Secure server communication¶

Unrestricted communication between servers expands the attack surface. A compromised instance can move laterally, reaching systems that would otherwise be inaccessible. Network segmentation limits that radius and applies least-privilege principles to the network layer.

Network isolation¶

Backend services (databases, internal APIs, message queues) belong in private subnets with no public IP addresses. Inbound traffic from the internet reaches a load balancer or API gateway; backend services are not exposed directly. Outbound traffic from private subnets passes through a NAT gateway when external access is needed.

For communication across account or team boundaries, private peering (AWS VPC peering, Azure VNet peering) keeps traffic off public routes:

aws ec2 create-vpc-peering-connection --vpc-id vpc-123 --peer-vpc-id vpc-456

Security groups and network ACLs restrict ingress and egress to the specific IPs and ports that each service actually uses:

aws ec2 authorize-security-group-ingress \
  --group-id sg-123 \
  --protocol tcp \
  --port 443 \
  --cidr 203.0.113.0/24

Internal traffic encryption¶

mTLS (mutual TLS) authenticates both sides of a service-to-service connection and encrypts it in transit. Service meshes like Istio and Linkerd handle certificate issuance and rotation automatically, and can enforce policies such as restricting service A to a specific port on service B. Cilium and Calico provide network-layer policy enforcement for the same purpose.

Traffic monitoring¶

VPC flow logs (AWS VPC Flow Logs, Azure NSG Flow Logs) record traffic at the network level. Aggregating these into a monitoring system makes anomaly detection practical: a backend service that begins initiating unexpected outbound connections is worth examining. Automated policy checks through AWS Config or Azure Policy can flag security group rules that have drifted toward permissiveness.

Administrative access¶

Direct server access for operations passes through controlled channels. AWS Systems Manager Session Manager removes the need for an exposed SSH port; bastion hosts with MFA provide an auditable alternative where SSM is not available. Hybrid environments connect on-premises networks to cloud resources through site-to-site VPNs or direct connections rather than public interfaces.