VPC Flow Logs are a powerful monitoring feature in AWS that captures information about IP traffic flowing to and from network interfaces within your Virtual Private Cloud (VPC). This capability is essential for SysOps Administrators who need to monitor network activity, troubleshoot connectivity is…VPC Flow Logs are a powerful monitoring feature in AWS that captures information about IP traffic flowing to and from network interfaces within your Virtual Private Cloud (VPC). This capability is essential for SysOps Administrators who need to monitor network activity, troubleshoot connectivity issues, and maintain security compliance.
Flow Logs can be created at three levels: VPC level (captures all traffic), subnet level (captures traffic for specific subnets), or network interface level (captures traffic for individual ENIs). The captured data includes source and destination IP addresses, ports, protocol numbers, packet counts, byte counts, and whether traffic was accepted or rejected by security groups or network ACLs.
Flow Log data can be published to three destinations: Amazon CloudWatch Logs, Amazon S3, or Amazon Kinesis Data Firehose. CloudWatch Logs enables real-time analysis and alerting through metric filters and alarms. S3 storage is cost-effective for long-term retention and analysis using services like Amazon Athena. Kinesis Data Firehose allows streaming to third-party tools or Amazon OpenSearch Service.
Key use cases include identifying overly restrictive security group rules by analyzing rejected traffic patterns, detecting unusual traffic volumes that might indicate security breaches, and auditing network access for compliance requirements. SysOps Administrators should understand that Flow Logs do not capture all traffic types - DNS traffic to Amazon DNS servers, DHCP traffic, and metadata service traffic are not logged.
For the certification exam, remember that enabling Flow Logs does not impact network performance or latency. Logs have a default aggregation interval of 10 minutes but can be configured for 1-minute intervals. Custom log formats allow you to select specific fields, reducing storage costs. IAM roles with appropriate permissions are required for publishing logs to the chosen destination. Understanding how to query and analyze Flow Log data is crucial for effective network troubleshooting and security monitoring.
VPC Flow Logs are a feature that enables you to capture information about the IP traffic going to and from network interfaces in your VPC. Flow log data can be published to Amazon CloudWatch Logs, Amazon S3, or Amazon Kinesis Data Firehose.
Why are VPC Flow Logs Important?
VPC Flow Logs are essential for: • Security monitoring - Detect suspicious traffic patterns and potential security threats • Troubleshooting connectivity issues - Identify why traffic is not reaching instances • Compliance requirements - Maintain audit trails of network traffic • Network optimization - Analyze traffic patterns to optimize your architecture • Cost analysis - Understand data transfer patterns
How VPC Flow Logs Work
Flow logs can be created at three levels: • VPC level - Captures all traffic for the entire VPC • Subnet level - Captures traffic for a specific subnet • Network interface level - Captures traffic for a specific ENI
Flow Log Record Format
Each record captures a specific traffic flow and includes: • Source and destination IP addresses • Source and destination ports • Protocol number • Number of packets and bytes • Start and end time • Action taken (ACCEPT or REJECT) • Log status
Key Characteristics to Remember
• Flow logs do not capture real-time data - there is a delay of several minutes • Flow logs do not capture traffic to Amazon DNS servers • Flow logs do not capture DHCP traffic • Flow logs do not capture traffic to the instance metadata service (169.254.169.254) • Flow logs do not capture traffic to the Amazon Time Sync Service • Once created, you cannot modify a flow log's configuration - you must delete and recreate it • Flow logs do not affect network throughput or latency
Destinations for Flow Logs
1. CloudWatch Logs - Best for real-time analysis and setting up metric filters and alarms 2. Amazon S3 - Best for long-term storage and analysis with Athena 3. Kinesis Data Firehose - Best for streaming to third-party tools or data lakes
IAM Requirements
• For CloudWatch Logs: The IAM role needs permissions to publish to CloudWatch Logs • For S3: The bucket policy must allow the flow logs service to write objects • The flow logs service principal is logs.amazonaws.com for CloudWatch and delivery.logs.amazonaws.com for S3
Exam Tips: Answering Questions on VPC Flow Logs
Scenario-based questions: • If asked about troubleshooting why traffic is being blocked, look for VPC Flow Logs showing REJECT entries • If Security Groups are rejecting traffic, you will see REJECT in flow logs • If NACLs are rejecting traffic, you will also see REJECT in flow logs • To determine if the issue is Security Group vs NACL, remember: Security Groups are stateful, NACLs are stateless
Storage and analysis questions: • For cost-effective long-term storage: Choose S3 • For querying flow logs with SQL: Use S3 + Amazon Athena • For real-time monitoring and alerting: Choose CloudWatch Logs • For streaming analytics: Choose Kinesis Data Firehose
Common exam traps: • Flow logs cannot be modified after creation - this is a frequent exam topic • Flow logs have a delay - they are not real-time • Peered VPC flow logs can only be captured if the peered VPC is in the same account • Flow logs do not capture all traffic types - remember the exclusions listed above
Troubleshooting scenarios: • ACCEPT + ACCEPT = Traffic allowed in both directions • REJECT + no corresponding entry = Security Group blocking (stateful, so no return traffic logged) • REJECT + REJECT = NACL blocking (stateless, both directions logged)
Best Practices for the Exam
• Always consider the aggregation interval - default is 10 minutes, can be set to 1 minute for more granular data • Remember that custom formats allow you to select only the fields you need, reducing storage costs • When questions mention compliance or auditing, VPC Flow Logs are typically part of the solution • For questions about identifying the source of DDoS attacks or unauthorized access attempts, VPC Flow Logs provide the necessary visibility