Hardening Docker Daemon Configuration

Overview

The Docker daemon (dockerd) runs with root privileges and controls all container operations. Hardening its configuration through /etc/docker/daemon.json, TLS certificates, user namespace remapping, and network restrictions is essential to prevent privilege escalation, lateral movement, and container breakout attacks.

Prerequisites

• Docker Engine 24.0+ installed
• Root or sudo access to the Docker host
• OpenSSL for TLS certificate generation
• Understanding of Linux namespaces and cgroups

Core Hardened daemon.json

{
  "icc": false,
  "userns-remap": "default",
  "no-new-privileges": true,
  "log-driver": "json-file",
  "log-opts": {
    "max-size": "10m",
    "max-file": "5"
  },
  "storage-driver": "overlay2",
  "live-restore": true,
  "userland-proxy": false,
  "default-ulimits": {
    "nofile": {
      "Name": "nofile",
      "Hard": 65536,
      "Soft": 32768
    },
    "nproc": {
      "Name": "nproc",
      "Hard": 4096,
      "Soft": 2048
    }
  },
  "seccomp-profile": "/etc/docker/seccomp/default.json",
  "default-address-pools": [
    {
      "base": "172.17.0.0/16",
      "size": 24
    }
  ],
  "iptables": true,
  "ip-forward": true,
  "ip-masq": true,
  "experimental": false,
  "metrics-addr": "127.0.0.1:9323",
  "max-concurrent-downloads": 3,
  "max-concurrent-uploads": 5,
  "default-runtime": "runc",
  "runtimes": {
    "runsc": {
      "path": "/usr/local/bin/runsc",
      "runtimeArgs": ["--platform=ptrace"]
    }
  }
}

Setting-by-Setting Explanation

Disable Inter-Container Communication (ICC)

{
  "icc": false
}

Prevents containers on the default bridge network from communicating. Each container must use explicit --link or user-defined networks with published ports.

Enable User Namespace Remapping

{
  "userns-remap": "default"
}

Maps container root (UID 0) to a high unprivileged UID on the host. This prevents a container breakout from gaining root on the host.

# Verify userns-remap is active
cat /etc/subuid
# Output: dockremap:100000:65536

cat /etc/subgid
# Output: dockremap:100000:65536

# Verify container UID mapping
docker run --rm alpine id
# uid=0(root) gid=0(root) -- but host UID is 100000+

Disable New Privilege Escalation

{
  "no-new-privileges": true
}

Prevents container processes from gaining additional privileges via setuid/setgid binaries or capability escalation.

Enable Live Restore

{
  "live-restore": true
}

Keeps containers running during daemon downtime, enabling daemon upgrades without container restart.

Disable Userland Proxy

{
  "userland-proxy": false
}

Uses iptables rules instead of docker-proxy for port forwarding, reducing attack surface and improving performance.

TLS Configuration for Remote Docker API

Generate CA and Server Certificates

# Create CA
openssl genrsa -aes256 -out ca-key.pem 4096
openssl req -new -x509 -days 365 -key ca-key.pem -sha256 -out ca.pem \
  -subj "/CN=Docker CA"

# Create server key and CSR
openssl genrsa -out server-key.pem 4096
openssl req -subj "/CN=docker-host" -sha256 -new -key server-key.pem -out server.csr

# Create extfile with SANs
echo "subjectAltName = DNS:docker-host,IP:10.0.0.5,IP:127.0.0.1" > extfile.cnf
echo "extendedKeyUsage = serverAuth" >> extfile.cnf

# Sign server 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

# Create client key and certificate
openssl genrsa -out key.pem 4096
openssl req -subj "/CN=client" -new -key key.pem -out client.csr
echo "extendedKeyUsage = clientAuth" > extfile-client.cnf
openssl x509 -req -days 365 -sha256 -in client.csr -CA ca.pem -CAkey ca-key.pem \
  -CAcreateserial -out cert.pem -extfile extfile-client.cnf

# Set permissions
chmod 0400 ca-key.pem key.pem server-key.pem
chmod 0444 ca.pem server-cert.pem cert.pem

# Move to Docker TLS directory
sudo mkdir -p /etc/docker/tls
sudo cp ca.pem server-cert.pem server-key.pem /etc/docker/tls/

Configure daemon.json for TLS

{
  "tls": true,
  "tlsverify": true,
  "tlscacert": "/etc/docker/tls/ca.pem",
  "tlscert": "/etc/docker/tls/server-cert.pem",
  "tlskey": "/etc/docker/tls/server-key.pem",
  "hosts": ["unix:///var/run/docker.sock", "tcp://0.0.0.0:2376"]
}

Client Connection

docker --tlsverify \
  --tlscacert=ca.pem \
  --tlscert=cert.pem \
  --tlskey=key.pem \
  -H=tcp://docker-host:2376 version

Docker Socket Protection

# Restrict socket ownership
sudo chown root:docker /var/run/docker.sock
sudo chmod 660 /var/run/docker.sock

# Audit Docker socket access
sudo auditctl -w /var/run/docker.sock -k docker-socket

# Never mount Docker socket into containers
# BAD: docker run -v /var/run/docker.sock:/var/run/docker.sock ...

Rootless Docker

# Install rootless Docker
curl -fsSL https://get.docker.com/rootless | sh

# Configure environment
export PATH=$HOME/bin:$PATH
export DOCKER_HOST=unix://$XDG_RUNTIME_DIR/docker.sock

# Start rootless daemon
systemctl --user start docker
systemctl --user enable docker

# Verify rootless mode
docker info | grep -i rootless
# Rootless: true

Content Trust (Image Signing)

# Enable Docker Content Trust
export DOCKER_CONTENT_TRUST=1

# Pull only signed images
docker pull library/alpine:3.18
# Will fail if image is not signed

# Sign and push image
docker trust sign myregistry/myapp:1.0

Seccomp Profile

# View default seccomp profile
docker info --format '{{.SecurityOptions}}'

# Use custom seccomp profile
docker run --security-opt seccomp=/etc/docker/seccomp/custom.json alpine

# Verify seccomp is enabled
docker inspect --format='{{.HostConfig.SecurityOpt}}' container_name

AppArmor Profile

# Check AppArmor status
sudo aa-status

# Use custom AppArmor profile
docker run --security-opt apparmor=docker-custom alpine

# Load custom profile
sudo apparmor_parser -r /etc/apparmor.d/docker-custom

Verification Commands

# Check daemon configuration
docker info

# Verify userns-remap
docker info --format '{{.SecurityOptions}}'

# Check ICC setting
docker network inspect bridge --format '{{.Options}}'

# Audit with Docker Bench
docker run --rm --net host --pid host \
  -v /var/run/docker.sock:/var/run/docker.sock \
  -v /etc:/etc:ro \
  docker/docker-bench-security

Best Practices

1. Never expose Docker daemon without TLS - Always use --tlsverify for remote access
2. Enable user namespace remapping - Map container root to unprivileged host UID
3. Disable ICC - Prevent default bridge network container-to-container communication
4. Use rootless mode - Run Docker daemon as non-root where possible
5. Enable content trust - Only pull signed images
6. Configure log rotation - Prevent log files from filling disk
7. Use seccomp profiles - Restrict syscalls available to containers
8. Audit Docker socket - Monitor access to /var/run/docker.sock
9. Run Docker Bench regularly - Automate CIS benchmark checks
10. Keep Docker updated - Apply security patches promptly