refactor: DNS

dns/transport/local/resolv_unix.go

@@ -0,0 +1,175 @@
+//go:build !windows
+
+package local
+
+import (
+	"bufio"
+	"net"
+	"net/netip"
+	"os"
+	"strings"
+	"time"
+	_ "unsafe"
+)
+
+func dnsReadConfig(name string) *dnsConfig {
+	conf := &dnsConfig{
+		ndots:    1,
+		timeout:  5 * time.Second,
+		attempts: 2,
+	}
+	file, err := os.Open(name)
+	if err != nil {
+		conf.servers = defaultNS
+		conf.search = dnsDefaultSearch()
+		conf.err = err
+		return conf
+	}
+	defer file.Close()
+	fi, err := file.Stat()
+	if err == nil {
+		conf.mtime = fi.ModTime()
+	} else {
+		conf.servers = defaultNS
+		conf.search = dnsDefaultSearch()
+		conf.err = err
+		return conf
+	}
+	reader := bufio.NewReader(file)
+	var (
+		prefix   []byte
+		line     []byte
+		isPrefix bool
+	)
+	for {
+		line, isPrefix, err = reader.ReadLine()
+		if err != nil {
+			break
+		}
+		if isPrefix {
+			prefix = append(prefix, line...)
+			continue
+		} else if len(prefix) > 0 {
+			line = append(prefix, line...)
+			prefix = nil
+		}
+		if len(line) > 0 && (line[0] == ';' || line[0] == '#') {
+			continue
+		}
+		f := strings.Fields(string(line))
+		if len(f) < 1 {
+			continue
+		}
+		switch f[0] {
+		case "nameserver":
+			if len(f) > 1 && len(conf.servers) < 3 {
+				if _, err := netip.ParseAddr(f[1]); err == nil {
+					conf.servers = append(conf.servers, net.JoinHostPort(f[1], "53"))
+				}
+			}
+		case "domain":
+			if len(f) > 1 {
+				conf.search = []string{ensureRooted(f[1])}
+			}
+
+		case "search":
+			conf.search = make([]string, 0, len(f)-1)
+			for i := 1; i < len(f); i++ {
+				name := ensureRooted(f[i])
+				if name == "." {
+					continue
+				}
+				conf.search = append(conf.search, name)
+			}
+
+		case "options":
+			for _, s := range f[1:] {
+				switch {
+				case strings.HasPrefix(s, "ndots:"):
+					n, _, _ := dtoi(s[6:])
+					if n < 0 {
+						n = 0
+					} else if n > 15 {
+						n = 15
+					}
+					conf.ndots = n
+				case strings.HasPrefix(s, "timeout:"):
+					n, _, _ := dtoi(s[8:])
+					if n < 1 {
+						n = 1
+					}
+					conf.timeout = time.Duration(n) * time.Second
+				case strings.HasPrefix(s, "attempts:"):
+					n, _, _ := dtoi(s[9:])
+					if n < 1 {
+						n = 1
+					}
+					conf.attempts = n
+				case s == "rotate":
+					conf.rotate = true
+				case s == "single-request" || s == "single-request-reopen":
+					conf.singleRequest = true
+				case s == "use-vc" || s == "usevc" || s == "tcp":
+					conf.useTCP = true
+				case s == "trust-ad":
+					conf.trustAD = true
+				case s == "edns0":
+				case s == "no-reload":
+					conf.noReload = true
+				default:
+					conf.unknownOpt = true
+				}
+			}
+
+		case "lookup":
+			conf.lookup = f[1:]
+
+		default:
+			conf.unknownOpt = true
+		}
+	}
+	if len(conf.servers) == 0 {
+		conf.servers = defaultNS
+	}
+	if len(conf.search) == 0 {
+		conf.search = dnsDefaultSearch()
+	}
+	return conf
+}
+
+//go:linkname defaultNS net.defaultNS
+var defaultNS []string
+
+func dnsDefaultSearch() []string {
+	hn, err := os.Hostname()
+	if err != nil {
+		return nil
+	}
+	if i := strings.IndexRune(hn, '.'); i >= 0 && i < len(hn)-1 {
+		return []string{ensureRooted(hn[i+1:])}
+	}
+	return nil
+}
+
+func ensureRooted(s string) string {
+	if len(s) > 0 && s[len(s)-1] == '.' {
+		return s
+	}
+	return s + "."
+}
+
+const big = 0xFFFFFF
+
+func dtoi(s string) (n int, i int, ok bool) {
+	n = 0
+	for i = 0; i < len(s) && '0' <= s[i] && s[i] <= '9'; i++ {
+		n = n*10 + int(s[i]-'0')
+		if n >= big {
+			return big, i, false
+		}
+	}
+	if i == 0 {
+		return 0, 0, false
+	}
+	return n, i, true
+}