Tirith (StackGuardian Policy Framework)
Maintainers
This project is maintained by StackGuardian.
A call for contributors
We are calling for contributors to help build out new features, review pull requests, fix bugs, and maintain overall code quality. If you're interested, please email us at team[at]stackguardian.io or get started by reading the contributing.md.
Tirith scans declarative Infrastructure as Code (IaC) configurations like Terraform against policies defined using JSON.
Content
- What is Tirith?
- Features
- Installation
- Usage
- Example Tirith policies
- Getting Started
- Want to contribute?
- Submitting a feedback
- Support
- License
What is Tirith?
Tirith is a policy framework developed by StackGuardian for enforcing policies on infrastructure configurations such as Terraform, CloudFormation, Kubernetes etc. It simplifies policy creation and enforcement ensuring compliance with infrastructure policies through a user-friendly approach.
Who is the project for?
- DevSecOps engineers
- Infrastructure architects
- Cloud administrators
- Anyone involved in managing and enforcing infrastructure guardrails
Why is it important and useful for users?
- Simplifies Policy Management: Managing policies in IaC can be complex and costly, requiring multiple codebases. Tirith abstracts these complexities, allowing for centralized and streamlined policy management.
- Extends Beyond Resource Configurations: Tirith's policies cover more than just resource configurations, including cost management and CI/CD definitions, offering a comprehensive compliance solution.
- Cost-Efficient: Maintaining policies within IaC logic is expensive. Tirith reduces costs by centralizing policy management, eliminating the need for duplicate policies across different IaC codebases.
- Eases Policy Creation: Writing Policy as Code is challenging. Tirith simplifies this by providing an intuitive, declarative approach, making it easier to ensure compliance and security.
Features
- An easy to read and simple way to define policy as code against structured formats.
- Use providers to define policies for terraform plan, infracost or any abstract JSON.
- Easily evaluate inputs against policy using pre-defined evaluators like ContainedIn, Equals, RegexMatch etc.
- Write your own provider (plugin) by leveraging a highly extensible and pluggable architecture to support any input formats.
Installation
For users
pip install git+https://github.com/StackGuardian/tirith.gitFor developers
Running the Dev Container
- Clone the repository to your local machine:
git clone <repository-url>
cd <repository-folder>-
Start the Docker Engine using docker desktop or CLI.
-
Open the project folder in Visual Studio Code
-
Once inside VS Code, open the Command Palette
(Ctrl+Shift+P or Cmd+Shift+P on macOS)and search for Dev Containers: Rebuild and Reopen in Container. Select this option. -
VS Code will build the dev container based on the devcontainer.json file or Docker configuration provided in the project. This may take a few minutes.
-
Once the container is up, you will have a fully configured development environment running inside Docker.
Reference Links:
https://code.visualstudio.com/docs/devcontainers/containers#_managing-containers
Manual Installation
Here we are going to install Tirith in a Python virtual environment.
-
Clone the Tirith repository to your system
git clone https://github.com/StackGuardian/tirith.git -
Change directory to the cloned repository
cd tirith -
Setup a virtualenv
virtualenv .venv -
Activate the virtualenv
source .venv/bin/activate -
Install Tirith in the virtualenv
# The -e is optional, just in case you wanna make some changes to the codebase pip install -e . -
Verify that Tirith is installed
tirith --version
1.0.0-beta.12
Congratulations! Tirith has been setup in your system
Usage
usage: tirith [-h] [-policy-path PATH] [-input-path PATH] [--json] [--verbose] [--version]
Tirith (StackGuardian Policy Framework)
options:
-h, --help show this help message and exit
-policy-path PATH Path containing Tirith policy as code
-input-path PATH Input file path
--json Only print the result in JSON form (useful for passing output to other programs)
--verbose Show detailed logs of from the run
--version show program's version number and exit
About Tirith:
* Abstract away the implementation complexity of policy engine underneath.
* Simplify creation of declarative policies that are easy to read and interpret.
* Provide a standard framework for scanning various configurations with granularity.
* Provide modularity to enable easy extensibility
* Github - https://github.com/StackGuardian/tirith
* Docs - https://docs.stackguardian.io/docs/tirith/overviewExample Tirith policies
Examples using various providers
Terraform plan provider
#### Example 1:
VPC and EC2 instance policy
- AWS VPC instance_tenancy is "default"
- EC2 instance cannot be destroyed
```json
{
"meta": {
"required_provider": "stackguardian/terraform_plan",
"version": "v1"
},
"evaluators": [
{
"id": "check_ec2_tenancy",
"provider_args": {
"operation_type": "attribute",
"terraform_resource_type": "aws_vpc",
"terraform_resource_attribute": "instance_tenancy"
},
"condition": {
"type": "Equals",
"value": "default"
}
},
{
"id": "destroy_ec2",
"provider_args": {
"operation_type": "action",
"terraform_resource_type": "aws_instance"
},
"condition": {
"type": "ContainedIn",
"value": ["destroy"]
}
}
],
"eval_expression": "check_ec2_tenancy && !destroy_ec2"
}
```
Make sure that all `aws_s3_bucket` are referenced by `aws_s3_bucket_intelligent_tiering_configuration` (using Terraform plan provider)
```json
{
"meta": {
"required_provider": "stackguardian/terraform_plan",
"version": "v1"
},
"evaluators": [
{
"id": "s3HasLifeCycleIntelligentTiering",
"description": "Make sure all aws_s3_bucket are referenced by aws_s3_bucket_intelligent_tiering_configuration",
"provider_args": {
"operation_type": "direct_references",
"terraform_resource_type": "aws_s3_bucket",
"referenced_by": "aws_s3_bucket_intelligent_tiering_configuration"
},
"condition": {
"type": "Equals",
"value": true,
"error_tolerance": 0
}
}
],
"eval_expression": "s3HasLifeCycleIntelligentTiering"
}
```
#### Example 2:
Make sure that all AWS ELBs are attached to security group (using Terraform plan provider)
```json
{
"meta": {
"version": "v1",
"required_provider": "stackguardian/terraform_plan"
},
"evaluators": [
{
"id": "aws_elbs_have_direct_references_to_security_group",
"provider_args": {
"operation_type": "direct_references",
"terraform_resource_type": "aws_elb",
"references_to": "aws_security_group"
},
"condition": {
"type": "Equals",
"value": true,
"error_tolerance": 0
}
}
],
"eval_expression": "aws_elbs_have_direct_references_to_security_group"
}
```
#### Example 3:
Policy:
```json
{
"meta": {
"version": "v1",
"required_provider": "stackguardian/terraform_plan"
},
"evaluators": [
{
"id": "check1",
"provider_args": {
"operation_type": "attribute",
"terraform_resource_type": "aws_vpc",
"terraform_resource_attribute": "instance_tenancy"
},
"condition": {
"type": "Equals",
"value": "default"
}
},
"..."
{
"id": "check22",
"provider_args": {
"operation_type": "attribute",
"terraform_resource_type": "aws_vpc",
"terraform_resource_attribute": "intra_dedicated_network_acl"
},
"condition": {
"type": "Equals",
"value": false
}
}
],
"eval_expression": "check1 && check11 && check111 & check2 & check22"
}
```
Input:
```json
{
"format_version": "0.1",
"terraform_version": "0.14.11",
"variables": {
"amazon_side_asn": {
"value": "64512"
},
"assign_ipv6_address_on_creation": {
"value": false
},
"azs": {
"value": []
},
"cidr": {
"value": "10.0.0.0/18"
},
"create_database_internet_gateway_route": {
"value": false
},
"..."
"vpn_gateway_id": {
"default": "",
"description": "ID of VPN Gateway to attach to the VPC"
},
"vpn_gateway_tags": {
"default": {},
"description": "Additional tags for the VPN gateway"
}
}
}
```
Output:
JSON Output:
```json
{
"final_result": false,
"evaluators": [
{
"id": "check1",
"passed": true,
"result": [
{
"passed": true,
"message": "default is equal to default",
"meta": {
"address": "aws_vpc.this[0]",
"mode": "managed",
"type": "aws_vpc",
"name": "this",
"index": 0,
"provider_name": "registry.terraform.io/hashicorp/aws",
"change": {
"actions": [
"create"
],
"before": null,
"after": {
"assign_generated_ipv6_cidr_block": false,
"cidr_block": "10.0.0.0/18",
"enable_dns_hostnames": false,
"enable_dns_support": true,
"instance_tenancy": "default",
"tags": {
"Name": ""
},
"tags_all": {}
},
"after_unknown": {
"arn": true,
"default_network_acl_id": true,
"default_route_table_id": true,
"default_security_group_id": true,
"dhcp_options_id": true,
"enable_classiclink": true,
"enable_classiclink_dns_support": true,
"id": true,
"ipv6_association_id": true,
"ipv6_cidr_block": true,
"main_route_table_id": true,
"owner_id": true,
"tags": {},
"tags_all": {
"Name": true
}
}
}
}
},
{
"passed": true,
"message": "default is equal to default",
"meta": {
"address": "aws_vpc.this[0]",
"mode": "managed",
"type": "aws_vpc",
"name": "this",
"index": 1,
"provider_name": "registry.terraform.io/hashicorp/aws",
"change": {
"actions": [
"create"
],
"before": null,
"after": {
"assign_generated_ipv6_cidr_block": false,
"cidr_block": "10.0.0.0/18",
"enable_dns_hostnames": false,
"enable_dns_support": true,
"instance_tenancy": "default",
"tags": {
"Name": ""
},
"tags_all": {}
},
"after_unknown": {
"arn": true,
"default_network_acl_id": true,
"default_route_table_id": true,
"default_security_group_id": true,
"dhcp_options_id": true,
"enable_classiclink": true,
"enable_classiclink_dns_support": true,
"id": true,
"ipv6_association_id": true,
"ipv6_cidr_block": true,
"main_route_table_id": true,
"owner_id": true,
"tags": {},
"tags_all": {
"Name": true
}
}
}
}
}
],
"description": null
},
"..."
{
"id": "check2",
"passed": false,
"result": [
{
"message": "attribute: 'intra_acl_tags' is not found",
"passed": false
}
],
"description": null
},
{
"id": "check22",
"passed": false,
"result": [
{
"message": "attribute: 'intra_dedicated_network_acl' is not found",
"passed": false
}
],
"description": null
}
],
"errors": [],
"eval_expression": "check1 && check11 && check111 & check2 & check22"
}
```
Infracost Provider
Cost control policy
#### Example 1
- EC2 instance cost is lower than 100 USD per month
```json
{
"meta": {
"required_provider": "stackguardian/infracost",
"version": "v1"
},
"evaluators": [
{
"id": "ec2_cost_below_100_per_month",
"provider_args": {
"operation_type": "total_monthly_cost",
"resource_type": ["aws_ec2"]
},
"condition": {
"type": "LessThanEqualTo",
"value": 100
}
}
],
"eval_expression": "ec2_cost_below_100_per_month"
}
```
#### Example 2
Policy:
```json
{
"meta": {
"version": "v1",
"required_provider": "stackguardian/infracost"
},
"evaluators": [
{
"id": "cost_check_1",
"provider_args": {
"operation_type": "total_monthly_cost",
"resource_type": [
"*"
]
},
"condition": {
"type": "LessThanEqualTo",
"value": 20
}
},
{
"id": "cost_check_2",
"provider_args": {
"operation_type": "total_monthly_cost",
"resource_type": [
"aws_eks_cluster",
"aws_s3_bucket"
]
},
"condition": {
"type": "LessThanEqualTo",
"value": -1
}
}
],
"eval_expression": "cost_check_1 && cost_check_2"
}
```
Input:
```json
{
"timeGenerated": "2022-04-03T15:19:53.271995639Z",
"summary": {
"totalUnsupportedResources": 0.0,
"totalUsageBasedResources": 1.0,
"totalNoPriceResources": 1.0,
"noPriceResourceCounts": {
"aws_s3_bucket_public_access_block": 1.0
},
"totalDetectedResources": 2.0,
"totalSupportedResources": 1.0,
"unsupportedResourceCounts": {}
},
"diffTotalHourlyCost": "0",
"projects": [
{
"name": "github.com/StackGuardian/template-tf-aws-s3-demo-website/tf_plan.json",
"pastBreakdown": {
...
}
}],
"pastTotalHourlyCost": "0",
"totalMonthlyCost": "100",
"diffTotalMonthlyCost": "0",
"currency": "USD",
"totalHourlyCost": "0",
"pastTotalMonthlyCost": "0",
"version": "0.2"
}
```
Output:
JSON Output:
```json
{
"meta": {
"version": "v1",
"required_provider": "stackguardian/infracost"
},
"final_result": false,
"evaluators": [
{
"id": "cost_check_1",
"passed": false,
"result": [
{
"passed": false,
"message": "300.1 is not less than or equal to 20",
"meta": null
}
],
"description": null
},
{
"id": "cost_check_2",
"passed": false,
"result": [
{
"passed": false,
"message": "100.1 is not less than or equal to -1",
"meta": null
}
],
"description": null
}
],
"errors": [],
"eval_expression": "cost_check_1 && cost_check_2"
}
```
StackGuardian Workflow Policy (using SG workflow provider)
- Terraform Workflow should require an approval to create or destroy resources
```json
{
"meta": {
"required_provider": "stackguardian/sg_workflow",
"version": "v1"
},
"evaluators": [
{
"id": "require_approval_before_creating_ec2",
"provider_args": {
"operation_type": "attribute",
"workflow_attribute": "approvalPreApply"
},
"condition": {
"type": "Equals",
"value": true
}
}
],
"eval_expression": "require_approval_before_creating_ec2"
}
```
#### Example 2
Policy:
```json
{
"meta": {
"version": "v1",
"required_provider": "stackguardian/sg_workflow"
},
"evaluators": [
{
"id": "wf_check_1",
"provider_args": {
"operation_type": "attribute",
"workflow_attribute": "useMarketplaceTemplate"
},
"condition": {
"type": "Equals",
"value": true
}
},
"..."
{
"id": "wf_check_14",
"provider_args": {
"operation_type": "attribute",
"workflow_attribute": "iacTemplateId"
},
"condition": {
"type": "Equals",
"value": "/stackguardian/s3-website:19"
}
}
],
"eval_expression": "wf_check_1 && wf_check_2 && wf_check_3 && wf_check_4 && wf_check_5 && wf_check_6 && wf_check_7 && wf_check_8 && wf_check_9 && wf_check_10 && wf_check_11 && wf_check_12 && wf_check_13 && wf_check_14"
}
```
Example Input:
```json
{
"DeploymentPlatformConfig": [
{
"config": {
"integrationId": "/integrations/aws-qa"
},
"kind": "AWS_RBAC"
}
],
"Description": "test",
"DocVersion": "V3.BETA",
"EnvironmentVariables": [
{
"config": {
"textValue": "eu-central-1",
"varName": "AWS_DEFAULT_REGION"
}}]
"..."
{
"schemaType": "FORM_JSONSCHEMA"
},
"iacVCSConfig": {
"iacTemplateId": "/stackguardian/s3-website:19",
"useMarketplaceTemplate": true
},
"WfStepsConfig": [],
"WfType": "TERRAFORM",
"_SGInternals": {}
}
```
Output:
JSON Output:
```json
{
"meta": {
"version": "v1",
"required_provider": "stackguardian/sg_workflow"
},
"final_result": false,
"evaluators": [
{
"id": "wf_check_1",
"passed": true,
"result": [
{
"passed": true,
"message": "True is equal to True",
"meta": null
}
],
"description": null
},
"..."
{
"id": "wf_check_11",
"passed": false,
"result": [
{
"passed": false,
"message": "True is not equal to False",
"meta": null
}
],
"description": null
},
],
"errors": [],
"eval_expression": "wf_check_1 && wf_check_2 && wf_check_3 && wf_check_4 && wf_check_5 && wf_check_6 && wf_check_7 && wf_check_8 && wf_check_9 && wf_check_10 && wf_check_11 && wf_check_12 && wf_check_13 && wf_check_14"
}
```
JSON
Example Policy
```json
{
"meta": {
"version": "v1",
"required_provider": "stackguardian/json"
},
"evaluators": [
{
"id": "check0",
"provider_args": {
"operation_type": "get_value",
"key_path": "z.b"
},
"condition": {
"type": "LessThanEqualTo",
"value": 1,
"error_tolerance": 2
}
},
{
"id": "check1",
"provider_args": {
"operation_type": "get_value",
"key_path": "a.b"
},
"condition": {
"type": "LessThanEqualTo",
"value": 1
}
},
{
"id": "check2",
"provider_args": {
"operation_type": "get_value",
"key_path": "c"
},
"condition": {
"type": "Contains",
"value": "aa"
}
},
{
"id": "check3",
"provider_args": {
"operation_type": "get_value",
"key_path": "nested_map.e.f"
},
"condition": {
"type": "Equals",
"value": "3"
}
},
{
"id": "check4",
"provider_args": {
"operation_type": "get_value",
"key_path": "list_of_dict.*.key1"
},
"condition": {
"type": "Equals",
"value": "value1"
}
},
{
"id": "check5",
"provider_args": {
"operation_type": "get_value",
"key_path": "nested_map"
},
"condition": {
"type": "Equals",
"value": { "e": { "f": "3" } }
}
}
],
"eval_expression": "check1 && check2 && check3 && check4 && check5"
}
```
Example Input
```json
{
"a": {
"b": 1
},
"c": ["aa", "bb"],
"nested_map": {
"e": {
"f": "3"
}
},
"list_of_dict": [
{
"key1": "value1"
},
{
"key1": "value1"
}
]
}
```
Output:
JSON Output
```json
{
"meta": {
"version": "v1",
"required_provider": "stackguardian/json"
},
"final_result": true,
"evaluators": [
{
"id": "check0",
"passed": null,
"result": [
{
"message": "key_path: `z.b` is not found (severity: 2)",
"passed": null
}
],
"description": null
},
{
"id": "check1",
"passed": true,
"result": [
{
"passed": true,
"message": "1 is less than equal to 1",
"meta": null
}
],
"description": null
},
{
"id": "check2",
"passed": true,
"result": [
{
"passed": true,
"message": "Found aa inside ['aa', 'bb']",
"meta": null
}
],
"description": null
},
{
"id": "check3",
"passed": true,
"result": [
{
"passed": true,
"message": "3 is equal to 3",
"meta": null
}
],
"description": null
},
{
"id": "check4",
"passed": true,
"result": [
{
"passed": true,
"message": "value1 is equal to value1",
"meta": null
},
{
"passed": true,
"message": "value1 is equal to value1",
"meta": null
}
],
"description": null
},
{
"id": "check5",
"passed": true,
"result": [
{
"passed": true,
"message": "{'e': {'f': '3'}} is equal to {'e': {'f': '3'}}",
"meta": null
}
],
"description": null
}
],
"errors": [],
"eval_expression": "check1 && check2 && check3 && check4 && check5"
```
Kubernetes
Kubernetes (using Kubernetes provider)
#### Example 1
- Make sure that all pods have a liveness probe defined
```json
{
"meta": {
"version": "v1",
"required_provider": "stackguardian/kubernetes"
},
"evaluators": [
{
"id": "kinds_have_null_liveness_probe",
"provider_args": {
"operation_type": "attribute",
"kubernetes_kind": "Pod",
"attribute_path": "spec.containers.*.livenessProbe"
},
"condition": {
"type": "Contains",
"value": null,
"error_tolerance": 2
}
}
],
"eval_expression": "!kinds_have_null_liveness_probe"
}
```
#### Example 2
Example Policy:
```json
{
"meta": {
"version": "v1",
"required_provider": "stackguardian/kubernetes"
},
"evaluators": [
{
"id": "kinds_have_null_liveness_probe",
"provider_args": {
"operation_type": "attribute",
"kubernetes_kind": "Pod",
"attribute_path": "spec.containers.*.livenessProbe"
},
"condition": {
"type": "Contains",
"value": null,
"error_tolerance": 2
}
}
],
"eval_expression": "!kinds_have_null_liveness_probe"
}
```
Example Input:
```yml
apiVersion: v1
kind: ServiceAccount
metadata:
name: wfs-demp-wfs-demo
labels:
helm.sh/chart: wfs-demo-0.1.0
app.kubernetes.io/name: wfs-demo
app.kubernetes.io/instance: wfs-demp
app.kubernetes.io/version: "1.16.0"
app.kubernetes.io/managed-by: Helm
---
# Source: wfs-demo/templates/user-acces.yaml
apiVersion: rbac.authorization.k8s.io/v1
...
- name: wget
image: busybox
command: ['wget']
args: ['wfs-demp-wfs-demo:80']
livenessProbe:
exec:
command:
- cat
- /tmp/healthy
initialDelaySeconds: 5
periodSeconds: 5
restartPolicy: Never
```
Output:
JSON Output:
```json
{
"meta": {
"version": "v1",
"required_provider": "stackguardian/kubernetes"
},
"final_result": false,
"evaluators": [
{
"id": "kinds_have_null_liveness_probe",
"passed": true,
"result": [
{
"passed": true,
"message": "Found None inside [None, {'exec': {'command': ['/tmp/healthy', 'cat']}, 'initialDelaySeconds': 5, 'periodSeconds': 5}]",
"meta": null
}
],
"description": null
}
],
"errors": [],
"eval_expression": "!kinds_have_null_liveness_probe"
}
```
Getting Started
This is a short getting started guide for Tirith. We will take a look on how we can use Tirith to guardrail a JSON input.
Create two files, one for input.json one for policy.json.
input.json
{
"path": "/stackguardian/wfgrps/test",
"verb": "POST",
"meta": {
"epoch": 1718860398,
"User-Agent": {
"name": "User-Agent",
"value": "PostmanRuntime/7.26.8"
}
}
}policy.json
{
"meta": {
"version": "v1",
"required_provider": "stackguardian/json"
},
"evaluators": [
{
"id": "can_post",
"provider_args": {
"operation_type": "get_value",
"key_path": "verb"
},
"condition": {
"type": "Equals",
"value": "POST"
}
},
{
"id": "wfgrps_path",
"provider_args": {
"operation_type": "get_value",
"key_path": "path"
},
"condition": {
"type": "RegexMatch",
"value": "/stackguardian/wfgrps/test.*"
}
},
{
"id": "epoch_less_than_8th_july_2024",
"provider_args": {
"operation_type": "get_value",
"key_path": "meta.epoch"
},
"condition": {
"type": "LessThan",
"value": 1720415598
}
}
],
"eval_expression": "can_post && wfgrps_path && epoch_less_than_8th_july_2024"
}Evaluating the policy against the input
To evaluate the policy against the input, run the following command:
tirith -input-path input.json -policy-path policy.jsonExplanation:
-
tirith:- This is the command to run the Tirith program, which is part of the StackGuardian Policy Framework.
-
-input-path input.json:- The
-input-pathoption specifies the path to the input file. - input.json is the file that contains the input data to be scanned by Tirith.
- The
-
-policy-path policy.json:- The
-policy-path optionspecifies the path to the policy file. - policy.json is the file that contains the policies (rules) defined in Tirith\'s policy as code.
- The
It should print:
Check: can_post
PASSED
Results:
1. PASSED: POST is equal to POST
Check: wfgrps_path
PASSED
Results:
1. PASSED: /stackguardian/wfgrps/test matches regex pattern /stackguardian/wfgrps/test.*
Check: epoch_less_than_8th_july_2024
PASSED
Results:
1. PASSED: 1718860398 is less than 1720415598
Passed: 3 Failed: 0 Skipped: 0
Final expression used:
-> can_post && wfgrps_path && epoch_less_than_8th_july_2024
✔ Passed final evaluatorWant to contribute?
If you're interested, please email us at team[at]stackguardian.io or get started by reading the contributing.md.
Getting an issue assigned
Go to the Tirith Repository and in the issues tab describe any bug or feature you want to add. If found relevant, the maintainers will assign the issue to you and you may start working on it as mentioned in the next section.
The kinds of issues a contributor can open:
- Report Bugs
- Feature Enhancement
- If any "help" is needed with using Tirith
A bug report
Head over to the Tirith repository and in the issues tab describe the bug you encountered and we will be happy to take a look into it.
Opening a Pull Request and getting it merged?
- Go to the repository and fork it.
- Clone the repository in your local machine.
- Open your terminal and
cd tirith - Create your own branch to work on the changes you intend to perform. For e.g. if you want some changes or bug fix to any function in the evaluators, name your branch with something relevant like,
git branch bug-fix-equals-evaluator - After necessary changes,
git push --set-upstream origin bug-fix-equals-evaluator,git checkout mainandgit merge bug-fix-equals-evaluatoror use the GUI to create a "Pull Request" after pushing it in the respective branch. - A review request will be sent to the repository maintainers and your changes will be merged if found relevant.
Submitting a Feedback
Wanna submit a feedback? It's as simple as writing and posting it in the feedback section.
Your feedback will help us improve
Support
License
Apache License 2.0
The Apache License is a permissive free software license written by the Apache Software Foundation (ASF). It allows all users to use the software for any purpose, to distribute it, to modify it, and to distribute modified versions of the software under the terms of the license, without concern for royalties.