--- title: 'Advanced | Kubernetes' --- ## Introduction Kubernetes (also known by its abbreviation K8s) is a production-grade orchestrating tool for containers. This article describes how to deploy `docker-mailserver` to K8s. K8s differs from Docker especially when it comes to separation of concerns: Whereas with Docker Compose, you can fit everything in one file, with K8s, the information is split. This may seem (too) verbose, but actually provides a clear structure with more features and scalability. We are going to have a look at how to deploy one instance of `docker-mailserver` to your cluster. We assume basic knowledge about K8s from the reader. If you're not familiar with K8s, we highly recommend starting with something less complex, like Docker Compose. !!! warning "About Support for K8s" Please note that Kubernetes **is not** officially supported and we do not build images specifically designed for it. When opening an issue, please remember that only Docker & Docker Compose are officially supported. This content is entirely community-supported. If you find errors, please open an issue and provide a PR. ## Manifests ### Configuration We want to provide the basic configuration in the form of environment variables with a `ConfigMap`. Note that this is just an example configuration; tune the `ConfigMap` to your needs. ```yaml --- apiVersion: v1 kind: ConfigMap metadata: name: mailserver.environment immutable: true # turn off during development data: TLS_LEVEL: modern POSTSCREEN_ACTION: drop OVERRIDE_HOSTNAME: mail.example.com FAIL2BAN_BLOCKTYPE: drop POSTMASTER_ADDRESS: postmaster@example.com UPDATE_CHECK_INTERVAL: 10d POSTFIX_INET_PROTOCOLS: ipv4 ONE_DIR: '1' DMS_DEBUG: '0' ENABLE_CLAMAV: '1' ENABLE_POSTGREY: '0' ENABLE_FAIL2BAN: '1' AMAVIS_LOGLEVEL: '-1' SPOOF_PROTECTION: '1' MOVE_SPAM_TO_JUNK: '1' ENABLE_UPDATE_CHECK: '1' ENABLE_SPAMASSASSIN: '1' SUPERVISOR_LOGLEVEL: warn SPAMASSASSIN_SPAM_TO_INBOX: '1' ``` We can also make use of user-provided configuration files, e.g. `user-patches.sh`, `postfix-accounts.cf` and more, to adjust `docker-mailserver` to our likings. We encourage you to have a look at [Kustomize][kustomize] for creating `ConfigMap`s from multiple files, but for now, we will provide a simple, hand-written example. This example is absolutely minimal and only goes to show what can be done. ```yaml --- apiVersion: v1 kind: ConfigMap metadata: name: mailserver.files data: postfix-accounts.cf: | test@example.com|{SHA512-CRYPT}$6$someHashValueHere other@example.com|{SHA512-CRYPT}$6$someOtherHashValueHere ``` ### Persistence Thereafter, we need persistence for our data. ```yaml --- apiVersion: v1 kind: PersistentVolumeClaim metadata: name: data spec: storageClassName: local-path accessModes: - ReadWriteOnce resources: requests: storage: 25Gi ``` ### Service A `Service` is required for getting the traffic to the pod itself. The service is somewhat crucial. Its configuration determines whether the original IP from the sender will be kept. [More about this further down below](#exposing-your-mail-server-to-the-outside-world). The configuration you're seeing does keep the original IP, but you will not be able to scale this way. We have chosen to go this route in this case because we think most K8s users will only want to have one instance anyway, and users that need high availability know how to do it anyways. ```yaml --- apiVersion: v1 kind: Service metadata: name: mailserver labels: app: mailserver spec: type: LoadBalancer externalTrafficPolicy: Local selector: app: mailserver ports: # Transfer - name: transfer port: 25 targetPort: transfer protocol: TCP # ESMTP with implicit TLS - name: esmtp-implicit port: 465 targetPort: esmtp-implicit protocol: TCP # ESMTP with explicit TLS (STARTTLS) - name: esmtp-explicit port: 587 targetPort: esmtp-explicit protocol: TCP # IMAPS with implicit TLS - name: imap-implicit port: 993 targetPort: imap-implicit protocol: TCP ``` ### Deployments Last but not least, the `Deployment` becomes the most complex component. It instructs Kubernetes how to run the docker-mailserver container and how to apply your ConfigMaps and persisted storage. Additionally, we can set options to enforce runtime security here. ```yaml --- apiVersion: apps/v1 kind: Deployment metadata: name: mailserver annotations: ignore-check.kube-linter.io/run-as-non-root: >- 'mailserver' needs to run as root ignore-check.kube-linter.io/privileged-ports: >- 'mailserver' needs privilegdes ports ignore-check.kube-linter.io/no-read-only-root-fs: >- There are too many files written to make The root FS read-only spec: replicas: 1 selector: matchLabels: app: mailserver template: metadata: labels: app: mailserver annotations: container.apparmor.security.beta.kubernetes.io/mailserver: runtime/default spec: hostname: mail containers: - name: mailserver image: docker.io/mailserver/docker-mailserver:latest imagePullPolicy: IfNotPresent securityContext: allowPrivilegeEscalation: false readOnlyRootFilesystem: false runAsUser: 0 runAsGroup: 0 runAsNonRoot: false privileged: false capabilities: add: # file permission capabilities - CHOWN - FOWNER - MKNOD - SETGID - SETUID - DAC_OVERRIDE # network capabilities - NET_ADMIN # needed for F2B - NET_RAW # needed for F2B - NET_BIND_SERVICE # miscellaneous capabilities - SYS_CHROOT - SYS_PTRACE - KILL drop: [ALL] seccompProfile: type: RuntimeDefault # You want to tune this to your needs. If you disable ClamAV, # you can use less RAM and CPU. This becomes important in # case you're low on resources and Kubernetes refuses to # schedule new pods. resources: limits: memory: 4Gi cpu: 1500m requests: memory: 2Gi cpu: 600m volumeMounts: - name: files subPath: postfix-accounts.cf mountPath: /tmp/docker-mailserver/postfix-accounts.cf readOnly: true # PVCs - name: data mountPath: /var/mail subPath: data readOnly: false - name: data mountPath: /var/mail-state subPath: state readOnly: false - name: data mountPath: /var/log/mail subPath: log readOnly: false # other - name: tmp-files mountPath: /tmp readOnly: false ports: - name: transfer containerPort: 25 protocol: TCP - name: esmtp-implicit containerPort: 465 protocol: TCP - name: esmtp-explicit containerPort: 587 - name: imap-implicit containerPort: 993 protocol: TCP envFrom: - configMapRef: name: mailserver.environment restartPolicy: Always volumes: # configuration files - name: files configMap: name: mailserver.files # PVCs - name: data persistentVolumeClaim: claimName: data # other - name: tmp-files emptyDir: {} ``` ### Sensitive Data By now, `docker-mailserver` starts, but does not really work for long (or at all), because we're lacking certificates. The [TLS docs page][docs-tls] provides guidance for various approaches. !!! attention "Sensitive Data" For storing OpenDKIM keys, TLS certificates or any sort of sensitive data, you should be using `Secret`s. You can mount secrets like `ConfigMap`s and use them the same way. ## Exposing your Mail-Server to the Outside World The more difficult part with K8s is to expose a deployed `docker-mailserver` to the outside world. K8s provides multiple ways for doing that; each has downsides and complexity. The major problem with exposing `docker-mailserver` to outside world in K8s is to [preserve the real client IP][k8s-service-source-ip]. The real client IP is required by `docker-mailserver` for performing IP-based SPF checks and spam checks. If you do not require SPF checks for incoming mails, you may disable them in your [Postfix configuration][docs-postfix] by dropping the line that states: `check_policy_service unix:private/policyd-spf`. The easiest approach was covered above, using `#!yaml externalTrafficPolicy: Local`, which disables the service proxy, but makes the service local as well (which does not scale). This approach only works when you are given the correct (that is, a public and routable) IP address by a load balancer (like MetalLB). In this sense, the approach above is similar to the next example below. We want to provide you with a few alternatives too. **But** we also want to communicate the idea of another simple method: you could use a load-balancer without an external IP and DNAT the network traffic to the mail-server. After all, this does not interfere with SPF checks because it keeps the origin IP address. If no dedicated external IP address is available, you could try the latter approach, if one is available, use the former. ### External IPs Service The simplest way is to expose `docker-mailserver` as a [Service][k8s-network-service] with [external IPs][k8s-network-external-ip]. This is very similar to the approach taken above. Here, an external IP is given to the service directly by you. With the approach above, you tell your load-balancer to do this. ```yaml --- apiVersion: v1 kind: Service metadata: name: mailserver labels: app: mailserver spec: selector: app: mailserver ports: - name: smtp port: 25 targetPort: smtp # ... externalIPs: - 80.11.12.10 ``` This approach - does not preserve the real client IP, so SPF check of incoming mail will fail. - requires you to specify the exposed IPs explicitly. ### Proxy port to Service The [proxy pod][k8s-proxy-service] helps to avoid the necessity of specifying external IPs explicitly. This comes at the cost of complexity; you must deploy a proxy pod on each [Node][k8s-nodes] you want to expose `docker-mailserver` on. This approach - does not preserve the real client IP, so SPF check of incoming mail will fail. ### Bind to concrete Node and use host network One way to preserve the real client IP is to use `hostPort` and `hostNetwork: true`. This comes at the cost of availability; you can reach `docker-mailserver` from the outside world only via IPs of [Node][k8s-nodes] where `docker-mailserver` is deployed. ```yaml --- apiVersion: extensions/v1beta1 kind: Deployment metadata: name: mailserver # ... spec: hostNetwork: true # ... containers: # ... ports: - name: smtp containerPort: 25 hostPort: 25 - name: smtp-auth containerPort: 587 hostPort: 587 - name: imap-secure containerPort: 993 hostPort: 993 # ... ``` With this approach, - it is not possible to access `docker-mailserver` via other cluster Nodes, only via the Node `docker-mailserver` was deployed at. - every Port within the Container is exposed on the Host side. ### Proxy Port to Service via PROXY Protocol This way is ideologically the same as [using a proxy pod](#proxy-port-to-service), but instead of a separate proxy pod, you configure your ingress to proxy TCP traffic to the `docker-mailserver` pod using the PROXY protocol, which preserves the real client IP. #### Configure your Ingress With an [NGINX ingress controller][k8s-nginx], set `externalTrafficPolicy: Local` for its service, and add the following to the TCP services config map (as described [here][k8s-nginx-expose]): ```yaml 25: "mailserver/mailserver:25::PROXY" 465: "mailserver/mailserver:465::PROXY" 587: "mailserver/mailserver:587::PROXY" 993: "mailserver/mailserver:993::PROXY" ``` !!! help "HAProxy" With [HAProxy][dockerhub-haproxy], the configuration should look similar to the above. If you know what it actually looks like, add an example here. :smiley: #### Configure the Mailserver Then, configure both [Postfix][docs-postfix] and [Dovecot][docs-dovecot] to expect the PROXY protocol: ??? example "HAProxy Example" ```yaml kind: ConfigMap apiVersion: v1 metadata: name: mailserver.config labels: app: mailserver data: postfix-main.cf: | postscreen_upstream_proxy_protocol = haproxy postfix-master.cf: | smtp/inet/postscreen_upstream_proxy_protocol=haproxy submission/inet/smtpd_upstream_proxy_protocol=haproxy smtps/inet/smtpd_upstream_proxy_protocol=haproxy dovecot.cf: | # Assuming your ingress controller is bound to 10.0.0.0/8 haproxy_trusted_networks = 10.0.0.0/8, 127.0.0.0/8 service imap-login { inet_listener imap { haproxy = yes } inet_listener imaps { haproxy = yes } } # ... --- kind: Deployment apiVersion: extensions/v1beta1 metadata: name: mailserver spec: template: spec: containers: - name: docker-mailserver volumeMounts: - name: config subPath: postfix-main.cf mountPath: /tmp/docker-mailserver/postfix-main.cf readOnly: true - name: config subPath: postfix-master.cf mountPath: /tmp/docker-mailserver/postfix-master.cf readOnly: true - name: config subPath: dovecot.cf mountPath: /tmp/docker-mailserver/dovecot.cf readOnly: true ``` With this approach, - it is not possible to access `docker-mailserver` via cluster-DNS, as the PROXY protocol is required for incoming connections. [kustomize]: https://kustomize.io/ [docs-tls]: ../security/ssl.md [docs-dovecot]: ./override-defaults/dovecot.md [docs-postfix]: ./override-defaults/postfix.md [dockerhub-haproxy]: https://hub.docker.com/_/haproxy [k8s-nginx]: https://kubernetes.github.io/ingress-nginx [k8s-nginx-expose]: https://kubernetes.github.io/ingress-nginx/user-guide/exposing-tcp-udp-services [k8s-network-service]: https://kubernetes.io/docs/concepts/services-networking/service [k8s-network-external-ip]: https://kubernetes.io/docs/concepts/services-networking/service/#external-ips [k8s-nodes]: https://kubernetes.io/docs/concepts/architecture/nodes [k8s-proxy-service]: https://github.com/kubernetes/contrib/tree/master/for-demos/proxy-to-service [k8s-service-source-ip]: https://kubernetes.io/docs/tutorials/services/source-ip