github.com/xuyutom/docker@v1.6.0/docs/sources/userguide/dockerlinks.md (about) 1 page_title: Linking Containers Together 2 page_description: Learn how to connect Docker containers together. 3 page_keywords: Examples, Usage, user guide, links, linking, docker, documentation, examples, names, name, container naming, port, map, network port, network 4 5 # Linking Containers Together 6 7 In [the Using Docker section](/userguide/usingdocker), you saw how you can 8 connect to a service running inside a Docker container via a network 9 port. But a port connection is only one way you can interact with services and 10 applications running inside Docker containers. In this section, we'll briefly revisit 11 connecting via a network port and then we'll introduce you to another method of access: 12 container linking. 13 14 ## Connect using Network port mapping 15 16 In [the Using Docker section](/userguide/usingdocker), you created a 17 container that ran a Python Flask application: 18 19 $ sudo docker run -d -P training/webapp python app.py 20 21 > **Note:** 22 > Containers have an internal network and an IP address 23 > (as we saw when we used the `docker inspect` command to show the container's 24 > IP address in the [Using Docker](/userguide/usingdocker/) section). 25 > Docker can have a variety of network configurations. You can see more 26 > information on Docker networking [here](/articles/networking/). 27 28 When that container was created, the `-P` flag was used to automatically map 29 any network port inside it to a random high port within an *ephemeral port 30 range* on your Docker host. Next, when `docker ps` was run, you saw that port 31 5000 in the container was bound to port 49155 on the host. 32 33 $ sudo docker ps nostalgic_morse 34 CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES 35 bc533791f3f5 training/webapp:latest python app.py 5 seconds ago Up 2 seconds 0.0.0.0:49155->5000/tcp nostalgic_morse 36 37 You also saw how you can bind a container's ports to a specific port using 38 the `-p` flag: 39 40 $ sudo docker run -d -p 5000:5000 training/webapp python app.py 41 42 And you saw why this isn't such a great idea because it constrains you to 43 only one container on that specific port. 44 45 There are also a few other ways you can configure the `-p` flag. By 46 default the `-p` flag will bind the specified port to all interfaces on 47 the host machine. But you can also specify a binding to a specific 48 interface, for example only to the `localhost`. 49 50 $ sudo docker run -d -p 127.0.0.1:5000:5000 training/webapp python app.py 51 52 This would bind port 5000 inside the container to port 5000 on the 53 `localhost` or `127.0.0.1` interface on the host machine. 54 55 Or, to bind port 5000 of the container to a dynamic port but only on the 56 `localhost`, you could use: 57 58 $ sudo docker run -d -p 127.0.0.1::5000 training/webapp python app.py 59 60 You can also bind UDP ports by adding a trailing `/udp`. For example: 61 62 $ sudo docker run -d -p 127.0.0.1:5000:5000/udp training/webapp python app.py 63 64 You also learned about the useful `docker port` shortcut which showed us the 65 current port bindings. This is also useful for showing you specific port 66 configurations. For example, if you've bound the container port to the 67 `localhost` on the host machine, then the `docker port` output will reflect that. 68 69 $ sudo docker port nostalgic_morse 5000 70 127.0.0.1:49155 71 72 > **Note:** 73 > The `-p` flag can be used multiple times to configure multiple ports. 74 75 ## Connect with the linking system 76 77 Network port mappings are not the only way Docker containers can connect 78 to one another. Docker also has a linking system that allows you to link 79 multiple containers together and send connection information from one to another. 80 When containers are linked, information about a source container can be sent to a 81 recipient container. This allows the recipient to see selected data describing 82 aspects of the source container. 83 84 ### The importance of naming 85 86 To establish links, Docker relies on the names of your containers. 87 You've already seen that each container you create has an automatically 88 created name; indeed you've become familiar with our old friend 89 `nostalgic_morse` during this guide. You can also name containers 90 yourself. This naming provides two useful functions: 91 92 1. It can be useful to name containers that do specific functions in a way 93 that makes it easier for you to remember them, for example naming a 94 container containing a web application `web`. 95 96 2. It provides Docker with a reference point that allows it to refer to other 97 containers, for example, you can specify to link the container `web` to container `db`. 98 99 You can name your container by using the `--name` flag, for example: 100 101 $ sudo docker run -d -P --name web training/webapp python app.py 102 103 This launches a new container and uses the `--name` flag to 104 name the container `web`. You can see the container's name using the 105 `docker ps` command. 106 107 $ sudo docker ps -l 108 CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES 109 aed84ee21bde training/webapp:latest python app.py 12 hours ago Up 2 seconds 0.0.0.0:49154->5000/tcp web 110 111 You can also use `docker inspect` to return the container's name. 112 113 $ sudo docker inspect -f "{{ .Name }}" aed84ee21bde 114 /web 115 116 > **Note:** 117 > Container names have to be unique. That means you can only call 118 > one container `web`. If you want to re-use a container name you must delete 119 > the old container (with `docker rm`) before you can create a new 120 > container with the same name. As an alternative you can use the `--rm` 121 > flag with the `docker run` command. This will delete the container 122 > immediately after it is stopped. 123 124 ## Communication across links 125 126 Links allow containers to discover each other and securely transfer information about one 127 container to another container. When you set up a link, you create a conduit between a 128 source container and a recipient container. The recipient can then access select data 129 about the source. To create a link, you use the `--link` flag. First, create a new 130 container, this time one containing a database. 131 132 $ sudo docker run -d --name db training/postgres 133 134 This creates a new container called `db` from the `training/postgres` 135 image, which contains a PostgreSQL database. 136 137 Now, you need to delete the `web` container you created previously so you can replace it 138 with a linked one: 139 140 $ sudo docker rm -f web 141 142 Now, create a new `web` container and link it with your `db` container. 143 144 $ sudo docker run -d -P --name web --link db:db training/webapp python app.py 145 146 This will link the new `web` container with the `db` container you created 147 earlier. The `--link` flag takes the form: 148 149 --link <name or id>:alias 150 151 Where `name` is the name of the container we're linking to and `alias` is an 152 alias for the link name. You'll see how that alias gets used shortly. 153 154 Next, inspect your linked containers with `docker inspect`: 155 156 $ sudo docker inspect -f "{{ .HostConfig.Links }}" web 157 [/db:/web/db] 158 159 You can see that the `web` container is now linked to the `db` container 160 `web/db`. Which allows it to access information about the `db` container. 161 162 So what does linking the containers actually do? You've learned that a link allows a 163 source container to provide information about itself to a recipient container. In 164 our example, the recipient, `web`, can access information about the source `db`. To do 165 this, Docker creates a secure tunnel between the containers that doesn't need to 166 expose any ports externally on the container; you'll note when we started the 167 `db` container we did not use either the `-P` or `-p` flags. That's a big benefit of 168 linking: we don't need to expose the source container, here the PostgreSQL database, to 169 the network. 170 171 Docker exposes connectivity information for the source container to the 172 recipient container in two ways: 173 174 * Environment variables, 175 * Updating the `/etc/hosts` file. 176 177 ### Environment Variables 178 179 Docker creates several environment variables when you link containers. Docker 180 automatically creates environment variables in the target container based on 181 the `--link` parameters. It will also expose all environment variables 182 originating from Docker from the source container. These include variables from: 183 184 * the `ENV` commands in the source container's Dockerfile 185 * the `-e`, `--env` and `--env-file` options on the `docker run` 186 command when the source container is started 187 188 These environment variables enable programmatic discovery from within the 189 target container of information related to the source container. 190 191 > **Warning**: 192 > It is important to understand that *all* environment variables originating 193 > from Docker within a container are made available to *any* container 194 > that links to it. This could have serious security implications if sensitive 195 > data is stored in them. 196 197 Docker sets an `<alias>_NAME` environment variable for each target container 198 listed in the `--link` parameter. For example, if a new container called 199 `web` is linked to a database container called `db` via `--link db:webdb`, 200 then Docker creates a `WEBDB_NAME=/web/webdb` variable in the `web` container. 201 202 Docker also defines a set of environment variables for each port exposed by the 203 source container. Each variable has a unique prefix in the form: 204 205 `<name>_PORT_<port>_<protocol>` 206 207 The components in this prefix are: 208 209 * the alias `<name>` specified in the `--link` parameter (for example, `webdb`) 210 * the `<port>` number exposed 211 * a `<protocol>` which is either TCP or UDP 212 213 Docker uses this prefix format to define three distinct environment variables: 214 215 * The `prefix_ADDR` variable contains the IP Address from the URL, for 216 example `WEBDB_PORT_8080_TCP_ADDR=172.17.0.82`. 217 * The `prefix_PORT` variable contains just the port number from the URL for 218 example `WEBDB_PORT_8080_TCP_PORT=8080`. 219 * The `prefix_PROTO` variable contains just the protocol from the URL for 220 example `WEBDB_PORT_8080_TCP_PROTO=tcp`. 221 222 If the container exposes multiple ports, an environment variable set is 223 defined for each one. This means, for example, if a container exposes 4 ports 224 that Docker creates 12 environment variables, 3 for each port. 225 226 Additionally, Docker creates an environment variable called `<alias>_PORT`. 227 This variable contains the URL of the source container's first exposed port. 228 The 'first' port is defined as the exposed port with the lowest number. 229 For example, consider the `WEBDB_PORT=tcp://172.17.0.82:8080` variable. If 230 that port is used for both tcp and udp, then the tcp one is specified. 231 232 Finally, Docker also exposes each Docker originated environment variable 233 from the source container as an environment variable in the target. For each 234 variable Docker creates an `<alias>_ENV_<name>` variable in the target 235 container. The variable's value is set to the value Docker used when it 236 started the source container. 237 238 Returning back to our database example, you can run the `env` 239 command to list the specified container's environment variables. 240 241 ``` 242 $ sudo docker run --rm --name web2 --link db:db training/webapp env 243 . . . 244 DB_NAME=/web2/db 245 DB_PORT=tcp://172.17.0.5:5432 246 DB_PORT_5432_TCP=tcp://172.17.0.5:5432 247 DB_PORT_5432_TCP_PROTO=tcp 248 DB_PORT_5432_TCP_PORT=5432 249 DB_PORT_5432_TCP_ADDR=172.17.0.5 250 . . . 251 ``` 252 253 You can see that Docker has created a series of environment variables with 254 useful information about the source `db` container. Each variable is prefixed 255 with 256 `DB_`, which is populated from the `alias` you specified above. If the `alias` 257 were `db1`, the variables would be prefixed with `DB1_`. You can use these 258 environment variables to configure your applications to connect to the database 259 on the `db` container. The connection will be secure and private; only the 260 linked `web` container will be able to talk to the `db` container. 261 262 ### Important notes on Docker environment variables 263 264 Unlike host entries in the [`/etc/hosts` file](#updating-the-etchosts-file), 265 IP addresses stored in the environment variables are not automatically updated 266 if the source container is restarted. We recommend using the host entries in 267 `/etc/hosts` to resolve the IP address of linked containers. 268 269 These environment variables are only set for the first process in the 270 container. Some daemons, such as `sshd`, will scrub them when spawning shells 271 for connection. 272 273 ### Updating the `/etc/hosts` file 274 275 In addition to the environment variables, Docker adds a host entry for the 276 source container to the `/etc/hosts` file. Here's an entry for the `web` 277 container: 278 279 $ sudo docker run -t -i --rm --link db:webdb training/webapp /bin/bash 280 root@aed84ee21bde:/opt/webapp# cat /etc/hosts 281 172.17.0.7 aed84ee21bde 282 . . . 283 172.17.0.5 webdb 6e5cdeb2d300 db 284 285 You can see two relevant host entries. The first is an entry for the `web` 286 container that uses the Container ID as a host name. The second entry uses the 287 link alias to reference the IP address of the `db` container. In addition to 288 the alias you provide, the linked container's name--if unique from the alias 289 provided to the `--link` parameter--and the linked container's hostname will 290 also be added in `/etc/hosts` for the linked container's IP address. You can ping 291 that host now via any of these entries: 292 293 root@aed84ee21bde:/opt/webapp# apt-get install -yqq inetutils-ping 294 root@aed84ee21bde:/opt/webapp# ping webdb 295 PING webdb (172.17.0.5): 48 data bytes 296 56 bytes from 172.17.0.5: icmp_seq=0 ttl=64 time=0.267 ms 297 56 bytes from 172.17.0.5: icmp_seq=1 ttl=64 time=0.250 ms 298 56 bytes from 172.17.0.5: icmp_seq=2 ttl=64 time=0.256 ms 299 300 > **Note:** 301 > In the example, you'll note you had to install `ping` because it was not included 302 > in the container initially. 303 304 Here, you used the `ping` command to ping the `db` container using its host entry, 305 which resolves to `172.17.0.5`. You can use this host entry to configure an application 306 to make use of your `db` container. 307 308 > **Note:** 309 > You can link multiple recipient containers to a single source. For 310 > example, you could have multiple (differently named) web containers attached to your 311 >`db` container. 312 313 If you restart the source container, the linked containers `/etc/hosts` files 314 will be automatically updated with the source container's new IP address, 315 allowing linked communication to continue. 316 317 $ sudo docker restart db 318 db 319 $ sudo docker run -t -i --rm --link db:db training/webapp /bin/bash 320 root@aed84ee21bde:/opt/webapp# cat /etc/hosts 321 172.17.0.7 aed84ee21bde 322 . . . 323 172.17.0.9 db 324 325 # Next step 326 327 Now that you know how to link Docker containers together, the next step is 328 learning how to manage data, volumes and mounts inside your containers. 329 330 Go to [Managing Data in Containers](/userguide/dockervolumes). 331