natcheck

NAT type diagnosis CLI. One command tells you why your WebRTC, P2P, or VPN connections struggle — and whether TURN is required.

Built on pion/stun. Pure Go, single static binary. No cgo, no services, no config.

v0.1.3. Spec: docs/design.md. Samples: docs/samples/.

Why natcheck

• One command, one answer. No more piecing together stun-client output with a dusty online NAT classifier and RFC 5780.
• Honest outputs. Reports unknown when classification can't be determined from the data, rather than guessing.
• Human-readable by default, --json on request. Clean output for terminals, schema-stable JSON for CI.

Quick start

Homebrew (recommended on macOS):

brew tap 1mb-dev/tap
brew install natcheck

Go install (any platform with Go 1.25+):

go install github.com/1mb-dev/natcheck/cmd/natcheck@latest

Then run:

natcheck

Example output on a healthy home network:

Direct P2P: likely
NAT type: Endpoint-Independent Mapping (cone)
Public endpoint: 203.0.113.45:51820

Probes:
  stun.l.google.com:19302   rtt=24ms  mapped=203.0.113.45:51820
  stun.cloudflare.com:3478  rtt=31ms  mapped=203.0.113.45:51820

Filtering not tested.

The Direct P2P: line leads so you get the answer on line 1.

Pointing --server at a STUN server that advertises OTHER-ADDRESS (e.g. coturn — see docs/coturn-setup.md) adds a one-line Filtering: verdict per RFC 5780 §4.4. Default servers (Google, Cloudflare) don't, so filtering classification is skipped and adds zero latency for default-server users.

Flags

Flag	Purpose
--json	Emit JSON instead of human-readable report
--verbose	Log each STUN transaction to stderr
--server host:port	Add a custom STUN server (repeatable; overrides defaults)
--timeout duration	Total probe timeout (default 5s)
--version	Print version and exit
--help	Print flag reference

Exit codes

Code	Meaning	When
0	P2P-friendly	Direct P2P: likely or possible
1	P2P-hostile	Direct P2P: unlikely or unknown
2	Probe or flag error	All probes failed, invalid flag, or bad --server

Scripts that ask "did the tool run?" check $? -ne 2. Scripts that ask "can I use direct P2P?" check $? -eq 0.

CI usage

verdict=$(natcheck --json | jq -r '.webrtc_forecast.direct_p2p')
if [ "$verdict" != "likely" ]; then
  echo "NAT not P2P-friendly: $verdict"
  exit 1
fi

The --json schema (nat_type, public_endpoint, probes[], webrtc_forecast, warnings[], filtering) is a public contract from v0.1 onward — additive changes only after release. Real captures live under docs/samples/.

Scope

natcheck classifies NAT mapping behavior (Endpoint-Independent, Address-Dependent, Address-and-Port-Dependent per RFC 5780) and reports a WebRTC direct-P2P forecast. Filtering behavior (RFC 5780 §4.4) is classified when the configured --server advertises OTHER-ADDRESS; otherwise the verdict is untested. Hairpinning is out of scope. On CGNAT networks (100.64.0.0/10), the forecast is unknown. IPv6 works when the network supports it but is not exhaustively tested.

See docs/design.md for full architecture and testing details.

Acknowledgements

Default STUN servers courtesy of Google (stun.l.google.com:19302) and Cloudflare (stun.cloudflare.com:3478). For high-frequency automation, self-host coturn and pass it via --server.

License

MIT. See LICENSE.