github-com-docker-mailserver/docker-mailserver
1
0
Fork 0
mirror of https://github.com/docker-mailserver/docker-mailserver.git synced 2024-01-19 02:48:50 +00:00
Code Issues Projects Releases Wiki Activity

chore(docs): adjust K8s documentation (#2111)

Browse source
This commit is contained in:
Georg Lauterbach 2021-08-13 01:01:49 +02:00 committed by GitHub
parent 392ee076ec
commit 78c4bc8282
No known key found for this signature in database
GPG key ID: 4AEE18F83AFDEB23
1 changed files with 330 additions and 404 deletions
Show stats Download patch file Download diff file Expand all files Collapse all files

734
docs/content/config/advanced/kubernetes.md
View file

@ -2,392 +2,377 @@
title: 'Advanced | Kubernetes'
---
## Deployment Example
## Introduction
There is nothing much in deploying mailserver to Kubernetes itself. The things are pretty same as in [`docker-compose.yml`][github-file-compose], but with Kubernetes syntax.
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.
??? example "ConfigMap"
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.
```yaml
apiVersion: v1
kind: Namespace
!!! 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: >-
The mailserver needs to run as root
ignore-check.kube-linter.io/privileged-ports: >-
The 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:
name: mailserver
---
kind: ConfigMap
apiVersion: v1
metadata:
name: mailserver.env.config
namespace: mailserver
labels:
app: mailserver
data:
OVERRIDE_HOSTNAME: example.com
ENABLE_FETCHMAIL: "0"
FETCHMAIL_POLL: "120"
ENABLE_SPAMASSASSIN: "0"
ENABLE_CLAMAV: "0"
ENABLE_FAIL2BAN: "0"
ENABLE_POSTGREY: "0"
ONE_DIR: "1"
DMS_DEBUG: "0"
---
kind: ConfigMap
apiVersion: v1
metadata:
name: mailserver.config
namespace: mailserver
labels:
app: mailserver
data:
postfix-accounts.cf: |
user1@example.com|{SHA512-CRYPT}$6$2YpW1nYtPBs2yLYS$z.5PGH1OEzsHHNhl3gJrc3D.YMZkvKw/vp.r5WIiwya6z7P/CQ9GDEJDr2G2V0cAfjDFeAQPUoopsuWPXLk3u1
annotations:
container.apparmor.security.beta.kubernetes.io/mailserver: runtime/default
postfix-virtual.cf: |
alias1@example.com user1@dexample.com
#dovecot.cf: |
# service stats {
# unix_listener stats-reader {
# group = docker
# mode = 0666
# }
# unix_listener stats-writer {
# group = docker
# mode = 0666
# }
# }
SigningTable: |
*@example.com mail._domainkey.example.com
KeyTable: |
mail._domainkey.example.com example.com:mail:/etc/opendkim/keys/example.com-mail.key
TrustedHosts: |
127.0.0.1
localhost
#user-patches.sh: |
# #!/bin/bash
#fetchmail.cf: |
```
??? example "Secret"
```yaml
apiVersion: v1
kind: Namespace
metadata:
name: mailserver
---
kind: Secret
apiVersion: v1
metadata:
name: mailserver.opendkim.keys
namespace: mailserver
labels:
app: mailserver
type: Opaque
data:
example.com-mail.key: 'base64-encoded-DKIM-key'
```
??? example "Service"
```yaml
apiVersion: v1
kind: Namespace
metadata:
name: mailserver
---
kind: Service
apiVersion: v1
metadata:
name: mailserver
namespace: mailserver
labels:
app: mailserver
spec:
selector:
app: mailserver
ports:
- name: smtp
port: 25
targetPort: smtp
- name: smtp-secure
port: 465
targetPort: smtp-secure
- name: smtp-auth
port: 587
targetPort: smtp-auth
- name: imap
port: 143
targetPort: imap
- name: imap-secure
port: 993
targetPort: imap-secure
```
hostname: mailserver
containers:
- name: mailserver
image: ghcr.io/docker-mailserver/docker-mailserver:10.0.0
imagePullPolicy: IfNotPresent
??? example "Deployment"
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
```yaml
apiVersion: v1
kind: Namespace
metadata:
name: mailserver
---
apiVersion: apps/v1
kind: Deployment
metadata:
name: mailserver
namespace: mailserver
spec:
replicas: 1
selector:
matchLabels:
app: mailserver
template:
metadata:
labels:
app: mailserver
role: mail
tier: backend
spec:
#nodeSelector:
# kubernetes.io/hostname: local.k8s
#initContainers:
#- name: init-myservice
# image: busybox
# command: ["/bin/sh", "-c", "cp /tmp/user-patches.sh /tmp/files"]
# volumeMounts:
# - name: config
# subPath: user-patches.sh
# mountPath: /tmp/user-patches.sh
# readOnly: true
# - name: tmp-files
# mountPath: /tmp/files
containers:
- name: docker-mailserver
image: mailserver/docker-mailserver:latest
imagePullPolicy: Always
securityContext:
capabilities:
# If Fail2Ban is not enabled, you can remove NET_ADMIN.
# If you are running on CRI-O, you will need the SYS_CHROOT capability,
# as it is no longer a default capability.
add: ["NET_ADMIN", "SYS_PTRACE", "SYS_CHROOT" ]
volumeMounts:
- name: config
subPath: postfix-accounts.cf
mountPath: /tmp/docker-mailserver/postfix-accounts.cf
readOnly: true
#- name: config
# subPath: postfix-main.cf
# mountPath: /tmp/docker-mailserver/postfix-main.cf
# readOnly: true
- name: config
subPath: postfix-virtual.cf
mountPath: /tmp/docker-mailserver/postfix-virtual.cf
readOnly: true
- name: config
subPath: fetchmail.cf
mountPath: /tmp/docker-mailserver/fetchmail.cf
readOnly: true
- name: config
subPath: dovecot.cf
mountPath: /tmp/docker-mailserver/dovecot.cf
readOnly: true
#- name: config
# subPath: user1.example.com.dovecot.sieve
# mountPath: /tmp/docker-mailserver/user1@example.com.dovecot.sieve
# readOnly: true
#- name: tmp-files
# subPath: user-patches.sh
# mountPath: /tmp/docker-mailserver/user-patches.sh
- name: config
subPath: SigningTable
mountPath: /tmp/docker-mailserver/opendkim/SigningTable
readOnly: true
- name: config
subPath: KeyTable
mountPath: /tmp/docker-mailserver/opendkim/KeyTable
readOnly: true
- name: config
subPath: TrustedHosts
mountPath: /tmp/docker-mailserver/opendkim/TrustedHosts
readOnly: true
- name: opendkim-keys
mountPath: /tmp/docker-mailserver/opendkim/keys
readOnly: true
- name: data
mountPath: /var/mail
subPath: data
- name: data
mountPath: /var/mail-state
subPath: state
- name: data
mountPath: /var/log/mail
subPath: log
ports:
- name: smtp
containerPort: 25
protocol: TCP
- name: smtp-secure
containerPort: 465
protocol: TCP
- name: smtp-auth
containerPort: 587
- name: imap
containerPort: 143
protocol: TCP
- name: imap-secure
containerPort: 993
protocol: TCP
envFrom:
- configMapRef:
name: mailserver.env.config
volumes:
- name: config
configMap:
name: mailserver.config
- name: opendkim-keys
secret:
secretName: mailserver.opendkim.keys
# 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
persistentVolumeClaim:
claimName: mail-storage
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
emptyDir: {}
```
mountPath: /tmp
readOnly: false
!!! warning
Any sensitive data (keys, etc) should be deployed via [Secrets][k8s-config-secret]. Other configuration just fits well into [ConfigMaps][k8s-config-pod].
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
!!! note
Make sure that [Pod][k8s-workload-pod] is [assigned][k8s-assign-pod-node] to specific [Node][k8s-nodes] in case you're using volume for data directly with `hostPath`. Otherwise Pod can be rescheduled on a different Node and previous data won't be found. Except the case when you're using some shared filesystem on your Nodes.
!!! note
If you experience issues with processes crashing showing an error like `operation not permitted` or `postfix/pickup[987]: fatal: chroot(/var/spool/postfix): Operation not permitted`, then you should add the `SYS_CHROOT` capability. Runtimes like CRI-O do not ship with this capability by default.
envFrom:
- configMapRef:
name: mailserver.environment
## Exposing to the Outside World
restartPolicy: Always
The hard part with Kubernetes is to expose deployed mailserver to outside world. Kubernetes provides multiple ways for doing that. Each has its downsides and complexity.
volumes:
# configuration files
- name: files
configMap:
name: mailserver.files
The major problem with exposing mailserver to outside world in Kubernetes is to [preserve real client IP][k8s-service-source-ip]. Real client IP is required by mailserver for performing IP-based SPF checks and spam checks.
# PVCs
- name: data
persistentVolumeClaim:
claimName: data
Preserving real client IP is relatively [non-trivial in Kubernetes][k8s-service-source-ip] and most exposing ways do not provide it. So, it's up to you to decide which exposing way suits better your needs in a price of complexity.
# other
- name: tmp-files
emptyDir: {}
```
If you do not require SPF checks for incoming mails you may disable them in [Postfix configuration][docs-postfix] by dropping following line (which removes `check_policy_service unix:private/policyd-spf` option):
### Sensitive Data
!!! example
By now, the mailserver starts, but does not really work for long (or at all), because we're lacking certificates. You will need to choose yourself, which approach you'd want to go with. The [TLS][docs-tls] section provides you with an overview.
```yaml
kind: ConfigMap
apiVersion: v1
metadata:
name: mailserver.config
labels:
app: mailserver
data:
postfix-main.cf: |
smtpd_recipient_restrictions = permit_sasl_authenticated, permit_mynetworks, reject_unauth_destination, reject_unauth_pipelining, reject_invalid_helo_hostname, reject_non_fqdn_helo_hostname, reject_unknown_recipient_domain, reject_rbl_client zen.spamhaus.org, reject_rbl_client bl.spamcop.net
# ...
!!! 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.
kind: Deployment
apiVersion: extensions/v1beta1
metadata:
name: mailserver
# ...
volumeMounts:
- name: config
subPath: postfix-main.cf
mountPath: /tmp/docker-mailserver/postfix-main.cf
readOnly: true
```
## Exposing your Mailserver to the Outside World
The more difficult part with K8s is to expose a deployed mailserver to the outside world. K8s provides multiple ways for doing that; each has downsides and complexity. The major problem with exposing the mailserver to outside world in K8s is to [preserve the real client IP][k8s-service-source-ip]. The real client IP is required by the 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 mailserver as a [Service][k8s-network-service] with [external IPs][k8s-network-external-ip].
The simplest way is to expose the 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.
!!! example
```yaml
---
apiVersion: v1
kind: Service
```yaml
kind: Service
apiVersion: v1
metadata:
name: mailserver
labels:
app: mailserver
spec:
selector:
app: mailserver
ports:
- name: smtp
port: 25
targetPort: smtp
metadata:
name: mailserver
labels:
app: mailserver
spec:
selector:
app: mailserver
ports:
- name: smtp
port: 25
targetPort: smtp
# ...
externalIPs:
- 80.11.12.10
```
**Downsides**
externalIPs:
- 80.11.12.10
```
- __Real client IP is not preserved__, so SPF check of incoming mail will fail.
This approach
- Requirement to specify exposed IPs explicitly.
- 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 necessity of specifying external IPs explicitly. This comes in price of complexity: you must deploy Proxy Pod on each [Node][k8s-nodes] you want to expose mailserver on.
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 mailserver on.
**Downsides**
This approach
- __Real client IP is not preserved__, so SPF check of incoming mail will fail.
- does not preserve the real client IP, so SPF check of incoming mail will fail.
### Bind to concrete Node and use host network
The simplest way to preserve real client IP is to use `hostPort` and `hostNetwork: true` in the mailserver [Pod][k8s-workload-pod]. This comes in price of availability: you can talk to mailserver from outside world only via IPs of [Node][k8s-nodes] where mailserver is deployed.
One way to preserve the real client IP is to use `hostPort` and `hostNetwork: true`. This comes at the cost of availability; you can talk to the mailserver from outside world only via IPs of [Node][k8s-nodes] where mailserver is deployed.
!!! example
```yaml
---
apiVersion: extensions/v1beta1
kind: Deployment
```yaml
kind: Deployment
apiVersion: extensions/v1beta1
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
# ...
```
metadata:
name: mailserver
**Downsides**
# ...
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
# ...
```
- Not possible to access mailserver via other cluster Nodes, only via the one mailserver deployed at.
- Every Port within the Container is exposed on the Host side, regardless of what the `ports` section in the Configuration defines.
With this approach,
- it is not possible to access mailserver via other cluster Nodes, only via the one mailserver 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 Proxy Pod](#proxy-port-to-service), but instead of a separate proxy pod, you configure your ingress to proxy TCP traffic to the mailserver pod using the PROXY protocol, which preserves the real client IP.
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 mailserver pod using the PROXY protocol, which preserves the real client IP.
#### Configure your Ingress
@ -400,13 +385,14 @@ With an [NGINX ingress controller][k8s-nginx], set `externalTrafficPolicy: Local
993: "mailserver/mailserver:993::PROXY"
```
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:
!!! 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
??? example "HAProxy Example"
```yaml
kind: ConfigMap
@ -460,79 +446,19 @@ Then, configure both [Postfix][docs-postfix] and [Dovecot][docs-dovecot] to expe
readOnly: true
```
**Downsides**
With this approach,
- Not possible to access mailserver via inner cluster Kubernetes DNS, as PROXY protocol is required for incoming connections.
## Let's Encrypt Certificates
[Kube-Lego][kube-lego] may be used for a role of Let's Encrypt client. It works with Kubernetes [Ingress Resources][k8s-network-ingress] and automatically issues/manages certificates/keys for exposed services via Ingresses.
!!! example
```yaml
kind: Ingress
apiVersion: extensions/v1beta1
metadata:
name: mailserver
labels:
app: mailserver
annotations:
kubernetes.io/tls-acme: 'true'
spec:
rules:
- host: example.com
http:
paths:
- path: /
backend:
serviceName: default-backend
servicePort: 80
tls:
- secretName: mailserver.tls
hosts:
- example.com
```
Now, you can use Let's Encrypt cert and key from `mailserver.tls` [Secret][k8s-config-secret] in your [Pod][k8s-workload-pod] spec:
!!! example
```yaml
# ...
env:
- name: SSL_TYPE
value: 'manual'
- name: SSL_CERT_PATH
value: '/etc/ssl/mailserver/tls.crt'
- name: SSL_KEY_PATH
value: '/etc/ssl/mailserver/tls.key'
# ...
volumeMounts:
- name: tls
mountPath: /etc/ssl/mailserver
readOnly: true
# ...
volumes:
- name: tls
secret:
secretName: mailserver.tls
```
- it is not possible to access the 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
[github-file-compose]: https://github.com/docker-mailserver/docker-mailserver/blob/master/docker-compose.yml
[dockerhub-haproxy]: https://hub.docker.com/_/haproxy
[kube-lego]: https://github.com/jetstack/kube-lego
[k8s-assign-pod-node]: https://kubernetes.io/docs/concepts/configuration/assign-pod-node
[k8s-config-pod]: https://kubernetes.io/docs/tasks/configure-pod-container/configmap
[k8s-config-secret]: https://kubernetes.io/docs/concepts/configuration/secret
[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-ingress]: https://kubernetes.io/docs/concepts/services-networking/ingress
[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
[k8s-workload-pod]: https://kubernetes.io/docs/concepts/workloads/pods/pod