Controlling Access with Firewalls

A firewall provides identity-agnostic access control for your App Engine app. The App Engine firewall allows you to define up to 1000 individual rules that either allow or restrict a range of IP addresses and subnets.

To configure a firewall, you need to define rules that specify a priority value, the range of IP addresses to either allow or deny, and an optional description.

To ensure that you've securely configured your app and set the appropriate levels of access, review Application Security as well as Access Control.

Structuring firewall rules

An App Engine firewall consists of an ordered list of rules that can allow or deny access from the specified IP address or range to your app. The rule applies to all resources of the App Engine application.

The firewall rules are ordered by importance, which you define as a numerical value in each rule's priority. You must specify a unique priority value for each rule as it defines the importance relative to the other rules in the firewall. The values for a rule's priority scale from the most important value of 1 up to the least important at value 2147483647.

Each firewall includes a default rule that is automatically created with the 2147483647 priority and applies to the entire IP range of your app. The default rule is always evaluated after all the other rules in the firewall and applied to all requests across all IP addresses.

The firewall evaluates the highest priority rule first. All the remaining rules in the firewall are sequentially evaluated until a rule matches the IP range of that request. When a matching rule is found, the connection is either allowed or denied, and all the remaining rules in the firewall are then skipped. If none of the manually defined rules in the firewall match the request, the default rule is evaluated.

For example, if you create a rule with priority 1 it is always evaluated first. If an incoming request matches the rule with priority 1, only that rule is evaluated and all the other rules in the firewall are skipped, including the default rule.

The example firewall below shows how a rule's priority can change the behavior of your firewall.

Before you begin

Before you can create a firewall for your app, you must have one of the following App Engine IAM roles, which include the necessary privileges for creating or modifying firewall rules:

  • App Engine Admin
  • Editor
  • Owner

If you want to use the gcloud tool or Admin API, you must first install and configure that tooling:

Preventing access to cached content

The App Engine firewall sits behind mechanisms that cache content, for example web proxies and browsers. When content is cached, that content is served publicly from the specific URL until it expires and can be accessed even after creating new firewall rules.

For apps running in the standard environment, the default expiration time for cached content is 10 minutes. For information about configuring the expiration time, see Static cache expiration.

To prevent your content from being cached, use the Cache-Control and Expires HTTP response headers. For more information about these HTTP headers, including how to control caching, see Avoiding caching.

For static content, you can configure your standard environment app to set HTTP response headers. For information about configuring HTTP response headers in your app's configuration file, see How Requests are Handled and the app.yaml reference.

Allowing requests from your services

You should consider the following points when creating rules:

  • By default, any request that does not match a rule is allowed access to your app. If you want to block all requests that do not match a specific rule, you must set the default rule to deny.
  • Task Queues and Cron traffic will be allowed by the firewall, even when the default rule is set to deny.
  • To control the access of requests from other App Engine apps or services, you might need to create rules to accommodate the IP addresses that are used for service-to-service communication. If your app communicates with other apps or services in App Engine, you must consider how to handle requests from the following IP addresses:

    • Requests from the URL Fetch service: 0.1.0.40
      • Requests received in the standard environment: 0.1.0.40
      • Requests received in the flexible environment: 0.1.0.40 and 10.0.0.1
    • Requests from the Blobstore or Cloud Storage: 0.1.0.30
    • Requests received only in the standard environment:
      • App deployment requests: 10.1.0.41

    Example

    Your app has a backend service running in the standard environment (backend_std). The service uses the URL Fetch service to communicate with backend_flex.

    You need to create two firewall rules to allow requests:

    • 0.1.0.40 - A rule to allow backend_flex to receive URL Fetch requests from backend_std.
    • 10.0.0.1 - A rule to allow the service-to-service communication for the URL Fetch requests in backend_flex.

Creating firewall rules

Use one of the following methods to create a firewall rule. Repeat these steps for each additional rule:

Console

Use the Firewall rules page in GCP Console to create a firewall rule:

  1. Go to the Create a firewall rule page in GCP Console:

    Go to the Create a firewall rule page

  2. Specify the details of the firewall rule:

    1. In Priority, enter an integer to specify the relative importance of the rule and define the order of when the rule is evaluated.

      Valid values are 1 to 2147483646. Priority 1 is the first rule evaluated. Priority 2147483647 is the last rule evaluated and is reserved for the default rule.

    2. In Action on match, specify whether to allow or deny access for requests that match the rule. Rules set to allow forward the request to the app. Rules set to deny respond to requests with a 403 Forbidden error.
    3. In IP range, define the range of IP addresses that apply to the rule. The IP address range must be defined in CIDR notation, can include subnet masks, and support both IPv4 and IPv6.
    4. Optional: In Description, include a description of the rule that is no longer than 100 characters.
  3. Click Save to create the rule.
  4. Test the rule to ensure that the priority and action provide the expected behavior:
    1. Click Test IP address.
    2. Enter the IP address that you want to validate and then click Test to ensure that the corresponding rule gets correctly evaluated.
gcloud

Run the following gcloud app firewall-rules commands to create a firewall rule:

  1. Run the following command to create a firewall rule:

    gcloud app firewall-rules create PRIORITY --action ALLOW_OR_DENY --source-range IP_RANGE --description DESCRIPTION
    where:
    • PRIORITY is an integer between 1 and 2147483646 that defines the rule's importance and order for which the rule is evaluated. Priority 1 is the first rule evaluated. Priority 2147483647 is the last rule evaluated and is reserved for the default rule.
    • ALLOW_OR_DENY specifies whether to allow or deny access for requests that match the rule. Valid values are allow or deny. Rules set to allow forward the request to the app. Rules set to deny respond to requests with a 403 Forbidden error.
    • IP_RANGE defines the range of IP addresses that apply to the rule. The IP range must be defined in CIDR notation, can include subnet masks, and support both IPv4 and IPv6.
    • DESCRIPTION is an optional description of the rule that is no longer than 100 characters.
  2. Run the following command to test your rule and ensure that the priority and action provide the expected behavior:
    gcloud app firewall-rules test-ip IP_ADDRESS
    where IP_ADDRESS is the IP address that you want to test against your firewall.
  3. Run the following command to view a list of the existing rules:
    gcloud app firewall-rules list
  4. Run the following command to delete an existing rule:
    gcloud app firewall-rules delete PRIORITY
    where PRIORITY is the priority value of the rule that you want to delete.
Examples:
Use the following examples to help you create your firewall:
  • Add a rule that allows an IPv6 address and subnet mask, and then test that rule to ensure it gets evaluated prior to your other rules:

    gcloud app firewall-rules create 123 --source-range fe80::3636:3bff:fecc:8778/128 --action allow
    gcloud app firewall-rules test-ip fe80::3636:3bff:fecc:8778
  • Add a rule to deny an IPv4 address and subnet mask, and then test that rule to ensure that it gets appropriately evaluated:

    gcloud app firewall-rules create 123456 --source-range "74.125.0.0/16" --action deny
    gcloud app firewall-rules test-ip 74.125.0.8
  • Update and then test the default rule to ensure that it restricts all IP addresses that don't match any other rules:

    gcloud app firewall-rules update default --action deny
    gcloud app firewall-rules test-ip 123.456.7.89
API

To programmatically create firewall rules for your App Engine app, you can use the apps.firewall.ingressRules methods in the Admin API.

To test a firewall rule and ensure that the priority and action provide the expected behavior, you can use the apps.firewall.ingressRules.list method and specify the IP address that you want to test within the matchingAddress parameter.

Example firewall

In this example, a company has set up a firewall to grant access to the engineering team and internal corporate network to their in-development app. The firewall rules have been created with large gaps between each priority to allow for growth.

Priority Action IP range Description
1000 Deny 192.0.2.1 Denies access to a DoS attacker.
2000 Allow 198.51.100.2 Allows access to an engineer in the satellite office.
3000 Deny 198.51.100.0/24 Denies access to all non-engineering buildings.
5000 Allow 203.0.113.0/24 Allows access to the main building's network.
2147483647 Deny * Default Action

After the firewall is created, assume that the following requests are directed at the sample app and note the app's response:

  • Request from 198.51.100.2 matches rule with priority 2000 and is allowed.
  • Request from 198.51.100.100 matches rule with priority 3000 and gets denied.
  • Request from 203.0.113.54 matches rule with priority 5000 and is allowed.
  • Request from 45.123.35.242 matches the default rule and gets denied.

Resolving conflicting rules

For example, assume that two of the priorities in the company's firewall are swapped. If the rules for priorities 2000 and 3000 are swapped, notice the unintended behavior.

Priority Action IP range Description
1000 Deny 192.0.2.1 Denies access to a DoS attacker.
2000 Deny 198.51.100.0/24 Denies access to all non-engineering buildings.
3000 Allow 198.51.100.2 Allows access to an engineer in the satellite office.
5000 Allow 203.0.113.0/24 Allows access to the main building's network.
2147483647 Deny * Default Action

The engineer in the satellite office will not be able to access the company's app as the rule's new priority means it will never be evaluated. The engineer's IP address 198.51.100.2 matches the rule that denies all non-engineers in the range 198.51.100.0/24 before the rule that allows access to the engineer's IP address.

To fix this, you must set the priority of the rule that allows access to 198.51.100.2 to be higher than the rule that denies access for the IP range 198.51.100.0/24.

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