Document updated on Jun 29, 2022

Forwarding query strings and headers

KrakenD is an API Gateway with a zero-trust policy, and when it comes to forward query strings, cookies, and headers, you need to define what is allowed.

Part of the zero-trust policy implies that KrakenD does not forward any unexpected query string, headers, or cookies. See below how to set the forwarding rules.

Configuration to enable parameter forwarding

You can change the default behavior according to your needs and define which elements can pass from the client to your backends. To do that, add the following configuration options under your endpoint definition:

• input_query_strings (array): Defines the exact list of query strings that are allowed to reach the backend when passed
• input_headers (array): Defines the list of all headers allowed to reach the backend when passed
• A single star element (["*"]) as the value of the options above, forwards everything to the backend (it’s safer avoiding this option)

Example:

Send the query strings items and page to the backend, and also User-Agent and Accept headers:

{
  "version": 3,
  "endpoints": [
    {
      "endpoint": "/v1/foo",
      "input_query_strings": [
        "items",
        "page"
      ],
      "input_headers": [
        "User-Agent",
        "Accept"
      ],
      "backend": [
        {
          "url_pattern": "/catalog",
          "host": [
            "http://some.api.com:9000"
          ]
        }
      ]
    }
  ]
}

Read below for further details and examples.

Query string forwarding

The zero-trust policy implies that, for instance, if a KrakenD endpoint /foo' receives the request /foo?items=10&page=2, all its declared backends are not going to see either itemsorpage`, unless otherwise configured.

To enable the transition of query strings to your backend, add the list input_query_strings in your endpoint definition. For instance, let’s forward ?items=10&page=2 to the backends now:

{
  "version": 3,
  "endpoints": [
    {
      "endpoint": "/v1/foo",
      "input_query_strings": [
        "items",
        "page"
      ],
      "backend": [
        {
          "url_pattern": "/catalog",
          "host": [
            "http://some.api.com:9000"
          ]
        }
      ]
    }
  ]
}

The input_query_strings list has the following behavior:

• Items in the list are forwarded to your backend when passed
• Additional query strings not in the list are removed from the final call
• Writing a single star element ("input_query_strings":["*"]) instead of individual strings, forwards everything to the backend

With this configuration, given a request like http://krakend:8080/v1/foo?items=10&page=2&evil=here, the backend receives items and page, but evil is missing.

Also, if a request like http://krakend:8080/v1/foo?items=10 does not include page, this parameter is simply missing in the backend request as well.

By definition, query string parameters are always optional, and the user can pass a subset of them, all or none. Suppose you want to enforce that the user provides a query string parameter. In that case, you must validate it with the Common Expression Language (faster) or with a Lua script (slower).

Sending all query string parameters

While the default policy prevents from sending unrecognized query string parameters, setting an asterisk * as the parameter name makes the gateway to forward any query string to the backends:

{
  "endpoint": "/foo",
  "input_query_strings":[
      "*"
  ]
}

Enabling the wildcard pollutes your backends, as any query string sent by end-users or malicious attackers gets through the gateway and impacts the backends behind. Our recommendation is to let the gateway know which query strings are in the API contract and specify them in the list, even when the list is long, and not use the wildcard. If the decision is to go with the wildcard, make sure your backends can handle abuse attempts from clients.

Mandatory query string parameters

When your backend requires mandatory query string parameters and you want to make them mandatory in KrakenD, the only way to enforce this (without scripting) is using the {variable} placeholders in the endpoints definition. Mandatory means that the endpoint won’t exist unless the parameter is passed. For instance:

{
  "endpoint": "/v3/{channel}/foo",
  "backend": [
    {
            "host": ["http://backend"],
            "url_pattern": "/foo?channel={channel}"
    }
  ]
}

The parameter is mandatory as if a value for channel is not provided the server replies with a 404.

With the configuration above a request to the KrakenD endpoint such as http://krakend/v3/iOS/foo?limit=10&evil=here makes a call to the backend with only the channel query string:

/foo?channel=iOS

Nevertheless, the input_query_strings could also be added in this configuration, creating a special case of optional and mandatory parameters! You would be passing query strings both hardcoded in the url_pattern and generated from the user input. In this strange case, if the user passes a single optional query string parameter that is declared in input_query_strings, then the mandatory value is lost. The mandatory value is used if the request does not contain any known optional parameter. For instance:

{
    "endpoint": "/v3/{channel}/foo",
    "input_query_strings": [
        "page",
        "limit"
    ],
    "backend": [
        {
            "host": [
                "http://backend"
            ],
            "url_pattern": "/foo?channel={channel}"
        }
    ]
}

With http://krakend/v3/iOS/foo?limit=10&evil=here the backend receives:

/foo?limit=10

No mandatory channel here! Because the optional parameter limit has been declared.

On the other hand, http://krakend/v3/iOS/foo?evil=here produces:

/foo?channel=iOS

No optional parameter has been passed, so the mandatory one is used.

Read the /__debug/ endpoint to understand how to test query string parameters.

Headers forwarding

KrakenD does not send client headers to the backend, unless they are under the input_headers list. The list of headers sent by the client that you want to let pass to the backend must be written as an entry of the input_headers array (or there is an "*" entry).

A client request from a browser or a mobile client contains a lot of headers, including cookies. Typical examples of the variety of headers that clients send are Host, Connection, Content-Type,Accept, Cache-Control, Cookie… and a long, long, etcetera. Remember that unless explicitly defined, KrakenD won’t let them pass. This security policy will save you from a lot of trouble.

Default headers sent from KrakenD to Backends

KrakenD will act as an independent client connecting to your backends and will send this headers with its own values:

• Accept-Encoding
• Host
• User-Agent (KrakenD Version v2.0)
• X-Forwarded-For
• X-Forwarded-Host
• X-Forwarded-Via (only when User-Agent is in the input_headers)

In addition, when you use tracing, you might also see arrive B3 propagation headers in your backends, e.g.:

• X-B3-Sampled
• X-B3-Spanid
• X-B3-Traceid

Overriding headers sent from KrakenD to Backends

When you use the input_headers, consider that any of the headers listed above are replaced with the ones you declare.

An example of passing the User-Agent to the backend:

{
  "version": 3,
  "endpoints": [
    {
      "endpoint": "/v1/foo",
      "input_headers": [
        "User-Agent"
      ],
      "backend": [
        {
          "url_pattern": "/catalog",
          "host": [
            "http://some.api.com:9000"
          ]
        }
      ]
    }
  ]
}

This setting changes the headers received by the backend to:

Accept-Encoding: gzip
Host: localhost:8080
User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10_13_4) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/67.0.3396.99 Safari/537.36
X-Forwarded-For: ::1

The User-Agent is no longer a KrakenD user-agent but a Mozilla one.

Read the /__debug/ endpoint to understand how to test headers.

Sending all client headers to the backends

While the default policy prevents forwarding unrecognized headers, setting an asterisk * as the parameter name makes the gateway to forward any header to the backends, including cookies:

{
  "endpoint": "/foo",
  "input_headers":[
      "*"
  ]
}

Enabling the wildcard pollutes your backends, as any header sent by end-users or malicious attackers gets through the gateway and impacts the backends behind (a famous exploit is the Log4J vulnerability). We recommend letting the gateway know which headers are in the API contract and specify them in the list, even when the list is long try to not use the wildcard. If the decision is to go with the wildcard, make sure your backends can handle abuse attempts from clients.

Cookies forwarding

A cookie is just some content passing inside the Cookie header. If you want cookies to reach your backend, add the Cookie header under input_headers, just as you would do with any other header.

When doing this, all your cookies are sent to all backends inside the endpoint. Use this option wisely!

Example:

{
  "version": 3,
  "endpoints": [
    {
      "endpoint": "/v1/foo",
      "input_headers": [
        "Cookie"
      ],
      "backend": [
        {
          "url_pattern": "/catalog",
          "host": [
            "http://some.api.com:9000"
          ]
        }
      ]
    }
  ]
}