1. Creating and Using Containers 🏠

Table of Contents

• Creating and Using Containers
  • Check our Docker Install and Config
  • Starting a Nginx Web Server
  • Containers v/s VMs
  • CLI Process Monitoring
  • Getting into a Shell inside Containers
  • Docker Networking
  • CLI Management of Virtual Networks
  • Docker Networks: DNS
  • Docker DNS

Table of contents generated with markdown-toc

Creating and Using Containers

Check our Docker Install and Config

docker version - Check your versions and that docker is working.

Aditya :: System32 » docker version
Client: Docker Engine - Community
 Cloud integration: 1.0.4
 Version:           20.10.0
 API version:       1.41
 Go version:        go1.13.15
 Git commit:        7287ab3
 Built:             Tue Dec  8 18:55:31 2020
 OS/Arch:           windows/amd64
 Context:           default
 Experimental:      true

Server: Docker Engine - Community
 Engine:
  Version:          20.10.0
  API version:      1.41 (minimum version 1.12)
  Go version:       go1.13.15
  Git commit:       eeddea2
  Built:            Tue Dec  8 18:58:04 2020
  OS/Arch:          linux/amd64
  Experimental:     false
 containerd:
  Version:          v1.4.3
  GitCommit:        269548fa27e0089a8b8278fc4fc781d7f65a939b
 runc:
  Version:          1.0.0-rc92
  GitCommit:        ff819c7e9184c13b7c2607fe6c30ae19403a7aff
 docker-init:
  Version:          0.19.0
  GitCommit:        de40ad0

docker info - Shows most configuration values for the engine.

Aditya :: System32 » docker info
Client:
 Context:    default
 Debug Mode: false
 Plugins:
  app: Docker App (Docker Inc., v0.9.1-beta3)
  buildx: Build with BuildKit (Docker Inc., v0.4.2-docker)
  scan: Docker Scan (Docker Inc., v0.5.0)

Server:
 Containers: 36
  Running: 36
  Paused: 0
  Stopped: 0
 Images: 11
 Server Version: 20.10.0
 Storage Driver: overlay2
  Backing Filesystem: extfs
  Supports d_type: true
  Native Overlay Diff: true
 Logging Driver: json-file
 Cgroup Driver: cgroupfs
 Cgroup Version: 1
 Plugins:
  Volume: local
  Network: bridge host ipvlan macvlan null overlay
  Log: awslogs fluentd gcplogs gelf journald json-file local logentries splunk syslog
 Swarm: inactive
 Runtimes: io.containerd.runc.v2 io.containerd.runtime.v1.linux runc
 Default Runtime: runc
 Init Binary: docker-init
 containerd version: 269548fa27e0089a8b8278fc4fc781d7f65a939b
 runc version: ff819c7e9184c13b7c2607fe6c30ae19403a7aff
 init version: de40ad0
 Security Options:
  seccomp
   Profile: default
 Kernel Version: 4.19.128-microsoft-standard
 Operating System: Docker Desktop
 OSType: linux
 Architecture: x86_64
 CPUs: 12
 Total Memory: 12.39GiB
 Name: docker-desktop
 ID: HHTP:JIH2:BC6E:DDT7:RAFS:XFNT:7L4P:WFZY:LLJC:BSVD:JSRW:RNAN
 Docker Root Dir: /var/lib/docker
 Debug Mode: false
 Registry: https://index.docker.io/v1/
 Labels:
 Experimental: false
 Insecure Registries:
  127.0.0.0/8
 Live Restore Enabled: false
 Product License: Community Engine

WARNING: No blkio weight support
WARNING: No blkio weight_device support
WARNING: No blkio throttle.read_bps_device support
WARNING: No blkio throttle.write_bps_device support
WARNING: No blkio throttle.read_iops_device support
WARNING: No blkio throttle.write_iops_device support
WARNING: bridge-nf-call-iptables is disabled
WARNING: bridge-nf-call-ip6tables is disabled

docker command formats:

1. new management commands format:

docker <command> <sub-command> (OPTIONS)

2. old way (still works):

docker <command> (OPTIONS)

Starting a Nginx Web Server

To create a simple container of nginx, we run:

docker container run --publish 8080:80 --name webhost --detach nginx:1.11 nginx -T

What actually happens in docker container run ?

1. Looks for that image locally in image cache, doesn't find anything.
2. Then looks in remote image repository (defaults to Docker Hub)
3. Downloads the latest version (nginx:latest by default)
4. Create new container based on that image and prepares to start
5. Gives it a virtual IP on a private network inside docker engine
6. Opens up port 80 on host and forwards to port 80 in container
7. Starts container by using the CMD in the image Dockerfile

Containers v/s VMs

docker run --name mongo -d mongo creates a mongodb container.

Aditya :: System32 » docker container run --name mongo -d mongo
Unable to find image 'mongo:latest' locally
latest: Pulling from library/mongo
f22ccc0b8772: Pull complete
3cf8fb62ba5f: Pull complete
e80c964ece6a: Pull complete
329e632c35b3: Pull complete
3e1bd1325a3d: Pull complete
4aa6e3d64a4a: Pull complete
035bca87b778: Pull complete
874e4e43cb00: Pull complete
08cb97662b8b: Pull complete
f623ce2ba1e1: Pull complete
f100ac278196: Pull complete
6f5539f9b3ee: Pull complete
Digest: sha256:02e9941ddcb949424fa4eb01f9d235da91a5b7b64feb5887eab77e1ef84a3bad
Status: Downloaded newer image for mongo:latest
da0fb8ecc5cf57014ec25dc606fdb85da259596302127bc4b4c940ec4114add0

docker top da0fb8ecc5cf57014ec25dc606fdb85da259596302127bc4b4c940ec4114add0 gives a list of running processes in specific container.

Aditya :: System32 » docker top da0fb8ecc5cf57014ec25dc606fdb85da259596302127bc4b4c940ec4114add0
PID                 USER                TIME                COMMAND
15592               999                 0:01                mongod --bind_ip_all

docker ps show all containers (default shows just running)

Aditya :: System32 » docker ps
CONTAINER ID   IMAGE     COMMAND                  CREATED         STATUS         PORTS       NAMES
da0fb8ecc5cf   mongo     "docker-entrypoint.s…"   2 minutes ago   Up 2 minutes   27017/tcp   mongo

For Linux, use ps aux|grep "mongod".

For Windows, tasklist | findstr "mongod".

We can see here that that mongo container is running as a process.

Aditya :: System32 » tasklist | findstr "mongo"
mongod.exe                    4296 Services                   0     20,044 K

If we stop the above container.....

docker container stop mongo

and we will get the same process but running on a different port.

If we want to remove all containers at once, we type `docker containers

Assignment: Manage multiple containers

CLI Process Monitoring

Let's spin up two containers - one from nginx and other from mysql.

Aditya :: learning-docker » docker container run --publish 81:80 --detach --name nginx_app nginx
2bb4c649c17b63433241dcfec77179fd60d0ecb1e3456d40e8dd5741bb240047

Aditya :: learning-docker » docker run --name mysql_app --publish 3306:3306 --env MYSQL_RANDOM_ROOT_PASSWORD=yes --detach mysql
fbff2b981ad16bafc5105c484e109be2229e27d1c7853e962da9e863acdf2f04

Running docker container ls -aq, we should see two running containers.

Aditya :: learning-docker » docker container ls -aq
fbff2b981ad1
2bb4c649c17b

docker container top mysql_app lists the processes running in that specific container.

Aditya :: learning-docker » docker container top mysql_app
PID                 USER                TIME                COMMAND
14910               999                 0:02                mysqld

Similarly, docker container top nginx_app lists the processes running in that specific container.

Aditya :: learning-docker » docker container top nginx_app
PID                 USER                TIME                COMMAND
14618               root                0:00                nginx: master process nginx -g daemon off;
14689               101                 0:00                nginx: worker process

docker container inspect nginx_app shows metadata about the container.

Aditya :: System32 » docker container inspect nginx_app
[
    {
        "Id": "2bb4c649c17b63433241dcfec77179fd60d0ecb1e3456d40e8dd5741bb240047",
        "Created": "2021-01-04T02:43:38.52542Z",
        "Path": "/docker-entrypoint.sh",
        "Args": [
            "nginx",
            "-g",
            "daemon off;"
        ],
        "State": {
            "Status": "running",
            "Running": true,
            "Paused": false,
            "Restarting": false,
            "OOMKilled": false,
            "Dead": false,
            "Pid": 14618,
            "ExitCode": 0,
            "Error": "",
            "StartedAt": "2021-01-04T02:43:39.1400873Z",
            "FinishedAt": "0001-01-01T00:00:00Z"
        },
        "Image": "sha256:ae2feff98a0cc5095d97c6c283dcd33090770c76d63877caa99aefbbe4343bdd",
        "ResolvConfPath": "/var/lib/docker/containers/2bb4c649c17b63433241dcfec77179fd60d0ecb1e3456d40e8dd5741bb240047/resolv.conf",
        "HostnamePath": "/var/lib/docker/containers/2bb4c649c17b63433241dcfec77179fd60d0ecb1e3456d40e8dd5741bb240047/hostname",
        "HostsPath": "/var/lib/docker/containers/2bb4c649c17b63433241dcfec77179fd60d0ecb1e3456d40e8dd5741bb240047/hosts",
        "LogPath": "/var/lib/docker/containers/2bb4c649c17b63433241dcfec77179fd60d0ecb1e3456d40e8dd5741bb240047/2bb4c649c17b63433241dcfec77179fd60d0ecb1e3456d40e8dd5741bb240047-json.log",
        "Name": "/nginx_app",
        "RestartCount": 0,
        "Driver": "overlay2",
        "Platform": "linux",
        "MountLabel": "",
        "ProcessLabel": "",
        "AppArmorProfile": "",
        "ExecIDs": null,
        "HostConfig": {
            "Binds": null,
            "ContainerIDFile": "",
            "LogConfig": {
                "Type": "json-file",
                "Config": {}
            },
            "NetworkMode": "default",
            "PortBindings": {
                "80/tcp": [
                    {
                        "HostIp": "",
                        "HostPort": "81"
                    }
                ]
            },
            "RestartPolicy": {
                "Name": "no",
                "MaximumRetryCount": 0
            },
            "AutoRemove": false,
            "VolumeDriver": "",
            "VolumesFrom": null,
            "CapAdd": null,
            "CapDrop": null,
            "Capabilities": null,
            "Dns": [],
            "DnsOptions": [],
            "DnsSearch": [],
            "ExtraHosts": null,
            "GroupAdd": null,
            "IpcMode": "private",
            "Cgroup": "",
            "Links": null,
            "OomScoreAdj": 0,
            "PidMode": "",
            "Privileged": false,
            "PublishAllPorts": false,
            "ReadonlyRootfs": false,
            "SecurityOpt": null,
            "UTSMode": "",
            "UsernsMode": "",
            "ShmSize": 67108864,
            "Runtime": "runc",
            "ConsoleSize": [
                52,
                102
            ],
            "Isolation": "",
            "CpuShares": 0,
            "Memory": 0,
            "NanoCpus": 0,
            "CgroupParent": "",
            "BlkioWeight": 0,
            "BlkioWeightDevice": [],
            "BlkioDeviceReadBps": null,
            "BlkioDeviceWriteBps": null,
            "BlkioDeviceReadIOps": null,
            "BlkioDeviceWriteIOps": null,
            "CpuPeriod": 0,
            "CpuQuota": 0,
            "CpuRealtimePeriod": 0,
            "CpuRealtimeRuntime": 0,
            "CpusetCpus": "",
            "CpusetMems": "",
            "Devices": [],
            "DeviceCgroupRules": null,
            "DeviceRequests": null,
            "KernelMemory": 0,
            "KernelMemoryTCP": 0,
            "MemoryReservation": 0,
            "MemorySwap": 0,
            "MemorySwappiness": null,
            "OomKillDisable": false,
            "PidsLimit": null,
            "Ulimits": null,
            "CpuCount": 0,
            "CpuPercent": 0,
            "IOMaximumIOps": 0,
            "IOMaximumBandwidth": 0,
            "MaskedPaths": [
                "/proc/asound",
                "/proc/acpi",
                "/proc/kcore",
                "/proc/keys",
                "/proc/latency_stats",
                "/proc/timer_list",
                "/proc/timer_stats",
                "/proc/sched_debug",
                "/proc/scsi",
                "/sys/firmware"
            ],
            "ReadonlyPaths": [
                "/proc/bus",
                "/proc/fs",
                "/proc/irq",
                "/proc/sys",
                "/proc/sysrq-trigger"
            ]
        },
        "GraphDriver": {
            "Data": {
                "LowerDir": "/var/lib/docker/overlay2/48a173e4e2293d5d7b57589771f9c00a06e68766b9e0b885968d49ea6160b8eb-init/diff:/var/lib/docker/overlay2/19a72843084a455cff1ed0e52b208c53854528818ccddf8a252fe6d054ac00d8/diff:/var/lib/docker/overlay2/af57f5fe5d0ff52731939a282bec22e9bbd45c78d280be114d1cf58994da88b5/diff:/var/lib/docker/overlay2/186e8478e1cc2447a09e1c7cb2c3a35b39b05bc7dd62e5fbd123784224fbe623/diff:/var/lib/docker/overlay2/19b26dfa4018c8aa6dfb50b4952c23d2593ad84e67ed5580db9fd4e604f92c22/diff:/var/lib/docker/overlay2/49a98dde916cafa3956d366bf884db8d9638e38ed7616ec7af98770d6386f031/diff",
                "MergedDir": "/var/lib/docker/overlay2/48a173e4e2293d5d7b57589771f9c00a06e68766b9e0b885968d49ea6160b8eb/merged",
                "UpperDir": "/var/lib/docker/overlay2/48a173e4e2293d5d7b57589771f9c00a06e68766b9e0b885968d49ea6160b8eb/diff",
                "WorkDir": "/var/lib/docker/overlay2/48a173e4e2293d5d7b57589771f9c00a06e68766b9e0b885968d49ea6160b8eb/work"
            },
            "Name": "overlay2"
        },
        "Mounts": [],
        "Config": {
            "Hostname": "2bb4c649c17b",
            "Domainname": "",
            "User": "",
            "AttachStdin": false,
            "AttachStdout": false,
            "AttachStderr": false,
            "ExposedPorts": {
                "80/tcp": {}
            },
            "Tty": false,
            "OpenStdin": false,
            "StdinOnce": false,
            "Env": [
                "PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin",
                "NGINX_VERSION=1.19.6",
                "NJS_VERSION=0.5.0",
                "PKG_RELEASE=1~buster"
            ],
            "Cmd": [
                "nginx",
                "-g",
                "daemon off;"
            ],
            "Image": "nginx",
            "Volumes": null,
            "WorkingDir": "",
            "Entrypoint": [
                "/docker-entrypoint.sh"
            ],
            "OnBuild": null,
            "Labels": {
                "maintainer": "NGINX Docker Maintainers \u003cdocker-maint@nginx.com\u003e"
            },
            "StopSignal": "SIGQUIT"
        },
        "NetworkSettings": {
            "Bridge": "",
            "SandboxID": "22d0b5d5b1552a4e7318675584e0fd668d0e12413b58b190a0c93bd06a56b7b8",
            "HairpinMode": false,
            "LinkLocalIPv6Address": "",
            "LinkLocalIPv6PrefixLen": 0,
            "Ports": {
                "80/tcp": [
                    {
                        "HostIp": "0.0.0.0",
                        "HostPort": "81"
                    }
                ]
            },
            "SandboxKey": "/var/run/docker/netns/22d0b5d5b155",
            "SecondaryIPAddresses": null,
            "SecondaryIPv6Addresses": null,
            "EndpointID": "1aa574223d640c714ae83a0aec7e2c183ace23e40827d5bb8de7b83aedf9e5ab",
            "Gateway": "172.17.0.1",
            "GlobalIPv6Address": "",
            "GlobalIPv6PrefixLen": 0,
            "IPAddress": "172.17.0.2",
            "IPPrefixLen": 16,
            "IPv6Gateway": "",
            "MacAddress": "02:42:ac:11:00:02",
            "Networks": {
                "bridge": {
                    "IPAMConfig": null,
                    "Links": null,
                    "Aliases": null,
                    "NetworkID": "5f10213090984174d45c417b17efd155f20d7cdd03667fd30d6cfbcfc39c9c34",
                    "EndpointID": "1aa574223d640c714ae83a0aec7e2c183ace23e40827d5bb8de7b83aedf9e5ab",
                    "Gateway": "172.17.0.1",
                    "IPAddress": "172.17.0.2",
                    "IPPrefixLen": 16,
                    "IPv6Gateway": "",
                    "GlobalIPv6Address": "",
                    "GlobalIPv6PrefixLen": 0,
                    "MacAddress": "02:42:ac:11:00:02",
                    "DriverOpts": null
                }
            }
        }
    }
]

docker container stats nginx_app shows live performance data for all containers.

CONTAINER ID   NAME        CPU %     MEM USAGE / LIMIT     MEM %     NET I/O      BLOCK I/O   PIDS
2bb4c649c17b   nginx_app   0.00%     4.324MiB / 12.39GiB   0.03%     1.8kB / 0B   0B / 0B     2

Getting into a Shell inside Containers

docker container run -it starts a new container interactively.

Aditya :: System32 » docker container run -it --name proxy nginx bash
root@18b8618b88cb:/#_

Now, if we did docker container inspect on both nginx_app and proxy, we'd get:

Aditya :: System32 » docker container inspect nginx_app
[
    {
        "Id": "2bb4c649c17b63433241dcfec77179fd60d0ecb1e3456d40e8dd5741bb240047",
        "Created": "2021-01-04T02:43:38.52542Z",
        "Path": "/docker-entrypoint.sh",
        "Args": [
            "nginx",						👈
            "-g",							👈
            "daemon off;"					👈
        ], ....
Aditya :: System32 » docker container inspect proxy
[
    {
        "Id": "18b8618b88cbbaa8f417ef164c079b05dbe94281a841d11132b1e55e4d5d25c7",
        "Created": "2021-01-04T13:04:26.9538719Z",
        "Path": "/docker-entrypoint.sh",
        "Args": [
            "bash"							👈
        ],        

The way it does this is docker container run -it --name proxy nginx bash , by replacing the default command nginx -g daemon off; by bash and hence, that is how we can exec into the container.

To start a stopped container, we say docker container start -ai nginx_app.

docker container exec -it runs additional command in existing container.

Aditya :: System32 » docker container exec -it proxy bash
root@700fd7a62e0e:/#

Tip:

To get into alpine container, we do docker container run -it alpine sh not bash.

Docker Networking

• Each container connected to a private virtual network bridge.
• Each virtual network routes through a NAT firewall on host IP.
• All containers on a virtual network can talk to each other without -p.

Batteries Included, But Removable. Defaults work well in many cases, but easy to swap out parts to customize it.

• Make new virtual networks.
• Attach containers to more than one virtual network (or none).
• Skip virtual networks and use host IP(--net=host).
• Use different Docker network drivers to gain new abilities.

Best practice is to create a new virtual network for each app:

• network my_web_app for mysql and php/apache containers.
• network my_api for mongo and nodejs containers.

For exposing ports on containers , we use --publish. The syntax remains as docker container run --publish HOST_PORT:CONTAINER_PORT IMAGE_NAME.

To check the port mapping, we use port management command.

Aditya :: System32 » docker container port af5a5abab1de02fed5d1bee3c19ed49054587ec2efa1c42f1fae5e1adfa30de4
80/tcp -> 0.0.0.0:8080

To get the container IP, we can inspet the running container and --format out the IPAddress.

Aditya :: System32 » docker container inspect --format "{{ .NetworkSettings.IPAddress }}" proxy
172.17.0.2

CLI Management of Virtual Networks

• docker network ls shows networks.

  Aditya :: learning-docker » docker network ls
  NETWORK ID     NAME      DRIVER    SCOPE
  de2e69bf271d   bridge    bridge    local
  6c467bd0fda4   host      host      local
  202ce568f0b7   none      null      local

  --network bridge Default Docker virtual network, which is NAT'd behind the Host IP.

• docker network inspect NETWORK_NAME inspects a network.

  Aditya :: learning-docker » docker network inspect bridge
  [
      {
          "Name": "bridge",
          "Id": "de2e69bf271d31e1a39f0f21d1a973f438d0cb1c76d04d3775338fbc82023ba3",
          "Created": "2021-01-06T02:42:08.6789436Z",
          "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": {
              "af5a5abab1de02fed5d1bee3c19ed49054587ec2efa1c42f1fae5e1adfa30de4": {
                  "Name": "proxy",
                  "EndpointID": "463928ba98a0fb740e32de3cc6c2ed0f9c2ba4c73dcda1cc306037a42093d072",
                  "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": {}
      }
  ]

  --network host gains performance by skipping virtual networks but sacrifices security of container model.

  --network none removes eth0 and only leaves you with localhost interface in container.

• docker network create --driver DRIVER_NAME create a network, spawns a new virtual network for you to attach containers.

  Aditya :: learning-docker » docker network create my_app_net
  973176dc6a251d003a8b649b2c495c3d800ad4878b867a25e509fe858b9cf2f1

  Aditya :: learning-docker » docker network ls
  NETWORK ID     NAME         DRIVER    SCOPE
  de2e69bf271d   bridge       bridge    local
  6c467bd0fda4   host         host      local
  973176dc6a25   my_app_net   bridge    local
  202ce568f0b7   none         null      local

  network driver They are built-in or third-party extensions that give you virtual network features.

• docker network connect attaches a network to container.

  Aditya :: System32 » docker network connect my_app_net proxy
  Aditya :: System32 » docker container inspect proxy
  [
      {
          ....,
          "NetworkSettings": {
              ....,
              "Networks": {
                  "bridge": {
                      "IPAMConfig": null,
                      "Links": null,
                      "Aliases": null,
                      "NetworkID": "4b999276351265f09f9698b36eb2b73257279ca97b461d201dabc949c196f52c",                    "EndpointID": "71c5679c7a8cdb376644ca9569a15e22a38a0d21dac01e069cc438e22fd63029",
                      "Gateway": "172.17.0.1",
                      "IPAddress": "172.17.0.2",
                      "IPPrefixLen": 16,
                      "IPv6Gateway": "",
                      "GlobalIPv6Address": "",
                      "GlobalIPv6PrefixLen": 0,
                      "MacAddress": "02:42:ac:11:00:02",
                      "DriverOpts": null
                  },
                  "my_app_net": {
                      "IPAMConfig": {},
                      "Links": null,
                      "Aliases": [
                          "af5a5abab1de"
                      ],
                      "NetworkID": "bd382ea27e51ee7563c708fbfb94183ac626eb652d3430e9db433f23dc4f0b78",                    "EndpointID": "45ad21a59a842c7b3f3a20d3e45224cd76a9203766bb4da91547e8e002d1ba61",
                      "Gateway": "172.18.0.1",
                      "IPAddress": "172.18.0.2",
                      "IPPrefixLen": 16,
                      "IPv6Gateway": "",
                      "GlobalIPv6Address": "",
                      "GlobalIPv6PrefixLen": 0,
                      "MacAddress": "02:42:ac:12:00:02",
                      "DriverOpts": {}
                 }
              }
          }
      }
  ]

• docker network disconnect detach a network from container.

Aditya :: System32 » docker network disconnect my_app_net proxy
Aditya :: System32 » docker container inspect proxy
[
    {
        .....,
        "NetworkSettings": {
            ....,
            "Networks": {
                "bridge": {
                    "IPAMConfig": null,
                    "Links": null,
                    "Aliases": null,
                    "NetworkID": "4b999276351265f09f9698b36eb2b73257279ca97b461d201dabc949c196f52c",
                    "EndpointID": "71c5679c7a8cdb376644ca9569a15e22a38a0d21dac01e069cc438e22fd63029",
                    "Gateway": "172.17.0.1",
                    "IPAddress": "172.17.0.2",
                    "IPPrefixLen": 16,
                    "IPv6Gateway": "",
                    "GlobalIPv6Address": "",
                    "GlobalIPv6PrefixLen": 0,
                    "MacAddress": "02:42:ac:11:00:02",
                    "DriverOpts": null
                }
            }
        }
    }
]

Docker Networks: DNS

Static IP's and using IP's for talking to containers is an anti-pattern. Do your best to avoid it.

Docker DNS

Docker daemon has a built-in DNS server that containers use by-default.

Aditya :: System32 » docker network connect my_app_net proxy
Aditya :: System32 » docker network inspect my_app_net
[
    {
        "Name": "my_app_net",
        "Id": "bd382ea27e51ee7563c708fbfb94183ac626eb652d3430e9db433f23dc4f0b78",
        "Created": "2021-01-06T09:19:25.4270614Z",
        "Scope": "local",
        "Driver": "bridge",
        "EnableIPv6": false,
        "IPAM": {
            "Driver": "default",
            "Options": {},
            "Config": [
                {
                    "Subnet": "172.18.0.0/16",
                    "Gateway": "172.18.0.1"
                }
            ]
        },
        "Internal": false,
        "Attachable": false,
        "Ingress": false,
        "ConfigFrom": {
            "Network": ""
        },
        "ConfigOnly": false,
        "Containers": {
            "af5a5abab1de02fed5d1bee3c19ed49054587ec2efa1c42f1fae5e1adfa30de4": {
                "Name": "proxy",
                "EndpointID": "b8dc65eaa0d7bf351d7d3e7a61b5ab9e5a7e4baaa3391279cf2daa0118e1bdba",
                "MacAddress": "02:42:ac:12:00:02",
                "IPv4Address": "172.18.0.2/16",
                "IPv6Address": ""
            }
        },
        "Options": {},
        "Labels": {}
    }
]

Docker defaults the hostname to the container's name, but you can also set aliases.

Let's create another container under my_app_net network.

Aditya :: System32 » docker container run --publish 5001:80 --name app1 --detach --network my_app_net nginx:alpine
8e71ef45d862e3a15e10317e5568b125add2cb959644688aadf9244688495fdd
Aditya :: System32 » docker container run --publish 5002:80 --name app2 --detach --network my_app_net nginx:alpine
a6c80a468966c83f1686da5db59ae50d479afd251f67bb932f1c9ff0e1149ab2
Aditya :: System32 » docker network inspect my_app_net
[
    {
        "Name": "my_app_net",
        "Id": "bd382ea27e51ee7563c708fbfb94183ac626eb652d3430e9db433f23dc4f0b78",
        "Created": "2021-01-06T09:19:25.4270614Z",
        "Scope": "local",
        "Driver": "bridge",
        "EnableIPv6": false,
        "IPAM": {
            "Driver": "default",
            "Options": {},
            "Config": [
                {
                    "Subnet": "172.18.0.0/16",
                    "Gateway": "172.18.0.1"
                }
            ]
        },
        "Internal": false,
        "Attachable": false,
        "Ingress": false,
        "ConfigFrom": {
            "Network": ""
        },
        "ConfigOnly": false,
        "Containers": {
            "8e71ef45d862e3a15e10317e5568b125add2cb959644688aadf9244688495fdd": {
                "Name": "app1",
                "EndpointID": "9eb326edbc0954bc9cdaeffa45a94eeb9fdb001cfebba38e88c16c153a0be0ca",
                "MacAddress": "02:42:ac:12:00:02",
                "IPv4Address": "172.18.0.2/16",
                "IPv6Address": ""
            },
            "a6c80a468966c83f1686da5db59ae50d479afd251f67bb932f1c9ff0e1149ab2": {
                "Name": "app2",
                "EndpointID": "1f401641d87f82671758ae6f922a2a53b37e8b798fa2d5d1d1a4f30cd92faf66",
                "MacAddress": "02:42:ac:12:00:03",
                "IPv4Address": "172.18.0.3/16",
                "IPv6Address": ""
            },
            "c92399081ac7dd63ef330d4255e896fda423d29b9c882ed915b47aa120d621db": {
                "Name": "app3",
                "EndpointID": "128326b21568c7de832289e8ef29b9aafac8d9fe3b3cc7bd1bb0d72d8004bf44",
                "MacAddress": "02:42:ac:12:00:04",
                "IPv4Address": "172.18.0.4/16",
                "IPv6Address": ""
            }
        },
        "Options": {},
        "Labels": {}
    }
]

Aditya :: System32 » docker container run --publish 5003:80 --name app3 -it --network my_app_net nginx:alpine ping app1
PING app1 (172.18.0.2): 56 data bytes
64 bytes from 172.18.0.2: seq=0 ttl=64 time=0.130 ms
64 bytes from 172.18.0.2: seq=1 ttl=64 time=0.082 ms
64 bytes from 172.18.0.2: seq=2 ttl=64 time=0.187 ms
64 bytes from 172.18.0.2: seq=3 ttl=64 time=0.187 ms
64 bytes from 172.18.0.2: seq=4 ttl=64 time=0.242 ms
64 bytes from 172.18.0.2: seq=5 ttl=64 time=0.184 ms
64 bytes from 172.18.0.2: seq=6 ttl=64 time=0.162 ms
64 bytes from 172.18.0.2: seq=7 ttl=64 time=0.182 ms
64 bytes from 172.18.0.2: seq=8 ttl=64 time=0.140 ms
64 bytes from 172.18.0.2: seq=9 ttl=64 time=0.183 ms
^C
--- app1 ping statistics ---
10 packets transmitted, 10 packets received, 0% packet loss
round-trip min/avg/max = 0.082/0.167/0.242 ms

The default network bridge does not have DNS built into it, however a work-around has been provided. We could --link for our containers; but it s better to create a new network.

Assignment: CLI App Testing

Assignment: DNS Round Robin Test

docker container run --rm alpine nslookup search automatically remove the container when it exits.