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Parameter Forwarding

Document updated on Jun 4, 2024

KrakenD is an API Gateway with a zero-trust security 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.

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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:

Fields of "endpoint"
* required fields
input_headers

array
Defines the list of all headers allowed to reach the backend when passed.
By default, KrakenD won’t pass any header from the client to the backend. This list is case-insensitive. You can declare headers in lowercase, uppercase, or mixed.
An entry ["Cookie"] forwards all cookies, and a single star element ["*"] as value forwards everything to the backend (it’s safer to avoid this option), including cookies. See headers forwarding
Defaults to []
input_query_strings

array
Defines the exact list of quey strings parameters that are allowed to reach the backend. This list is case-sensitive.
By default, KrakenD won’t pass any query string to the backend.
A single star element ["*"] as value forwards everything to the backend (it’s safer to avoid this option)
Defaults to []

Case-sensitive and case-insensitive parameters

  • The input_query_strings list is case sensitive, as per the RFC specification. For instance, a request ?Page=1 and ?page=1 are considered different parameters, and only the latter will pass when setting "input_query_strings": ["page"]. If you expect multiple cases, add them all.
  • The input_headers is case-insensitive, as per its RFC specification. It allows the passing of user headers in uppercase, lowercase, or mixed. Nevertheless, when the header is forwarded to the backend or used in other components, they receive it normalized in the canonical format of the MIME header, so users can mix capitalization and yet receive a consistent format.
Canonical Headers

While the input_headers declaration does not care about how you write the header (upper/lowercase), and KrakenD will access either way, when accessing or checking a header name through any components in KrakenD, you must write its canonical form regardless of what’s being provided by the user.

The canonicalization converts the first letter and any letter following a hyphen to upper case; the rest are converted to lowercase. For example, the canonical key for accept-encoding, ACCEPT-ENCODING, or ACCept-enCODING is Accept-Encoding. MIME header keys are assumed to be ASCII only. If the header contains a space or invalid header field bytes, it is returned without modifications.

If you get used to writing headers in canonical format, you will save yourself from a lot of trouble.

Example:

Send the query strings items and page to the backend, as well as 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 items or page, unless otherwise configured.

Add the list input_query_strings in your endpoint definition to enable the transition of query strings to your backend. 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.

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 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. We recommend letting the gateway know which query strings are in the API contract and specify them in the list, even when it is long, and not use the wildcard. If you decide to go with the wildcard, ensure your backends can handle client abuse attempts.

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 compulsory; 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 pass query strings hardcoded in the url_pattern and generated from the user input. In this strange case, the mandatory value is lost if the user passes a single optional query string parameter that is declared in input_query_strings. The compulsory value is used if the request contains no 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 headers sent by the client that you want to let pass to the backend must be written explicitly in the input_headers. See below how to forward all client headers (and why is it a bad idea).

A client request from a browser or a mobile client contains a lot of headers, including cookies. Typical examples of the variety of headers clients send are Host, Connection, Content-Type, Accept, Cache-Control, Cookie… and a 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 these headers setting its own values:

  • Accept-Encoding
  • Host
  • User-Agent (KrakenD Version 2.6.3)
  • X-Forwarded-For
  • X-Forwarded-Host
  • X-Forwarded-Via (only when User-Agent is in the input_headers)

Except for the X-Forwarded-like headers that are controlled by forwarded_by_client_ip, you can override the rest when present in the input_headers.

There are a few KrakenD components that support setting attributes in headers, like for instance propagate_claims in JWT. These components will send the additional headers you configure to the backend automatically.

Overriding headers sent from KrakenD to Backends

When you use the input_headers, consider that any 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). Let the gateway know which headers are in the API contract and specify them in the list. Even when the list is long, try not to use the wildcard. If the decision is to go with the wildcard, make sure your backends can handle client abuse attempts, and do not discard adding a second input_headers list in the backend (not all backends in aggregation might need every header).

Exception for X-Forwarded-like headers

Even though you allow to pass all headers with "input_headers": ["*"], you cannot override the headers X-Forwarded-For, X-Forwarded-Host, and X-Forwarded-Via which are automatically calculated by KrakenD.

If you’d like to take those headers into account, use the flag forwarded_by_client_ip under the router section

Granular header filtering

All headers listed in the input_headers parameter hit every single backend of the endpoint. If you want to add a second level of filtering, you can configure the input_headers list in the backend section too. Doing this allows you to have backends that receive fewer headers than other backends in the same endpoint.

For instance, the following endpoint allows passing two headers to its backends, but the second backend allows a single header to pass:

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

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 with any other header.

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

Example:

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

Scarf

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