Passthrough Network Load Balancer (NLB)

 A passthrough Network Load Balancer (NLB) is a Layer 4 (OSI model transport layer) load balancer that distributes incoming network traffic directly to backend virtual machines or servers without acting as an intermediate proxy. [1, 2]

How It Works

No Connection Termination: Unlike a proxy load balancer, a passthrough NLB does not terminate the client connection. [1]
Unmodified Packets: It receives traffic and forwards the packets to backend instances with the original source and destination IP addresses, protocols, and ports unchanged. [1, 2]
Direct Server Return (DSR): Backend servers process the requests and send responses directly to the client rather than routing the return traffic back through the load balancer. [1]

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Key Characteristics

Layer 4 Operation: Operates at the transport layer, handling protocols like TCP, UDP, ESP, and ICMP.
Source IP Preservation: Because packets are not translated or proxied, backend applications see the real, original client IP address. This is vital for logging, security auditing, and geolocation tracking.
High Performance & Low Latency: Bypassing a proxy layer reduces overhead, delivering maximum throughput and minimal delay. [1, 2, 3, 4]

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Common Use Cases

High-Performance Applications: Real-time online gaming, video streaming, and high-frequency financial trading.
Non-HTTP/S Protocols: Custom TCP/UDP services, mail servers (SMTP/IMAP), and VPN gateways.
Strict Source IP Requirement: Architectures that require backend servers or firewalls to inspect the true client IP address. [1, 2]