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44c2b7dc79
Further refinement for tailscale/corp#1662. Signed-off-by: David Anderson <danderson@tailscale.com>
429 lines
12 KiB
Go
429 lines
12 KiB
Go
// Copyright (c) 2020 Tailscale Inc & AUTHORS All rights reserved.
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// Use of this source code is governed by a BSD-style
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// license that can be found in the LICENSE file.
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package dns
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import (
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"errors"
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"fmt"
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"os/exec"
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"sort"
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"strings"
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"syscall"
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"time"
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"golang.org/x/sys/windows"
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"golang.org/x/sys/windows/registry"
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"golang.zx2c4.com/wireguard/windows/tunnel/winipcfg"
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"inet.af/netaddr"
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"tailscale.com/types/logger"
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"tailscale.com/util/dnsname"
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)
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const (
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ipv4RegBase = `SYSTEM\CurrentControlSet\Services\Tcpip\Parameters`
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ipv6RegBase = `SYSTEM\CurrentControlSet\Services\Tcpip6\Parameters`
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// the GUID is randomly generated. At present, Tailscale installs
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// zero or one NRPT rules, so hardcoding a single GUID everywhere
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// is fine.
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nrptBase = `SYSTEM\CurrentControlSet\services\Dnscache\Parameters\DnsPolicyConfig\{5abe529b-675b-4486-8459-25a634dacc23}`
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nrptOverrideDNS = 0x8 // bitmask value for "use the provided override DNS resolvers"
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versionKey = `SOFTWARE\Microsoft\Windows NT\CurrentVersion`
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)
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type windowsManager struct {
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logf logger.Logf
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guid string
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nrptWorks bool
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}
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func NewOSConfigurator(logf logger.Logf, interfaceName string) (OSConfigurator, error) {
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ret := windowsManager{
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logf: logf,
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guid: interfaceName,
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nrptWorks: !isWindows7(),
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}
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// Best-effort: if our NRPT rule exists, try to delete it. Unlike
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// per-interface configuration, NRPT rules survive the unclean
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// termination of the Tailscale process, and depending on the
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// rule, it may prevent us from reaching login.tailscale.com to
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// boot up. The bootstrap resolver logic will save us, but it
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// slows down start-up a bunch.
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if ret.nrptWorks {
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ret.delKey(nrptBase)
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}
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return ret, nil
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}
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// keyOpenTimeout is how long we wait for a registry key to
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// appear. For some reason, registry keys tied to ephemeral interfaces
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// can take a long while to appear after interface creation, and we
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// can end up racing with that.
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const keyOpenTimeout = 20 * time.Second
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func (m windowsManager) openKey(path string) (registry.Key, error) {
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key, err := openKeyWait(registry.LOCAL_MACHINE, path, registry.SET_VALUE, keyOpenTimeout)
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if err != nil {
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return 0, fmt.Errorf("opening %s: %w", path, err)
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}
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return key, nil
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}
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func (m windowsManager) ifPath(basePath string) string {
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return fmt.Sprintf(`%s\Interfaces\%s`, basePath, m.guid)
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}
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func (m windowsManager) delKey(path string) error {
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if err := registry.DeleteKey(registry.LOCAL_MACHINE, path); err != nil && err != registry.ErrNotExist {
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return err
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}
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return nil
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}
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func delValue(key registry.Key, name string) error {
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if err := key.DeleteValue(name); err != nil && err != registry.ErrNotExist {
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return err
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}
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return nil
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}
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// setSplitDNS configures an NRPT (Name Resolution Policy Table) rule
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// to resolve queries for domains using resolvers, rather than the
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// system's "primary" resolver.
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//
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// If no resolvers are provided, the Tailscale NRPT rule is deleted.
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func (m windowsManager) setSplitDNS(resolvers []netaddr.IP, domains []dnsname.FQDN) error {
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if len(resolvers) == 0 {
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return m.delKey(nrptBase)
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}
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servers := make([]string, 0, len(resolvers))
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for _, resolver := range resolvers {
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servers = append(servers, resolver.String())
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}
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doms := make([]string, 0, len(domains))
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for _, domain := range domains {
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// NRPT rules must have a leading dot, which is not usual for
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// DNS search paths.
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doms = append(doms, "."+domain.WithoutTrailingDot())
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}
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// CreateKey is actually open-or-create, which suits us fine.
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key, _, err := registry.CreateKey(registry.LOCAL_MACHINE, nrptBase, registry.SET_VALUE)
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if err != nil {
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return fmt.Errorf("opening %s: %w", nrptBase, err)
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}
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defer key.Close()
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if err := key.SetDWordValue("Version", 1); err != nil {
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return err
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}
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if err := key.SetStringsValue("Name", doms); err != nil {
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return err
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}
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if err := key.SetStringValue("GenericDNSServers", strings.Join(servers, "; ")); err != nil {
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return err
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}
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if err := key.SetDWordValue("ConfigOptions", nrptOverrideDNS); err != nil {
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return err
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}
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return nil
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}
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// setPrimaryDNS sets the given resolvers and domains as the Tailscale
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// interface's DNS configuration.
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// If resolvers is non-empty, those resolvers become the system's
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// "primary" resolvers.
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// domains can be set without resolvers, which just contributes new
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// paths to the global DNS search list.
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func (m windowsManager) setPrimaryDNS(resolvers []netaddr.IP, domains []dnsname.FQDN) error {
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var ipsv4 []string
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var ipsv6 []string
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for _, ip := range resolvers {
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if ip.Is4() {
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ipsv4 = append(ipsv4, ip.String())
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} else {
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ipsv6 = append(ipsv6, ip.String())
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}
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}
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domStrs := make([]string, 0, len(domains))
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for _, dom := range domains {
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domStrs = append(domStrs, dom.WithoutTrailingDot())
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}
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key4, err := m.openKey(m.ifPath(ipv4RegBase))
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if err != nil {
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return err
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}
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defer key4.Close()
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if len(ipsv4) == 0 {
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if err := delValue(key4, "NameServer"); err != nil {
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return err
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}
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} else if err := key4.SetStringValue("NameServer", strings.Join(ipsv4, ",")); err != nil {
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return err
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}
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if len(domains) == 0 {
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if err := delValue(key4, "SearchList"); err != nil {
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return err
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}
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} else if err := key4.SetStringValue("SearchList", strings.Join(domStrs, ",")); err != nil {
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return err
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}
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key6, err := m.openKey(m.ifPath(ipv6RegBase))
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if err != nil {
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return err
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}
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defer key6.Close()
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if len(ipsv6) == 0 {
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if err := delValue(key6, "NameServer"); err != nil {
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return err
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}
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} else if err := key6.SetStringValue("NameServer", strings.Join(ipsv6, ",")); err != nil {
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return err
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}
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if len(domains) == 0 {
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if err := delValue(key6, "SearchList"); err != nil {
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return err
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}
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} else if err := key6.SetStringValue("SearchList", strings.Join(domStrs, ",")); err != nil {
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return err
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}
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// Disable LLMNR on the Tailscale interface. We don't do
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// multicast, and we certainly don't do LLMNR, so it's pointless
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// to make Windows try it.
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if err := key4.SetDWordValue("EnableMulticast", 0); err != nil {
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return err
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}
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if err := key6.SetDWordValue("EnableMulticast", 0); err != nil {
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return err
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}
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return nil
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}
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func (m windowsManager) SetDNS(cfg OSConfig) error {
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// We can configure Windows DNS in one of two ways:
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//
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// - In primary DNS mode, we set the NameServer and SearchList
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// registry keys on our interface. Because our interface metric
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// is very low, this turns us into the one and only "primary"
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// resolver for the OS, i.e. all queries flow to the
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// resolver(s) we specify.
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// - In split DNS mode, we set the Domain registry key on our
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// interface (which adds that domain to the global search list,
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// but does not contribute other DNS configuration from the
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// interface), and configure an NRPT (Name Resolution Policy
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// Table) rule to route queries for our suffixes to the
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// provided resolver.
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//
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// When switching modes, we delete all the configuration related
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// to the other mode, so these two are an XOR.
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//
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// Windows actually supports much more advanced configurations as
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// well, with arbitrary routing of hosts and suffixes to arbitrary
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// resolvers. However, we use it in a "simple" split domain
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// configuration only, routing one set of things to the "split"
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// resolver and the rest to the primary.
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if len(cfg.MatchDomains) == 0 {
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if err := m.setSplitDNS(nil, nil); err != nil {
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return err
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}
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if err := m.setPrimaryDNS(cfg.Nameservers, cfg.SearchDomains); err != nil {
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return err
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}
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} else if !m.nrptWorks {
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return errors.New("cannot set per-domain resolvers on Windows 7")
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} else {
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if err := m.setSplitDNS(cfg.Nameservers, cfg.MatchDomains); err != nil {
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return err
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}
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// Still set search domains on the interface, since NRPT only
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// handles query routing and not search domain expansion.
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if err := m.setPrimaryDNS(nil, cfg.SearchDomains); err != nil {
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return err
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}
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}
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// Force DNS re-registration in Active Directory. What we actually
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// care about is that this command invokes the undocumented hidden
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// function that forces Windows to notice that adapter settings
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// have changed, which makes the DNS settings actually take
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// effect.
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//
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// This command can take a few seconds to run, so run it async, best effort.
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//
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// After re-registering DNS, also flush the DNS cache to clear out
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// any cached split-horizon queries that are no longer the correct
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// answer.
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go func() {
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t0 := time.Now()
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m.logf("running ipconfig /registerdns ...")
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cmd := exec.Command("ipconfig", "/registerdns")
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cmd.SysProcAttr = &syscall.SysProcAttr{HideWindow: true}
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err := cmd.Run()
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d := time.Since(t0).Round(time.Millisecond)
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if err != nil {
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m.logf("error running ipconfig /registerdns after %v: %v", d, err)
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} else {
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m.logf("ran ipconfig /registerdns in %v", d)
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}
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t0 = time.Now()
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m.logf("running ipconfig /registerdns ...")
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cmd = exec.Command("ipconfig", "/flushdns")
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cmd.SysProcAttr = &syscall.SysProcAttr{HideWindow: true}
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err = cmd.Run()
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d = time.Since(t0).Round(time.Millisecond)
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if err != nil {
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m.logf("error running ipconfig /flushdns after %v: %v", d, err)
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} else {
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m.logf("ran ipconfig /flushdns in %v", d)
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}
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}()
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return nil
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}
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func (m windowsManager) SupportsSplitDNS() bool {
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return m.nrptWorks
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}
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func (m windowsManager) Close() error {
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return m.SetDNS(OSConfig{})
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}
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func (m windowsManager) GetBaseConfig() (OSConfig, error) {
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resolvers, err := m.getBasePrimaryResolver()
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if err != nil {
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return OSConfig{}, err
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}
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return OSConfig{
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Nameservers: resolvers,
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// Don't return any search domains here, because even Windows
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// 7 correctly handles blending search domains from multiple
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// sources, and any search domains we add here will get tacked
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// onto the Tailscale config unnecessarily.
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}, nil
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}
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// getBasePrimaryResolver returns a guess of the non-Tailscale primary
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// resolver on the system.
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// It's used on Windows 7 to emulate split DNS by trying to figure out
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// what the "previous" primary resolver was. It might be wrong, or
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// incomplete.
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func (m windowsManager) getBasePrimaryResolver() (resolvers []netaddr.IP, err error) {
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tsGUID, err := windows.GUIDFromString(m.guid)
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if err != nil {
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return nil, err
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}
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tsLUID, err := winipcfg.LUIDFromGUID(&tsGUID)
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if err != nil {
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return nil, err
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}
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ifrows, err := winipcfg.GetIPInterfaceTable(windows.AF_INET)
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if err == windows.ERROR_NOT_FOUND {
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// IPv4 seems disabled, try to get interface metrics from IPv6 instead.
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ifrows, err = winipcfg.GetIPInterfaceTable(windows.AF_INET6)
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}
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if err != nil {
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return nil, err
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}
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type candidate struct {
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id winipcfg.LUID
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metric uint32
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}
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var candidates []candidate
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for _, row := range ifrows {
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if !row.Connected {
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continue
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}
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if row.InterfaceLUID == tsLUID {
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continue
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}
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candidates = append(candidates, candidate{row.InterfaceLUID, row.Metric})
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}
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if len(candidates) == 0 {
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// No resolvers set outside of Tailscale.
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return nil, nil
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}
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sort.Slice(candidates, func(i, j int) bool { return candidates[i].metric < candidates[j].metric })
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for _, candidate := range candidates {
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ips, err := candidate.id.DNS()
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if err != nil {
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return nil, err
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}
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ipLoop:
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for _, stdip := range ips {
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ip, ok := netaddr.FromStdIP(stdip)
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if !ok {
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continue
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}
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// Skip IPv6 site-local resolvers. These are an ancient
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// and obsolete IPv6 RFC, which Windows still faithfully
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// implements. The net result is that some low-metric
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// interfaces can "have" DNS resolvers, but they're just
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// site-local resolver IPs that don't go anywhere. So, we
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// skip the site-local resolvers in order to find the
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// first interface that has real DNS servers configured.
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for _, sl := range siteLocalResolvers {
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if ip.WithZone("") == sl {
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continue ipLoop
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}
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}
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resolvers = append(resolvers, ip)
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}
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if len(resolvers) > 0 {
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// Found some resolvers, we're done.
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break
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}
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}
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return resolvers, nil
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}
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var siteLocalResolvers = []netaddr.IP{
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netaddr.MustParseIP("fec0:0:0:ffff::1"),
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netaddr.MustParseIP("fec0:0:0:ffff::2"),
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netaddr.MustParseIP("fec0:0:0:ffff::3"),
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}
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func isWindows7() bool {
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key, err := registry.OpenKey(registry.LOCAL_MACHINE, versionKey, registry.READ)
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if err != nil {
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// Fail safe, assume Windows 7.
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return true
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}
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ver, _, err := key.GetStringValue("CurrentVersion")
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if err != nil {
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return true
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}
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// Careful to not assume anything about version numbers beyond
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// 6.3, Microsoft deprecated this registry key and locked its
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// value to what it was in Windows 8.1. We can only use this to
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// probe for versions before that. Good thing we only need Windows
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// 7 (so far).
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//
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// And yes, Windows 7 is version 6.1. Don't ask.
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return ver == "6.1"
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}
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