diff --git a/internal/checker/checker.go b/internal/checker/checker.go
index c2d8eba..392b046 100644
--- a/internal/checker/checker.go
+++ b/internal/checker/checker.go
@@ -18,6 +18,10 @@ const (
 	TypeTCP  = "tcp"
 	TypeICMP = "icmp"
 	TypeTLS  = "tls"
+
+	// UserAgent identifies Vantage monitor traffic so a WAF rule can single it
+	// out. Match on a prefix, not equality: the version moves.
+	UserAgent = "Vantage-Monitor/1.0 (+https://vantage.hostxtra.co.uk)"
 )
 
 
@@ -84,6 +88,7 @@ func runHTTP(ctx context.Context, s Spec) Result {
 	if err != nil {
 		return Result{Message: err.Error()}
 	}
+	req.Header.Set("User-Agent", UserAgent)
 	resp, err := client.Do(req)
 	if err != nil {
 		return Result{LatencyMs: msSince(start), Message: err.Error()}