vxlan: Generate MAC address before creating a link

systemd 242+ assigns MAC addresses for all virtual devices which don't
have the address assigned already. That resulted in systemd overriding
MAC addresses of flannel.* interfaces. The fix which prevents systemd
from setting the address is to define the concrete MAC address when
creating the link.

Fixes: flannel-io#1155
Ref: k3s-io/k3s#4188
Signed-off-by: Michal Rostecki <mrostecki@opensuse.org>

backend/vxlan/device.go

@@ -13,7 +13,6 @@
 // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 // See the License for the specific language governing permissions and
 // limitations under the License.
-// +build !windows
 
 package vxlan
 
@@ -24,6 +23,7 @@ import (
 
 	"github.com/containernetworking/plugins/pkg/utils/sysctl"
 	"github.com/flannel-io/flannel/pkg/ip"
+	"github.com/flannel-io/flannel/pkg/mac"
 	"github.com/vishvananda/netlink"
 	log "k8s.io/klog"
 )
@@ -44,9 +44,15 @@ type vxlanDevice struct {
 }
 
 func newVXLANDevice(devAttrs *vxlanDeviceAttrs) (*vxlanDevice, error) {
+	hardwareAddr, err := mac.NewHardwareAddr()
+	if err != nil {
+		return nil, err
+	}
+
 	link := &netlink.Vxlan{
 		LinkAttrs: netlink.LinkAttrs{
-			Name: devAttrs.name,
+			Name:         devAttrs.name,
+			HardwareAddr: hardwareAddr,
 		},
 		VxlanId:      int(devAttrs.vni),
 		VtepDevIndex: devAttrs.vtepIndex,
@@ -56,7 +62,7 @@ func newVXLANDevice(devAttrs *vxlanDeviceAttrs) (*vxlanDevice, error) {
 		GBP:          devAttrs.gbp,
 	}
 
-	link, err := ensureLink(link)
+	link, err = ensureLink(link)
 	if err != nil {
 		return nil, err
 	}

pkg/mac/mac.go

@@ -0,0 +1,35 @@
+// Copyright 2021 flannel authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package mac
+
+import (
+	"crypto/rand"
+	"fmt"
+	"net"
+)
+
+// NewHardwareAddr generates a new random hardware (MAC) address, local and
+// unicast.
+func NewHardwareAddr() (net.HardwareAddr, error) {
+	hardwareAddr := make(net.HardwareAddr, 6)
+	if _, err := rand.Read(hardwareAddr); err != nil {
+		return nil, fmt.Errorf("could not generate random MAC address: %w", err)
+	}
+
+	// Ensure that address is locally administered and unicast.
+	hardwareAddr[0] = (hardwareAddr[0] & 0xfe) | 0x02
+
+	return hardwareAddr, nil
+}

pkg/mac/mac_test.go

@@ -0,0 +1,28 @@
+// Copyright 2021 flannel authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package mac
+
+import (
+	"testing"
+)
+
+func TestNewHardwareAddr(t *testing.T) {
+	// Ignore the actual address, since it's random.
+	// But an error should never be returned.
+	_, err := NewHardwareAddr()
+	if err != nil {
+		t.Fatalf(err)
+	}
+}