f-46: dhcpbackend: drive dnsmasq or the built-in server through one interface #46

internal/subnet et internal/vm ne parlent plus à dnsmasq en direct mais à une
interface Backend, choisie au démarrage par dhcp.backend. Le défaut restant
dnsmasq, un agent.yml de 0.1.0 se comporte à l'identique ; ValidBackend refuse
toute autre valeur que dnsmasq ou two.

Les entrées ip→mac sont écrites hors du backend : elles font autorité pour les
deux, internal/vm/data.go les lisant par GetMACForIP quel que soit le serveur.
D'où l'extraction de dhcp.Entries, que GenerateConfig mêlait à l'écriture du
fichier dnsmasq.

Reservation porte Index et DefaultRoute : dnsmasq en dérive ses tags et n'en
pose que sur les interfaces sans route par défaut, le backend two transmet le
drapeau tel quel. La notion de tag ne remonte plus jusqu'à internal/vm.

Two.ConfigureSubnet sonde get-state avant de pousser la configuration : le job
systemd d'une unit Type=simple est done dès le fork, avant que la socket
existe. TeardownSubnet arrête l'unit avant de supprimer le .state.

Le backend two est testé contre un vrai serveur sur une vraie socket. Onze
mutations, toutes détectées.

Signed-off-by: GnomeZworc <nicolas.boufidjeline@g3e.fr>

internal/dhcpbackend/backend.go

@@ -0,0 +1,53 @@
+package dhcpbackend
+
+import (
+	"fmt"
+	"net"
+
+	configuration "git.g3e.fr/syonad/two/internal/config/agent"
+)
+
+type Subnet struct {
+	Name           string
+	VPC            string
+	Bridge         string
+	Network        *net.IPNet
+	InterfaceIP    net.IP
+	VPCRoute       *net.IPNet
+	DefaultGateway net.IP
+}
+
+func (s Subnet) Instance() string {
+	return s.VPC + "_" + s.Bridge
+}
+
+type Reservation struct {
+	Index        int
+	MAC          string
+	IP           string
+	DefaultRoute bool
+}
+
+type Backend interface {
+	Unit(s Subnet) string
+	ConfigureSubnet(s Subnet) error
+	TeardownSubnet(s Subnet) error
+	SetVM(s Subnet, vmName string, res []Reservation) error
+	DelVM(s Subnet, vmName string, res []Reservation) error
+}
+
+func New(cfg *configuration.Config) (Backend, error) {
+	if cfg == nil {
+		return nil, fmt.Errorf("configuration is required to pick a dhcp backend")
+	}
+	if err := configuration.ValidBackend(cfg.DHCP.Backend); err != nil {
+		return nil, err
+	}
+
+	switch cfg.DHCP.Backend {
+	case configuration.BackendTwo:
+		return Two{}, nil
+	default:
+		return Dnsmasq{}, nil
+	}
+}

internal/dhcpbackend/backend_test.go

@@ -0,0 +1,99 @@
+package dhcpbackend
+
+import (
+	"net"
+	"testing"
+
+	configuration "git.g3e.fr/syonad/two/internal/config/agent"
+)
+
+func testSubnet(t *testing.T) Subnet {
+	t.Helper()
+	_, network, err := net.ParseCIDR("10.0.5.0/24")
+	if err != nil {
+		t.Fatalf("ParseCIDR: %v", err)
+	}
+	_, vpcRoute, err := net.ParseCIDR("10.0.0.0/16")
+	if err != nil {
+		t.Fatalf("ParseCIDR: %v", err)
+	}
+	return Subnet{
+		Name:           "sn-000001",
+		VPC:            "vp-admin",
+		Bridge:         "br-000001",
+		Network:        network,
+		InterfaceIP:    net.ParseIP("10.0.5.1"),
+		VPCRoute:       vpcRoute,
+		DefaultGateway: net.ParseIP("10.0.5.254"),
+	}
+}
+
+func configFor(backend string) *configuration.Config {
+	cfg := &configuration.Config{}
+	cfg.DHCP.Backend = backend
+	return cfg
+}
+
+func TestInstance_JoinsVPCAndBridge(t *testing.T) {
+	if got := testSubnet(t).Instance(); got != "vp-admin_br-000001" {
+		t.Errorf("Instance = %s, want vp-admin_br-000001", got)
+	}
+}
+
+func TestNew_DnsmasqIsTheDefault(t *testing.T) {
+	backend, err := New(configFor(configuration.BackendDnsmasq))
+	if err != nil {
+		t.Fatalf("New: %v", err)
+	}
+	if _, ok := backend.(Dnsmasq); !ok {
+		t.Errorf("backend = %T, want Dnsmasq", backend)
+	}
+}
+
+func TestNew_ReturnsTheTwoBackendWhenAsked(t *testing.T) {
+	backend, err := New(configFor(configuration.BackendTwo))
+	if err != nil {
+		t.Fatalf("New: %v", err)
+	}
+	if _, ok := backend.(Two); !ok {
+		t.Errorf("backend = %T, want Two", backend)
+	}
+}
+
+func TestNew_RejectsAnUnknownBackend(t *testing.T) {
+	if _, err := New(configFor("dhcpd")); err == nil {
+		t.Fatal("an unknown backend must be reported rather than silently defaulted")
+	}
+}
+
+func TestNew_RejectsAnEmptyBackend(t *testing.T) {
+	if _, err := New(configFor("")); err == nil {
+		t.Fatal("an empty backend must be reported: the config default is what fills it")
+	}
+}
+
+func TestNew_RejectsANilConfig(t *testing.T) {
+	if _, err := New(nil); err == nil {
+		t.Fatal("a nil config must be reported")
+	}
+}
+
+func TestUnit_NamesADistinctUnitPerBackend(t *testing.T) {
+	s := testSubnet(t)
+
+	if got := (Dnsmasq{}).Unit(s); got != "dnsmasq@vp-admin_br-000001.service" {
+		t.Errorf("dnsmasq unit = %s", got)
+	}
+	if got := (Two{}).Unit(s); got != "dhcp@vp-admin_br-000001.service" {
+		t.Errorf("two unit = %s", got)
+	}
+}
+
+func TestTag_IsPerInterfaceNotPerVM(t *testing.T) {
+	if tag("vm-web", 0) == tag("vm-web", 1) {
+		t.Error("two interfaces of the same vm must get distinct tags")
+	}
+	if got := tag("vm-web", 1); got != "vm-web-1" {
+		t.Errorf("tag = %s, want vm-web-1", got)
+	}
+}

internal/dhcpbackend/dnsmasq.go

@@ -0,0 +1,127 @@
+package dhcpbackend
+
+import (
+	"fmt"
+
+	"git.g3e.fr/syonad/two/internal/dhcp"
+	"git.g3e.fr/syonad/two/pkg/systemd"
+)
+
+type Dnsmasq struct {
+	ConfDir string
+}
+
+func (b Dnsmasq) confDir() string {
+	if b.ConfDir == "" {
+		return dhcp.DefaultConfDir
+	}
+	return b.ConfDir
+}
+
+func tag(vmName string, index int) string {
+	return fmt.Sprintf("%s-%d", vmName, index)
+}
+
+func (Dnsmasq) Unit(s Subnet) string {
+	return dhcp.UnitName(s.Instance())
+}
+
+func (b Dnsmasq) config(s Subnet) dhcp.Config {
+	return dhcp.Config{
+		Network:        s.Network,
+		Name:           s.Instance(),
+		ConfDir:        b.confDir(),
+		InterfaceIP:    s.InterfaceIP,
+		VPCRoute:       s.VPCRoute,
+		DefaultGateway: s.DefaultGateway,
+	}
+}
+
+func (b Dnsmasq) ConfigureSubnet(s Subnet) error {
+	if _, _, err := dhcp.GenerateConfig(b.config(s)); err != nil {
+		return fmt.Errorf("generate dhcp config: %w", err)
+	}
+
+	svc, err := systemd.New()
+	if err != nil {
+		return fmt.Errorf("connect to systemd: %w", err)
+	}
+	defer svc.Close()
+
+	if err := svc.Start(b.Unit(s)); err != nil {
+		return fmt.Errorf("start dnsmasq: %w", err)
+	}
+	return nil
+}
+
+func (b Dnsmasq) TeardownSubnet(s Subnet) error {
+	svc, err := systemd.New()
+	if err != nil {
+		return fmt.Errorf("connect to systemd: %w", err)
+	}
+	defer svc.Close()
+
+	unit := b.Unit(s)
+	if status, err := svc.Status(unit); err == nil && status.ActiveState == "active" {
+		if err := svc.Stop(unit); err != nil {
+			return fmt.Errorf("stop dnsmasq: %w", err)
+		}
+	}
+
+	if err := dhcp.RemoveConfig(b.confDir(), s.Instance()); err != nil {
+		return err
+	}
+	return dhcp.RemoveSubnetDirs(b.confDir(), s.Instance())
+}
+
+func (b Dnsmasq) SetVM(s Subnet, vmName string, res []Reservation) error {
+	instance := s.Instance()
+
+	reservations := make([]dhcp.Reservation, 0, len(res))
+	var tags []string
+	for _, r := range res {
+		reservations = append(reservations, dhcp.Reservation{
+			MAC: r.MAC, IP: r.IP, Tag: tag(vmName, r.Index),
+		})
+		if !r.DefaultRoute {
+			tags = append(tags, tag(vmName, r.Index))
+		}
+	}
+
+	if err := dhcp.WriteReservations(b.confDir(), instance, vmName, reservations); err != nil {
+		return fmt.Errorf("write dhcp reservations on %s: %w", instance, err)
+	}
+
+	options := dhcp.Config{InterfaceIP: s.InterfaceIP, VPCRoute: s.VPCRoute}
+	if err := dhcp.WriteVMOptions(b.confDir(), instance, vmName, tags, options); err != nil {
+		return fmt.Errorf("write dhcp options on %s: %w", instance, err)
+	}
+	return nil
+}
+
+func (b Dnsmasq) DelVM(s Subnet, vmName string, _ []Reservation) error {
+	if err := dhcp.RemoveReservations(b.confDir(), s.Instance(), vmName); err != nil {
+		return err
+	}
+
+	svc, err := systemd.New()
+	if err != nil {
+		return fmt.Errorf("connect to systemd: %w", err)
+	}
+	defer svc.Close()
+
+	unit := b.Unit(s)
+	status, err := svc.Status(unit)
+	if err != nil || status.ActiveState != "active" {
+		return nil
+	}
+	if err := svc.Restart(unit); err != nil {
+		return fmt.Errorf("restart %s: %w", unit, err)
+	}
+	if status, err := svc.Status(unit); err != nil {
+		return fmt.Errorf("status %s after restart: %w", unit, err)
+	} else if status.ActiveState != "active" {
+		return fmt.Errorf("%s is %s after restart", unit, status.ActiveState)
+	}
+	return nil
+}

internal/dhcpbackend/dnsmasq_test.go

@@ -0,0 +1,113 @@
+package dhcpbackend
+
+import (
+	"os"
+	"path/filepath"
+	"strings"
+	"testing"
+)
+
+func dnsmasqBackend(t *testing.T) Dnsmasq {
+	t.Helper()
+	return Dnsmasq{ConfDir: t.TempDir()}
+}
+
+func readFile(t *testing.T, path string) string {
+	t.Helper()
+	raw, err := os.ReadFile(path)
+	if err != nil {
+		t.Fatalf("read %s: %v", path, err)
+	}
+	return string(raw)
+}
+
+func TestDnsmasq_ConfDirDefaultsToTheSystemOne(t *testing.T) {
+	if got := (Dnsmasq{}).confDir(); got != "/etc/dnsmasq.d" {
+		t.Errorf("confDir = %s, want /etc/dnsmasq.d", got)
+	}
+}
+
+func TestDnsmasqSetVM_WritesOneReservationPerInterface(t *testing.T) {
+	b := dnsmasqBackend(t)
+	s := testSubnet(t)
+
+	res := []Reservation{
+		{Index: 0, MAC: "00:22:33:00:00:0a", IP: "10.0.5.10", DefaultRoute: true},
+		{Index: 1, MAC: "00:22:33:00:00:0b", IP: "10.0.5.11"},
+	}
+	if err := b.SetVM(s, "vm-web", res); err != nil {
+		t.Fatalf("SetVM: %v", err)
+	}
+
+	hosts := readFile(t, filepath.Join(b.ConfDir, s.Instance()+".hosts.d", "vm-web"))
+	for _, want := range []string{"00:22:33:00:00:0a,10.0.5.10,set:vm-web-0", "00:22:33:00:00:0b,10.0.5.11,set:vm-web-1"} {
+		if !strings.Contains(hosts, want) {
+			t.Errorf("hosts file missing %q:\n%s", want, hosts)
+		}
+	}
+}
+
+func TestDnsmasqSetVM_TagsOnlyTheInterfacesWithoutADefaultRoute(t *testing.T) {
+	b := dnsmasqBackend(t)
+	s := testSubnet(t)
+
+	res := []Reservation{
+		{Index: 0, MAC: "00:22:33:00:00:0a", IP: "10.0.5.10", DefaultRoute: true},
+		{Index: 1, MAC: "00:22:33:00:00:0b", IP: "10.0.5.11"},
+	}
+	if err := b.SetVM(s, "vm-web", res); err != nil {
+		t.Fatalf("SetVM: %v", err)
+	}
+
+	opts := readFile(t, filepath.Join(b.ConfDir, s.Instance()+".opts.d", "vm-web"))
+	if strings.Contains(opts, "tag:vm-web-0") {
+		t.Errorf("the interface carrying the default route must get no override:\n%s", opts)
+	}
+	if !strings.Contains(opts, "tag:vm-web-1,3\n") {
+		t.Errorf("the secondary interface must get a bare option 3:\n%s", opts)
+	}
+	if !strings.Contains(opts, "tag:vm-web-1,121,") {
+		t.Errorf("the secondary interface must get its own option 121:\n%s", opts)
+	}
+}
+
+func TestDnsmasqSetVM_SecondaryOptionsKeepTheMetadataRoute(t *testing.T) {
+	b := dnsmasqBackend(t)
+	s := testSubnet(t)
+
+	res := []Reservation{{Index: 1, MAC: "00:22:33:00:00:0b", IP: "10.0.5.11"}}
+	if err := b.SetVM(s, "vm-web", res); err != nil {
+		t.Fatalf("SetVM: %v", err)
+	}
+
+	opts := readFile(t, filepath.Join(b.ConfDir, s.Instance()+".opts.d", "vm-web"))
+	if !strings.Contains(opts, "169.254.169.254/32,10.0.5.1") {
+		t.Errorf("overriding option 121 without the metadata route breaks cloud-init:\n%s", opts)
+	}
+	if strings.Contains(opts, "0.0.0.0/0") {
+		t.Errorf("a secondary interface must not receive a default route:\n%s", opts)
+	}
+}
+
+func TestDnsmasqSetVM_AllInterfacesDefaultRoutedWritesNoOptions(t *testing.T) {
+	b := dnsmasqBackend(t)
+	s := testSubnet(t)
+
+	res := []Reservation{{Index: 0, MAC: "00:22:33:00:00:0a", IP: "10.0.5.10", DefaultRoute: true}}
+	if err := b.SetVM(s, "vm-web", res); err != nil {
+		t.Fatalf("SetVM: %v", err)
+	}
+
+	path := filepath.Join(b.ConfDir, s.Instance()+".opts.d", "vm-web")
+	if _, err := os.Stat(path); !os.IsNotExist(err) {
+		t.Errorf("no options file expected, got %v: the subnet-wide options already carry the default route", err)
+	}
+}
+
+func TestDnsmasqSetVM_RejectsAVMWithoutReservation(t *testing.T) {
+	b := dnsmasqBackend(t)
+
+	if err := b.SetVM(testSubnet(t), "vm-web", nil); err == nil {
+		t.Fatal("a vm with no reservation would get no address: that must be reported")
+	}
+}

internal/dhcpbackend/two.go

@@ -0,0 +1,139 @@
+package dhcpbackend
+
+import (
+	"fmt"
+	"time"
+
+	dhcpapi "git.g3e.fr/syonad/two/internal/api/dhcp"
+	dhcpclient "git.g3e.fr/syonad/two/internal/client/dhcp"
+	"git.g3e.fr/syonad/two/pkg/db/statefile"
+	"git.g3e.fr/syonad/two/pkg/systemd"
+)
+
+const (
+	readyTimeout = 5 * time.Second
+	readyPoll    = 50 * time.Millisecond
+)
+
+type Two struct {
+	RunDir string
+}
+
+func (b Two) runDir() string {
+	if b.RunDir == "" {
+		return dhcpapi.DefaultRunDir
+	}
+	return b.RunDir
+}
+
+func (b Two) Unit(s Subnet) string {
+	return dhcpapi.Unit(s.Instance())
+}
+
+func (b Two) client(s Subnet) *dhcpclient.Client {
+	return dhcpclient.New(dhcpapi.SocketPath(b.runDir(), s.Instance()))
+}
+
+func (b Two) statePath(s Subnet) string {
+	return dhcpapi.StatePath(b.runDir(), s.Instance())
+}
+
+func (b Two) waitReady(s Subnet, timeout, poll time.Duration) error {
+	client := b.client(s)
+	deadline := time.Now().Add(timeout)
+
+	var err error
+	for {
+		if _, _, err = client.GetState(); err == nil {
+			return nil
+		}
+		if time.Now().After(deadline) {
+			return fmt.Errorf("dhcp server for %s did not answer within %s: %w", s.Instance(), timeout, err)
+		}
+		time.Sleep(poll)
+	}
+}
+
+func (b Two) ConfigureSubnet(s Subnet) error {
+	if err := statefile.Remove(b.statePath(s)); err != nil {
+		return fmt.Errorf("remove residual state: %w", err)
+	}
+
+	svc, err := systemd.New()
+	if err != nil {
+		return fmt.Errorf("connect to systemd: %w", err)
+	}
+	defer svc.Close()
+
+	if err := svc.Start(b.Unit(s)); err != nil {
+		return fmt.Errorf("start dhcp: %w", err)
+	}
+	if err := b.waitReady(s, readyTimeout, readyPoll); err != nil {
+		return err
+	}
+
+	return b.pushSubnet(s)
+}
+
+func (b Two) pushSubnet(s Subnet) error {
+	subnet := dhcpapi.Subnet{
+		Network:     s.Network.String(),
+		InterfaceIP: s.InterfaceIP.String(),
+	}
+	if s.VPCRoute != nil {
+		subnet.VPCRoute = s.VPCRoute.String()
+	}
+	if s.DefaultGateway != nil {
+		subnet.DefaultGateway = s.DefaultGateway.String()
+	}
+
+	if err := b.client(s).SetSubnet(subnet); err != nil {
+		return fmt.Errorf("configure dhcp for %s: %w", s.Instance(), err)
+	}
+	return nil
+}
+
+func (b Two) TeardownSubnet(s Subnet) error {
+	svc, err := systemd.New()
+	if err != nil {
+		return fmt.Errorf("connect to systemd: %w", err)
+	}
+	defer svc.Close()
+
+	unit := b.Unit(s)
+	if status, err := svc.Status(unit); err == nil && status.ActiveState == "active" {
+		if err := svc.Stop(unit); err != nil {
+			return fmt.Errorf("stop dhcp: %w", err)
+		}
+	}
+
+	return statefile.Remove(b.statePath(s))
+}
+
+func (b Two) SetVM(s Subnet, vmName string, res []Reservation) error {
+	client := b.client(s)
+
+	for _, r := range res {
+		host := dhcpapi.Host{
+			MAC:          r.MAC,
+			IP:           r.IP,
+			VM:           vmName,
+			DefaultRoute: r.DefaultRoute,
+		}
+		if err := client.SetHost(host); err != nil {
+			return fmt.Errorf("reserve %s for vm %s on %s: %w", r.MAC, vmName, s.Instance(), err)
+		}
+	}
+	return nil
+}
+
+func (b Two) DelVM(s Subnet, vmName string, res []Reservation) error {
+	client := b.client(s)
+
+	for _, r := range res {
+		if err := client.DelHost(r.MAC); err != nil {
+			return fmt.Errorf("release %s of vm %s on %s: %w", r.MAC, vmName, s.Instance(), err)
+		}
+	}
+	return nil
+}

internal/dhcpbackend/two_test.go

@@ -0,0 +1,254 @@
+package dhcpbackend
+
+import (
+	"io"
+	"log/slog"
+	"net"
+	"os"
+	"testing"
+	"time"
+
+	dhcpapi "git.g3e.fr/syonad/two/internal/api/dhcp"
+	"git.g3e.fr/syonad/two/internal/dhcpd"
+)
+
+func shortTempDir(t *testing.T) string {
+	t.Helper()
+	dir, err := os.MkdirTemp("", "dhcpd")
+	if err != nil {
+		t.Fatalf("MkdirTemp: %v", err)
+	}
+	t.Cleanup(func() { os.RemoveAll(dir) })
+	return dir
+}
+
+func twoBackend(t *testing.T) (Two, Subnet, *dhcpd.Store) {
+	t.Helper()
+
+	b := Two{RunDir: shortTempDir(t)}
+	s := testSubnet(t)
+
+	store := dhcpd.NewStore(dhcpapi.StatePath(b.RunDir, s.Instance()))
+	if err := store.Load(); err != nil {
+		t.Fatalf("Load: %v", err)
+	}
+
+	server, err := dhcpapi.Listen(store, dhcpapi.SocketPath(b.RunDir, s.Instance()), slog.New(slog.NewTextHandler(io.Discard, nil)))
+	if err != nil {
+		t.Fatalf("Listen: %v", err)
+	}
+	go server.Serve()
+	t.Cleanup(func() { server.Close() })
+
+	return b, s, store
+}
+
+func TestTwo_RunDirDefaultsToTheSharedOne(t *testing.T) {
+	if got := (Two{}).runDir(); got != dhcpapi.DefaultRunDir {
+		t.Errorf("runDir = %s, want %s", got, dhcpapi.DefaultRunDir)
+	}
+}
+
+func TestTwoWaitReady_ReturnsOnceTheServerAnswers(t *testing.T) {
+	b, s, _ := twoBackend(t)
+
+	if err := b.waitReady(s, time.Second, 10*time.Millisecond); err != nil {
+		t.Fatalf("waitReady: %v", err)
+	}
+}
+
+func TestTwoWaitReady_TimesOutWhenNothingListens(t *testing.T) {
+	b := Two{RunDir: shortTempDir(t)}
+	s := testSubnet(t)
+
+	start := time.Now()
+	err := b.waitReady(s, 200*time.Millisecond, 10*time.Millisecond)
+	if err == nil {
+		t.Fatal("waitReady must report a server that never came up")
+	}
+	if elapsed := time.Since(start); elapsed > 2*time.Second {
+		t.Errorf("waitReady took %s, want the 200ms budget to apply", elapsed)
+	}
+}
+
+func TestTwoPushSubnet_ReachesTheStore(t *testing.T) {
+	b, s, store := twoBackend(t)
+
+	if err := b.pushSubnet(s); err != nil {
+		t.Fatalf("pushSubnet: %v", err)
+	}
+
+	got, configured := store.Subnet()
+	if !configured {
+		t.Fatal("the subnet configuration did not reach the server")
+	}
+	if !got.InterfaceIP.Equal(net.ParseIP("10.0.5.1")) {
+		t.Errorf("interface ip = %s, want 10.0.5.1", got.InterfaceIP)
+	}
+	if got.VPCRoute == nil || got.VPCRoute.String() != "10.0.0.0/16" {
+		t.Errorf("vpc route = %v, want 10.0.0.0/16", got.VPCRoute)
+	}
+	if !got.DefaultGateway.Equal(net.ParseIP("10.0.5.254")) {
+		t.Errorf("default gateway = %s, want 10.0.5.254", got.DefaultGateway)
+	}
+}
+
+func TestTwoPushSubnet_OmitsAnAbsentVPCRouteAndGateway(t *testing.T) {
+	b, s, store := twoBackend(t)
+	s.VPCRoute = nil
+	s.DefaultGateway = nil
+
+	if err := b.pushSubnet(s); err != nil {
+		t.Fatalf("pushSubnet: %v", err)
+	}
+
+	got, _ := store.Subnet()
+	if got.VPCRoute != nil {
+		t.Errorf("vpc route = %v, want none", got.VPCRoute)
+	}
+	if got.DefaultGateway != nil {
+		t.Errorf("default gateway = %v, want none: a bridge subnet has no gateway of ours", got.DefaultGateway)
+	}
+}
+
+func TestTwoSetVM_ReservesEveryInterface(t *testing.T) {
+	b, s, store := twoBackend(t)
+	if err := b.pushSubnet(s); err != nil {
+		t.Fatalf("pushSubnet: %v", err)
+	}
+
+	res := []Reservation{
+		{Index: 0, MAC: "00:22:33:00:00:0a", IP: "10.0.5.10", DefaultRoute: true},
+		{Index: 1, MAC: "00:22:33:00:00:0b", IP: "10.0.5.11"},
+	}
+	if err := b.SetVM(s, "vm-web", res); err != nil {
+		t.Fatalf("SetVM: %v", err)
+	}
+
+	hosts := store.Hosts()
+	if len(hosts) != 2 {
+		t.Fatalf("hosts = %d, want 2", len(hosts))
+	}
+	for _, h := range hosts {
+		if h.VM != "vm-web" {
+			t.Errorf("host %s carries vm %q, want vm-web", h.MAC, h.VM)
+		}
+	}
+}
+
+func TestTwoSetVM_CarriesTheDefaultRouteFlagPerInterface(t *testing.T) {
+	b, s, store := twoBackend(t)
+	if err := b.pushSubnet(s); err != nil {
+		t.Fatalf("pushSubnet: %v", err)
+	}
+
+	res := []Reservation{
+		{Index: 0, MAC: "00:22:33:00:00:0a", IP: "10.0.5.10", DefaultRoute: true},
+		{Index: 1, MAC: "00:22:33:00:00:0b", IP: "10.0.5.11"},
+	}
+	if err := b.SetVM(s, "vm-web", res); err != nil {
+		t.Fatalf("SetVM: %v", err)
+	}
+
+	primary, known := store.Lookup(mustMAC(t, "00:22:33:00:00:0a"))
+	if !known || !primary.DefaultRoute {
+		t.Errorf("primary interface = %+v, want the default route", primary)
+	}
+	secondary, known := store.Lookup(mustMAC(t, "00:22:33:00:00:0b"))
+	if !known || secondary.DefaultRoute {
+		t.Errorf("secondary interface = %+v, want no default route", secondary)
+	}
+}
+
+func TestTwoSetVM_IsIdempotent(t *testing.T) {
+	b, s, store := twoBackend(t)
+	if err := b.pushSubnet(s); err != nil {
+		t.Fatalf("pushSubnet: %v", err)
+	}
+
+	res := []Reservation{{Index: 0, MAC: "00:22:33:00:00:0a", IP: "10.0.5.10", DefaultRoute: true}}
+	for range 3 {
+		if err := b.SetVM(s, "vm-web", res); err != nil {
+			t.Fatalf("SetVM: %v", err)
+		}
+	}
+	if got := len(store.Hosts()); got != 1 {
+		t.Errorf("hosts = %d, want 1", got)
+	}
+}
+
+func TestTwoSetVM_RejectsAnInvalidMAC(t *testing.T) {
+	b, s, _ := twoBackend(t)
+	if err := b.pushSubnet(s); err != nil {
+		t.Fatalf("pushSubnet: %v", err)
+	}
+
+	res := []Reservation{{Index: 0, MAC: "nope", IP: "10.0.5.10"}}
+	if err := b.SetVM(s, "vm-web", res); err == nil {
+		t.Fatal("an invalid mac must be reported")
+	}
+}
+
+func TestTwoDelVM_ReleasesEveryInterface(t *testing.T) {
+	b, s, store := twoBackend(t)
+	if err := b.pushSubnet(s); err != nil {
+		t.Fatalf("pushSubnet: %v", err)
+	}
+
+	res := []Reservation{
+		{Index: 0, MAC: "00:22:33:00:00:0a", IP: "10.0.5.10", DefaultRoute: true},
+		{Index: 1, MAC: "00:22:33:00:00:0b", IP: "10.0.5.11"},
+	}
+	if err := b.SetVM(s, "vm-web", res); err != nil {
+		t.Fatalf("SetVM: %v", err)
+	}
+	if err := b.DelVM(s, "vm-web", res); err != nil {
+		t.Fatalf("DelVM: %v", err)
+	}
+
+	if got := len(store.Hosts()); got != 0 {
+		t.Errorf("hosts = %d, want 0", got)
+	}
+}
+
+func TestTwoDelVM_LeavesOtherVMsAlone(t *testing.T) {
+	b, s, store := twoBackend(t)
+	if err := b.pushSubnet(s); err != nil {
+		t.Fatalf("pushSubnet: %v", err)
+	}
+
+	web := []Reservation{{Index: 0, MAC: "00:22:33:00:00:0a", IP: "10.0.5.10", DefaultRoute: true}}
+	db := []Reservation{{Index: 0, MAC: "00:22:33:00:00:0b", IP: "10.0.5.11", DefaultRoute: true}}
+	for name, res := range map[string][]Reservation{"vm-web": web, "vm-db": db} {
+		if err := b.SetVM(s, name, res); err != nil {
+			t.Fatalf("SetVM %s: %v", name, err)
+		}
+	}
+
+	if err := b.DelVM(s, "vm-web", web); err != nil {
+		t.Fatalf("DelVM: %v", err)
+	}
+
+	hosts := store.Hosts()
+	if len(hosts) != 1 || hosts[0].VM != "vm-db" {
+		t.Errorf("remaining hosts = %+v, want only vm-db", hosts)
+	}
+}
+
+func TestTwoDelVM_OnAnUnknownMACIsNotAnError(t *testing.T) {
+	b, s, _ := twoBackend(t)
+
+	res := []Reservation{{Index: 0, MAC: "00:22:33:ff:ff:ff", IP: "10.0.5.99"}}
+	if err := b.DelVM(s, "vm-gone", res); err != nil {
+		t.Errorf("releasing an absent reservation must be idempotent, got %v", err)
+	}
+}
+
+func mustMAC(t *testing.T, s string) net.HardwareAddr {
+	t.Helper()
+	m, err := net.ParseMAC(s)
+	if err != nil {
+		t.Fatalf("ParseMAC(%q): %v", s, err)
+	}
+	return m
+}