init

internal/resolv/resolv.go

@@ -0,0 +1,183 @@
+package resolv
+
+import (
+	"bufio"
+	"errors"
+	"fmt"
+	"net"
+	"os"
+	"slices"
+	"strings"
+	"sync"
+)
+
+const DefaultPath = "/run/dns/resolv.conf"
+
+var (
+	ErrInvalidAddress = errors.New("invalid nameserver address")
+	ErrAlreadyExists  = errors.New("nameserver already exists")
+	ErrNotFound       = errors.New("nameserver not found")
+)
+
+type Manager struct {
+	path string
+	mu   sync.Mutex
+}
+
+func New(path string) *Manager {
+	if path == "" {
+		path = DefaultPath
+	}
+	return &Manager{path: path}
+}
+
+func (m *Manager) List() ([]string, error) {
+	m.mu.Lock()
+	defer m.mu.Unlock()
+
+	_, servers, err := m.read()
+	if err != nil {
+		return nil, err
+	}
+	return servers, nil
+}
+
+func (m *Manager) Add(addr string) error {
+	addr = strings.TrimSpace(addr)
+	if !isValidIP(addr) {
+		return fmt.Errorf("%w: %q", ErrInvalidAddress, addr)
+	}
+
+	m.mu.Lock()
+	defer m.mu.Unlock()
+
+	lines, servers, err := m.read()
+	if err != nil {
+		return err
+	}
+	if slices.Contains(servers, addr) {
+		return fmt.Errorf("%w: %s", ErrAlreadyExists, addr)
+	}
+	lines = append(lines, "nameserver "+addr)
+	return m.write(lines)
+}
+
+func (m *Manager) Remove(addr string) error {
+	addr = strings.TrimSpace(addr)
+	if !isValidIP(addr) {
+		return fmt.Errorf("%w: %q", ErrInvalidAddress, addr)
+	}
+
+	m.mu.Lock()
+	defer m.mu.Unlock()
+
+	lines, _, err := m.read()
+	if err != nil {
+		return err
+	}
+
+	out := make([]string, 0, len(lines))
+	removed := false
+	for _, line := range lines {
+		if !removed && parseNameserver(line) == addr {
+			removed = true
+			continue
+		}
+		out = append(out, line)
+	}
+	if !removed {
+		return fmt.Errorf("%w: %s", ErrNotFound, addr)
+	}
+	return m.write(out)
+}
+
+func (m *Manager) read() ([]string, []string, error) {
+	f, err := os.Open(m.path)
+	if err != nil {
+		if os.IsNotExist(err) {
+			return nil, nil, nil
+		}
+		return nil, nil, err
+	}
+	defer f.Close()
+
+	var lines, servers []string
+	sc := bufio.NewScanner(f)
+	for sc.Scan() {
+		line := sc.Text()
+		lines = append(lines, line)
+		if ns := parseNameserver(line); ns != "" {
+			servers = append(servers, ns)
+		}
+	}
+	if err := sc.Err(); err != nil {
+		return nil, nil, err
+	}
+	return lines, servers, nil
+}
+
+func (m *Manager) write(lines []string) error {
+	dir := dirOf(m.path)
+	tmp, err := os.CreateTemp(dir, ".resolv.conf.tmp.*")
+	if err != nil {
+		return err
+	}
+	tmpName := tmp.Name()
+	cleanup := func() { _ = os.Remove(tmpName) }
+
+	w := bufio.NewWriter(tmp)
+	for _, line := range lines {
+		if _, err := w.WriteString(line); err != nil {
+			tmp.Close()
+			cleanup()
+			return err
+		}
+		if _, err := w.WriteString("\n"); err != nil {
+			tmp.Close()
+			cleanup()
+			return err
+		}
+	}
+	if err := w.Flush(); err != nil {
+		tmp.Close()
+		cleanup()
+		return err
+	}
+	if err := tmp.Sync(); err != nil {
+		tmp.Close()
+		cleanup()
+		return err
+	}
+	if err := tmp.Close(); err != nil {
+		cleanup()
+		return err
+	}
+	if err := os.Rename(tmpName, m.path); err != nil {
+		cleanup()
+		return err
+	}
+	return nil
+}
+
+func parseNameserver(line string) string {
+	s := strings.TrimSpace(line)
+	if s == "" || strings.HasPrefix(s, "#") || strings.HasPrefix(s, ";") {
+		return ""
+	}
+	fields := strings.Fields(s)
+	if len(fields) < 2 || fields[0] != "nameserver" {
+		return ""
+	}
+	return fields[1]
+}
+
+func isValidIP(s string) bool {
+	return s != "" && net.ParseIP(s) != nil
+}
+
+func dirOf(path string) string {
+	if i := strings.LastIndex(path, "/"); i >= 0 {
+		return path[:i]
+	}
+	return "."
+}