system default nameserver in windows

cmd/doggo/nameservers.go

@@ -1,18 +1,14 @@
 package main
 
 import (
-	"errors"
 	"fmt"
 	"net"
 	"net/url"
-	"runtime"
 
-	"github.com/miekg/dns"
+	"github.com/mr-karan/doggo/pkg/config"
 )
 
 const (
-	//DefaultResolvConfPath specifies path to default resolv config file on UNIX.
-	DefaultResolvConfPath = "/etc/resolv.conf"
 	// DefaultTLSPort specifies the default port for a DNS server connecting over TCP over TLS
 	DefaultTLSPort = "853"
 	// DefaultUDPPort specifies the default port for a DNS server connecting over UDP
@@ -58,28 +54,20 @@ func (hub *Hub) loadNameservers() error {
 	return nil
 }
 
-// getDefaultServers reads the `resolv.conf`
-// file and returns a list of nameservers with it's config.
 func getDefaultServers() ([]Nameserver, int, []string, error) {
-	if runtime.GOOS == "windows" {
-		// TODO: Add a method for reading system default nameserver in windows.
-		return nil, 0, nil, errors.New(`unable to read default nameservers in this machine`)
-	}
-	// if no nameserver is provided, take it from `resolv.conf`
-	cfg, err := dns.ClientConfigFromFile(DefaultResolvConfPath)
+	dnsServers, ndots, search, err := config.GetDefaultServers()
 	if err != nil {
 		return nil, 0, nil, err
 	}
-	servers := make([]Nameserver, 0, len(cfg.Servers))
-	for _, s := range cfg.Servers {
-		addr := net.JoinHostPort(s, cfg.Port)
+	servers := make([]Nameserver, 0, len(dnsServers))
+	for _, s := range dnsServers {
 		ns := Nameserver{
 			Type:    UDPResolver,
-			Address: addr,
+			Address: net.JoinHostPort(s, DefaultUDPPort),
 		}
 		servers = append(servers, ns)
 	}
-	return servers, cfg.Ndots, cfg.Search, nil
+	return servers, ndots, search, nil
 }
 
 func initNameserver(n string) (Nameserver, error) {

go.mod

@@ -11,4 +11,5 @@ require (
 	github.com/sirupsen/logrus v1.7.0
 	github.com/spf13/pflag v1.0.5
 	github.com/stretchr/testify v1.6.1 // indirect
+	golang.org/x/sys v0.0.0-20200331124033-c3d80250170d
 )

pkg/config/config_unix.go

@@ -0,0 +1,19 @@
+// +build !windows
+
+package config
+
+import (
+	"github.com/miekg/dns"
+)
+
+//DefaultResolvConfPath specifies path to default resolv config file on UNIX.
+const DefaultResolvConfPath = "/etc/resolv.conf"
+
+func GetDefaultServers() ([]string, int, []string, error) {
+	// if no nameserver is provided, take it from `resolv.conf`
+	cfg, err := dns.ClientConfigFromFile(DefaultResolvConfPath)
+	if err != nil {
+		return nil, 0, nil, err
+	}
+	return cfg.Servers, cfg.Ndots, cfg.Search, nil
+}

pkg/config/config_windows.go

@@ -0,0 +1,123 @@
+package config
+
+import (
+	"net"
+	"os"
+	"syscall"
+	"unsafe"
+
+	"golang.org/x/sys/windows"
+)
+
+const GAA_FLAG_INCLUDE_GATEWAYS = 0x00000080
+
+type IpAdapterWinsServerAddress struct {
+	Length  uint32
+	_       uint32
+	Next    *IpAdapterWinsServerAddress
+	Address windows.SocketAddress
+}
+
+type IpAdapterGatewayAddress struct {
+	Length  uint32
+	_       uint32
+	Next    *IpAdapterGatewayAddress
+	Address windows.SocketAddress
+}
+
+type IpAdapterAddresses struct {
+	Length                uint32
+	IfIndex               uint32
+	Next                  *IpAdapterAddresses
+	AdapterName           *byte
+	FirstUnicastAddress   *windows.IpAdapterUnicastAddress
+	FirstAnycastAddress   *windows.IpAdapterAnycastAddress
+	FirstMulticastAddress *windows.IpAdapterMulticastAddress
+	FirstDnsServerAddress *windows.IpAdapterDnsServerAdapter
+	DnsSuffix             *uint16
+	Description           *uint16
+	FriendlyName          *uint16
+	PhysicalAddress       [syscall.MAX_ADAPTER_ADDRESS_LENGTH]byte
+	PhysicalAddressLength uint32
+	Flags                 uint32
+	Mtu                   uint32
+	IfType                uint32
+	OperStatus            uint32
+	Ipv6IfIndex           uint32
+	ZoneIndices           [16]uint32
+	FirstPrefix           *windows.IpAdapterPrefix
+	/* more fields might be present here. */
+	TransmitLinkSpeed      uint64
+	ReceiveLinkSpeed       uint64
+	FirstWINSServerAddress *IpAdapterWinsServerAddress
+	FirstGatewayAddress    *IpAdapterGatewayAddress
+}
+
+func adapterAddresses() ([]*IpAdapterAddresses, error) {
+	var b []byte
+	// https://docs.microsoft.com/en-us/windows/win32/api/iphlpapi/nf-iphlpapi-getadaptersaddresses
+	// #define WORKING_BUFFER_SIZE 15000
+	l := uint32(15000)
+	for {
+		b = make([]byte, l)
+		err := windows.GetAdaptersAddresses(syscall.AF_UNSPEC, GAA_FLAG_INCLUDE_GATEWAYS|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)
+		}
+	}
+	aas := make([]*IpAdapterAddresses, 0, uintptr(l)/unsafe.Sizeof(IpAdapterAddresses{}))
+	for aa := (*IpAdapterAddresses)(unsafe.Pointer(&b[0])); aa != nil; aa = aa.Next {
+		aas = append(aas, aa)
+	}
+	return aas, nil
+}
+
+/// As per [RFC 3879], the whole `FEC0::/10` prefix is
+/// deprecated. New software must not support site-local
+/// addresses.
+///
+/// [RFC 3879]: https://tools.ietf.org/html/rfc3879
+func isUnicastLinkLocal(ip net.IP) bool {
+	return len(ip) == net.IPv6len && ip[0] == 0xfe && ip[1] == 0xc0
+}
+
+func GetDefaultDnsServers() ([]string, error) {
+	ifs, err := adapterAddresses()
+	if err != nil {
+		return nil, err
+	}
+	dnsServers := make([]string, 0)
+	for _, ifi := range ifs {
+		if ifi.OperStatus != windows.IfOperStatusUp {
+			continue
+		}
+
+		if ifi.FirstGatewayAddress == nil {
+			continue
+		}
+
+		for dnsServer := ifi.FirstDnsServerAddress; dnsServer != nil; dnsServer = dnsServer.Next {
+			ip := dnsServer.Address.IP()
+			if isUnicastLinkLocal(ip) {
+				continue
+			}
+			dnsServers = append(dnsServers, ip.String())
+		}
+	}
+	return dnsServers, nil
+}
+
+func GetDefaultServers() ([]string, int, []string, error) {
+	// TODO: DNS Suffix
+	servers, err := GetDefaultDnsServers()
+	return servers, 0, nil, err
+}