---
draft: true
title: "Encrypting a Docker API for Remote Access Using Portainer"
date: "2021-05-16"
---

tl;dr [This script](/scripts/docker-tcp.sh) has everything you need, just run
`./docker-tcp.sh -h` (after making it executable) for any help.

## Introduction

To manage my little army of servers, I use
[Portainer CE](https://www.portainer.io/). It's an open-source management tool
for controlling Dockerized applications across multiple hosts. It can handle
regular Docker containers, compose stacks, Kubernetes clusters or Docker swarm
mode. It's a really useful tool to keep track of everything, and nowadays, I
really can't miss it.

Before we can add a host to Portainer, its Docker API has to be exposed to the
public, and in order to do this, we need to protect it using encryption (unless
of course you like random people controlling your server). This post will
explain how this can be done, and I've also written a script that can automate
the "heavy" lifting.

**Note**: This tutorial is only for Linux. I have no experience with managing a
Windows server and therefore can't confirm these steps will also work on a
Windows machine.

I recommend running these commands on your local Linux machine and just copying
the certificates to the server later, as you'll need all the files in order to
add the host to Portainer later.

## Server-side

To make the connection as secure as possible, we'll use both a server- & a
client-side certificate. This first section describes how to generate the
former:

```shell
openssl genrsa -aes256 -out ca-key.pem 4096
openssl req -new -x509 -days 365 -key ca-key.pem -sha256 -out ca.pem
```

These first two commands generate the
[CA](https://en.wikipedia.org/wiki/Certificate_authority) key. You'll be asked
for some basic information, e.g. your country, state, city, organization, etc.
The most important one is the password. Keep this one safe, as you'll be asked
for it later when creating the client key.

One thing to note here is the `-days 365` flag. This defines after how many
days this certificate will expire (but only when the `-x509` flag is
specified). By default, its value is set at 30 days, but I find this to be
rather short. After this time, you'll have to repeat these steps and generate a
new certificate. You'll have to figure out for yourself how long you'd like
your certificate to be valid for.

Now we can generate the server key:

```shell
openssl genrsa -out server-key.pem 4096
openssl req -subj "/CN=<HOST>" -sha256 -new -key server-key.pem -out server.csr
```

In the above snippet, replace `<HOST>` with the hostname (output of the
`hostname` command) of the machine who's API you want to expose. Now we've
created `server-key.pem` and `server.csr`.

As a final step, we need to create a file named `extfile.cnf` with the
following content:

```
subjectAltName = DNS:<HOST>,IP:<IP>,IP:127.0.0.1 >> extfile.cnf
extendedKeyUsage = serverAuth
```

Here, we once again replace `<HOST>` with the machine's hostname, and `<IP>`
with the machine's public IP.