Note: If you have missed my previous articles on Docker, you can find them here.
Application deployment models evolution.
Getting started with Docker.
Docker file and images.
Publishing images to Docker Hub and re-using them
Docker- Find out what's going on
Docker Networking- Part 1
In part 1 of the Docker Networking Series, I gave an overview of Docker Networking. In this article, I am going to show things in action. If you are fellow brethren in networking field and trying to make sense of all the madness around containers, you can be assured you are hearing this from one of your own kind :)
In this part of the series, I intend to talk about the following "network" requirements:
Communicate with other containers on the same host- Docker Bridge. This is a docker0 interface.
Communicate with the outside world - Leverage the host's network to communicate with the outside world with docker Host Networking.
Let's spin up 2 busybox containers
root@ubuntu20-docker1:/home/sathish# docker container run --name linux1 -it --detach busybox
root@ubuntu20-docker1:/home/sathish# docker container run --name linux2 -it --detach busyboxand get a shell inside the container to find out what's going on
root@ubuntu20-docker1:/home/sathish/dockerbuilds# docker container exec -it linux1 sh
/ # ifconfig
eth0 Link encap:Ethernet HWaddr 02:42:AC:11:00:02
inet addr:172.17.0.2 Bcast:172.17.255.255 Mask:255.255.0.0
UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
RX packets:11 errors:0 dropped:0 overruns:0 frame:0
TX packets:0 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:0
RX bytes:946 (946.0 B) TX bytes:0 (0.0 B)
lo Link encap:Local Loopback
inet addr:127.0.0.1 Mask:255.0.0.0
UP LOOPBACK RUNNING MTU:65536 Metric:1
RX packets:0 errors:0 dropped:0 overruns:0 frame:0
TX packets:0 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:1000
RX bytes:0 (0.0 B) TX bytes:0 (0.0 B)
# iproute
default via 172.17.0.1 dev eth0
172.17.0.0/16 dev eth0 scope link src 172.17.0.2172.17.0.2 is the IP address assigned to the container, this is attached to the Docker0 interface. The default gateway for this is 172.17.0.1.
Back in the host, We can see Docker0 interface uses an IP of 172.17.0.1 which is the default route of containers associated with the bridge:
root@ubuntu20-docker1:/home/sathish# ifconfig
docker0: flags=4099<UP,BROADCAST,MULTICAST> mtu 1500
inet 172.17.0.1 netmask 255.255.0.0 broadcast 172.17.255.255
ether 02:42:3c:2e:1f:de txqueuelen 0 (Ethernet)
RX packets 0 bytes 0 (0.0 B)
RX errors 0 dropped 0 overruns 0 frame 0
TX packets 0 bytes 0 (0.0 B)
TX errors 0 dropped 0 overruns 0 carrier 0 collisions 0Containers communicate with the outside world with the host's physical interface. The host performs NAT to make this happen.
root@ubuntu20-docker1:/home/sathish# iptables -t nat -L
Chain PREROUTING (policy ACCEPT)
target prot opt source destination
DOCKER all -- anywhere anywhere ADDRTYPE match dst-type LOCAL
Chain INPUT (policy ACCEPT)
target prot opt source destination
Chain OUTPUT (policy ACCEPT)
target prot opt source destination
DOCKER all -- anywhere !localhost/8 ADDRTYPE match dst-type LOCAL
Chain POSTROUTING (policy ACCEPT)
target prot opt source destination
MASQUERADE all -- 172.17.0.0/16 anywhere
Chain DOCKER (2 references)
target prot opt source destination
RETURN all -- anywhere anywhereThose familiar with IPTABLES will notice, with"outbound" NAT- Any packet from containers is forwarded to outside networks.
To check out inbound NAT in action- let's spin up an HTTP server that exposes port 80.
root@ubuntu20-docker1:/home/sathish# docker run -detach --name web -p 80:80 httpdAs we can see a new rule gets added to allow this
root@ubuntu20-docker1:/home/sathish# iptables -t nat -L
Chain PREROUTING (policy ACCEPT)
target prot opt source destination
DOCKER all -- anywhere anywhere ADDRTYPE match dst-type LOCAL
Chain INPUT (policy ACCEPT)
target prot opt source destination
Chain OUTPUT (policy ACCEPT)
target prot opt source destination
DOCKER all -- anywhere !localhost/8 ADDRTYPE match dst-type LOCAL
Chain POSTROUTING (policy ACCEPT)
target prot opt source destination
MASQUERADE all -- 172.17.0.0/16 anywhere
MASQUERADE tcp -- 172.17.0.4 172.17.0.4 tcp dpt:http
Chain DOCKER (2 references)
target prot opt source destination
RETURN all -- anywhere anywhere
DNAT tcp -- anywhere anywhere tcp dpt:http to:172.17.0.4:80172.17.0.4 is the eth0 IP address of the "web" container.
Let's now look at how Docker looks at things.
root@ubuntu20-docker1:/home/sathish# docker network ls
NETWORK ID NAME DRIVER SCOPE
cd789c842a3d bridge bridge local
bfaf94bd6cff host host local
c26f01b036e0 none null local
Well, there is the bridge, host interfaces, and none driver.
Note: The none driver is used when a container does not need any kind of network access. Only a loopback interface will be created inside cotainer. Let's look at the bridge interface with the inspect command.
root@ubuntu20-docker1:/home/sathish# docker network inspect bridge
[
{
"Name": "bridge",
"Id": "cd789c842a3d292ba7e6d34e0e54cbdf2c3962c6fe54d7a75dbd5e24946930ab",
"Created": "2020-08-09T12:32:30.01803359Z",
"Scope": "local",
"Driver": "bridge",
"EnableIPv6": false,
"IPAM": {
"Driver": "default",
"Options": null,
"Config": [
{
"Subnet": "172.17.0.0/16",
"Gateway": "172.17.0.1"
}
]
},
"Internal": false,
"Attachable": false,
"Ingress": false,
"ConfigFrom": {
"Network": ""
},
"ConfigOnly": false,
"Containers": {
"02f571db76d1c6a8e288f51520af689d9b5c1e14bd4dd1e3ba89d69a6f10ae1f": {
"Name": "web",
"EndpointID": "118815e10ee1275f904adb946fdd69ae99a8b6ac5e40d6e581199b17455c19c6",
"MacAddress": "02:42:ac:11:00:04",
"IPv4Address": "172.17.0.4/16",
"IPv6Address": ""
},
"d98dce33019d09f53fc5105aa4c88bd72668f7b7cd12072128e5b6b6acf927fe": {
"Name": "linux2",
"EndpointID": "b0300e80e95d22511a446035a1847a45b56690340f73107c3904ac0a358bef39",
"MacAddress": "02:42:ac:11:00:03",
"IPv4Address": "172.17.0.3/16",
"IPv6Address": ""
},
"e70937b55c641cdd2e611489dce0fb72e9dcd2e7bcca564589a82dc1f25bc67e": {
"Name": "linux1",
"EndpointID": "ae7bc12fb0afada9bbc1ee1a25e92f6c7f32c2268640be7f0018bf389403d5c5",
"MacAddress": "02:42:ac:11:00:02",
"IPv4Address": "172.17.0.2/16",
"IPv6Address": ""
}
},
"Options": {
"com.docker.network.bridge.default_bridge": "true",
"com.docker.network.bridge.enable_icc": "true",
"com.docker.network.bridge.enable_ip_masquerade": "true",
"com.docker.network.bridge.host_binding_ipv4": "0.0.0.0",
"com.docker.network.bridge.name": "docker0",
"com.docker.network.driver.mtu": "1500"
},
"Labels": {}
}
]The inspect command shows the containers we have spun up are associated with this bridge. Let's look at a few other important things
"com.docker.network.bridge.default_bridge": "true", means this is default bridge
"com.docker.network.bridge.enable_icc": "true"- setting allows communication between container on this bridge
"com.docker.network.bridge.enable_ip_masquerade"- This setting tells Docker to perform NAT and this is how I was able to ping google.com from within the container.
Let's get back inside linux1 container and check things out.
root@ubuntu20-docker1:/home/sathish# docker container exec -it linux1 sh
/ # ping -c 1 google.com
PING google.com (172.217.163.142): 56 data bytes
64 bytes from 172.217.163.142: seq=0 ttl=112 time=39.624 ms
--- google.com ping statistics ---
1 packets transmitted, 1 packets received, 0% packet loss
round-trip min/avg/max = 39.624/39.624/39.624 ms
/ # ping -c 1 172.17.0.3
PING 172.17.0.3 (172.17.0.3): 56 data bytes
64 bytes from 172.17.0.3: seq=0 ttl=64 time=0.040 msWe can ping both the outside world and other containers connected to the Docker0 bridge. Ping to google is possible by the post routing iptable NAT rule on the host, while docker 0 bridge allows ping to the linux2 container.
That's all for today folks !! In the next article, I will give an intro to Docker Swarm and then will talk about multi-host container networking.
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