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, defaulting to 127.0.0.1.
Loopback is the only default that does not silently expose that
listener; reaching webhooker from another host becomes a deliberate
act. A container must set BIND_ADDRESS=0.0.0.0, since a loopback bind
inside a network namespace is unreachable even with -p, and the
documented `docker run` does. 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. Two goroutines reached that field with nothing ordering
them — the serving goroutine assigning it, the fx stop hook calling
Shutdown on it — which is a data race the race detector reports as
soon as anything starts and stops the server, and a nil dereference if
a stop arrived first.

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.

README.md

@@ -104,6 +104,7 @@ TTY detection, and security headers are always applied.
 | ----------------------- | ----------------------------------- | -------- |
 | `WEBHOOKER_ENVIRONMENT` | `dev` or `prod`                     | `dev`    |
 | `PORT`                  | HTTP listen port                    | `8080`   |
+| `BIND_ADDRESS`          | IP address the HTTP listener binds. Loopback by default, so the cleartext listener is not published on every interface. Containers must set `0.0.0.0`. See [Bind address](#bind-address) | `127.0.0.1` |
 | `DATA_DIR`              | Directory for all SQLite databases  | `/var/lib/webhooker` |
 | `DEBUG`                 | Enable debug logging                | `false`  |
 | `MAINTENANCE_MODE`      | Report `maintenanceMode: true` in the healthcheck JSON. It does not change how any request is served — no maintenance page exists | `false`  |
@@ -225,6 +226,58 @@ A set but unparseable value aborts startup. When the list is non-empty
 webhooker logs it at startup, blocks and all, so the hole is visible in
 the log of any deployment that has one.
 
+#### Bind address
+
+`BIND_ADDRESS` is the IP address the HTTP listener binds. It defaults
+to `127.0.0.1`, so out of the box webhooker is reachable only from the
+host it runs on.
+
+That listener speaks **cleartext**, and it serves both the admin UI and
+the unauthenticated webhook receiver. webhooker terminates no TLS
+itself; a production deployment puts a reverse proxy in front of it
+(see
+[Deployment behind a reverse proxy](#deployment-behind-a-reverse-proxy)),
+and the proxy reaches it over loopback. A default that bound every
+interface would leave that cleartext port answering the internet
+alongside the proxy — the admin login form and the receiver, in the
+clear, on a port nobody chose to publish. Reaching webhooker from
+another host is therefore something you configure, not something you
+get by default.
+
+**A container must set `BIND_ADDRESS=0.0.0.0`.** A process bound to
+loopback inside a container is unreachable from outside its network
+namespace even with `-p`, because the published port maps to the
+container's external address and nothing is listening there. The
+container's namespace is its own boundary, and `-p` is the exposure
+decision. The `docker run` command in
+[Running with Docker](#running-with-docker) sets it.
+
+The value must be an IP address literal:
+
+- `127.0.0.1` — loopback only (the default). Use this with a reverse
+  proxy on the same host.
+- `0.0.0.0` — every IPv4 address. Required in a container; on a bare
+  host, only with a firewall in front of the port.
+- `::` — every address, IPv6 and (on Linux, with the default
+  `net.ipv6.bindv6only=0`) IPv4 as well.
+- A specific address such as `10.0.0.5` — that interface only.
+
+Hostnames are **not** accepted. `localhost` aborts startup rather than
+being resolved: which of `127.0.0.1` and `::1` it means differs by
+host, a name can resolve to several addresses of which only one could
+be bound, and the answer can change under a running process. A value
+carrying a port (`127.0.0.1:8080`) is likewise rejected — the port is
+`PORT`'s business. Any unparseable value aborts startup; see
+[Invalid values abort startup](#invalid-values-abort-startup).
+
+An address that parses but is not assigned to this host — say
+`10.0.0.5` on a machine that has no such interface — is a valid
+literal, so it reaches the listener and fails there. The process logs
+the bind error and exits non-zero rather than staying up with nothing
+listening. The effective value is in the `bindAddress` field of the
+startup log line, which is the way to check what a running deployment
+actually bound.
+
 #### Metrics credentials
 
 `METRICS_USERNAME` and `METRICS_PASSWORD` are set together or not at
@@ -382,8 +435,11 @@ additionally be a number in the range 1–65535,
 `RECEIVER_RATE_LIMIT` must be at least 1,
 `RETENTION_SWEEP_INTERVAL` must be greater than zero (it is a ticker
 period, so `0s` or a negative value would crash the reaper after
-startup), and every entry in `TRUSTED_PROXIES` and
+startup), every entry in `TRUSTED_PROXIES` and
-`ALLOWED_EGRESS_CIDRS` must be a CIDR block or a bare IP address.
+`ALLOWED_EGRESS_CIDRS` must be a CIDR block or a bare IP address, and
+`BIND_ADDRESS` must be an IP address literal — `localhost`,
+`127.0.0.1:8080` and `10.0.0.0/8` are each rejected rather than
+resolved, split, or narrowed to something they do not say.
 `SESSION_IDLE_TIMEOUT` is the exception: a
 non-positive value there means idle expiry is disabled, not invalid.
 
@@ -528,12 +584,32 @@ decision:
 
 ```bash
 docker run -d \
-  -p 8080:8080 \
+  -p 127.0.0.1:8080:8080 \
   -v /path/to/data:/var/lib/webhooker \
   -e WEBHOOKER_ENVIRONMENT=prod \
+  -e BIND_ADDRESS=0.0.0.0 \
   webhooker:latest
 ```
 
+`BIND_ADDRESS=0.0.0.0` is **required** in a container and is not the
+default. webhooker binds loopback unless told otherwise (see
+[Bind address](#bind-address)), and a loopback-bound process inside a
+container is unreachable from outside its network namespace even with
+`-p`: the port is published and nothing answers on it. The container's
+health check fails too — it requests `http://localhost:8080`, and
+`localhost` resolves to `::1` first, which a `127.0.0.1` bind is not
+listening on — so the container goes `unhealthy` about 95 seconds
+after start. A container that is `unhealthy` with `connection refused`
+in its health log, or a published port that resets connections, is
+this.
+
+Publishing to `127.0.0.1:8080` rather than `8080` keeps Docker from
+opening the cleartext port on every interface of the host, which is
+what a bare `-p 8080:8080` does — including through firewall rules,
+since Docker's forwarding rules are inserted ahead of most host
+firewalls. Bind it to the host address your reverse proxy connects
+from, and nothing wider.
+
 The container runs as a non-root user (`webhooker`, UID 1000), exposes
 port 8080, and includes a health check against
 `/.well-known/healthcheck`. The `/var/lib/webhooker` volume holds all
@@ -543,6 +619,137 @@ databases written by `database` targets (`archive-{uuid}.db`). Mount
 this as a persistent volume to preserve data across container
 restarts.
 
+## Deployment behind a reverse proxy
+
+webhooker terminates no TLS of its own. It serves plaintext HTTP and
+expects a reverse proxy in front of it, which is the deployment it is
+built for: the proxy holds the certificate, and webhooker binds
+loopback where only the proxy can reach it.
+
+Five things have to be right. Each one is silent when it is wrong —
+the service comes up, serves pages, and is broken in a way nothing
+reports.
+
+1. **Bind or firewall the app port.** `BIND_ADDRESS` defaults to
+   `127.0.0.1` so the cleartext listener is not published beside the
+   proxy. If you must widen it — a container, or a proxy on another
+   host — firewall the port to the proxy's address. A cleartext port
+   answering the internet serves the admin login form and the
+   unauthenticated receiver with no TLS at all, and the proxy in front
+   of it changes nothing about that.
+2. **Set `WEBHOOKER_ENVIRONMENT=prod`.** It defaults to `dev`, and the
+   session cookie's `Secure` flag depends on it — see the table in
+   [Configuration](#configuration). Left at the default, a session
+   cookie can be sent over plaintext HTTP.
+3. **Set `TRUSTED_PROXIES` to the proxy's address.** Unset, every rate
+   limiter keys on the connecting peer, which behind a proxy is the
+   proxy on every request: all clients collapse into one global bucket
+   per limit and the receiver's per-IP limits become service-wide
+   ceilings. See [Trusted proxies](#trusted-proxies). List the proxy
+   and nothing else.
+4. **Send `Host` as `$http_host`, not `$host`.** `$host` strips the
+   port. webhooker's Origin/Referer check compares against the host it
+   was given, so on any port other than 443 `$host` makes every form
+   POST — including login — fail with `403 origin invalid`, with
+   nothing in the error naming the cause.
+5. **Keep the proxy's access log.** webhooker's own access log records
+   the peer address, which behind a proxy is always the proxy. The
+   proxy's log is the only record of which client sent what. nginx's
+   default `combined` format already logs `$remote_addr`; do not
+   replace it with one that drops the client address, and retain those
+   logs as long as you would want to answer a question about traffic.
+
+### nginx
+
+Complete server block. Replace the `server_name` and the two
+certificate paths.
+
+```nginx
+server {
+    listen 443 ssl;
+    listen [::]:443 ssl;
+    http2 on;  # nginx 1.25.1+; older: listen 443 ssl http2;
+
+    server_name webhooker.example.com;
+
+    ssl_certificate     /etc/ssl/certs/webhooker.example.com.crt;
+    ssl_certificate_key /etc/ssl/private/webhooker.example.com.key;
+    ssl_protocols       TLSv1.2 TLSv1.3;
+
+    # webhooker caps form POST bodies at 1 MB. nginx's default happens
+    # to match, so leaving this out breaks nothing today — but if you
+    # ever raise webhooker's cap, this is the limit you will still be
+    # hitting, and the rejection is nginx's HTML page rather than
+    # webhooker's message.
+    client_max_body_size 1m;
+
+    # $remote_addr is the client. webhooker's own log records this
+    # proxy and nothing else, so this file is the only place the
+    # client's address is written down.
+    access_log /var/log/nginx/webhooker.access.log combined;
+
+    location / {
+        # A literal address, not localhost: with BIND_ADDRESS at its
+        # 127.0.0.1 default, a localhost that resolves to ::1 first
+        # gets connection refused.
+        proxy_pass http://127.0.0.1:8080;
+
+        # $http_host, NOT $host. $host drops the port and every form
+        # POST fails with 403 origin invalid on any port but 443.
+        proxy_set_header Host              $http_host;
+        proxy_set_header X-Forwarded-For   $proxy_add_x_forwarded_for;
+        proxy_set_header X-Forwarded-Proto $scheme;
+
+        # Above webhooker's own 60s request timeout, so its 503
+        # reaches the client instead of nginx cutting the connection
+        # first and answering 504.
+        proxy_read_timeout 70s;
+    }
+}
+
+server {
+    listen 80;
+    listen [::]:80;
+    server_name webhooker.example.com;
+    return 308 https://$host$request_uri;
+}
+```
+
+`X-Forwarded-For` must be **appended** to, which
+`$proxy_add_x_forwarded_for` does. webhooker reads no other forwarded
+client header: `X-Real-IP` and `True-Client-IP` are ignored from every
+peer, so setting them has no effect. See
+[Trusted proxies](#trusted-proxies) for how the chain is walked.
+
+`X-Forwarded-Proto: https` is what tells webhooker the request arrived
+over TLS, which decides the CSRF cookie's `Secure` flag and the strict
+Origin/Referer mode. Without it, requests are treated as plaintext.
+
+With that block, webhooker's environment is:
+
+```sh
+WEBHOOKER_ENVIRONMENT=prod
+BIND_ADDRESS=127.0.0.1   # the default; stated here to be explicit
+TRUSTED_PROXIES=127.0.0.1
+```
+
+If nginx runs on another host, `BIND_ADDRESS` becomes the address it
+connects to, `TRUSTED_PROXIES` becomes nginx's address, and the port
+must be firewalled to that address — the traffic between them is
+cleartext.
+
+### HSTS is always sent, and is not configurable
+
+Every response carries
+`Strict-Transport-Security: max-age=63072000; includeSubDomains; preload`.
+Two years, every subdomain, and a `preload` token. There is no setting
+that changes or suppresses it.
+
+This is worth knowing before the first request reaches a browser: a
+client that sees it once will refuse plaintext HTTP to that hostname —
+and to every subdomain of it — for two years, whatever else is served
+there. Terminate TLS on a hostname you are prepared to keep on HTTPS.
+
 ## Backup, Restore, and Upgrades
 
 ### What to back up
@@ -678,6 +885,16 @@ Upgrade procedure:
 4. Confirm `database migrations completed` in the logs before putting
    traffic back on it.
 
+**Upgrading past the introduction of `BIND_ADDRESS`:** earlier versions
+always bound every interface. The listener now binds `127.0.0.1` unless
+`BIND_ADDRESS` says otherwise, so a deployment that relied on the old
+behaviour becomes unreachable from other hosts until it sets one. In a
+container that means `BIND_ADDRESS=0.0.0.0`; without it the container
+also goes `unhealthy`, since its health check reaches the app over
+`localhost`, which resolves to `::1` before `127.0.0.1`. On a bare
+host, set the address the proxy connects to. See
+[Bind address](#bind-address).
+
 **Downgrade is unsupported.** Once a newer binary has migrated the files
 there is no way to move them back. `AutoMigrate` is additive — it adds
 tables, columns and indexes and never drops or rewrites them — so an

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,

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 {
+	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,
 		},
-	}
+		{
-
+			name:  "valid BIND_ADDRESS is used",
-	for _, tt := range tests {
+			key:   envKeyBindAddress,
-		t.Run(tt.name, func(t *testing.T) {
+			value: bindAddressWildcard,
-			// Cannot use t.Parallel() here because t.Setenv
+			check: func(t *testing.T, cfg *config.Config) {
-			// is incompatible with parallel subtests.
+				t.Helper()
-			t.Setenv("WEBHOOKER_ENVIRONMENT", "dev")
+				assert.Equal(
-			t.Setenv(tt.key, tt.value)
+					t, bindAddressWildcard, cfg.BindAddress,
-
+				)
-			cfg, err := buildConfig(t)
+			},
-
+		},
-			if tt.expectError {
+		{
-				require.Error(t, err)
+			name:        "unparseable BIND_ADDRESS aborts startup",
-				assert.Contains(t, err.Error(), tt.key)
+			key:         envKeyBindAddress,
-				assert.Contains(t, err.Error(), tt.value)
+			value:       "not-an-address",
-
+			expectError: true,
-				return
+		},
-			}
+		{
-
+			name:        "hostname BIND_ADDRESS aborts startup",
-			require.NoError(t, err)
+			key:         envKeyBindAddress,
-			require.NotNil(t, cfg)
+			value:       "localhost",
-			tt.check(t, cfg)
+			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

internal/server/bind_address_test.go

@@ -0,0 +1,336 @@
+package server_test
+
+import (
+	"context"
+	"net"
+	"net/http"
+	"strconv"
+	"testing"
+	"time"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+	"go.uber.org/fx"
+	"sneak.berlin/go/webhooker/internal/config"
+	"sneak.berlin/go/webhooker/internal/globals"
+	"sneak.berlin/go/webhooker/internal/server"
+)
+
+const (
+	// loopbackV4 is the shipped BIND_ADDRESS default.
+	loopbackV4 = "127.0.0.1"
+
+	// wildcardV4 is the value a container deployment must set,
+	// where a loopback-bound process is unreachable from outside
+	// its network namespace even with a published port.
+	wildcardV4 = "0.0.0.0"
+
+	// unavailableAddr is a TEST-NET-1 address (RFC 5737). It is a
+	// well-formed literal that no host is assigned, so binding it
+	// fails with EADDRNOTAVAIL rather than succeeding somewhere
+	// unexpected.
+	unavailableAddr = "192.0.2.1"
+
+	// listenReadyTimeout bounds the wait for the listener to accept
+	// connections. The bind itself is immediate; this only covers
+	// goroutine scheduling.
+	listenReadyTimeout = 3 * time.Second
+
+	// listenPollInterval is how often the readiness wait retries.
+	listenPollInterval = 10 * time.Millisecond
+
+	// dialTimeout bounds a single connection attempt in these
+	// tests. Everything dialled here is on this host, so a dial
+	// that is not answered immediately is a failure, not slowness.
+	dialTimeout = time.Second
+)
+
+// freePort returns a TCP port that is free on every local address at
+// the moment it returns, by taking one on the wildcard and releasing
+// it. The window between release and re-bind is the standard one
+// every "pick a free port" helper carries.
+func freePort(t *testing.T) int {
+	t.Helper()
+
+	var listenCfg net.ListenConfig
+
+	l, err := listenCfg.Listen(t.Context(), "tcp", "0.0.0.0:0")
+	require.NoError(t, err)
+
+	addr, ok := l.Addr().(*net.TCPAddr)
+	require.True(t, ok, "listener is not TCP")
+	require.NoError(t, l.Close())
+
+	return addr.Port
+}
+
+// otherLocalAddr returns a local IPv4 address that is not
+// loopbackV4, or skips the test when the host has none.
+//
+// The bind-address tests need a second address of this host to stand
+// in for "another interface": what a wildcard bind claims and a
+// loopback bind does not. 127.0.0.2 is that address on Linux, where
+// the whole 127.0.0.0/8 is local; elsewhere an interface address is
+// used instead. Each candidate is proven bindable before it is
+// returned, so a host that offers neither skips rather than fails on
+// something that was never about the code under test.
+func otherLocalAddr(t *testing.T) string {
+	t.Helper()
+
+	candidates := []string{"127.0.0.2"}
+
+	ifaceAddrs, err := net.InterfaceAddrs()
+	require.NoError(t, err)
+
+	for _, a := range ifaceAddrs {
+		ipNet, ok := a.(*net.IPNet)
+		if !ok {
+			continue
+		}
+
+		ip4 := ipNet.IP.To4()
+		if ip4 == nil || ip4.String() == loopbackV4 {
+			continue
+		}
+
+		candidates = append(candidates, ip4.String())
+	}
+
+	var listenCfg net.ListenConfig
+
+	for _, candidate := range candidates {
+		l, listenErr := listenCfg.Listen(
+			t.Context(), "tcp", net.JoinHostPort(candidate, "0"),
+		)
+		if listenErr != nil {
+			continue
+		}
+
+		require.NoError(t, l.Close())
+
+		return candidate
+	}
+
+	t.Skip("host has no second local IPv4 address to bind")
+
+	return ""
+}
+
+// startBoundServer starts the wired app with the given bind address
+// on a free port and returns that port. The app is stopped on
+// cleanup.
+func startBoundServer(t *testing.T, bindAddress string) int {
+	t.Helper()
+
+	port := freePort(t)
+
+	env := newTestEnv(t)
+	env.cfg.BindAddress = bindAddress
+	env.cfg.Port = port
+
+	app := fx.New(
+		fx.NopLogger,
+		fx.Supply(env.log, env.cfg, env.mw, env.hnd),
+		fx.Provide(globals.New, server.New),
+		fx.Invoke(func(*server.Server) {}),
+	)
+
+	startCtx, cancelStart := context.WithTimeout(
+		context.Background(), lifecycleTimeout,
+	)
+	defer cancelStart()
+
+	require.NoError(t, app.Start(startCtx))
+
+	t.Cleanup(func() {
+		stopCtx, cancelStop := context.WithTimeout(
+			context.Background(), lifecycleTimeout,
+		)
+		defer cancelStop()
+
+		require.NoError(t, app.Stop(stopCtx))
+	})
+
+	return port
+}
+
+// dialable reports whether a TCP connection to addr succeeds.
+func dialable(ctx context.Context, addr string) bool {
+	dialer := net.Dialer{Timeout: dialTimeout}
+
+	conn, err := dialer.DialContext(ctx, "tcp", addr)
+	if err != nil {
+		return false
+	}
+
+	_ = conn.Close()
+
+	return true
+}
+
+// requireDialable waits for addr to accept connections, failing the
+// test if it never does.
+func requireDialable(t *testing.T, addr string) {
+	t.Helper()
+
+	deadline := time.Now().Add(listenReadyTimeout)
+	for time.Now().Before(deadline) {
+		if dialable(t.Context(), addr) {
+			return
+		}
+
+		time.Sleep(listenPollInterval)
+	}
+
+	t.Fatalf("nothing accepted connections on %s", addr)
+}
+
+// TestListenAddr pins how BindAddress and Port are rendered into the
+// listen address.
+//
+// The defect this covers was a bare fmt.Sprintf(":%d", port), which
+// binds every interface with no way to say otherwise. The IPv6 rows
+// are here because an unbracketed IPv6 host would produce an address
+// net.Listen rejects, turning a valid configuration into a startup
+// failure.
+func TestListenAddr(t *testing.T) {
+	t.Parallel()
+
+	tests := []struct {
+		name        string
+		bindAddress string
+		port        int
+		expected    string
+	}{
+		{
+			name:        "loopback default",
+			bindAddress: loopbackV4,
+			port:        8080,
+			expected:    "127.0.0.1:8080",
+		},
+		{
+			name:        "ipv4 wildcard",
+			bindAddress: wildcardV4,
+			port:        8080,
+			expected:    "0.0.0.0:8080",
+		},
+		{
+			name:        "ipv6 wildcard is bracketed",
+			bindAddress: "::",
+			port:        8080,
+			expected:    "[::]:8080",
+		},
+		{
+			name:        "ipv6 literal is bracketed",
+			bindAddress: "2001:db8::5",
+			port:        9001,
+			expected:    "[2001:db8::5]:9001",
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			t.Parallel()
+
+			assert.Equal(t, tt.expected, server.ListenAddrForTest(
+				&config.Config{
+					BindAddress: tt.bindAddress,
+					Port:        tt.port,
+				},
+			))
+		})
+	}
+}
+
+// TestBindAddress_LoopbackIsNotOnOtherAddresses proves the fix end to
+// end: with BIND_ADDRESS at its loopback default, the cleartext
+// listener answers on loopback and has not claimed any other address
+// of this host.
+//
+// The second address is proven free by binding it on the same port
+// while the server runs. That is the assertion that fails against the
+// old wildcard bind — a wildcard listener owns the port on every
+// address, so this bind would return EADDRINUSE. Dialling from
+// another machine is what the operator cares about, and this is the
+// in-process form of it: the socket the remote host would connect to
+// does not exist.
+func TestBindAddress_LoopbackIsNotOnOtherAddresses(t *testing.T) {
+	t.Parallel()
+
+	other := otherLocalAddr(t)
+	port := startBoundServer(t, loopbackV4)
+
+	// Positive control: the service really is up and serving.
+	requireDialable(t, net.JoinHostPort(loopbackV4, strconv.Itoa(port)))
+
+	var listenCfg net.ListenConfig
+
+	l, err := listenCfg.Listen(
+		t.Context(), "tcp",
+		net.JoinHostPort(other, strconv.Itoa(port)),
+	)
+	require.NoError(
+		t, err,
+		"port %d on %s is taken while bound to %s: the listener "+
+			"claimed more than its configured address",
+		port, other, loopbackV4,
+	)
+
+	require.NoError(t, l.Close())
+}
+
+// TestBindAddress_WildcardReachesOtherAddresses is the counterpart:
+// the value a container deployment sets does reach the addresses the
+// default withholds. Without this, a loopback-only bind would pass
+// the test above by never listening at all.
+func TestBindAddress_WildcardReachesOtherAddresses(t *testing.T) {
+	t.Parallel()
+
+	other := otherLocalAddr(t)
+	port := startBoundServer(t, wildcardV4)
+
+	requireDialable(t, net.JoinHostPort(other, strconv.Itoa(port)))
+}
+
+// TestBindAddress_ServesRequestsOnConfiguredAddress proves the bound
+// listener serves the application rather than merely accepting TCP,
+// so a bind address that is honoured cannot be mistaken for one that
+// is honoured and broken.
+func TestBindAddress_ServesRequestsOnConfiguredAddress(t *testing.T) {
+	t.Parallel()
+
+	port := startBoundServer(t, loopbackV4)
+	addr := net.JoinHostPort(loopbackV4, strconv.Itoa(port))
+	requireDialable(t, addr)
+
+	req, err := http.NewRequestWithContext(
+		t.Context(), http.MethodGet,
+		"http://"+addr+"/.well-known/healthcheck", nil,
+	)
+	require.NoError(t, err)
+
+	client := &http.Client{Timeout: dialTimeout}
+
+	resp, err := client.Do(req)
+	require.NoError(t, err)
+
+	defer func() { _ = resp.Body.Close() }()
+
+	assert.Equal(t, http.StatusOK, resp.StatusCode)
+}
+
+// TestBindAddress_UnavailableAddressShutsDownTheApp covers the half
+// of the fail-loud rule that configuration parsing cannot reach. A
+// syntactically valid address that is not assigned to this host
+// parses fine and fails at bind time, after fx has already reported
+// RUNNING. It must end the process non-zero rather than leave it
+// alive with nothing listening.
+func TestBindAddress_UnavailableAddressShutsDownTheApp(t *testing.T) {
+	t.Parallel()
+
+	env := newTestEnv(t)
+	env.cfg.BindAddress = unavailableAddr
+	env.cfg.Port = freePort(t)
+
+	requireListenFailureExit(t, env)
+}

internal/server/export_test.go

@@ -55,6 +55,16 @@ func NewRouterForTest(
 	return s.router
 }
 
+// ListenAddrForTest exposes the address the HTTP listener binds for
+// a given Config, so the rendering of host and port — IPv6
+// bracketing above all — can be pinned without standing up a
+// listener.
+func ListenAddrForTest(cfg *config.Config) string {
+	s := &Server{params: ServerParams{Config: cfg}}
+
+	return s.listenAddr()
+}
+
 // ProbePattern is the route NewRouterWithProbeForTest adds to the
 // production route tree.
 const ProbePattern = "/probe"

internal/server/http.go

@@ -2,8 +2,9 @@ package server
 
 import (
 	"errors"
-	"fmt"
+	"net"
 	"net/http"
+	"strconv"
 	"time"
 )
 
@@ -24,21 +25,47 @@ const (
 	httpMaxHeaderBytes = 1 << 20
 )
 
-func (s *Server) serveUntilShutdown() {
+// listenAddr renders the address the HTTP listener binds.
-	listenAddr := fmt.Sprintf(":%d", s.params.Config.Port)
+//
-	s.httpServer = &http.Server{
+// The host half is always present: an empty host would be the
-		Addr:           listenAddr,
+// wildcard, and the whole point of BIND_ADDRESS is that binding every
+// interface is a choice the operator makes rather than one the
+// process makes for them. Config guarantees a literal, so
+// JoinHostPort's bracketing is enough to make IPv6 well formed.
+func (s *Server) listenAddr() string {
+	return net.JoinHostPort(
+		s.params.Config.BindAddress,
+		strconv.Itoa(s.params.Config.Port),
+	)
+}
+
+// newHTTPServer builds the HTTP server for this Server's
+// configuration.
+//
+// It is called from New, on the constructing goroutine, rather than
+// from the serving goroutine that used to assign s.httpServer
+// directly. Two goroutines reach that field — the serving goroutine
+// and the fx stop hook, which calls Shutdown on it — with nothing
+// ordering them. Constructing it during New puts the write before
+// every hook fx will later run, which both removes the race and rules
+// out the nil dereference a stop that arrived before the serving
+// goroutine had run would have caused.
+func (s *Server) newHTTPServer() *http.Server {
+	return &http.Server{
+		Addr:           s.listenAddr(),
 		ReadTimeout:    httpReadTimeout,
 		WriteTimeout:   httpWriteTimeout,
 		MaxHeaderBytes: httpMaxHeaderBytes,
 		Handler:        s,
 	}
+}
 
+func (s *Server) serveUntilShutdown() {
 	// add routes
 	// this does any necessary setup in each handler
 	s.SetupRoutes()
 
-	s.log.Info("http begin listen", "listenaddr", listenAddr)
+	s.log.Info("http begin listen", "listenaddr", s.httpServer.Addr)
 
 	err := s.httpServer.ListenAndServe()
 	if err != nil && !errors.Is(err, http.ErrServerClosed) {

internal/server/listen_failure_test.go

@@ -36,9 +36,9 @@ const lifecycleTimeout = 15 * time.Second
 //
 // The port is occupied by a listener this test holds open, on a
 // kernel-chosen port, so the failure is the real EADDRINUSE the
-// operator hits when a second instance starts. Loopback is enough to
+// operator hits when a second instance starts. The server is pointed
-// collide with the server's wildcard bind: a listening socket on a
+// at the same loopback address, so the collision is a direct one on
-// specific address blocks the wildcard from claiming the same port.
+// the exact address it asks the kernel for.
 func TestListenFailure_ShutsDownTheApp(t *testing.T) {
 	t.Parallel()
 
@@ -55,11 +55,27 @@ func TestListenFailure_ShutsDownTheApp(t *testing.T) {
 	require.True(t, ok, "listener is not TCP")
 
 	// The collaborators come from the wired graph rather than stubs,
-	// so the Server under test is the one that ships. Only the port
+	// so the Server under test is the one that ships. Only the
-	// is test-specific.
+	// listen address is test-specific.
 	env := newTestEnv(t)
+	env.cfg.BindAddress = loopbackV4
 	env.cfg.Port = addr.Port
 
+	requireListenFailureExit(t, env)
+}
+
+// requireListenFailureExit starts the wired app over env and asserts
+// that it gives up on its own with the listen-failure status, then
+// completes its stop sequence.
+//
+// Two different listen failures share it — a port already in use and
+// an address that is not on this host — because what has to hold for
+// both is the same: the failure is discovered after fx has already
+// reported RUNNING, so the only thing that can turn it into a visible
+// exit is the shutdown path under test.
+func requireListenFailureExit(t *testing.T, env *testEnv) {
+	t.Helper()
+
 	app := fx.New(
 		fx.NopLogger,
 		fx.Supply(env.log, env.cfg, env.mw, env.hnd),

internal/server/server.go

@@ -107,6 +107,7 @@ func New(lc fx.Lifecycle, params ServerParams) (*Server, error) {
 	s.mw = params.Middleware
 	s.h = params.Handlers
 	s.log = params.Logger.Get()
+	s.httpServer = s.newHTTPServer()
 
 	lc.Append(fx.Hook{
 		OnStart: func(_ context.Context) error {