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Add more info about the LINK env vars created 

@SvenDowideit see what you think. I'd like to get your take on this
before I submit the PR.

Closes #4789

Signed-off-by: Doug Davis <dug@us.ibm.com>
This commit is contained in:
Doug Davis 2014-10-11 09:15:54 -07:00
parent 7fa7c42ce2
commit 61387427cb
1 changed files with 41 additions and 1 deletions
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42
docs/sources/userguide/dockerlinks.md
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@ -179,7 +179,45 @@ recipient container in two ways:
* Environment variables,
* Updating the `/etc/hosts` file.
Docker can set a number of environment variables. You run the `env`
### Environment Variables
When two containers are linked, Docker will set some envrionment variables
in the target container to enable programmatic discovery of information
related to the source container.
First, Docker will set a `<alias>_NAME` environment variable specifying the
alias of each target container that was given in a `--link` parameter. So,
for example, if a new container called `web` is being linked to a database
container called `db` via `--link db:webdb` then in the `web` container
would be `WEBDB_NAME=/web/webdb`.
Docker will then also define a set of environment variables for each
port that is exposed by the source container. The pattern followed is:
* `<name>_PORT_<port>_<protocol>` will contain a URL reference to the
port. Where `<name>` is the alias name specified in the `--link` parameter
(e.g. `webdb`), `<port>` is the port number being exposed, and `<protocol>`
is either `TCP` or `UDP`. The format of the URL will be:
`<protocol>://<container_ip_address>:<port>`
(e.g. `tcp://172.17.0.82:8080`). This URL will then be
split into the following 3 environment variables for convinience:
* `<name>_PORT_<port>_<protocol>_ADDR` will contain just the IP address
from the URL (e.g. `WEBDB_PORT_8080_TCP_ADDR=172.17.0.82`).
* `<name>_PORT_<port>_<protocol>_PORT` will contain just the port number
from the URL (e.g. `WEBDB_PORT_8080_TCP_PORT=8080`).
* `<name>_PORT_<port>_<protocol>_PROTO` will contain just the protocol
from the URL (e.g. `WEBDB_PORT_8080_TCP_PROTO=tcp`).
If there are multiple ports exposed then the above set of environment
variables will be defined for each one.
Finally, there will be an environment variable called `<alias>_PORT` that will
contain the URL of the first exposed port of the source container.
For example, `WEBDB_PORT=tcp://172.17.0.82:8080`. In this case, 'first'
is defined as the lowest numbered port that is exposed. If that port is
used for both tcp and udp, then the tcp one will be specified.
Returning back to our database example, you can run the `env`
command to list the specified container's environment variables.
```
@ -207,6 +245,8 @@ environment variables to configure your applications to connect to the database
on the `db` container. The connection will be secure and private; only the
linked `web` container will be able to talk to the `db` container.
### 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: