Web-based SSH clients basically consist of the following parts:
- Client-side: Typically JavaScript and dynamic HTML pages are used to capture keystrokes, transmit messages to/from the server and display the results in the user's web browser.
- Server-side/Web application: Incoming requests are processed on the web application server. Keyboard events are forwarded to a secure shell client communicating with the connectedSSH server. Terminal output is either passed to the client where it is converted into HTML via JavaScript or it is translated into HTML by the server before it is transmitted to the client.
Client-side terminal emulation
Web-based SSH servers that utilize client-side terminal emulation typically transmit the raw terminal output from the SSH server directly to the client. This has the advantage of offloading the process of translating terminal output into HTML on to the client. The disadvantage of this method is that it is limited by the capabilities of JavaScript and it will use up a non-trivial amount of the client's CPU and memory to process the incoming character stream. It also relies on the client to keep track of the terminal state and respond to escape sequences.
Client-side terminal emulator example:
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Server-side terminal emulation
Web-based SSH servers that utilize server-side terminal emulation typically keep track of the terminal screen and state in memory and convert it to HTML either when a screen update occurs or when the client expressly requests an update. The advantage of this method is that the state of the terminal remains persistent even if the user connects to their existing session(s) from a different web browser. It also enables the server to act upon terminal output even if the user is disconnected[1]. The disadvantage of this method is that it uses up more CPU and memory on the server.
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