The Right Way of Setting <A Href=""> When It's a Local File

How can I create a link to a local file on a locally-run web page?

You need to use the file:/// protocol (yes, that's three slashes) if you want to link to local files.

<a href="file:///C:\Programs\sort.mw">Link 1</a>
<a href="file:///C:\Videos\lecture.mp4">Link 2</a>

These will never open the file in your local applications automatically. That's for security reasons which I'll cover in the last section. If it opens, it will only ever open in the browser. If your browser can display the file, it will, otherwise it will probably ask you if you want to download the file.

You cannot cross from http(s) to the file protocol

Modern versions of many browsers (e.g. Firefox and Chrome) will refuse to cross from the http(s) protocol to the file protocol to prevent malicious behaviour.

This means a webpage hosted on a website somewhere will never be able to link to files on your hard drive. You'll need to open your webpage locally using the file protocol if you want to do this stuff at all.

Why does it get stuck without file:///?

The first part of a URL is the protocol. A protocol is a few letters, then a colon and two slashes. HTTP:// and FTP:// are valid protocols; C:/ isn't and I'm pretty sure it doesn't even properly resemble one.

C:/ also isn't a valid web address. The browser could assume it's meant to be http://c/ with a blank port specified, but that's going to fail.

Your browser may not assume it's referring to a local file. It has little reason to make that assumption because webpages generally don't try to link to peoples' local files.

So if you want to access local files: tell it to use the file protocol.

Why three slashes?

Because it's part of the File URI scheme. You have the option of specifying a host after the first two slashes. If you skip specifying a host it will just assume you're referring to a file on your own PC. This means file:///C:/etc is a shortcut for file://localhost/C:/etc.

These files will still open in your browser and that is good

Your browser will respond to these files the same way they'd respond to the same file anywhere on the internet. These files will not open in your default file handler (e.g. MS Word or VLC Media Player), and you will not be able to do anything like ask File Explorer to open the file's location.

This is an extremely good thing for your security.

Sites in your browser cannot interact with your operating system very well. If a good site could tell your machine to open lecture.mp4 in VLC.exe, a malicious site could tell it to open virus.bat in CMD.exe. Or it could just tell your machine to run a few Uninstall.exe files or open File Explorer a million times.

This may not be convenient for you, but HTML and browser security weren't really designed for what you're doing. If you want to be able to open lecture.mp4 in VLC.exe consider writing a desktop application instead.

How to specify a local file within html using the file: scheme?

The file: URL scheme refers to a file on the client machine. There is no hostname in the file: scheme; you just provide the path of the file. So, the file on your local machine would be file:///~User/2ndFile.html. Notice the three slashes; the hostname part of the URL is empty, so the slash at the beginning of the path immediately follows the double slash at the beginning of the URL. You will also need to expand the user's path; ~ does no expand in a file: URL. So you would need file:///home/User/2ndFile.html (on most Unixes), file:///Users/User/2ndFile.html (on Mac OS X), or file:///C:/Users/User/2ndFile.html (on Windows).

Many browsers, for security reasons, do not allow linking from a file that is loaded from a server to a local file. So, you may not be able to do this from a page loaded via HTTP; you may only be able to link to file: URLs from other local pages.

How to write out the absolute path of a local file correctly when linking css file to html?

I guess this is a popular misconception between server-side and client-side execution context.

1. The webserver provides an html document for http://localhost/whatever/index.htm
2. Your browser reads that document and it's href-attributes. It tries to download the linked files too.
   
   
   • href="styles/style.css" a relative path. The browser will request the file at http://localhost/whatever/styles/style.css
   • href="/styles/style.css" a kind-of-absolute path. The browser will request the file at the root of the servers url http://localhost/styles/style.css
   • href="http://localhost/foobar/styles/style.css" an absolute path. The browser will try to download exactly from there.
   • href="file:///C:\...." a local path on your system. The server has nothing to do with it. The page may only work on your own system not when other people access it through the server from other computers.

Your browser should come some development tools. You can inspect what your browser is requesting and what your server is responding with that tools.

The answer is: use relative or kind-of-absolute urls here.

Creating download link to a file on a file server

You can use ASHX file (say, downloadfile.ashx) and use the following code (not tested, but it will be something like that) in it:

 Response.Clear();
 Response.ContentType = "application/octet-stream";
 Response.AddHeader("Content-Disposition", "attachment; filename=abc.txt");                                            
 Response.WriteFile(Server.MapPath("\\servername\folder1\folder2\folder3\abc.txt"));
 Response.End();

and then use this in your anchor tag like:

<a href="downloadfile.ashx"  target=""_blank"">Click me</a>

Note: You can also pass parameters for downloading different files like:

<a href="downloadfile.ashx?file=abc.txt"  target=""_blank"">Click me</a>

and then, in ashx file, use the file name to download the appropriate file.

How to use a relative path to link to another local file?

Remove the file:/// part to have just ../June/foo.txt.

This should help you out.

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