Protect the Docker daemon socket

By default, Docker runs through a non-networked UNIX socket. It can also optionally communicate using SSH or a TLS (HTTPS) socket.

Use SSH to protect the Docker daemon socket

Note

The given USERNAME must have permissions to access the docker socket on the remote machine. Refer to manage Docker as a non-root user to learn how to give a non-root user access to the docker socket.

The following example creates a docker context to connect with a remote dockerd daemon on host1.example.com using SSH, and as the docker-user user on the remote machine:

$ docker context create \
    --docker host=ssh://docker-user@host1.example.com \
    --description="Remote engine" \
    my-remote-engine

my-remote-engine
Successfully created context "my-remote-engine"

After creating the context, use docker context use to switch the docker CLI to use it, and to connect to the remote engine:

$ docker context use my-remote-engine
my-remote-engine
Current context is now "my-remote-engine"

$ docker info
<prints output of the remote engine>

Use the default context to switch back to the default (local) daemon:

$ docker context use default
default
Current context is now "default"

Alternatively, use the DOCKER_HOST environment variable to temporarily switch the docker CLI to connect to the remote host using SSH. This does not require creating a context, and can be useful to create an ad-hoc connection with a different engine:

$ export DOCKER_HOST=ssh://docker-user@host1.example.com
$ docker info
<prints output of the remote engine>

SSH Tips

For the best user experience with SSH, configure ~/.ssh/config as follows to allow reusing a SSH connection for multiple invocations of the docker CLI:

ControlMaster     auto
ControlPath       ~/.ssh/control-%C
ControlPersist    yes

Use TLS (HTTPS) to protect the Docker daemon socket

If you need Docker to be reachable through HTTP rather than SSH in a safe manner, you can enable TLS (HTTPS) by specifying the tlsverify flag and pointing Docker's tlscacert flag to a trusted CA certificate.

In the daemon mode, it only allows connections from clients authenticated by a certificate signed by that CA. In the client mode, it only connects to servers with a certificate signed by that CA.

Important

Using TLS and managing a CA is an advanced topic. Familiarize yourself with OpenSSL, x509, and TLS before using it in production.

Create a CA, server and client keys with OpenSSL

Note

Replace all instances of $HOST in the following example with the DNS name of your Docker daemon's host.

First, on the Docker daemon's host machine, generate CA private and public keys:

$ 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:

$ openssl req -new -x509 -days 365 -key ca-key.pem -sha256 -out ca.pem
Enter pass phrase for ca-key.pem:
You are about to be asked to enter information that will be incorporated
into your certificate request.
What you are about to enter is what is called a Distinguished Name or a DN.
There are quite a few fields but you can leave some blank
For some fields there will be a default value,
If you enter '.', the field will be left blank.
-----
Country Name (2 letter code) [AU]:
State or Province Name (full name) [Some-State]:Queensland
Locality Name (eg, city) []:Brisbane
Organization Name (eg, company) [Internet Widgits Pty Ltd]:Docker Inc
Organizational Unit Name (eg, section) []:Sales
Common Name (e.g. server FQDN or YOUR name) []:$HOST
Email Address []:Sven@home.org.au

Now that you have a CA, you can create a server key and certificate signing request (CSR). Make sure that "Common Name" matches the hostname you use to connect to Docker:

Note

Replace all instances of $HOST in the following example with the DNS name of your Docker daemon's host.

$ openssl genrsa -out server-key.pem 4096
Generating RSA private key, 4096 bit long modulus
.....................................................................++
.................................................................................................++
e is 65537 (0x10001)

$ openssl req -subj "/CN=$HOST" -sha256 -new -key server-key.pem -out server.csr

Next, we're going to sign the public key with our CA:

Since TLS connections can be made through IP address as well as DNS name, the IP addresses need to be specified when creating the certificate. For example, to allow connections using 10.10.10.20 and 127.0.0.1:

$ echo subjectAltName = DNS:$HOST,IP:10.10.10.20,IP:127.0.0.1 >> extfile.cnf

Set the Docker daemon key's extended usage attributes to be used only for server authentication:

$ echo extendedKeyUsage = serverAuth >> extfile.cnf

Now, generate the signed certificate:

$ openssl x509 -req -days 365 -sha256 -in server.csr -CA ca.pem -CAkey ca-key.pem \
  -CAcreateserial -out server-cert.pem -extfile extfile.cnf
Signature ok
subject=/CN=your.host.com
Getting CA Private Key
Enter pass phrase for ca-key.pem:

Authorization plugins offer more fine-grained control to supplement authentication from mutual TLS. In addition to other information described in the above document, authorization plugins running on a Docker daemon receive the certificate information for connecting Docker clients.

For client authentication, create a client key and certificate signing request:

Note

For simplicity of the next couple of steps, you may perform this step on the Docker daemon's host machine as well.

$ openssl genrsa -out key.pem 4096
Generating RSA private key, 4096 bit long modulus
.........................................................++
................++
e is 65537 (0x10001)

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

To make the key suitable for client authentication, create a new extensions config file:

$ echo extendedKeyUsage = clientAuth > extfile-client.cnf

Now, generate the signed certificate:

$ openssl x509 -req -days 365 -sha256 -in client.csr -CA ca.pem -CAkey ca-key.pem \
  -CAcreateserial -out cert.pem -extfile extfile-client.cnf
Signature ok
subject=/CN=client
Getting CA Private Key
Enter pass phrase for ca-key.pem:

After generating cert.pem and server-cert.pem you can safely remove the two certificate signing requests and extensions config files:

$ rm -v client.csr server.csr extfile.cnf extfile-client.cnf

With a default umask of 022, your secret keys are world-readable and writable for you and your group.

To protect your keys from accidental damage, remove their write permissions. To make them only readable by you, change file modes as follows:

$ chmod -v 0400 ca-key.pem key.pem server-key.pem

Certificates can be world-readable, but you might want to remove write access to prevent accidental damage:

$ chmod -v 0444 ca.pem server-cert.pem cert.pem

Now you can make the Docker daemon only accept connections from clients providing a certificate trusted by your CA:

$ dockerd \
    --tlsverify \
    --tlscacert=ca.pem \
    --tlscert=server-cert.pem \
    --tlskey=server-key.pem \
    -H=0.0.0.0:2376

To connect to Docker and validate its certificate, provide your client keys, certificates and trusted CA:

Tip

This step should be run on your Docker client machine. As such, you need to copy your CA certificate, your server certificate, and your client certificate to that machine.

Note

Replace all instances of $HOST in the following example with the DNS name of your Docker daemon's host.

$ docker --tlsverify \
    --tlscacert=ca.pem \
    --tlscert=cert.pem \
    --tlskey=key.pem \
    -H=$HOST:2376 version

Note

Docker over TLS should run on TCP port 2376.

Warning

As shown in the example above, you don't need to run the docker client with sudo or the docker group when you use certificate authentication. That means anyone with the keys can give any instructions to your Docker daemon, giving them root access to the machine hosting the daemon. Guard these keys as you would a root password!

Secure by default

If you want to secure your Docker client connections by default, you can move the files to the .docker directory in your home directory --- and set the DOCKER_HOST and DOCKER_TLS_VERIFY variables as well (instead of passing -H=tcp://$HOST:2376 and --tlsverify on every call).

$ mkdir -pv ~/.docker
$ cp -v {ca,cert,key}.pem ~/.docker

$ export DOCKER_HOST=tcp://$HOST:2376 DOCKER_TLS_VERIFY=1

Docker now connects securely by default:

$ docker ps

Other modes

If you don't want to have complete two-way authentication, you can run Docker in various other modes by mixing the flags.

Daemon modes

• tlsverify, tlscacert, tlscert, tlskey set: Authenticate clients
• tls, tlscert, tlskey: Do not authenticate clients

Client modes

• tls: Authenticate server based on public/default CA pool
• tlsverify, tlscacert: Authenticate server based on given CA
• tls, tlscert, tlskey: Authenticate with client certificate, do not authenticate server based on given CA
• tlsverify, tlscacert, tlscert, tlskey: Authenticate with client certificate and authenticate server based on given CA

If found, the client sends its client certificate, so you just need to drop your keys into ~/.docker/{ca,cert,key}.pem. Alternatively, if you want to store your keys in another location, you can specify that location using the environment variable DOCKER_CERT_PATH.

$ export DOCKER_CERT_PATH=~/.docker/zone1/
$ docker --tlsverify ps

Connecting to the secure Docker port using curl

To use curl to make test API requests, you need to use three extra command line flags:

$ curl https://$HOST:2376/images/json \
  --cert ~/.docker/cert.pem \
  --key ~/.docker/key.pem \
  --cacert ~/.docker/ca.pem

Related information

• Using certificates for repository client verification
• Use trusted images