user-data, cloud-init and CloudFormation

Show Notes

Instance meta-data can be accessed by hitting this URL from within an EC2 instance:

http://169.254.169.254/

On Ubuntu, you can use the ec2metadata script to list out one or all of the fields. On Amazon Linux, use the ec2-metadata script.

$ ec2metadata
ami-id: ami-a73264ce
ami-launch-index: 0
ami-manifest-path: (unknown)
ancestor-ami-ids: unavailable
availability-zone: us-east-1a
block-device-mapping: ami
root
instance-action: none
instance-id: i-91d66fe8
instance-type: t1.micro
local-hostname: ip-10-73-174-101.ec2.internal
local-ipv4: 10.73.174.101
kernel-id: aki-88aa75e1
mac: unavailable
profile: default-paravirtual
product-codes: unavailable
public-hostname: ec2-54-205-181-234.compute-1.amazonaws.com
public-ipv4: 54.205.181.234
public-keys: ['ssh-rsa AAAAB...']
ramdisk-id: unavailable
reserveration-id: unavailable
security-groups: mysg
user-data: unavailable

Both Amazon Linux and Ubuntu come with a package called cloud-init that checks the meta-data for user-data, and performs various actions including running scripts and chef recipes on boot.

An example user-data script to update Ubuntu packages and install Apache is:

#!/bin/bash
apt-get update
apt-get upgrade -y
apt-get install apache2 -y
echo "<html><body><h1>Welcome</h1>" > /var/www/index.html
echo "I was generated from user-data and cloud-init" >> /var/www/index.html
echo "</body></html>" >> /var/www/index.html

You can pass CloudFormation parameters to instances via user-data by using either the AWS::EC2::Instance or AWS::AutoScaling::LaunchConfiguration resources in your template. Strings inside the template can use the Fn::Join intrinsic function. An example template is here:

{
  "Description": "Episode 4 example of user-data and cloud-init",
  "Parameters": {
    "KeyPair": {
      "Description": "Name of the keypair to use for SSH access",
      "Type": "String"
    },
    "Environment": {
      "Description": "Name of this environment",
      "Type": "String",
      "Default": "Production"
    },
    "Role": {
      "Description": "The role this server should be",
      "Type": "String",
      "Default": "Web"
    }
  },
  "Resources": {
    "MyElasticLoadBalancer": {
      "Properties": {
        "AvailabilityZones": {
          "Fn::GetAZs": ""
        },
        "HealthCheck": {
          "HealthyThreshold": 3,
          "Interval": 30,
          "Target": "HTTP:80/",
          "Timeout": 5,
          "UnhealthyThreshold": 5
        },
        "Listeners": [
          {
            "InstancePort": 80,
            "LoadBalancerPort": 80,
            "Protocol": "HTTP"
          }
        ]
      },
      "Type": "AWS::ElasticLoadBalancing::LoadBalancer"
    },
    "MySecurityGroup": {
      "Properties": {
        "GroupDescription": "Allow access to MyInstance",
        "SecurityGroupIngress": [
          {
            "CidrIp": "0.0.0.0/0",
            "FromPort": 22,
            "IpProtocol": "tcp",
            "ToPort": 22
          },
          {
            "FromPort": 80,
            "IpProtocol": "tcp",
            "SourceSecurityGroupName": {
              "Fn::GetAtt": [
                "MyElasticLoadBalancer",
                "SourceSecurityGroup.GroupName"
              ]
            },
            "SourceSecurityGroupOwnerId": {
              "Fn::GetAtt": [
                "MyElasticLoadBalancer",
                "SourceSecurityGroup.OwnerAlias"
              ]
            },
            "ToPort": 80
          }
        ]
      },
      "Type": "AWS::EC2::SecurityGroup"
    },
    "MyASG": {
      "Properties": {
        "AvailabilityZones": {
          "Fn::GetAZs": ""
        },
        "Cooldown": 120,
        "LaunchConfigurationName": {
          "Ref": "MyLaunchConfig"
        },
        "LoadBalancerNames": [
          {
            "Ref": "MyElasticLoadBalancer"
          }
        ],
        "MaxSize": 1,
        "MinSize": 1,
        "Tags": [
          {
            "Key": "Name",
            "PropagateAtLaunch": "true",
            "Value": "Episode 4"
          }
        ]
      },
      "Type": "AWS::AutoScaling::AutoScalingGroup"
    },
    "MyLaunchConfig": {
      "Properties": {
        "ImageId": "ami-ef277b86",
        "InstanceType": "t1.micro",
        "KeyName": {
          "Ref": "KeyPair"
        },
        "SecurityGroups": [
          {
            "Ref": "MySecurityGroup"
          }
        ],
        "UserData": {
          "Fn::Base64": {
            "Fn::Join": [
              "\n",
              [
                "#!/bin/bash",
                "apt-get update",
                "apt-get upgrade -y",
                "apt-get install apache2 -y",
                "echo \"<html><body><h1>Welcome</h1>\" > /var/www/index.html",
                {
                  "Fn::Join": [
                    "",
                    [
                      "echo \"<h2>Environment: ",
                      {
                        "Ref": "Environment"
                      },
                      "</h2>\" >> /var/www/index.html"
                    ]
                  ]
                },
                {
                  "Fn::Join": [
                    "",
                    [
                      "echo \"<h2>Role: ",
                      {
                        "Ref": "Role"
                      },
                      "</h2>\" >> /var/www/index.html"
                    ]
                  ]
                },
                "echo \"</body></html>\" >> /var/www/index.html"
              ]
            ]
          }
        }
      },
      "Type": "AWS::AutoScaling::LaunchConfiguration"
    }
  },
  "Outputs": {
    "URL": {
      "Description": "URL of the sample website",
      "Value": {
        "Fn::Join": [
          "",
          [
            "http://",
            {
              "Fn::GetAtt": [
                "MyElasticLoadBalancer",
                "DNSName"
              ]
            }
          ]
        ]
      }
    }
  }
}

The same template and user-data script is easier to write using troposphere.

#4 user-data, cloud-init and CloudFormation

Show Notes

Resources