mesh-controller/internal/broker/broker.go

// Package broker is what the control plane knows about the broker nodes dial.
//
// Two facts, and a token needs both (novox/hq ADR 0004): where it is, and what certificate to
// expect there. They are the two parts of a token this control plane does not generate itself.
//
// The address is configuration. The fingerprint is **not** — it is derived from the certificate
// the broker is actually serving. Configuring a fingerprint separately would let it drift from
// the certificate it describes, and a drifted pin is worse than none: every node issued a token
// during the drift refuses to connect, and the failure looks like an attack.
package broker

import (
	"crypto/sha256"
	"crypto/x509"
	"encoding/hex"
	"encoding/pem"
	"errors"
	"fmt"
	"github.com/novox/mesh-controller/internal/envfile"
	"os"
	"strings"
)

// Where the two settings come from.
const (
	AddressVar     = "MESH_BROKER_ADDRESS"
	CertificateVar = "MESH_BROKER_CERTIFICATE"
)

// Broker is what a token needs to say about it.
type Broker struct {
	Address     string
	Fingerprint string
}

// ErrNotConfigured means this control plane has not been told where its broker is.
//
// Not a failure to start. A control plane can hold node records and a signing key without one;
// what it cannot do is issue a token anybody could use, and that is where this surfaces.
var ErrNotConfigured = errors.New("this control plane has not been told about its broker")

// FromEnvironment reads the two settings, if they are there.
//
// The address's port follows MESH_BROKER_ADDRESS_PORT when the node's settings moved the bus
// (novox/hq 04-ISSUES/102): the address genesis wrote is a public name and the port genesis
// chose, and only the port is the node's to move.
func FromEnvironment() (Broker, error) {
	address, err := envfile.Placed(AddressVar)
	if err != nil {
		return Broker{}, err
	}
	path := strings.TrimSpace(os.Getenv(CertificateVar))

	if address == "" && path == "" {
		return Broker{}, ErrNotConfigured
	}
	// One without the other is worse than neither: a token with an address and no fingerprint
	// invites a node to connect to something it cannot check.
	if address == "" || path == "" {
		return Broker{}, fmt.Errorf(
			"%s and %s must be set together — an address with nothing to check the certificate "+
				"against is a node connecting to whatever answers", AddressVar, CertificateVar)
	}

	fingerprint, err := FingerprintOf(path)
	if err != nil {
		return Broker{}, err
	}
	return Broker{Address: address, Fingerprint: fingerprint}, nil
}

// FingerprintOf reads a PEM certificate and returns what a client pins.
//
// SHA-256 over the DER bytes, which is what a TLS client can compute from the certificate the
// server presents — so the two are comparing the same thing. A digest over the PEM text would
// not be: the same certificate re-wrapped with different line endings would hash differently
// while being the same certificate.
func FingerprintOf(path string) (string, error) {
	raw, err := os.ReadFile(path)
	if err != nil {
		return "", fmt.Errorf("cannot read the broker's certificate at %s: %w", path, err)
	}

	block, _ := pem.Decode(raw)
	if block == nil || block.Type != "CERTIFICATE" {
		return "", fmt.Errorf(
			"%s does not contain a PEM certificate. If this is a private key, it is the wrong "+
				"file — what a node pins is the certificate the broker presents", path)
	}
	// Parsed rather than hashed straight from the block, so a malformed certificate is caught
	// here rather than becoming a pin that matches nothing.
	if _, err := x509.ParseCertificate(block.Bytes); err != nil {
		return "", fmt.Errorf("the certificate at %s could not be parsed: %w", path, err)
	}

	sum := sha256.Sum256(block.Bytes)
	return "sha256:" + hex.EncodeToString(sum[:]), nil
}