internal/monitor/checker.go

@@ -37,19 +37,26 @@ type CheckResult struct {
 }
 
 func RunCheck(ctx context.Context, site models.SiteConfig, strict, insecure *http.Client, globalInsecure, allowPrivate bool) CheckResult {
+	// Resolve + validate once for non-HTTP types to prevent DNS-rebind TOCTOU:
+	// a second resolve in the check function could return a different (private) IP.
+	// HTTP is safe — SafeDialContext resolves and validates at dial time.
+	var pinnedIP net.IP
 	if site.Type != "http" && site.Type != "dns" && !allowPrivate {
 		host := site.Hostname
 		if host == "" {
 			host = site.URL
 		}
 		if host != "" {
-			if ips, err := net.LookupIP(host); err == nil {
+			ips, err := net.LookupIP(host)
-				for _, ip := range ips {
+			if err != nil {
-					if isPrivateIP(ip) {
+				return CheckResult{SiteID: site.ID, Status: string(models.StatusDown), ErrorReason: "resolve failed: " + err.Error()}
-						return CheckResult{SiteID: site.ID, Status: string(models.StatusDown), ErrorReason: "target resolves to private IP"}
+			}
-					}
+			for _, ip := range ips {
+				if isPrivateIP(ip) {
+					return CheckResult{SiteID: site.ID, Status: string(models.StatusDown), ErrorReason: "target resolves to private IP"}
 				}
 			}
+			pinnedIP = ips[0]
 		}
 	}
 
@@ -57,9 +64,9 @@ func RunCheck(ctx context.Context, site models.SiteConfig, strict, insecure *htt
 	case "http":
 		return runHTTPCheck(ctx, site, strict, insecure, globalInsecure)
 	case "ping":
-		return runPingCheck(ctx, site)
+		return runPingCheck(ctx, site, pinnedIP)
 	case "port":
-		return runPortCheck(ctx, site)
+		return runPortCheck(ctx, site, pinnedIP)
 	case "dns":
 		return runDNSCheck(ctx, site, allowPrivate)
 	default:
@@ -130,7 +137,7 @@ func runHTTPCheck(ctx context.Context, site models.SiteConfig, strict, insecure
 	return result
 }
 
-func runPingCheck(_ context.Context, site models.SiteConfig) CheckResult {
+func runPingCheck(_ context.Context, site models.SiteConfig, pinnedIP net.IP) CheckResult {
 	host := site.Hostname
 	if host == "" {
 		host = site.URL
@@ -140,6 +147,9 @@ func runPingCheck(_ context.Context, site models.SiteConfig) CheckResult {
 	if err != nil {
 		return CheckResult{SiteID: site.ID, Status: string(models.StatusDown), ErrorReason: "ping setup: " + err.Error()}
 	}
+	if pinnedIP != nil {
+		pinger.SetIPAddr(&net.IPAddr{IP: pinnedIP})
+	}
 	pinger.Count = 1
 	pinger.Timeout = siteTimeout(site)
 	pinger.SetPrivileged(false)
@@ -159,11 +169,14 @@ func runPingCheck(_ context.Context, site models.SiteConfig) CheckResult {
 	return CheckResult{SiteID: site.ID, Status: string(models.StatusUp), LatencyNs: stats.AvgRtt.Nanoseconds()}
 }
 
-func runPortCheck(_ context.Context, site models.SiteConfig) CheckResult {
+func runPortCheck(_ context.Context, site models.SiteConfig, pinnedIP net.IP) CheckResult {
 	host := site.Hostname
 	if host == "" {
 		host = site.URL
 	}
+	if pinnedIP != nil {
+		host = pinnedIP.String()
+	}
 	addr := net.JoinHostPort(host, strconv.Itoa(site.Port))
 	timeout := siteTimeout(site)
 

internal/monitor/checker_test.go

@@ -161,6 +161,43 @@ func TestRunCheck_Port_Closed(t *testing.T) {
 	}
 }
 
+func TestRunPortCheck_UsesPinnedIP(t *testing.T) {
+	ln, err := net.Listen("tcp", "127.0.0.1:0")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer ln.Close()
+
+	_, portStr, _ := net.SplitHostPort(ln.Addr().String())
+	port, _ := strconv.Atoi(portStr)
+
+	// Pass a pinned IP — runPortCheck should dial it instead of resolving Hostname.
+	site := models.SiteConfig{ID: 1, Type: "port", Hostname: "will-not-resolve.invalid", Port: port, Timeout: 2}
+	result := runPortCheck(context.Background(), site, net.ParseIP("127.0.0.1"))
+
+	if result.Status != "UP" {
+		t.Errorf("expected UP when pinned IP used, got %s: %s", result.Status, result.ErrorReason)
+	}
+}
+
+func TestRunPortCheck_NilPinnedIP_UsesHostname(t *testing.T) {
+	ln, err := net.Listen("tcp", "127.0.0.1:0")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer ln.Close()
+
+	_, portStr, _ := net.SplitHostPort(ln.Addr().String())
+	port, _ := strconv.Atoi(portStr)
+
+	site := models.SiteConfig{ID: 1, Type: "port", Hostname: "127.0.0.1", Port: port, Timeout: 2}
+	result := runPortCheck(context.Background(), site, nil)
+
+	if result.Status != "UP" {
+		t.Errorf("expected UP with nil pinnedIP fallback, got %s: %s", result.Status, result.ErrorReason)
+	}
+}
+
 func TestRunCheck_Port_BlocksPrivateByDefault(t *testing.T) {
 	ln, err := net.Listen("tcp", "127.0.0.1:0")
 	if err != nil {