What Is a 511 Status Code?

The 511 Network Authentication Required status code indicates that the client must authenticate to gain network access. This status is most commonly returned by intercepting proxies, such as captive portals on public Wi-Fi or enterprise networks, not by origin servers. It is defined in RFC 6585, Section 6.

A 511 response is returned when:
The client attempts to access the web or another internet-facing resource.
The client has not authenticated with the local network (e.g., via a captive portal).
The network intercepts the request and redirects it to a login page using a 511 status, alerting non-browser software that this response is from the network layer — not the destination server.

Key Characteristics

Not cacheable: 511 responses must not be stored by caches.
Not shown with a challenge: Unlike a 401 or 407, the 511 response doesn’t include a standard WWW-Authenticate challenge header.
Usually redirects to a login page: The response often contains an HTML meta refresh or a link to a login server.
Only for intercepting proxies: This code is not used by websites directly; it’s intended for intermediate infrastructure that controls access.

Example Use Case

A user connects to a hotel Wi-Fi and tries to visit any webpage. Before they reach the site, they get redirected:

HTTP/1.1 511 Network Authentication Required
Content-Type: text/html

<html>
  <head>
    <meta http-equiv="refresh" content="0; url=https://login.hotelwifi.com/">
  </head>
  <body>
    <p>Please <a href="https://login.hotelwifi.com/">log in to access the network</a>.</p>
  </body>
</html>

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511 CODE REFERENCES

While the 511 Network Authentication Required status code is not widely used in standard web development, it is defined across multiple languages and frameworks—particularly those that support HTTP specification extensions or proxy-related middleware.

Here’s how the 511 status is referenced in various environments:

Common Language & Framework Constants for 511

Environment / Framework	Reference Constant
Symfony (PHP)	`Response::HTTP_NETWORK_AUTHENTICATION_REQUIRED`
.NET / C#	`HttpStatusCode.NetworkAuthenticationRequired`
Rust	`http::StatusCode::NETWORK_AUTHENTICATION_REQUIRED`
Rails	`:network_authentication_required`
Go	`http.StatusNetworkAuthenticationRequired`
Python 3.5+	`http.HTTPStatus.NETWORK_AUTHENTICATION_REQUIRED`
Apache HttpComponents	`org.apache.hc.core5.http.HttpStatus.SC_NETWORK_AUTHENTICATION_REQUIRED`
Angular	`HttpStatusCode.NetworkAuthenticationRequired`

Node.js/Express doesn’t offer a predefined constant for 511, but developers can manually send it using res.status(511).
This status is rarely generated by application logic—it is usually configured in network-level systems, such as captive portals, ISP firewalls, or enterprise gateways.

What Causes a 511 Status Code?

A 511 Network Authentication Required status code is typically returned by intercepting proxies, not origin servers. It signals that the client must authenticate to access the broader network, rather than just a particular site or service.

Common Causes:

Captive Portals
The most frequent use case. These are network setups (e.g., in hotels, airports, or cafes) that block internet access until the user logs in or accepts terms. Until authenticated, all HTTP traffic is rerouted to a login interface.
Corporate Firewall or Gateway Policies
In enterprise networks, clients may be blocked from general internet access unless authenticated via a local gateway or VPN.
ISP-Level Restrictions
Some ISPs enforce temporary blocks requiring identity verification or terms acceptance before re-enabling internet access.
MAC Address Filtering
Networks may classify users as “unknown” based on their device’s MAC address, denying access and triggering a 511 status until the client authenticates.
Custom Proxy Middleware
Developers implementing custom proxies or middleboxes can configure them to issue 511 errors as part of a broader access control mechanism.

Important:

The 511 response should not be used by origin servers like websites or APIs. It is meant exclusively for intercepting network infrastructure to alert the client to authentication requirements before normal web traffic is allowed.

How to Fix a 511 Status Code

A 511 status code means your device or client must authenticate with the network before accessing the requested resource. This is commonly encountered in public Wi-Fi setups, enterprise networks, or ISP-controlled environments.

For End Users

Open a Browser Window
Manually visit any non-HTTPS website like neverssl.com. This often triggers the captive portal or login page.
Authenticate via Captive Portal
Enter your login credentials, accept terms, or perform whatever action the network requires. Once complete, access will typically be granted.
Reconnect to the Network
Disconnect and reconnect to the Wi-Fi or network, which may refresh your session and prompt the authentication page again.
Disable VPN or Proxy Temporarily
These services can block captive portal detection. Disable them briefly to allow the authentication to go through.
Contact the Network Administrator
In offices or enterprise settings, your device may need to be registered or approved before accessing the internet.

For Developers / Admins

Implement Standard Captive Portal Behavior
Redirect unknown clients only through port 80 (HTTP) and respond with a 511 plus a proper HTML message or meta redirect to a login URL.
Avoid Full Page Login in the 511 Response
Include only redirection instructions (e.g., a meta refresh)—not full login interfaces—within the 511 message to prevent user agent misinterpretation.
Exclude 511 Responses from Caching
Ensure proxies and client-side caches do not store the 511 response using Cache-Control: no-store.
Log and Monitor Authentication Events
Track unauthenticated requests and successful logins to help debug or refine the login experience.

What Is the Difference Between a 401 and 511 Status Code?

Although both the 401 and 511 status codes relate to authentication, they serve distinct purposes and occur in different contexts:

Feature	401 Unauthorized	511 Network Authentication Required
Purpose	Client is not authenticated to the server or resource.	Client is not authenticated to the network itself.
Who Generates It	Origin server	Intercepting proxy or captive portal
When It Occurs	User tries to access a protected page without valid credentials	Client tries to access the internet without authenticating on the network
Authentication Location	Server-side credentials (e.g., login to app)	Network login page (e.g., Wi-Fi login splash page)
HTTP Header Used	`WWW-Authenticate`	Typically a `meta` redirect, no auth header
Common Scenario	Accessing a private API without a token	Connecting to hotel Wi-Fi without logging in

Summary

401: Used by servers to enforce access control for content or APIs.
511: Used by networks (especially via captive portals) to enforce gateway-level access control before internet access is allowed.

Frequently Asked Questions (FAQ)

What does a 511 status code mean?

A 511 status code means the client must authenticate to access the network. It’s commonly returned by captive portals like those on public Wi-Fi that require login before browsing the internet.

Is a 511 status code a client or server error?

It’s neither in the traditional sense. The 511 status is generated by an intercepting proxy (like a captive portal), not by a typical origin server or client error.

Can a 511 error be cached?

No. According to the RFC, responses with a 511 status must not be cached. This ensures clients always get the updated authentication state.

How do I resolve a 511 Network Authentication Required error?

To fix it, connect to the network’s login page and complete authentication. Usually, this means accepting terms or entering login credentials via a redirect page.

What causes a 511 status code on a private network?

It typically occurs when a device attempts to access the web without authenticating through the network’s captive portal, which is common in guest Wi-Fi or hotel networks.

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