Yea, an option to enable this can certainly be added. I'm not sure what you mean by auto magically, though. Can you elaborate on what you are picturing with that?

@dougwilson what @NoelDavies means is that you can automatically enable and add the required headers if you detect private IP like 127.0.0.1 or in the private ranges as specified here: https://wicg.github.io/private-network-access/#ip-address-space-heading

I'm personally against that as it defies the purpose of having these headers in the first place. But we can have some property like allowPrivateNetworkAccess if set to true CORS module can auto-add the required header Access-Control-Allow-Private-Network: true.

As I understand in the preflight request the client needs to use the header Access-Control-Request-Private-Network for OPTIONS preflight request to pass.

Chrome 98 seems to have started to add and require Access-Control-Request-Private-Network header.
https://developer.chrome.com/blog/private-network-access-preflight/

After disabling chrome://flags/#block-insecure-private-network-requests , Chrome still adds and requires the header.

For future visitors, you can do:

app.use('/', function (req, res, next) {
  if (req.headers["access-control-request-private-network"]) {
    res.setHeader("access-control-allow-private-network", "true");
  }
  next(null);
})

But I hope to see some flag in cors package.

Chrome Beta version 102, started the experiment again and the preflight requests are sent with Access-Control-Request-Private-Network header.

For future readers, disabling both the following flags are required, in case you wanna disable requesting the headers from the Chrome browser for now.

chrome://flags/#block-insecure-private-network-requests
chrome://flags/#private-network-access-send-preflights

I created PR #274 for this.

About the "maybe auto-magically?": user agents should only send the Access-Control-Request-Private-Network-header when they do requests to private networks. So, in my PR Access-Control-Allow-Private-Network: true is only returned when the user agent requested it in the CORS preflight.