Readme

README.md

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+# UDPGW (NapsterV framing) — `udpgw.go`
+
+A lightweight TCP↔UDP gateway that speaks the “NapsterV” UDPGW framing:
+
+- Listens on TCP (default `0.0.0.0:7400`).
+- For each TCP client, opens **one UDP socket** (stable source port) and forwards framed payloads to UDP targets.
+- Tracks a short-lived mapping so UDP replies are routed back to the correct `connID` + `X` byte.
+- **IPv4 only** (UDP replies are dropped if they’re not IPv4).
+
+## Build
+
+Single-file build:
+
+```bash
+# Linux static-ish build (recommended)
+CGO_ENABLED=0 go build -trimpath -ldflags="-s -w" -o udpgw ./udpgw.go
+```
+
+Run help:
+
+```bash
+./udpgw -h
+```
+
+## Minimal RAM/CPU start command (recommended profile)
+
+This profile is tuned to **minimize per-client kernel buffer memory** and reduce background CPU (slower map reaping), while keeping behavior safe for most traffic:
+
+```bash
+GOMAXPROCS=1 ./udpgw   -listen 0.0.0.0:7400   -debug=false   -write-chan 256   -udp-rbuf $((256*1024))   -udp-wbuf $((256*1024))   -map-ttl 60s   -reap-every 30s
+```
+
+Notes:
+- `GOMAXPROCS=1` limits Go’s scheduler to one OS thread for CPU predictability.
+- `-udp-rbuf/-udp-wbuf` are the biggest memory knobs because they are applied **per TCP client UDP socket**.
+- If you see packet loss under load, raise `-udp-rbuf/-udp-wbuf` to `524288` (512 KiB) or `1048576` (1 MiB) first.
+
+## How framing works
+
+Each TCP frame is:
+
+- `LEN` (2 bytes, little-endian)
+- `PAYLOAD` (`LEN` bytes)
+
+`PAYLOAD` format for both directions:
+
+- `connID` (2 bytes, big-endian)
+- `X` (1 byte)
+- `IPv4` (4 bytes)
+- `port` (2 bytes, big-endian)
+- `data` (remaining bytes)
+
+(See `readPayload()` and `buildFrame()` in the source.)
+
+## Configuration flags
+
+All runtime knobs are flags (defaults shown):
+
+- `-listen 0.0.0.0:7400`  
+  TCP listen address.
+
+- `-max-frame 65536`  
+  Max frame payload length (DoS protection). Frames larger than this are rejected.
+
+- `-debug=false`  
+  Verbose logs.
+
+- `-hexdump 64`  
+  When `-debug=true`, hex-dump the first N bytes of payload.
+
+- `-write-chan 4096`  
+  TCP write queue depth. Larger can help bursty traffic (e.g., games) but increases worst-case RAM.
+
+- `-udp-bind ""`  
+  Bind the per-client UDP socket to a specific local IP (optional).
+
+- `-udp-rbuf 8388608`  
+  UDP read buffer bytes (socket option). **Applied per TCP client**.
+
+- `-udp-wbuf 8388608`  
+  UDP write buffer bytes (socket option). **Applied per TCP client**.
+
+- `-map-ttl 90s`  
+  Destination→`connID` mapping TTL.
+
+- `-reap-every 10s`  
+  How often to delete expired mappings.
+
+## Tuning guide (what affects RAM/CPU the most)
+
+### RAM hotspots
+
+1. **Kernel UDP buffers per TCP client**  
+   Roughly proportional to: `udp-rbuf + udp-wbuf` *per connected client*.  
+   Defaults are high (8 MiB + 8 MiB). If you expect many concurrent TCP clients, reduce these.
+
+2. **TCP write queue (`-write-chan`)**  
+   Each queued element is a `[]byte` frame. Bigger `-write-chan` allows more buffering but can grow memory.
+
+3. **Per-client TCP reader buffer (fixed)**  
+   The code uses `bufio.NewReaderSize(conn, 256*1024)` per client (256 KiB). This is not configurable via flags.
+
+### CPU hotspots
+
+1. `-debug=true` (logging)  
+   Turn off for production.
+
+2. Reaper frequency (`-reap-every`)  
+   A longer interval reduces wakeups/CPU at the cost of keeping expired mappings a bit longer.
+
+## Recommended profiles
+
+### 1) Ultra-low memory (many clients, light traffic)
+
+```bash
+GOMAXPROCS=1 ./udpgw   -listen 0.0.0.0:7400   -write-chan 128   -udp-rbuf $((128*1024))   -udp-wbuf $((128*1024))   -map-ttl 45s   -reap-every 45s
+```
+
+### 2) Balanced (good default for most servers)
+
+```bash
+./udpgw   -listen 0.0.0.0:7400   -write-chan 512   -udp-rbuf $((1024*1024))   -udp-wbuf $((1024*1024))   -map-ttl 90s   -reap-every 15s
+```
+
+### 3) Low-latency / games (avoid drops under bursts)
+
+```bash
+./udpgw   -listen 0.0.0.0:7400   -write-chan 4096   -udp-rbuf $((4*1024*1024))   -udp-wbuf $((4*1024*1024))   -map-ttl 120s   -reap-every 10s
+```
+
+## Operational notes
+
+- Run behind a firewall; expose only the TCP listen port you need.
+- If you bind `-udp-bind`, ensure that IP is present on the host.
+- The program will drop UDP replies that are not IPv4.
+
+## systemd (optional)
+
+Example unit (adjust paths/flags):
+
+```ini
+[Unit]
+Description=udpgw (NapsterV)
+After=network.target
+
+[Service]
+Type=simple
+WorkingDirectory=/opt/udpgw
+Environment=GOMAXPROCS=1
+ExecStart=/opt/udpgw/udpgw -listen 0.0.0.0:7400 -write-chan 256 -udp-rbuf 262144 -udp-wbuf 262144 -map-ttl 60s -reap-every 30s
+Restart=on-failure
+NoNewPrivileges=true
+PrivateTmp=true
+
+[Install]
+WantedBy=multi-user.target
+```