Protect the Docker daemon socket

Estimated reading time: 6 minutes

By default, Docker runs via a non-networked Unix socket. It can also optionally communicate using an HTTP socket.

If you need Docker to be reachable via the network in a safe manner, you can enable TLS by specifying the tlsverify flag and pointing Docker’s tlscacert flag to a trusted CA certificate.

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

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

Warning: These TLS commands will only generate a working set of certificates on Linux. macOS comes with a version of OpenSSL that is incompatible with the certificates that Docker requires.

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 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 we have a CA, you can create a server key and certificate signing request (CSR). Make sure that “Common Name” (i.e., server FQDN or YOUR name) matches the hostname you will 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 via IP address as well as DNS name, they 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

$ 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:

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

$ 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 an extensions config file:

$ echo extendedKeyUsage = clientAuth > extfile.cnf

Now sign the public key:

$ openssl x509 -req -days 365 -sha256 -in client.csr -CA ca.pem -CAkey ca-key.pem \
  -CAcreateserial -out cert.pem -extfile extfile.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:

$ rm -v client.csr server.csr

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

In order to protect your keys from accidental damage, you will want to 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 our CA:

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

To be able to connect to Docker and validate its certificate, you now need to provide your client keys, certificates and trusted CA:

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 have 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 will now connect 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 will send 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
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