ipswap/internal/netcfg/netcfg_windows.go

//go:build windows

package netcfg

import (
	"errors"
	"fmt"
	"log"
	"net"
	"os/exec"
	"strings"
	"syscall"
	"time"
	"unsafe"

	"golang.org/x/sys/windows"
	"golang.org/x/sys/windows/registry"

	"gitea.apointless.space/bsncubed/ipswap/internal/preset"
)

// ErrElevationRequired is returned when netsh refuses because the process token
// is not elevated. The caller turns this into the "Relaunch as administrator"
// offer rather than a bare failure message.
var ErrElevationRequired = errors.New("this operation requires an elevated process")

// ipAdapterDHCPEnabled is IP_ADAPTER_DHCP_ENABLED from iptypes.h.
const ipAdapterDHCPEnabled = 0x00000004

// Interface types we never want in the adapter picker.
const (
	ifTypeSoftwareLoopback = 24
	ifTypeTunnel           = 131
)

// applyTimeout bounds a whole plan. A single netsh call is normally well under
// a second; anything past this is a hung call, not a slow one.
const applyTimeout = 30 * time.Second

// Windows is the real Manager.
type Windows struct{}

// New returns the platform Manager.
func New() Manager { return Windows{} }

// Adapters lists IPv4-capable, non-loopback, non-tunnel adapters.
func (Windows) Adapters() ([]Adapter, error) {
	rows, err := adapterRows()
	if err != nil {
		return nil, err
	}

	var out []Adapter
	for _, a := range rows {
		if a.IfType == ifTypeSoftwareLoopback || a.IfType == ifTypeTunnel {
			continue
		}
		out = append(out, Adapter{
			Name:        windows.UTF16PtrToString(a.FriendlyName),
			Description: windows.UTF16PtrToString(a.Description),
			GUID:        windows.BytePtrToString(a.AdapterName),
			Up:          a.OperStatus == windows.IfOperStatusUp,
		})
	}
	return out, nil
}

// Current reads one adapter's live IPv4 configuration.
func (Windows) Current(adapter string) (LiveConfig, error) {
	rows, err := adapterRows()
	if err != nil {
		return LiveConfig{}, err
	}

	for _, a := range rows {
		if !strings.EqualFold(windows.UTF16PtrToString(a.FriendlyName), adapter) {
			continue
		}

		cfg := LiveConfig{
			Adapter: adapter,
			DHCP:    a.Flags&ipAdapterDHCPEnabled != 0,
		}

		for ua := a.FirstUnicastAddress; ua != nil; ua = ua.Next {
			ip := ua.Address.IP()
			if ip == nil || ip.To4() == nil {
				continue // v1 is IPv4 only
			}
			cfg.Addresses = append(cfg.Addresses, preset.Address{
				Address: ip.String(),
				Prefix:  int(ua.OnLinkPrefixLength),
			})
		}

		for ga := a.FirstGatewayAddress; ga != nil; ga = ga.Next {
			if ip := ga.Address.IP(); ip != nil && ip.To4() != nil {
				cfg.Gateways = append(cfg.Gateways, ip.String())
			}
		}

		for da := a.FirstDnsServerAddress; da != nil; da = da.Next {
			if ip := da.Address.IP(); ip != nil && ip.To4() != nil {
				cfg.DNS = append(cfg.DNS, ip.String())
			}
		}

		// GetAdaptersAddresses reports the resolvers in use but not whether
		// they were configured or leased, and that distinction decides whether
		// a DNS-from-DHCP preset matches. The registry holds it: a non-empty
		// NameServer value means statically configured.
		cfg.DNSFromDHCP = dnsIsFromDHCP(windows.BytePtrToString(a.AdapterName))

		return cfg, nil
	}

	return LiveConfig{}, fmt.Errorf("adapter %q not found", adapter)
}

// Apply runs each command in order, stopping at the first failure.
func (Windows) Apply(plan []Command) error {
	for _, c := range plan {
		log.Printf("netsh: %s (%s)", c, c.Desc)

		out, err := runNetsh(c)
		if out != "" {
			log.Printf("netsh output: %s", out)
		}
		if err != nil {
			if isElevationError(out) {
				return fmt.Errorf("%w\n\ncommand: %s\n%s", ErrElevationRequired, c, out)
			}
			return fmt.Errorf("%s failed: %w\n\n%s", c.Desc, err, out)
		}
	}
	return nil
}

func runNetsh(c Command) (string, error) {
	cmd := exec.Command("netsh", c.Args...)
	// The binary is linked with -H windowsgui and has no console of its own, so
	// without this every netsh call flashes a console window on screen.
	cmd.SysProcAttr = &syscall.SysProcAttr{HideWindow: true}

	done := make(chan struct{})
	var out []byte
	var err error
	go func() {
		out, err = cmd.CombinedOutput()
		close(done)
	}()

	select {
	case <-done:
	case <-time.After(applyTimeout):
		if cmd.Process != nil {
			_ = cmd.Process.Kill()
		}
		<-done
		return string(out), fmt.Errorf("timed out after %s", applyTimeout)
	}

	return strings.TrimSpace(string(out)), err
}

// isElevationError sniffs netsh's refusal. netsh exits non-zero with a message
// rather than a distinguishable code, and the message is localised, so this
// also accepts the English text as the common case and falls back to the
// Win32 error number that appears in several translations.
func isElevationError(out string) bool {
	l := strings.ToLower(out)
	return strings.Contains(l, "requires elevation") ||
		strings.Contains(l, "requested operation requires") ||
		strings.Contains(l, "access is denied") ||
		strings.Contains(l, "740")
}

func dnsIsFromDHCP(guid string) bool {
	if guid == "" {
		return true
	}
	key, err := registry.OpenKey(
		registry.LOCAL_MACHINE,
		`SYSTEM\CurrentControlSet\Services\Tcpip\Parameters\Interfaces\`+guid,
		registry.QUERY_VALUE,
	)
	if err != nil {
		// Not readable without admin on some machines; assume DHCP rather
		// than falsely reporting a static configuration.
		return true
	}
	defer key.Close()

	ns, _, err := key.GetStringValue("NameServer")
	if err != nil {
		return true
	}
	return strings.TrimSpace(ns) == ""
}

// adapterRows walks the GetAdaptersAddresses linked list into a slice.
func adapterRows() ([]*windows.IpAdapterAddresses, error) {
	flags := uint32(windows.GAA_FLAG_INCLUDE_GATEWAYS |
		windows.GAA_FLAG_SKIP_ANYCAST |
		windows.GAA_FLAG_SKIP_MULTICAST)

	size := uint32(15 * 1024)
	var buf []byte
	for attempt := 0; attempt < 4; attempt++ {
		buf = make([]byte, size)
		err := windows.GetAdaptersAddresses(
			windows.AF_INET, flags, 0,
			(*windows.IpAdapterAddresses)(unsafe.Pointer(&buf[0])),
			&size,
		)
		if err == nil {
			break
		}
		// The call reports the required size in `size`; retry with it.
		if err == windows.ERROR_BUFFER_OVERFLOW {
			continue
		}
		return nil, fmt.Errorf("GetAdaptersAddresses: %w", err)
	}

	var out []*windows.IpAdapterAddresses
	for a := (*windows.IpAdapterAddresses)(unsafe.Pointer(&buf[0])); a != nil; a = a.Next {
		out = append(out, a)
	}
	return out, nil
}

// compile-time assertion that net is used even if the address helpers change.
var _ = net.IP{}