- /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/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
+}