Option to avoid deleting the kernel_ll address from bridges.

integration/networking/bridge_test.go

@@ -469,7 +469,6 @@ func TestDefaultBridgeAddresses(t *testing.T) {
 	skip.If(t, testEnv.DaemonInfo.OSType == "windows")
 
 	ctx := setupTest(t)
-	d := daemon.New(t)
 
 	type testStep struct {
 		stepName    string
@@ -487,21 +486,21 @@ func TestDefaultBridgeAddresses(t *testing.T) {
 				{
 					stepName:    "Set up initial UL prefix",
 					fixedCIDRV6: "fd1c:f1a0:5d8d:aaaa::/64",
-					expAddrs:    []string{"fd1c:f1a0:5d8d:aaaa::1/64", "fe80::1/64"},
+					expAddrs:    []string{"fd1c:f1a0:5d8d:aaaa::1/64", "fe80::"},
 				},
 				{
 					// Modify that prefix, the default bridge's address must be deleted and re-added.
 					stepName:    "Modify UL prefix - address change",
 					fixedCIDRV6: "fd1c:f1a0:5d8d:bbbb::/64",
-					expAddrs:    []string{"fd1c:f1a0:5d8d:bbbb::1/64", "fe80::1/64"},
+					expAddrs:    []string{"fd1c:f1a0:5d8d:bbbb::1/64", "fe80::"},
 				},
 				{
 					// Modify the prefix length, the default bridge's address should not change.
 					stepName:    "Modify UL prefix - no address change",
 					fixedCIDRV6: "fd1c:f1a0:5d8d:bbbb::/80",
 					// The prefix length displayed by 'ip a' is not updated - it's informational, and
 					// can't be changed without unnecessarily deleting and re-adding the address.
-					expAddrs: []string{"fd1c:f1a0:5d8d:bbbb::1/64", "fe80::1/64"},
+					expAddrs: []string{"fd1c:f1a0:5d8d:bbbb::1/64", "fe80::"},
 				},
 			},
 		},
@@ -511,47 +510,63 @@ func TestDefaultBridgeAddresses(t *testing.T) {
 				{
 					stepName:    "Standard LL subnet prefix",
 					fixedCIDRV6: "fe80::/64",
-					expAddrs:    []string{"fe80::1/64"},
+					expAddrs:    []string{"fe80::"},
 				},
 				{
 					// Modify that prefix, the default bridge's address must be deleted and re-added.
 					// The bridge must still have an address in the required (standard) LL subnet.
 					stepName:    "Nonstandard LL prefix - address change",
 					fixedCIDRV6: "fe80:1234::/32",
-					expAddrs:    []string{"fe80:1234::1/32", "fe80::1/64"},
+					expAddrs:    []string{"fe80:1234::1/32", "fe80::"},
 				},
 				{
 					// Modify the prefix length, the addresses should not change.
 					stepName:    "Modify LL prefix - no address change",
 					fixedCIDRV6: "fe80:1234::/64",
 					// The prefix length displayed by 'ip a' is not updated - it's informational, and
 					// can't be changed without unnecessarily deleting and re-adding the address.
-					expAddrs: []string{"fe80:1234::1/", "fe80::1/64"},
+					expAddrs: []string{"fe80:1234::1/", "fe80::"},
 				},
 			},
 		},
 	}
 
-	for _, tc := range testcases {
-		t.Run(tc.name, func(t *testing.T) {
+	for _, preserveKernelLL := range []bool{false, true} {
+		var dopts []daemon.Option
+		if preserveKernelLL {
+			dopts = append(dopts, daemon.WithEnvVars("DOCKER_BRIDGE_PRESERVE_KERNEL_LL=1"))
+		}
+		d := daemon.New(t, dopts...)
+		c := d.NewClientT(t)
+
+		for _, tc := range testcases {
 			for _, step := range tc.steps {
-				// Check that the daemon starts - regression test for:
-				//   https://github.com/moby/moby/issues/46829
-				d.Start(t, "--experimental", "--ipv6", "--ip6tables", "--fixed-cidr-v6="+step.fixedCIDRV6)
-				d.Stop(t)
-
-				// Check that the expected addresses have been applied to the bridge. (Skip in
-				// rootless mode, because the bridge is in a different network namespace.)
-				if !testEnv.IsRootless() {
-					res := testutil.RunCommand(ctx, "ip", "-6", "addr", "show", "docker0")
-					assert.Equal(t, res.ExitCode, 0, step.stepName)
-					stdout := res.Stdout()
-					for _, expAddr := range step.expAddrs {
-						assert.Check(t, is.Contains(stdout, expAddr))
+				tcName := fmt.Sprintf("kernel_ll_%v/%s/%s", preserveKernelLL, tc.name, step.stepName)
+				t.Run(tcName, func(t *testing.T) {
+					ctx := testutil.StartSpan(ctx, t)
+					// Check that the daemon starts - regression test for:
+					//   https://github.com/moby/moby/issues/46829
+					d.StartWithBusybox(ctx, t, "--experimental", "--ipv6", "--ip6tables", "--fixed-cidr-v6="+step.fixedCIDRV6)
+
+					// Start a container, so that the bridge is set "up" and gets a kernel_ll address.
+					cID := container.Run(ctx, t, c)
+					defer c.ContainerRemove(ctx, cID, containertypes.RemoveOptions{Force: true})
+
+					d.Stop(t)
+
+					// Check that the expected addresses have been applied to the bridge. (Skip in
+					// rootless mode, because the bridge is in a different network namespace.)
+					if !testEnv.IsRootless() {
+						res := testutil.RunCommand(ctx, "ip", "-6", "addr", "show", "docker0")
+						assert.Equal(t, res.ExitCode, 0, step.stepName)
+						stdout := res.Stdout()
+						for _, expAddr := range step.expAddrs {
+							assert.Check(t, is.Contains(stdout, expAddr))
+						}
 					}
-				}
+				})
 			}
-		})
+		}
 	}
 }
 

libnetwork/drivers/bridge/interface_linux.go

@@ -5,6 +5,7 @@ import (
 	"fmt"
 	"net"
 	"net/netip"
+	"os"
 
 	"github.com/containerd/log"
 	"github.com/docker/docker/errdefs"
@@ -73,18 +74,20 @@ func (i *bridgeInterface) addresses(family int) ([]netlink.Addr, error) {
 func getRequiredIPv6Addrs(config *networkConfiguration) (requiredAddrs map[netip.Addr]netip.Prefix, err error) {
 	requiredAddrs = make(map[netip.Addr]netip.Prefix)
 
-	// Always give the bridge 'fe80::1' - every interface is required to have an
-	// address in 'fe80::/64'. Linux may assign an address, but we'll replace it with
-	// 'fe80::1'. Then, if the configured prefix is 'fe80::/64', the IPAM pool
-	// assigned address will not be a second address in the LL subnet.
-	ra, ok := netiputil.ToPrefix(bridgeIPv6)
-	if !ok {
-		err = fmt.Errorf("Failed to convert Link-Local IPv6 address to netip.Prefix")
-		return nil, err
+	if os.Getenv("DOCKER_BRIDGE_PRESERVE_KERNEL_LL") != "1" {
+		// Always give the bridge 'fe80::1' - every interface is required to have an
+		// address in 'fe80::/64'. Linux may assign an address, but we'll replace it with
+		// 'fe80::1'. Then, if the configured prefix is 'fe80::/64', the IPAM pool
+		// assigned address will not be a second address in the LL subnet.
+		ra, ok := netiputil.ToPrefix(bridgeIPv6)
+		if !ok {
+			err = fmt.Errorf("Failed to convert Link-Local IPv6 address to netip.Prefix")
+			return nil, err
+		}
+		requiredAddrs[ra.Addr()] = ra
 	}
-	requiredAddrs[ra.Addr()] = ra
 
-	ra, ok = netiputil.ToPrefix(config.AddressIPv6)
+	ra, ok := netiputil.ToPrefix(config.AddressIPv6)
 	if !ok {
 		err = fmt.Errorf("failed to convert bridge IPv6 address '%s' to netip.Prefix", config.AddressIPv6.String())
 		return nil, err
@@ -116,6 +119,14 @@ func (i *bridgeInterface) programIPv6Addresses(config *networkConfiguration) err
 		if !ok {
 			return errdefs.System(fmt.Errorf("Failed to convert IPv6 address '%s' to netip.Addr", config.AddressIPv6))
 		}
+		// Optionally, avoid deleting the kernel-assigned link local address.
+		// (Don't delete fe80::1 either - if it was previously assigned to the bridge, and the
+		// kernel_ll address was deleted, the bridge won't get a new kernel_ll address.)
+		if os.Getenv("DOCKER_BRIDGE_PRESERVE_KERNEL_LL") == "1" {
+			if p, _ := ea.Prefix(64); p == linkLocalPrefix {
+				continue
+			}
+		}
 		// Ignore the prefix length when comparing addresses, it's informational
 		// (RFC-5942 section 4), and removing/re-adding an address that's still valid
 		// would disrupt traffic on live-restore.

libnetwork/drivers/bridge/setup_ipv6_linux.go

@@ -4,6 +4,7 @@ import (
 	"context"
 	"fmt"
 	"net"
+	"net/netip"
 	"os"
 
 	"github.com/containerd/log"
@@ -13,6 +14,9 @@ import (
 // bridgeIPv6 is the default, link-local IPv6 address for the bridge (fe80::1/64)
 var bridgeIPv6 = &net.IPNet{IP: net.ParseIP("fe80::1"), Mask: net.CIDRMask(64, 128)}
 
+// Standard link local prefix
+var linkLocalPrefix = netip.MustParsePrefix("fe80::/64")
+
 const (
 	ipv6ForwardConfPerm    = 0o644
 	ipv6ForwardConfDefault = "/proc/sys/net/ipv6/conf/default/forwarding"