DB_PORT_5432_TCP=tcp://172.17.0.5:5432
DB_PORT_5432_TCP_PROTO=tcp
DB_PORT_5432_TCP_PORT=5432
DB_PORT_5432_TCP_ADDR=172.17.0.5
<...>
```

You can see that Docker has created a series of environment variables with
useful information about the source `db` container. Each variable is prefixed
with
`DB_`, which is populated from the `alias` you specified above. If the `alias`
were `db1`, the variables would be prefixed with `DB1_`. You can use these
environment variables to configure your applications to connect to the database
on the `db` container. The connection is secure and private; only the
linked `web` container can communicate with the `db` container.

### Important notes on Docker environment variables

Unlike host entries in the [`/etc/hosts` file](#updating-the-etchosts-file),
IP addresses stored in the environment variables are not automatically updated
if the source container is restarted. We recommend using the host entries in
`/etc/hosts` to resolve the IP address of linked containers.

These environment variables are only set for the first process in the
container. Some daemons, such as `sshd`, scrub them when spawning shells
for connection.

### Updating the `/etc/hosts` file

In addition to the environment variables, Docker adds a host entry for the
source container to the `/etc/hosts` file. Here's an entry for the `web`
container:

```console
$ docker run -t -i --rm --link db:webdb training/webapp /bin/bash

root@aed84ee21bde:/opt/webapp# cat /etc/hosts
172.17.0.7  aed84ee21bde
<...>
172.17.0.5  webdb 6e5cdeb2d300 db
```

You can see two relevant host entries. The first is an entry for the `web`
container that uses the Container ID as a host name. The second entry uses the
link alias to reference the IP address of the `db` container. In addition to
the alias you provide, the linked container's name, if unique from the alias
provided to the `--link` parameter, and the linked container's hostname are
also added to `/etc/hosts` for the linked container's IP address. You can ping
that host via any of these entries:

```console
root@aed84ee21bde:/opt/webapp# apt-get install -yqq inetutils-ping
root@aed84ee21bde:/opt/webapp# ping webdb

PING webdb (172.17.0.5): 48 data bytes
56 bytes from 172.17.0.5: icmp_seq=0 ttl=64 time=0.267 ms
56 bytes from 172.17.0.5: icmp_seq=1 ttl=64 time=0.250 ms
56 bytes from 172.17.0.5: icmp_seq=2 ttl=64 time=0.256 ms
```

> [!NOTE]
>
> In the example, you had to install `ping` because it was not included
> in the container initially.

Here, you used the `ping` command to ping the `db` container using its host entry,
which resolves to `172.17.0.5`. You can use this host entry to configure an application
to make use of your `db` container.

> [!NOTE]
>
> You can link multiple recipient containers to a single source. For
> example, you could have multiple (differently named) web containers attached to your
>`db` container.

If you restart the source container, the `/etc/hosts` files on the linked containers
are automatically updated with the source container's new IP address,
allowing linked communication to continue.

```console
$ docker restart db
db

$ docker run -t -i --rm --link db:db training/webapp /bin/bash

root@aed84ee21bde:/opt/webapp# cat /etc/hosts
172.17.0.7  aed84ee21bde
<...>
172.17.0.9  db
```