- /control/dhcp/find_active_dhcp: fix DHCP server detection

Before this patch we couldn't receive incoming DHCP packets.
Now we bind() to 0.0.0.0 and set the required network interface
 using SO_BINDTODEVICE option.

As an improvement, we now wait until a reply to our request is
 received and skip all unknown packets.

dhcpd/check_other_dhcp.go

@@ -1,6 +1,7 @@
 package dhcpd
 
 import (
+	"bytes"
 	"crypto/rand"
 	"encoding/binary"
 	"fmt"
@@ -11,6 +12,7 @@ import (
 
 	"github.com/AdguardTeam/golibs/log"
 	"github.com/krolaw/dhcp4"
+	"golang.org/x/net/ipv4"
 )
 
 // CheckIfOtherDHCPServersPresent sends a DHCP request to the specified network interface,
@@ -32,13 +34,13 @@ func CheckIfOtherDHCPServersPresent(ifaceName string) (bool, error) {
 	dst := "255.255.255.255:67"
 
 	// form a DHCP request packet, try to emulate existing client as much as possible
-	xID := make([]byte, 8)
+	xID := make([]byte, 4)
 	n, err := rand.Read(xID)
-	if n != 8 && err == nil {
+	if n != 4 && err == nil {
-		err = fmt.Errorf("Generated less than 8 bytes")
+		err = fmt.Errorf("Generated less than 4 bytes")
 	}
 	if err != nil {
-		return false, wrapErrPrint(err, "Couldn't generate 8 random bytes")
+		return false, wrapErrPrint(err, "Couldn't generate random bytes")
 	}
 	hostname, err := os.Hostname()
 	if err != nil {
@@ -89,29 +91,31 @@ func CheckIfOtherDHCPServersPresent(ifaceName string) (bool, error) {
 
 	// bind to 0.0.0.0:68
 	log.Tracef("Listening to udp4 %+v", udpAddr)
-	c, err := net.ListenPacket("udp4", src)
+	c, err := newBroadcastPacketConn(net.IPv4(0, 0, 0, 0), 68, ifaceName)
 	if c != nil {
 		defer c.Close()
 	}
 	// spew.Dump(c, err)
 	// spew.Printf("net.ListenUDP returned %v, %v\n", c, err)
 	if err != nil {
-		return false, wrapErrPrint(err, "Couldn't listen to %s", src)
+		return false, wrapErrPrint(err, "Couldn't listen on :68")
 	}
 
 	// send to 255.255.255.255:67
-	_, err = c.WriteTo(packet, dstAddr)
+	cm := ipv4.ControlMessage{}
+	_, err = c.WriteTo(packet, &cm, dstAddr)
 	// spew.Dump(n, err)
 	if err != nil {
 		return false, wrapErrPrint(err, "Couldn't send a packet to %s", dst)
 	}
 
+	for {
 		// wait for answer
 		log.Tracef("Waiting %v for an answer", defaultDiscoverTime)
 		// TODO: replicate dhclient's behaviour of retrying several times with progressively bigger timeouts
 		b := make([]byte, 1500)
-	c.SetReadDeadline(time.Now().Add(defaultDiscoverTime))
+		_ = c.SetReadDeadline(time.Now().Add(defaultDiscoverTime))
-	n, _, err = c.ReadFrom(b)
+		n, _, _, err = c.ReadFrom(b)
 		if isTimeout(err) {
 			// timed out -- no DHCP servers
 			return false, nil
@@ -119,28 +123,31 @@ func CheckIfOtherDHCPServersPresent(ifaceName string) (bool, error) {
 		if err != nil {
 			return false, wrapErrPrint(err, "Couldn't receive packet")
 		}
-	if n > 0 {
-		b = b[:n]
-	}
 		// spew.Dump(n, fromAddr, err, b)
 
+		log.Tracef("Received packet (%v bytes)", n)
+
 		if n < 240 {
 			// packet too small for dhcp
-		return false, wrapErrPrint(err, "got packet that's too small for DHCP")
+			continue
 		}
 
 		response := dhcp4.Packet(b[:n])
-	if response.HLen() > 16 {
+		if response.OpCode() != dhcp4.BootReply ||
-		// invalid size
+			response.HType() != 1 /*Ethernet*/ ||
-		return false, wrapErrPrint(err, "got malformed packet with HLen() > 16")
+			response.HLen() > 16 ||
+			!bytes.Equal(response.CHAddr(), iface.HardwareAddr) ||
+			!bytes.Equal(response.XId(), xID) {
+			continue
 		}
 
 		parsedOptions := response.ParseOptions()
-	_, ok := parsedOptions[dhcp4.OptionDHCPMessageType]
+		if t := parsedOptions[dhcp4.OptionDHCPMessageType]; len(t) != 1 {
-	if !ok {
+			continue //packet without DHCP message type
-		return false, wrapErrPrint(err, "got malformed packet without DHCP message type")
 		}
 
+		log.Tracef("The packet is from an active DHCP server")
 		// that's a DHCP server there
 		return true, nil
 	}
+}

dhcpd/os_unix.go

@@ -0,0 +1,47 @@
+// +build aix darwin dragonfly freebsd linux netbsd openbsd solaris
+
+package dhcpd
+
+import (
+	"net"
+	"os"
+	"syscall"
+
+	"golang.org/x/net/ipv4"
+)
+
+// Create a socket for receiving broadcast packets
+func newBroadcastPacketConn(bindAddr net.IP, port int, ifname string) (*ipv4.PacketConn, error) {
+	s, err := syscall.Socket(syscall.AF_INET, syscall.SOCK_DGRAM, syscall.IPPROTO_UDP)
+	if err != nil {
+		return nil, err
+	}
+
+	if err := syscall.SetsockoptInt(s, syscall.SOL_SOCKET, syscall.SO_BROADCAST, 1); err != nil {
+		return nil, err
+	}
+	if err := syscall.SetsockoptInt(s, syscall.SOL_SOCKET, syscall.SO_REUSEADDR, 1); err != nil {
+		return nil, err
+	}
+	if err := syscall.SetsockoptString(s, syscall.SOL_SOCKET, syscall.SO_BINDTODEVICE, ifname); err != nil {
+		return nil, err
+	}
+
+	addr := syscall.SockaddrInet4{Port: port}
+	copy(addr.Addr[:], bindAddr.To4())
+	err = syscall.Bind(s, &addr)
+	if err != nil {
+		syscall.Close(s)
+		return nil, err
+	}
+
+	f := os.NewFile(uintptr(s), "")
+	c, err := net.FilePacketConn(f)
+	f.Close()
+	if err != nil {
+		return nil, err
+	}
+
+	p := ipv4.NewPacketConn(c)
+	return p, nil
+}

dhcpd/os_windows.go

@@ -0,0 +1,12 @@
+package dhcpd
+
+import (
+	"net"
+
+	"golang.org/x/net/ipv4"
+)
+
+// Create a socket for receiving broadcast packets
+func newBroadcastPacketConn(bindAddr net.IP, port int, ifname string) (*ipv4.PacketConn, error) {
+	return nil, nil
+}