The metadataproxy is used to allow containers to acquire IAM roles.

From inside of the repo run the following commands:

mkdir -p /srv/metadataproxy
cd /srv/metadataproxy
virtualenv venv
source venv/bin/activate
pip install metadataproxy
deactivate

See the settings file for specific configuration options.

The metadataproxy has two basic modes of operation:

1. Running in AWS where it simply proxies most routes to the real metadata service.
2. Running outside of AWS where it mocks out most routes.

To enable mocking, use the environment variable:

export MOCK_API=true

metadataproxy relies on boto configuration for its AWS credentials. If metadata IAM credentials are available, it will use this. Otherwise, you'll need to use .aws/credentials, .boto, or environment variables to specify the IAM credentials before the service is started.

For IAM routes, the metadataproxy will use STS to assume roles for containers. To do so it takes the incoming IP address of metadata requests and finds the running docker container associated with the IP address. It uses the value of the container's IAM_ROLE environment variable as the role it will assume. It then assumes the role and gives back STS credentials in the metadata response.

So, to specify the role of a container, simply launch it with the IAM_ROLE environment variable set to the IAM role you wish the container to run with.

If you'd like containers to fallback to a default role if no role is specified, you can use the following configuration option:

export DEFAULT_ROLE=my-default-role

A useful way to deploy this metadataproxy is with a two-tier role structure:

1. The first tier is the EC2 service role for the instances running your containers. Call it DockerHostRole. Your instances must be launched with a policy that assigns this role.

2. The second tier is the role that each container will use. These roles must trust your own account ("Role for Cross-Account Access" in AWS terms). Call it ContainerRole1.

3. metadataproxy needs to query and assume the container role. So the DockerHostRole policy must permit this for each container role. For example:

   "Statement": [ {
       "Effect": "Allow",
       "Action": [
           "iam:GetRole",
           "sts:AssumeRole"
       ],
       "Resource": [
           "arn:aws:iam::012345678901:role/ContainerRole1",
           "arn:aws:iam::012345678901:role/ContainerRole2"
       ]
   } ]
   

4. Now customize ContainerRole1 & friends as you like

Using iptables, we can forward traffic meant to 169.254.169.254 from docker0 to the metadataproxy. The following example assumes the metadataproxy is run on the host, and not in a container:

/sbin/iptables \
  --append PREROUTING \
  --destination 169.254.169.254 \
  --dport 80 \
  --in-interface docker0 \
  --jump DNAT \
  --protocol tcp \
  --table nat \
  --to-destination 127.0.0.1:8000 \
  --wait

If you'd like to start the metadataproxy in a container, it's recommended to use host-only networking. Also, it's necessary to volume mount in the docker socket, as metadataproxy must be able to interact with docker.

Be aware that non-host-mode containers will not be able to contact 127.0.0.1 in the host network stack. As an alternative, you can use the meta-data service to find the local address. In this case, you probably want to restrict proxy access to the docker0 interface!

LOCAL_IPV4=$(curl http://169.254.169.254/latest/meta-data/local-ipv4)

/sbin/iptables \
  --append PREROUTING \
  --destination 169.254.169.254 \
  --dport 80 \
  --in-interface docker0 \
  --jump DNAT \
  --protocol tcp \
  --table nat \
  --to-destination $LOCAL_IPV4:8000 \
  --wait

/sbin/iptables
  --wait \
  --insert INPUT 1
  --protocol tcp \
  --dport 80 \
  \! \
  --in-interface docker0 \
  --jump DROP

In the following we assume my_config is a bash file with exports for all of the necessary settings discussed in the configuration section.

source my_config
cd /srv/metadataproxy
source venv/bin/activate
gunicorn metadataproxy:app --workers=2 -k gevent

For production purposes, you'll want to kick up a container to run. You can build one with the included Dockerfile. To run, do something like:

docker run --net=host \
    -v /var/run/docker.sock:/var/run/docker.sock \
    metadataproxy

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