What is the relation between docker0 and eth0? - networking

Question

I know by default docker creates a virtual bridge docker0, and all container network are linked to docker0.
As illustrated above:
container eth0 is paired with vethXXX
vethXXX is linked to docker0 same as a machine linked to switch
But what is the relation between docker0 and host eth0?
More specifically:
When a packet flows from container to docker0, how does it know it will be forwarded to eth0, and then to the outside world?
When an external packet arrives to eth0, why it is forwarded to docker0 then container? instead of processing it or drop it?
Question 2 can be a little confusing, I will keep it there and explained a little more:
It is a return packet that initialed by container(in question 1): since the outside does not know container network, the packet is sent to host eth0. How it is forwarded to container? I mean, there must be some place to store the information, how can I check it?
Thanks in advance!
After reading the answer and official network articles, I find the following diagram more accurate that docker0 and eth0 has no direct link,instead they can forward packets:
http://dockerone.com/uploads/article/20150527/e84946a8e9df0ac6d109c35786ac4833.png

Answer 1

There is no direct link between the default docker0 bridge and the hosts ethernet devices. If you use the --net=host option for a container then the hosts network stack will be available in the container.
When a packet flows from container to docker0, how does it know it will be forwarded to eth0, and then to the outside world?
The docker0 bridge has the .1 address of the Docker network assigned to it, this is usually something around a 172.17 or 172.18.
$ ip address show dev docker0
8: docker0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue state UP group default
link/ether 02:42:03:47:33:c1 brd ff:ff:ff:ff:ff:ff
inet 172.17.0.1/16 scope global docker0
valid_lft forever preferred_lft forever
Containers are assigned a veth interface which is attached to the docker0 bridge.
$ bridge link
10: vethcece7e5 state UP #(null): <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 master docker0 state forwarding priority 32 cost 2
Containers created on the default Docker network receive the .1 address as their default route.
$ docker run busybox ip route show
default via 172.17.0.1 dev eth0
172.17.0.0/16 dev eth0 src 172.17.0.3
Docker uses NAT MASQUERADE for outbound traffic from there and it will follow the standard outbound routing on the host, which may or may not involve eth0.
$ iptables -t nat -vnL POSTROUTING
Chain POSTROUTING (policy ACCEPT 0 packets, 0 bytes)
pkts bytes target prot opt in out source destination
0 0 MASQUERADE all -- * !docker0 172.17.0.0/16 0.0.0.0/0
iptables handles the connection tracking and return traffic.
When an external packet arrives to eth0, why it is forwarded to docker0 then container? instead of processing it or drop it?
If you are asking about the return path for outbound traffic from the container, see iptables above as the MASQUERADE will map the connection back through.
If you mean new inbound traffic, Packets are not forwarded into a container by default. The standard way to achieve this is to setup a port mapping. Docker launches a daemon that listens on the host on port X and forwards to the container on port Y.
I'm not sure why NAT wasn't used for inbound traffic as well. I've run into some issues trying to map large numbers of ports into containers which led to mapping real world interfaces completely into containers.

Answer 2

You can detect the relation by network interface iflink from a container and ifindex on the host machine.
Get iflink from a container:
$ docker exec ID cat /sys/class/net/eth0/iflink
17253
Then find this ifindex among interfaces on the host machine:
$ grep -l 17253 /sys/class/net/veth*/ifindex
/sys/class/net/veth02455a1/ifindex