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.

When to Use

• When deploying or configuring hardening docker daemon configuration capabilities in your environment
• When establishing security controls aligned to compliance requirements
• When building or improving security architecture for this domain
• When conducting security assessments that require this implementation

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/

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