diff --git a/api.go b/api.go
index ab294b4..66fa0ff 100644
--- a/api.go
+++ b/api.go
@@ -136,7 +136,7 @@ func handleWrite(rw http.ResponseWriter, r *http.Request) {
 	}
 
 	dec := lineprotocol.NewDecoderWithBytes(bytes)
-	if err := decodeLine(dec); err != nil {
+	if err := decodeLine(dec, r.URL.Query().Get("cluster")); err != nil {
 		http.Error(rw, err.Error(), http.StatusBadRequest)
 		return
 	}
diff --git a/lineprotocol.go b/lineprotocol.go
index a1829f1..d8b4af5 100644
--- a/lineprotocol.go
+++ b/lineprotocol.go
@@ -20,7 +20,7 @@ type Metric struct {
 // Connect to a nats server and subscribe to "updates". This is a blocking
 // function. handleLine will be called for each line recieved via nats.
 // Send `true` through the done channel for gracefull termination.
-func ReceiveNats(conf *NatsConfig, handleLine func(dec *lineprotocol.Decoder) error, workers int, ctx context.Context) error {
+func ReceiveNats(conf *NatsConfig, handleLine func(*lineprotocol.Decoder, string) error, workers int, ctx context.Context) error {
 	var opts []nats.Option
 	if conf.Username != "" && conf.Password != "" {
 		opts = append(opts, nats.UserInfo(conf.Username, conf.Password))
@@ -44,7 +44,7 @@ func ReceiveNats(conf *NatsConfig, handleLine func(dec *lineprotocol.Decoder) er
 			go func() {
 				for m := range msgs {
 					dec := lineprotocol.NewDecoderWithBytes(m.Data)
-					if err := handleLine(dec); err != nil {
+					if err := handleLine(dec, conf.ClusterTag); err != nil {
 						log.Printf("error: %s\n", err.Error())
 					}
 				}
@@ -59,7 +59,7 @@ func ReceiveNats(conf *NatsConfig, handleLine func(dec *lineprotocol.Decoder) er
 	} else {
 		sub, err = nc.Subscribe(conf.SubscribeTo, func(m *nats.Msg) {
 			dec := lineprotocol.NewDecoderWithBytes(m.Data)
-			if err := handleLine(dec); err != nil {
+			if err := handleLine(dec, conf.ClusterTag); err != nil {
 				log.Printf("error: %s\n", err.Error())
 			}
 		})
@@ -104,7 +104,9 @@ func reorder(buf, prefix []byte) []byte {
 	}
 }
 
-func decodeLine(dec *lineprotocol.Decoder) error {
+// Decode lines using dec and make write calls to the MemoryStore.
+// If a line is missing its cluster tag, use clusterDefault as default.
+func decodeLine(dec *lineprotocol.Decoder, clusterDefault string) error {
 	// Reduce allocations in loop:
 	t := time.Now()
 	metrics := make([]Metric, 0, 10)
@@ -138,7 +140,7 @@ func decodeLine(dec *lineprotocol.Decoder) error {
 		}
 
 		typeBuf, subTypeBuf := typeBuf[:0], subTypeBuf[:0]
-		var cluster, host string
+		cluster, host := clusterDefault, ""
 		for {
 			key, val, err := dec.NextTag()
 			if err != nil {
diff --git a/lineprotocol_test.go b/lineprotocol_test.go
index 83efe84..520003c 100644
--- a/lineprotocol_test.go
+++ b/lineprotocol_test.go
@@ -10,7 +10,7 @@ import (
 const TestDataClassicFormat string = `
 m1,cluster=ctest,hostname=htest1,type=node value=1 123456789
 m2,cluster=ctest,hostname=htest1,type=node value=2 123456789
-m3,cluster=ctest,hostname=htest2,type=node value=3 123456789
+m3,hostname=htest2,type=node value=3 123456789
 m4,cluster=ctest,hostname=htest2,type=core,type-id=1 value=4 123456789
 m4,cluster=ctest,hostname=htest2,type-id=2,type=core value=5 123456789
 `
@@ -29,7 +29,7 @@ func TestLineprotocolDecoder(t *testing.T) {
 	})
 
 	dec := lineprotocol.NewDecoderWithBytes([]byte(TestDataClassicFormat))
-	if err := decodeLine(dec); err != nil {
+	if err := decodeLine(dec, "ctest"); err != nil {
 		log.Fatal(err)
 	}
 
diff --git a/metric-store.go b/metric-store.go
index ecec370..28f3e9a 100644
--- a/metric-store.go
+++ b/metric-store.go
@@ -41,6 +41,9 @@ type NatsConfig struct {
 	// Channel name
 	SubscribeTo string `json:"subscribe-to"`
 
+	// Allow lines without a cluster tag, use this as default, optional
+	ClusterTag string `json:"cluster-tag"`
+
 	// Username/Password, optional
 	Username string `json:"username"`
 	Password string `json:"password"`
diff --git a/openapi.yaml b/openapi.yaml
index e3d31f6..02b2e71 100644
--- a/openapi.yaml
+++ b/openapi.yaml
@@ -10,6 +10,11 @@ paths:
     post:
       operationId: 'writeMetrics'
       description: 'Recieves metrics in the influx line-protocol using [this format](https://github.com/ClusterCockpit/cc-specifications/blob/master/metrics/lineprotocol_alternative.md)'
+      parameters:
+        - name: cluster
+          in: query
+          schema: { type: string }
+          description: "If the lines in the body do not have a cluster tag, use this value instead."
       requestBody:
         required: true
         content: