Can Codespaces be configured to forward ports to localhost instead of 127.0.0.1

[andrewalderson]

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This is a (lazy) Developer Experience issue.

The Identity Provider I am using (Entra External Id) only allows insecure redirect urls to start with http://localhost. Currently, automatically opening a browser when serving the app will open it at http://127.0.0.1. This includes the http://localhost:{PORT} link printed in the console and the VS Code toast message to open a browser. This causes the authentication to fail. I can manually open the http://localhost url but that's just getting to be annoying.

I have been searching for a solution but currently myself and my trusty Copilot are stumped.

Is there a way to configure Codespaces to forward ports to http://localhost instead of http://127.0.0.1? Basically I would like the same experience in Codespaces that I have when I develop locally in a devcontainer.

[jhapendra-kandel]

No direct config to force Codespaces forward ports to http://localhost/ instead of http://127.0.0.1/ (they're basically the same thing — localhost resolves to 127.0.0.1 anyway).
But here's the easy fix for your Entra ID (Azure AD) insecure redirect issue:
Use the VS Code desktop app to connect to your Codespace (not the browser version).

Private forwarded ports bind to 127.0.0.1:PORT (or localhost:PORT) on your local machine automatically.
The toast message / link will use that localhost URL.
Entra External ID allows http://localhost/ (or better, register http://127.0.0.1:PORT as redirect URI — Microsoft recommends the IP literal to avoid glitches).

In browser Codespaces, it always uses the remote *.app.github.dev domain (secure, but not localhost).
Quick steps:

Install VS Code desktop if you don't have it.
In GitHub → Codespaces → Open in VS Code (desktop).
Run your app → port auto-forwards to localhost on your machine.
Update Entra app registration: Add http://127.0.0.1:YOUR_PORT (or localhost) as allowed redirect URI (http scheme OK for loopback during dev).

This gives the exact same dev experience as local devcontainer. No need to manually open links anymore!
If you're stuck on browser-only, no perfect workaround — you'd have to use the Codespaces domain for redirects (but Entra might not allow insecure http there).
Try desktop VS Code — it solves this lazy DX pain for most people.

[andrewalderson]

@jhapendra-kandel I am using the desktop version of VS Code. Also, Microsoft doesn't just recommend the localhost string literal, they actually enforce it

There is form validation preventing me from adding `http://127.0.0.1

I am a frontend dev and my knowledge of networking and port forwarding is limited. I know that localhost is the hostname for 127.0.0.1. When I develop locally in devcontainers ports get forwarded to the localhost hostname but in Codespaces all ports get forwarded to 127.0.0.1. Why does Codespaces use 127.0.0.1 instead of localhost?

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[jhapendra-kandel]

Hey @andrewalderson,
yeah Entra being picky with only accepting "localhost" sucks.
Quick try: in your app code, make sure it logs/prints the URL as http://localhost:PORT (not 127.0.0.1). A lot of frameworks default to the IP, change that and the toast/link in VS Code desktop usually switches to localhost too.
If it's still showing 127.0.0.1 in the Ports tab:

Right-click the port → Stop Forwarding
Forward it again manually
Sometimes that forces it to use the hostname version.

Worst case, just edit the URL in your browser from 127.0.0.1 → localhost each time, it works since they're the same.

[Rajveer-code]

Yeah, this is a pretty common annoyance with Codespaces + auth providers that are strict about localhost.

Short answer: there’s no built-in way to force Codespaces to use http://localhost instead of 127.0.0.1 for forwarded ports. It defaults to 127.0.0.1, and that’s what VS Code/toasts will use.

That said, a few workarounds that usually help:

1. Bind your app explicitly to localhost

Instead of binding to 0.0.0.0 or letting it default, try:

--host localhost

or in code/config:

host: "localhost"

Some frameworks will then print/use localhost URLs instead of 127.0.0.1.

2. Manually override the redirect URI (most reliable)

In your auth config (Entra External ID), explicitly set:

http://localhost:{PORT}/callback

and make sure your app uses that exact value for redirects, regardless of what Codespaces opens automatically.

3. Use the forwarded public URL (alternative approach)

Codespaces also gives you a URL like:

https://-.app.github.dev

If your IdP allows HTTPS redirects (most do), you can:

register that URL instead of localhost
avoid the localhost restriction entirely
4. Small DX workaround

Yeah, it’s a bit annoying, but the simplest flow right now is:

ignore the auto-opened tab
open http://localhost:{PORT} manually

You can also copy/paste once and reuse the tab during dev.

TL;DR
Codespaces currently defaults to 127.0.0.1 and isn’t configurable
Best fixes are:
bind explicitly to localhost
or use the forwarded HTTPS URL
or hardcode redirect URI

Would definitely be nice if Codespaces let us configure this — feels like a small DX improvement that would save a lot of friction.

[Aj-ziad]

Yes! In your devcontainer config, you can set the forwardPorts option and use "portsAttributes" to control behavior, but for the localhost vs 127.0.0.1 distinction specifically, you can configure it in your devcontainer.json:
json"forwardPorts": [3000],
"portsAttributes": {
"3000": {
"label": "App",
"onAutoForward": "openBrowser"
}
}
However, Codespaces forwards to localhost by default in the browser — localhost and 127.0.0.1 should resolve the same way in most cases. If you're hitting issues, the common fix is to make your app listen on 0.0.0.0 instead of 127.0.0.1, which ensures Codespaces can properly intercept and forward the port