Docker Networking Enhancements: Gateway Modes, IPv6 Support, and Address Management

- Docker will now only set the `ip6tables` policy for the `FORWARD` chain in the `filter` table to `DROP` if it enables IP forwarding on the host itself (sysctls `net.ipv6.conf.all.forwarding` and `net.ipv6.conf.default.forwarding`). This is now aligned with existing IPv4 behaviour. [moby/moby#48594](https://github.com/moby/moby/pull/48594) - If IPv6 forwarding is enabled on your host, but you were depending on Docker to set the ip6tables filter-FORWARD policy to `DROP`, you may need to update your host's configuration to make sure it is secure. - Direct routed access to container ports that are not exposed using `p`/`-publish` is now blocked in the `DOCKER` iptables chain. [moby/moby#48724](https://github.com/moby/moby/pull/48724) - If the default iptables filter-FORWARD policy was previously left at `ACCEPT` on your host, and direct routed access to a container's unpublished ports from a remote host is still required, options are: - Publish the ports you need. - Use the new `gateway_mode_ipv[46]=nat-unprotected`, described below. - Container ports published to host addresses will continue to be accessible via those host addresses, using NAT or the userland proxy. - Unpublished container ports continue to be directly accessible from the Docker host via the container's IP address. - Networks created with `gateway_mode_ipv[46]=routed` are now accessible from other bridge networks running on the same Docker host, as well as from outside the host. [moby/moby#48596](https://github.com/moby/moby/pull/48596) - Bridge driver options `com.docker.network.bridge.gateway_mode_ipv4` and `com.docker.network.bridge.gateway_mode_ipv6` now accept mode `nat-unprotected`. [moby/moby#48597](https://github.com/moby/moby/pull/48597) - `nat-unprotected` is similar to the default `nat` mode, but no per port/protocol rules are set up. This means any port on a container can be accessed by direct-routing from a remote host. - Bridge driver options `com.docker.network.bridge.gateway_mode_ipv4` and `com.docker.network.bridge.gateway_mode_ipv6` now accept mode `isolated`, when the network is also `internal`. [moby/moby#49262](https://github.com/moby/moby/pull/49262) - An address is normally assigned to the bridge device in an `internal` network. So, processes on the Docker host can access the network, and containers in the network can access host services listening on that bridge address (including services listening on "any" host address, `0.0.0.0` or `::`). - An `internal` bridge network created with gateway mode `isolated` does not have an address on the Docker host. - When a port mapping includes a host IP address or port number that cannot be used because NAT from the host is disabled using `--gateway_mode_ipv[46]`, container creation will no longer fail. The unused fields may be needed if the gateway endpoint changes when networks are connected or disconnected. A message about the unused fields will be logged. [moby/moby#48575](https://github.com/moby/moby/pull/48575) - Do not create iptables nat-POSTROUTING masquerade rules for a container's own published ports, when the userland proxy is enabled. [moby/moby#48854](https://github.com/moby/moby/pull/48854) #### IPv6 - Add `docker network create` option `--ipv4`. To disable IPv4 address assignment for a network, use `docker network create --ipv4=false [...]`. [docker/cli#5599](https://github.com/docker/cli/pull/5599) - Daemon option `--ipv6` (`"ipv6": true` in `daemon.json`) can now be used without `fixed-cidr-v6`. [moby/moby#48319](https://github.com/moby/moby/pull/48319) - IPAM now handles subnets bigger than "/64". [moby/moby#49223](https://github.com/moby/moby/pull/49223) - Duplicate address detection (DAD) is now disabled for addresses assigned to the bridges belonging to bridge networks. [moby/moby#48609](https://github.com/moby/moby/pull/48609) - Modifications to `host-gateway`, for compatibility with IPv6-only networks. [moby/moby#48807](https://github.com/moby/moby/pull/48807)

This section covers various enhancements in Docker networking. It details how Docker now aligns IPv6 `ip6tables` policy with IPv4, blocking direct routed access to unpublished container ports and introducing the `nat-unprotected` gateway mode. It explains how networks with `gateway_mode_ipv[46]=routed` are accessible from other bridge networks and from outside the host. It also discusses the `isolated` gateway mode for internal networks, handling of unused host IP addresses in port mappings, and the prevention of masquerade rules for published ports with userland proxy enabled. Additionally, it includes enhancements to IPv6 support, such as the `--ipv4` option to disable IPv4, using `--ipv6` without `fixed-cidr-v6`, handling larger subnets, disabling duplicate address detection for bridge networks, and modifications for IPv6-only networks.