each frame:

  • Data
  • Size
  • OpCode
  • MaskBit
  • FinBit
  • Time

the Data and Time columns are visible by default, but you can customize the interface to see more columns by choosing which ones to show from the Messages panel context menu.

columns in the messages panel

Expanding each frame

Selecting a frame in the list shows a preview of the data being sent in that frame at the bottom of the Messages panel.

web socket payload preview

Supported WS protocols

The inspector currently supports the following web socket protocols:

  • Plain JSON
  • Socket.IO
  • SockJS
  • SignalR
  • WAMP

The payload based on those protocols is parsed and displayed as an expandable tree for easy inspection, although you can of course still see the raw data (as sent over the wire) as well.

web socket protocol preview