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TL;DR - Why does Docker call it's default networking as Bridge Networking when it seems to be a lot like NAT Network.

Let start by looking at how -
1) VMWare or VirtualBox handles networking for virtual machines. Say the Host IP is some random 152.50.60.21 and the network CIDR is 152.50.60.0/24.
Bridge Network - Any VM connected through this interface can have any free IP on the network the host is connected to. So if IP 152.50.60.30 is free, then VM can bind to this IP. Similarly, a second VM can have an IP 152.50.60.32 if this IP is free. The Bridge Network connects the VM's on to the same network the host is connected to. Any machine on the internet can reach the VM's and the VM's can reach the internet directly (of course if the HOST network is connected to internet).
NAT Network - NAT is a separate network from the network the host is connected to. And VMWare can accept any valid CIDR (to not complicate things I will refer to the private reserved blocks only. Though, if am right, any CIDR is fine.) Safely, this new NAT Network being created on host and accessible only on the host can have CIDR 10.0.0.0/8 or 172.16.0.0/12 or 192.168.0.0/16 (or any subnet of these networks). I am picking 10.0.0.0/8. So two VM's spinning up on the host and connected through NAT Network can have IP's 10.0.3.3 and 10.0.3.6 Being on the NAT Network the VM's are not visible to the outer world beyond the host i.e the VM's are not reachable to the outer world (except with DNAT/Port Forwarding configuration on the Host). But the VM's can access the outer world/internet/intranet though SNAT provided by HOST i.e the IP's of VM's are never exposed to the outer world.

VMWare Doc's reference: Understanding Common Networking Configurations

Next, let's look on the Docker side -
Dockers default networking
When an image is run on the HOST (whose IP from above is 152.50.60.21) using Dockers default networking (which it calls as Bridge Network), the new container can get an IP (say) 172.17.0.13 from the network - 172.16.0.0/12 (at least on my env). Similarly, a second container can get an IP 172.17.0.23. For accessing the internet these containers rely on SNAT provided by HOST. And any machine on the internet/intranet can't access the Containers except through port forwarding provided by the HOST. So the containers are not visible to the world except for HOST. Looking at this I would assume that the default networking provided by Docker is NAT Network, but Docker likes to call it as Bridge Network.

So, Could anyone say where things are messed up or how I got to look at Bridge/NAT Networks?

samshers
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2 Answers2

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Docker's equivalent of VMWare or VirtualBox bridge network is macvlan.

From the docs:

...you can use the macvlan network driver to assign a MAC address to each container’s virtual network interface, making it appear to be a physical network interface directly connected to the physical network. In this case, you need to designate a physical interface on your Docker host to use for the macvlan, as well as the subnet and gateway of the macvlan. You can even isolate your macvlan networks using different physical network interfaces.

When you create a macvlan network, it can either be in bridge mode or 802.1q trunk bridge mode.

In bridge mode, macvlan traffic goes through a physical device on the host.

In the simple bridge example, your traffic flows through eth0 and Docker routes traffic to your container using its MAC address. To network devices on your network, your container appears to be physically attached to the network.

Example of macvlan bridge mode:

$ docker network create -d macvlan \
  --subnet=172.16.86.0/24 \
  --gateway=172.16.86.1 \
  -o parent=eth0 \
  my-macvlan-net

This command creates MacVLAN network on top of eth0 with the network name of my-macvlan-net.

In 802.1q trunk bridge mode, traffic goes through an 802.1q sub-interface which Docker creates on the fly. This allows you to control routing and filtering at a more granular level.

In the 802.1q trunked bridge example, your traffic flows through a sub-interface of eth0 (called eth0.10 in the example below) and Docker routes traffic to your container using its MAC address. To network devices on your network, your container appears to be physically attached to the network.

Example of macvlan 802.1q trunk bridge mode:

$ docker network create -d macvlan \
  --subnet=172.16.86.0/24 \
  --gateway=172.16.86.1 \
  -o parent=eth0.10 \
  my-8021q-macvlan-net

This creates macvlan network, and has parent eth0.10.

The naming seems confusing to someone coming from VMWare or VirtualBox, but it exists.

You can see another tutorial of macvlan (includes assigning IP addr to the container) here.

Christophorus Reyhan
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3

Adding this as an answer because it is too long to be a comment.

After hours of googling and reading docs, since I could find any concrete answer, my theory is that the difference is because no standards are defining these virtual interfaces, unlike the physical interfaces which are defined by IEEE and IETF.

There is a list of Linux virtual network interfaces by RedHat. Most of the network interface types that you see in docker and vmware/vbox will be a subset of this, except for some. overlay network, for example -- which only exists in docker to serve its special purpose by connecting container on different hosts to a single network.

Back to the point - it is not like these network interfaces are defined by Cisco or IETF or IEEE or anything/anyone. Physical networking interfaces are implemented at OS/other networking device level and they are defined standards, for other products -- be it a virtualization platform (vmware/vbox) or containerization (docker), they need to implement their own networking solution. At this point, they can implement their interfaces, give it some fancy name, or they can call it the same thing it's called at the OS level.

I am certain docker devs would not be stupid enough to know the diff between NAT and bridge, they probably knew that the network they are calling bridge is a NAT network yet decided to call it a bridge for some reason. And since there exists no centralized definition of a bridge or NAT network, you can't tell that docker is wrong. They wanted to call this network bridge, they call it a bridge. but yes, indeed it can be confusing to people coming from a virtualization/Linux networking background. but it is what it is.

Christophorus Reyhan already wrote an amazing answer about the bridge equivalent in docker and how to create it. I am only adding this because I was not satisfied with the WHY of the question. why is it not the same? Yes, my answer is based on some assumptions, but this is the closest that I could think of the be the reason. No way to confirm the hypothesis unless one day some docker dev sees this and decides to reply.

One more thing, for vmware/vbox, the host is the host physical machine itself, and thus when using bridge mode, it connects to the host's network. but for containers, the host is not the physical host but rather the docker host/engine. so when using bridge mode, containers attach to the host's network, but it would be docker host in this case. So it is still a bridge network, but the hierarchy is different this time. container -> docker host -> host os, and not guest os -> host os. This could be another possibility as well.

From the docker documentation:

macvlan driver is sometimes the best choice when dealing with legacy applications that expect to be directly connected to the physical network, rather than routed through the Docker host’s network stack.

"rather than through the Docker host's network stack" and

Macvlan networks are best when you are migrating from a VM setup or need your containers to look like physical hosts on your network, each with a unique MAC address.

which indicates that networking is not the same as in VMs in the world of docker though they may sometimes share some similarities or the same names.

Aniket Kariya
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