# Understand Docker container networks To build web applications that act in concert but do so securely, use the Docker networks feature. Networks, by definition, provide complete isolation for containers. So, it is important to have control over the networks your applications run on. Docker container networks give you that control. This section provides an overview of the default networking behavior that Docker Engine delivers natively. It describes the type of networks created by default and how to create your own, user--defined networks. It also describes the resources required to create networks on a single host or across a cluster of hosts. ## Default Networks When you install Docker, it creates three networks automatically. You can list these networks using the `docker network ls` command: ``` $ docker network ls NETWORK ID NAME DRIVER 7fca4eb8c647 bridge bridge 9f904ee27bf5 none null cf03ee007fb4 host host ``` Historically, these three networks are part of Docker's implementation. When you run a container you can use the `--net` flag to specify which network you want to run a container on. These three networks are still available to you. The `bridge` network represents the `docker0` network present in all Docker installations. Unless you specify otherwise with the `docker run --net=` option, the Docker daemon connects containers to this network by default. You can see this bridge as part of a host's network stack by using the `ifconfig` command on the host. ``` ubuntu@ip-172-31-36-118:~$ ifconfig docker0 Link encap:Ethernet HWaddr 02:42:47:bc:3a:eb inet addr:172.17.0.1 Bcast:0.0.0.0 Mask:255.255.0.0 inet6 addr: fe80::42:47ff:febc:3aeb/64 Scope:Link UP BROADCAST RUNNING MULTICAST MTU:9001 Metric:1 RX packets:17 errors:0 dropped:0 overruns:0 frame:0 TX packets:8 errors:0 dropped:0 overruns:0 carrier:0 collisions:0 txqueuelen:0 RX bytes:1100 (1.1 KB) TX bytes:648 (648.0 B) ``` The `none` network adds a container to a container-specific network stack. That container lacks a network interface. Attaching to such a container and looking at it's stack you see this: ``` ubuntu@ip-172-31-36-118:~$ docker attach nonenetcontainer / # cat /etc/hosts 127.0.0.1 localhost ::1 localhost ip6-localhost ip6-loopback fe00::0 ip6-localnet ff00::0 ip6-mcastprefix ff02::1 ip6-allnodes ff02::2 ip6-allrouters / # ifconfig lo Link encap:Local Loopback inet addr:127.0.0.1 Mask:255.0.0.0 inet6 addr: ::1/128 Scope:Host 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:0 RX bytes:0 (0.0 B) TX bytes:0 (0.0 B) / # ``` >**Note**: You can detach from the container and leave it running with `CTRL-p CTRL-q`. The `host` network adds a container on the hosts network stack. You'll find the network configuration inside the container is identical to the host. With the exception of the the `bridge` network, you really don't need to interact with these default networks. While you can list and inspect them, you cannot remove them. They are required by your Docker installation. However, you can add your own user-defined networks and these you can remove when you no longer need them. Before you learn more about creating your own networks, it is worth looking at the `default` network a bit. ### The default bridge network in detail The default bridge network is present on all Docker hosts. The `docker network inspect` ``` $ docker network inspect bridge [ { "Name": "bridge", "Id": "f7ab26d71dbd6f557852c7156ae0574bbf62c42f539b50c8ebde0f728a253b6f", "Scope": "local", "Driver": "bridge", "IPAM": { "Driver": "default", "Config": [ { "Subnet": "172.17.0.1/16", "Gateway": "172.17.0.1" } ] }, "Containers": {}, "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": "9001" } } ] ``` The Engine automatically creates a `Subnet` and `Gateway` to the network. The `docker run` command automatically adds new containers to this network. ``` $ docker run -itd --name=container1 busybox 3386a527aa08b37ea9232cbcace2d2458d49f44bb05a6b775fba7ddd40d8f92c $ docker run -itd --name=container2 busybox 94447ca479852d29aeddca75c28f7104df3c3196d7b6d83061879e339946805c ``` Inspecting the `bridge` network again after starting two containers shows both newly launched containers in the network. Their ids show up in the container ``` $ docker network inspect bridge {[ { "Name": "bridge", "Id": "f7ab26d71dbd6f557852c7156ae0574bbf62c42f539b50c8ebde0f728a253b6f", "Scope": "local", "Driver": "bridge", "IPAM": { "Driver": "default", "Config": [ { "Subnet": "172.17.0.1/16", "Gateway": "172.17.0.1" } ] }, "Containers": { "3386a527aa08b37ea9232cbcace2d2458d49f44bb05a6b775fba7ddd40d8f92c": { "EndpointID": "647c12443e91faf0fd508b6edfe59c30b642abb60dfab890b4bdccee38750bc1", "MacAddress": "02:42:ac:11:00:02", "IPv4Address": "172.17.0.2/16", "IPv6Address": "" }, "94447ca479852d29aeddca75c28f7104df3c3196d7b6d83061879e339946805c": { "EndpointID": "b047d090f446ac49747d3c37d63e4307be745876db7f0ceef7b311cbba615f48", "MacAddress": "02:42:ac:11:00:03", "IPv4Address": "172.17.0.3/16", "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": "9001" } } ] ``` The `docker network inspect` command above shows all the connected containers and their network resources on a given network. Containers in this default network are able to communicate with each other using IP addresses. Docker does not support automatic service discovery on the default bridge network. If you want to communicate with container names in this default bridge network, you must connect the containers via the legacy `docker run --link` option. You can `attach` to a running `container` and investigate its configuration: ``` $ docker attach container1 / # ifconfig ifconfig eth0 Link encap:Ethernet HWaddr 02:42:AC:11:00:02 inet addr:172.17.0.2 Bcast:0.0.0.0 Mask:255.255.0.0 inet6 addr: fe80::42:acff:fe11:2/64 Scope:Link UP BROADCAST RUNNING MULTICAST MTU:9001 Metric:1 RX packets:16 errors:0 dropped:0 overruns:0 frame:0 TX packets:8 errors:0 dropped:0 overruns:0 carrier:0 collisions:0 txqueuelen:0 RX bytes:1296 (1.2 KiB) TX bytes:648 (648.0 B) lo Link encap:Local Loopback inet addr:127.0.0.1 Mask:255.0.0.0 inet6 addr: ::1/128 Scope:Host 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:0 RX bytes:0 (0.0 B) TX bytes:0 (0.0 B) ``` Then use `ping` for about 3 seconds to test the connectivity of the containers on this `bridge` network. ``` / # ping -w3 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=0.096 ms 64 bytes from 172.17.0.3: seq=1 ttl=64 time=0.080 ms 64 bytes from 172.17.0.3: seq=2 ttl=64 time=0.074 ms --- 172.17.0.3 ping statistics --- 3 packets transmitted, 3 packets received, 0% packet loss round-trip min/avg/max = 0.074/0.083/0.096 ms ``` Finally, use the `cat` command to check the `container1` network configuration: ``` / # cat /etc/hosts 172.17.0.2 3386a527aa08 127.0.0.1 localhost ::1 localhost ip6-localhost ip6-loopback fe00::0 ip6-localnet ff00::0 ip6-mcastprefix ff02::1 ip6-allnodes ff02::2 ip6-allrouters ``` To detach from a `container1` and leave it running use `CTRL-p CTRL-q`.Then, attach to `container2` and repeat these three commands. ``` $ docker attach container2 / # ifconfig eth0 Link encap:Ethernet HWaddr 02:42:AC:11:00:03 inet addr:172.17.0.3 Bcast:0.0.0.0 Mask:255.255.0.0 inet6 addr: fe80::42:acff:fe11:3/64 Scope:Link UP BROADCAST RUNNING MULTICAST MTU:9001 Metric:1 RX packets:15 errors:0 dropped:0 overruns:0 frame:0 TX packets:13 errors:0 dropped:0 overruns:0 carrier:0 collisions:0 txqueuelen:0 RX bytes:1166 (1.1 KiB) TX bytes:1026 (1.0 KiB) lo Link encap:Local Loopback inet addr:127.0.0.1 Mask:255.0.0.0 inet6 addr: ::1/128 Scope:Host 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:0 RX bytes:0 (0.0 B) TX bytes:0 (0.0 B) / # ping -w3 172.17.0.2 PING 172.17.0.2 (172.17.0.2): 56 data bytes 64 bytes from 172.17.0.2: seq=0 ttl=64 time=0.067 ms 64 bytes from 172.17.0.2: seq=1 ttl=64 time=0.075 ms 64 bytes from 172.17.0.2: seq=2 ttl=64 time=0.072 ms --- 172.17.0.2 ping statistics --- 3 packets transmitted, 3 packets received, 0% packet loss round-trip min/avg/max = 0.067/0.071/0.075 ms / # cat /etc/hosts 172.17.0.3 94447ca47985 127.0.0.1 localhost ::1 localhost ip6-localhost ip6-loopback fe00::0 ip6-localnet ff00::0 ip6-mcastprefix ff02::1 ip6-allnodes ff02::2 ip6-allrouters ``` The default `docker0` bridge network supports the use of port mapping and `docker run --link` to allow communications between containers in the `docker0` network. These techniques are cumbersome to set up and prone to error. While they are still available to you as techniques, it is better to avoid them and define your own bridge networks instead. ## User-defined networks You can create your own user-defined networks that better isolate containers. Docker provides some default **network drivers** for use creating these networks. You can create a new **bridge network** or **overlay network**. You can also create a **network plugin** or **remote network** written to your own specifications. You can create multiple networks. You can add containers to more than one network. Containers can only communicate within networks but not across networks. A container attached to two networks can communicate with member containers in either network. The next few sections describe each of Docker's built-in network drivers in greater detail. ### A bridge network The easiest user-defined network to create is a `bridge` network. This network is similar to the historical, default `docker0` network. There are some added features and some old features that aren't available. ``` $ docker network create --driver bridge isolated_nw c5ee82f76de30319c75554a57164c682e7372d2c694fec41e42ac3b77e570f6b $ docker network inspect isolated_nw [ { "Name": "isolated_nw", "Id": "c5ee82f76de30319c75554a57164c682e7372d2c694fec41e42ac3b77e570f6b", "Scope": "local", "Driver": "bridge", "IPAM": { "Driver": "default", "Config": [ {} ] }, "Containers": {}, "Options": {} } ] $ docker network ls NETWORK ID NAME DRIVER 9f904ee27bf5 none null cf03ee007fb4 host host 7fca4eb8c647 bridge bridge c5ee82f76de3 isolated_nw bridge ``` After you create the network, you can launch containers on it using the `docker run --net=` option. ``` $ docker run --net=isolated_nw -itd --name=container3 busybox 885b7b4f792bae534416c95caa35ba272f201fa181e18e59beba0c80d7d77c1d $ docker network inspect isolated_nw [ { "Name": "isolated_nw", "Id": "c5ee82f76de30319c75554a57164c682e7372d2c694fec41e42ac3b77e570f6b", "Scope": "local", "Driver": "bridge", "IPAM": { "Driver": "default", "Config": [ {} ] }, "Containers": { "885b7b4f792bae534416c95caa35ba272f201fa181e18e59beba0c80d7d77c1d": { "EndpointID": "514e1b419074397ea92bcfaa6698d17feb62db49d1320a27393b853ec65319c3", "MacAddress": "02:42:ac:15:00:02", "IPv4Address": "172.21.0.2/16", "IPv6Address": "" } }, "Options": {} } ] ``` The containers you launch into this network must reside on the same Docker host. Each container in the network can immediately communicate with other containers in the network. Though, the network itself isolates the containers from external networks. ![An isolated network](images/bridge_network.png) Within a user-defined bridge network, linking is not supported. You can expose and publish container ports on containers in this network. This is useful if you want to make a portion of the `bridge` network available to an outside network. ![Bridge network](images/network_access.png) A bridge network is useful in cases where you want to run a relatively small network on a single host. You can, however, create significantly larger networks by creating an `overlay` network. ### An overlay network Docker's `overlay` network driver supports multi-host networking natively out-of-the-box. This support is accomplished with the help of `libnetwork`, a built-in VXLAN-based overlay network driver, and Docker's `libkv` library. The `overlay` network requires a valid key-value store service. Currently, Docker's supports Consul, Etcd, and ZooKeeper (Distributed store). Before creating a network you must install and configure your chosen key-value store service. The Docker hosts that you intend to network and the service must be able to communicate. ![Key-value store](images/key_value.png) Each host in the network must run a Docker Engine instance. The easiest way to provision the hosts are with Docker Machine. ![Engine on each host](images/engine_on_net.png) You should open the following ports between each of your hosts. | Protocol | Port | Description | |----------|------|-----------------------| | udp | 4789 | Data plane (VXLAN) | | tcp/udp | 7946 | Control plane | Your key-value store service may require additional ports. Check your vendor's documentation and open any required ports. Once you have several machines provisioned, you can use Docker Swarm to quickly form them into a swarm which includes a discovery service as well. To create an overlay network, you configure options on the `daemon` on each Docker Engine for use with `overlay` network. There are two options to set:
Option Description 
--cluster-store=PROVIDER://URL
 Describes the location of the KV service. 
--cluster-advertise=HOST_IP|HOST_IFACE:PORT
 The IP address or interface of the HOST used for clustering.
Create an `overlay` network on one of the machines in the Swarm. $ docker network create --driver overlay my-multi-host-network This results in a single network spanning multiple hosts. An `overlay` network provides complete isolation for the containers. ![An overlay network](images/overlay_network.png) Then, on each host, launch containers making sure to specify the network name. $ docker run -itd --net=my-multi-host-network busybox Once connected, each container has access to all the containers in the network regardless of which Docker host the container was launched on. ![Published port](images/overlay-network-final.png) If you would like to try this for yourself, see the [Getting started for overlay](get-started-overlay.md). ### Custom network plugin If you like, you can write your own network driver plugin. A network driver plugin makes use of Docker's plugin infrastructure. In this infrastructure, a plugin is a process running on the same Docker host as the Docker `daemon`. Network plugins follow the same restrictions and installation rules as other plugins. All plugins make use of the plugin API. They have a lifecycle that encompasses installation, starting, stopping and activation. Once you have created and installed a custom network driver, you use it like the built-in network drivers. For example: $ docker network create --driver weave mynet You can inspect it, add containers too and from it, and so forth. Of course, different plugins may make use of different technologies or frameworks. Custom networks can include features not present in Docker's default networks. For more information on writing plugins, see [Extending Docker](../../extend/index.md) and [Writing a network driver plugin](../../extend/plugins_network.md). ## Legacy links Before the Docker network feature, you could use the Docker link feature to allow containers to discover each other and securely transfer information about one container to another container. With the introduction of Docker networks, you can still create links but they are only supported on the default `bridge` network named `bridge` and appearing in your network stack as `docker0`. While links are still supported in this limited capacity, you should avoid them in preference of Docker networks. The link feature is expected to be deprecated and removed in a future release. ## Related information - [Work with network commands](work-with-networks.md) - [Get started with multi-host networking](get-started-overlay.md) - [Managing Data in Containers](../dockervolumes.md) - [Docker Machine overview](https://docs.docker.com/machine) - [Docker Swarm overview](https://docs.docker.com/swarm) - [Investigate the LibNetwork project](https://github.com/docker/libnetwork)