Bind the app port deliberately and document the proxy deployment (closes #268)

The plaintext listener bound `:PORT`, so it answered on every
interface with no way to say otherwise. That published the admin UI
and the unauthenticated receiver in cleartext beside whatever TLS
proxy was in front of them, reachable from any host that could route
to the machine.

BIND_ADDRESS now selects the address. The binary defaults to
127.0.0.1, which is the safe answer for a bare host: reaching
webhooker from elsewhere becomes a deliberate act. The image sets
0.0.0.0, which is the correct answer inside a container, where the
network namespace is already the boundary and exposure is decided by
the publish flag instead — so `-p 127.0.0.1:8080:8080` is what the
README shows. Existing container deployments are unaffected. Only IP
address literals are accepted: hostnames, host:port and CIDR blocks
abort startup naming the variable and the value, and a literal that is
not an address of this host fails at listen and exits non-zero.

The http.Server is now built in New rather than in the serving
goroutine, and sentryEnabled is atomic. Both fields were written by
the serving goroutine and read by the fx stop hook with nothing
ordering them, and the OnStart hook returns before that goroutine has
necessarily run: cleanShutdown could dereference a nil httpServer on
an early SIGTERM, and both reads raced. No test started and stopped
the server, so nothing observed it.

Closes #226.

README gains a "Deployment behind a reverse proxy" section: a working
nginx server block, and the five things that are silent when wrong —
bind or firewall the app port, WEBHOOKER_ENVIRONMENT=prod,
TRUSTED_PROXIES, Host as $http_host rather than $host, and keeping the
proxy's access log because webhooker's own records only the proxy.

internal/config/config.go

@@ -33,6 +33,27 @@ const (
 	// defaultPort is the default HTTP listen port.
 	defaultPort = 8080
 
+	// defaultBindAddress is the interface the plaintext HTTP
+	// listener claims when BIND_ADDRESS is unset.
+	//
+	// Loopback, because the listener speaks cleartext and serves
+	// both the admin UI and the unauthenticated receiver: a
+	// wildcard default publishes them on every interface of every
+	// host that never configured anything, which is the failure
+	// this default exists to prevent. Reaching webhooker from off
+	// the host is then a deliberate act — a reverse proxy in front
+	// of it, or an explicit BIND_ADDRESS.
+	//
+	// A container needs BIND_ADDRESS=0.0.0.0 set explicitly: a
+	// loopback-bound process is unreachable from outside its
+	// network namespace even with -p. That is deliberate. The
+	// container fails its healthcheck immediately and visibly,
+	// where the wildcard default fails silently in the direction of
+	// exposure. There is no container auto-detection here, because
+	// a heuristic that guesses wrong opens the cleartext port
+	// exactly where nobody is looking.
+	defaultBindAddress = "127.0.0.1"
+
 	// defaultRetentionSweepInterval is how often the retention
 	// reaper deletes events older than each webhook's RetentionDays.
 	defaultRetentionSweepInterval = time.Hour
@@ -75,6 +96,10 @@ var ErrInvalidPort = errors.New("invalid port")
 // nor a bare IP address.
 var ErrInvalidCIDR = errors.New("invalid CIDR")
 
+// ErrInvalidBindAddress is returned when BIND_ADDRESS is set to
+// something that is not an IP address literal.
+var ErrInvalidBindAddress = errors.New("invalid bind address")
+
 // ErrIncompleteMetricsAuth is returned when exactly one of
 // METRICS_USERNAME and METRICS_PASSWORD carries a value. Neither
 // fallback is acceptable: serving /metrics on the username alone
@@ -105,6 +130,13 @@ type Config struct {
 	Port            int
 	SentryDSN       string
 
+	// BindAddress is the IP address the plaintext HTTP listener
+	// binds, as an address literal. It defaults to
+	// defaultBindAddress and is never empty: an empty string would
+	// mean the wildcard to net.Listen, which is the opposite of the
+	// default this ships.
+	BindAddress string
+
 	// RetentionSweepInterval is how often the retention reaper runs.
 	// Always positive: it becomes a time.NewTicker period.
 	RetentionSweepInterval time.Duration
@@ -387,6 +419,42 @@ func envPrefixList(key string) ([]netip.Prefix, error) {
 	return prefixes, nil
 }
 
+// envBindAddress returns the value of the named environment variable
+// parsed as an IP address literal. An unset (or empty, or
+// whitespace-only) value yields defaultValue.
+//
+// Only literals are accepted: no hostname is resolved, so `localhost`
+// is an error rather than a DNS lookup at startup whose answer could
+// be either loopback family, could change under the process, and
+// could return several addresses of which only one would be bound. A
+// value with a port in it (`127.0.0.1:8080`) is likewise an error —
+// the port is PORT's business, and silently accepting it would bind
+// something other than what was asked for.
+//
+// A set value that is not a literal is a hard error naming the key
+// and the bad value, so startup fails loudly rather than falling back
+// to a default the operator plainly did not want. A literal that is
+// not an address of this host parses here and fails at listen time
+// instead, which ends the process non-zero.
+func envBindAddress(key, defaultValue string) (string, error) {
+	v := strings.TrimSpace(os.Getenv(key))
+	if v == "" {
+		return defaultValue, nil
+	}
+
+	addr, err := netip.ParseAddr(v)
+	if err != nil {
+		return "", fmt.Errorf(
+			"%w: %s: %q must be an IP address literal such as "+
+				"127.0.0.1, 0.0.0.0 or ::, not a hostname and not "+
+				"host:port: %w",
+			ErrInvalidBindAddress, key, v, err,
+		)
+	}
+
+	return addr.String(), nil
+}
+
 // resolveMetricsAuth reads the /metrics basic-auth credentials and
 // rejects a half-set pair, naming both variables either way. The
 // error carries neither value: the password is a secret.
@@ -431,6 +499,27 @@ func resolveEnvironment() (string, error) {
 	return environment, nil
 }
 
+// resolveListener reads the two variables that describe the HTTP
+// listener: which port it claims and which address it claims it on.
+// They are read together because neither is meaningful alone, and
+// because a validation failure in either has to abort startup before
+// anything binds.
+func resolveListener() (int, string, error) {
+	port, err := envPort("PORT", defaultPort)
+	if err != nil {
+		return 0, "", err
+	}
+
+	bindAddress, err := envBindAddress(
+		"BIND_ADDRESS", defaultBindAddress,
+	)
+	if err != nil {
+		return 0, "", err
+	}
+
+	return port, bindAddress, nil
+}
+
 // loadFromEnv builds a Config from the environment. Every value that
 // needs parsing fails loudly when it is set but unparseable: the
 // documented defaults apply only to variables that are unset (or
@@ -442,7 +531,7 @@ func loadFromEnv() (*Config, error) {
 		return nil, err
 	}
 
-	port, err := envPort("PORT", defaultPort)
+	port, bindAddress, err := resolveListener()
 	if err != nil {
 		return nil, err
 	}
@@ -506,6 +595,7 @@ func loadFromEnv() (*Config, error) {
 		MetricsUsername:        metricsUsername,
 		MetricsPassword:        metricsPassword,
 		Port:                   port,
+		BindAddress:            bindAddress,
 		SentryDSN:              envString("SENTRY_DSN"),
 		RetentionSweepInterval: retentionSweepInterval,
 		SessionIdleTimeout:     sessionIdleTimeout,
@@ -625,6 +715,11 @@ func New(lc fx.Lifecycle, params ConfigParams) (*Config, error) {
 	log.Info("Configuration loaded",
 		"environment", s.Environment,
 		"port", s.Port,
+		// Logged because which interfaces the cleartext listener
+		// answers on is not otherwise observable from inside a
+		// container, and it decides whether anything but the local
+		// host can reach the admin UI.
+		"bindAddress", s.BindAddress,
 		"debug", s.Debug,
 		"maintenanceMode", s.MaintenanceMode,
 		"dataDir", s.DataDir,