Skills
skills/bagelhole/devops-security-agent-skills/aws-vpc

aws-vpc

Installation
SKILL.md

AWS VPC

Design and manage Virtual Private Cloud networking for production AWS environments with proper subnet isolation, routing, and security.

When to Use This Skill

  • Building a new VPC for production, staging, or development
  • Setting up public/private subnet architecture across multiple AZs
  • Configuring NAT Gateways for private subnet internet access
  • Creating security groups and NACLs for network segmentation
  • Setting up VPC peering or Transit Gateway for multi-VPC connectivity
  • Implementing VPC endpoints for private access to AWS services
  • Troubleshooting connectivity issues between resources

Prerequisites

  • AWS CLI v2 installed and configured
  • IAM permissions: ec2:* (or scoped to VPC-related actions)
  • CIDR range planning completed (avoid overlaps with on-premises or other VPCs)
  • For VPC peering: access to both VPCs (same or different accounts)

Network Architecture

VPC (10.0.0.0/16) - 65,536 IPs
├── Public Subnets (internet-facing via IGW)
│   ├── 10.0.1.0/24 (us-east-1a) - 256 IPs - ALBs, NAT GW, bastion
│   ├── 10.0.2.0/24 (us-east-1b) - 256 IPs
│   └── 10.0.3.0/24 (us-east-1c) - 256 IPs
├── Private Subnets (app tier, NAT GW for outbound)
│   ├── 10.0.11.0/24 (us-east-1a) - 256 IPs - ECS, EC2, Lambda
│   ├── 10.0.12.0/24 (us-east-1b) - 256 IPs
│   └── 10.0.13.0/24 (us-east-1c) - 256 IPs
├── Data Subnets (isolated, no internet)
│   ├── 10.0.21.0/24 (us-east-1a) - 256 IPs - RDS, ElastiCache
│   ├── 10.0.22.0/24 (us-east-1b) - 256 IPs
│   └── 10.0.23.0/24 (us-east-1c) - 256 IPs
├── Internet Gateway
├── NAT Gateways (one per AZ for HA)
├── Route Tables (public, private, data)
└── VPC Flow Logs → CloudWatch / S3

Create a VPC with CLI

# Create the VPC
VPC_ID=$(aws ec2 create-vpc \
  --cidr-block 10.0.0.0/16 \
  --tag-specifications 'ResourceType=vpc,Tags=[{Key=Name,Value=production-vpc},{Key=Environment,Value=production}]' \
  --query 'Vpc.VpcId' --output text)

# Enable DNS support and hostnames
aws ec2 modify-vpc-attribute --vpc-id $VPC_ID --enable-dns-support '{"Value":true}'
aws ec2 modify-vpc-attribute --vpc-id $VPC_ID --enable-dns-hostnames '{"Value":true}'

# Create public subnets
PUB_SUB_A=$(aws ec2 create-subnet \
  --vpc-id $VPC_ID \
  --cidr-block 10.0.1.0/24 \
  --availability-zone us-east-1a \
  --tag-specifications 'ResourceType=subnet,Tags=[{Key=Name,Value=public-a},{Key=Tier,Value=public}]' \
  --query 'Subnet.SubnetId' --output text)

PUB_SUB_B=$(aws ec2 create-subnet \
  --vpc-id $VPC_ID \
  --cidr-block 10.0.2.0/24 \
  --availability-zone us-east-1b \
  --tag-specifications 'ResourceType=subnet,Tags=[{Key=Name,Value=public-b},{Key=Tier,Value=public}]' \
  --query 'Subnet.SubnetId' --output text)

# Enable auto-assign public IP on public subnets
aws ec2 modify-subnet-attribute --subnet-id $PUB_SUB_A --map-public-ip-on-launch
aws ec2 modify-subnet-attribute --subnet-id $PUB_SUB_B --map-public-ip-on-launch

# Create private subnets
PRIV_SUB_A=$(aws ec2 create-subnet \
  --vpc-id $VPC_ID \
  --cidr-block 10.0.11.0/24 \
  --availability-zone us-east-1a \
  --tag-specifications 'ResourceType=subnet,Tags=[{Key=Name,Value=private-a},{Key=Tier,Value=private}]' \
  --query 'Subnet.SubnetId' --output text)

PRIV_SUB_B=$(aws ec2 create-subnet \
  --vpc-id $VPC_ID \
  --cidr-block 10.0.12.0/24 \
  --availability-zone us-east-1b \
  --tag-specifications 'ResourceType=subnet,Tags=[{Key=Name,Value=private-b},{Key=Tier,Value=private}]' \
  --query 'Subnet.SubnetId' --output text)

# Create data subnets (isolated)
DATA_SUB_A=$(aws ec2 create-subnet \
  --vpc-id $VPC_ID \
  --cidr-block 10.0.21.0/24 \
  --availability-zone us-east-1a \
  --tag-specifications 'ResourceType=subnet,Tags=[{Key=Name,Value=data-a},{Key=Tier,Value=data}]' \
  --query 'Subnet.SubnetId' --output text)

DATA_SUB_B=$(aws ec2 create-subnet \
  --vpc-id $VPC_ID \
  --cidr-block 10.0.22.0/24 \
  --availability-zone us-east-1b \
  --tag-specifications 'ResourceType=subnet,Tags=[{Key=Name,Value=data-b},{Key=Tier,Value=data}]' \
  --query 'Subnet.SubnetId' --output text)

Internet Gateway and NAT Gateway

# Create and attach Internet Gateway
IGW_ID=$(aws ec2 create-internet-gateway \
  --tag-specifications 'ResourceType=internet-gateway,Tags=[{Key=Name,Value=production-igw}]' \
  --query 'InternetGateway.InternetGatewayId' --output text)
aws ec2 attach-internet-gateway --vpc-id $VPC_ID --internet-gateway-id $IGW_ID

# Create public route table
PUB_RT=$(aws ec2 create-route-table \
  --vpc-id $VPC_ID \
  --tag-specifications 'ResourceType=route-table,Tags=[{Key=Name,Value=public-rt}]' \
  --query 'RouteTable.RouteTableId' --output text)
aws ec2 create-route --route-table-id $PUB_RT --destination-cidr-block 0.0.0.0/0 --gateway-id $IGW_ID
aws ec2 associate-route-table --route-table-id $PUB_RT --subnet-id $PUB_SUB_A
aws ec2 associate-route-table --route-table-id $PUB_RT --subnet-id $PUB_SUB_B

# Allocate Elastic IPs for NAT Gateways (one per AZ for HA)
EIP_A=$(aws ec2 allocate-address --domain vpc --query 'AllocationId' --output text)
EIP_B=$(aws ec2 allocate-address --domain vpc --query 'AllocationId' --output text)

# Create NAT Gateways in public subnets
NAT_A=$(aws ec2 create-nat-gateway \
  --subnet-id $PUB_SUB_A \
  --allocation-id $EIP_A \
  --tag-specifications 'ResourceType=natgateway,Tags=[{Key=Name,Value=nat-a}]' \
  --query 'NatGateway.NatGatewayId' --output text)

NAT_B=$(aws ec2 create-nat-gateway \
  --subnet-id $PUB_SUB_B \
  --allocation-id $EIP_B \
  --tag-specifications 'ResourceType=natgateway,Tags=[{Key=Name,Value=nat-b}]' \
  --query 'NatGateway.NatGatewayId' --output text)

# Wait for NAT Gateways
aws ec2 wait nat-gateway-available --nat-gateway-ids $NAT_A $NAT_B

# Create private route tables (one per AZ for HA NAT)
PRIV_RT_A=$(aws ec2 create-route-table \
  --vpc-id $VPC_ID \
  --tag-specifications 'ResourceType=route-table,Tags=[{Key=Name,Value=private-rt-a}]' \
  --query 'RouteTable.RouteTableId' --output text)
aws ec2 create-route --route-table-id $PRIV_RT_A --destination-cidr-block 0.0.0.0/0 --nat-gateway-id $NAT_A
aws ec2 associate-route-table --route-table-id $PRIV_RT_A --subnet-id $PRIV_SUB_A

PRIV_RT_B=$(aws ec2 create-route-table \
  --vpc-id $VPC_ID \
  --tag-specifications 'ResourceType=route-table,Tags=[{Key=Name,Value=private-rt-b}]' \
  --query 'RouteTable.RouteTableId' --output text)
aws ec2 create-route --route-table-id $PRIV_RT_B --destination-cidr-block 0.0.0.0/0 --nat-gateway-id $NAT_B
aws ec2 associate-route-table --route-table-id $PRIV_RT_B --subnet-id $PRIV_SUB_B

Security Groups

# ALB security group (public-facing)
ALB_SG=$(aws ec2 create-security-group \
  --group-name alb-sg \
  --description "Application Load Balancer" \
  --vpc-id $VPC_ID \
  --query 'GroupId' --output text)
aws ec2 authorize-security-group-ingress --group-id $ALB_SG --protocol tcp --port 443 --cidr 0.0.0.0/0
aws ec2 authorize-security-group-ingress --group-id $ALB_SG --protocol tcp --port 80 --cidr 0.0.0.0/0

# Application security group (only from ALB)
APP_SG=$(aws ec2 create-security-group \
  --group-name app-sg \
  --description "Application tier" \
  --vpc-id $VPC_ID \
  --query 'GroupId' --output text)
aws ec2 authorize-security-group-ingress \
  --group-id $APP_SG \
  --protocol tcp \
  --port 8080 \
  --source-group $ALB_SG

# Database security group (only from app tier)
DB_SG=$(aws ec2 create-security-group \
  --group-name db-sg \
  --description "Database tier" \
  --vpc-id $VPC_ID \
  --query 'GroupId' --output text)
aws ec2 authorize-security-group-ingress \
  --group-id $DB_SG \
  --protocol tcp \
  --port 5432 \
  --source-group $APP_SG

# List all security groups in the VPC
aws ec2 describe-security-groups \
  --filters "Name=vpc-id,Values=$VPC_ID" \
  --query "SecurityGroups[].{Name:GroupName,ID:GroupId,Description:Description}" \
  --output table

VPC Endpoints (Private Access to AWS Services)

# Gateway endpoint for S3 (free, route-table based)
aws ec2 create-vpc-endpoint \
  --vpc-id $VPC_ID \
  --service-name com.amazonaws.us-east-1.s3 \
  --route-table-ids $PRIV_RT_A $PRIV_RT_B \
  --tag-specifications 'ResourceType=vpc-endpoint,Tags=[{Key=Name,Value=s3-endpoint}]'

# Gateway endpoint for DynamoDB (free)
aws ec2 create-vpc-endpoint \
  --vpc-id $VPC_ID \
  --service-name com.amazonaws.us-east-1.dynamodb \
  --route-table-ids $PRIV_RT_A $PRIV_RT_B

# Interface endpoint for Secrets Manager (ENI-based, has hourly cost)
aws ec2 create-vpc-endpoint \
  --vpc-id $VPC_ID \
  --vpc-endpoint-type Interface \
  --service-name com.amazonaws.us-east-1.secretsmanager \
  --subnet-ids $PRIV_SUB_A $PRIV_SUB_B \
  --security-group-ids $APP_SG \
  --private-dns-enabled \
  --tag-specifications 'ResourceType=vpc-endpoint,Tags=[{Key=Name,Value=secretsmanager-endpoint}]'

VPC Flow Logs

# Enable VPC flow logs to CloudWatch
aws ec2 create-flow-log \
  --resource-type VPC \
  --resource-ids $VPC_ID \
  --traffic-type ALL \
  --log-destination-type cloud-watch-logs \
  --log-group-name /vpc/production-flow-logs \
  --deliver-logs-permission-arn arn:aws:iam::123456789012:role/VPCFlowLogRole \
  --max-aggregation-interval 60 \
  --tag-specifications 'ResourceType=vpc-flow-log,Tags=[{Key=Name,Value=production-flow-log}]'

# Enable VPC flow logs to S3 (cheaper for long-term storage)
aws ec2 create-flow-log \
  --resource-type VPC \
  --resource-ids $VPC_ID \
  --traffic-type ALL \
  --log-destination-type s3 \
  --log-destination arn:aws:s3:::my-flow-logs-bucket/vpc-logs/ \
  --max-aggregation-interval 60

VPC Peering

# Request peering connection
PEERING_ID=$(aws ec2 create-vpc-peering-connection \
  --vpc-id vpc-requester \
  --peer-vpc-id vpc-accepter \
  --peer-owner-id 987654321098 \
  --peer-region us-west-2 \
  --tag-specifications 'ResourceType=vpc-peering-connection,Tags=[{Key=Name,Value=prod-to-shared}]' \
  --query 'VpcPeeringConnection.VpcPeeringConnectionId' --output text)

# Accept peering (from the accepter account/region)
aws ec2 accept-vpc-peering-connection --vpc-peering-connection-id $PEERING_ID

# Add routes in both VPCs
aws ec2 create-route --route-table-id rtb-requester --destination-cidr-block 10.1.0.0/16 --vpc-peering-connection-id $PEERING_ID
aws ec2 create-route --route-table-id rtb-accepter --destination-cidr-block 10.0.0.0/16 --vpc-peering-connection-id $PEERING_ID

Terraform VPC Module

resource "aws_vpc" "main" {
  cidr_block           = "10.0.0.0/16"
  enable_dns_support   = true
  enable_dns_hostnames = true

  tags = {
    Name        = "production-vpc"
    Environment = "production"
  }
}

resource "aws_subnet" "public" {
  count                   = 3
  vpc_id                  = aws_vpc.main.id
  cidr_block              = cidrsubnet(aws_vpc.main.cidr_block, 8, count.index + 1)
  availability_zone       = data.aws_availability_zones.available.names[count.index]
  map_public_ip_on_launch = true

  tags = {
    Name = "public-${data.aws_availability_zones.available.names[count.index]}"
    Tier = "public"
  }
}

resource "aws_subnet" "private" {
  count             = 3
  vpc_id            = aws_vpc.main.id
  cidr_block        = cidrsubnet(aws_vpc.main.cidr_block, 8, count.index + 11)
  availability_zone = data.aws_availability_zones.available.names[count.index]

  tags = {
    Name = "private-${data.aws_availability_zones.available.names[count.index]}"
    Tier = "private"
  }
}

resource "aws_subnet" "data" {
  count             = 3
  vpc_id            = aws_vpc.main.id
  cidr_block        = cidrsubnet(aws_vpc.main.cidr_block, 8, count.index + 21)
  availability_zone = data.aws_availability_zones.available.names[count.index]

  tags = {
    Name = "data-${data.aws_availability_zones.available.names[count.index]}"
    Tier = "data"
  }
}

resource "aws_internet_gateway" "main" {
  vpc_id = aws_vpc.main.id
  tags   = { Name = "production-igw" }
}

resource "aws_eip" "nat" {
  count  = 2
  domain = "vpc"
  tags   = { Name = "nat-eip-${count.index}" }
}

resource "aws_nat_gateway" "main" {
  count         = 2
  allocation_id = aws_eip.nat[count.index].id
  subnet_id     = aws_subnet.public[count.index].id
  tags          = { Name = "nat-${count.index}" }
}

resource "aws_route_table" "public" {
  vpc_id = aws_vpc.main.id
  route {
    cidr_block = "0.0.0.0/0"
    gateway_id = aws_internet_gateway.main.id
  }
  tags = { Name = "public-rt" }
}

resource "aws_route_table_association" "public" {
  count          = 3
  subnet_id      = aws_subnet.public[count.index].id
  route_table_id = aws_route_table.public.id
}

resource "aws_route_table" "private" {
  count  = 2
  vpc_id = aws_vpc.main.id
  route {
    cidr_block     = "0.0.0.0/0"
    nat_gateway_id = aws_nat_gateway.main[count.index].id
  }
  tags = { Name = "private-rt-${count.index}" }
}

resource "aws_route_table_association" "private" {
  count          = 2
  subnet_id      = aws_subnet.private[count.index].id
  route_table_id = aws_route_table.private[count.index].id
}

resource "aws_vpc_endpoint" "s3" {
  vpc_id       = aws_vpc.main.id
  service_name = "com.amazonaws.${data.aws_region.current.name}.s3"
  route_table_ids = aws_route_table.private[*].id
  tags = { Name = "s3-endpoint" }
}

resource "aws_flow_log" "main" {
  vpc_id                 = aws_vpc.main.id
  traffic_type           = "ALL"
  log_destination_type   = "s3"
  log_destination        = "${aws_s3_bucket.flow_logs.arn}/vpc-logs/"
  max_aggregation_interval = 60
}

Troubleshooting

Problem Cause Fix
Cannot reach internet from private subnet NAT Gateway route missing Add 0.0.0.0/0 route to NAT GW in private route table
Cannot reach internet from public subnet IGW not attached or route missing Attach IGW; add 0.0.0.0/0 route to IGW in public RT
EC2 cannot reach S3 No VPC endpoint or NAT Add S3 gateway endpoint (free) or ensure NAT GW route
Security group rule not working Wrong direction (ingress vs egress) SG is stateful; check inbound rule on destination
NACL blocking traffic NACLs are stateless; need both directions Add matching inbound AND outbound rules with correct ports
VPC peering one-way only Routes missing in one VPC Add routes in BOTH VPC route tables
DNS resolution failing DNS hostnames not enabled on VPC Enable enableDnsHostnames on VPC
NAT Gateway charges high All AZs routing through one NAT Deploy NAT GW per AZ with separate route tables
Cross-AZ data transfer costs Resources in different AZs communicating Co-locate tightly coupled services in same AZ

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