hardening-docker-daemon-configuration

# 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 ```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) ```json { "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 ```json { "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. ```bash # 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 ```json { "no-new-privileges": true } ``` Prevents container processes from gaining additional privileges via setuid/setgid binaries or capability escalation. ### Enable Live Restore ```json { "live-restore": true } ``` Keeps containers running during daemon downtime, enabling daemon upgrades without container restart. ### Disable Userland Proxy ```json { "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 ```bash # 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 ```json { "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 ```bash docker --tlsverify \ --tlscacert=ca.pem \ --tlscert=cert.pem \ --tlskey=key.pem \ -H=tcp://docker-host:2376 version ``` ## Docker Socket Protection ```bash # 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 ```bash # 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) ```bash # 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 ```bash # 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 ```bash # 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 ```bash # 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