Add a read-only Settings page for the loaded configuration (closes #402)

The page at /settings, behind the login and linked from the
navigation bar, lists every field of the configuration the server
started with: each by its environment variable name, with the
README's description and the value in effect. METRICS_PASSWORD and
SENTRY_DSN show only as set or not set; their values never reach the
template. The handlers now take the loaded Config from the dependency
graph.

Model: opus-5-5

README.md

@@ -145,6 +145,11 @@ TTY detection, and security headers are always applied.
 | `TRUSTED_PROXIES`       | CIDRs whose forwarded headers are trusted. A set value replaces the default. If any client can reach webhooker, or the proxy in front of it, from an RFC 1918 source address, set it to the proxy's address alone. See [Trusted proxies](#trusted-proxies) | `10.0.0.0/8,172.16.0.0/12,192.168.0.0/16` (RFC 1918) |
 | `ALLOWED_EGRESS_CIDRS`  | CIDRs that delivery targets may reach despite the SSRF blocklist. Read [Allowing egress to your own network](#allowing-egress-to-your-own-network) before setting it | `""` (none) |
 
+The Settings page of the web UI (`/settings`, behind the login) lists
+every one of these with the value the running server loaded. It is
+read-only, and it shows `METRICS_PASSWORD` and `SENTRY_DSN` only as
+set or not set, never their values.
+
 #### Allowing egress to your own network
 
 By default every delivery target must resolve to a public address. The
@@ -158,20 +163,19 @@ WireServer, which serves an Azure VM its credentials. Because it is a
 public address, listing it in `ALLOWED_EGRESS_CIDRS` reopens it.
 
 That is all the default blocklist covers: the IPv4 private and reserved
-ranges; of IPv6, only loopback (`::1`), the unspecified address (`::`),
-unique local addresses (`fc00::/7`), link-local addresses (`fe80::/10`),
-multicast (`ff00::/8`) and documentation space (`2001:db8::/32`); and
-certain public addresses. A public address belongs on the default
-blocklist only if it hands credentials, user data or bootstrap material
-to whatever can reach it, without the caller presenting anything. A
-provider's other public addresses are not refused. IBM Cloud, for
-example, serves its package mirrors, time servers and object storage on
-`161.26.0.0/16`, and the private endpoints of its own cloud services on
-`166.8.0.0/14`. Neither range hands out credentials that way: the token
-service among those endpoints issues a token only in exchange for
-something the caller presents, such as an API key. Reaching these
-services can be a legitimate delivery, and every cloud has some, so a
-partial list would promise coverage it does not give.
+ranges; of IPv6, only loopback (`::1`), unique local addresses
+(`fc00::/7`) and link-local addresses (`fe80::/10`); and certain public
+addresses. A public address belongs on the default blocklist only if it
+hands credentials, user data or bootstrap material to whatever can reach
+it, without the caller presenting anything. A provider's other public
+addresses are not refused. IBM Cloud, for example, serves its package
+mirrors, time servers and object storage on `161.26.0.0/16`, and the
+private endpoints of its own cloud services on `166.8.0.0/14`. Neither
+range hands out credentials that way: the token service among those
+endpoints issues a token only in exchange for something the caller
+presents, such as an API key. Reaching these services can be a
+legitimate delivery, and every cloud has some, so a partial list would
+promise coverage it does not give.
 
 That default is also inconvenient for the thing webhooker is mostly
 for: taking a public webhook and forwarding it to something on your own
@@ -211,16 +215,16 @@ Two things this setting cannot do:
   the list is always an allowlist; an empty list (the default) means
   every private and reserved range stays refused. Note that
   `0.0.0.0/0` gets you most of the way there anyway, per above.
-- **It cannot open link-local, the unspecified addresses, or a cloud
-  metadata endpoint at a non-public address that discloses credentials
-  or user data.** A metadata address is on the list below when it is not
-  a public address and both of these hold: the provider fixes it, so it
-  cannot collide with anything you run; and reaching it hands out
-  credentials, user data or bootstrap material. Those stay blocked no
-  matter what you list, including when you list them outright or list a
-  supernet such as `0.0.0.0/0`, `::/0`, `fd00::/8` or `100.64.0.0/10`.
-  Treat this as best effort rather than a guarantee — it is a
-  hand-maintained list and the caveat below the table applies:
+- **It cannot open link-local, or a cloud metadata endpoint at a
+  non-public address that discloses credentials or user data.** An
+  address is on the list below when it is not a public address and both
+  of these hold: the provider fixes it, so it cannot collide with
+  anything you run; and reaching it hands out credentials, user data or
+  bootstrap material. Those stay blocked no matter what you list,
+  including when you list them outright or list a supernet such as
+  `0.0.0.0/0`, `::/0`, `fd00::/8` or `100.64.0.0/10`. Treat this as best
+  effort rather than a guarantee — it is a hand-maintained list and the
+  caveat below the table applies:
 
   | Blocked unconditionally | What it is |
   | ----------------------- | ---------- |
@@ -234,22 +238,14 @@ Two things this setting cannot do:
   | `fd00:a9fe:a9fe::1/128` | Linode/Akamai metadata over IPv6 |
   | `100.100.100.200/32` | Alibaba Cloud metadata, inside CGNAT |
   | `192.0.0.192/32` | Oracle Cloud Classic metadata |
-  | `0.0.0.0/32` | IPv4 unspecified address, which reaches this host's loopback on Linux |
-  | `::/128` | IPv6 unspecified address, which reaches this host's loopback on Linux |
   | `::a9fe:a9fe/128` | `169.254.169.254` as an IPv4-compatible IPv6 address |
   | `64:ff9b::a9fe:a9fe/128` | `169.254.169.254` behind the NAT64 well-known prefix |
 
   The IPv4-mapped form `::ffff:169.254.169.254` is covered by the
-  `169.254.0.0/16` entry. Reaching any of these but the two unspecified
-  addresses is credential or user-data theft rather than delivery to an
-  internal service. Every entry outside the two link-local blocks is a
-  single address, so blocking it costs you nothing else on the network
-  around it.
-
-  The unspecified addresses `0.0.0.0` and `::` hand out nothing
-  themselves, but no host can have either, and on Linux a connection to
-  one reaches this host's own loopback. They are listed so that the only
-  way to open loopback is to name it, as `127.0.0.0/8` or `::1`.
+  `169.254.0.0/16` entry. Reaching any of these is credential or
+  user-data theft rather than delivery to an internal service. Every
+  entry outside the two link-local blocks is a single address, so
+  blocking it costs you nothing else on the network around it.
 
   The six ULA entries, all inside `fd00::/8`, are why this matters in
   practice: `fd00::/8` is an ordinary block to allowlist for your own
@@ -2789,6 +2785,7 @@ returns to the page that was asked for.
 | ------ | ------------------------ | ----------- |
 | `GET`  | `/user/{username}`       | User profile page |
 | `POST` | `/user/{username}/password` | Change the user's password (5 per minute per bucket, then `429`; `503` if no verification slot frees up within 5s, or immediately if 16 requests are already queued for one) |
+| `GET`  | `/settings`              | Read-only list of the configuration the server is running with; `METRICS_PASSWORD` and `SENTRY_DSN` show only as set or not set |
 | `GET`  | `/hooks`                 | List user's webhooks |
 | `GET`  | `/hooks/new`             | Create webhook form |
 | `POST` | `/hooks/new`             | Create webhook submission |
@@ -2902,6 +2899,7 @@ webhooker/
 │   │   ├── healthcheck.go     # Health check handler
 │   │   ├── index.go           # Index page handler
 │   │   ├── profile.go         # User profile handler
+│   │   ├── settings.go        # Read-only Settings page handler
 │   │   ├── source_management.go  # Webhook CRUD handlers
 │   │   └── webhook.go         # Webhook receiver handler
 │   ├── healthcheck/
@@ -3011,14 +3009,14 @@ local record instead of nothing. What that placement gives up is
 recovery of a panic in the six entries above it, none of which does
 more than set a header or start a timer.
 
-Each admin page route group (`/pages`, `/user/*`, `/hooks`,
-`/hook/*`) starts with its own **Recoverer** and, if `SENTRY_DSN` is
-set, its own **Sentry** error reporting. That Recoverer answers a panic
+Each admin page route group (`/pages`, `/user/*`, `/settings`,
+`/hooks`, `/hook/*`) starts with its own **Recoverer** and, if
+`SENTRY_DSN` is set, its own **Sentry** error reporting. That Recoverer answers a panic
 with the `500` error page in the normal layout; the global one keeps
 the plain-text `500` for every other route.
 
-Additionally, form endpoints (`/pages`, `/user/*`, `/hooks`,
-`/hook/*`) apply a **MaxBodySize** middleware that limits
+Additionally, form endpoints (`/pages`, `/user/*`, `/settings`,
+`/hooks`, `/hook/*`) apply a **MaxBodySize** middleware that limits
 POST/PUT/PATCH request bodies to 1 MB. It is registered ahead of the
 CSRF middleware in every one of those route groups, because
 gorilla/csrf parses the form; if the cap were installed after it, form
@@ -3037,7 +3035,7 @@ declared length. A chunked request, or
 one that lies about its length, is hard-capped by
 `http.MaxBytesReader` and fails downstream at form-parse time.
 
-Those same four route groups then apply **CSRF** and **NoCache**
+Those same five route groups then apply **CSRF** and **NoCache**
 (`Cache-Control: no-store`, `Pragma: no-cache`), and every group except
 `/pages` applies **RequireAuth**. The rate limiters are per-route
 rather than global: **PasswordChangeRateLimit** on
@@ -3083,12 +3081,12 @@ check, see [The login endpoint](#the-login-endpoint).
   by middleware that runs before CSRF parses the form
 - **CSRF protection** via [gorilla/csrf](https://github.com/gorilla/csrf)
   on all state-changing forms (cookie-based double-submit tokens with
-  HMAC authentication). Applied to `/pages`, `/hooks`, `/hook`, and
-  `/user` routes. Excluded from `/h` (inbound webhook POSTs) and
-  `/api` (stateless API). The middleware detects TLS per-request through
-  `internal/reqtls.IsTLS` — the same predicate the session cookie uses —
-  to set appropriate cookie security flags and Origin/Referer validation
-  mode
+  HMAC authentication). Applied to `/pages`, `/hooks`, `/hook`,
+  `/settings`, and `/user` routes. Excluded from `/h` (inbound webhook
+  POSTs) and `/api` (stateless API). The middleware detects TLS
+  per-request through `internal/reqtls.IsTLS` — the same predicate the
+  session cookie uses — to set appropriate cookie security flags and
+  Origin/Referer validation mode
 - **The entrypoint URL is the receiver's only credential.** Nothing
   about an inbound request is verified; possession of the UUID
   authorises submission, and no shared secret or signature check will
@@ -3102,8 +3100,7 @@ check, see [The login endpoint](#the-login-endpoint).
   route through a single decision function, so they cannot disagree
   about a destination. An operator can permit specific blocks with
   [`ALLOWED_EGRESS_CIDRS`](#allowing-egress-to-your-own-network); the
-  guard cannot be switched off, and link-local, the unspecified
-  addresses `0.0.0.0` and `::`, and a
+  guard cannot be switched off, and link-local plus a
   [pinned set](#allowing-egress-to-your-own-network) of known cloud
   metadata endpoints — several of which are ULAs outside link-local —
   stay blocked whatever is listed, though listing `0.0.0.0/0` or

internal/config/config.go

@@ -196,11 +196,10 @@ type Config struct {
 	// otherwise refuse. The guard itself is always on: there is no
 	// setting that disables SSRF protection, and delivery's
 	// alwaysBlockedNetworks stays blocked no matter what is listed
-	// here. That set is link-local, the unspecified addresses
-	// 0.0.0.0 and ::, and the cloud metadata endpoints outside
-	// link-local that disclose credentials or user data at a
-	// provider-fixed, non-public address; it is not exhaustive of
-	// every cloud's metadata address. See
+	// here. That set is link-local plus the cloud metadata
+	// endpoints outside it that disclose credentials or user data
+	// at a provider-fixed, non-public address; it is not
+	// exhaustive of every cloud's metadata address. See
 	// alwaysBlockedNetworks for the authoritative list and the
 	// criterion it is built from.
 	AllowedEgressCIDRs []netip.Prefix

internal/delivery/ssrf.go

@@ -37,8 +37,8 @@ var (
 		"blocked cloud metadata address",
 	)
 	errBlockedMetadata = errors.New(
-		"blocked link-local, cloud instance metadata or " +
-			"unspecified address: ALLOWED_EGRESS_CIDRS cannot open it",
+		"blocked link-local or cloud instance metadata " +
+			"address: ALLOWED_EGRESS_CIDRS cannot open it",
 	)
 	errInvalidScheme = errors.New(
 		"only http and https are allowed",
@@ -72,16 +72,14 @@ var blockedNetworks []*net.IPNet
 var blockedPublicNetworks []*net.IPNet
 
 // alwaysBlockedNetworks are the ranges no configuration can
-// open: the link-local blocks, the cloud instance metadata
-// endpoints that live outside them, and the unspecified
-// addresses. Reaching a metadata endpoint is credential or
-// user-data theft rather than delivery to an internal service,
-// so a supplied CIDR that covers such an address still leaves it
-// blocked.
+// open: the link-local blocks and the cloud instance metadata
+// endpoints that live outside them. Reaching one is credential
+// or user-data theft rather than delivery to an internal
+// service, so a supplied CIDR that covers such an address still
+// leaves it blocked.
 //
 // Inclusion criterion — an address belongs here only if BOTH
-// hold, and every entry below but the unspecified addresses
-// satisfies both:
+// hold, and every entry below satisfies both:
 //
 //  1. It is a fixed address assigned by the provider, or a
 //     range reserved by IANA — never one the operator chose.
@@ -114,12 +112,6 @@ var blockedPublicNetworks []*net.IPNet
 // This is a criterion, not an enumeration of every metadata
 // address in existence.
 //
-// The unspecified addresses 0.0.0.0 and :: fail (2) and are here
-// anyway. No host can have either, and on Linux a connection to
-// one reaches this host's own loopback, so an allowlist opens
-// loopback only by naming it (127.0.0.0/8, ::1/128). Nothing else
-// lives at either address, so refusing them costs nothing.
-//
 // Every entry is either already in blockedNetworks — this list is
 // what makes it unconditional — or an alternate encoding of
 // 169.254.169.254 that Contains does not match against
@@ -139,46 +131,23 @@ var alwaysBlockedNetworks []*net.IPNet
 //nolint:gochecknoinits // init is the idiomatic way to parse CIDRs once at startup
 func init() {
 	blockedNetworks = mustParseCIDRs([]string{
-		// IPv4 loopback.
 		"127.0.0.0/8",
-		// RFC 1918 private network.
 		"10.0.0.0/8",
-		// RFC 1918 private network.
 		"172.16.0.0/12",
-		// RFC 1918 private network.
 		"192.168.0.0/16",
-		// IPv4 link-local.
 		"169.254.0.0/16",
-		// "This network", holding the IPv4 unspecified address 0.0.0.0.
 		"0.0.0.0/8",
-		// Carrier-grade NAT shared address space.
 		"100.64.0.0/10",
-		// IETF protocol assignments.
 		"192.0.0.0/24",
-		// IPv4 documentation (TEST-NET-1).
 		"192.0.2.0/24",
-		// Benchmarking.
 		"198.18.0.0/15",
-		// IPv4 documentation (TEST-NET-2).
 		"198.51.100.0/24",
-		// IPv4 documentation (TEST-NET-3).
 		"203.0.113.0/24",
-		// IPv4 multicast.
 		"224.0.0.0/4",
-		// Reserved, including the broadcast address.
 		"240.0.0.0/4",
-		// IPv6 loopback.
 		"::1/128",
-		// IPv6 unspecified address.
-		"::/128",
-		// IPv6 unique local addresses.
 		"fc00::/7",
-		// IPv6 link-local.
 		"fe80::/10",
-		// IPv6 multicast.
-		"ff00::/8",
-		// IPv6 documentation.
-		"2001:db8::/32",
 	})
 
 	blockedPublicNetworks = mustParseCIDRs([]string{
@@ -238,14 +207,6 @@ func init() {
 		// allowlist from opening it.
 		"192.0.0.192/32",
 
-		// The unspecified addresses, each of which reaches this
-		// host's loopback on Linux.
-		//
-		// IPv4 unspecified address, inside the blocked 0.0.0.0/8.
-		"0.0.0.0/32",
-		// IPv6 unspecified address.
-		"::/128",
-
 		// 169.254.169.254 as an IPv4-compatible IPv6 address.
 		"::a9fe:a9fe/128",
 		// 169.254.169.254 behind the NAT64 well-known prefix.
@@ -382,9 +343,8 @@ func (g *Guard) allows(ip net.IP) bool {
 // The order is the policy:
 //
 //  1. alwaysBlockedNetworks is refused before the allowlist is
-//     consulted, so no configured CIDR reaches link-local, a
-//     cloud metadata endpoint at a non-public address, or an
-//     unspecified address.
+//     consulted, so no configured CIDR reaches link-local or a
+//     cloud metadata endpoint at a non-public address.
 //  2. The allowlist is consulted next, so a listed private
 //     network, or a listed public address on the default
 //     blocklist, becomes reachable.

internal/delivery/ssrf_allowlist_test.go

@@ -524,10 +524,6 @@ func TestAlwaysBlockedNetworks_PinnedSet(t *testing.T) {
 		// Oracle Cloud Classic metadata, inside the blocked
 		// 192.0.0.0/24.
 		"192.0.0.192/32",
-		// The IPv4 and IPv6 unspecified addresses, each of
-		// which reaches this host's loopback on Linux.
-		"0.0.0.0/32",
-		"::/128",
 		// 169.254.169.254 as an IPv4-compatible IPv6 address.
 		"::a9fe:a9fe/128",
 		// 169.254.169.254 behind the NAT64 well-known prefix.
@@ -560,8 +556,7 @@ func TestDefaultBlocklist_PinnedSet(t *testing.T) {
 		{cidr: "172.16.0.0/12", reopenable: true},
 		{cidr: "192.168.0.0/16", reopenable: true},
 		{cidr: linkLocalIPv4, reopenable: false},
-		// Its first address, 0.0.0.0, is in the unconditional set.
-		{cidr: "0.0.0.0/8", reopenable: false},
+		{cidr: "0.0.0.0/8", reopenable: true},
 		{cidr: "100.64.0.0/10", reopenable: true},
 		{cidr: "192.0.0.0/24", reopenable: true},
 		{cidr: "192.0.2.0/24", reopenable: true},
@@ -571,11 +566,8 @@ func TestDefaultBlocklist_PinnedSet(t *testing.T) {
 		{cidr: "224.0.0.0/4", reopenable: true},
 		{cidr: "240.0.0.0/4", reopenable: true},
 		{cidr: "::1/128", reopenable: true},
-		{cidr: "::/128", reopenable: false},
 		{cidr: "fc00::/7", reopenable: true},
 		{cidr: "fe80::/10", reopenable: false},
-		{cidr: "ff00::/8", reopenable: true},
-		{cidr: "2001:db8::/32", reopenable: true},
 		{cidr: "168.63.129.16/32", public: true, reopenable: true},
 	}
 

internal/delivery/ssrf_test.go

@@ -101,42 +101,6 @@ func TestValidateTargetURL_Blocked(t *testing.T) {
 	}
 }
 
-// TestDefaultGuard_RefusesUnspecifiedMulticastAndDocumentation
-// covers the unspecified addresses and the IPv6 multicast and
-// documentation ranges: with no allowlist set, each is refused
-// both when a target is created and when a delivery dials it.
-func TestDefaultGuard_RefusesUnspecifiedMulticastAndDocumentation(
-	t *testing.T,
-) {
-	t.Parallel()
-
-	guard := delivery.NewTestGuard()
-
-	targets := []string{
-		// The unspecified addresses. On Linux a connection to
-		// either reaches this host's loopback.
-		"http://0.0.0.0:8080/hook",
-		"http://[::]:8080/hook",
-		// IPv6 multicast, all nodes.
-		"http://[ff02::1]/hook",
-		// IPv6 documentation.
-		"http://[2001:db8::1]/hook",
-	}
-
-	for _, target := range targets {
-		t.Run(target, func(t *testing.T) {
-			t.Parallel()
-
-			require.Error(t,
-				guard.ValidateTargetURL(context.Background(), target),
-				"%s must be refused at target creation", target,
-			)
-
-			assertDialRefused(t, guard, target)
-		})
-	}
-}
-
 func TestValidateTargetURL_Allowed(t *testing.T) {
 	t.Parallel()
 

internal/handlers/handlers.go

@@ -14,6 +14,7 @@ import (
 
 	"github.com/prometheus/client_golang/prometheus"
 	"go.uber.org/fx"
+	"sneak.berlin/go/webhooker/internal/config"
 	"sneak.berlin/go/webhooker/internal/database"
 	"sneak.berlin/go/webhooker/internal/delivery"
 	"sneak.berlin/go/webhooker/internal/globals"
@@ -57,6 +58,7 @@ type HandlersParams struct {
 
 	Logger       *logger.Logger
 	Globals      *globals.Globals
+	Config       *config.Config
 	Database     *database.Database
 	WebhookDBMgr *database.WebhookDBManager
 	Healthcheck  *healthcheck.Healthcheck
@@ -139,6 +141,7 @@ func New(
 	s.templates = map[string]*template.Template{
 		"login.html":         parsePageTemplate("login.html"),
 		"profile.html":       parsePageTemplate("profile.html"),
+		"settings.html":      parsePageTemplate("settings.html"),
 		"sources_list.html":  parsePageTemplate("sources_list.html"),
 		"sources_new.html":   parsePageTemplate("sources_new.html"),
 		"source_detail.html": parsePageTemplate("source_detail.html", "webhook_stats.html"),

internal/handlers/handlers_test.go

@@ -84,16 +84,25 @@ func newTestApp(
 ) *fxtest.App {
 	t.Helper()
 
+	return newTestAppWithConfig(
+		t, &config.Config{DataDir: t.TempDir()}, targets...,
+	)
+}
+
+// newTestAppWithConfig is newTestApp over a caller-supplied Config.
+func newTestAppWithConfig(
+	t *testing.T,
+	cfg *config.Config,
+	targets ...any,
+) *fxtest.App {
+	t.Helper()
+
 	return fxtest.New(
 		t,
 		fx.Provide(
 			globals.New,
 			logger.New,
-			func() *config.Config {
-				return &config.Config{
-					DataDir: t.TempDir(),
-				}
-			},
+			func() *config.Config { return cfg },
 			database.New,
 			database.NewWebhookDBManager,
 			healthcheck.New,

internal/handlers/settings.go

@@ -0,0 +1,130 @@
+package handlers
+
+import (
+	"net/http"
+	"net/netip"
+	"strconv"
+	"strings"
+
+	"sneak.berlin/go/webhooker/internal/config"
+)
+
+// notSet is what the Settings page shows for a value that is empty.
+const notSet = "not set"
+
+// settingRow is one line of the Settings page: an environment
+// variable, what it controls, and the value the server loaded for it.
+type settingRow struct {
+	Name        string
+	Description string
+	Value       string
+}
+
+// HandleSettings returns a handler for the read-only Settings page,
+// which lists the configuration the server started with.
+func (h *Handlers) HandleSettings() http.HandlerFunc {
+	return func(w http.ResponseWriter, r *http.Request) {
+		h.renderTemplate(w, r, "settings.html", map[string]any{
+			"Settings": settingRows(h.params.Config),
+		})
+	}
+}
+
+// settingRows lists every field of cfg under the environment variable
+// it is read from, in the order of the README's configuration table.
+// METRICS_PASSWORD and SENTRY_DSN are credentials, so their values
+// never reach the page: only whether they are set.
+func settingRows(cfg *config.Config) []settingRow {
+	metricsUsername := cfg.MetricsUsername
+	if metricsUsername == "" {
+		metricsUsername = notSet
+	}
+
+	return []settingRow{
+		{"WEBHOOKER_ENVIRONMENT", "dev or prod", cfg.Environment},
+		{"PORT", "HTTP listen port", strconv.Itoa(cfg.Port)},
+		{
+			"BIND_ADDRESS",
+			"IP address the HTTP listener binds",
+			cfg.BindAddress,
+		},
+		{
+			"DATA_DIR",
+			"Directory for all SQLite databases",
+			cfg.DataDir,
+		},
+		{
+			"DEBUG",
+			"Enable debug logging",
+			strconv.FormatBool(cfg.Debug),
+		},
+		{
+			"MAINTENANCE_MODE",
+			"Report maintenanceMode: true in the healthcheck JSON. " +
+				"It does not change how any request is served",
+			strconv.FormatBool(cfg.MaintenanceMode),
+		},
+		{
+			"METRICS_USERNAME",
+			"Basic auth username for /metrics",
+			metricsUsername,
+		},
+		{
+			"METRICS_PASSWORD",
+			"Basic auth password for /metrics",
+			setOrNotSet(cfg.MetricsPassword),
+		},
+		{
+			"SENTRY_DSN",
+			"Error reporting DSN. Unset leaves error reporting off",
+			setOrNotSet(cfg.SentryDSN),
+		},
+		{
+			"RETENTION_SWEEP_INTERVAL",
+			"How often the retention reaper and archive sweeper run",
+			cfg.RetentionSweepInterval.String(),
+		},
+		{
+			"SESSION_IDLE_TIMEOUT",
+			"Idle session timeout. Zero or negative disables idle " +
+				"expiry",
+			cfg.SessionIdleTimeout.String(),
+		},
+		{
+			"RECEIVER_RATE_LIMIT",
+			"Receiver requests per minute per IP per entrypoint " +
+				"(10x that per IP across the route)",
+			strconv.Itoa(cfg.ReceiverRateLimit),
+		},
+		{
+			"TRUSTED_PROXIES",
+			"CIDRs whose forwarded headers are trusted",
+			cidrList(cfg.TrustedProxies),
+		},
+		{
+			"ALLOWED_EGRESS_CIDRS",
+			"CIDRs that delivery targets may reach despite the " +
+				"SSRF blocklist",
+			cidrList(cfg.AllowedEgressCIDRs),
+		},
+	}
+}
+
+// setOrNotSet is how the Settings page shows a credential: whether it
+// has a value, never the value itself.
+func setOrNotSet(value string) string {
+	if value == "" {
+		return notSet
+	}
+
+	return "set"
+}
+
+// cidrList renders a CIDR list setting for the Settings page.
+func cidrList(prefixes []netip.Prefix) string {
+	if len(prefixes) == 0 {
+		return "none"
+	}
+
+	return strings.Join(config.PrefixStrings(prefixes), ", ")
+}

internal/handlers/settings_test.go

@@ -0,0 +1,151 @@
+package handlers_test
+
+import (
+	"context"
+	"html"
+	"net/http"
+	"net/http/httptest"
+	"net/netip"
+	"regexp"
+	"testing"
+	"time"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+	"sneak.berlin/go/webhooker/internal/config"
+	"sneak.berlin/go/webhooker/internal/handlers"
+	"sneak.berlin/go/webhooker/internal/session"
+)
+
+// settingsShown renders the Settings page over cfg as a logged-in user
+// and returns the value it shows for each variable name, plus the
+// whole page.
+func settingsShown(
+	t *testing.T, cfg *config.Config,
+) (map[string]string, string) {
+	t.Helper()
+
+	var h *handlers.Handlers
+
+	var sess *session.Session
+
+	app := newTestAppWithConfig(t, cfg, &h, &sess)
+	app.RequireStart()
+
+	t.Cleanup(app.RequireStop)
+
+	req := httptest.NewRequestWithContext(
+		context.Background(), http.MethodGet, "/settings", nil,
+	)
+	for _, c := range authenticatedCookies(t, sess, "id", "admin") {
+		req.AddCookie(c)
+	}
+
+	w := httptest.NewRecorder()
+	h.HandleSettings().ServeHTTP(w, req)
+	require.Equal(t, http.StatusOK, w.Code)
+
+	body := w.Body.String()
+
+	row := regexp.MustCompile(
+		`<code[^>]*>([A-Z_]+)</code>\s*<code[^>]*>([^<]*)</code>`,
+	)
+
+	shown := map[string]string{}
+	for _, match := range row.FindAllStringSubmatch(body, -1) {
+		shown[match[1]] = html.UnescapeString(match[2])
+	}
+
+	return shown, body
+}
+
+func TestSettingsPageShowsLoadedConfiguration(t *testing.T) {
+	t.Parallel()
+
+	// DEBUG and MAINTENANCE_MODE get opposite values, and each of
+	// METRICS_USERNAME, METRICS_PASSWORD and SENTRY_DSN is the only one
+	// of the three set in one of the content tests, so each row is
+	// checked against its own field.
+	cfg := &config.Config{
+		DataDir:                t.TempDir(),
+		Debug:                  true,
+		MaintenanceMode:        false,
+		Environment:            config.EnvironmentDev,
+		MetricsUsername:        "scraper",
+		MetricsPassword:        "",
+		Port:                   9123,
+		SentryDSN:              "",
+		BindAddress:            "192.0.2.10",
+		RetentionSweepInterval: 17 * time.Minute,
+		SessionIdleTimeout:     3 * time.Hour,
+		ReceiverRateLimit:      77,
+		TrustedProxies: []netip.Prefix{
+			netip.MustParsePrefix("10.1.0.0/16"),
+		},
+		AllowedEgressCIDRs: []netip.Prefix{
+			netip.MustParsePrefix("192.168.5.0/24"),
+			netip.MustParsePrefix("fd00::/8"),
+		},
+	}
+
+	shown, body := settingsShown(t, cfg)
+
+	assert.Equal(t, map[string]string{
+		"WEBHOOKER_ENVIRONMENT":    "dev",
+		"PORT":                     "9123",
+		"BIND_ADDRESS":             "192.0.2.10",
+		"DATA_DIR":                 cfg.DataDir,
+		"DEBUG":                    "true",
+		"MAINTENANCE_MODE":         "false",
+		"METRICS_USERNAME":         "scraper",
+		"METRICS_PASSWORD":         "not set",
+		"SENTRY_DSN":               "not set",
+		"RETENTION_SWEEP_INTERVAL": "17m0s",
+		"SESSION_IDLE_TIMEOUT":     "3h0m0s",
+		"RECEIVER_RATE_LIMIT":      "77",
+		"TRUSTED_PROXIES":          "10.1.0.0/16",
+		"ALLOWED_EGRESS_CIDRS":     "192.168.5.0/24, fd00::/8",
+	}, shown)
+
+	assert.Contains(
+		t, body, `href="/settings"`,
+		"the navigation bar links to the page",
+	)
+}
+
+func TestSettingsPageShowsUnsetValues(t *testing.T) {
+	t.Parallel()
+
+	const metricsPassword = "metrics-password-1f9a"
+
+	shown, body := settingsShown(t, &config.Config{
+		DataDir:         t.TempDir(),
+		MetricsPassword: metricsPassword,
+	})
+
+	assert.Equal(t, "not set", shown["METRICS_USERNAME"])
+	assert.Equal(t, "set", shown["METRICS_PASSWORD"])
+	assert.Equal(t, "not set", shown["SENTRY_DSN"])
+	assert.NotContains(t, body, metricsPassword)
+	assert.Equal(t, "none", shown["TRUSTED_PROXIES"])
+	assert.Equal(t, "none", shown["ALLOWED_EGRESS_CIDRS"])
+}
+
+func TestSettingsPageShowsSentryDSNOnlyAsSet(t *testing.T) {
+	t.Parallel()
+
+	const (
+		sentryKey = "dsnkey7c2e"
+		sentryDSN = "https://" + sentryKey + "@errors.example.com/42"
+	)
+
+	shown, body := settingsShown(t, &config.Config{
+		DataDir:   t.TempDir(),
+		SentryDSN: sentryDSN,
+	})
+
+	assert.Equal(t, "not set", shown["METRICS_USERNAME"])
+	assert.Equal(t, "not set", shown["METRICS_PASSWORD"])
+	assert.Equal(t, "set", shown["SENTRY_DSN"])
+	assert.NotContains(t, body, sentryKey)
+}

internal/handlers/source_management.go

@@ -1569,11 +1569,10 @@ func (h *Handlers) validateTargetURL(
 		msg := "Invalid target URL: " + err.Error()
 
 		// Only a private or reserved address's refusal says how
-		// to allow it. Other refusals never do: link-local, the
-		// unspecified addresses and the other unconditional
-		// metadata addresses cannot be opened, and the default
-		// blocklist's public addresses, which listing does open,
-		// hand out credentials.
+		// to allow it. Metadata refusals never do: link-local and
+		// the other unconditional metadata addresses cannot be
+		// opened, and the default blocklist's public addresses,
+		// which listing does open, hand out credentials.
 		if errors.Is(err, delivery.ErrBlockedPrivateOrReservedIP) {
 			msg += ". Private and reserved addresses are refused " +
 				"by default; the server's ALLOWED_EGRESS_CIDRS " +

internal/server/routes.go

@@ -14,10 +14,11 @@ import (
 // bytes) for form POST endpoints. 1 MB is generous for any form
 // submission while preventing abuse from oversized payloads.
 //
-// The four admin page route groups below (/pages, /user/{username},
-// /hooks and /hook/{sourceID}) install MaxBodySize(maxFormBodySize)
-// right after their recoverer and error reporting, ahead of both CSRF
-// and RequireAuth. Both orderings are deliberate.
+// The five admin page route groups below (/pages, /user/{username},
+// /settings, /hooks and /hook/{sourceID}) install
+// MaxBodySize(maxFormBodySize) right after their recoverer and error
+// reporting, ahead of both CSRF and RequireAuth. Both orderings are
+// deliberate.
 //
 // Ahead of CSRF because gorilla/csrf parses the form. The cap has to
 // be installed before anything reads the body, or the parse runs
@@ -154,6 +155,7 @@ func (s *Server) setupRoutes() {
 
 	s.setupPageRoutes()
 	s.setupUserRoutes()
+	s.setupSettingsRoutes()
 	s.setupSourceRoutes()
 	s.setupWebhookRoutes()
 }
@@ -201,6 +203,24 @@ func (s *Server) setupUserRoutes() {
 	})
 }
 
+// setupSettingsRoutes serves the Settings page. It is GET only:
+// configuration comes from the environment and nothing here changes
+// it.
+func (s *Server) setupSettingsRoutes() {
+	s.router.Route("/settings", func(r chi.Router) {
+		s.recoverPanics(
+			r, s.h.HandleErrorPage(http.StatusInternalServerError),
+		)
+		// MaxBodySize precedes CSRF and RequireAuth deliberately;
+		// see maxFormBodySize for why, and for what it costs.
+		r.Use(s.mw.MaxBodySize(maxFormBodySize))
+		r.Use(s.mw.CSRF(s.h.HandleErrorPage(http.StatusForbidden)))
+		r.Use(s.mw.NoCache())
+		r.Use(s.mw.RequireAuth())
+		r.Get("/", s.h.HandleSettings())
+	})
+}
+
 func (s *Server) setupSourceRoutes() {
 	s.router.Route("/hooks", func(r chi.Router) {
 		s.recoverPanics(

internal/server/settings_test.go

@@ -0,0 +1,24 @@
+package server_test
+
+import (
+	"net/http"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestSettingsPageIsBehindLogin(t *testing.T) {
+	t.Parallel()
+
+	env := newTestEnv(t)
+
+	w := env.get("/settings", nil)
+	assert.Equal(t, http.StatusSeeOther, w.Code)
+	assert.Equal(
+		t, "/pages/login?next=%2Fsettings", w.Header().Get("Location"),
+	)
+
+	w = env.get("/settings", env.authCookies(t, "id", "admin"))
+	assert.Equal(t, http.StatusOK, w.Code)
+	assert.Contains(t, w.Body.String(), "WEBHOOKER_ENVIRONMENT")
+}

templates/navbar.html

@@ -19,6 +19,7 @@
         <div class="hidden md:flex items-center gap-4">
             {{if .User}}
             <a href="/hooks" class="btn-text">Webhooks</a>
+            <a href="/settings" class="btn-text">Settings</a>
             <a href="/user/{{.User.Username}}" class="btn-text">
                 <svg class="w-5 h-5 mr-1" fill="currentColor" viewBox="0 0 16 16">
                     <path d="M11 6a3 3 0 1 1-6 0 3 3 0 0 1 6 0z"/>
@@ -43,6 +44,7 @@
         <div class="flex flex-col gap-2">
             {{if .User}}
             <a href="/hooks" class="btn-text w-full text-left">Webhooks</a>
+            <a href="/settings" class="btn-text w-full text-left">Settings</a>
             <a href="/user/{{.User.Username}}" class="btn-text w-full text-left">Profile</a>
             {{if .CSRFToken}}
             <form method="POST" action="/pages/logout">

templates/settings.html

@@ -0,0 +1,24 @@
+{{template "base" .}}
+
+{{define "title"}}Settings - Webhooker{{end}}
+
+{{define "content"}}
+<div class="max-w-6xl mx-auto px-6 py-8">
+    <h1 class="text-2xl font-medium text-gray-900">Settings</h1>
+    <p class="text-sm text-gray-500 mt-1 mb-6">The configuration this server started with. It is set in the server's environment and cannot be changed here.</p>
+
+    <div class="card">
+        <div class="divide-y divide-gray-100">
+            {{range .Settings}}
+            <div class="p-4">
+                <div class="flex justify-between items-start gap-4">
+                    <code class="text-sm font-medium text-gray-900">{{.Name}}</code>
+                    <code class="text-sm text-gray-900 break-all">{{.Value}}</code>
+                </div>
+                <p class="text-sm text-gray-500 mt-1">{{.Description}}</p>
+            </div>
+            {{end}}
+        </div>
+    </div>
+</div>
+{{end}}