feat: support system dns for windows

dns/system_windows.go

@@ -2,6 +2,111 @@
 
 package dns
 
-func loadSystemResolver() (clients []dnsClient, err error) {
-	return nil, errors.New("system resolver is not yet supported on Windows")
+import (
+	"fmt"
+	"net"
+	"os"
+	"syscall"
+	"unsafe"
+
+	"golang.org/x/sys/windows"
+)
+
+var (
+	defaultNS = []string{"114.114.114.114:53"}
+)
+
+type dnsConfig struct {
+	servers []string // server addresses (in host:port form) to use
+}
+
+func loadSystemResolver() (clients []dnsClient, err error) {
+	content, err := dnsReadConfig()
+	if err != nil {
+		err = fmt.Errorf("failed to read system DNS: %w", err)
+	}
+	nameservers := content.servers
+	if len(nameservers) == 0 {
+		err = fmt.Errorf("no nameserver found in windows system")
+		return
+	}
+	servers := make([]NameServer, 0, len(nameservers))
+	for _, addr := range nameservers {
+		servers = append(servers, NameServer{
+			Addr: addr,
+			Net:  "udp",
+		})
+	}
+	return transform(servers, nil), nil
+}
+
+func dnsReadConfig() (conf *dnsConfig, err error) {
+	conf = &dnsConfig{}
+	defer func() {
+		if len(conf.servers) == 0 {
+			conf.servers = defaultNS
+		}
+	}()
+	aas, err := adapterAddresses()
+	if err != nil {
+		return conf, err
+	}
+	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
+				//}
+				continue
+			default:
+				// Unexpected type.
+				continue
+			}
+			conf.servers = append(conf.servers, net.JoinHostPort(ip.String(), "53"))
+		}
+	}
+	return conf, nil
+}
+
+// adapterAddresses returns a list of IP adapter and address
+// structures. The structure contains an IP adapter and flattened
+// multiple IP addresses including unicast, anycast and multicast
+// addresses.
+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
 }