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---
title: 'Advanced | Kubernetes'
---
## Introduction
This article describes how to deploy DMS to Kubernetes. We highly recommend everyone to use our community [DMS Helm chart][github-web::docker-mailserver-helm].
!!! note "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_).
!!! warning "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.
<!-- This empty quote block is purely for a visual border -->
!!! quote ""
=== "`ConfigMap`"
Provide the basic configuration via environment variables with a `ConfigMap`.
!!! example
Below is only an example configuration, adjust the `ConfigMap` to your own needs.
```yaml
---
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.
??? example "Providing config files"
Here is a minimal example that supplies a `postfix-accounts.cf` file inline with two users:
```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
```
!!! warning "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.
!!! tip "Modularize your `ConfigMap`"
[Kustomize][kustomize] can be a useful tool as it supports creating a `ConfigMap` from multiple files.
=== "`PersistentVolumeClaim`"
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
```yaml
---
apiVersion: v1
kind: PersistentVolumeClaim
metadata:
name: data
spec:
storageClassName: local-path
accessModes:
- ReadWriteOnce
resources:
requests:
storage: 25Gi
```
=== "`Service`"
A [`Service`][k8s-docs::config::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](#exposing-your-mail-server-to-the-outside-world).
!!! example
```yaml
---
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
```
=== "`Certificate`"
!!! example "Using [`cert-manager`][cert-manager] to supply TLS certificates"
```yaml
---
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][docs-tls] provides guidance when it comes to certificates and transport layer security.
!!! tip "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`.
!!! warning "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][docs::k8s::config-deployment] tab.
=== "`Deployment`"
The [`Deployment`][k8s-docs::config::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
```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 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][k8s-docs::service-source-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.
<!-- This empty quote block is purely for a visual border -->
!!! quote ""
=== "Configure IP Manually"
???+ abstract "Advantages / Disadvantages"
- [x] 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_).
!!! info "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 Service"
The DMS `Service` is configured with an "[external IP][k8s-docs::network-external-ip]" manually. Append your externally reachable IP address to `spec.externalIPs`.
```yaml
---
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
```
=== "Load-Balancer"
The config differs depending on your choice of load balancer. This example uses [MetalLB][metallb-web].
```yaml
---
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 ]
```
=== "Host network"
???+ abstract "Advantages / Disadvantages"
- [x] 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][docker-docs::compose::network_mode].
```yaml
---
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
```
=== "Using the PROXY Protocol"
???+ abstract "Advantages / Disadvantages"
- [x] Preserves the origin IP address of clients (_which is crucial for DNS related checks_)
- [x] 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_)
- [x] Avoids the restraint of a single [node][k8s-docs::nodes] (_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][github-web::docker-mailserver::proxy-protocol-fail2ban].
??? question "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.
```mermaid
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][docs-mailserver-behind-proxy] on the topic.
???+ example "Configure the Ingress Controller"
=== "Traefik"
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`][traefik-docs::k8s::ingress-route-tcp] 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).
```yaml
---
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
```
!!! info "`*-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][docs::k8s::config-deployment], 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.
=== "NGINX"
With an [NGINX ingress controller][k8s-docs::nginx], add the following to the TCP services config map (_as described [here][k8s-docs::nginx-expose]_):
```yaml
25: "mailserver/mailserver:25::PROXY"
465: "mailserver/mailserver:465::PROXY"
587: "mailserver/mailserver:587::PROXY"
993: "mailserver/mailserver:993::PROXY"
```
???+ example "Adjust DMS config for Dovecot + Postfix"
??? warning "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_).
!!! tip "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 protocol"
!!! warning "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][docs-postfix], [Dovecot][docs-dovecot], and the required adjustments for the [`Deployment` manifest][docs::k8s::config-deployment]. The port names are adjusted here only to convey the additional context described earlier.
```yaml
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
```
=== "Separate PROXY protocol ports for ingress"
!!! 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:
```bash
#!/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`][docs-dovecot] to look like this:
```cf
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][docs::k8s::config-deployment] `ports` section by appending these new ports:
```yaml
- 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][docs-mailserver-behind-proxy] (_in the equivalent section of that page_).
[docs::k8s::config-deployment]: #deployment
[docs-tls]: ../security/ssl.md
[docs-dovecot]: ./override-defaults/dovecot.md
[docs-postfix]: ./override-defaults/postfix.md
[docs-mailserver-behind-proxy]: ../../examples/tutorials/mailserver-behind-proxy.md
[github-web::docker-mailserver-helm]: https://github.com/docker-mailserver/docker-mailserver-helm
[docker-docs::compose::network_mode]: https://docs.docker.com/compose/compose-file/compose-file-v3/#network_mode
[kustomize]: https://kustomize.io/
[cert-manager]: https://cert-manager.io/docs/
[metallb-web]: https://metallb.universe.tf/
[k8s-docs::config::service]: https://kubernetes.io/docs/concepts/services-networking/service
[k8s-docs::config::deployment]: https://kubernetes.io/docs/concepts/workloads/controllers/deployment/#creating-a-deployment
[k8s-docs::nodes]: https://kubernetes.io/docs/concepts/architecture/nodes
[k8s-docs::nginx]: https://kubernetes.github.io/ingress-nginx
[k8s-docs::nginx-expose]: https://kubernetes.github.io/ingress-nginx/user-guide/exposing-tcp-udp-services
[k8s-docs::service-source-ip]: https://kubernetes.io/docs/tutorials/services/source-ip
[k8s-docs::network-external-ip]: https://kubernetes.io/docs/concepts/services-networking/service/#external-ips
[traefik-docs::k8s::ingress-route-tcp]: https://doc.traefik.io/traefik/routing/providers/kubernetes-crd/#kind-ingressroutetcp
[github-web::docker-mailserver::proxy-protocol-fail2ban]: https://github.com/docker-mailserver/docker-mailserver/issues/1761#issuecomment-2016879319
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