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DNS Considerations for Kong Gateway

Kong Gateway provides the Kong Manager, which must be able to interact with the Admin API. This application is subject to security restrictions enforced by browsers, and Kong must send appropriate information to browsers in order for it to function properly.

These security restrictions use the applications’ DNS hostnames to evaluate whether the applications’ metadata satisfies the security constraints. As such, you must design your DNS structure to meet the requirements.

Quick guide

It is recommended you read through this document to understand why these requirements exist and how they function. In brief, your environment must meet one of the two criteria below:

  • Kong Manager and the Admin API are served from the same hostname, typically by placing the Admin API under an otherwise unused path, such as /_adminapi/.
  • Kong Manager and the Admin API are served from different hostnames with a shared suffix (e.g. kong.example for api.admin.kong.example and manager.admin.kong.example). Admin session configuration sets cookie_domain to the shared suffix.

The first option simplifies configuration in kong.conf, but requires an HTTP proxy in front of the applications (because it uses HTTP path-based routing). The Kong proxy can be used for this. The second option requires more configuration in kong.conf, but can be used without proxying the applications.

DNS clients

Two different DNS clients are available:

  • The legacy DNS client: This is the client enabled by default, its configuration is under DNS RESOLVER in kong.conf.
  • The new DNS client: This client follows the industry standard and is more performant. Its configuration is under New DNS RESOLVER. For more information, see Migrate to the new DNS client.

CORS

Understanding CORS

Cross-Origin Resource Sharing, or CORS, is a set of rules for web applications that make requests across origins, i.e. to URLs that do not share the same scheme, hostname, and port as the page making the request. When making a cross-origin request, browsers send an Origin request header, and servers must respond with a matching Access-Control-Allow-Origin (ACAO) header. If the two headers do not match, the browser will discard the response, and any application components that require that response’s data will not function properly.

For example, the following request/response pairs have matching CORS headers, and will succeed:

GET / HTTP/1.1
Host: example.com
Origin: http://example.net

HTTP/1.1 200 OK
Access-Control-Allow-Origin: http://example.net

GET / HTTP/1.1
Host: example.com
Origin: http://example.net

HTTP/1.1 200 OK
Access-Control-Allow-Origin: *

* indicates that any origin is allowed.

These requests do not have a matching pair of CORS headers, and will fail:

GET / HTTP/1.1
Host: example.com
Origin: http://example.net

HTTP/1.1 200 OK
Access-Control-Allow-Origin: http://badbadcors.example

GET / HTTP/1.1
Host: example.com
Origin: http://example.net

HTTP/1.1 200 OK

Missing CORS headers when CORS headers are expected results in failure.

CORS in the context of Kong Gateway

Kong Manager operate by issuing requests to the Admin API using JavaScript. These requests may be cross-origin depending on your environment. The Admin API obtains its ACAO header value from admin_gui_url in kong.conf.

You can configure your environment such that these requests are not cross-origin by accessing both the GUI and its associated API via the same hostname, e.g. by accessing Kong Manager at https://admin.kong.example/ and the Admin API at https://admin.kong.example/_api/. This option requires placing a proxy in front of both Kong Manager and the Admin API to handle path-based routing; you can use Kong’s proxy for this purpose. Note that the GUIs must be served at the root of their domains; you cannot place the APIs at the root and the GUI under a path.

Troubleshooting

CORS errors are shown in the browser developer console (for example, see documentation for Firefox and Chrome) with explanations of the specific issue. ACAO/Origin mismatches are most common, but other error conditions can appear as well.

For example, if you have mistyped your admin_api_uri, you will see something like the following:

Access to XMLHttpRequest at 'https://admin.kong.example' from origin 'https://manager.kong.example' has been blocked by CORS policy: Response to preflight request doesn't pass access control check: The 'Access-Control-Allow-Origin' header has a value 'https://typo.kong.example' that is not equal to the supplied origin.

These errors are generally self-explanatory, but if the issue is not clear, check the Network developer tool, find the requests for the path in the error, and compare its Origin request header and Access-Control-Allow-Origin response header (it may be missing entirely).

Cookies

Understanding cookies

Cookies are small pieces of data saved by browsers for use in future requests. Servers include a Set-Cookie header in their response headers to set cookies, and browsers include a Cookie header when making subsequent requests.

Cookies are used for a variety of purposes and offer many settings to tailor when a browser will include them to fit a particular use case. Of particular interest are the following directives:

  • Cookie scope, defined by the cookie’s Domain and Path directives. Absent these, cookies are sent only to the hostname that created them: a cookie created by example.com will not be sent with a request to www.example.com. When Domain is specified, cookies are sent to that hostname and its subdomains, so a cookie from example.com with Domain=example.com will be sent with requests to www.example.com.
  • The Secure directive, which determines whether a cookie can be sent over an unencrypted (HTTP rather than HTTPS) connection. A cookie with Secure cannot be sent over HTTP, and must be set using HTTPS.
  • The SameSite directive, which controls when a cookie can be sent with cross-origin requests. Note that cookies have a different notion of cross-origin than CORS and check against the domain suffix: while example.com sending a request to api.example.com is cross-origin for the purposes of CORS, a cookie with Domain=example.com is considered same-site for requests to api.example.com. SameSite=Strict cookies are only sent with same-site requests, Lax are sent when navigating to a link from another site, and None are sent with all cross-origin requests.

Cookies in the context of Kong Gateway

After you log in to Kong Manager, Kong stores session information in a cookie to recognize your browser during future requests. These cookies are created using the session plugin (via admin_gui_session_conf) or OpenID Connect plugin.

Configuration is more or less the same between each–the OpenID Connect plugin contains an embedded version of the session plugin, so while cookie handling code is the same, it is configured directly in the OpenID Connect plugin settings (admin_gui_auth_conf).

  • cookie_domain should match the common hostname suffix shared by the GUI and its API. For example, if you use api.admin.kong.example and manager.admin.kong.example for the Admin API and Kong Manager, cookie_domain should be admin.kong.example.

  • cookie_same_site should typically be left at its default, Strict.
    • None is not necessary if you have your DNS records and cookie_domain set following the examples in this document.
    • Lax is only needed if the GUI and API are on entirely separate hostnames, e.g. admin.kong.example for the API and manager.example.com for Kong Manager. This configuration is not recommended because Lax opens a vector for cross-site request forgery attacks. It may be needed in some development or testing environments, but should not be used in production.
  • cookie_secure controls whether cookies can be sent over unsecured (plaintext HTTP) requests. By default, it is set to true, which does not permit sending the cookie over unsecured connections. This setting should also remain on the default, but may be disabled in some development or testing environments where HTTPS is not used.

OpenID Connect uses the same settings, but prefixed with session_, e.g. session_cookie_name rather than cookie_name.

As with CORS, the above is not necessary if both the GUI and API use the same hostname, with both behind a proxy and the API under a specific path on the hostname.

Troubleshooting

Issues with session cookies broadly fall into cases where the cookie is not sent and cases where the cookie is not set. The network (for example, see documentation for Firefox or Chrome) and application/storage (see documentation for Firefox or Chrome) developer tools can assist with investigating these.

In the network tool, selecting individual requests will show their request and response headers. Successful authentication requests should see a Set-Cookie response header including a cookie whose name matches cookie_name setting, and subsequent requests should include the same cookie in the Cookie request header.

If Set-Cookie is not present, it may be being stripped by some intermediate proxy, or may indicate that the authentication handler encountered an error. There should typically be other evidence in the response status and body in the event of an error, and possible additional information in Kong’s error logs.

If the cookie is set but not sent, it may have been deleted or may not match requests that need it. The application/storage tool will show current cookies and their parameters. Review these to see if your requests do not meet the criteria to send the cookie (e.g. the cookie domain is not a suffix for a request that requires the cookie, or is not present) and adjust your session configuration accordingly.

If cookies are not present in application/storage, but were previously set with Set-Cookie, they may have since been deleted, or may have expired. Review the Set-Cookie information to see when the cookie was set to expire and subsequent requests to determine if any other response has issued a Set-Cookie that deleted it (by setting expiration to a date in the past).

This troubleshooting information may not immediately indicate the cause of the issue, but can help Kong Support pinpoint the cause. Please provide it in tickets if possible.