Always randomize MAC address (#251)

* Always randomize MAC address

* Worker: check DHCP lease time and print a warning if it's unconfigured

* Further improve the explanation

* Add two leases example to the explanation

* Add an example of the resulting /var/db/dhcpd_leases

internal/worker/dhcpleasetime/dhcpleasetime.go

@@ -0,0 +1,56 @@
+package dhcpleasetime
+
+import (
+	"errors"
+	"fmt"
+	"howett.net/plist"
+	"os"
+)
+
+const (
+	internetSharingPath = "/Library/Preferences/SystemConfiguration/com.apple.InternetSharing.default.plist"
+)
+
+type InternetSharing struct {
+	Bootpd Bootpd `plist:"bootpd"`
+}
+
+type Bootpd struct {
+	DHCPLeaseTimeSecs int `plist:"DHCPLeaseTimeSecs"`
+}
+
+func Check() error {
+	errDefault := fmt.Errorf("it seems that you're using a default DHCP lease time of 1 day which may result in " +
+		"failures communicating with the VMs, please read https://tart.run/faq/#changing-the-default-dhcp-lease-time " +
+		"for more details and intstructions on how to fix that")
+
+	plistBytes, err := os.ReadFile(internetSharingPath)
+	if err != nil {
+		if errors.Is(err, os.ErrNotExist) {
+			return errDefault
+		}
+
+		return fmt.Errorf("failed to check the default DHCP lease time: %w, please read "+
+			"https://tart.run/faq/#changing-the-default-dhcp-lease-time for more details", err)
+	}
+
+	var internetSharing InternetSharing
+
+	_, err = plist.Unmarshal(plistBytes, &internetSharing)
+	if err != nil {
+		return fmt.Errorf("failed to check the default DHCP lease time: %w, please read "+
+			"https://tart.run/faq/#changing-the-default-dhcp-lease-time for more details", err)
+	}
+
+	if internetSharing.Bootpd.DHCPLeaseTimeSecs == 0 {
+		return errDefault
+	}
+
+	if internetSharing.Bootpd.DHCPLeaseTimeSecs > 3600 {
+		return fmt.Errorf("it seems that you're using a DHCP lease time larger than 1 hour which may result in " +
+			"failures communicating with the VMs, please read https://tart.run/faq/#changing-the-default-dhcp-lease-time " +
+			"for more details and intstructions on how to fix that")
+	}
+
+	return nil
+}

internal/worker/vmmanager/vm.go

@@ -225,15 +225,67 @@ func (vm *VM) cloneAndConfigure(ctx context.Context) error {
 		}
 	}
 
-	// Randomize the VM's MAC-address when using bridged networking
-	// to avoid collisions when cloning from an OCI image on multiple hosts
+	// Randomize VM's MAC-address, this is important when using shared (NAT) networking
+	// with full /var/db/dhcpd_leases file (e.g. 256 entries) having an expired entry
+	// for a MAC address used by some OCI image, for example:
 	//
-	// See https://github.com/cirruslabs/orchard/issues/181 for more details.
-	if vm.Resource.NetBridged != "" {
-		_, _, err = tart.Tart(ctx, vm.logger, "set", "--random-mac", vm.id())
-		if err != nil {
-			return err
-		}
+	// {
+	//	name=adminsVlMachine
+	//	ip_address=192.168.64.2
+	//	hw_address=1,11:11:11:11:11:11
+	//	identifier=1,11:11:11:11:11:11
+	//	lease=0x1234
+	//}
+	//
+	// The next VM to start with a MAC address 22:22:22:22:22:22 will assume that
+	// 192.168.64.2 is free (because its lease expired a long time ago) and will
+	// add a new entry using its MAC address and 192.168.64.2 to the
+	// /var/db/dhcpd_leases and won't delete the old entry:
+	//
+	// {
+	//	name=adminsVlMachine
+	//	ip_address=192.168.64.2
+	//	hw_address=1,11:11:11:11:11:11
+	//	identifier=1,11:11:11:11:11:11
+	//	lease=0x1234
+	// }
+	// {
+	//	name=adminsVlMachine
+	//	ip_address=192.168.64.2
+	//	hw_address=1,22:22:22:22:22:22
+	//	identifier=1,22:22:22:22:22:22
+	//	lease=0x67ade532
+	// }
+	//
+	// Afterward, when an OCI VM with MAC address 11:11:11:11:11:11 is cloned and run,
+	// it will re-use the 192.168.64.2 entry instead of creating a new one, even through
+	// its lease had already expired. The resulting /var/db/dhcpd_leases will look like this:
+	//
+	// {
+	//	name=adminsVlMachine
+	//	ip_address=192.168.64.2
+	//	hw_address=1,11:11:11:11:11:11
+	//	identifier=1,11:11:11:11:11:11
+	//	lease=0x67ade5c6
+	// }
+	// {
+	//	name=adminsVlMachine
+	//	ip_address=192.168.64.2
+	//	hw_address=1,22:22:22:22:22:22
+	//	identifier=1,22:22:22:22:22:22
+	//	lease=0x67ade532
+	// }
+	//
+	// As a result, you will see two VMs with different MAC address using an identical
+	// IP address 192.168.64.2.
+	//
+	// Another scenarion when this is important is when using bridged networking
+	// to avoid collisions when cloning from an OCI image on multiple hosts[1].
+	//
+	// [1]: https://github.com/cirruslabs/orchard/issues/181
+	_, _, err = tart.Tart(ctx, vm.logger, "set", "--random-mac", vm.id())
+	if err != nil {
+		return err
 	}
 
 	if vm.Resource.RandomSerial {

internal/worker/worker.go

@@ -6,6 +6,7 @@ import (
 	"fmt"
 	"github.com/avast/retry-go/v4"
 	"github.com/cirruslabs/orchard/internal/opentelemetry"
+	"github.com/cirruslabs/orchard/internal/worker/dhcpleasetime"
 	"github.com/cirruslabs/orchard/internal/worker/iokitregistry"
 	"github.com/cirruslabs/orchard/internal/worker/ondiskname"
 	"github.com/cirruslabs/orchard/internal/worker/tart"
@@ -88,6 +89,10 @@ func New(client *client.Client, opts ...Option) (*Worker, error) {
 }
 
 func (worker *Worker) Run(ctx context.Context) error {
+	if err := dhcpleasetime.Check(); err != nil {
+		worker.logger.Warnf("%v", err)
+	}
+
 	for {
 		if err := worker.runNewSession(ctx); err != nil {
 			return err