whalewall

Automate management of firewall rules for Docker containers.

Requirements

Linux with a recent kernel, around 5.10 or newer.

Purpose

Docker by default creates iptables rules to handle container traffic that override almost all user-set rules. There are two main ways to get around this:

1. Prevent Docker from creating any iptables rules by setting "iptables": false in /etc/docker/daemon.json
   • This is the nuclear approach. It will break most networking for containers, and require that you manage iptables for containers manually, which can be a very involved process.
2. Add rules to the DOCKER-USER iptables chain
   • Docker ensures that rules in this chain are processed before any rules Docker creates.

Adding rules to the DOCKER-USER chain is what whalewall does to avoid managing more firewall rules than it needs to. You may be wondering if whalewall is necessary, after all it is very easy to add firewall rules to the DOCKER-USER chain yourself. Well, Docker containers and networks are ephemeral, meaning every time a container or network is destroyed and recreated, the IP address and subnet respectively will be randomized. Whalewall takes care of creating or deleting rules when containers are created or killed, which would be very tedious and error-prone manually. Finally, as well as managing firewall rules to limit traffic to and from localhost and external interfaces, whalewall can also enforce container network isolation by limiting traffic between containers.

Mechanism

Whalewall listens for Docker container start and die events and creates or deletes nftables rules appropriately. Why is nftables used instead of iptables? A few reasons:

• nftables can be configured programmatically unlike iptables, removing the need for whalewall to execute any binaries
• nftables allows for first-class sets and maps in firewall rules which can greatly speed up traffic matching in the kernel
• In most distros, iptables rules are translated to nftables rules under the hood, making iptables rules compatible with nftables rules

Whalewall stores details of containers it is managing rules for in a SQLite database. If containers are started or stopped while whalewall isn't running, whalewall will compare currently running containers to what was last saved to the database and create/delete firewall rules appropriately.

Security

Whalewall needs the NET_ADMIN capability to manage nftables rules. It also needs to be a member of the docker group in order to use /var/run/docker/docker.sock to receive events from the local Docker daemon.

To reduce attack surface, landlock and seccomp are leveraged to ensure only files and syscalls required by whalewall can be accessed and called respectively. This vastly limits what whalewall is able to do in the event an attacker is able to execute code in the context of its process. However, this will not prevent said attacker from taking advantage of the Docker socket whalewall has access to which can trivially lead to privilege escalation.

Installation

Docker image

Download the Docker image:

docker pull ghcr.io/capnspacehook/whalewall:0.2.0

Ensure whalewall is given necessary permissions, and that it is using host network mode. This allows the whalewall container to modify host firewall rules.

Example Docker compose file:

version: "3"
services:
  whalewall:
    cap_add: 
      - NET_ADMIN
    image: ghcr.io/capnspacehook/whalewall
    network_mode: host
    volumes:
      - whalewall_data:/data
      - /var/run/docker.sock:/var/run/docker.sock:ro

volumes:
  whalewall_data:

Binary install

If you want to run whalewall natively, download a release binary.

Or if you want to compile from source, assuming you have Go 1.19 installed:

go install github.com/capnspacehook/whalewall/cmd/whalewall@latest

After installing whalewall, grant it required permissions by running:

# this must be run first, it will erase any set capabilities
chgrp docker whalewall
setcap 'cap_net_admin=+ep' whalewall

Configuration

Whalewall uses Docker labels for configuration:

• whalewall.enabled is used to enable or disable firewall rules for a container. If this label is not present and set to true for a container, whalewall will not create any firewall rules for it.
• whalewall.rules specifies the firewall rules for a container. If this label is not specified but whalewall.enabled=true is, no traffic will be allowed to or from the container (unless another container has an output rule for this container).

The contents of the whalewall.rules label is a yaml config.

Whalewall creates rules with a default drop policy, meaning any traffic not explicitly allowed will be dropped.

Example

Below is an example Docker compose file that configures Miniflux, a feed reader. Miniflux needs to connect to a Postgresql database to store state and make outbound HTTPS connections to fetch articles, so that's only what is allowed.

version: "3"
services:
  miniflux:
    depends_on:
      - miniflux_db
    environment:
      - DATABASE_URL=postgres://miniflux:secret@miniflux_db/miniflux?sslmode=disable
      - RUN_MIGRATIONS=1
      - CREATE_ADMIN=1
      - ADMIN_USERNAME=admin
      - ADMIN_PASSWORD=password
    image: miniflux/miniflux:latest
    labels:
      whalewall.enabled: true
      whalewall.rules: |
        mapped_ports:
          # allow traffic to port 80 from localhost
          localhost:
            allow: true
          # allow traffic to port 80 from LAN
          external:
            allow: true
            ip: "192.168.1.0/24"
        output:
          # allow postgres connections
          - network: default
            container: miniflux_db
            proto: tcp
            port: 5432
          # allow DNS requests
          - log_prefix: "dns"
            proto: udp
            port: 53
          # allow HTTPS requests
          - log_prefix: "https"
            proto: tcp
            port: 443
    ports:
      - "80:8080/tcp"

  miniflux_db:
    environment:
      - POSTGRES_USER=miniflux
      - POSTGRES_PASSWORD=secret
    image: postgres:alpine
    labels:
      # no rules specified, drop all traffic
      whalewall.enabled: true

Note to make this Docker compose config as concise as possible, best practices were not followed. This is merely intended to be an example of whalewall rules, not how to setup Miniflux securely.

Rules config reference

# controls traffic from localhost or external networks to a container on mapped ports
mapped_ports:
  # controls traffic from localhost
  localhost:
    # required; allow traffic from localhost or not 
    allow: false
    # optional; log new inbound traffic that this rule will match
    log_prefix: ""
    # optional; settings that allow you to filter traffic further if desired
    verdict:
      # optional; a chain to jump to after matching traffic. This applies to new and established
      # inbound traffic, and established outbound traffic 
      chain: ""
      # optional; the userspace nfqueue to send new outbound packets to
      queue: 0
      # optional; the userspace nfqueue to send established inbound packets to. Required if
      # 'output_est_queue' is set
      input_est_queue: 0
      # optional; the userspace nfqueue to send established inbound packets to. Required if
      # 'input_est_queue' is set
      output_est_queue: 0
  # controls traffic from external networks (from any non-loopback network interface)
  external:
    # required; allow external traffic or not
    allow: false
    # optional; log new inbound traffic that this rule will match
    log_prefix: ""
    # optional; an IP address, CIDR, or range of IP addresses to allow traffic from
    ip: ""
    # optional; settings that allow you to filter traffic further if desired
    verdict:
      # optional; a chain to jump to after matching traffic. This applies to new and established
      # inbound traffic, and established outbound traffic 
      chain: ""
      # optional; the userspace nfqueue to send new outbound packets to
      queue: 0
      # optional; the userspace nfqueue to send established inbound packets to. Required if
      # 'output_est_queue' is set
      input_est_queue: 0
      # optional; the userspace nfqueue to send established inbound packets to. Required if
      # 'input_est_queue' is set
      output_est_queue: 0
# controls traffic from a container to localhost, another container, or the internet
output:
    # optional; log new outbound traffic that this rule will match
  - log_prefix: ""
    # optional; a Docker network traffic will be allowed out of. If unset, will default to all 
    # networks the container is a member of. Required if 'container' is set
    network: ""
    # optional; an IP address, CIDR, or range of IP addresses to allow traffic to
    ip: ""
    # optional; a container to allow traffic to. This can be either the name of the container or
    # the service name of the container is docker compose is used
    container: ""
    # required; either 'tcp' or 'udp'
    proto: ""
    # required; the port to allow traffic to
    port: 0
    # optional; settings that allow you to filter traffic further if desired
    verdict:
      # optional; a chain to jump to after matching traffic. This applies to new and established
      # inbound traffic, and established outbound traffic 
      chain: ""
      # optional; the userspace nfqueue to send new outbound packets to
      queue: 0
      # optional; the userspace nfqueue to send established inbound packets to. Required if
      # 'output_est_queue' is set
      input_est_queue: 0
      # optional; the userspace nfqueue to send established inbound packets to. Required if
      # 'input_est_queue' is set
      output_est_queue: 0

Tips

• Logged traffic is sent to the kernel log file, typically /var/log/kern.log for Debian based distros and /var/log/messages for RHEL based distros
• If you want a container to only be allowed outbound access on a port to localhost, use the IP of the docker0 network interface, which is often 172.17.0.1
• If no Docker networks are explicitly created, use the default network when creating container to container rules

Verifying releases

Starting from v0.2.0, all Docker images and binary checksum files are signed. You can verify images or released binaries to ensure they were not tampered with.

Verifying Docker images or binaries both require cosign.

Verifying Docker images

Simply check the signature of the image with cosign:

COSIGN_EXPERIMENTAL=true cosign verify ghcr.io/capnspacehook/whalewall:<version> | jq

You can verify the image was built by Github Actions by inspecting the Issuer and Subject fields of the output.

Verifying binaries

Download the checksums file, certificate, signature and the archive to the same directory.

Extract the binary from the archive, verify the checksums file and verify the contents of the binary:

tar xfs whalewall_<version>_linux_amd64.tar.gz
COSIGN_EXPERIMENTAL=true cosign verify-blob --certificate checksums.txt.crt --signature checksums.txt.sig checksums.txt
sha256sum -c checksums.txt