dashboard and status API show why a check failed (closes #225)

/api/v1/status now gives `error` for each nameserver entry and
certificate entry whose status is `error`, copied from the state, which
already kept it. The dashboard shows that reason in place of the records
for a failed nameserver, which used to show the same `-` as one that
answered with no records, and across the CN, issuer and expiry cells
for a failed certificate, wrapped at a width of 20rem so the long TLS
error does not narrow the Endpoint column. The dashboard stylesheet is a
trimmed build, so the new markup uses only classes the page already had.
README Web Dashboard and HTTP API say so.

Model: opus-5-5

README.md

@@ -230,9 +230,11 @@ dnswatcher includes an unauthenticated, read-only web dashboard at the root URL
 
 - **Summary counts** for monitored domains, hostnames, ports, and certificates.
 - **Domains** with their discovered nameservers.
-- **Hostnames** with per-nameserver DNS records and status.
+- **Hostnames** with per-nameserver DNS records and status. For a nameserver
+  whose query failed, the reason is shown in place of the records.
 - **Ports** with open/closed state and associated hostnames.
-- **TLS certificates** with CN, issuer, expiry, and status.
+- **TLS certificates** with CN, issuer, expiry, and status. For a failed check,
+  the reason is shown in place of CN, issuer and expiry.
 - **Recent alerts** (last 100 notifications sent since the process started),
   displayed in reverse chronological order.
 
@@ -259,6 +261,10 @@ dnswatcher exposes a lightweight HTTP API for operational visibility:
 | `GET /api/v1/status`           | Current monitoring state      |
 | `GET /metrics`                 | Prometheus metrics, see below |
 
+In `/api/v1/status`, each nameserver entry and certificate entry whose `status`
+is `error` also has `error`, the reason, as in the state file (see State File
+Format).
+
 `/metrics` is served only when `DNSWATCHER_METRICS_USERNAME` is set, behind
 Basic Auth. It has the Prometheus Go client's default metrics only (Go runtime,
 process, and counts of `/metrics` requests); dnswatcher records no metrics of

TODO.md

@@ -19,6 +19,8 @@ trial run of the finished image: https://git.eeqj.de/sneak/dnswatcher/issues/149
 
 # Completed Steps
 
+- 2026-10-02: the dashboard and `/api/v1/status` show why a nameserver query or
+  a certificate check failed, which only the state file showed (closes #225).
 - 2026-10-02: a name's CNAME is stored once per nameserver, not once per record
   type asked for; a state file with repeats loads each value once (closes #220).
 - 2026-10-02: a DNS lookup that shutdown cuts short logs no error; one that

internal/handlers/dashboard_test.go

@@ -1,6 +1,7 @@
 package handlers_test
 
 import (
+	"strings"
 	"testing"
 	"time"
 
@@ -78,3 +79,39 @@ func TestFormatRecords(t *testing.T) {
 		t.Errorf("unexpected format: %q", got)
 	}
 }
+
+// dashboardRow returns the table row of page that contains name.
+func dashboardRow(t *testing.T, page string, name string) string {
+	t.Helper()
+
+	for row := range strings.SplitSeq(page, "<tr") {
+		if strings.Contains(row, name) {
+			return row
+		}
+	}
+
+	t.Fatalf("dashboard has no row containing %q", name)
+
+	return ""
+}
+
+// TestDashboardShowsFailureReasons checks that the dashboard shows the
+// reason in the row of a failed nameserver and of a failed certificate,
+// and not in the row of a nameserver that answered.
+func TestDashboardShowsFailureReasons(t *testing.T) {
+	t.Parallel()
+
+	page := get(t, newHandlersWithFailures(t).HandleDashboard())
+
+	if !strings.Contains(dashboardRow(t, page, failedNS), nsFailureReason) {
+		t.Errorf("row of %s does not show %q", failedNS, nsFailureReason)
+	}
+
+	if strings.Contains(dashboardRow(t, page, answeringNS), nsFailureReason) {
+		t.Errorf("row of %s shows %q", answeringNS, nsFailureReason)
+	}
+
+	if !strings.Contains(dashboardRow(t, page, certKey), certFailedReason) {
+		t.Errorf("row of %s does not show %q", certKey, certFailedReason)
+	}
+}

internal/handlers/status.go

@@ -18,6 +18,7 @@ type statusDomainInfo struct {
 type statusHostnameNSInfo struct {
 	Records     map[string][]string `json:"records"`
 	Status      string              `json:"status"`
+	Error       string              `json:"error,omitempty"`
 	LastChecked time.Time           `json:"lastChecked"`
 }
 
@@ -41,6 +42,7 @@ type statusCertificateInfo struct {
 	NotAfter                time.Time `json:"notAfter"`
 	SubjectAlternativeNames []string  `json:"subjectAlternativeNames"`
 	Status                  string    `json:"status"`
+	Error                   string    `json:"error,omitempty"`
 	LastChecked             time.Time `json:"lastChecked"`
 }
 
@@ -144,6 +146,7 @@ func buildHostnames(
 			info.Nameservers[ns] = &statusHostnameNSInfo{
 				Records:     recs,
 				Status:      nsState.Status,
+				Error:       nsState.Error,
 				LastChecked: nsState.LastChecked,
 			}
 		}
@@ -183,6 +186,7 @@ func buildCertificates(
 			NotAfter:                cs.NotAfter,
 			SubjectAlternativeNames: sans,
 			Status:                  cs.Status,
+			Error:                   cs.Error,
 			LastChecked:             cs.LastChecked,
 		}
 	}

internal/handlers/status_test.go

@@ -0,0 +1,158 @@
+package handlers_test
+
+import (
+	"encoding/json"
+	"net/http"
+	"net/http/httptest"
+	"testing"
+	"time"
+
+	"go.uber.org/fx/fxtest"
+
+	"sneak.berlin/go/dnswatcher/internal/config"
+	"sneak.berlin/go/dnswatcher/internal/globals"
+	"sneak.berlin/go/dnswatcher/internal/handlers"
+	"sneak.berlin/go/dnswatcher/internal/logger"
+	"sneak.berlin/go/dnswatcher/internal/notify"
+	"sneak.berlin/go/dnswatcher/internal/state"
+)
+
+// The state the handler tests serve: www.example.com has one nameserver
+// that answered and one whose query failed, and its certificate check
+// failed.
+const (
+	testHostname     = "www.example.com"
+	answeringNS      = "ns1.example.com."
+	failedNS         = "ns2.example.com."
+	nsFailureReason  = "server returned a referral"
+	certKey          = "192.0.2.1:443:www.example.com"
+	certFailedReason = "x509: certificate has expired or is not yet valid"
+)
+
+// newHandlersWithFailures builds real Handlers whose state holds the
+// entries described above.
+func newHandlersWithFailures(t *testing.T) *handlers.Handlers {
+	t.Helper()
+
+	glob, err := globals.New(nil)
+	if err != nil {
+		t.Fatalf("globals.New: %v", err)
+	}
+
+	log, err := logger.New(nil, logger.Params{Globals: glob})
+	if err != nil {
+		t.Fatalf("logger.New: %v", err)
+	}
+
+	notifier, err := notify.New(fxtest.NewLifecycle(t), notify.Params{
+		Logger: log,
+		Config: &config.Config{},
+	})
+	if err != nil {
+		t.Fatalf("notify.New: %v", err)
+	}
+
+	st, err := state.New(fxtest.NewLifecycle(t), state.Params{
+		Logger: log,
+		Config: &config.Config{DataDir: t.TempDir()},
+	})
+	if err != nil {
+		t.Fatalf("state.New: %v", err)
+	}
+
+	now := time.Now()
+
+	st.SetHostnameState(testHostname, &state.HostnameState{
+		RecordsByNameserver: map[string]*state.NameserverRecordState{
+			answeringNS: {
+				Records:     map[string][]string{"A": {"192.0.2.1"}},
+				Status:      "ok",
+				LastChecked: now,
+			},
+			failedNS: {
+				Records:     map[string][]string{},
+				Status:      "error",
+				Error:       nsFailureReason,
+				LastChecked: now,
+			},
+		},
+		LastChecked: now,
+	})
+
+	st.SetCertificateState(certKey, &state.CertificateState{
+		Status:      "error",
+		Error:       certFailedReason,
+		LastChecked: now,
+	})
+
+	hnd, err := handlers.New(nil, handlers.Params{
+		Logger:  log,
+		Globals: glob,
+		State:   st,
+		Notify:  notifier,
+	})
+	if err != nil {
+		t.Fatalf("handlers.New: %v", err)
+	}
+
+	return hnd
+}
+
+// get serves one GET request to handler and returns the response body.
+func get(t *testing.T, handler http.HandlerFunc) string {
+	t.Helper()
+
+	rec := httptest.NewRecorder()
+	req := httptest.NewRequestWithContext(
+		t.Context(), http.MethodGet, "/", nil,
+	)
+
+	handler(rec, req)
+
+	if rec.Code != http.StatusOK {
+		t.Fatalf("status = %d, want 200", rec.Code)
+	}
+
+	return rec.Body.String()
+}
+
+// TestStatusGivesFailureReasons checks that /api/v1/status gives the
+// reason for a failed nameserver entry and a failed certificate entry,
+// and no error for a nameserver that answered.
+func TestStatusGivesFailureReasons(t *testing.T) {
+	t.Parallel()
+
+	body := get(t, newHandlersWithFailures(t).HandleStatus())
+
+	var resp struct {
+		Hostnames map[string]struct {
+			Nameservers map[string]map[string]any `json:"nameservers"`
+		} `json:"hostnames"`
+		Certificates map[string]map[string]any `json:"certificates"`
+	}
+
+	err := json.Unmarshal([]byte(body), &resp)
+	if err != nil {
+		t.Fatalf("decoding response: %v", err)
+	}
+
+	nameservers := resp.Hostnames[testHostname].Nameservers
+
+	got := nameservers[failedNS]["error"]
+	if got != nsFailureReason {
+		t.Errorf("failed nameserver error = %v, want %q",
+			got, nsFailureReason)
+	}
+
+	_, has := nameservers[answeringNS]["error"]
+	if has {
+		t.Errorf("answering nameserver has an error field: %v",
+			nameservers[answeringNS])
+	}
+
+	got = resp.Certificates[certKey]["error"]
+	if got != certFailedReason {
+		t.Errorf("failed certificate error = %v, want %q",
+			got, certFailedReason)
+	}
+}

internal/handlers/templates/dashboard.html

@@ -146,7 +146,11 @@
                 <td
                   class="py-2 px-3 text-slate-400 break-all max-w-xs"
                 >
+                  {{ if $nsr.Error }}
+                  <span class="text-red-400">{{ $nsr.Error }}</span>
+                  {{ else }}
                   {{ formatRecords $nsr.Records }}
+                  {{ end }}
                 </td>
                 <td class="py-2 px-3 text-slate-500 whitespace-nowrap">
                   {{ relTime $nsr.LastChecked }}
@@ -258,6 +262,11 @@
                   >
                   {{ end }}
                 </td>
+                {{ if $cs.Error }}
+                <td colspan="3" class="py-2 px-3 text-red-400 break-all">
+                  <div class="max-w-xs">{{ $cs.Error }}</div>
+                </td>
+                {{ else }}
                 <td class="py-2 px-3 text-slate-200">
                   {{ $cs.CommonName }}
                 </td>
@@ -285,6 +294,7 @@
                     {{ end }}
                   {{ end }}
                 </td>
+                {{ end }}
                 <td class="py-2 px-3 text-slate-500 whitespace-nowrap">
                   {{ relTime $cs.LastChecked }}
                 </td>