Running sshfs through ssh proxy and passing in passwords why does this work?
Running sshfs Through SSH Proxy with Password Passing: A Deep Dive
This article meticulously examines the mechanics of mounting remote directories using sshfs through an SSH proxy, specifically addressing the seemingly paradoxical behavior of password input when using password stdin redirection. We’ll explore the underlying mechanisms to clarify why this approach functions despite the expectation of requiring multiple password entries.
Understanding the Double Password Prompt with sshfs and SSH Proxies
When manually mounting a remote directory via sshfs with an SSH proxy configured, users frequently encounter two password prompts. This occurs because sshfs internally uses two separate SSH connections: one to establish the proxy connection and another to connect to the final server. Each connection, independently managed by the SSH client, requires authentication, hence the dual password prompts. This inherent separation is crucial to grasp the efficacy of the password_stdin approach. The proxy connection and the server connection are distinct authentication events, even though both use the same underlying SSH protocol. The distinction is critical to understanding why simply piping the password might seem insufficient.
Dissecting the SSH Proxy Mechanism
The ProxyCommand directive within your SSH configuration instructs the client to execute a command – in this instance, another ssh command – to create a forwarded connection. This creates a tunnel through the proxy server, masking your true location and directing traffic appropriately. The -W %h:%p argument is vital; it specifies that the forwarded connection should use the hostname (%h) and port (%p) of the target server. This dynamically adjusts the tunnel’s destination based on the command attempting to use the proxy. The implications are substantial in the context of sshfs, which requires a separate connection to the target server for file transfer operations. This means that the sshfs command isn’t directly interacting with the remote server; instead, it communicates through the proxy tunnel, adding an extra layer of complexity for authentication.
The Role of sshfs in the Process
sshfs leverages the SSH protocol for secure file transfers. It constructs an SSH connection to the target server, enabling read and write operations to its filesystem. When a proxy is involved, sshfs implicitly relies on the connection established by the ProxyCommand. This means sshfs itself doesn’t directly manage the proxy connection; it inherits it. This subtle distinction is significant. While sshfs needs its own authentication, the authentication for the proxy connection is handled by the initial ssh command invoked by the ProxyCommand, independent of sshfs.
Why Password Stdin Redirection Works
The method of using sshfs -o password_stdin with password redirection (<<< $user_password) bypasses the explicit, interactive password prompts, yet successfully authenticates both connections. This seemingly contradictory result is due to a well-defined interaction between the SSH client, the proxy command, and the password_stdin option. Let’s unravel the intricacies.
The Mechanism of password_stdin
The password_stdin option instructs the SSH client to read the password from the standard input. This implies that if a password is supplied through standard input, it will bypass interactive prompting. Crucially, this applies to both the proxy connection and the server connection initiated by sshfs. Since the proxy connection, initiated by ProxyCommand, is executed before sshfs is called, the -W %h:%p part of the ProxyCommand establishes a secure connection, transparently handling its password requirement from the piped input. This doesn’t directly pass the password twice, but rather handles authentication for both levels independently, yet concurrently.
Sequential Authentication: A Key Insight
The apparent ‘single password’ scenario is deceptive. The command structure ensures sequential authentication. First, the ssh command within ProxyCommand is executed, reading the password from standard input and establishing the proxy connection. The subsequent sshfs command inherits this established, authenticated connection. sshfs then initiates its own connection to the target server. However, this connection is already established through the proxy, so the sshfs command’s authentication, also reading from the standard input, is handled using the same password – the only password actually entered. No additional prompting is required as the password is already provided to the initial connection, which is transparently used by subsequent connections.
Importance of Process Order and Standard Input
The precise ordering of commands and the use of standard input are paramount. The bash script carefully orchestrates the password input before invoking the commands. The <<< $user_password syntax redirects the content of the user_password variable to the standard input of sshfs. This ensures that both the proxy and the server connection receive the password concurrently, preventing additional prompts. The redirection technique essentially pre-empts the interactive prompt by supplying the credentials directly. The timing and the use of standard input are absolutely crucial, therefore the seemingly singular password entry handles both authentication requests silently. The elegance of this method lies in its efficient utilization of the existing SSH functionality.
Why Expect Failed (Possible Explanations)
The failure of expect to handle the proxy authentication might stem from its inability to reliably interact with the dynamic proxy connection established by the ProxyCommand. Expect often relies on pattern matching within the standard output, which can be problematic with the indirect communication pathway established by an SSH proxy. In contrast, the standard input redirection used with sshfs directly provides the password, bypassing potential complexities in parsing the output from multiple SSH connections. This direct approach proves more robust and effective in this scenario.
Conclusion: Efficiency and Robustness of Password Stdin Redirection
Utilizing sshfs -o password_stdin with password redirection delivers an efficient solution for mounting remote directories through an SSH proxy. This method avoids the inconvenience of repetitive password entries, streamlining the workflow. The careful orchestration of commands and the clever use of standard input for password delivery effectively bypass the anticipated dual authentication prompts. By understanding the intricacies of SSH proxying and sshfs authentication mechanisms, we can appreciate the efficiency and elegance of this approach. While other approaches might be employed, this method provides a straightforward and reliable solution to manage authentication complexity within this specific environment. The careful understanding of the sequence of events, the role of standard input redirection, and the inherent functionality of SSH and sshfs provide the keys to understanding why this technique works so well.