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# Kubernetes

## Introduction

This article describes how to deploy DMS to Kubernetes. We highly recommend everyone to use our community DMS Helm chart.

Requirements

1. Basic knowledge about Kubernetes from the reader.
2. A basic understanding of mail servers.
3. Ideally, the reader has already deployed DMS before with a simpler setup (`docker run` or Docker Compose).

Limited Support

DMS does not officially support Kubernetes. This content is entirely community-supported. If you find errors, please open an issue and raise a PR.

## Manually Writing Manifests

If using our Helm chart is not viable for you, here is some guidance to start with your own manifests.

`ConfigMap``PersistentVolumeClaim``Service``Certificate``Deployment`

Provide the basic configuration via environment variables with a `ConfigMap`.

Example

Below is only an example configuration, adjust the `ConfigMap` to your own needs.

    ---
    apiVersion: v1
    kind: ConfigMap

    metadata:
      name: mailserver.environment

    immutable: false

    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
      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'

      # here, we provide an example for the SSL configuration
      SSL_TYPE: manual
      SSL_CERT_PATH: /secrets/ssl/rsa/tls.crt
      SSL_KEY_PATH: /secrets/ssl/rsa/tls.key

You can also make use of user-provided configuration files (e.g. `user-patches.sh`, `postfix-accounts.cf`, etc), to customize DMS to your needs.

Providing config files

Here is a minimal example that supplies a `postfix-accounts.cf` file inline with two users:

    ---
    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

Static Configuration

The inline `postfix-accounts.cf` config example above provides file content that is static. It is mounted as read-only at runtime, thus cannot support modifications.

For production deployments, use persistent volumes instead (via `PersistentVolumeClaim`). That will enable files like `postfix-account.cf` to add and remove accounts, while also persisting those changes externally from the container.

Modularize your `ConfigMap`

Kustomize can be a useful tool as it supports creating a `ConfigMap` from multiple files.

To persist data externally from the DMS container, configure a `PersistentVolumeClaim` (PVC).

Make sure you have a storage system (like Longhorn, Rook, etc.) and that you choose the correct `storageClassName` (according to your storage system).

Example

    ---
    apiVersion: v1
    kind: PersistentVolumeClaim

    metadata:
      name: data

    spec:
      storageClassName: local-path
      accessModes:
        - ReadWriteOnce
      resources:
        requests:
          storage: 25Gi

A `Service` is required for getting the traffic to the pod itself. It configures a load balancer with the ports you'll need.

The configuration for a `Service` affects if the original IP from a connecting client is preserved (this is important). More about this further down below.

Example

    ---
    apiVersion: v1
    kind: Service

    metadata:
      name: mailserver
      labels:
        app: mailserver

    spec:
      type: LoadBalancer

      selector:
        app: mailserver

      ports:
        # smtp
        - name: smtp
          port: 25
          targetPort: smtp
          protocol: TCP
        # submissions (ESMTP with implicit TLS)
        - name: submission
          port: 465
          targetPort: submissions
          protocol: TCP
        # submission (ESMTP with explicit TLS)
        - name: submission
          port: 587
          targetPort: submission
          protocol: TCP
        # imaps (implicit TLS)
        - name: imaps
          port: 993
          targetPort: imaps
          protocol: TCP

Using `cert-manager` to supply TLS certificates

    ---
    apiVersion: cert-manager.io/v1
    kind: Certificate

    metadata:
      name: mail-tls-certificate-rsa

    spec:
      secretName: mail-tls-certificate-rsa
      isCA: false
      privateKey:
        algorithm: RSA
        encoding: PKCS1
        size: 2048
      dnsNames: [mail.example.com]
      issuerRef:
        name: mail-issuer
        kind: Issuer

The TLS docs page provides guidance when it comes to certificates and transport layer security.

ECDSA + RSA (fallback)

You could supply RSA certificates as fallback certificates instead, with ECDSA as the primary. DMS supports dual certificates via the ENV `SSL_ALT_CERT_PATH` and `SSL_ALT_KEY_PATH`.

Always provide sensitive information via a `Secret`

For storing OpenDKIM keys, TLS certificates, or any sort of sensitive data - you should be using `Secret`s.

A `Secret` is similar to `ConfigMap`, it can be used and mounted as a volume as demonstrated in the `Deployment` manifest tab.

The `Deployment` config is the most complex component.

- It instructs Kubernetes how to run the DMS container and how to apply your `ConfigMap`s, persisted storage, etc.
- Additional options can be set to enforce runtime security.

Example

    ---
    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 privileged 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: ghcr.io/docker-mailserver/docker-mailserver:latest
              imagePullPolicy: IfNotPresent

              securityContext:
                # `allowPrivilegeEscalation: true` is required to support SGID via the `postdrop`
                # executable in `/var/mail-state` for Postfix (maildrop + public dirs):
                # https://github.com/docker-mailserver/docker-mailserver/pull/3625
                allowPrivilegeEscalation: true
                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
                    - KILL
                  drop: [ALL]
                seccompProfile:
                  type: RuntimeDefault

              # 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

                # certificates
                - name: certificates-rsa
                  mountPath: /secrets/ssl/rsa/
                  readOnly: true

              ports:
                - name: smtp
                  containerPort: 25
                  protocol: TCP
                - name: submissions
                  containerPort: 465
                  protocol: TCP
                - name: submission
                  containerPort: 587
                - name: imaps
                  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

            # certificates
            - name: certificates-rsa
              secret:
                secretName: mail-tls-certificate-rsa
                items:
                  - key: tls.key
                    path: tls.key
                  - key: tls.crt
                    path: tls.crt

## Exposing your Mail Server to the Outside World

The more difficult part with Kubernetes is to expose a deployed DMS instance to the outside world.

The major problem with exposing DMS to the outside world in Kubernetes is to preserve the real client IP. The real client IP is required by DMS for performing IP-based DNS and spam checks.

Kubernetes provides multiple ways to address this; each has its upsides and downsides.

Configure IP ManuallyHost networkUsing the PROXY Protocol
Advantages / Disadvantages

- Simple
- Requires the node to have a dedicated, publicly routable IP address
- Limited to a single node (associated to the dedicated IP address)
- Your deployment requires an explicit IP in your configuration (or an entire Load Balancer).

Requirements

1. You can dedicate a publicly routable IP address for the DMS configured `Service`.
2. A dedicated IP is required to allow your mail server to have matching `A` and `PTR` records (which other mail servers will use to verify trust when they receive mail sent from your DMS instance).

Example

Assign the DMS `Service` an external IP directly, or delegate an LB to assign the IP on your behalf.

External-IP ServiceLoad-Balancer

The DMS `Service` is configured with an "external IP" manually. Append your externally reachable IP address to `spec.externalIPs`.

    ---
    apiVersion: v1
    kind: Service

    metadata:
      name: mailserver
      labels:
        app: mailserver

    spec:
      selector:
        app: mailserver
      ports:
        - name: smtp
          port: 25
          targetPort: smtp
        # ...

      externalIPs:
        - 10.20.30.40

The config differs depending on your choice of load balancer. This example uses MetalLB.

    ---
    apiVersion: v1
    kind: Service

    metadata:
      name: mailserver
      labels:
        app: mailserver
      annotations:
        metallb.universe.tf/address-pool: mailserver

    # ...

    ---
    apiVersion: metallb.io/v1beta1
    kind: IPAddressPool

    metadata:
      name: mail
      namespace: metallb-system

    spec:
      addresses: [ <YOUR PUBLIC DEDICATED IP IN CIDR NOTATION> ]
      autoAssign: true

    ---
    apiVersion: metallb.io/v1beta1
    kind: L2Advertisement

    metadata:
      name: mail
      namespace: metallb-system

    spec:
      ipAddressPools: [ mailserver ]

Advantages / Disadvantages

- Simple
- Requires the node to have a dedicated, publicly routable IP address
- Limited to a single node (associated to the dedicated IP address)
- It is not possible to access DMS via other cluster nodes, only via the node that DMS was deployed on
- Every port within the container is exposed on the host side

Example

Using `hostPort` and `hostNetwork: true` is a similar approach to `network_mode: host` with Docker Compose.

    ---
    apiVersion: apps/v1
    kind: Deployment

    metadata:
      name: mailserver

    # ...
        spec:
          hostNetwork: true
        # ...
          containers:
            # ...
              ports:
                - name: smtp
                  containerPort: 25
                  hostPort: 25
                - name: submissions
                  containerPort: 465
                  hostPort: 465
                - name: submission
                  containerPort: 587
                  hostPort: 587
                - name: imaps
                  containerPort: 993
                  hostPort: 993

Advantages / Disadvantages

- Preserves the origin IP address of clients (which is crucial for DNS related checks)
- Aligns with a best practice for Kubernetes by using a dedicated ingress, routing external traffic to the k8s cluster (with the benefits of flexible routing rules)
- Avoids the restraint of a single node (as a workaround to preserve the original client IP)
- Introduces complexity by requiring:
  - A reverse-proxy / ingress controller (potentially extra setup)
  - Kubernetes manifest changes for the DMS configured `Service`
  - DMS configuration changes for Postfix and Dovecot
- To keep support for direct connections to DMS services internally within cluster, service ports must be "duplicated" to offer an alternative port for connections using PROXY protocol
- Custom Fail2Ban required: Because the traffic to DMS is now coming from the proxy, banning the origin IP address will have no effect; you'll need to implement a custom solution for your setup.

What is the PROXY protocol?

PROXY protocol is a network protocol for preserving a client’s IP address when the client’s TCP connection passes through a proxy.

It is a common feature supported among reverse-proxy services (NGINX, HAProxy, Traefik), which you may already have handling ingress traffic for your cluster.

    flowchart LR
        A(External Mail Server) -->|Incoming connection| B
        subgraph cluster
        B("Ingress Acting as a Proxy") -->|PROXY protocol connection| C(DMS)
        end

For more information on the PROXY protocol, refer to our dedicated docs page on the topic.

Configure the Ingress Controller
TraefikNGINX

On Traefik's side, the configuration is very simple.

- Create an entrypoint for each port that you want to expose (probably 25, 465, 587 and 993).
- Each entrypoint should configure an `IngressRouteTCP` that routes to the equivalent internal DMS `Service` port which supports PROXY protocol connections.

The below snippet demonstrates an example for two entrypoints, `submissions` (port 465) and `imaps` (port 993).

    ---
    apiVersion: v1
    kind: Service

    metadata:
      name: mailserver

    spec:
      # This an optimization to get rid of additional routing steps.
      # Previously "type: LoadBalancer"
      type: ClusterIP

    ---
    apiVersion: traefik.io/v1alpha1
    kind: IngressRouteTCP

    metadata:
      name: smtp

    spec:
      entryPoints: [ submissions ]
      routes:
        - match: HostSNI(`*`)
          services:
            - name: mailserver
              namespace: mail
              port: subs-proxy # note the 15 character limit here
              proxyProtocol:
                version: 2

    ---
    apiVersion: traefik.io/v1alpha1
    kind: IngressRouteTCP

    metadata:
      name: imaps

    spec:
      entryPoints: [ imaps ]
      routes:
        - match: HostSNI(`*`)
          services:
            - name: mailserver
              namespace: mail
              port: imaps-proxy
              proxyProtocol:
                version: 2

`*-proxy` port name suffix

The `IngressRouteTCP` example configs above reference ports with a `*-proxy` suffix.

- These port variants will be defined in the `Deployment` manifest, and are scoped to the `mailserver` service (via `spec.routes.services.name`).
- The suffix is used to distinguish that these ports are only compatible with connections using the PROXY protocol, which is what your ingress controller should be managing for you by adding the correct PROXY protocol headers to TCP connections it routes to DMS.

With an NGINX ingress controller, add the following to the TCP services config map (as described here):

    25:  "mailserver/mailserver:25::PROXY"
    465: "mailserver/mailserver:465::PROXY"
    587: "mailserver/mailserver:587::PROXY"
    993: "mailserver/mailserver:993::PROXY"

Adjust DMS config for Dovecot + Postfix
Only ingress should connect to DMS with PROXY protocol

While Dovecot will restrict connections via PROXY protocol to only clients trusted configured via `haproxy_trusted_networks`, Postfix does not have an equivalent setting. Public clients should always route through ingress to establish a PROXY protocol connection.

You are responsible for properly managing traffic inside your cluster and to ensure that only trustworthy entities can connect to the designated PROXY protocol ports.

With Kubernetes, this is usually the task of the CNI (container network interface).

Advised approach

The "Separate PROXY protocol ports" tab below introduces a little more complexity, but provides better compatibility for internal connections to DMS.

Only accept connections with PROXY protocolSeparate PROXY protocol ports for ingress

Connections to DMS within the internal cluster will be rejected

The services for these ports can only enable PROXY protocol support by mandating the protocol on all connections for these ports.

This can be problematic when you also need to support internal cluster traffic directly to DMS (instead of routing indirectly through the ingress controller).

Here is an example configuration for Postfix, Dovecot, and the required adjustments for the `Deployment` manifest. The port names are adjusted here only to convey the additional context described earlier.

    kind: ConfigMap
    apiVersion: v1
    metadata:
      name: mailserver-extra-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
        submissions/inet/smtpd_upstream_proxy_protocol=haproxy
      dovecot.cf: |
        haproxy_trusted_networks = <YOUR POD CIDR>
        service imap-login {
          inet_listener imap {
            haproxy = yes
          }
          inet_listener imaps {
            haproxy = yes
          }
        }
    # ...

    ---
    kind: Deployment
    apiVersion: apps/v1
    metadata:
      name: mailserver
    spec:
      template:
        spec:
          containers:
            - name: docker-mailserver
              # ...
              ports:
                - name: smtp-proxy
                  containerPort: 25
                  protocol: TCP
                - name: imap-proxy
                  containerPort: 143
                  protocol: TCP
                - name: subs-proxy
                  containerPort: 465
                  protocol: TCP
                - name: sub-proxy
                  containerPort: 587
                  protocol: TCP
                - name: imaps-proxy
                  containerPort: 993
                  protocol: TCP
              # ...
              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

Info

Supporting internal cluster connections to DMS without using PROXY protocol requires both Postfix and Dovecot to be configured with alternative ports for each service port (which only differ by enforcing PROXY protocol connections).

- The ingress controller will route public connections to the internal alternative ports for DMS (`*-proxy` variants).
- Internal cluster connections will instead use the original ports configured for the DMS container directly (which are private to the cluster network).

In this example we'll create a copy of the original service ports with PROXY protocol enabled, and increment the port number assigned by `10000`.

Create a `user-patches.sh` file to apply these config changes during container startup:

    #!/bin/bash

    # Duplicate the config for the submission(s) service ports (587 / 465) with adjustments for the PROXY ports (10587 / 10465) and `syslog_name` setting:
    postconf -Mf submission/inet | sed -e s/^submission/10587/ -e 's/submission/submission-proxyprotocol/' >> /etc/postfix/master.cf
    postconf -Mf submissions/inet | sed -e s/^submissions/10465/ -e 's/submissions/submissions-proxyprotocol/' >> /etc/postfix/master.cf
    # Enable PROXY Protocol support for these new service variants:
    postconf -P 10587/inet/smtpd_upstream_proxy_protocol=haproxy
    postconf -P 10465/inet/smtpd_upstream_proxy_protocol=haproxy

    # Create a variant for port 25 too (NOTE: Port 10025 is already assigned in DMS to Amavis):
    postconf -Mf smtp/inet | sed -e s/^smtp/12525/ >> /etc/postfix/master.cf
    # Enable PROXY Protocol support (different setting as port 25 is handled via postscreen), optionally configure a `syslog_name` to distinguish in logs:
    postconf -P 12525/inet/postscreen_upstream_proxy_protocol=haproxy 12525/inet/syslog_name=smtp-proxyprotocol

For Dovecot, you can configure `dovecot.cf` to look like this:

    haproxy_trusted_networks = <YOUR POD CIDR>

    service imap-login {
      inet_listener imap-proxied {
        haproxy = yes
        port = 10143
      }

      inet_listener imaps-proxied {
        haproxy = yes
        port = 10993
        ssl = yes
      }
    }

Update the `Deployment` manifest `ports` section by appending these new ports:

    - name: smtp-proxy
      # not 10025 in this example due to a possible clash with Amavis
      containerPort: 12525
      protocol: TCP
    - name: imap-proxy
      containerPort: 10143
      protocol: TCP
    - name: subs-proxy
      containerPort: 10465
      protocol: TCP
    - name: sub-proxy
      containerPort: 10587
      protocol: TCP
    - name: imaps-proxy
      containerPort: 10993
      protocol: TCP

Note

If you use other Dovecot ports (110, 995, 4190), you may want to configure those similar to above. The `dovecot.cf` config for these ports is documented here (in the equivalent section of that page).