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/dhcp/generate.go

@@ -29,12 +29,7 @@ func GenerateConfig(c Config) (string, map[string]string, error) {
 	fmt.Fprintf(&sb, "dhcp-hostsdir=%s\n", HostsDir(c.ConfDir, c.Name))
 	fmt.Fprintf(&sb, "dhcp-optsdir=%s\n", OptsDir(c.ConfDir, c.Name))
 
-	entries := make(map[string]string)
-	i := 0
-	for ip := cloneIP(c.Network.IP); c.Network.Contains(ip); incrementIP(ip) {
-		entries[ip.String()] = fmt.Sprintf("00:22:33:%02X:%02X:%02X", (i>>16)&0xFF, (i>>8)&0xFF, i&0xFF)
-		i++
-	}
+	entries := Entries(c.Network)
 
 	for _, dir := range []string{c.ConfDir, HostsDir(c.ConfDir, c.Name), OptsDir(c.ConfDir, c.Name)} {
 		if err := os.MkdirAll(dir, 0755); err != nil {
@@ -46,6 +41,16 @@ func GenerateConfig(c Config) (string, map[string]string, error) {
 	return outPath, entries, os.WriteFile(outPath, []byte(sb.String()), 0644)
 }
 
+func Entries(network *net.IPNet) map[string]string {
+	entries := make(map[string]string)
+	i := 0
+	for ip := cloneIP(network.IP); network.Contains(ip); incrementIP(ip) {
+		entries[ip.String()] = fmt.Sprintf("00:22:33:%02X:%02X:%02X", (i>>16)&0xFF, (i>>8)&0xFF, i&0xFF)
+		i++
+	}
+	return entries
+}
+
 func classlessRoutes(c Config) []string {
 	nextHop := c.InterfaceIP.String()
 

internal/dhcp/reservation.go

@@ -117,3 +117,11 @@ func RemoveSubnetDirs(confDir, name string) error {
 	}
 	return nil
 }
+
+func RemoveConfig(confDir, name string) error {
+	path := filepath.Join(confDir, name+".conf")
+	if err := os.Remove(path); err != nil && !os.IsNotExist(err) {
+		return fmt.Errorf("remove %s: %w", path, err)
+	}
+	return nil
+}

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
+}

internal/dispatcher/agent/subnet_commands.go

@@ -84,7 +84,7 @@ func (c CreateSubnetCommand) Execute(db *badger.DB, cfg *configuration.Config) e
 		case <-time.After(time.Duration(cfg.Dispatcher.PollSeconds) * time.Second):
 		}
 	}
-	return subnet.CreateSubnet(db, c.Name)
+	return subnet.CreateSubnet(db, c.Name, cfg)
 }
 
 type DeleteSubnetCommand struct {
@@ -104,8 +104,8 @@ func (c DeleteSubnetCommand) Prepare(db *badger.DB, _ *configuration.Config) err
 	return state.Set(db, c.Key(), state.Deleting)
 }
 
-func (c DeleteSubnetCommand) Execute(db *badger.DB, _ *configuration.Config) error {
-	if err := subnet.DeleteSubnet(db, c.Name); err != nil {
+func (c DeleteSubnetCommand) Execute(db *badger.DB, cfg *configuration.Config) error {
+	if err := subnet.DeleteSubnet(db, c.Name, cfg); err != nil {
 		return err
 	}
 	current, err := state.Get(db, c.Key())

internal/subnet/create.go

@@ -3,17 +3,18 @@ package subnet
 import (
 	"fmt"
 
+	configuration "git.g3e.fr/syonad/two/internal/config/agent"
 	"git.g3e.fr/syonad/two/internal/dhcp"
+	"git.g3e.fr/syonad/two/internal/dhcpbackend"
 	"git.g3e.fr/syonad/two/internal/ebtables"
 	"git.g3e.fr/syonad/two/internal/netif"
 	"git.g3e.fr/syonad/two/internal/netns"
 	"git.g3e.fr/syonad/two/internal/state"
-	"git.g3e.fr/syonad/two/pkg/systemd"
 
 	"github.com/dgraph-io/badger/v4"
 )
 
-func CreateSubnet(db *badger.DB, subnetName string) error {
+func CreateSubnet(db *badger.DB, subnetName string, cfg *configuration.Config) error {
 	current, err := state.Get(db, "subnet/"+subnetName)
 	if err != nil {
 		return err
@@ -27,14 +28,19 @@ func CreateSubnet(db *badger.DB, subnetName string) error {
 		return err
 	}
 
-	if err := createSubnet(db, subnetName, d); err != nil {
+	backend, err := dhcpbackend.New(cfg)
+	if err != nil {
+		return err
+	}
+
+	if err := createSubnet(db, subnetName, d, backend); err != nil {
 		return err
 	}
 
 	return state.Set(db, "subnet/"+subnetName, state.Running)
 }
 
-func createSubnet(db *badger.DB, subnetName string, d subnetData) error {
+func createSubnet(db *badger.DB, subnetName string, d subnetData, backend dhcpbackend.Backend) error {
 	vethE := "v-" + d.subnetID + "-e"
 	vethI := "v-" + d.subnetID + "-i"
 
@@ -110,7 +116,7 @@ func createSubnet(db *badger.DB, subnetName string, d subnetData) error {
 		}
 	}
 
-	return startDHCP(db, subnetName, d)
+	return startDHCP(db, subnetName, d, backend)
 }
 
 func setupVxlanHost(d subnetData, vethE string) error {
@@ -136,35 +142,23 @@ func setupVxlanHost(d subnetData, vethE string) error {
 	return nil
 }
 
-func startDHCP(db *badger.DB, subnetName string, d subnetData) error {
-	conf := dhcp.Config{
-		Network:     d.cidr,
-		Name:        d.vpc + "_" + d.bridge,
-		ConfDir:     dhcp.DefaultConfDir,
-		InterfaceIP: d.interfaceIP,
-	}
+func startDHCP(db *badger.DB, subnetName string, d subnetData, backend dhcpbackend.Backend) error {
 	defaultGateway, vpcRoute, err := dhcpRouting(d, netif.GetDefaultGateway)
 	if err != nil {
 		return err
 	}
-	conf.DefaultGateway = defaultGateway
-	conf.VPCRoute = vpcRoute
-	_, entries, err := dhcp.GenerateConfig(conf)
-	if err != nil {
-		return fmt.Errorf("generate dhcp config: %w", err)
-	}
-	if err := dhcp.StoreDHCPEntries(db, subnetName, entries); err != nil {
+
+	if err := dhcp.StoreDHCPEntries(db, subnetName, dhcp.Entries(d.cidr)); err != nil {
 		return fmt.Errorf("store dhcp entries: %w", err)
 	}
 
-	svc, err := systemd.New()
-	if err != nil {
-		return fmt.Errorf("connect to systemd: %w", err)
-	}
-	defer svc.Close()
-
-	if err := svc.Start("dnsmasq@" + conf.Name + ".service"); err != nil {
-		return fmt.Errorf("start dnsmasq: %w", err)
-	}
-	return nil
+	return backend.ConfigureSubnet(dhcpbackend.Subnet{
+		Name:           subnetName,
+		VPC:            d.vpc,
+		Bridge:         d.bridge,
+		Network:        d.cidr,
+		InterfaceIP:    d.interfaceIP,
+		VPCRoute:       vpcRoute,
+		DefaultGateway: defaultGateway,
+	})
 }

internal/subnet/delete.go

@@ -2,21 +2,19 @@ package subnet
 
 import (
 	"fmt"
-	"os"
-	"path/filepath"
 
-	"git.g3e.fr/syonad/two/internal/dhcp"
+	configuration "git.g3e.fr/syonad/two/internal/config/agent"
+	"git.g3e.fr/syonad/two/internal/dhcpbackend"
 	"git.g3e.fr/syonad/two/internal/ebtables"
 	"git.g3e.fr/syonad/two/internal/netif"
 	"git.g3e.fr/syonad/two/internal/netns"
 	"git.g3e.fr/syonad/two/internal/state"
 	"git.g3e.fr/syonad/two/pkg/db/kv"
-	"git.g3e.fr/syonad/two/pkg/systemd"
 
 	"github.com/dgraph-io/badger/v4"
 )
 
-func DeleteSubnet(db *badger.DB, subnetName string) error {
+func DeleteSubnet(db *badger.DB, subnetName string, cfg *configuration.Config) error {
 	current, err := state.Get(db, "subnet/"+subnetName)
 	if err != nil {
 		return err
@@ -30,7 +28,12 @@ func DeleteSubnet(db *badger.DB, subnetName string) error {
 		return err
 	}
 
-	if err := stopDHCP(db, subnetName, d); err != nil {
+	backend, err := dhcpbackend.New(cfg)
+	if err != nil {
+		return err
+	}
+
+	if err := stopDHCP(db, subnetName, d, backend); err != nil {
 		return err
 	}
 
@@ -50,26 +53,13 @@ func DeleteSubnet(db *badger.DB, subnetName string) error {
 	return state.Set(db, "subnet/"+subnetName, state.Deleted)
 }
 
-func stopDHCP(db *badger.DB, subnetName string, d subnetData) error {
-	svc, err := systemd.New()
-	if err != nil {
-		return fmt.Errorf("connect to systemd: %w", err)
-	}
-	defer svc.Close()
-
-	svcName := "dnsmasq@" + d.vpc + "_" + d.bridge + ".service"
-	if status, err := svc.Status(svcName); err == nil && status.ActiveState == "active" {
-		if err := svc.Stop(svcName); err != nil {
-			return fmt.Errorf("stop dnsmasq: %w", err)
-		}
-	}
-
-	if err := os.Remove(filepath.Join(dhcp.DefaultConfDir, d.vpc+"_"+d.bridge+".conf")); err != nil && !os.IsNotExist(err) {
-		return fmt.Errorf("remove dnsmasq config: %w", err)
-	}
-
-	if err := dhcp.RemoveSubnetDirs(dhcp.DefaultConfDir, d.vpc+"_"+d.bridge); err != nil {
-		return fmt.Errorf("remove dnsmasq dirs: %w", err)
+func stopDHCP(db *badger.DB, subnetName string, d subnetData, backend dhcpbackend.Backend) error {
+	if err := backend.TeardownSubnet(dhcpbackend.Subnet{
+		Name:   subnetName,
+		VPC:    d.vpc,
+		Bridge: d.bridge,
+	}); err != nil {
+		return err
 	}
 
 	if err := kv.DeleteInDB(db, "subnet/"+subnetName+"/dhcp"); err != nil {

internal/vm/create.go

@@ -8,7 +8,7 @@ import (
 	"path/filepath"
 
 	configuration "git.g3e.fr/syonad/two/internal/config/agent"
-	"git.g3e.fr/syonad/two/internal/dhcp"
+	"git.g3e.fr/syonad/two/internal/dhcpbackend"
 	"git.g3e.fr/syonad/two/internal/iptables"
 	"git.g3e.fr/syonad/two/internal/metadata"
 	"git.g3e.fr/syonad/two/internal/netif"
@@ -34,6 +34,11 @@ func StartVM(db *badger.DB, name string, cfg *configuration.Config) error {
 	}
 	nic := d.primary()
 
+	backend, err := dhcpbackend.New(cfg)
+	if err != nil {
+		return err
+	}
+
 	for _, n := range d.nics {
 		if err := netif.CreateTap(n.tapID, n.bridge, n.vpcName); err != nil {
 			return fmt.Errorf("create tap of interface %d: %w", n.index, err)
@@ -54,7 +59,7 @@ func StartVM(db *badger.DB, name string, cfg *configuration.Config) error {
 		return fmt.Errorf("add metadata redirect: %w", err)
 	}
 
-	if err := writeDHCPFiles(d, name); err != nil {
+	if err := writeDHCPFiles(d, name, backend); err != nil {
 		return err
 	}
 
@@ -109,53 +114,37 @@ func StartVM(db *badger.DB, name string, cfg *configuration.Config) error {
 	return state.Set(db, "vm/"+name, state.Running)
 }
 
-// writeDHCPFiles écrit, pour chaque subnet touché par la VM, les réservations
-// de ses interfaces et les options qui suppriment la route par défaut sur les
-// interfaces non primaires. Le subnet de l'interface primaire ne reçoit aucune
-// option : les options non taggées du subnet portent déjà la route par défaut.
-func writeDHCPFiles(d vmData, name string) error {
-	type subnetFiles struct {
-		nic          nicData
-		reservations []dhcp.Reservation
-		tags         []string
-	}
-	bySubnet := make(map[string]*subnetFiles)
+func dhcpReservations(d vmData) map[string]*subnetReservations {
+	bySubnet := make(map[string]*subnetReservations)
 
 	for _, n := range d.nics {
-		confName := n.vpcName + "_" + n.bridge
-		if bySubnet[confName] == nil {
-			bySubnet[confName] = &subnetFiles{nic: n}
+		key := n.vpcName + "_" + n.bridge
+		if bySubnet[key] == nil {
+			bySubnet[key] = &subnetReservations{subnet: dhcpbackend.Subnet{
+				Name:        n.subnetName,
+				VPC:         n.vpcName,
+				Bridge:      n.bridge,
+				InterfaceIP: net.ParseIP(n.interfaceIP),
+				VPCRoute:    n.vpcCIDR,
+			}}
 		}
-		f := bySubnet[confName]
-		f.reservations = append(f.reservations, dhcp.Reservation{
-			MAC: n.mac, IP: n.ip, Tag: nicTag(name, n.index),
+		f := bySubnet[key]
+		f.reservations = append(f.reservations, dhcpbackend.Reservation{
+			Index: n.index, MAC: n.mac, IP: n.ip, DefaultRoute: n.primary,
 		})
-		if !n.primary {
-			f.tags = append(f.tags, nicTag(name, n.index))
-		}
 	}
+	return bySubnet
+}
 
-	for confName, f := range bySubnet {
-		if err := dhcp.WriteReservations(dhcp.DefaultConfDir, confName, name, f.reservations); err != nil {
-			return fmt.Errorf("write dhcp reservations on %s: %w", confName, err)
-		}
-		if err := dhcp.WriteVMOptions(dhcp.DefaultConfDir, confName, name, f.tags, dhcp.Config{
-			InterfaceIP: net.ParseIP(f.nic.interfaceIP),
-			VPCRoute:    f.nic.vpcCIDR,
-		}); err != nil {
-			return fmt.Errorf("write dhcp options on %s: %w", confName, err)
+func writeDHCPFiles(d vmData, name string, backend dhcpbackend.Backend) error {
+	for _, f := range dhcpReservations(d) {
+		if err := backend.SetVM(f.subnet, name, f.reservations); err != nil {
+			return err
 		}
 	}
 	return nil
 }
 
-// nicTag identifie une interface auprès de dnsmasq. Il est par interface et non
-// par VM : deux interfaces d'une même VM peuvent partager un subnet, et n'y
-// avoir pas le même rôle.
-func nicTag(vmName string, index int) string {
-	return fmt.Sprintf("%s-%d", vmName, index)
-}
-
 func copyFile(src, dst string) error {
 	if err := os.MkdirAll(filepath.Dir(dst), 0755); err != nil {
 		return err

internal/vm/delete.go

@@ -7,14 +7,13 @@ import (
 	"time"
 
 	configuration "git.g3e.fr/syonad/two/internal/config/agent"
-	"git.g3e.fr/syonad/two/internal/dhcp"
+	"git.g3e.fr/syonad/two/internal/dhcpbackend"
 	"git.g3e.fr/syonad/two/internal/iptables"
 	"git.g3e.fr/syonad/two/internal/metadata"
 	"git.g3e.fr/syonad/two/internal/netif"
 	"git.g3e.fr/syonad/two/internal/netns"
 	"git.g3e.fr/syonad/two/internal/qmp"
 	"git.g3e.fr/syonad/two/internal/state"
-	"git.g3e.fr/syonad/two/pkg/systemd"
 
 	"github.com/dgraph-io/badger/v4"
 )
@@ -34,6 +33,11 @@ func StopVM(db *badger.DB, name string, cfg *configuration.Config) error {
 	}
 	nic := d.primary()
 
+	backend, err := dhcpbackend.New(cfg)
+	if err != nil {
+		return err
+	}
+
 	socketPath := filepath.Join(cfg.QEMU.QMPDir, name+".sock")
 
 	if _, err := os.Stat(socketPath); err == nil {
@@ -69,7 +73,7 @@ func StopVM(db *badger.DB, name string, cfg *configuration.Config) error {
 		}
 	}
 
-	if err := removeDHCPFiles(d, name); err != nil {
+	if err := removeDHCPFiles(d, name, backend); err != nil {
 		return err
 	}
 
@@ -81,52 +85,15 @@ func StopVM(db *badger.DB, name string, cfg *configuration.Config) error {
 	return state.Set(db, "vm/"+name, state.Deleted)
 }
 
-// removeDHCPFiles retire les fichiers de la VM dans chaque subnet qu'elle
-// touche, puis redémarre les dnsmasq concernés : un fichier ajouté dans un
-// dhcp-hostsdir est relu à chaud, un fichier retiré ne l'est pas (vérifié sur
-// dnsmasq 2.90).
-func removeDHCPFiles(d vmData, name string) error {
-	seen := make(map[string]bool)
-	for _, n := range d.nics {
-		confName := n.vpcName + "_" + n.bridge
-		if seen[confName] {
-			continue
-		}
-		seen[confName] = true
-		if err := removeDHCPReservation(confName, name); err != nil {
+func removeDHCPFiles(d vmData, name string, backend dhcpbackend.Backend) error {
+	for _, f := range dhcpReservations(d) {
+		if err := backend.DelVM(f.subnet, name, f.reservations); err != nil {
 			return err
 		}
 	}
 	return nil
 }
 
-func removeDHCPReservation(confName, name string) error {
-	if err := dhcp.RemoveReservations(dhcp.DefaultConfDir, confName, name); err != nil {
-		return err
-	}
-
-	svc, err := systemd.New()
-	if err != nil {
-		return fmt.Errorf("connect to systemd: %w", err)
-	}
-	defer svc.Close()
-
-	unit := dhcp.UnitName(confName)
-	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
-}
-
 func waitQMPDead(socketPath string, timeout, poll time.Duration) {
 	timer := time.After(timeout)
 	for {

internal/vm/dhcp.go

@@ -0,0 +1,10 @@
+package vm
+
+import (
+	"git.g3e.fr/syonad/two/internal/dhcpbackend"
+)
+
+type subnetReservations struct {
+	subnet       dhcpbackend.Subnet
+	reservations []dhcpbackend.Reservation
+}

internal/vm/dhcp_test.go

@@ -0,0 +1,122 @@
+package vm
+
+import (
+	"net"
+	"testing"
+)
+
+func nic(idx int, subnet, vpc, bridge, ip, mac string, primary bool) nicData {
+	return nicData{
+		index:       idx,
+		subnetName:  subnet,
+		vpcName:     vpc,
+		bridge:      bridge,
+		interfaceIP: "10.0.5.1",
+		ip:          ip,
+		mac:         mac,
+		primary:     primary,
+	}
+}
+
+func group(t *testing.T, groups map[string]*subnetReservations, key string) *subnetReservations {
+	t.Helper()
+	g, ok := groups[key]
+	if !ok || g == nil {
+		t.Fatalf("no group %q, got %v", key, keysOf(groups))
+	}
+	return g
+}
+
+func keysOf(groups map[string]*subnetReservations) []string {
+	keys := make([]string, 0, len(groups))
+	for k := range groups {
+		keys = append(keys, k)
+	}
+	return keys
+}
+
+func TestDHCPReservations_GroupsInterfacesBySubnet(t *testing.T) {
+	d := vmData{nics: []nicData{
+		nic(0, "sn-000001", "vp-admin", "br-000001", "10.0.5.10", "00:22:33:00:00:0a", true),
+		nic(1, "sn-000001", "vp-admin", "br-000001", "10.0.5.11", "00:22:33:00:00:0b", false),
+		nic(2, "sn-000002", "vp-admin", "br-000002", "10.0.6.10", "00:22:33:00:00:0c", false),
+	}}
+
+	got := dhcpReservations(d)
+	if len(got) != 2 {
+		t.Fatalf("groups = %d, want one per subnet", len(got))
+	}
+	if n := len(group(t, got, "vp-admin_br-000001").reservations); n != 2 {
+		t.Errorf("br-000001 carries %d reservations, want 2", n)
+	}
+	if n := len(group(t, got, "vp-admin_br-000002").reservations); n != 1 {
+		t.Errorf("br-000002 carries %d reservations, want 1", n)
+	}
+}
+
+func TestDHCPReservations_OnlyThePrimaryCarriesTheDefaultRoute(t *testing.T) {
+	d := vmData{nics: []nicData{
+		nic(0, "sn-000001", "vp-admin", "br-000001", "10.0.5.10", "00:22:33:00:00:0a", true),
+		nic(1, "sn-000001", "vp-admin", "br-000001", "10.0.5.11", "00:22:33:00:00:0b", false),
+	}}
+
+	res := group(t, dhcpReservations(d), "vp-admin_br-000001").reservations
+	byMAC := map[string]bool{}
+	for _, r := range res {
+		byMAC[r.MAC] = r.DefaultRoute
+	}
+	if !byMAC["00:22:33:00:00:0a"] {
+		t.Error("the primary interface must carry the default route")
+	}
+	if byMAC["00:22:33:00:00:0b"] {
+		t.Error("a secondary interface must not carry the default route")
+	}
+}
+
+func TestDHCPReservations_KeepsTheInterfaceIndex(t *testing.T) {
+	d := vmData{nics: []nicData{
+		nic(3, "sn-000001", "vp-admin", "br-000001", "10.0.5.10", "00:22:33:00:00:0a", true),
+	}}
+
+	res := group(t, dhcpReservations(d), "vp-admin_br-000001").reservations
+	if res[0].Index != 3 {
+		t.Errorf("index = %d, want 3: the dnsmasq tag is derived from it", res[0].Index)
+	}
+}
+
+func TestDHCPReservations_CarriesTheSubnetIdentity(t *testing.T) {
+	_, vpcCIDR, err := net.ParseCIDR("10.0.0.0/16")
+	if err != nil {
+		t.Fatalf("ParseCIDR: %v", err)
+	}
+	n := nic(0, "sn-000001", "vp-admin", "br-000001", "10.0.5.10", "00:22:33:00:00:0a", true)
+	n.vpcCIDR = vpcCIDR
+
+	g := group(t, dhcpReservations(vmData{nics: []nicData{n}}), "vp-admin_br-000001")
+	if g.subnet.Name != "sn-000001" {
+		t.Errorf("subnet name = %s, want sn-000001", g.subnet.Name)
+	}
+	if !g.subnet.InterfaceIP.Equal(net.ParseIP("10.0.5.1")) {
+		t.Errorf("interface ip = %s, want 10.0.5.1", g.subnet.InterfaceIP)
+	}
+	if g.subnet.VPCRoute == nil || g.subnet.VPCRoute.String() != "10.0.0.0/16" {
+		t.Errorf("vpc route = %v, want 10.0.0.0/16", g.subnet.VPCRoute)
+	}
+}
+
+func TestDHCPReservations_SameBridgeInTwoVPCsStaysSeparate(t *testing.T) {
+	d := vmData{nics: []nicData{
+		nic(0, "sn-000001", "vp-admin", "br-000001", "10.0.5.10", "00:22:33:00:00:0a", true),
+		nic(1, "sn-000009", "vp-other", "br-000001", "10.9.5.10", "00:22:33:00:00:0d", false),
+	}}
+
+	if got := len(dhcpReservations(d)); got != 2 {
+		t.Errorf("groups = %d, want 2: the vpc is part of the instance identity", got)
+	}
+}
+
+func TestDHCPReservations_NoNICYieldsNoGroup(t *testing.T) {
+	if got := len(dhcpReservations(vmData{})); got != 0 {
+		t.Errorf("groups = %d, want none", got)
+	}
+}

pkg/db/statefile/statefile.go

@@ -85,8 +85,12 @@ func (f *File[T]) Save(value T) error {
 }
 
 func (f *File[T]) Remove() error {
-	if err := os.Remove(f.path); err != nil && !errors.Is(err, os.ErrNotExist) {
-		return fmt.Errorf("remove %s: %w", f.path, err)
+	return Remove(f.path)
+}
+
+func Remove(path string) error {
+	if err := os.Remove(path); err != nil && !errors.Is(err, os.ErrNotExist) {
+		return fmt.Errorf("remove %s: %w", path, err)
 	}
 	return nil
 }