Split getting DNS configuration.

 For UNIX systems read and parse `/etc/resolv.conf` file.
 On Windows use 'GetAdaptersAddresses' syscall to get DNS configuration

Signed-off-by: Yevhen Vydolob <[email protected]>

pkg/services/dns/dns.go

@@ -22,7 +22,7 @@ type dnsHandler struct {
 	nameserver string
 }
 
-func newDnsHandler(zones []types.Zone) *dnsHandler {
+func newDNSHandler(zones []types.Zone) *dnsHandler {
 
 	dnsClient, nameserver := readAndCreateClient()
 
@@ -35,23 +35,16 @@ func newDnsHandler(zones []types.Zone) *dnsHandler {
 }
 
 func readAndCreateClient() (*dns.Client, string) {
-	conf, err := dns.ClientConfigFromFile("/etc/resolv.conf")
+
+	nameserver, port, err := GetDNSHostAndPort()
 	if err != nil {
-		fmt.Fprintln(os.Stderr, err)
 		os.Exit(2)
 	}
-	nameserver := conf.Servers[0]
 
-	// if the nameserver is from /etc/resolv.conf the [ and ] are already
-	// added, thereby breaking net.ParseIP. Check for this and don't
-	// fully qualify such a name
-	if nameserver[0] == '[' && nameserver[len(nameserver)-1] == ']' {
-		nameserver = nameserver[1 : len(nameserver)-1]
-	}
 	if i := net.ParseIP(nameserver); i != nil {
-		nameserver = net.JoinHostPort(nameserver, conf.Port)
+		nameserver = net.JoinHostPort(nameserver, port)
 	} else {
-		nameserver = dns.Fqdn(nameserver) + ":" + conf.Port
+		nameserver = dns.Fqdn(nameserver) + ":" + port
 	}
 	client := new(dns.Client)
 	client.Net = "udp"
@@ -163,7 +156,7 @@ type Server struct {
 }
 
 func New(udpConn net.PacketConn, tcpLn net.Listener, zones []types.Zone) (*Server, error) {
-	handler := newDnsHandler(zones)
+	handler := newDNSHandler(zones)
 	return &Server{udpConn: udpConn, tcpLn: tcpLn, handler: handler}, nil
 }
 

pkg/services/dns/dns_config_unix.go

@@ -0,0 +1,28 @@
+//go:build !windows
+
+package dns
+
+import (
+	"fmt"
+	"os"
+
+	"github.com/miekg/dns"
+)
+
+func GetDNSHostAndPort() (string, string, error) {
+	conf, err := dns.ClientConfigFromFile("/etc/resolv.conf")
+	if err != nil {
+		fmt.Fprintln(os.Stderr, err)
+		return "", "", err
+	}
+	nameserver := conf.Servers[0]
+
+	// if the nameserver is from /etc/resolv.conf the [ and ] are already
+	// added, thereby breaking net.ParseIP. Check for this and don't
+	// fully qualify such a name
+	if nameserver[0] == '[' && nameserver[len(nameserver)-1] == ']' {
+		nameserver = nameserver[1 : len(nameserver)-1]
+	}
+
+	return nameserver, conf.Port, nil
+}

pkg/services/dns/dns_config_windows.go

@@ -0,0 +1,296 @@
+//go:build windows
+
+package dns
+
+import (
+	"errors"
+	"fmt"
+	"net/netip"
+	"strconv"
+	"syscall"
+	"unsafe"
+)
+
+func GetDNSHostAndPort() (string, string, error) {
+	nameservers := getDNSServers()
+
+	var nameserver netip.AddrPort
+	for _, n := range nameservers {
+		// return first non ipv6 nameserver
+		if n.Addr().Is4() {
+			nameserver = n
+			break
+		}
+	}
+
+	return nameserver.Addr().String(), strconv.Itoa(int(nameserver.Port())), nil
+
+}
+
+// copied from https://github.com/qdm12/dns/blob/v2.0.0-beta/pkg/nameserver/getlocal_windows.go
+// this function will use windows syscall to get DNS configuration
+func getDNSServers() (nameservers []netip.AddrPort) {
+	const defaultDNSPort = 53
+	defaultLocalNameservers := []netip.AddrPort{
+		netip.AddrPortFrom(netip.AddrFrom4([4]byte{127, 0, 0, 1}), defaultDNSPort),
+		netip.AddrPortFrom(netip.AddrFrom16([16]byte{0, 0, 0, 0, 0, 0, 0, 1}), defaultDNSPort),
+	}
+
+	adapterAddresses, err := getAdapterAddresses()
+	if err != nil {
+		return defaultLocalNameservers
+	}
+
+	for _, adapterAddress := range adapterAddresses {
+		const statusUp = 0x01
+		if adapterAddress.operStatus != statusUp {
+			continue
+		}
+
+		if adapterAddress.firstGatewayAddress == nil {
+			// Only search DNS servers for adapters having a gateway
+			continue
+		}
+
+		dnsServerAddress := adapterAddress.firstDnsServerAddress
+		for dnsServerAddress != nil {
+			ip, ok := sockAddressToIP(dnsServerAddress.address.rawSockAddrAny)
+			if !ok || ipIsSiteLocalAnycast(ip) {
+				// fec0/10 IPv6 addresses are site local anycast DNS
+				// addresses Microsoft sets by default if no other
+				// IPv6 DNS address is set. Site local anycast is
+				// deprecated since 2004, see
+				// https://datatracker.ietf.org/doc/html/rfc3879
+				dnsServerAddress = dnsServerAddress.next
+				continue
+			}
+
+			nameserver := netip.AddrPortFrom(ip, defaultDNSPort)
+			nameservers = append(nameservers, nameserver)
+			dnsServerAddress = dnsServerAddress.next
+		}
+	}
+
+	if len(nameservers) == 0 {
+		return defaultLocalNameservers
+	}
+	return nameservers
+}
+
+var (
+	errBufferOverflowUnexpected = errors.New("unexpected buffer overflowed because buffer was large enough")
+)
+
+func getAdapterAddresses() (
+	adapterAddresses []*ipAdapterAddresses, err error) {
+	var buffer []byte
+	const initialBufferLength uint32 = 15000
+	sizeVar := initialBufferLength
+
+	for {
+		buffer = make([]byte, sizeVar)
+		err := runProcGetAdaptersAddresses(
+			(*ipAdapterAddresses)(unsafe.Pointer(&buffer[0])),
+			&sizeVar)
+		if err != nil {
+			if err.(syscall.Errno) == syscall.ERROR_BUFFER_OVERFLOW {
+				if sizeVar <= uint32(len(buffer)) {
+					return nil, fmt.Errorf("%w: buffer size variable %d is "+
+						"equal or lower to the buffer current length %d",
+						errBufferOverflowUnexpected, sizeVar, len(buffer))
+				}
+				continue
+			}
+			return nil, fmt.Errorf("getting adapters addresses: %w", err)
+		}
+
+		noDataFound := sizeVar == 0
+		if noDataFound {
+			return nil, nil
+		}
+		break
+	}
+
+	adapterAddress := (*ipAdapterAddresses)(unsafe.Pointer(&buffer[0]))
+	for adapterAddress != nil {
+		adapterAddresses = append(adapterAddresses, adapterAddress)
+		adapterAddress = adapterAddress.next
+	}
+
+	return adapterAddresses, nil
+}
+
+var (
+	procGetAdaptersAddresses = syscall.NewLazyDLL("iphlpapi.dll").
+		NewProc("GetAdaptersAddresses")
+)
+
+func runProcGetAdaptersAddresses(adapterAddresses *ipAdapterAddresses,
+	sizePointer *uint32) (errcode error) {
+	const family = syscall.AF_UNSPEC
+	const GAA_FLAG_SKIP_UNICAST = 0x0001
+	const GAA_FLAG_SKIP_ANYCAST = 0x0002
+	const GAA_FLAG_SKIP_MULTICAST = 0x0004
+	const GAA_FLAG_SKIP_FRIENDLY_NAME = 0x0020
+	const GAA_FLAG_INCLUDE_GATEWAYS = 0x0080
+	const flags = GAA_FLAG_SKIP_UNICAST | GAA_FLAG_SKIP_ANYCAST |
+		GAA_FLAG_SKIP_MULTICAST | GAA_FLAG_SKIP_FRIENDLY_NAME |
+		GAA_FLAG_INCLUDE_GATEWAYS
+	const reserved = 0
+	// See https://learn.microsoft.com/en-us/windows/win32/api/iphlpapi/nf-iphlpapi-getadaptersaddresses
+	r1, _, err := syscall.SyscallN(procGetAdaptersAddresses.Addr(),
+		uintptr(family), uintptr(flags), uintptr(reserved),
+		uintptr(unsafe.Pointer(adapterAddresses)),
+		uintptr(unsafe.Pointer(sizePointer)))
+	switch {
+	case err != 0:
+		return err
+	case r1 != 0:
+		return syscall.Errno(r1)
+	default:
+		return nil
+	}
+}
+
+func sockAddressToIP(rawSockAddress *syscall.RawSockaddrAny) (ip netip.Addr, ok bool) {
+	if rawSockAddress == nil {
+		return netip.Addr{}, false
+	}
+
+	sockAddress, err := rawSockAddress.Sockaddr()
+	if err != nil {
+		return netip.Addr{}, false
+	}
+
+	switch sockAddress := sockAddress.(type) {
+	case *syscall.SockaddrInet4:
+		return netip.AddrFrom4([4]byte{
+				sockAddress.Addr[0], sockAddress.Addr[1], sockAddress.Addr[2], sockAddress.Addr[3]}),
+			true
+	case *syscall.SockaddrInet6:
+		return netip.AddrFrom16([16]byte{
+				sockAddress.Addr[0], sockAddress.Addr[1], sockAddress.Addr[2], sockAddress.Addr[3],
+				sockAddress.Addr[4], sockAddress.Addr[5], sockAddress.Addr[6], sockAddress.Addr[7],
+				sockAddress.Addr[8], sockAddress.Addr[9], sockAddress.Addr[10], sockAddress.Addr[11],
+				sockAddress.Addr[12], sockAddress.Addr[13], sockAddress.Addr[14], sockAddress.Addr[15]}),
+			true
+	default:
+		return netip.Addr{}, false
+	}
+}
+
+func ipIsSiteLocalAnycast(ip netip.Addr) bool {
+	if !ip.Is6() {
+		return false
+	}
+
+	array := ip.As16()
+	return array[0] == 0xfe && array[1] == 0xc0
+}
+
+// See https://learn.microsoft.com/en-us/windows/win32/api/iptypes/ns-iptypes-ip_adapter_addresses_lh
+type ipAdapterAddresses struct {
+	// The order of fields DOES matter since they are read
+	// raw from a bytes buffer. However, we are only interested
+	// in a few select fields, so unneeded fields are either
+	// named as "_" or removed if they are after the fields
+	// we are interested in.
+	_                     uint32
+	_                     uint32
+	next                  *ipAdapterAddresses
+	_                     *byte
+	_                     *ipAdapterUnicastAddress
+	_                     *ipAdapterAnycastAddress
+	_                     *ipAdapterMulticastAddress
+	firstDnsServerAddress *ipAdapterDnsServerAdapter
+	_                     *uint16
+	_                     *uint16
+	_                     *uint16
+	_                     [syscall.MAX_ADAPTER_ADDRESS_LENGTH]byte
+	_                     uint32
+	_                     uint32
+	_                     uint32
+	_                     uint32
+	operStatus            uint32
+	_                     uint32
+	_                     [16]uint32
+	_                     *ipAdapterPrefix
+	_                     uint64
+	_                     uint64
+	_                     *ipAdapterWinsServerAddress
+	firstGatewayAddress   *ipAdapterGatewayAddress
+	// Additional fields not needed here
+}
+
+type ipAdapterUnicastAddress struct {
+	// The order of fields DOES matter since they are read raw
+	// from a bytes buffer. However, we are not interested in
+	// the value of any field, so they are all named as "_".
+	_ uint32
+	_ uint32
+	_ *ipAdapterUnicastAddress
+	_ ipAdapterSocketAddress
+	_ int32
+	_ int32
+	_ int32
+	_ uint32
+	_ uint32
+	_ uint32
+	_ uint8
+}
+
+type ipAdapterAnycastAddress struct {
+	// The order of fields DOES matter since they are read raw
+	// from a bytes buffer. However, we are not interested in
+	// the value of any field, so they are all named as "_".
+	_ uint32
+	_ uint32
+	_ *ipAdapterAnycastAddress
+	_ ipAdapterSocketAddress
+}
+
+type ipAdapterMulticastAddress struct {
+	// The order of fields DOES matter since they are read raw
+	// from a bytes buffer. However, we are only interested in
+	// a few select fields, so unneeded fields are named as "_".
+	_ uint32
+	_ uint32
+	_ *ipAdapterMulticastAddress
+	_ ipAdapterSocketAddress
+}
+
+type ipAdapterDnsServerAdapter struct {
+	// The order of fields DOES matter since they are read raw
+	// from a bytes buffer. However, we are only interested in
+	// a few select fields, so unneeded fields are named as "_".
+	_       uint32
+	_       uint32
+	next    *ipAdapterDnsServerAdapter
+	address ipAdapterSocketAddress
+}
+
+type ipAdapterPrefix struct {
+	_ uint32
+	_ uint32
+	_ *ipAdapterPrefix
+	_ ipAdapterSocketAddress
+	_ uint32
+}
+
+type ipAdapterWinsServerAddress struct {
+	_ uint32
+	_ uint32
+	_ *ipAdapterWinsServerAddress
+	_ ipAdapterSocketAddress
+}
+
+type ipAdapterGatewayAddress struct {
+	_ uint32
+	_ uint32
+	_ *ipAdapterGatewayAddress
+	_ ipAdapterSocketAddress
+}
+
+type ipAdapterSocketAddress struct {
+	rawSockAddrAny *syscall.RawSockaddrAny
+}