Rework os resolver

Currently, os resolver not only handle A and AAAA records, but also does
it wrongly, since when it packs AAAA record to a dns.A record.

This commit reworks os resolver to make it works with all supported
record types.

nameservers_windows.go

@@ -0,0 +1,71 @@
+package ctrld
+
+import (
+	"net"
+	"os"
+	"syscall"
+	"unsafe"
+
+	"golang.org/x/sys/windows"
+)
+
+func nameservers() []string {
+	aas, err := adapterAddresses()
+	if err != nil {
+		return nil
+	}
+	ns := make([]string, 0, len(aas))
+	for _, aa := range aas {
+		for dns := aa.FirstDnsServerAddress; dns != nil; dns = dns.Next {
+			sa, err := dns.Address.Sockaddr.Sockaddr()
+			if err != nil {
+				continue
+			}
+			var ip net.IP
+			switch sa := sa.(type) {
+			case *syscall.SockaddrInet4:
+				ip = net.IPv4(sa.Addr[0], sa.Addr[1], sa.Addr[2], sa.Addr[3])
+			case *syscall.SockaddrInet6:
+				ip = make(net.IP, net.IPv6len)
+				copy(ip, sa.Addr[:])
+				if ip[0] == 0xfe && ip[1] == 0xc0 {
+					// Ignore these fec0/10 ones. Windows seems to
+					// populate them as defaults on its misc rando
+					// interfaces.
+					continue
+				}
+			default:
+				// Unexpected type.
+				continue
+			}
+			ns = append(ns, net.JoinHostPort(ip.String(), "53"))
+		}
+	}
+	return ns
+}
+
+func adapterAddresses() ([]*windows.IpAdapterAddresses, error) {
+	var b []byte
+	l := uint32(15000) // recommended initial size
+	for {
+		b = make([]byte, l)
+		err := windows.GetAdaptersAddresses(syscall.AF_UNSPEC, windows.GAA_FLAG_INCLUDE_PREFIX, 0, (*windows.IpAdapterAddresses)(unsafe.Pointer(&b[0])), &l)
+		if err == nil {
+			if l == 0 {
+				return nil, nil
+			}
+			break
+		}
+		if err.(syscall.Errno) != syscall.ERROR_BUFFER_OVERFLOW {
+			return nil, os.NewSyscallError("getadaptersaddresses", err)
+		}
+		if l <= uint32(len(b)) {
+			return nil, os.NewSyscallError("getadaptersaddresses", err)
+		}
+	}
+	var aas []*windows.IpAdapterAddresses
+	for aa := (*windows.IpAdapterAddresses)(unsafe.Pointer(&b[0])); aa != nil; aa = aa.Next {
+		aas = append(aas, aa)
+	}
+	return aas, nil
+}