Network Address Translation (NAT) instances in AWS sometimes encounter connectivity issues when forwarding traffic from private EC2 instances. Here's a detailed analysis of common problems and their solutions.
Key Configuration Points
Source/Destination Check
- Disable source/destination checking on the NAT instance, as this is often the root cause of connectivity issues[3].
Network Configuration
- Ensure the private subnet's route table directs 0.0.0.0/0 traffic to the NAT instance
- The NAT instance must reside in a public subnet with proper routing to the Internet Gateway
Essential IPTables Configuration
# Enable IP forwarding
echo 1 > /proc/sys/net/ipv4/ip_forward
# Configure NAT rules
iptables -t nat -A POSTROUTING -o eth0 -s 10.0.0.0/16 -j MASQUERADE
iptables -A FORWARD -i eth0 -o eth1 -m state --state RELATED,ESTABLISHED -j ACCEPT
iptables -A FORWARD -i eth1 -o eth0 -j ACCEPT
Security Considerations
Security Group Settings
- NAT instance security group must permit inbound traffic from private subnets
- Allow all outbound traffic
- Private instance security groups need outbound rules to NAT instance[1]
Connection Timeout Issues
The NAT gateway terminates idle connections after 350 seconds. To prevent this:
- Implement TCP keepalive with values under 350 seconds
- Maintain active traffic flow over the connection[1]
Advanced Troubleshooting
Network Connectivity Tests
ping 8.8.8.8
traceroute 8.8.8.8
Consider using AWS NAT Gateway as an alternative for improved reliability and simplified management[1].
情報源
[1] Troubleshoot NAT gateways - Amazon Virtual Private Cloud https://docs.aws.amazon.com/vpc/latest/userguide/nat-gateway-troubleshooting.html
[2] NAT ゲートウェイのトラブルシューティング https://docs.aws.amazon.com/ja_jp/vpc/latest/userguide/nat-gateway-troubleshooting.html
[3] プライベートリソースが VPC の外側で通信できるようにする https://docs.aws.amazon.com/ja_jp/vpc/latest/userguide/work-with-nat-instances.html
[4] nat-instance(terraform-aws-nat-instance)を利用して発生した ... - Qiita https://qiita.com/fkdfkdfkd/items/09a15f32002dc44b6b18
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