Make nodenames unique in Gossip cluster

Signed-off-by: Santhosh Manohar <santhosh@docker.com>

agent.go

@@ -9,6 +9,7 @@ import (
 	"sort"
 
 	"github.com/Sirupsen/logrus"
+	"github.com/docker/docker/pkg/stringid"
 	"github.com/docker/go-events"
 	"github.com/docker/libnetwork/datastore"
 	"github.com/docker/libnetwork/discoverapi"
@@ -246,10 +247,25 @@ func (c *controller) agentInit(bindAddrOrInterface, advertiseAddr string) error
 	}
 
 	keys, tags := c.getKeys(subsysGossip)
+
+	// Make the hostname unique in the gossip cluster by appending a 12
+	// byte random ID. Between distros there is no consensus if the
+	// hostname should be an FQDN or shortname. Limit the hostname to
+	// RFC 1123 compliant 63 bytes
 	hostname, _ := os.Hostname()
+	id := stringid.TruncateID(stringid.GenerateRandomID())
+
+	var nodeName string
+	if len(hostname)+len(id) > 63 {
+		nodeName = hostname[:63-len(id)] + "-" + id
+	} else {
+		nodeName = hostname + "-" + id
+	}
+	logrus.Infof("Gossip cluster hostname ", nodeName)
+
 	nDB, err := networkdb.New(&networkdb.Config{
 		AdvertiseAddr: advertiseAddr,
-		NodeName:      hostname,
+		NodeName:      nodeName,
 		Keys:          keys,
 	})