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https://github.com/kubemod/kubemod
Universal Kubernetes mutating operator
https://github.com/kubemod/kubemod
kubernetes kubernetes-admission-webhook kubernetes-operators kubernetes-resource
Last synced: 24 days ago
JSON representation
Universal Kubernetes mutating operator
- Host: GitHub
- URL: https://github.com/kubemod/kubemod
- Owner: kubemod
- License: bsd-3-clause
- Created: 2020-10-07T15:12:49.000Z (about 4 years ago)
- Default Branch: master
- Last Pushed: 2023-11-03T21:33:41.000Z (about 1 year ago)
- Last Synced: 2024-08-05T08:08:11.966Z (4 months ago)
- Topics: kubernetes, kubernetes-admission-webhook, kubernetes-operators, kubernetes-resource
- Language: Go
- Homepage:
- Size: 376 KB
- Stars: 264
- Watchers: 5
- Forks: 14
- Open Issues: 10
-
Metadata Files:
- Readme: README.md
- Changelog: CHANGELOG.md
- Contributing: CONTRIBUTING.md
- License: LICENSE
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README
[![Build Status][ci-img]][ci] [![Go Report Card][goreport-img]][goreport] [![Docker Image][docker-img]][docker]
# KubeMod
KubeMod is a universal [Kubernetes mutating operator](https://kubernetes.io/docs/reference/access-authn-authz/extensible-admission-controllers/).
It introduces `ModRule` - a custom Kubernetes resource that can intercept the deployment of any Kubernetes object and apply targeted modifications to it, or reject it before it is deployed to the cluster.
Use KubeMod to:
* Customize opaque Helm charts and Kubernetes operators.
* Build a system of policy rules to reject misbehaving resources.
* Develop your own sidecar container injections - no coding required.
* Derive metadata from external manifests.
* ...exercise your imagination :)
## Table of contents
* [Installation](#installation)
* [Deploying our first ModRule](#deploying-our-first-modrule)
* [Common use cases](#common-use-cases)
* [Modification of behavior](#modification-of-behavior)
* [Modification of metadata](#modification-of-metadata)
* [Sidecar injection](#sidecar-injection)
* [Resource rejection](#resource-rejection)
* [Anatomy of a ModRule](#anatomy-of-a-modrule)
* [Match section](#match-section)
* [Patch section](#patch-section)
* [Miscellaneous](#miscellaneous)
* [Operation type](#operation-type)
* [Execution tiers](#execution-tiers)
* [Namespaced and cluster-wide resources](#namespaced-and-cluster-wide-resources)
* [Synthetic references](#synthetic-references)
* [Target resources](#target-resources)
* [Note on idempotency](#note-on-idempotency)
* [Debugging ModRules](#debugging-modrules)
* [KubeMod's version of JSONPath](#kubemods-version-of-jsonpath)
* [Declarative kubectl apply](#declarative-kubectl-apply)
---
## Installation
KubeMod requires Kubernetes 1.21 or later, architecture AMD64 or ARM64.
As a Kubernetes operator, KubeMod is deployed into its own namespace — `kubemod-system`.
Run the following commands to deploy KubeMod.
```bash
# Make KubeMod ignore Kubernetes' system namespace.
kubectl label namespace kube-system admission.kubemod.io/ignore=true --overwrite
# Deploy KubeMod.
kubectl apply -f https://raw.githubusercontent.com/kubemod/kubemod/v0.20.0/bundle.yaml
```
By default KubeMod allows you to target a limited set of high-level resource types, such as deployments and services.
See [target resources](#target-resources) for the full list as well as instructions on how to expand or limit it.
### Upgrade
If you are upgrading from a previous version of KubeMod, run the following:
```bash
# Delete the KubeMod certificate generation job in case KubeMod has already been installed.
kubectl delete job kubemod-crt-job -n kubemod-system
# Make KubeMod ignore Kubernetes' system namespace.
kubectl label namespace kube-system admission.kubemod.io/ignore=true --overwrite
# Upgrade KubeMod operator.
kubectl apply -f https://raw.githubusercontent.com/kubemod/kubemod/v0.20.0/bundle.yaml
```
### Uninstall
To uninstall KubeMod and all its resources, run:
```bash
kubectl delete -f https://raw.githubusercontent.com/kubemod/kubemod/v0.20.0/bundle.yaml
```
**Note**: Uninstalling KubeMod will also remove all your ModRules deployed to all Kubernetes namespaces.
## Deploying our first ModRule
Once KubeMod is installed, you can deploy ModRules to intercept the creation and update of specific resources and perform modifications on them.
For example, here's a ModRule which intercepts the creation of Deployment resources whose `app` labels equal `nginx` and include at least one container of `nginx` version `1.14.*`.
The ModRule patches the matching Deployments on-the-fly to enforce a specific `securityContext` and add annotation `my-annotation`.
Since KubeMod intercepts and patches resources **before** they are deployed to Kubernetes, we are able to patch read-only fields such as `securityContext` without the need to drop and recreate existing resources.
```yaml
apiVersion: api.kubemod.io/v1beta1
kind: ModRule
metadata:
name: my-modrule
spec:
type: Patch
match:
# Match deployments ...
- select: '$.kind'
matchValue: Deployment
# ... with label app = nginx ...
- select: '$.metadata.labels.app'
matchValue: 'nginx'
# ... and at least one container whose image matches nginx:1.14.* ...
- select: '$.spec.template.spec.containers[*].image'
matchRegex: 'nginx:1\.14\..*'
# ... but has no explicit runAsNonRoot security context.
# Note: "negate: true" flips the meaning of the match.
- select: '$.spec.template.spec.securityContext.runAsNonRoot == true'
negate: true
patch:
# Add custom annotation.
- op: add
path: /metadata/annotations/my-annotation
value: whatever
# Enforce non-root securityContext and make nginx run as user 101.
- op: add
path: /spec/template/spec/securityContext
value: |-
fsGroup: 101
runAsGroup: 101
runAsUser: 101
runAsNonRoot: true
```
Save the above `ModRule` to file `my-modrule.yaml` and deploy it to the default namespace of your Kubernetes cluster:
```bash
kubectl apply -f my-modrule.yaml
```
After the `ModRule` is created, the creation of any nginx Kubernetes `Deployment` resource in the same namespace will be intercepted by KubeMod, and if the `Deployment` resource matches the ModRule's `match` section, the resource will be patched with the collection of `patch` operations.
To list all ModRules deployed to a namespace, run the following:
```bash
kubectl get modrules
```
## Common use cases
The development of KubeMod was motivated by the proliferation of Kubernetes Operators and Helm charts which are sometimes opaque to customizations and lead to runtime issues.
For example, consider these issues:
* [https://github.com/elastic/cloud-on-k8s/issues/2328](https://github.com/elastic/cloud-on-k8s/issues/2328)
* [https://github.com/jaegertracing/jaeger-operator/issues/1096](https://github.com/jaegertracing/jaeger-operator/issues/1096)
Oftentimes these issues are showstoppers that render the chart/operator impossible to use for certain use cases.
With the help of KubeMod we can make those charts and operators work for us. Just deploy a `ModRule` which targets the problematic primitive resource and patch it on the fly at the time it is created.
See the following sections for a number of typical use cases for KubeMod.
### Modification of behavior
Here's a typical black-box operator issue which can be fixed with KubeMod: [https://github.com/elastic/cloud-on-k8s/issues/2328](https://github.com/elastic/cloud-on-k8s/issues/2328).
The issue is that when the [Elastic Search operator](https://github.com/elastic/cloud-on-k8s) creates Persistent Volume Claims, it attaches an `ownerReference` to them such that they are garbage-collected after the operator removes the Elastic Search stack of resources.
This makes sense when we plan to dynamically scale Elastic Search up and down, but it doesn't make sense if we don't plan to scale dynamically, but we do want to keep the Elastic Search indexes during Elastic Search reinstallation \(see comments [here](https://github.com/elastic/cloud-on-k8s/issues/2328#issuecomment-583254122) and [here](https://github.com/elastic/cloud-on-k8s/issues/2328#issuecomment-650335893)\).
A solution to this issue would be the following ModRule which simply removes the `ownerReference` from PVCs created by the Elastic Search operator at the time they are deployed, thus excluding those resources from Kubernetes garbage collection:
```yaml
apiVersion: api.kubemod.io/v1beta1
kind: ModRule
metadata:
name: my-modrule
spec:
type: Patch
matches:
# Match persistent volume claims ...
- select: '$.kind'
matchValue: PersistentVolumeClaim
# ... created by the elasticsearch operator.
- select: '$.metadata.labels["common.k8s.elastic.co/type"]'
matchValue: elasticsearch
patch:
# Remove the ownerReference if it exists, thus excluding the resource from Kubernetes garbage collection.
- op: remove
path: /metadata/ownerReferences/0
```
### Modification of metadata
With the help of ModRules, one can dynamically modify the resources generated by one operator such that another operator can detect those resources.
For example, [Istio's sidecar injection](https://istio.io/latest/docs/setup/additional-setup/sidecar-injection/) can be controlled by pod annotation `sidecar.istio.io/inject`. If another operator creates a deployment which we want to explicitly exclude from Istio's injection mechanism, we can create a ModRule which modifies that deployment by adding this annotation with value `"false"`.
The following ModRule explicitly excludes the Jaeger collector deployment created by the [Jaeger Operator](https://www.jaegertracing.io/docs/1.18/operator/) from Istio sidecar injection:
```yaml
apiVersion: api.kubemod.io/v1beta1
kind: ModRule
metadata:
name: my-modrule
spec:
type: Patch
match:
# Match deployments ...
- select: '$.kind'
matchValue: Deployment
# ... with label app = jaeger ...
- select: '$.metadata.labels.app'
matchValue: jaeger
# ... and label app.kubernetes.io/component = collector ...
- select: '$.metadata.labels["app.kubernetes.io/component"]'
matchValue: collector
# ... but with and no annotation sidecar.istio.io/inject.
- select: '$.metadata.annotations["sidecar.istio.io/inject"]'
negate: true
patch:
# Add Istio annotation sidecar.istio.io/inject=false to exclude this deployment from Istio injection.
- op: add
path: /metadata/annotations/sidecar.istio.io~1inject
value: '"false"'
```
### Sidecar injection
With the help of ModRules, one can dynamically inject arbitrary sidecar containers into `Deployment` and `StatefulSet` resources. The `patch` part of the ModRule is a [Golang template](https://golang.org/pkg/text/template/) which takes the target resource object as an intrinsic context allowing for powerful declarative rules such as the following one which injects a [Jaeger Agent](https://www.jaegertracing.io/docs/1.19/architecture/#agent) sidecar into any Deployment tagged with annotation `my-inject-annotation` set to `"true"`:
```yaml
apiVersion: api.kubemod.io/v1beta1
kind: ModRule
metadata:
name: my-modrule
spec:
type: Patch
match:
# Match deployments ...
- select: '$.kind'
matchValue: Deployment
# ... with annotation my-inject-annotation = true ...
- select: '$.metadata.annotations["my-inject-annotation"]'
matchValue: '"true"'
# ... but ensure that there isn't already a jaeger-agent container injected in the pod template to avoid adding more containers on UPDATE operations.
- select: '$.spec.template.spec.containers[*].name'
matchValue: 'jaeger-agent'
negate: true
patch:
- op: add
# Use -1 to insert the new container at the end of the containers list.
path: /spec/template/spec/containers/-1
value: |-
name: jaeger-agent
image: jaegertracing/jaeger-agent:1.18.1
imagePullPolicy: IfNotPresent
args:
- --jaeger.tags=deployment.name={{ .Target.metadata.name }},pod.namespace={{ .Namespace }},pod.id=${POD_ID:},host.ip=${HOST_IP:}
- --reporter.grpc.host-port=dns:///jaeger-collector-headless.{{ .Namespace }}:14250
- --reporter.type=grpc
env:
- name: POD_ID
valueFrom:
fieldRef:
apiVersion: v1
fieldPath: metadata.uid
- name: HOST_IP
valueFrom:
fieldRef:
apiVersion: v1
fieldPath: status.hostIP
ports:
- containerPort: 6832
name: jg-binary-trft
protocol: UDP
```
Note the use of `{{ .Target.metadata.name }}` in the patch `value` to dynamically access the name of the deployment being patched and pass it to the Jaeger agent as a tracer tag.
When a patch is evaluated, KubeMod executes the patch value as a [Golang template](https://golang.org/pkg/text/template/) and passes the following intrinsic items accessible through the template's context:
* `.Target` — the original resource object being patched with all its properties.
* `.Namespace` — the namespace of the resource object.
* `.SelectedItem` — when `select` was used for the patch, `.SelectedItem` yields the current result of the select evaluation. See second example below.
* `.SelectKeyParts` — when `select` was used for the patch, `.SelectKeyParts` can be used in `value` to access
the wildcard/filter values captured for this patch operation.
### Resource rejection
There are two types of ModRules — `Patch` and `Reject`.
All of the examples we've seen so far have been of type `Patch`.
`Reject` ModRules are simpler as they only have a `match` section.
If a resource matches the `match` section of a `Reject` ModRule, its creation/update will be rejected. This enables the development of a system of policy ModRules which enforce certain security restrictions in the namespace they are deployed to.
For example, here's a `Reject` ModRule which prevents the infamous [CVE-2020-8554: Man in the middle using ExternalIPs](https://github.com/kubernetes/kubernetes/issues/97076):
```yaml
apiVersion: api.kubemod.io/v1beta1
kind: ModRule
metadata:
name: reject-malicious-external-ips
spec:
type: Reject
rejectMessage: 'One or more of the following external IPs are not allowed {{ .Target.spec.externalIPs }}'
match:
# Reject Service resources...
- select: '$.kind'
matchValue: Service
# ...with non-empty externalIPs...
- select: 'length($.spec.externalIPs) > 0'
# ...where some of the IPs were not part of the allowed subnet 123.45.67.0/24.
- select: '$.spec.externalIPs[*]'
matchFor: All
matchRegex: '123\.45\.67\.*'
negate: true
```
Here's another ModRule which rejects the deployment of any `Deployment` or `StatefulSet` resource that does not explicitly require non-root containers:
```yaml
apiVersion: api.kubemod.io/v1beta1
kind: ModRule
metadata:
name: reject-root-access-workloads
spec:
type: Reject
rejectMessage: 'All workloads must run as non-root user'
match:
# Reject Deployments and StatefulSets...
- select: '$.kind'
matchValues:
- Deployment
- StatefulSet
# ...that have no explicit runAsNonRoot security context.
- select: "$.spec.template.spec.securityContext.runAsNonRoot == true"
negate: true
```
---
## Anatomy of a ModRule
A `ModRule` consists of a `type`, a `match` section, and a `patch` section.
It also includes the optional `targetNamespaceRegex` and `rejectMessage` fields.
```yaml
apiVersion: api.kubemod.io/v1beta1
kind: ModRule
spec:
type: ...
admissionOperations: ...
executionTier: ...
match:
...
patch:
...
```
The `type` of a `ModRule` can be one of the following:
* `Patch` — this type of `ModRule` applies patches to objects that match the `match` section of the rule. Section `patch` is required for `Patch` ModRules.
* `Reject` — this type of `ModRule` rejects objects which match the `match` section. When `type` is `Reject`, the spec accepts an optional `rejectMessage` field.
Section [`match`](#match-section) is an array of individual criteria items used to determine if the `ModRule` applies to a Kubernetes object.
Section [`patch`](#patch-section) is an array of patch operations.
### Match section
Section `match` of a `ModRule` is an array of individual criteria items.
When a new object is deployed to Kubernetes, or an existing one is updated, KubeMod intercepts the operation and attempts to match the object's definition against all `ModRules` deployed to the namespace where the object is being deployed.
A `ModRule` is considered to have a match with the Kubernetes object definition when all criteria items in its `match` section yield a positive match.
A criteria item contains a required `select` expression and optional `matchValue`, `matchValues`, `matchRegex` and `negate` fields.
For example, the following `match` section has two criteria items. This `ModRule` will match all resources whose `kind` is equal to `Deployment` **and** have a container name that's either `container-1` or `container-2` .
```yaml
...
match:
- select: '$.kind'
matchValue: Deployment
- select: '$.spec.template.spec.containers[*].name'
matchValues:
- 'container-1'
- 'container-2'
```
A criteria item is considered a positive match when:
* its `select` expression yields a single boolean `true` value.
* its `select` expression yields one or more non-boolean values **and** one of the following is true:
* Fields `matchValue`, `matchValues` and `matchRegex` are not specified.
* `matchValue` is specified and:
* `matchFor` is set to `Any` (or unspecified) and one or more of the values resulting from `select` exactly matches the value of `matchValue`.
* `matchFor` is set to `All` and all of the values resulting from `select` exactly match the value of `matchValue`.
* `matchValues` is specified and:
* `matchFor` is set to `Any` (or unspecified) and one or more of the values resulting from `select` exactly matches one of the values in `matchValues`.
* `matchFor` is set to `All` and all of the values resulting from `select` exactly match one of the values in `matchValues`.
* `matchRegex` is specified and:
* `matchFor` is set to `Any` (or unspecified) and one or more of the values resulting from `select` matches that regular expression.
* `matchFor` is set to `All` and all of the values resulting from `select` match that regular expression.
The result of a criteria item can be inverted by setting its `negate` field to `true`.
A criteria item whose `select` expression yields no results is considered non-matching unless it is `negated`.
#### `select` \(string : required\)
The `select` field of a criteria item is a [JSONPath](https://goessner.net/articles/JsonPath/) expression.
[See more on KubeMod's version of JSONPath](#kubemods-version-of-jsonpath).
When a `select` expression is evaluated against a Kubernetes object definition, it yields zero or more values.
Let's consider the following JSONPath select expression:
```javascript
$.spec.template.spec.containers[*].name
```
When this expression is evaluated against a `Deployment` resource definition whose specification includes three containers, the result of this `select` expression will be a list of the names of those three containers.
If `select` yields a single boolean value, that value is considered to be the result of the match regardless of the values of `matchValue`, `matchValues` and `matchRegex`.
For any other case, KubeMod converts the result of the `select` expression to a list of strings, regardless of what the original type of the target field is.
Here's another example:
```javascript
$.spec.template.spec.containers[*].ports[*].containerPort
```
This expression will yield a list of all `containerPort` values for all ports and all containers. The values in the list will be the string representation of those port numbers.
##### `select` filters
KubeMod `select` expressions includes an extension to JSONPath — filters.
A filter is an expression in the form of `[? ]` and can be used in place of JSONPath's `[*]` to filter the elements of an array.
Let's take a look at the following `select` expression:
```javascript
$.spec.template.spec.containers[*].ports[? @.containerPort == 8080]
```
The expression `[? @.containerPort == 8080]` filters the result of the `select` to include only containers that include a port with whose `containerPort` field equals `8080`.
The filter expression could be any JavaScript boolean expression.
The special character `@` represents the current object the filter is iterating over. In the above filter expression, that is the current element of the `ports` array.
#### `matchFor` \(string: optional\)
Field `matchFor` controls how `select` results are evaluated against `matchValue`, `matchValues` and `matchRegex`.
The value of `matchFor` can be either `Any` or `All`. When not specified, `matchFor` defaults to `Any`.
See below for more information on how `matchFor` impacts the results of a match.
#### `matchValue` \(string: optional\)
When present, the value of field `matchValue` is matched against the results of `select`.
If `matchFor` is set to `Any` and any of the items returned by `select` match `matchValue`, the match criteria is considered a positive match.
If `matchFor` is set to `All` and all of the items returned by `select` match `matchValue`, the match criteria is considered a positive match.
The match performed by `matchValue` is case sensitive. If you need case insensitive matches, use `matchRegex`.
#### `matchValues` \(array of strings: optional\)
Field `matchValues` is an array of strings which are tested against the results of `select`.
If `matchFor` is set to `Any` and any of the items returned by `select` match any of the `matchValues`, the match criteria is considered a positive match.
If `matchFor` is set to `All` and all of the items returned by `select` match any of the `matchValues`, the match criteria is considered a positive match.
This match is case sensitive. If you need case insensitive matches, use `matchRegex`.
#### `matchRegex` \(string: optional\)
Field `matchRegex` is a regular expression matched against the results of `select`.
If `matchFor` is set to `Any` and any of the items returned by `select` match `matchRegex`, the match criteria is considered a positive match.
If `matchFor` is set to `All` and all of the items returned by `select` match `matchRegex`, the match criteria is considered a positive match.
#### `negate` \(boolean: optional\)
Field `negate` can be used to flip the outcome of the criteria item match. Its default value is `false`.
### Patch section
Section `patch` is an array of [RFC6902 JSON Patch](https://tools.ietf.org/html/rfc6902) operations.
KubeMod's variant of JSON Patch includes the following extensions to RFC6902:
* Negative array indices mean starting at the end of the array.
* Operations which attempt to remove a non-existent path in the JSON object are ignored.
A patch operation contains fields `op`, `select`, `path` and `value`.
For example, the following `patch` section applies two patch operations executed against every `Deployment` object deployed to the namespace where the `ModRule` resides.
```yaml
...
match:
- select: '$.kind'
matchValue: Deployment
patch:
- op: add
path: /metadata/labels/color
value: blue
# Change all nginx containers' ports from 80 to 8080
- op: add
select: '$.spec.template.spec.containers[? @.image =~ "nginx" ].ports[? @.containerPort == 80]'
path: '/spec/template/spec/containers/#0/ports/#1/containerPort'
value: '8080'
```
The first patch operation adds a new `color` label to the target object.
The second one switches the `containerPort` of all `nginx` containers present in the target `Deployment` object from `80` to `8080`.
Following is a break-down of each field of a patch operation.
#### `op` \(string: required\)
Field `op` indicates the type of patch operation to be performed against the target object. It can be one of the following:
* `replace` — this type of operation replaces the value of element represented by `path` with the value of field `value`. If `path` points to a non-existent element, the operation fails.
* `add` — this type of operation adds the element represented by `path` with the value of field `value`. If the element already exists, `add` behaves like `replace`.
* `remove` — this type of operation removes the element represented by `path`. If `path` points to a non-existent element, the operation is ignored.
#### `select` \(string: optional\) and `path` \(string: required\)
The `select` field of a patch item is a [JSONPath](https://goessner.net/articles/JsonPath/) expression.
When a `select` expression is evaluated against a Kubernetes object definition, it yields zero or more values.
For more information about `select` expressions, see [Match item select expressions](#select-string--required).
When `select` is used in a patch operation, the patch is executed once for each item yielded by `select`.
If the `select` field of a patch item uses JSONPatch wildcards \(such as `..` or `[*]`\) and/or [select filters](#select-filters), KubeMod captures the zero-based index of each wildcard/filter result and makes it available for use in the target `path` field.
The `path` field of a patch item points to the target element which should be patched.
The path components are separated by slashes (`/`). A slash in the name of a `path` component is escaped with the special `~1`.
When targeting elements of an array, index `-1` is relative and means "the element after the last one in the array".
Let's consider the following example:
```yaml
op: add
select: '$.spec.template.spec.containers[*].ports[? @.containerPort == 80]'
path: '/spec/template/spec/containers/#0/ports/#1/containerPort'
value: '8080'
```
The `select` expression includes a wildcard to loop over all containers \(`containers[*]`\), and then a filter \(`ports[? @.containerPort == 80]`\) to select only the ports whose `containerPort` is equal to `80`.
If we evaluate this `select` expression against a `Deployment` with the following four containers and port objects...
```yaml
...
containers:
- name: c1
ports:
- containerPort: 100
name: abc
- containerPort: 200
name: xyz
- name: c2
ports:
- containerPort: 100
name: abc
- containerPort: 80
name: xyz
- name: c3
ports:
- containerPort: 100
name: abc
- containerPort: 200
name: xyz
- name: c4
ports:
- containerPort: 80
name: abc
- containerPort: 200
name: xyz
- containerPort: 300
name: foo
...
```
... the `select` expression will yield the following two items:
* Item 1: The second port object of the second container
* Item 2: The first port object of the fourth container
The zero-based wildcard/filter indexes captured by KubeMod for this `select` will be as follows:
* Item 1:
* container index: 1
* port index: 1
* Item 2:
* container index: 3
* port index: 0
The indexes above can be used when constructing the `path` of the patch operation.
For example:
```yaml
path: '/spec/template/spec/containers/#0/ports/#1/containerPort'
```
The index placeholders `#0` and `#1` refers to the first and the second index captured by KubeMod when evaluating the `select` expression.
In our previous `select` example, `#0` would be the placeholder for the container index, and `#1` would be the placeholder for the port index.
When KubeMod performs patch operations, it constructs the `path` by replacing the index placeholders with the value of the corresponding indexes.
For the above `select` and `path` examples executed against our sample `Deployment`, KubeMod will generate two patch operations which will target the following paths:
* `/spec/template/spec/containers/1/ports/1/containerPort`
* `/spec/template/spec/containers/3/ports/0/containerPort`
Combining `select` expressions with `path`s with index placeholders gives us the ability to perform sophisticated targeted resource modifications.
If `select` is not specified, `path` is rendered as-is and is not subject to index placeholder interpolation.
#### `value` \(string\)
`value` is required for `add` and `replace` operations.
`value` is the **string representation** of a `YAML` value. It can represent a primitive value or a complex `YAML` object or array.
Here are a few examples:
**Number** \(note the quotes - `value` itself is a string, but its "value" evaluates to a `YAML` number\):
```yaml
value: '8080'
```
**String**:
```yaml
value: hello
# or
value: 'hello'
```
String representation of a number \(note the double-quotes\):
```yaml
value: '"8080"'
```
**Boolean**:
```yaml
value: 'false'
```
String representation of a boolean:
```yaml
value: '"false"'
```
**YAML object** \(note `|-` which makes `value` a multi-line string\):
```yaml
value: |-
name: jaeger-agent
image: jaegertracing/jaeger-agent:1.18.1
imagePullPolicy: IfNotPresent
ports:
- containerPort: 6832
name: jg-binary-trft
protocol: UDP
```
#### Golang Template
When `value` contains `{{ ... }}`, it is evaluated as a [Golang template](https://golang.org/pkg/text/template/).
In addition, the Golang template engine used by KubeMod is extended with the [Sprig library of template functions](http://masterminds.github.io/sprig/).
The following intrinsic items are accessible through the template's context:
* `.Target` — the original resource object being patched.
* `.Namespace` — the namespace of the target object.
* `.SelectedItem` — when `select` was used for the patch, `.SelectedItem` yields the current result of the select evaluation. See second example below.
* `.SelectKeyParts` — when `select` was used for the patch, `.SelectKeyParts` can be used in `value` to access
the wildcard/filter values captured for this patch operation.
For example, the following excerpt of a Jaeger side-car injection `ModRule` includes a `value` which uses `{{ .Target.metadata.name }}` to access the name of the `Deployment` being patched.
```yaml
...
value: |-
name: jaeger-agent
image: jaegertracing/jaeger-agent:1.18.1
imagePullPolicy: IfNotPresent
args:
- --jaeger.tags=deployment.name={{ .Target.metadata.name }}
ports:
- containerPort: 6832
name: jg-binary-trft
protocol: UDP
...
```
See full example of the above ModRule [here](#sidecar-injection).
#### Advanced use of SelectedItem
The presence of `.SelectedItem` in the `value` template unlocks some advanced scenarios.
For example, the following `patch` rule will match all containers from image repository `their-repo` and will replace the repository part of the image with `my-repo`,
keeping the rest of the image name intact:
```yaml
...
patch:
- op: replace
# Select only containers whose image belongs to container registry "their-repo".
select: '$.spec.containers[? @.image =~ "their-repo/.+"].image'
path: /spec/containers/#0/image
# Replace the existing value by running Sprig's regexReplaceAll function against .SelectedItem.
value: '{{ regexReplaceAll "(.+)/(.*)" .SelectedItem "my-repo/${2}" }}'
```
Note that `.SelectedItem` points to the part of the resource selected by the `select` expression.
In the above example, the `select` expression is `$.spec.containers[? @.image =~ "repo-1/.+"].image` so `.SelectedItem` is a string with the value of the image field.
On the other hand, if the `select` expression was `$.spec.containers[? @.image =~ "repo-1/.+"]`, then `.SelectedItem` would be a map with the named properties
of the `container` object.
In that case, to access any of the properties of the container, one would use the `index` Golang template function.
For example, `{{ index .SelectedItem "image" }}` or `{{ index .SelectedItem "imagePullPolicy" }}`.
### `targetNamespaceRegex` \(string: optional\)
Field `targetNamespaceRegex` is an optional regular expression which is used to match namespaced object.
It only applies to ModRules deployed to namespace `kubemod-system`.
Setting this field allows for the deployment of ModRules which apply to resources deployed across namespaces.
### `rejectMessage` \(string: optional\)
Field `rejectMessage` is an optional message displayed when a resource is rejected by a `Reject` ModRule.
The field is a Golang template evaluated in the context of the object being rejected
## Miscellaneous
### Operation type
Admission requests have different operation types. A `ModRule` will handle `CREATE` and `UPDATE` operations by default.
If you want to limit the `ModRule` to a specific list of operations, you can do so through the `admissionOperations` property.
Allowed values are:
```yaml
admissionOperations:
- CREATE
- UPDATE
- DELETE
```
This property is optional and will default to an empty list, which will execute the `ModRule` against `CREATE` and `UPDATE` operations
### Execution tiers
`ModRules` are matched and executed in tiers.
When executing `ModRules` against a Kubernetes resource, KubeMod executes all `ModRules` in the lowest tier first, then passes the patched results to the next tier of `ModRules`.
This cascading execution continues until the highest tier of `ModRules` has been executed.
Set the `executionTier` property of a `ModRule` to control which tier it belongs to.
The execution tier of a ModRule can be set to any integer value between `-32767` and `32766`. It defaults to `0`.
The execution tier pipeline can be used to create powerful systems of `ModRules`.
For example, let's say that we have a `ModRule` with `executionTier` of `1` which replaces all Docker Hub container images of all deployments with corresponding images hosted in a private container registry.
Then another `ModRule` with `executionTier` of `2` can be created to detect deployments with private container registry images and inject an appropriate `imagePullSecrets` secret into the Deployment's pods.
The second `ModRule` will trigger for all deployments which refer to the private container registry, including the ones that originally used Docker Hub images, but were modified by the first `ModRule` to use the private registry. This tiered execution behavior allows us to develop less redundant and more generic `ModRules`.
#### Gotchas
When multiple `ModRules` in the same execution tier match the same resource object, all of the ModRule patches are executed against the object in an indeterminate order.
Be careful when creating `ModRules` int the same execution tier - make sure that their match criteria and patch sections don't overlap leading to unexpected behavior.
### Namespaced and cluster-wide resources
KubeMod can patch/reject both namespaced and cluster-wide resources.
If a ModRule is deployed to any namespace other than `kubemod-system`, the ModRule applies only to objects deployed/updated in that same namespace.
ModRules deployed to namespace `kubemod-system` are treated differently.
- If a ModRule is deployed to `kubemod-system` and its `targetNamespaceRegex` is empty or equal to `.*`, this rule applies to cluster-wide resources such as `Namespace` or `ClusterRole`.
- If a ModRule is deployed to `kubemod-system` and its `targetNamespaceRegex` is non-empty, this rule applies to all namespaced resources in namespaces that match the regular expression in `targetNamespaceRegex`.
Note that matching `targetNamespaceRegex` to the namespace of a resource does not guarantee an actual match for the rule. It only guarantees that the rule will be considered for a match — the final outcome will be decided by evaluating the rule's `Match`criteria against the resource's definition.
**Note on ignored namespaces**
If a namespace has label `admission.kubemod.io/ignore` equal to `"true"`, KubeMod will not monitor resources created in that namespace.
By default, the following namespaces are tagged with the above label:
- `kube-system`
- `kubemod-system`
### Synthetic references
KubeMod 0.17.0 introduced `syntheticRefs` - a map of external resource manifests injected at the root of every Kubernetes resource processed by KubeMod.
Synthetic references unlock use cases where a ModRule can be matched against objects not only based on their own manifest, but also the manifests of other related resources.
In addition, since `syntheticRefs` exists in the body of the target resource, it can be used when constructing `patch` values.
Currently KubeMod injects the following manifests in `syntheticRefs`:
- `namespace`: The manifest of the namespace of the target, if the target is a namespaced object.
- `node`: The manifest of the node of a pod (See [Node synthetic references](#node-synthetic-references) below for more information).
Here's an example ModRule which matches all pods created in namespaces labeled with `color` equal to `blue`.
The ModRule mutates those pods by tagging them with a label `flavor`, whose value is inherited from the `flavor` label of the pod's namespace.
```yaml
apiVersion: api.kubemod.io/v1beta1
kind: ModRule
metadata:
name: add-flavor-modrule
# This is a cluster-wide rule - we need to create it in the kubemod-system namespace.
namespace: kubemod-system
spec:
type: Patch
# We need to set targetNamespaceRegex to a regular expression,
# otherwise the namespace will only apply to non-namespaced objects.
targetNamespaceRegex: ".*"
match:
# Match pods...
- select: '$.kind'
matchValue: 'Pod'
# ...which are created/updated in a namespace whose "color" label is set to "blue"...
- select: '$.syntheticRefs.namespace.metadata.labels.color'
matchValue: 'blue'
# ...and have a "flavor" label.
- select: '$.syntheticRefs.namespace.metadata.labels.flavor'
patch:
# Mutate the pod by setting its "flavor" label to the value of its namespace's "flavor" label.
- op: add
path: /metadata/labels/flavor
value: '{{ .Target.syntheticRefs.namespace.metadata.labels.flavor }}'
```
**Note 1**:
This particular ModRule targets resources created in multiple namespaces - this is the reason we need to create it in the `kubemod-system` namespace (see note on [namespaced and cluster-wide resources](#namespaced-and-cluster-wide-resources)).
In addition, we set `targetNamespaceRegex` to a regular expression. Leaving `targetNamespaceRegex` blank would instruct KubeMod to use this ModRule only against non-namespaced objects.
In this case we set the regular expression to match all namespaces (`.*`) - we narrow down the filter to the `blue` colored namespaces in the `match` section.
**Note 2**:
The `syntheticRefs` map exists in the object's manifest only for the purpose of participating in KubeMod's `match` and `patch` processing.
It is not actually inserted in the resulting resource manifest ultimately sent to the cluster.
#### Node synthetic references
KubeMod 0.19.0 introduced `node` synthetic reference for ModRules targeting pod manifests.
In order to capture the node which a pod has been scheduled on, KubeMod listens to pod scheduling events.
If KubeMod intercepts a pod scheduling event for a pod which has annotation `ref.kubemod.io/inject-node-ref` set to `"true"`, KubeMod updates the pod by injecting annotation `ref.kubemod.io/node` whose value is set to the name of the node.
This triggers an `UPDATE` operation, which is again captured by KubeMod. When KubeMod intercepts pod operations for pods with annotation `ref.kubemod.io/node`, it injects the node manifest into the pod's synthetic references, thus making them available for ModRule matching and patching.
This enables a wide array of use cases not natively supported by Kubernetes (see https://github.com/kubernetes/kubernetes/issues/40610).
For example, the following cluster-wide ModRule will inject a pod with it's node's availability region and zone, as soon as the pod gets scheduled on a node:
```yaml
apiVersion: api.kubemod.io/v1beta1
kind: ModRule
metadata:
name: inject-node-annotations
namespace: kubemod-system
spec:
type: Patch
targetNamespaceRegex: ".*"
admissionOperations:
- UPDATE
match:
# Match pods...
- select: '$.kind'
matchValue: 'Pod'
# ...which have access to the node's manifest through the synthetic ref injected by KubeMod.
- select: '$.syntheticRefs.node.metadata.labels'
patch:
# Grab the node's region and zone and put them in the pod's corresponding labels.
- op: add
path: /metadata/labels/topology.kubernetes.io~1region
value: '"{{ index .Target.syntheticRefs.node.metadata.labels "topology.kubernetes.io/region"}}"'
- op: add
path: /metadata/labels/topology.kubernetes.io~1zone
value: '"{{ index .Target.syntheticRefs.node.metadata.labels "topology.kubernetes.io/zone"}}"'
```
The above ModRule will apply to any pod created in any namespace as long as it has the following annotation:
```yaml
ref.kubemod.io/inject-node-ref: "true"
```
If you want to have this rule apply to pods which don't have this annotation, you can create another ModRule which injects `ref.kubemod.io/inject-node-ref` into any pod that matches a given criteria, or to all pods that are created in the cluster.
Here's a cluster-wide ModRule which will inject `ref.kubemod.io/inject-node-ref` into every Pod created in the cluster.
Note the double-quotes surrounded by single quotes around the value of `true` at the bottom of the ModRule.
This ensures that annotation `ref.kubemod.io/inject-node-ref` is set to string value `"true"` instead of boolean value `true`.
```yaml
apiVersion: api.kubemod.io/v1beta1
kind: ModRule
metadata:
name: patch-pod-with-inject-node-ref
namespace: kubemod-system
spec:
type: Patch
targetNamespaceRegex: ".*"
admissionOperations:
- CREATE
match:
- select: '$.kind'
matchValue: 'Pod'
- select: '$.metadata.annotations["ref.kubemod.io/inject-node-ref"]'
matchValue: 'true'
negate: true
patch:
- op: add
path: /metadata/annotations/ref.kubemod.io~1inject-node-ref
value: '"true"'
```
### Target resources
By default, KubeMod targets the following list of resources:
- namespaces
- nodes
- configmaps
- persistentvolumeclaims
- persistentvolumes
- secrets
- services
- daemonsets
- deployments
- replicasets
- statefulsets
- horizontalpodautoscalers
- ingresses
- pods
- cronjobs
- jobs
- serviceaccounts
- clusterrolebindings
- clusterroles
- rolebindings
- roles
If you need to expand or limit this list create a patch file `patch.yaml` with the following content and populate the resources list with the full list of resources you want to target:
```yaml
webhooks:
- name: dragnet.kubemod.io
rules:
- apiGroups:
- '*'
apiVersions:
- '*'
scope: '*'
operations:
- CREATE
- UPDATE
resources:
- namespaces
- nodes
- configmaps
- persistentvolumeclaims
- persistentvolumes
- secrets
- services
- daemonsets
- deployments
- replicasets
- statefulsets
- horizontalpodautoscalers
- ingresses
- pods
- cronjobs
- jobs
- serviceaccounts
- clusterrolebindings
- clusterroles
- rolebindings
- roles
```
Save the file and run the following:
```bash
kubectl patch mutatingwebhookconfiguration kubemod-mutating-webhook-configuration --patch "$(cat patch.yaml)"
```
You can get the full list of Kubernetes API resources by running:
```yaml
kubectl api-resources --verbs list -o name
```
### Note on idempotency
Make sure your patch ModRules are idempotent - executing them multiple times against the same object should lead to no changes beyond the first execution.
This is important because Kubernetes will pass the same object through KubeMod every time its state changes. For example, when a `Deployment` resource is created, its `status` field changes multiple times after its creation.
We want to make sure KubeMod will not apply cumulative patch operations against objects that have already been patched.
Here's an example of an idempotent sidecar injection ModRule:
```yaml
apiVersion: api.kubemod.io/v1beta1
kind: ModRule
metadata:
name: my-sidecar-injection-rule
spec:
type: Patch
match:
# Match Deployments...
- select: '$.kind'
matchValue: Deployment
# ...which have label app = whatever...
- select: '$.metadata.labels.app'
matchValue: 'whatever'
# ...and have not yet received the injection.
- select: '$.spec.template.spec.containers[*].name'
matchValue: 'my-sidecar'
negate: true
patch:
# Operations on non-array fields are idempotent by default.
- op: add
path: /metadata/labels/color
value: blue
# Careful - add and remove operations against relative array elements (-1) are not idempotent.
# We need to protect against cumulative patches through a negate select rule in the match section (see above).
- op: add
path: /spec/template/spec/containers/-1
value: |-
name: my-sidecar
image: alpine:3
command:
- sh
- -c
- while true; do sleep 5; done;
```
The first `add` operation in section `patch` is performed against a non-array field `/metadata/labels/color`.
Such an operation is idempotent by default. If a `color` label does not exist, it will be created and its value will be set to `blue`.
If it does exist, its value will be overwritten by value `blue`.
Now let's take a look at the next `add` operation.
It targets path `/spec/template/spec/containers/-1` to inject a sidecar container into the Deployment's manifest.
Index `-1` is relative. It indicates the index after the last element of an array.
This rule is not idempotent.
Running it multiple times against the same deployment will inject `my-sidecar` container multiple times.
To prevent that, we add a `negate:true` select statement in the `match` section, which basically says "don't run this rule against objects that already have a container named `my-sidecar`".
### Debugging ModRules
To list the ModRules deployed to a namespace, run the following command:
```bash
kubectl get modrules
```
When a ModRule does not behave as expected, your best bet is to analyze KubeMod's operator logs.
Follow these steps:
* Deploy your ModRule to the namespace where you will be deploying the Kubernetes resources the ModRule should intercept.
* Find the KubeMod operator pod - run the following command and grab the name of the pod that begins with `kubemod-operator`:
```bash
kubectl get pods -n kubemod-system
```
* Tail the logs of the pod:
```bash
kubectl logs kubemod-operator-xxxxxxxx-xxxx -n kubemod-system -f
```
* In another terminal deploy the Kubernetes object your ModRule is designed to intercept.
* Watch the operator pod logs.
If your ModRule is a Patch rule, KubeMod operator will log the full JSON Patch applied to the target Kubernetes object at the time of interception.
If there are any errors at the time the patch is calculated, you will see them in the logs.
If the operator log is silent at the time you deploy the target object, this means that your ModRule's `match` criteria did not yield a positive match for the target object.
### Declarative `kubectl apply`
KubeMod is aligned with Kubernetes' approach to [declarative object management](https://kubernetes.io/docs/tasks/manage-kubernetes-objects/declarative-config/).
When an object is patched by a ModRule, if the object has a `kubectl.kubernetes.io/last-applied-configuration` annotation, KubeMod patches the contents of that annotation as well.
KubeMod supports both client-side and server-side declarative management through `kubectl apply`.
### KubeMod's version of JSONPath
KubeMod implements a modified (extended) version of [JSONPath](https://goessner.net/articles/JsonPath/).
This version introduces the following new features:
#### Value `undefined`
- Includes internal representation of value `undefined`.
- Resolves each path that includes undefined properties to value `undefined`.
For example `$.a.b.c` will resolve to `undefined` if any of the `a`, `b` or `c` properties do not exist.
- Filters out all `undefined` values on partial matches.
- Makes all equality and arithmetic comparisons to `undefined` return `false`.
For example, assuming that `$.a.b.c` is a path to undefined property, all of the following expressions will yield `false`:
- `$.a.b.c == 12`
- `$.a.b.c != 12`
- `$.a.b.c > 12`
- `$.a.b.c < 12`
- `$.a.b.c == true`
- `$.a.b.c == false`
- Makes all boolean operators (`&&` and `||`) require boolean operands.
#### `undefined`-based functions
- `isDefined()` - returns `true` if the passed in path leads to a defined property, otherwise return `false`.
- `isUndefined()` - returns `true` if the passed in path leads to an undefined property, otherwise return `false`.
- `isEmpty()` - returns `true` if the passed in path leads to one of the following values:
- An empty array
- An empty object
- An empty string
- Null
- `undefined`
- `isNotEmpty()` - equivalent to evaluating `!isEmpty()`
- `length()` - returns the length of arrays, objects and strings. Returns `0` for Nulls and `undefined`.
#### Note on presence check
KubeMod uses the above `undefined` based functions to provide both presence (`isDefined`) and negative-presence (`isUndefined`) filters - see next section for an example.
These functions should be used in place of the standard JSONPath's presence-based `[?(@.property)]` filter [discussed here](https://goessner.net/articles/JsonPath/).
#### Usage in ModRules
For example, to patch all deployments' containers which have either no `securityContext` defined, or `securityContext` is empty, one would use the following KubeMod rule.
```yaml
apiVersion: api.kubemod.io/v1beta1
kind: ModRule
metadata:
name: kubemod-patch-deployments-containers-securitycontext
namespace: kubemod-system
spec:
targetNamespaceRegex: .*
type: Patch
match:
- matchValue: Deployment
select: $.kind
patch:
- op: add
select: '$.spec.template.spec.containers[? isEmpty(@.securityContext)]'
path: '/spec/template/spec/containers/#0/securityContext'
value: |-
runAsNonRoot: true
capabilities:
drop:
- ALL
```
The rule uses `isEmpty` which returns `true` when the passed in path is not defined or if it points to an empty object.
If we wanted to only patch the containers which have no `securityContext` defined, but leave the ones which have an empty `securityContext`, we would use the following `select`:
```yaml
select: '$.spec.template.spec.containers[? isUndefined(@.securityContext)]'
```
If we wanted to only patch the containers which have an empty `securityContext`, but leave the ones which have no `securityContext` defined, we would use the following `select`:
```yaml
select: '$.spec.template.spec.containers[? isDefined(@.securityContext) && isEmpty(@.securityContext)]'
```
[ci-img]: https://github.com/kubemod/kubemod/actions/workflows/main-workflow.yaml/badge.svg
[ci]: https://github.com/kubemod/kubemod/actions
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