refactor: DNS

dns/transport/local/resolv.go

@@ -0,0 +1,146 @@
+package local
+
+import (
+	"os"
+	"runtime"
+	"strings"
+	"sync"
+	"sync/atomic"
+	"time"
+)
+
+const (
+	// net.maxDNSPacketSize
+	maxDNSPacketSize = 1232
+)
+
+type resolverConfig struct {
+	initOnce    sync.Once
+	ch          chan struct{}
+	lastChecked time.Time
+	dnsConfig   atomic.Pointer[dnsConfig]
+}
+
+var resolvConf resolverConfig
+
+func getSystemDNSConfig() *dnsConfig {
+	resolvConf.tryUpdate("/etc/resolv.conf")
+	return resolvConf.dnsConfig.Load()
+}
+
+func (conf *resolverConfig) init() {
+	conf.dnsConfig.Store(dnsReadConfig("/etc/resolv.conf"))
+	conf.lastChecked = time.Now()
+	conf.ch = make(chan struct{}, 1)
+}
+
+func (conf *resolverConfig) tryUpdate(name string) {
+	conf.initOnce.Do(conf.init)
+
+	if conf.dnsConfig.Load().noReload {
+		return
+	}
+	if !conf.tryAcquireSema() {
+		return
+	}
+	defer conf.releaseSema()
+
+	now := time.Now()
+	if conf.lastChecked.After(now.Add(-5 * time.Second)) {
+		return
+	}
+	conf.lastChecked = now
+	if runtime.GOOS != "windows" {
+		var mtime time.Time
+		if fi, err := os.Stat(name); err == nil {
+			mtime = fi.ModTime()
+		}
+		if mtime.Equal(conf.dnsConfig.Load().mtime) {
+			return
+		}
+	}
+	dnsConf := dnsReadConfig(name)
+	conf.dnsConfig.Store(dnsConf)
+}
+
+func (conf *resolverConfig) tryAcquireSema() bool {
+	select {
+	case conf.ch <- struct{}{}:
+		return true
+	default:
+		return false
+	}
+}
+
+func (conf *resolverConfig) releaseSema() {
+	<-conf.ch
+}
+
+type dnsConfig struct {
+	servers       []string
+	search        []string
+	ndots         int
+	timeout       time.Duration
+	attempts      int
+	rotate        bool
+	unknownOpt    bool
+	lookup        []string
+	err           error
+	mtime         time.Time
+	soffset       uint32
+	singleRequest bool
+	useTCP        bool
+	trustAD       bool
+	noReload      bool
+}
+
+func (c *dnsConfig) serverOffset() uint32 {
+	if c.rotate {
+		return atomic.AddUint32(&c.soffset, 1) - 1 // return 0 to start
+	}
+	return 0
+}
+
+func (conf *dnsConfig) nameList(name string) []string {
+	l := len(name)
+	rooted := l > 0 && name[l-1] == '.'
+	if l > 254 || l == 254 && !rooted {
+		return nil
+	}
+
+	if rooted {
+		if avoidDNS(name) {
+			return nil
+		}
+		return []string{name}
+	}
+
+	hasNdots := strings.Count(name, ".") >= conf.ndots
+	name += "."
+	// l++
+
+	names := make([]string, 0, 1+len(conf.search))
+	if hasNdots && !avoidDNS(name) {
+		names = append(names, name)
+	}
+	for _, suffix := range conf.search {
+		fqdn := name + suffix
+		if !avoidDNS(fqdn) && len(fqdn) <= 254 {
+			names = append(names, fqdn)
+		}
+	}
+	if !hasNdots && !avoidDNS(name) {
+		names = append(names, name)
+	}
+	return names
+}
+
+func avoidDNS(name string) bool {
+	if name == "" {
+		return true
+	}
+	if name[len(name)-1] == '.' {
+		name = name[:len(name)-1]
+	}
+	return strings.HasSuffix(name, ".onion")
+}