├── .gitignore ├── Dockerfile.in ├── README.md ├── example.yaml ├── Makefile └── LICENSE /.gitignore: -------------------------------------------------------------------------------- 1 | /.push-* 2 | /.container-* 3 | /.dockerfile-* 4 | /.buildx-initialized 5 | -------------------------------------------------------------------------------- /Dockerfile.in: -------------------------------------------------------------------------------- 1 | # Copyright 2024, Tim Hockin 2 | # 3 | # Licensed under the Apache License, Version 2.0 (the "License"); 4 | # you may not use this file except in compliance with the License. 5 | # You may obtain a copy of the License at 6 | # 7 | # http://www.apache.org/licenses/LICENSE-2.0 8 | # 9 | # Unless required by applicable law or agreed to in writing, software 10 | # distributed under the License is distributed on an "AS IS" BASIS, 11 | # WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. 12 | # See the License for the specific language governing permissions and 13 | # limitations under the License. 14 | 15 | # Debian is very standard but has lots of tools. 16 | # Alpine is less expansive, but smaller. 17 | FROM debian AS base 18 | 19 | # We can be sloppy about layers because this is a multi-stage build. 20 | RUN apt-get -y -qq -o Dpkg::Use-Pty=0 update 21 | RUN apt-get -y -qq -o Dpkg::Use-Pty=0 -y upgrade 22 | 23 | # Install the tools you need 24 | RUN apt-get -y -qq -o Dpkg::Use-Pty=0 -y install \ 25 | bash \ 26 | grep \ 27 | sed \ 28 | gawk \ 29 | coreutils \ 30 | jq \ 31 | diffutils 32 | RUN rm -rf /var/lib/apt/lists/* 33 | 34 | # Update this as needed. 35 | FROM registry.k8s.io/kubernetes/kubectl:{ARG_VERSION} AS kubectl 36 | 37 | FROM scratch 38 | 39 | COPY --from=base / / 40 | COPY --from=kubectl /bin/kubectl /bin/kubectl 41 | 42 | # This image has no ENTRYPOINT or CMD. 43 | -------------------------------------------------------------------------------- /README.md: -------------------------------------------------------------------------------- 1 | # kubectl-sidecar 2 | 3 | This repo has all the base tools to build a kubectl "sidecar" container. 4 | 5 | ## Why would I want this? 6 | 7 | Often a Kubernetes Pod wants to know something about itself or its environment. 8 | Kubernetes offers a "downward API" which can publish some information via 9 | environment variables or files. This information is very limited in scope and 10 | not very flexible in formatting. It's not uncommon to need information that 11 | isn't offered or to need it formatted in some particular way. 12 | 13 | Instead of asking Kubelet to fetch the data, which poses a risk of violating 14 | authorization policies and exposing information that should not be exposed, and 15 | instead of teaching kubelet to do arbitrary formatting and template expansion, 16 | this project offers a different approach. 17 | 18 | Do it yourself! 19 | 20 | Only the author of a Pod can really know what information they need or how they 21 | want it formatted. Perhaps more important than that, we already have a nice 22 | mechanism for controlling what information a given Pod is allowed to know - the 23 | kube-apiserver's configured authorization policies (e.g. RBAC). If the pod 24 | itself requests the information it needs, the cluster administrators can decide 25 | if that level of information is allowed. 26 | 27 | That said, it's unreasonable to change arbitrary apps to become Kubernetes 28 | clients. Fortunately, this is exactly what sidecar containers are good for. A 29 | Pod can run an additional container whose job is to fetch information from the 30 | API, format it, and write it to a shared volume. The main app can consume the 31 | information from the volume. 32 | 33 | ## How to use this 34 | 35 | This repo offers an example of how you might build such a sidecar. Only you 36 | know which data processing and formatting tools you need, and where your app is 37 | allowed to pull images from. Including too much in a container can pose 38 | security risks. Including too little makes it not useful. You are best 39 | positioned to make those tradeoffs. 40 | 41 | ## Example 42 | 43 | Here is a working [example](example.yaml) which you can use to understand this 44 | approach. After applying the manifest you can check it using: 45 | 46 | ``` 47 | kubectl port-forward deployment/demo-kubectl-sidecar 8080:80 48 | curl http://localhost:8080/this-pod-status.json 49 | curl http://localhost:8080/this-node-status.json 50 | ``` 51 | -------------------------------------------------------------------------------- /example.yaml: -------------------------------------------------------------------------------- 1 | # This is the identity the Pods will run as. 2 | apiVersion: v1 3 | kind: ServiceAccount 4 | metadata: 5 | name: demo-kubectl-sidecar 6 | namespace: default 7 | --- 8 | # This defines the namespace-scope permissions granted. 9 | apiVersion: rbac.authorization.k8s.io/v1 10 | kind: Role 11 | metadata: 12 | name: demo-kubectl-sidecar 13 | namespace: default 14 | rules: 15 | - apiGroups: 16 | - '' 17 | resources: 18 | - pods 19 | verbs: 20 | - get 21 | - watch 22 | --- 23 | # This joins the ServiceAccount to the Role above. 24 | apiVersion: rbac.authorization.k8s.io/v1 25 | kind: RoleBinding 26 | metadata: 27 | name: demo-kubectl-sidecar 28 | namespace: default 29 | roleRef: 30 | apiGroup: rbac.authorization.k8s.io 31 | kind: Role 32 | name: demo-kubectl-sidecar 33 | subjects: 34 | - kind: ServiceAccount 35 | name: demo-kubectl-sidecar 36 | --- 37 | # This defines the cluster-scope permissions granted. 38 | apiVersion: rbac.authorization.k8s.io/v1 39 | kind: ClusterRole 40 | metadata: 41 | name: demo-kubectl-sidecar 42 | rules: 43 | - apiGroups: 44 | - '' 45 | resources: 46 | - nodes 47 | verbs: 48 | - get 49 | - watch 50 | --- 51 | # This joins the ServiceAccount to the ClusterRole above. 52 | apiVersion: rbac.authorization.k8s.io/v1 53 | kind: ClusterRoleBinding 54 | metadata: 55 | name: demo-kubectl-sidecar 56 | roleRef: 57 | apiGroup: rbac.authorization.k8s.io 58 | kind: ClusterRole 59 | name: demo-kubectl-sidecar 60 | subjects: 61 | - kind: ServiceAccount 62 | name: demo-kubectl-sidecar 63 | namespace: default 64 | --- 65 | # This is the actual workload. 66 | apiVersion: apps/v1 67 | kind: Deployment 68 | metadata: 69 | name: demo-kubectl-sidecar 70 | namespace: default 71 | spec: 72 | replicas: 1 73 | selector: 74 | matchLabels: 75 | app: demo-kubectl-sidecar 76 | template: 77 | metadata: 78 | labels: 79 | app: demo-kubectl-sidecar 80 | spec: 81 | serviceAccountName: demo-kubectl-sidecar 82 | securityContext: 83 | # Set this to any valid GID, and two things happen: 84 | # 1) The volume "content" is group-owned by this GID. 85 | # 2) This GID is added to each container. 86 | fsGroup: 9376 87 | containers: 88 | - name: server 89 | image: nginx 90 | volumeMounts: 91 | - mountPath: /usr/share/nginx/html 92 | name: content 93 | readOnly: true 94 | initContainers: 95 | - name: sidecar 96 | image: thockin/kubectl-sidecar:v1.31.3-1 97 | restartPolicy: Always 98 | env: 99 | - name: MYPOD 100 | valueFrom: 101 | fieldRef: 102 | fieldPath: metadata.name 103 | - name: MYNS 104 | valueFrom: 105 | fieldRef: 106 | fieldPath: metadata.namespace 107 | - name: MYNODE 108 | valueFrom: 109 | fieldRef: 110 | fieldPath: spec.nodeName 111 | args: 112 | - bash 113 | - -c 114 | - | 115 | while true; do 116 | kubectl -n $MYNS get pod $MYPOD -o json | jq '.status' > /data/this-pod-status.json 117 | kubectl get node $MYNODE -o json | jq '.status' > /data/this-node-status.json 118 | sleep 30 119 | done 120 | volumeMounts: 121 | - name: content 122 | mountPath: /data 123 | securityContext: 124 | # This doesn't need to run as root. 125 | runAsUser: 9376 126 | runAsGroup: 9376 127 | volumes: 128 | - name: content 129 | emptyDir: {} 130 | terminationGracePeriodSeconds: 5 131 | -------------------------------------------------------------------------------- /Makefile: -------------------------------------------------------------------------------- 1 | # Where to push the docker image. 2 | REGISTRY ?= thockin 3 | NAME ?= kubectl-sidecar 4 | VERSION ?= v1.31.3 5 | RELEASE ?= 1 6 | 7 | # Set these to cross-compile. 8 | GOOS ?= 9 | GOARCH ?= 10 | 11 | # Set this to 1 to build a debugger-friendly binary. 12 | DBG ?= 13 | 14 | ### 15 | ### These variables should not need tweaking. 16 | ### 17 | 18 | ALL_PLATFORMS := linux/amd64 linux/arm64 19 | 20 | # Used internally. Users should pass GOOS and/or GOARCH. 21 | OS := $(if $(GOOS),$(GOOS),$(shell go env GOOS)) 22 | ARCH := $(if $(GOARCH),$(GOARCH),$(shell go env GOARCH)) 23 | 24 | IMAGE := $(REGISTRY)/$(NAME) 25 | TAG := $(VERSION)-$(RELEASE) 26 | OS_ARCH_TAG := $(TAG)__$(OS)_$(ARCH) 27 | 28 | DBG_MAKEFILE ?= 29 | ifneq ($(DBG_MAKEFILE),1) 30 | # If we're not debugging the Makefile, don't echo recipes. 31 | MAKEFLAGS += -s 32 | endif 33 | 34 | all: container 35 | 36 | # For the following OS/ARCH expansions, we transform OS/ARCH into OS_ARCH 37 | # because make pattern rules don't match with embedded '/' characters. 38 | 39 | container-%: 40 | $(MAKE) container \ 41 | --no-print-directory \ 42 | GOOS=$(firstword $(subst _, ,$*)) \ 43 | GOARCH=$(lastword $(subst _, ,$*)) 44 | 45 | push-%: 46 | $(MAKE) push \ 47 | --no-print-directory \ 48 | GOOS=$(firstword $(subst _, ,$*)) \ 49 | GOARCH=$(lastword $(subst _, ,$*)) 50 | 51 | all-container: $(addprefix container-, $(subst /,_, $(ALL_PLATFORMS))) 52 | 53 | all-push: $(addprefix push-, $(subst /,_, $(ALL_PLATFORMS))) 54 | 55 | # Used to track state in hidden files. 56 | DOTFILE_IMAGE = $(subst /,_,$(IMAGE))-$(OS_ARCH_TAG) 57 | 58 | container: .container-$(DOTFILE_IMAGE) container-name 59 | .container-$(DOTFILE_IMAGE): Dockerfile.in .buildx-initialized 60 | sed \ 61 | -e 's|{ARG_NAME}|$(NAME)|g' \ 62 | -e 's|{ARG_ARCH}|$(ARCH)|g' \ 63 | -e 's|{ARG_OS}|$(OS)|g' \ 64 | -e 's|{ARG_VERSION}|$(VERSION)|g' \ 65 | Dockerfile.in > .dockerfile-$(OS)_$(ARCH) 66 | docker buildx build \ 67 | --builder thockin \ 68 | --progress=plain \ 69 | --load \ 70 | --platform "$(OS)/$(ARCH)" \ 71 | -t $(IMAGE):$(OS_ARCH_TAG) \ 72 | -f .dockerfile-$(OS)_$(ARCH) \ 73 | . 74 | docker images -q $(IMAGE):$(OS_ARCH_TAG) > $@ 75 | 76 | container-name: 77 | echo "container: $(IMAGE):$(OS_ARCH_TAG)" 78 | echo 79 | 80 | push: .push-$(DOTFILE_IMAGE) push-name 81 | .push-$(DOTFILE_IMAGE): .container-$(DOTFILE_IMAGE) 82 | docker push $(IMAGE):$(OS_ARCH_TAG) 83 | docker images -q $(IMAGE):$(OS_ARCH_TAG) > $@ 84 | 85 | push-name: 86 | echo "pushed: $(IMAGE):$(OS_ARCH_TAG)" 87 | echo 88 | 89 | # This depends on github.com/estesp/manifest-tool/v2/cmd/manifest-tool in $PATH. 90 | manifest-list: all-push 91 | echo "manifest-list: $(REGISTRY)/$(NAME):$(TAG)" 92 | platforms=$$(echo $(ALL_PLATFORMS) | sed 's/ /,/g'); \ 93 | manifest-tool \ 94 | push from-args \ 95 | --platforms "$$platforms" \ 96 | --template $(REGISTRY)/$(NAME):$(TAG)__OS_ARCH \ 97 | --target $(REGISTRY)/$(NAME):$(TAG) 98 | 99 | # Help set up multi-arch build tools. This assumes you have the tools 100 | # installed. If you already have a buildx builder available, you don't need 101 | # this. See https://medium.com/@artur.klauser/building-multi-architecture-docker-images-with-buildx-27d80f7e2408 102 | # for great context. 103 | .buildx-initialized: 104 | docker buildx create --name thockin --node thockin-0 >/dev/null 105 | docker run --rm --privileged multiarch/qemu-user-static --reset -p yes >/dev/null 106 | date > $@ 107 | 108 | clean: 109 | rm -rf .container-* .dockerfile-* .push-* .buildx-initialized 110 | -------------------------------------------------------------------------------- /LICENSE: -------------------------------------------------------------------------------- 1 | Apache License 2 | Version 2.0, January 2004 3 | http://www.apache.org/licenses/ 4 | 5 | TERMS AND CONDITIONS FOR USE, REPRODUCTION, AND DISTRIBUTION 6 | 7 | 1. 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