Enabling remote access

Instead of logging in remotely to our Docker host to run containers, we will access the Docker host by enabling the remote API in Docker Engine. This allows us to manage our Docker containers from our client workstation or continuous delivery server. We will then interact with our Docker host to represent our production environment. The remote API will then be used from our client workstation to perform deployments of our Docker containers.

This section will cover the steps to secure and enable remote access to our Docker host:

• Setting up a certificate authority
• Reconfiguring Docker Engine to enable remote access
• Configuring the Docker client for remote access

Setting up a certificate authority

For the rest of this section, we will be installing TLS certificates in both our Docker host server and client. To ensure trusted communication between the server and client, we will be setting up a Public Key Infrastructure (PKI). This will allow both the Docker engine running in our host and our Docker client to make the connection to verify the identity of each other. 

The first step in building the PKI is setting up the Certificate Authority (CA). A CA is a trusted third party that issues digital certificates to members of our PKI, namely our Docker host and client.

In the next few steps, we will set up our CA inside of our client workstation:

1. First, we will create a directory for our PKI and generate the CA's private key in a file called ca-key.pem:

client$ mkdir ~/ca
client$ cd ~/ca
client$ openssl genrsa -aes256 -out ca-key.pem 4096

Generating RSA private key, 4096 bit long modulus
......................................++
................................................++
e is 65537 (0x10001)
      
Enter pass phrase for ca-key.pem: ****
Verifying - Enter pass phrase for ca-key.pem: ****

We need to remember the passphrases set for our CA's private key, as we will always need them for the rest of this chapter.

2. Next, we make sure that this private key is secure by restricting read and write access to us:

client$ chmod 600 ca-key.pem
client$ ls -l ca-key.pem
-rw-------. 1 dockeruser group 3326 Dec  2 20:45 ca-key.pem

3. Finally, we will generate a certificate for our CA that is self-signed. Let's type the following command to place the self-signed certificate in a file called ca.pem:

client$ openssl req -key ca-key.pem -new -x509  \
-subj '/CN=Certificate Authority' \
-sha256 -days 365 -out ca.pem
Enter pass phrase for ca-key.pem: ****

We now have a CA, and we will be distributing its ca.pem certificate to our Docker host and client later in this section. For now, let's inspect the generate certificate:

client$ cat ca.pem

-----BEGIN CERTIFICATE-----
      MIIFEzCCAvugAwIBAgIJAM19ce5sap+kMA0GCSqGSIb3DQEBCwUAMCAxHjAcBgNV
      BAMMFUNlcnRpZmljYXRlIEF1dGhvcml0eTAeFw0xODEyMDYwMzQ5MTNaFw0xOTEy
      MDYwMzQ5MTNaMCAxHjAcBgNVBAMMFUNlcnRpZmljYXRlIEF1dGhvcml0eTCCAiIw
      DQYJKoZIhvcNAQEBBQADggIPADCCAgoCggIBAKCESs7QpRZ78v8p2nKomCGABqCN
      b3E0vBpjveTnjA4kOEWVsHloq2o66yuuNff75GNWghzq791KyKJOy/dehNL9DauA
      DD3DJh0+uaOGn547W827Z37wJ64acNyvIQjyiyeLrpF4BzzxaZ/AJFVgqar5Kuqc
      qiOG3GUYcnfu6mpmlKoa1XqBtSQ+A2fd4/mpXC0zrDrz9MSEOCs5/Xm6/faexYae
      V8gBkCYWiUVUi+RRRc2vU1LzuiI5FsXmD3kNHCjNIbYIoyqKMzbTJjEffhN+5B/V
      Rc3qfRmfoEv8P0Hc4Wx55qH8BLWwhvNFAZ+nre+j7zPz+dTfLVyveOPxErHaI1V8
      WH9qEVf+haNqUBrjNCuL+xyVNx7evPygD88jyZDWLK5Y0JTh2GSPqMeVi3hSKzNP
      GbVjT8tmkCUEsYbSJg2vkPYJR4aC8LLdJsjr7wkWBF1IcYYZpLo3EsUnkjNi7MGS
      pGdLob3UToekXaA1D6esDhlEB+3Tt/RWJkS91ijUiDs2kTSmDfnxUQGyeD4wx/rj
      lPFRSLdUUYiFcdI5VegZVSqYxW/Qw2/t+GvoLkrOrggqY1f++XUgK5hSoT8EqgiG
      SjapkgphMEquVP8UlZ3jC0VmgwFnRUEdqau6yLWMYG6TvLkyVi1Vmfam7CoB1aDn
      TccUszk+rezX+1nJAgMBAAGjUDBOMB0GA1UdDgQWBBTrqfPKO0i2peZ6Hd/BYOMq
      WXD9kDAfBgNVHSMEGDAWgBTrqfPKO0i2peZ6Hd/BYOMqWXD9kDAMBgNVHRMEBTAD
      AQH/MA0GCSqGSIb3DQEBCwUAA4ICAQCa6SPGncEZSWu0WLfkh1mERa9JfBQzJFpv
      1E7M3tZeFyJS7LfXdcf9WEAaWqTpha87A+5g9uBi/whYk47dyTik07/k+CyF112i
      9GXK8j/UNCjAMOSluOCxpIsmMXp2Dn+ma21msN1K/lHK0ZhGWB9ZDggvdzRRPjic
      Dq3aQ49ATHQHGg9cqgZO0zXtcQYaHfCNds5YLNVL66eDhuN91V2MEqWtRDHfr0vA
      F3KldXfQ/clnrjGLqo7a3oR1R4QofQ03bV+PRIgub+l3Fee1D68BqF9dLRjUABd2
      zm5OzNAmmHPSGWGvOxylvPrUS0ulUzMUWdoXN85SDdLHFXTXwpbD/GgqK+Y3BTgO
      7d+mOoTHVEdw2gUXLaqeEchBge2Kh/LQtiN7Zp8OY7snX66Z8tF6W2MKhnSpDzcW
      J4WMbmaRqsTEeaRk0aTWkhBZukSZf4zjaa/abF+iRvU5c1OGS9GmYfuGq3Tlj+Xo
      JZNuKp9HzOPaj8qiD0DJW9EnuZ24zzpDSiSdmOdARcaaFFKhW8i+SVP6VqrAR3Nb
      OL8ne6w6kdoiq4+hPKfWVS9Yh0aQstJMNP91Nnw3J+aRz9eN03jpl/z18vHhW/xl
      nYJrB2KlC7SOnUT7TMJr4O5Aw1SidxMH6NLiiC1jbTWXDMuYL8UghDIk9Ne/WhBd
qg0sW+boLw==
-----END CERTIFICATE-----

Enabling remote access in Docker Engine

Now that we have a CA for our PKI, we can use this CA to verify the identity of our Docker host. The following steps will prepare the identity of our Docker host:

1. First, we log in to our Docker host. In here, we will generate a private key that will secure the remote API being served by running Docker Engine. The following command will save the private key into a file called /etc/docker/server-key.pem:

dockerhost$ openssl genrsa -out /etc/docker/server-key.pem 2048

Generating RSA private key, 2048 bit long modulus
................................+++
........+++
e is 65537 (0x10001)

2. Next, we make sure that this file is secure and can only be accessed by the Docker Engine daemon (through the root user):

dockerhost$ chmod 600 /etc/docker/server-key.pem
dockerhost$ ls -l /etc/docker/server-key.pem

-rw-------. 1 root root 1675 Dec  2 21:09 /etc/docker/server-key.pem

3. Now that the private key is ready, we will use this file to generate a Certificate Signing Request (CSR). openssl req following command will generate a CSR:

dockerhost$ openssl req -key /etc/docker/server-key.pem 
-new -subj "/CN=dockerhost" -sha256 -out           
dockerhost.csr
dockerhost$ ls -l dockerhost.csr
-rw-r--r--. 1 root root 891 Dec  2 21:33 dockerhost.csr

4. Next, we go back to our client workstation where our CA's files are hosted. In here, we will download the CSR from our Docker host:

client$ scp dockerhost:~/dockerhost.csr dockerhost.csr
dockerhost.csr           100%  891     1.5MB/s   00:00

5. We now prepare an OpenSSL configuration server-ext.cnf file that indicates that the certificates our CA will issue are used for server authentication:

extendedKeyUsage = serverAuth

6. Finally, we can sign the CSR with our CA. The following command will place our Docker host's signed certificate in a file called dockerhost.pem:

client$ cd ~/ca
client$ openssl x509 -req -CA ca.pem -CAkey ca-key.pem \
-CAcreateserial -extfile server-ext.cnf \
-in dockerhost.csr -out dockerhost.pem
      
Signature ok
subject=/CN=dockerhost
Getting CA Private Key
Enter pass phrase for ca-key.pem: ****

Now that we have the identity of our Docker host verified by our CA, we can now enable the secure TCP port in our Docker host. We will bring up the secure remote API with the following steps:

1. Let's now go back into our Docker host. Here, we will copy the certificates of our Docker host and CA from our client workstation:

dockerhost$ scp client:~/ca/ca.pem /etc/docker/ca.pem
ca.pem                  100% 1911     1.1MB/s   00:00
dockerhost$ scp client:~/ca/dockerhost.pem /etc/docker/server.pem
dockerhost.pem                      100% 1428     1.2MB/s   00:00

2. Now that our TLS assets are in place, let's now reconfigure the Docker Engine daemon file, /etc/docker/daemon.json, to use those certificates:

{
  "tlsverify": true,
  "tlscacert": "/etc/docker/ca.pem",
  "tlskey": "/etc/docker/server-key.pem",
  "tlscert": "/etc/docker/server.pem"
}

3. Next, we configure the Docker Engine daemon to listen to a secure port by creating a systemd override file /etc/systemd/system/docker.service.d/override.conf:

[Service]
ExecStart=
ExecStart=/usr/bin/dockerd -H unix:// -H tcp://0.0.0.0:2376

4. Finally, we are now ready to restart Docker Engine:

dockerhost$ systemctl daemon-reload
dockerhost$ systemctl restart docker.service

Our Docker host is now ready and serving a secure API.

Connecting remotely from the Docker client

Now that our Docker host is secure, it won't respond to requests from our Docker client yet. The Docker host will only respond to requests being made by clients verified by our CA.

The following steps will generate an identity for our Docker client:

1. First, let's generate the private key of our Docker client in ~/.docker/key.pem:

client$ openssl genrsa -out ~/.docker/key.pem 4096

Generating RSA private key, 4096 bit long modulus
...................++
      .........................................................++
e is 65537 (0x10001)

2. Next, we make sure that this private key is restricted to us for viewing:

client$ chmod 600 ~/.docker/key.pem

3. We now generate the CSR for client in a file called client.csr:

client$ openssl req -subj '/CN=client' -new \
-key ~/.docker/key.pem -out client.csr

4. Now that our CSR is ready, we will now create an OpenSSL configuration to indicate that certificates will be used for client authentication. The OpenSSL command following creates this configuration in a file called ~/ca/client-ext.cnf:

extendedKeyUsage = clientAuth

5. Finally, we are ready to issue the certificate for our Docker client. The following command writes our Docker client's certificate to ~/.docker/cert.pem:

 client$ openssl x509 -req -CA ca.pem 
      -CAkey ca-key.pem -CAcreateserial 
      -extfile client-ext.cnf -in       
      ~/client.csr -out ~/.docker/cert.pem
      Signature ok
      subject=/CN=client
      Getting CA Private Key
      Enter pass phrase for ca-key.pem: ****

6. To complete our client's TLS configuration, we will also deploy our CA's certificate in our ~/.docker directory file:

      client$ cp ca.pem ~/.docker/ca.pem

7. Finally, we indicate to our Docker client that we will be connecting securely to our remote Docker host by exporting the following environment variables:

      client$ export DOCKER_HOST=tcp://dockerhost:2376
      client$ export DOCKER_TLS_VERIFY=true

Congratulations! We now have a secure communication channel between our Docker client and Docker host. To verify the connection, we can run the following command and show information about our remote Docker host:

client$ docker info