Add hosts file as source for hostname resolver

internal/clientinfo/hostsfile.go

@@ -0,0 +1,86 @@
+package clientinfo
+
+import (
+	"os"
+
+	"github.com/fsnotify/fsnotify"
+	"github.com/txn2/txeh"
+
+	"github.com/Control-D-Inc/ctrld"
+)
+
+// hostsFile provides client discovery functionality using system hosts file.
+type hostsFile struct {
+	h       *txeh.Hosts
+	watcher *fsnotify.Watcher
+}
+
+// init performs initialization works, which is necessary before hostsFile can be fully operated.
+func (hf *hostsFile) init() error {
+	h, err := txeh.NewHostsDefault()
+	if err != nil {
+		return err
+	}
+	hf.h = h
+	watcher, err := fsnotify.NewWatcher()
+	if err != nil {
+		return err
+	}
+	hf.watcher = watcher
+	if err := hf.watcher.Add(hf.h.ReadFilePath); err != nil {
+		return err
+	}
+	return nil
+}
+
+// refresh reloads hosts file entries.
+func (hf *hostsFile) refresh() error {
+	return hf.h.Reload()
+}
+
+// watchChanges watches and updates hosts file data if any changes happens.
+func (hf *hostsFile) watchChanges() {
+	if hf.watcher == nil {
+		return
+	}
+	for {
+		select {
+		case event, ok := <-hf.watcher.Events:
+			if !ok {
+				return
+			}
+			if event.Has(fsnotify.Write) || event.Has(fsnotify.Rename) || event.Has(fsnotify.Chmod) || event.Has(fsnotify.Remove) {
+				if err := hf.refresh(); err != nil && !os.IsNotExist(err) {
+					ctrld.ProxyLogger.Load().Err(err).Msg("hosts file changed but failed to update client info")
+				}
+			}
+		case err, ok := <-hf.watcher.Errors:
+			if !ok {
+				return
+			}
+			ctrld.ProxyLogger.Load().Err(err).Msg("could not watch client info file")
+		}
+	}
+
+}
+
+// LookupHostnameByIP returns hostname for given IP from current hosts file entries.
+func (hf *hostsFile) LookupHostnameByIP(ip string) string {
+	hf.h.Lock()
+	defer hf.h.Unlock()
+
+	if names := hf.h.ListHostsByIP(ip); len(names) > 0 {
+		return names[0]
+	}
+	return ""
+}
+
+// LookupHostnameByMac returns hostname for given Mac from current hosts file entries.
+func (hf *hostsFile) LookupHostnameByMac(mac string) string {
+	return ""
+}
+
+// String returns human-readable format of hostsFile.
+func (hf *hostsFile) String() string {
+	return "hosts"
+}