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AWS EC2 IMDS Backend

This backend retrieves configuration data from the AWS EC2 Instance Metadata Service version 2 (IMDSv2).

Overview

The IMDS backend provides access to comprehensive EC2 instance metadata including:

  • Instance information (ID, type, AMI ID, etc.)
  • IAM role credentials
  • Instance tags
  • Network configuration
  • Placement information
  • User data
  • Dynamic instance identity data

Features

  • IMDSv2 Support: Automatically handles session token management via AWS SDK
  • Caching: In-memory cache with configurable TTL to reduce API calls
  • No Credentials Required: Uses network-based authentication (169.254.169.254)
  • Recursive Directory Walking: Automatically discovers nested metadata paths
  • Thread-Safe: Safe for concurrent access

Usage

Basic Example

# Run with polling interval
confd imds --interval 300

# One-time execution
confd imds --onetime

# Custom cache TTL
confd imds --interval 300 --imds-cache-ttl 2m

Template Resource Configuration

Create a template resource file at /etc/confd/conf.d/instance.toml:

[template]
src = "instance.tmpl"
dest = "/etc/app/instance.conf"
keys = [
    "/meta-data/instance-id",
    "/meta-data/instance-type",
    "/meta-data/ami-id",
    "/meta-data/tags/instance/Name",
    "/meta-data/tags/instance/Environment",
    "/meta-data/placement/availability-zone",
    "/meta-data/local-ipv4"
]
reload_cmd = "systemctl reload app"

Template Example

Create a template at /etc/confd/templates/instance.tmpl:

# Instance Configuration
instance_id={{ getv "/meta-data/instance-id" }}
instance_type={{ getv "/meta-data/instance-type" }}
ami_id={{ getv "/meta-data/ami-id" }}
name={{ getv "/meta-data/tags/instance/Name" }}
environment={{ getv "/meta-data/tags/instance/Environment" }}
availability_zone={{ getv "/meta-data/placement/availability-zone" }}
local_ip={{ getv "/meta-data/local-ipv4" }}

Available Metadata Paths

Instance Information

  • /meta-data/instance-id - Instance ID (e.g., i-1234567890abcdef0)
  • /meta-data/instance-type - Instance type (e.g., t3.micro)
  • /meta-data/ami-id - AMI ID used to launch the instance
  • /meta-data/ami-launch-index - Launch index
  • /meta-data/hostname - Private hostname
  • /meta-data/public-hostname - Public hostname

Network Information

  • /meta-data/local-ipv4 - Private IPv4 address
  • /meta-data/public-ipv4 - Public IPv4 address (if assigned)
  • /meta-data/mac - MAC address of primary network interface
  • /meta-data/network/interfaces/macs/{mac}/ - Network interface details
    • /meta-data/network/interfaces/macs/{mac}/local-ipv4 - Interface private IP
    • /meta-data/network/interfaces/macs/{mac}/subnet-id - Subnet ID
    • /meta-data/network/interfaces/macs/{mac}/vpc-id - VPC ID
    • /meta-data/network/interfaces/macs/{mac}/security-group-ids - Security groups

Placement Information

  • /meta-data/placement/availability-zone - Availability zone
  • /meta-data/placement/region - AWS region

Instance Tags

  • /meta-data/tags/instance/ - All instance tags (directory)
  • /meta-data/tags/instance/{tag-name} - Specific tag value

IAM Credentials

  • /meta-data/iam/security-credentials/ - List available IAM roles
  • /meta-data/iam/security-credentials/{role-name} - Temporary credentials (JSON)

Dynamic Data

  • /dynamic/instance-identity/document - Instance identity document (JSON)
  • /dynamic/instance-identity/signature - Instance identity signature

User Data

  • /user-data - User data script provided at instance launch

Configuration Options

Command-Line Flags

  • --imds-cache-ttl - Cache TTL for metadata (default: 60s)
    • Examples: 30s, 2m, 1h

Environment Variables

  • IMDS_ENDPOINT - Custom IMDS endpoint (for testing only)
    • Default: AWS SDK handles endpoint automatically

Caching Behavior

The IMDS backend implements intelligent caching to minimize API calls:

  1. Cache TTL: Metadata is cached for the configured duration (default 60 seconds)
  2. Per-Value Caching: Each metadata path is cached independently
  3. Directory Walking: When fetching a directory path (e.g., /meta-data/tags/instance/), all discovered values are cached
  4. Thread-Safe: Cache uses read-write locks for safe concurrent access
  5. Expiration: Cache entries are validated on each read; expired entries trigger refetch

Cache Performance

  • First Request: Fetches from IMDS and populates cache
  • Subsequent Requests: Served from cache until TTL expires
  • Directory Requests: Single IMDS request can populate multiple cache entries

Limitations

Watch Mode Not Supported

The IMDS backend only supports polling mode, not watch mode:

# This works
confd imds --interval 300

# This will error
confd imds --watch

Rationale: Continuous watching of IMDS would be inefficient and expensive. Use --interval with an appropriate duration instead.

EC2 Instance Required

This backend only works on EC2 instances where IMDS is available. It will fail during initialization if IMDS is not accessible.

IMDSv2 Required

The backend uses IMDSv2, which requires instance metadata options:

  • HttpTokens: required (recommended for security)
  • Instance metadata service enabled

Security Considerations

IAM Role Credentials

The /meta-data/iam/security-credentials/{role-name} path returns temporary AWS credentials. When using these in templates:

[template]
src = "aws-credentials.tmpl"
dest = "/etc/app/aws-credentials"
keys = ["/meta-data/iam/security-credentials/my-app-role"]
mode = "0600"  # Restrict permissions
reload_cmd = "systemctl reload app"

Network-Based Authentication

  • IMDS is only accessible from the instance itself (169.254.169.254)
  • No AWS credentials required for IMDS access
  • IMDSv2 uses session tokens for additional security

Sensitive Data

Instance tags and user data may contain sensitive information. Use appropriate file permissions:

[template]
mode = "0600"
uid = 0
gid = 0

Monitoring

Metrics

When metrics are enabled (--metrics-addr), the IMDS backend exposes:

  • confd_backend_request_duration_seconds - IMDS request duration
  • confd_backend_request_total - Total IMDS requests
  • confd_backend_errors_total - IMDS errors
  • confd_backend_healthy - Health status

Health Checks

  • /health - Basic IMDS availability check
  • /ready/detailed - Detailed status including cache statistics

Example:

curl http://localhost:9100/health
curl http://localhost:9100/ready/detailed

Examples

Dynamic Service Discovery

Use instance tags for service configuration:

[template]
src = "service.tmpl"
dest = "/etc/app/service.conf"
keys = [
    "/meta-data/tags/instance/ServiceName",
    "/meta-data/tags/instance/Environment",
    "/meta-data/placement/availability-zone"
]

Regional Configuration

Configure based on instance region:

[template]
src = "region.tmpl"
dest = "/etc/app/region.conf"
keys = [
    "/meta-data/placement/region",
    "/meta-data/placement/availability-zone"
]

Template:

region={{ getv "/meta-data/placement/region" }}
az={{ getv "/meta-data/placement/availability-zone" }}
endpoint=https://api.{{ getv "/meta-data/placement/region" }}.amazonaws.com

Network Configuration

[template]
src = "network.tmpl"
dest = "/etc/app/network.conf"
keys = [
    "/meta-data/local-ipv4",
    "/meta-data/public-ipv4",
    "/meta-data/mac"
]

Troubleshooting

IMDS Not Available

Error: IMDS not available: RequestError

Solutions:

  1. Verify you're running on an EC2 instance
  2. Check instance metadata service is enabled
  3. Verify IMDSv2 is configured (HttpTokens)
  4. Check security groups allow outbound to 169.254.169.254

Cache Not Working

If seeing too many IMDS requests:

  1. Increase cache TTL: --imds-cache-ttl 5m
  2. Check logs for cache hit/miss: --log-level debug
  3. Verify concurrent requests aren't bypassing cache

404 Errors

Error: path not found

Solutions:

  1. Verify the metadata path exists on your instance type
  2. Check instance has tags if accessing /meta-data/tags/instance/
  3. Verify IAM role is attached if accessing credentials
  4. Some paths only exist on specific instance types

Testing

Unit Tests

go test ./pkg/backends/imds/

Integration Tests

./test/integration/imds/test.sh

Manual Testing (on EC2)

# Enable debug logging
confd imds --onetime --log-level debug

# Check generated files
cat /etc/app/instance.conf

# Verify metrics
confd imds --interval 300 --metrics-addr :9100 &
curl http://localhost:9100/metrics | grep confd_backend

References