Stream file from FTP server using FluentFTP to web client in ASP.NET Core

Question

I have a shared webhosting service which my ASP.NET Core app runs, and an FTP server. I want to serve the files that clients need to download from the site. I want the files not to be accessible to everyone, so this is what I do (I use FluentFTP):

var cred = new NetworkCredential(_config.UserName, _config.Password);
FtpClient client = new FtpClient(_config.Host, int.Parse(_config.Port), cred);
await client.ConnectAsync();
var remotePath = $"{_config.Folder}{FILE_DIR}/{filename}";
var result = await client.DownloadAsync(remotePath: remotePath, 0L);
await client.DisconnectAsync();
if (result != null)
{
     return File(result, "application/force-download", filename)
}
else
{
    throw new Exception("Failed");
}

The problem is, the server tries to download the file from the FTP server, then sends it to client. And there is the same problem for upload too, the client needs to upload file to the server, and then server uploads it to the FTP server. I think this can be very time-consuming with large files.

Is there any better way to achieve this goal? Or is it possible to write the file being downloaded from FTP simultaneously to the client response so the client downloads any bit of file downloaded in server and doesn't have to wait for the download to server to finish to start the download?

Thanks in advance.

Martin Prikryl · Accepted Answer · 2023-06-28T05:57:12.217

Download the file directly to the HTTP output stream with use of FtpClient.DownloadStream (FtpClient.Download in older versions) and HttpResponse.OutputStream:

Response.AddHeader("Content-Disposition", $"attachment; filename={filename}");

client.DownloadStream(Response.OutputStream, remotePath);

(where Response is ASP.NET HttpResponse).

A similar question for native .NET FtpWebRequest/WebClient:
Download file from FTP and how prompt user to save/open file in ASP.NET C#

Also note that the application/force-download seems like a hack.
Use Content-Disposition: attachment to force the download.

Stream file from FTP server using FluentFTP to web client in ASP.NET Core

1 Answers1