net/dns: [win] add MagicDNS entries to etc/hosts

This works around the 2.3s delay in short name lookups when SNR is
enabled.
C:\Windows\System32\drivers\etc\hosts file. We only add known hosts that
match the search domains, and we populate the list in order of
Search Domains so that our matching algorithm mimics what Windows would
otherwise do itself if SNR was off.

Updates #1659

Signed-off-by: Maisem Ali <maisem@tailscale.com>

net/dns/manager.go

@@ -16,6 +16,7 @@ import (
 	"sync/atomic"
 	"time"
 
+	"golang.org/x/exp/slices"
 	"tailscale.com/health"
 	"tailscale.com/net/dns/resolver"
 	"tailscale.com/net/packet"
@@ -137,6 +138,47 @@ func (m *Manager) Set(cfg Config) error {
 	return nil
 }
 
+// compileHostEntries creates a list of single-label resolutions possible
+// from the configured hosts and search domains.
+// The entries are compiled in the order of the search domains, then the hosts.
+// The returned list is sorted by the first hostname in each entry.
+func compileHostEntries(cfg Config) (hosts []*HostEntry) {
+	didLabel := make(map[string]bool, len(cfg.Hosts))
+	for _, sd := range cfg.SearchDomains {
+		for h, ips := range cfg.Hosts {
+			if !sd.Contains(h) || h.NumLabels() != (sd.NumLabels()+1) {
+				continue
+			}
+			ipHosts := []string{string(h.WithTrailingDot())}
+			if label := dnsname.FirstLabel(string(h)); !didLabel[label] {
+				didLabel[label] = true
+				ipHosts = append(ipHosts, label)
+			}
+			for _, ip := range ips {
+				if cfg.OnlyIPv6 && ip.Is4() {
+					continue
+				}
+				hosts = append(hosts, &HostEntry{
+					Addr:  ip,
+					Hosts: ipHosts,
+				})
+				// Only add IPv4 or IPv6 per host, like we do in the resolver.
+				break
+			}
+		}
+	}
+	slices.SortFunc(hosts, func(a, b *HostEntry) bool {
+		if len(a.Hosts) == 0 {
+			return false
+		}
+		if len(b.Hosts) == 0 {
+			return true
+		}
+		return a.Hosts[0] < b.Hosts[0]
+	})
+	return hosts
+}
+
 // compileConfig converts cfg into a quad-100 resolver configuration
 // and an OS-level configuration.
 func (m *Manager) compileConfig(cfg Config) (rcfg resolver.Config, ocfg OSConfig, err error) {
@@ -154,6 +196,9 @@ func (m *Manager) compileConfig(cfg Config) (rcfg resolver.Config, ocfg OSConfig
 	}
 	// Similarly, the OS always gets search paths.
 	ocfg.SearchDomains = cfg.SearchDomains
+	if runtime.GOOS == "windows" {
+		ocfg.Hosts = compileHostEntries(cfg)
+	}
 
 	// Deal with trivial configs first.
 	switch {