This post introduces Docker Engine’s network feature in general and specifically introduces configuring DNS in containers. This post assumes that you have the Docker Engine installed and that you know the basics<\/a> of working with containers<\/a>.<\/p>\n We will not discuss service discovery from other networks here since that needs a deeper knowledge of Docker tools like Docker Swarm. Since these posts focus on the basics of Docker we will focus on networking cotainers within the same host. So let’s get started with understanding the basics of networking in Docker.\n<\/p>\n The Docker Engine provides 3 networks by default – The Every container created by the Docker engine is added to the default Note the Apart from the default networks, users can create new networks with the Let us now add a container to this network and inspect again. The command The container is added into the network You can see that the All containers are added to the Containers So as you can see, containers added into the same network can automatically find each other through their IP addresses. Next, let us see how to setup DNS in Docker so that we need use the IP addresses to communicate.<\/p>\n Docker engine uses these 3 files within every container to configure it’s DNS.<\/p>\n These files can be setup and manipulated using the The entry of But the fun with discovering containers in the There are some important differences between connecting containers in user-defined networks and within 2. Basics of Networking Configurations in Docker Engine<\/h2>\n
bridge<\/code>, host<\/code> and none<\/code>. The command docker network ls<\/code> lists out all networks created by Docker. So on a default installation you will see something like this.<\/p>\n$ docker network ls\nNETWORK ID NAME DRIVER SCOPE\n36d51e62e518 bridge bridge local \n18e8e68644ea host host local \n1f87f168df62 none null local\n<\/pre>\n
bridge<\/code> network is mapped to the docker0<\/code> network bridge on the Docker Engine’s host. The ip address<\/code> command can be used to check the details of docker0.<\/p>\n$ ip address show label docker0\n7: docker0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue state UP \n link\/ether 02:45:01:c0:ba:c7 brd ff:ff:ff:ff:ff:ff\n inet 172.17.0.1\/16 scope global docker0\n valid_lft forever preferred_lft forever\n inet6 fe80::42:1ff:fec0:bac7\/64 scope link \n valid_lft forever preferred_lft forever\n<\/pre>\n
2.1. Inspect Networks<\/h3>\n
bridge<\/code> network. The command docker network inspect<\/code> can be used to inspect the network like so…<\/p>\n$ docker network inspect bridge\n[\n {\n \"Name\": \"bridge\",\n \"Id\": \"36d51e62e518d5430c798ce6eb68dc2737e9ad0555dd45097b04ef7597bce644\",\n \"Scope\": \"local\",\n \"Driver\": \"bridge\",\n \"EnableIPv6\": false,\n \"IPAM\": {\n \"Driver\": \"default\",\n \"Options\": null,\n \"Config\": [\n {\n \"Subnet\": \"172.17.0.0\/16\",\n \"Gateway\": \"172.17.0.1\"\n }\n ]\n },\n \"Internal\": false,\n \"Containers\": {},\n \"Options\": {\n \"com.docker.network.bridge.default_bridge\": \"true\",\n \"com.docker.network.bridge.enable_icc\": \"true\",\n \"com.docker.network.bridge.enable_ip_masquerade\": \"true\",\n \"com.docker.network.bridge.host_binding_ipv4\": \"0.0.0.0\",\n \"com.docker.network.bridge.name\": \"docker0\",\n \"com.docker.network.driver.mtu\": \"1500\"\n },\n \"Labels\": {}\n }\n]\n<\/pre>\n\"Containers\"<\/code> section in the above picture. It lists the containers attached to the bridge<\/code> network. This section will be empty in a fresh Docker installation or when all containers are stopped.<\/p>\n2.2. Create User Defined Networks<\/h3>\n
docker network create<\/code> command. Let us create a new network called example-network and inspect it’s contents now.<\/p>\n$ docker network create example-network && docker network inspect example-network\n301ffd3fbc1e323d40d2bc687f2e5c6341c16011e91d57151577fa08c914a157\n[\n {\n \"Name\": \"example-network\",\n \"Id\": \"301ffd3fbc1e323d40d2bc687f2e5c6341c16011e91d57151577fa08c914a157\",\n \"Scope\": \"local\",\n \"Driver\": \"bridge\",\n \"EnableIPv6\": false,\n \"IPAM\": {\n \"Driver\": \"default\",\n \"Options\": {},\n \"Config\": [\n {\n \"Subnet\": \"172.18.0.0\/16\",\n \"Gateway\": \"172.18.0.1\/16\"\n }\n ]\n },\n \"Internal\": false,\n \"Containers\": {},\n \"Options\": {},\n \"Labels\": {}\n }\n]\n<\/pre>\ndocker run --network=<\/code> can be used to attach a container to a network of our choice, like below:$ docker run -itd --name=infinite_loop --network=example-network enhariharan\/infinite-loop\n8da6157240e09d69c7490c1af4ce483b30ff6f7ba7804b45818c0975e7b8ba25\n<\/pre>\n
example-network<\/code>. Let us now investigate the network settings again.<\/p>\n$ docker network inspect example-network\n[\n {\n \"Name\": \"example-network\",\n \"Id\": \"301ffd3fbc1e323d40d2bc687f2e5c6341c16011e91d57151577fa08c914a157\",\n \"Scope\": \"local\",\n \"Driver\": \"bridge\",\n \"EnableIPv6\": false,\n \"IPAM\": {\n \"Driver\": \"default\",\n \"Options\": {},\n \"Config\": [\n {\n \"Subnet\": \"172.18.0.0\/16\",\n \"Gateway\": \"172.18.0.1\/16\"\n }\n ]\n },\n \"Internal\": false,\n \"Containers\": {\n \"8da6157240e09d69c7490c1af4ce483b30ff6f7ba7804b45818c0975e7b8ba25\": {\n \"Name\": \"infinite_loop\",\n \"EndpointID\": \"ccb0318b24dc3b0f1676f5aa16e5ea839adb715baa6bcd4ad16cdadb4aabd973\",\n \"MacAddress\": \"02:42:ac:12:00:02\",\n \"IPv4Address\": \"172.18.0.2\/16\",\n \"IPv6Address\": \"\"\n }\n },\n \"Options\": {},\n \"Labels\": {}\n }\n]\n<\/pre>\n\"Containers\"<\/code> section now has the container infinite_loop<\/code> created from the image.<\/p>\n2.3. Connect Containers Within a Network<\/h3>\n
bridge<\/code> network by default. Containers within the same Docker network can connect to each other. Let us see that through a simple example. We will create 2 simple containers from the image enhariharan\/infinite-loop<\/code> into the custom network example-network<\/code> created earlier. Then we’ll inspect the network to see that the containers were added as expected.<\/p>\n$ docker run -itd --name=infinite_loop_1 --network=example-network enhariharan\/infinite-loop\n3c23fb845274e24ac8bdaa3216affc3b6d2755b58e6a8a4424c4008ca3c8022c\n$\n$ docker run -itd --name=infinite_loop_2 --network=example-network enhariharan\/infinite-loop\n9bd946658d4456618f60413e0cb41f6b4502eb60cfbff325a1bd7f218bffdd45\n$\n$ docker network inspect example-network\n...\n \"Containers\": {\n \"3c23fb845274e24ac8bdaa3216affc3b6d2755b58e6a8a4424c4008ca3c8022c\": {\n \"Name\": \"infinite_loop_1\",\n \"EndpointID\": \"0c5fc22252e5d88ae746fb07d9dc9827ab6840a40db956eff2ed8bcfa042a099\",\n \"MacAddress\": \"02:42:ac:12:00:02\",\n \"IPv4Address\": \"172.18.0.2\/16\",\n \"IPv6Address\": \"\"\n },\n \"9bd946658d4456618f60413e0cb41f6b4502eb60cfbff325a1bd7f218bffdd45\": {\n \"Name\": \"infinite_loop_2\",\n \"EndpointID\": \"1eecb3c8414026bfeaf63cfb23fb5319ca068c2c3c220ad486f19de02b2225bd\",\n \"MacAddress\": \"02:42:ac:12:00:03\",\n \"IPv4Address\": \"172.18.0.3\/16\",\n \"IPv6Address\": \"\"\n }\n },\n...\n<\/pre>\ninfinite_loop_1<\/code> and infinite_loop_2<\/code> were created in detached mode. They have been added into example-network<\/code> with IP address 172.18.0.2<\/code> and 172.18.0.3<\/code> respectively, as confirmed above. If the --network=<\/code> is not provided then containers are added to the bridge<\/code> network by default. Now, let us open a session into the container infinite_loop_1<\/code> and try to ping infinite_loop_2<\/code>.<\/p>\n$ docker exec -it infinite_loop_1 sh\n\/ # ping -c 5 172.18.0.3\nPING 172.18.0.3 (172.18.0.3): 56 data bytes\n64 bytes from 172.18.0.3: seq=0 ttl=64 time=0.165 ms\n64 bytes from 172.18.0.3: seq=1 ttl=64 time=0.124 ms\n64 bytes from 172.18.0.3: seq=2 ttl=64 time=0.115 ms\n64 bytes from 172.18.0.3: seq=3 ttl=64 time=0.115 ms\n64 bytes from 172.18.0.3: seq=4 ttl=64 time=0.116 ms\n\n--- 172.18.0.3 ping statistics ---\n5 packets transmitted, 5 packets received, 0% packet loss\nround-trip min\/avg\/max = 0.106\/0.117\/0.137 ms\n<\/pre>\n
3. Setup Container DNS in Bridge Network<\/h2>\n
\n
\/etc\/hostname<\/code> – This file maps the container IP address to a name.<\/li>\n\/etc\/hosts<\/code> – This file maps other container’s IP addresses to names.<\/li>\n\/etc\/resolv.conf<\/code> – This file contains the IP addresses of other DNS servers to refer to if the container cannot resolve a name to IP address.<\/li>\n<\/ul>\ndocker run<\/code> command. The hostname for a container is set into \/etc\/hostname<\/code> using the --hostname=<\/code> option.[ulp id=’6PVIvOz3kDbYmNRn’]<\/p>\n$ docker run -itd --name=infinite_loop_1 --hostname=container1 enhariharan\/infinite-loop\n986d6b95a283493edd6d331d3c9f0c31595b9ec2b4ffae737ad99f5b1b090c46\n$\n$ docker exec -it infinite_loop_1 cat \/etc\/hostname\ncontainer1\n<\/pre>\n
container1<\/code> can be put into the \/etc\/hosts<\/code> file of container2<\/code> using the --link=<\/code> option of docker run<\/code>. Let us try one more example now, we will link infinite_loop_1<\/code> to infinite_loop_2<\/code> and ping container1<\/code> from within container2<\/code> using the hostname of infinite_loop_1<\/code>.<\/p>\n$ docker run -itd --name=infinite_loop_1 --hostname=container1 enhariharan\/infinite-loop\nc97fe6577f1e8b2ca90b04157a299c45becadea5a5ccd61564c8d279d98c3719\n$\n$ docker run -itd --name=infinite_loop_2 --hostname=container2 --link=infinite_loop_1 enhariharan\/infinite-loop\n2a26adfae9fb06038a823e4fea9a5118875abbdb8e023502eda96c91d4c79d6c\n$\n$ docker exec -it infinite_loop_2 cat \/etc\/hosts\n127.0.0.1\tlocalhost\n::1\tlocalhost ip6-localhost ip6-loopback\nfe00::0\tip6-localnet\nff00::0\tip6-mcastprefix\nff02::1\tip6-allnodes\nff02::2\tip6-allrouters\n172.17.0.3\tinfinite_loop_1 container1\n172.17.0.4\tcontainer2\n$\n$ docker exec -it infinite_loop_2 ping -c 5 container1\nPING container1 (172.17.0.3): 56 data bytes\n64 bytes from 172.17.0.3: seq=0 ttl=64 time=0.175 ms\n64 bytes from 172.17.0.3: seq=1 ttl=64 time=0.118 ms\n64 bytes from 172.17.0.3: seq=2 ttl=64 time=0.115 ms\n64 bytes from 172.17.0.3: seq=3 ttl=64 time=0.123 ms\n64 bytes from 172.17.0.3: seq=4 ttl=64 time=0.118 ms\n\n--- container1 ping statistics ---\n5 packets transmitted, 5 packets received, 0% packet loss\nround-trip min\/avg\/max = 0.115\/0.129\/0.175 ms\n<\/pre>\n
bridge<\/code> network sort of stops here. To connect to containers without using their IP addreses and without linking to them at docker run<\/code> as above we need to do user-defined networks of Docker. Let us see the basics of that next.<\/p>\n4. Setup Container DNS in a User Defined Network<\/h2>\n
bridge<\/code> network. One such point is that the --link<\/code> option of docker run<\/code> will not work in user-defined networks. Let us see how we can get the same functionality as above using user-defined networks. We will add the containers infinite_loop_1<\/code> and infinite_loop_2<\/code> into a user-defined network called example_network<\/code> and ping infinite_loop_2<\/code> from within infinite_loop_1<\/code>.<\/p>\n$ docker run -itd --name=infinite_loop_1 --hostname=container1 --network=example_network enhariharan\/infinite-loop\nd91757ae6e4dab01d9a37e73ec9a314c15ed195e5763fb6d9898ceb45dbc0964\n$\n$ docker run -itd --name=infinite_loop_2 --hostname=container2 --network=example_network enhariharan\/infinite-loop\n3ab4dad71bad7114025db7330686cb23114c76f705f9e3489a2763aa74970f2b\n$\n$ docker exec -it infinite_loop_2 ping -c 4 infinite_loop_1\nPING infinite_loop_1 (172.20.0.2): 56 data bytes\n64 bytes from 172.20.0.2: seq=0 ttl=64 time=0.111 ms\n64 bytes from 172.20.0.2: seq=1 ttl=64 time=0.122 ms\n64 bytes from 172.20.0.2: seq=2 ttl=64 time=0.119 ms\n64 bytes from 172.20.0.2: seq=3 ttl=64 time=0.131 ms\n\n--- infinite_loop_1 ping statistics ---\n4 packets transmitted, 4 packets received, 0% packet loss\nround-trip min\/avg\/max = 0.111\/0.120\/0.131 ms\n$\n$ docker exec -it infinite_loop_2 ping -c 4 container1\nping: bad address 'container1'\n<\/pre>\n