internal/clientinfo: ignoring localhost entry for hostsfile mapping

Otherwise, actual hostname will be overriden with "localhost", which is
rather confusing/bad for UX.

internal/clientinfo/hostsfile.go

@@ -10,6 +10,12 @@ import (
 	"github.com/Control-D-Inc/ctrld"
 )
 
+const (
+	ipv4LocalhostName = "localhost"
+	ipv6LocalhostName = "ip6-localhost"
+	ipv6LoopbackName  = "ip6-loopback"
+)
+
 // hostsFile provides client discovery functionality using system hosts file.
 type hostsFile struct {
 	watcher *fsnotify.Watcher
@@ -80,7 +86,15 @@ func (hf *hostsFile) LookupHostnameByIP(ip string) string {
 	hf.mu.Lock()
 	defer hf.mu.Unlock()
 	if names := hf.m[ip]; len(names) > 0 {
-		return normalizeHostname(names[0])
+		isLoopback := ip == "127.0.0.1" || ip == "::1"
+		for _, hostname := range names {
+			name := normalizeHostname(hostname)
+			// Ignoring ipv4/ipv6 loopback entry.
+			if isLoopback && isLocalhostName(name) {
+				continue
+			}
+			return name
+		}
 	}
 	return ""
 }
@@ -94,3 +108,13 @@ func (hf *hostsFile) LookupHostnameByMac(mac string) string {
 func (hf *hostsFile) String() string {
 	return "hosts"
 }
+
+// isLocalhostName reports whether the given hostname represents localhost.
+func isLocalhostName(hostname string) bool {
+	switch hostname {
+	case ipv4LocalhostName, ipv6LocalhostName, ipv6LoopbackName:
+		return true
+	default:
+		return false
+	}
+}

internal/clientinfo/hostsfile_test.go

@@ -0,0 +1,33 @@
+package clientinfo
+
+import (
+	"testing"
+)
+
+func Test_hostsFile_LookupHostnameByIP(t *testing.T) {
+	tests := []struct {
+		name             string
+		ip               string
+		hostnames        []string
+		expectedHostname string
+	}{
+		{"ipv4 loopback", "127.0.0.1", []string{ipv4LocalhostName}, ""},
+		{"ipv6 loopback", "::1", []string{ipv6LocalhostName, ipv6LoopbackName}, ""},
+		{"non-localhost", "::1", []string{"foo"}, "foo"},
+		{"multiple hostnames", "::1", []string{ipv4LocalhostName, "foo"}, "foo"},
+	}
+
+	for _, tc := range tests {
+		tc := tc
+		t.Run(tc.name, func(t *testing.T) {
+			t.Parallel()
+			hf := &hostsFile{m: make(map[string][]string)}
+			hf.mu.Lock()
+			hf.m[tc.ip] = tc.hostnames
+			hf.mu.Unlock()
+			if got := hf.LookupHostnameByIP(tc.ip); got != tc.expectedHostname {
+				t.Errorf("unpexpected result, want: %q, got: %q", tc.expectedHostname, got)
+			}
+		})
+	}
+}