Rework getAvailableIp

This commit reworks getAvailableIp with a "simpler" version that will
look for the first available IP address in our IP Prefix.

There is a couple of ideas behind this:

* Make the host IPs reasonably predictable and in within similar
  subnets, which should simplify ACLs for subnets
* The code is not random, but deterministic so we can have tests
* The code is a bit more understandable (no bit shift magic)

app_test.go

@@ -38,7 +38,7 @@ func (s *Suite) ResetDB(c *check.C) {
 		c.Fatal(err)
 	}
 	cfg := Config{
-		IPPrefix: netaddr.MustParseIPPrefix("127.0.0.1/32"),
+		IPPrefix: netaddr.MustParseIPPrefix("10.27.0.0/23"),
 	}
 
 	h = Headscale{

cli_test.go

@@ -15,6 +15,7 @@ func (s *Suite) TestRegisterMachine(c *check.C) {
 		DiscoKey:    "faa",
 		Name:        "testmachine",
 		NamespaceID: n.ID,
+		IPAddress:   "10.0.0.1",
 	}
 	h.db.Save(&m)
 

utils.go

@@ -7,18 +7,11 @@ package headscale
 
 import (
 	"crypto/rand"
-	"encoding/binary"
 	"encoding/json"
-	"errors"
 	"fmt"
 	"io"
-	"net"
-	"time"
-
-	mathrand "math/rand"
 
 	"golang.org/x/crypto/nacl/box"
-	"gorm.io/gorm"
 	"inet.af/netaddr"
 	"tailscale.com/types/wgkey"
 )
@@ -78,47 +71,73 @@ func encodeMsg(b []byte, pubKey *wgkey.Key, privKey *wgkey.Private) ([]byte, err
 	return msg, nil
 }
 
-func (h *Headscale) getAvailableIP() (*net.IP, error) {
-	i := 0
+func (h *Headscale) getAvailableIP() (*netaddr.IP, error) {
+	ipPrefix := h.cfg.IPPrefix
+
+	usedIps, err := h.getUsedIPs()
+	if err != nil {
+		return nil, err
+	}
+
+	// for _, ip := range usedIps {
+	// 	nextIP := ip.Next()
+
+	// 	if !containsIPs(usedIps, nextIP) && ipPrefix.Contains(nextIP) {
+	// 		return &nextIP, nil
+	// 	}
+	// }
+
+	// // If there are no IPs in use, we are starting fresh and
+	// // can issue IPs from the beginning of the prefix.
+	// ip := ipPrefix.IP()
+	// return &ip, nil
+
+	// return nil, fmt.Errorf("failed to find any available IP in %s", ipPrefix)
+
+	// Get the first IP in our prefix
+	ip := ipPrefix.IP()
+
 	for {
-		ip, err := getRandomIP(h.cfg.IPPrefix)
-		if err != nil {
-			return nil, err
+		if !ipPrefix.Contains(ip) {
+			return nil, fmt.Errorf("could not find any suitable IP in %s", ipPrefix)
 		}
-		m := Machine{}
-		if result := h.db.First(&m, "ip_address = ?", ip.String()); errors.Is(result.Error, gorm.ErrRecordNotFound) {
-			return ip, nil
+
+		if ip.IsZero() &&
+			ip.IsLoopback() {
+			continue
 		}
-		i++
-		if i == 100 { // really random number
-			break
+
+		if !containsIPs(usedIps, ip) {
+			return &ip, nil
 		}
+
+		ip = ip.Next()
 	}
-	return nil, errors.New(fmt.Sprintf("Could not find an available IP address in %s", h.cfg.IPPrefix.String()))
 }
 
-func getRandomIP(ipPrefix netaddr.IPPrefix) (*net.IP, error) {
-	mathrand.Seed(time.Now().Unix())
-	ipo, ipnet, err := net.ParseCIDR(ipPrefix.String())
-	if err == nil {
-		ip := ipo.To4()
-		// fmt.Println("In Randomize IPAddr: IP ", ip, " IPNET: ", ipnet)
-		// fmt.Println("Final address is ", ip)
-		// fmt.Println("Broadcast address is ", ipb)
-		// fmt.Println("Network address is ", ipn)
-		r := mathrand.Uint32()
-		ipRaw := make([]byte, 4)
-		binary.LittleEndian.PutUint32(ipRaw, r)
-		// ipRaw[3] = 254
-		// fmt.Println("ipRaw is ", ipRaw)
-		for i, v := range ipRaw {
-			// fmt.Println("IP Before: ", ip[i], " v is ", v, " Mask is: ", ipnet.Mask[i])
-			ip[i] = ip[i] + (v &^ ipnet.Mask[i])
-			// fmt.Println("IP After: ", ip[i])
+func (h *Headscale) getUsedIPs() ([]netaddr.IP, error) {
+	var addresses []string
+	h.db.Model(&Machine{}).Pluck("ip_address", &addresses)
+
+	ips := make([]netaddr.IP, len(addresses))
+	for index, addr := range addresses {
+		ip, err := netaddr.ParseIP(addr)
+		if err != nil {
+			return nil, fmt.Errorf("failed to parse ip from database, %w", err)
+		}
+
+		ips[index] = ip
+	}
+
+	return ips, nil
+}
+
+func containsIPs(ips []netaddr.IP, ip netaddr.IP) bool {
+	for _, v := range ips {
+		if v == ip {
+			return true
 		}
-		// fmt.Println("FINAL IP: ", ip.String())
-		return &ip, nil
 	}
 
-	return nil, err
+	return false
 }

utils_test.go

@@ -0,0 +1,105 @@
+package headscale
+
+import (
+	"gopkg.in/check.v1"
+	"inet.af/netaddr"
+)
+
+func (s *Suite) TestGetAvailableIp(c *check.C) {
+	ip, err := h.getAvailableIP()
+
+	c.Assert(err, check.IsNil)
+
+	expected := netaddr.MustParseIP("10.27.0.0")
+
+	c.Assert(ip.String(), check.Equals, expected.String())
+}
+
+func (s *Suite) TestGetUsedIps(c *check.C) {
+	ip, err := h.getAvailableIP()
+	c.Assert(err, check.IsNil)
+
+	n, err := h.CreateNamespace("test_ip")
+	c.Assert(err, check.IsNil)
+
+	pak, err := h.CreatePreAuthKey(n.Name, false, false, nil)
+	c.Assert(err, check.IsNil)
+
+	_, err = h.GetMachine("test", "testmachine")
+	c.Assert(err, check.NotNil)
+
+	m := Machine{
+		ID:             0,
+		MachineKey:     "foo",
+		NodeKey:        "bar",
+		DiscoKey:       "faa",
+		Name:           "testmachine",
+		NamespaceID:    n.ID,
+		Registered:     true,
+		RegisterMethod: "authKey",
+		AuthKeyID:      uint(pak.ID),
+		IPAddress:      ip.String(),
+	}
+	h.db.Save(&m)
+
+	ips, err := h.getUsedIPs()
+
+	c.Assert(err, check.IsNil)
+
+	expected := netaddr.MustParseIP("10.27.0.0")
+
+	c.Assert(ips[0], check.Equals, expected)
+}
+
+func (s *Suite) TestGetMultiIp(c *check.C) {
+	n, err := h.CreateNamespace("test-ip-multi")
+	c.Assert(err, check.IsNil)
+
+	for i := 1; i <= 350; i++ {
+		ip, err := h.getAvailableIP()
+		c.Assert(err, check.IsNil)
+
+		pak, err := h.CreatePreAuthKey(n.Name, false, false, nil)
+		c.Assert(err, check.IsNil)
+
+		_, err = h.GetMachine("test", "testmachine")
+		c.Assert(err, check.NotNil)
+
+		m := Machine{
+			ID:             0,
+			MachineKey:     "foo",
+			NodeKey:        "bar",
+			DiscoKey:       "faa",
+			Name:           "testmachine",
+			NamespaceID:    n.ID,
+			Registered:     true,
+			RegisterMethod: "authKey",
+			AuthKeyID:      uint(pak.ID),
+			IPAddress:      ip.String(),
+		}
+		h.db.Save(&m)
+	}
+
+	ips, err := h.getUsedIPs()
+
+	c.Assert(err, check.IsNil)
+
+	c.Assert(len(ips), check.Equals, 350)
+
+	c.Assert(ips[0], check.Equals, netaddr.MustParseIP("10.27.0.0"))
+	c.Assert(ips[9], check.Equals, netaddr.MustParseIP("10.27.0.9"))
+	c.Assert(ips[300], check.Equals, netaddr.MustParseIP("10.27.1.44"))
+
+	expectedNextIP := netaddr.MustParseIP("10.27.1.94")
+	nextIP, err := h.getAvailableIP()
+	c.Assert(err, check.IsNil)
+
+	c.Assert(nextIP.String(), check.Equals, expectedNextIP.String())
+
+	// If we call get Available again, we should receive
+	// the same IP, as it has not been reserved.
+	nextIP2, err := h.getAvailableIP()
+	c.Assert(err, check.IsNil)
+
+	c.Assert(nextIP2.String(), check.Equals, expectedNextIP.String())
+}