External references

This topic describes the external references feature for F5 WAF for NGINX.

External references in policy are code blocks that can be used as part of a policy without being part of the policy file. This means that you can have a set of pre-defined configurations for parts of the policy, and you can incorporate them as part of the policy by simply referencing them.

It allows you to separate policy information into smaller files, which can be easier to maintain for a large, complex policy. Another use case for external references is to build a dynamic policy that requires replaceable files.

You can create and populate specific files with the configuration relevant to your policy, and then compile the policy to include the latest version of these files, ensuring that your policy is always up to date when it comes to a constantly changing environment.

Updating a single file referenced in the policy will not trigger a policy compilation. This action needs to be done actively by reloading the NGINX configuration.

To use external references, replace the direct property in the policy file with the <ext-ref>Reference property, where <ext-ref> defines the replacement text for the property changed to singular (if originally plural) and notation converted from snake case to camelCase.

For example, a modifications section could be replaced by modificationsReference and data-guard could be replaced by dataGuardReference.

External reference types

There are different implementations based on the type of references that are being made.

URL references

URL reference is the method of referencing an external source by providing its full URL.

This is a useful method when trying to combine or consolidate parts of the policy that are present on different host machines.

You need to make sure that the server where the resource files are located is always available when you are compiling your policy.

This example creates a skeleton policy, enabling the file type violation.

It does not specify the file types as these file types depend on a separate application that defines these types. It populates this section from an external reference instead.

Note that the filetypes section is replaced by the filetypeReference section. In the filetypeReference section, there is key/value pair defining the type of reference and actual URL to use to reach that reference item.

For the content of the file itself, it is an extension of the original JSON format for the policy.

json{
    "name": "external_resources_file_types",
    "template": {
        "name": "POLICY_TEMPLATE_NGINX_BASE"
    },
    "applicationLanguage": "utf-8",
    "enforcementMode": "blocking",
    "blocking-settings": {
        "violations": [
            {
                "name": "VIOL_FILETYPE",
                "alarm": true,
                "block": true
            }
        ]
    },
    "filetypeReference": {
        "link": "http://domain.com:8081/file-types.txt"
    }
}

The referenced file-types.txt file contains the following code:

json[
    {
        "name": "*",
        "type": "wildcard",
        "allowed": true,
        "checkPostDataLength": false,
        "postDataLength": 4096,
        "checkRequestLength": false,
        "requestLength": 8192,
        "checkUrlLength": true,
        "urlLength": 2048,
        "checkQueryStringLength": true,
        "queryStringLength": 2048,
        "responseCheck": false
    },
    {
        "name": "pat",
        "allowed": false
    },
    {
        "name": "mat",
        "allowed": false
    }
]

HTTPS references

HTTPS references are a special case of URL references. It uses the HTTPS protocol instead of the HTTP protocol. Make sure that the webserver you are downloading the resources from does also support HTTPS protocol and has certificates setup properly.

Certificates must be valid in date (not expired) during the policy compilation.
Certificates must be signed by a trusted CA.
For Self-signed certificates, you need to make sure to add your certificates to the trusted CA of the machine where F5 WAF for NGINX is installed.
Certificates must use the exact domain name that the certificate was issued for. For example, SSL will differentiate between domain.com and www.domain.com, considering each a different domain name.

The following configuration builds on the default policy by defining a custom response page using an external file located on an HTTPS web server.

The external reference file contains the custom response page configuration.

json{
    "name": "external_references_custom_response",
    "template": {
        "name": "POLICY_TEMPLATE_NGINX_BASE"
    },
    "applicationLanguage": "utf-8",
    "enforcementMode": "blocking",
    "responsePageReference": {
        "link": "https://securedomain.com:8081/response-pages.txt"
    }
}

Content of the referenced file response-pages.txt:

json[
    {
        "responseContent": "<html><head><title>Custom Reject Page</title></head><body>This is a custom response page, it is supposed to overwrite the default page with custom text.<br><br>Your support ID is: <%TS.request.ID()%><br><br><a href='javascript:history.back();'>[Go Back]</a></body></html>",
        "responseHeader": "HTTP/1.1 200 OK\r\nCache-Control: no-cache\r\nPragma: no-cache\r\nConnection: close",
        "responseActionType": "custom",
        "responsePageType": "default"
    }
]

The next example enables all attack signatures while excluding specific signatures from being enforced:

json{
    "policy": {
        "name": "external_resources_signature_modification",
        "template": {
            "name": "POLICY_TEMPLATE_NGINX_BASE"
        },
        "applicationLanguage": "utf-8",
        "enforcementMode": "blocking",
        "signature-sets": [
            {
                "name": "All Signatures",
                "block": true,
                "alarm": true
            }
        ]
    },
    "modificationsReference": {
        "link": "http://my-domain.com:8081/modifications.txt"
    }
}

Content of the referenced file modifications.txt:

json{
    "modifications": [
        {
            "entityChanges": {
                "enabled": false
            },
            "entity": {
                "signatureId": 200001834
            },
            "entityType": "signature",
            "action": "add-or-update"
        }
    ]
}

File references

File references access local resources on the same machine, as opposed to accessing a remote resource on another server/machine.

File references do not work with remote hosts.

You can specify any location that is accessible by F5 WAF for NGINX except for the root folder ("/").

If a full path is not provided, the default path (/etc/app_protect/conf) will be used for resolution.

Here are some examples of typical cases:

Examples	File path	Notes
file:///foo.json	/etc/app_protect/conf/foo.json	Default directory assumed
file://foo.json	/etc/app_protect/conf/foo.json	Formally illegal, but tolerated as long as there is no trailing slash.
file:///etc/app_protect/conf/foo.json	/etc/app_protect/conf/foo.json	Full path, using default directory
file:///bar/foo.json	/bar/foo.json	Non-default path
file://etc/app_protect/conf/foo.json	Not accepted	“etc” is interpreted as remote host name

The next example enables all attack signatures, excluding specific signatures from enforcement using a reference to a local file on the machine.

json{
    "policy": {
        "name": "external_resources_signature_modification",
        "template": {
            "name": "POLICY_TEMPLATE_NGINX_BASE"
        },
        "applicationLanguage": "utf-8",
        "enforcementMode": "blocking",
        "signature-sets": [
            {
                "name": "All Signatures",
                "block": true,
                "alarm": true
            }
        ]
    },
    "modificationsReference": {
        "link": "file:///modifications.txt"
    }
}

Content of the referenced modifications.txt file:

json{
    "modifications": [
        {
            "entityChanges": {
                "enabled": false
            },
            "entity": {
                "signatureId": 200001834
            },
            "entityType": "signature",
            "action": "add-or-update"
        }
    ]
}

If the configuration is incorrect, the policy compilation process will fail with the following error:

APP_PROTECT { "event": "configuration_load_failure" ...

The error will vary based on the conditions of the failure. If the policy compilation process fails, the compiler will revert to the last working policy and the changes for the last policy compilation attempt will be discarded.

OpenAPI references

F5 WAF for NGINX can reference an OpenAPI specification file used to describe RESTful APIs, using it for API protection.

The specification file can be written in JSON or YAML format: the details can be used to create a protection policy.

Based on the content of the OpenAPI specification file, F5 WAF for NGINX will automatically create a policy for the following properties:

Methods
URLs
Parameters
JSON profiles

A policy template is provided with F5 WAF for NGINX packages containing violations related to OpenAPI configured to block (enforced) and is located on the path /etc/app_protect/conf/NginxApiSecurityPolicy.json.

F5 WAF for NGINX supports only one OpenAPI specification file reference per policy.

Any update of an OpenAPI Specification file referenced in the policy will not trigger a policy compilation. This action needs to be done actively by reloading the NGINX configuration.

Configuring and referencing OpenAPI specification files are similar to other external references.

URL references

This example adds an OpenAPI specification file reference using the link http://127.0.0.1:8088/myapi.yaml.

The referenced file configures the allowed data types for query_int and query_str parameter values.

json{
    "policy": {
        "name": "petstore_api_security_policy",
        "description": "F5 WAF for NGINX API Security Policy for the Petstore API",
        "template": {
            "name": "POLICY_TEMPLATE_NGINX_BASE"
        },
        "open-api-files": [
            {
                "link": "http://127.0.0.1:8088/myapi.yaml"
            }
        ],
        "blocking-settings": {
            "violations": [
                {
                    "block": true,
                    "description": "Disallowed file upload content detected in body",
                    "name": "VIOL_FILE_UPLOAD_IN_BODY"
                },
                {
                    "block": true,
                    "description": "Mandatory request body is missing",
                    "name": "VIOL_MANDATORY_REQUEST_BODY"
                },
                {
                    "block": true,
                    "description": "Illegal parameter location",
                    "name": "VIOL_PARAMETER_LOCATION"
                },
                {
                    "block": true,
                    "description": "Mandatory parameter is missing",
                    "name": "VIOL_MANDATORY_PARAMETER"
                },
                {
                    "block": true,
                    "description": "JSON data does not comply with JSON schema",
                    "name": "VIOL_JSON_SCHEMA"
                },
                {
                    "block": true,
                    "description": "Illegal parameter array value",
                    "name": "VIOL_PARAMETER_ARRAY_VALUE"
                },
                {
                    "block": true,
                    "description": "Illegal Base64 value",
                    "name": "VIOL_PARAMETER_VALUE_BASE64"
                },
                {
                    "block": true,
                    "description": "Disallowed file upload content detected",
                    "name": "VIOL_FILE_UPLOAD"
                },
                {
                    "block": true,
                    "description": "Illegal request content type",
                    "name": "VIOL_URL_CONTENT_TYPE"
                },
                {
                    "block": true,
                    "description": "Illegal static parameter value",
                    "name": "VIOL_PARAMETER_STATIC_VALUE"
                },
                {
                    "block": true,
                    "description": "Illegal parameter value length",
                    "name": "VIOL_PARAMETER_VALUE_LENGTH"
                },
                {
                    "block": true,
                    "description": "Illegal parameter data type",
                    "name": "VIOL_PARAMETER_DATA_TYPE"
                },
                {
                    "block": true,
                    "description": "Illegal parameter numeric value",
                    "name": "VIOL_PARAMETER_NUMERIC_VALUE"
                },
                {
                    "block": true,
                    "description": "Parameter value does not comply with regular expression",
                    "name": "VIOL_PARAMETER_VALUE_REGEXP"
                },
                {
                    "block": true,
                    "description": "Illegal URL",
                    "name": "VIOL_URL"
                },
                {
                    "block": true,
                    "description": "Illegal parameter",
                    "name": "VIOL_PARAMETER"
                },
                {
                    "block": true,
                    "description": "Illegal empty parameter value",
                    "name": "VIOL_PARAMETER_EMPTY_VALUE"
                },
                {
                    "block": true,
                    "description": "Illegal repeated parameter name",
                    "name": "VIOL_PARAMETER_REPEATED"
                }
            ]
        }
    }
}

Content of the referenced file myapi.yaml:

yamlopenapi: 3.0.1
info:
  title: 'Primitive data types'
  description: 'Primitive data types.'
  version: '2.5.0'
servers:
  - url: http://localhost
paths:
  /query:
    get:
      tags:
        - query_int_str
      description: query_int_str
      operationId: query_int_str
      parameters:
        - name: query_int
          in: query
          required: false
          allowEmptyValue: false
          schema:
            type: integer
        - name: query_str
          in: query
          required: false
          allowEmptyValue: true
          schema:
            type: string
      responses:
        200:
          description: OK
        404:
          description: NotFound

The following request will trigger an Illegal parameter data type violation, F5 WAF for NGINX expects to have an integer value in the query_int parameter:

http://localhost/query?query_int=abc

The link option is also available with the openApiFileReference property and synonymous with the one in open-api-files. openApiFileReference is not an array.

This next example references the same OpenAPI specification file as the previous one, using the openApiFileReference property.

json{
    "name": "openapifilereference-yaml",
    "template": {
        "name": "POLICY_TEMPLATE_NGINX_BASE"
    },
    "openApiFileReference": {
        "link": "http://127.0.0.1:8088/ref.txt"
    }
}

Content of the file ref.txt:

json[
    {
        "link": "http://127.0.0.1:8088/myapi.yaml"
    }
]

File references

This example adds an OpenAPI specification file reference to the default policy.

json{
    "name": "openapi-file-reference-json",
    "template": {
        "name": "POLICY_TEMPLATE_NGINX_BASE"
    },
    "open-api-files": [
        {
            "link": "file:///myapi2.json"
        }
    ]
}

Content of the referenced file myapi2.json:

json{
    "openapi": "3.0.1",
    "info": {
        "title": "Primitive data types2",
        "description": "Primitive data types.",
        "version": "2.5.1"
    },
    "servers": [
        {
            "url": "http://localhost"
        }
    ],
    "paths": {
        "/query": {
            "get": {
                "tags": [
                    "query_bool"
                ],
                "description": "query_bool",
                "operationId": "query_bool",
                "parameters": [
                    {
                        "name": "query_bool",
                        "in": "query",
                        "required": false,
                        "schema": {
                            "type": "boolean"
                        }
                    }
                ],
                "responses": {
                    "200": {
                        "description": "OK"
                    },
                    "404": {
                        "description": "NotFound"
                    }
                }
            }
        }
    }
}

The following request will trigger an Illegal repeated parameter name violation, as the OpenAPI specification doesn’t allow repeated parameters.

http://localhost/query?a=true&a=false

The request will not be blocked because this violation is set to alarm in the default policy.

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