How to Overcome NAT Barriers: VPS Relays and Nginx UDP Proxy Explained

In the first part we saw how NAT affects UDP, especially how Symmetric NAT makes peer‑to‑peer connections almost impossible. This article explores engineering solutions for those problems.

Solution Overview

Depending on the network environment and requirements, different UDP solutions can be chosen:

Home/friendly network (Full Cone, Port‑Restricted NAT) – direct P2P connection (low latency, but depends on NAT type).

Enterprise/mobile network (Symmetric NAT) – UDP hole punching (possible but success rate varies, requires a coordination server).

High‑reliability requirement (any NAT) – VPS relay (100 % reliable, higher latency, needs an extra server).

Service proxy / load balancing (N/A) – Nginx UDP proxy (simple configuration, supports load balancing, requires a proxy server).

Full Cone NAT Is Ideal; Public‑Space Proxy Solves Symmetric NAT

Full Cone NAT is friendly to P2P but less secure, and Symmetric NAT is far more common in practice. When both peers sit behind Symmetric NAT, establishing a direct P2P link is virtually impossible. A publicly reachable VPS can act as a reliable relay server to ensure connection establishment.

VPS Relay Diagram

VPS relay operation:

Connection establishment phase:

Client A and Client B each initiate a connection to the public VPS.

Symmetric NAT creates separate port mappings for each connection.

The VPS records each client’s public address and port.

Data relay phase:

When Client A sends data to Client B, the data first goes to the VPS.

The VPS looks up Client B’s address information and forwards the packet to B’s NAT.

Because B previously opened a connection to the VPS, the Symmetric NAT allows inbound traffic from the VPS.

Key advantages:

Independent of NAT‑traversal success; 100 % reliable.

All traffic passes through the VPS, making monitoring and management easy.

Supports every NAT type, including the strictest Symmetric NAT.

VPS relay configuration example (using socat ):

# Set up UDP relay on the VPS
# Forward traffic from port 10000 to Player B’s address
socat UDP4-LISTEN:10000,fork UDP4:203.0.113.2:66666 &

# Player A connects to VPS:10000, all data is forwarded to Player B

Although the VPS relay adds latency because traffic passes through an intermediate node, it is the most reliable solution for Symmetric NAT and is commonly used in commercial applications that require high reliability.

Nginx UDP Proxy: Which NAT Mode Does It Resemble?

Nginx can proxy UDP via its stream module, maintaining a session table keyed by the five‑tuple (protocol, source IP, source port, destination IP, destination port). This table uniquely identifies each UDP session, and Nginx creates a session entry for every new tuple, linking it to a backend server.

Nginx UDP Proxy Configuration Details

stream {
    # Global UDP settings
    proxy_udp_timeout 10s;
    proxy_udp_buffer_size 64k;

    # UDP load‑balancing backend
    upstream udp_backend {
        hash $remote_addr;
        server backend1.example.com:53 max_fails=3 fail_timeout=30s;
        server backend2.example.com:53 max_fails=3 fail_timeout=30s;
        server backend3.example.com:53 backup;
    }

    # Basic UDP proxy server
    server {
        listen 53 udp reuseport;
        proxy_pass udp_backend;
        proxy_timeout 5s;
        proxy_responses 1;
        proxy_udp_packet_buffer 128;
        allow all;
        # allow 192.168.0.0/16;
        # allow 10.0.0.0/8;
        # deny all;
    }
}

Comparison of Nginx UDP Proxy to NAT

Similarities:

Port mapping – Nginx listens on a port and forwards traffic, similar to NAT port mapping.

Session tracking – Maintains state based on the five‑tuple, akin to NAT mapping tables.

Timeout mechanism – Sessions are cleared after a timeout, like NAT mapping expiration.

Key differences:

Asymmetry – Nginx can act as both client and server proxy, while NAT only handles outbound connections.

Load balancing – Nginx can distribute traffic among multiple backends; NAT is typically 1:1.

Application‑layer awareness – Nginx can apply complex routing logic, whereas NAT operates at the transport layer.

Effectively, Nginx’s UDP proxy behaves like an application‑layer Full Cone NAT.