Bind the app port deliberately and document the proxy deployment (closes #268) (closes #226)

internal/config/config.go

@@ -33,6 +33,34 @@ 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.
+	//
+	// This is the binary's default only. The Dockerfile ships
+	// ENV BIND_ADDRESS=0.0.0.0, so a container deployment needs
+	// nothing set and is unaffected by this constant. The two
+	// differ because they answer different questions: a container's
+	// network namespace is already the boundary this default is
+	// reaching for, so binding every address inside it exposes
+	// nothing, and what decides exposure there is the publish flag
+	// (-p 127.0.0.1:8080:8080). A loopback bind inside a container
+	// buys no security and makes the process unreachable through
+	// its own published port.
+	//
+	// The split is expressed as two explicit defaults rather than
+	// container auto-detection, 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 +103,10 @@ 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 +137,13 @@ 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 +426,42 @@ 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 +506,27 @@ 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 +538,7 @@ func loadFromEnv() (*Config, error) {
 		return nil, err
 	}
 
-	port, err := envPort("PORT", defaultPort)
+	port, bindAddress, err := resolveListener()
 	if err != nil {
 		return nil, err
 	}
@@ -506,6 +602,7 @@ func loadFromEnv() (*Config, error) {
 		MetricsUsername:        metricsUsername,
 		MetricsPassword:        metricsPassword,
 		Port:                   port,
+		BindAddress:            bindAddress,
 		SentryDSN:              envString("SENTRY_DSN"),
 		RetentionSweepInterval: retentionSweepInterval,
 		SessionIdleTimeout:     sessionIdleTimeout,
@@ -625,6 +722,11 @@ 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,

internal/config/env_test.go

@@ -21,6 +21,22 @@ const (
 	envKeyPort            = "PORT"
 	envKeyDebug           = "DEBUG"
 	envKeyMaintenanceMode = "MAINTENANCE_MODE"
+	envKeyBindAddress     = "BIND_ADDRESS"
+)
+
+// Sample BIND_ADDRESS values used by the tables below.
+const (
+	// bindAddressDefault is the shipped default. It is asserted
+	// against the package's own constant in
+	// TestNewUsesDefaultsWhenUnset, so the two cannot drift.
+	bindAddressDefault = "127.0.0.1"
+
+	// bindAddressWildcard is the value a container deployment sets.
+	bindAddressWildcard = "0.0.0.0"
+
+	// bindAddressSample is an arbitrary specific address, standing
+	// for "one interface of several".
+	bindAddressSample = "10.1.2.3"
 )
 
 // envBoolCase is one row of the envBool table.
@@ -291,6 +307,160 @@ func TestEnvPort(t *testing.T) {
 	}
 }
 
+// TestEnvBindAddress covers BIND_ADDRESS parsing.
+//
+// Only IP address literals are accepted. Every rejection below is a
+// value an operator plausibly writes — a hostname, a host:port, a
+// CIDR block — and each has to abort startup rather than fall back to
+// the default, because falling back would bind an address other than
+// the one asked for and, in the wildcard-default case this setting
+// exists to end, publish cleartext on every interface.
+func TestEnvBindAddress(t *testing.T) {
+	for _, tt := range envBindAddressCases() {
+		t.Run(tt.name, func(t *testing.T) {
+			// Cannot use t.Parallel() here because t.Setenv
+			// is incompatible with parallel subtests.
+			if tt.set {
+				t.Setenv(testEnvKey, tt.value)
+			} else {
+				require.NoError(t, os.Unsetenv(testEnvKey))
+			}
+
+			got, err := config.EnvBindAddressForTest(
+				testEnvKey, bindAddressDefault,
+			)
+
+			if tt.expectError {
+				require.Error(t, err)
+				require.ErrorIs(t, err, config.ErrInvalidBindAddress)
+				assert.Contains(t, err.Error(), testEnvKey)
+				assert.Contains(t, err.Error(), tt.value)
+
+				return
+			}
+
+			require.NoError(t, err)
+			assert.Equal(t, tt.expected, got)
+		})
+	}
+}
+
+// envBindAddressCase is one row of the envBindAddress table.
+type envBindAddressCase struct {
+	name        string
+	set         bool
+	value       string
+	expectError bool
+	expected    string
+}
+
+// envBindAddressCases is the envBindAddress table, kept out of the
+// test body so the test itself stays readable.
+func envBindAddressCases() []envBindAddressCase {
+	return append(
+		envBindAddressAcceptedCases(),
+		envBindAddressRejectedCases()...,
+	)
+}
+
+// envBindAddressAcceptedCases are the values that parse: the three
+// spellings of "unset" that take the default, and the literals.
+func envBindAddressAcceptedCases() []envBindAddressCase {
+	return []envBindAddressCase{
+		{
+			name:     "unset returns the default",
+			expected: bindAddressDefault,
+		},
+		{
+			name:     "empty returns the default",
+			set:      true,
+			value:    "",
+			expected: bindAddressDefault,
+		},
+		{
+			name:     "whitespace returns the default",
+			set:      true,
+			value:    "   ",
+			expected: bindAddressDefault,
+		},
+		{
+			name:     "ipv4 wildcard is parsed",
+			set:      true,
+			value:    bindAddressWildcard,
+			expected: bindAddressWildcard,
+		},
+		{
+			name:     "ipv4 literal is parsed",
+			set:      true,
+			value:    bindAddressSample,
+			expected: bindAddressSample,
+		},
+		{
+			name:     "surrounding whitespace is trimmed",
+			set:      true,
+			value:    "  " + bindAddressSample + "  ",
+			expected: bindAddressSample,
+		},
+		{
+			name:     "ipv6 wildcard is parsed",
+			set:      true,
+			value:    "::",
+			expected: "::",
+		},
+		{
+			name:     "ipv6 literal is parsed",
+			set:      true,
+			value:    "2001:db8::5",
+			expected: "2001:db8::5",
+		},
+	}
+}
+
+// envBindAddressRejectedCases are the values that abort startup.
+// Each is something an operator plausibly writes, and none may fall
+// back to the default: the default is loopback, so a silent fallback
+// would bind somewhere other than what was asked for.
+func envBindAddressRejectedCases() []envBindAddressCase {
+	return []envBindAddressCase{
+		{
+			name:        "garbage is rejected",
+			set:         true,
+			value:       "not-an-address",
+			expectError: true,
+		},
+		{
+			name:        "hostname is rejected",
+			set:         true,
+			value:       "localhost",
+			expectError: true,
+		},
+		{
+			name:        "unresolvable hostname is rejected",
+			set:         true,
+			value:       "no-such-host.invalid",
+			expectError: true,
+		},
+		{
+			name:        "host and port is rejected",
+			set:         true,
+			value:       bindAddressDefault + ":8080",
+			expectError: true,
+		},
+		{
+			name:        "bracketed ipv6 is rejected",
+			set:         true,
+			value:       "[::1]",
+			expectError: true,
+		},
+		{
+			name:        "CIDR block is rejected",
+			set:         true,
+			value:       "10.0.0.0/8",
+			expectError: true,
+		},
+	}
+}
+
 // buildConfig constructs a Config through fx exactly as the
 // application does, returning the config and any construction error.
 func buildConfig(t *testing.T) (*config.Config, error) {
@@ -312,13 +482,45 @@ func buildConfig(t *testing.T) (*config.Config, error) {
 }
 
 func TestNewRejectsBadEnvValues(t *testing.T) {
-	tests := []struct {
-		name        string
-		key         string
-		value       string
-		expectError bool
-		check       func(t *testing.T, cfg *config.Config)
-	}{
+	for _, tt := range badEnvValueCases() {
+		t.Run(tt.name, func(t *testing.T) {
+			// Cannot use t.Parallel() here because t.Setenv
+			// is incompatible with parallel subtests.
+			t.Setenv("WEBHOOKER_ENVIRONMENT", "dev")
+			t.Setenv(tt.key, tt.value)
+
+			cfg, err := buildConfig(t)
+
+			if tt.expectError {
+				require.Error(t, err)
+				assert.Contains(t, err.Error(), tt.key)
+				assert.Contains(t, err.Error(), tt.value)
+
+				return
+			}
+
+			require.NoError(t, err)
+			require.NotNil(t, cfg)
+			tt.check(t, cfg)
+		})
+	}
+}
+
+// badEnvValueCase is one row of the config.New table: a variable, the
+// value it is set to, and either the assertion that startup fails
+// naming both, or a check on the Config that resulted.
+type badEnvValueCase struct {
+	name        string
+	key         string
+	value       string
+	expectError bool
+	check       func(t *testing.T, cfg *config.Config)
+}
+
+// badEnvValueCases is the config.New table, kept out of the test body
+// so the test itself stays readable.
+func badEnvValueCases() []badEnvValueCase {
+	return []badEnvValueCase{
 		{
 			name:  "valid PORT is used",
 			key:   envKeyPort,
@@ -361,29 +563,35 @@ func TestNewRejectsBadEnvValues(t *testing.T) {
 			value:       "sometimes",
 			expectError: true,
 		},
-	}
-
-	for _, tt := range tests {
-		t.Run(tt.name, func(t *testing.T) {
-			// Cannot use t.Parallel() here because t.Setenv
-			// is incompatible with parallel subtests.
-			t.Setenv("WEBHOOKER_ENVIRONMENT", "dev")
-			t.Setenv(tt.key, tt.value)
-
-			cfg, err := buildConfig(t)
-
-			if tt.expectError {
-				require.Error(t, err)
-				assert.Contains(t, err.Error(), tt.key)
-				assert.Contains(t, err.Error(), tt.value)
-
-				return
-			}
-
-			require.NoError(t, err)
-			require.NotNil(t, cfg)
-			tt.check(t, cfg)
-		})
+		{
+			name:  "valid BIND_ADDRESS is used",
+			key:   envKeyBindAddress,
+			value: bindAddressWildcard,
+			check: func(t *testing.T, cfg *config.Config) {
+				t.Helper()
+				assert.Equal(
+					t, bindAddressWildcard, cfg.BindAddress,
+				)
+			},
+		},
+		{
+			name:        "unparseable BIND_ADDRESS aborts startup",
+			key:         envKeyBindAddress,
+			value:       "not-an-address",
+			expectError: true,
+		},
+		{
+			name:        "hostname BIND_ADDRESS aborts startup",
+			key:         envKeyBindAddress,
+			value:       "localhost",
+			expectError: true,
+		},
+		{
+			name:        "BIND_ADDRESS with a port aborts startup",
+			key:         envKeyBindAddress,
+			value:       bindAddressDefault + ":8080",
+			expectError: true,
+		},
 	}
 }
 
@@ -395,6 +603,7 @@ func TestNewUsesDefaultsWhenUnset(t *testing.T) {
 
 	for _, key := range []string{
 		envKeyPort, envKeyDebug, envKeyMaintenanceMode,
+		envKeyBindAddress,
 	} {
 		require.NoError(t, os.Unsetenv(key))
 	}
@@ -406,4 +615,15 @@ func TestNewUsesDefaultsWhenUnset(t *testing.T) {
 	assert.Equal(t, 8080, cfg.Port)
 	assert.False(t, cfg.Debug)
 	assert.False(t, cfg.MaintenanceMode)
+
+	// Loopback, not the wildcard: the default must not publish the
+	// cleartext admin UI and the unauthenticated receiver on every
+	// interface of a host that configured nothing. The value is read
+	// from the package rather than repeated, so the README's
+	// documented default and the compiled-in one are pinned to the
+	// same constant.
+	assert.Equal(
+		t, config.DefaultBindAddressForTest, cfg.BindAddress,
+	)
+	assert.Equal(t, bindAddressDefault, cfg.BindAddress)
 }

internal/config/export_test.go

@@ -50,3 +50,13 @@ func EnvPositiveIntForTest(key string, defaultValue int) (int, error) {
 func EnvPortForTest(key string, defaultValue int) (int, error) {
 	return envPort(key, defaultValue)
 }
+
+// EnvBindAddressForTest exposes envBindAddress.
+func EnvBindAddressForTest(key, defaultValue string) (string, error) {
+	return envBindAddress(key, defaultValue)
+}
+
+// DefaultBindAddressForTest exposes the compiled-in BIND_ADDRESS
+// default, so a test pins the documented value rather than repeating
+// a literal that could drift from it.
+const DefaultBindAddressForTest = defaultBindAddress