Basic platform?

Former-commit-id: 423e18edc3

dns/platform/file.go

@@ -0,0 +1,192 @@
+//go:build (linux && !android) || freebsd
+
+package dns
+
+import (
+	"fmt"
+	"net/netip"
+	"os"
+	"strings"
+)
+
+const (
+	resolvConfPath       = "/etc/resolv.conf"
+	resolvConfBackupPath = "/etc/resolv.conf.olm.backup"
+	resolvConfHeader     = "# Generated by Olm DNS Manager\n# Original file backed up to " + resolvConfBackupPath + "\n\n"
+)
+
+// FileDNSConfigurator manages DNS settings by directly modifying /etc/resolv.conf
+type FileDNSConfigurator struct {
+	originalState *DNSState
+}
+
+// NewFileDNSConfigurator creates a new file-based DNS configurator
+func NewFileDNSConfigurator() (*FileDNSConfigurator, error) {
+	return &FileDNSConfigurator{}, nil
+}
+
+// Name returns the configurator name
+func (f *FileDNSConfigurator) Name() string {
+	return "file-resolv.conf"
+}
+
+// SetDNS sets the DNS servers and returns the original servers
+func (f *FileDNSConfigurator) SetDNS(servers []netip.Addr) ([]netip.Addr, error) {
+	// Get current DNS settings before overriding
+	originalServers, err := f.GetCurrentDNS()
+	if err != nil {
+		return nil, fmt.Errorf("get current DNS: %w", err)
+	}
+
+	// Backup original resolv.conf if not already backed up
+	if !f.isBackupExists() {
+		if err := f.backupResolvConf(); err != nil {
+			return nil, fmt.Errorf("backup resolv.conf: %w", err)
+		}
+	}
+
+	// Store original state
+	f.originalState = &DNSState{
+		OriginalServers:  originalServers,
+		ConfiguratorName: f.Name(),
+	}
+
+	// Write new resolv.conf
+	if err := f.writeResolvConf(servers); err != nil {
+		return nil, fmt.Errorf("write resolv.conf: %w", err)
+	}
+
+	return originalServers, nil
+}
+
+// RestoreDNS restores the original DNS configuration
+func (f *FileDNSConfigurator) RestoreDNS() error {
+	if !f.isBackupExists() {
+		return fmt.Errorf("no backup file exists")
+	}
+
+	// Copy backup back to original location
+	if err := copyFile(resolvConfBackupPath, resolvConfPath); err != nil {
+		return fmt.Errorf("restore from backup: %w", err)
+	}
+
+	// Remove backup file
+	if err := os.Remove(resolvConfBackupPath); err != nil {
+		return fmt.Errorf("remove backup file: %w", err)
+	}
+
+	return nil
+}
+
+// GetCurrentDNS returns the currently configured DNS servers
+func (f *FileDNSConfigurator) GetCurrentDNS() ([]netip.Addr, error) {
+	content, err := os.ReadFile(resolvConfPath)
+	if err != nil {
+		return nil, fmt.Errorf("read resolv.conf: %w", err)
+	}
+
+	return f.parseNameservers(string(content)), nil
+}
+
+// backupResolvConf creates a backup of the current resolv.conf
+func (f *FileDNSConfigurator) backupResolvConf() error {
+	// Get file info for permissions
+	info, err := os.Stat(resolvConfPath)
+	if err != nil {
+		return fmt.Errorf("stat resolv.conf: %w", err)
+	}
+
+	if err := copyFile(resolvConfPath, resolvConfBackupPath); err != nil {
+		return fmt.Errorf("copy file: %w", err)
+	}
+
+	// Preserve permissions
+	if err := os.Chmod(resolvConfBackupPath, info.Mode()); err != nil {
+		return fmt.Errorf("chmod backup: %w", err)
+	}
+
+	return nil
+}
+
+// writeResolvConf writes a new resolv.conf with the specified DNS servers
+func (f *FileDNSConfigurator) writeResolvConf(servers []netip.Addr) error {
+	if len(servers) == 0 {
+		return fmt.Errorf("no DNS servers provided")
+	}
+
+	// Get file info for permissions
+	info, err := os.Stat(resolvConfPath)
+	if err != nil {
+		return fmt.Errorf("stat resolv.conf: %w", err)
+	}
+
+	var content strings.Builder
+	content.WriteString(resolvConfHeader)
+
+	// Write nameservers
+	for _, server := range servers {
+		content.WriteString("nameserver ")
+		content.WriteString(server.String())
+		content.WriteString("\n")
+	}
+
+	// Write the file
+	if err := os.WriteFile(resolvConfPath, []byte(content.String()), info.Mode()); err != nil {
+		return fmt.Errorf("write resolv.conf: %w", err)
+	}
+
+	return nil
+}
+
+// isBackupExists checks if a backup file exists
+func (f *FileDNSConfigurator) isBackupExists() bool {
+	_, err := os.Stat(resolvConfBackupPath)
+	return err == nil
+}
+
+// parseNameservers extracts nameserver entries from resolv.conf content
+func (f *FileDNSConfigurator) parseNameservers(content string) []netip.Addr {
+	var servers []netip.Addr
+
+	lines := strings.Split(content, "\n")
+	for _, line := range lines {
+		line = strings.TrimSpace(line)
+
+		// Skip comments and empty lines
+		if line == "" || strings.HasPrefix(line, "#") {
+			continue
+		}
+
+		// Look for nameserver lines
+		if strings.HasPrefix(line, "nameserver") {
+			fields := strings.Fields(line)
+			if len(fields) >= 2 {
+				if addr, err := netip.ParseAddr(fields[1]); err == nil {
+					servers = append(servers, addr)
+				}
+			}
+		}
+	}
+
+	return servers
+}
+
+// copyFile copies a file from src to dst
+func copyFile(src, dst string) error {
+	content, err := os.ReadFile(src)
+	if err != nil {
+		return fmt.Errorf("read source: %w", err)
+	}
+
+	// Get source file permissions
+	info, err := os.Stat(src)
+	if err != nil {
+		return fmt.Errorf("stat source: %w", err)
+	}
+
+	if err := os.WriteFile(dst, content, info.Mode()); err != nil {
+		return fmt.Errorf("write destination: %w", err)
+	}
+
+	return nil
+}