docker-net-dhcp

docker-net-dhcp is a Docker plugin providing a network driver which allocates IP addresses (IPv4 and optionally IPv6) via an existing DHCP server (e.g. your router).

When configured correctly, this allows you to spin up a container (e.g. docker run ... or docker-compose up ...) and access it on your network as if it was any other machine! Probably not a great idea for production, but it's pretty handy for home deployment.

Usage

Installation

The plugin can be installed with the docker plugin install command:

$ docker plugin install ghcr.io/devplayer0/docker-net-dhcp:release-linux-amd64
Plugin "ghcr.io/devplayer0/docker-net-dhcp:release-linux-amd64" is requesting the following privileges:
 - network: [host]
 - host pid namespace: [true]
 - mount: [/var/run/docker.sock]
 - capabilities: [CAP_NET_ADMIN CAP_SYS_ADMIN]
Do you grant the above permissions? [y/N] y
release-linux-amd64: Pulling from ghcr.io/devplayer0/docker-net-dhcp
Digest: sha256:<some hash>
<some id>: Complete
Installed plugin ghcr.io/devplayer0/docker-net-dhcp:release-linux-amd64
$

Other tags

There are a number of supported tags for different architectures and versions, the format is <version>-<os>-<architecture>. For example, latest-linux-arm-v7 would install the newest build for ARMv7 (e.g. for Raspberry Pi).

Version

• release: The latest release (can be upgraded via docker plugin upgrade)
• x.y.z: A specific (semver) release (e.g. 0.1.0)
• latest: Build of the newest commit

OS

Currently only linux is supported.

Architecture

• amd64: Intel / AMD 64-bit
• 386: Intel / AMD legacy 32-bit
• arm64-v8: ARMv8 64-bit
• arm-v7: ARMv7 (e.g. Raspberry Pi)

Unfortunately Docker plugin images don't support multiple architectures per tag.

Network creation

In order to create a Docker network using net-dhcp, you'll need a pre-configured bridge interface on the host. How you set this up will depend on your system, but the following (manual) instructions should work on most Linux distros:

# Create the bridge
$ sudo ip link add my-bridge type bridge
$ sudo ip link set my-bridge up

# Assuming 'eth0' is connected to your LAN (where the DHCP server is)
$ sudo ip link set eth0 up
# Attach your network card to the bridge
$ sudo ip link set eth0 master my-bridge

# If your firewall's policy for forwarding is to drop packets, you'll need to add an ACCEPT rule
$ sudo iptables -A FORWARD -i my-bridge -j ACCEPT

# Get an IP for the host (will go out to the DHCP server since eth0 is attached to the bridge)
# Replace this step with whatever network configuration you were using for eth0
$ sudo dhcpcd my-bridge

Once the bridge is ready, you can create the network:

$ docker network create -d ghcr.io/devplayer0/docker-net-dhcp:release-linux-amd64 --ipam-driver null -o bridge=my-bridge my-dhcp-net
<some network id>
$

# With IPv6 enabled
# Although `docker network create` has a `--ipv6` flag, it doesn't work with the null IPAM driver
$ docker network create -d ghcr.io/devplayer0/docker-net-dhcp:release-linux-amd64 --ipam-driver null -o bridge=test -o ipv6=true my-dhcp-net
<some network id>
$

Note: The null IPAM driver must be used, or else Docker will try to allocate IP addresses from its choice of subnet - this can cause IP conflicts since the bridge is connected to your local network!

Container creation

Once you've set up a network, you can create some containers:

$ docker run --rm -ti --network my-dhcp-net alpine
/ # ip address show
1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN qlen 1000
    link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
    inet 127.0.0.1/8 scope host lo
       valid_lft forever preferred_lft forever
159: my-bridge0@if160: <BROADCAST,MULTICAST,UP,LOWER_UP,M-DOWN> mtu 1500 qdisc noqueue state UP qlen 1000
    link/ether 86:41:68:f8:85:b9 brd ff:ff:ff:ff:ff:ff
    inet 10.255.0.246/24 brd 10.255.0.255 scope global test0
       valid_lft forever preferred_lft forever
/ # ip route show
default via 10.255.0.123 dev my-bridge0
10.255.0.0/24 dev my-bridge0 scope link  src 10.255.0.246
/ #

Note:

• It will take a bit longer than usual for the container to start, as a DHCP lease needs to be obtained before creating it
• Once created, a persistent DHCP client will renew the DHCP lease (and then update the default gateway in the container) when necessary - this client runs separately from the container
• Use --mac-address to specify a MAC address if you've configured reserved IP addresses on your DHCP server, or if you want a container to re-use an old lease
• If the docker run command times out waiting for a lease, you can try increasing the initial timeout value by passing -o lease_timeout=60s (e.g. to increase to 60 seconds)

Debugging

To read the plugin's log, do cat /var/lib/docker/plugins/*/rootfs/var/log/net-dhcp.log (as root). You can also use docker plugin set ghcr.io/devplayer0/docker-net-dhcp:release-linux-amd64 LOG_LEVEL=trace to increase log verbosity.

Implementation

Fundamentally, the same mechanism is used by net-dhcp as Docker's bridge driver to wire up networking to containers. That is, a bridge on the host is used as a switch so that containers can communicate with each other - veth pairs connect each container's network namespace to the bridge.

• While Docker creates and manages its own bridges (and routes and filters traffic), net-dhcp uses an existing bridge on the host, bridged with the desired local network.
• Instead of allocating IP addresses from a static pool stored on the Docker host, net-dhcp relies on an external DHCP server to provide IP addresses

Flow

1. Container creation request is made
2. A veth pair is created and the host end is connected to the bridge (at this point both interfaces are still in the host namespace)
3. A DHCP client (BusyBox udhcpc) is started on the container end (still in the host namespace) - initial IP address is provided to Docker by the plugin
4. Docker moves the container end of the veth pair into the container's network namespace and sets the IP address - at this point udhcpc must be stopped
5. net-dhcp starts udhcpc on the container end of the veth pair in the container's network namespace (but still in the plugin PID namespace - this means that the container can't see the DHCP client)
6. udhcpc continues to run, renewing the lease when required, until the container shuts down