Securing the docker daemon with TLS certificates

docker-tls.md

Secure the docker daemon with TLS

first, let's create some directories to work in

mkdir -p ./docker_certs/ca ./docker_certs/server ./docker_certs/client
cd ./docker_certs

Generate Root CA certificate

Generate the CA private key and certificate:

# Generate CA private key
openssl genpkey -algorithm RSA -out ca/local-root-ca.key -aes256

# Generate CA certificate
openssl req -new -x509 -days 3650 -key ca/local-root-ca.key -sha256 -out ca/local-root-ca.pem

Add the CA to the trusted Root CAs (Redhat flavor)

sudo cp ca/local-root-ca.pem /etc/pki/ca-trust/source/anchors/local-root-ca.pem
sudo update-ca-trust

Generate Docker Server certificate

Generate the server private key:

openssl genpkey -algorithm RSA -out server/docker-daemon.key

Create a certificate signing request (CSR) for the server:

openssl req -new -key server/docker-daemon.key -out server/docker-daemon.csr

Create a configuration file for the server certificate (server/server-ext.cnf):

authorityKeyIdentifier=keyid,issuer
basicConstraints=CA:FALSE
keyUsage = digitalSignature, nonRepudiation, keyEncipherment, dataEncipherment
extendedKeyUsage = serverAuth
subjectAltName = @alt_names

[alt_names]
IP.1 = 127.0.0.1
IP.2 = 192.168.122.1
IP.3 = 192.168.85.30

Generate the server certificate:

openssl x509 -req -days 3650 \
  -in server/docker-daemon.csr \
  -CA ca/local-root-ca.pem \
  -CAkey ca/local-root-ca.key \
  -CAcreateserial \
  -out server/docker-daemon.pem \
  -extfile server/server-ext.cnf

Verify the generated server certificate:

openssl x509 -noout -text -in server/docker-daemon.pem

Summary of Files

• ./ca/local-root-ca.key: CA private key
• ./ca/local-root-ca.pem: CA certificate
• ./server/docker-daemon.key: Server private key
• ./server/docker-daemon.pem: Server certificate

Configure the Docker daemon to use the Certificates

Copy the certificates to the Docker daemon:

sudo mkdir -p /etc/docker/certs.d
sudo cp ./server/docker-daemon.pem /etc/docker/certs.d/
sudo cp ./server/docker-daemon.key /etc/docker/certs.d/
sudo cp ./ca/local-root-ca.pem /etc/docker/certs.d/ca.pem

Edit the Docker daemon configuration (/etc/docker/daemon.json):

{
    "tls": true,
    "tlscert": "/etc/docker/certs.d/docker-daemon.pem",
    "tlskey": "/etc/docker/certs.d/docker-daemon.key",
    "tlscacert": "/etc/docker/certs.d/ca.pem"
}

Restart the Docker daemon:

sudo systemctl restart docker

Now the Docker daemon should be using the generated certificates for secure communication.

Generate a Client Certificate

Generate the client private key:

openssl genpkey -algorithm RSA -out client/docker-client.key

Create a certificate signing request (CSR) for the client:

openssl req -new -key client/docker-client.key -out client/client.csr

Create a configuration file for the client certificate (client/client-ext.cnf):

authorityKeyIdentifier=keyid,issuer
basicConstraints=CA:FALSE
keyUsage = digitalSignature, nonRepudiation, keyEncipherment, dataEncipherment
extendedKeyUsage = clientAuth

Generate the client certificate:

openssl x509 -req -days 3650 \
  -in client/client.csr \
  -CA ca/local-root-ca.pem \
  -CAkey ca/local-root-ca.key \
  -CAcreateserial \
  -out client/docker-client.pem \
  -extfile client/client-ext.cnf

Summary of Client Certificate Files

• docker-client.key: Client private key
• docker-client.pem: Client certificate
• local-root-ca.pem: CA certificate (same as for the server)

Verifying the client certificate using cURL

Use the following curl command, providing the CA certificate, client certificate, and client key:

curl \
  --cacert ca/local-root-ca.pem \
  --cert client/docker-client.pem \
  --key client/docker-client.key \
  https://192.168.122.1:2375

Create a Docker Context that uses the Client Certificate

Create a Directory for Docker Client Certificates

Store your client certificates and keys in a dedicated directory:

mkdir -p ~/.docker/certs
cp client/docker-client.pem ~/.docker/certs/
cp client/docker-client.key ~/.docker/certs/
cp ca/local-root-ca.pem ~/.docker/certs/

Create a Docker Context

Use the docker context command to create a new context with the specified certificates:

docker context create docker-tls \
    --description "Context for secure Docker daemon at 192.168.122.1" \
    --docker "host=tcp://192.168.122.1:2375,ca=~/.docker/certs/local-root-ca.pem,cert=~/.docker/certs/docker-client.pem,key=~/.docker/certs/docker-client.key"

Verify the New Context

List all available Docker contexts to verify the new context has been created:

docker context ls

You should see an output similar to:

NAME                DESCRIPTION                                 DOCKER ENDPOINT                      KUBERNETES ENDPOINT   ORCHESTRATOR
default *           Current DOCKER_HOST based configuration     unix:///var/run/docker.sock                               swarm
docker-tls          Context for secure Docker daemon at 192.168.122.1   tcp://192.168.122.1:2375                             swarm

Use the New Context

docker context use docker-tls
docker info

In the /etc/docker/daemon config.json needs to add an option:

"hosts": [
  "unix:///var/run/docker.sock",
  "tcp://<my-ip>:2375"
]

you're right. Another option would be to add this in the systemd unit as drop-in

sudo systemctl edit docker

### Editing /etc/systemd/system/docker.service.d/override.conf
### Anything between here and the comment below will become the contents of the drop-in file

[Service]
ExecStart=
ExecStart=/usr/bin/dockerd -H fd:// -H tcp://192.168.122.1:2375

### Edits below this comment will be discarded

I am using this setup to share the docker daemon on my main development machine (Fedora) with a windows-VM for a legacy project. Running Docker Desktop for Windows in kvm is possible but slow, since it requires nested virtualization.

The ExecStart= and ExecStart=/usr/bin/dockerd options will be enough.

https://gist.github.com/marcwittke/716fecdf87d6b17ea298e503c774cdf9#create-a-docker-context

lsb_release --all
No LSB modules are available.
Distributor ID: Ubuntu
Description:    Ubuntu 24.04 LTS
Release:        24.04
Codename:       noble

docker --version
Docker version 27.1.1, build 6312585

The ~/ construction does not work. Only the absolute path.