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Opa Introduction

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OPA Filter (dgp.filter.http.opa)

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English

Overview

The dgp.filter.http.opa filter delegates authorization decisions to Open Policy Agent (OPA) via a Rego policy. This filter evaluates requests and determines whether to allow or deny based on the policy defined in Rego. The policy is provided as an inline Rego module and evaluated using OPA's built-in query engine.

What the filter does (current behavior)

• Loads a Rego module string from config.policy.
• Builds a Rego query from config.entrypoint.
• For each incoming request, constructs an input object and evaluates the query.
• If the query result is true, the request is allowed. Otherwise, the request is denied.

There is no built-in support for external policy files or URIs, custom HTTP status codes, or custom error bodies.

Configuration schema

Add the filter under your HTTP connection manager’s http_filters list.

filters:
  - name: dgp.filter.httpconnectionmanager
    config:
      route_config:
        # ... your routes
      http_filters:
        - name: dgp.filter.http.opa
          config:
            policy: |
              package http.authz

              default allow = false

              allow {
                input.method == "GET"
                input.path == "/status"
              }
            entrypoint: "data.http.authz.allow"
        # HTTP proxy filter should be after OPA filter
        - name: dgp.filter.http.proxy
          config:
          	# ... proxy config

Fields

• policy (string, required)
  • Meaning: The Rego module source code (inline string). Loaded via rego.Module("policy.rego", policy).
  • Datatype: string (multiline YAML recommended with |).
  • Notes: File paths or bundle URIs are not supported.
• entrypoint (string, required)
  • Meaning: The Rego query string passed to rego.Query(...). Should be a valid query like data.<package>.<rule> (e.g., data.http.authz.allow).
  • Datatype: string.

Decision contract

• If the query result is a non-empty set whose first expression value is true, the request continues.
• Otherwise (empty results or value ≠ true), the filter stops (request denied).

Policy input

The filter constructs an input object with the following keys, which correspond to the HTTP request.

input.method       # HTTP method string
input.path         # URL path (string)
input.headers      # map[string][]string
input.client_ip    # string
input.query        # map[string][]string (URL query)
input.host         # string
input.remote_addr  # string
input.user_agent   # string
input.route        # route entry object (opaque to policy; structure may change)
input.api          # API object (opaque)
input.params       # route params map

Minimal examples

1) Allow only GET /status

- name: dgp.filter.http.opa
  config:
    policy: |
      package http.authz
      default allow = false
      allow { input.method == "GET"; input.path == "/status" }
    entrypoint: "data.http.authz.allow"

2) Allow requests with a specific header value

- name: dgp.filter.http.opa
  config:
    policy: |
      package http.authz
      default allow = false
      allow {
        input.headers["x-api-key"][0] == "secret"
      }
    entrypoint: "data.http.authz.allow"

Limitations and notes

• Return type must be boolean: Only true allows; objects (e.g., {allow: true}) will not be interpreted specially.
• No custom deny status/body: The filter does not map policy outputs to HTTP status or body.
• Module-only loading: Policies are loaded from the inline policy string only.

Troubleshooting

• Denied unexpectedly: Confirm the query is correct (e.g., data.http.authz.allow), and that the policy returns true for the given input.
• Policy compile errors: Validate the Rego module with opa eval locally before embedding.
• Nil/empty results: Re-check access to headers/query (they are maps of lists), and confirm path/method match.