smb: allow SPN (service principal name) to be configured

This enables connection to clusters.

Fixes #6515

backend/smb/connpool.go

@@ -34,9 +34,10 @@ func (f *Fs) dial(ctx context.Context, network, addr string) (*conn, error) {
 
 	d := &smb2.Dialer{
 		Initiator: &smb2.NTLMInitiator{
-			User:     f.opt.User,
-			Password: pass,
-			Domain:   f.opt.Domain,
+			User:      f.opt.User,
+			Password:  pass,
+			Domain:    f.opt.Domain,
+			TargetSPN: f.opt.SPN,
 		},
 	}
 

backend/smb/smb.go

@@ -60,6 +60,17 @@ func init() {
 			Name:    "domain",
 			Help:    "Domain name for NTLM authentication.",
 			Default: "WORKGROUP",
+		}, {
+			Name: "spn",
+			Help: `Service principal name.
+
+Rclone presents this name to the server. Some servers use this as further
+authentication, and it often needs to be set for clusters. For example:
+
+    cifs/remotehost:1020
+
+Leave blank if not sure.
+`,
 		}, {
 			Name:    "idle_timeout",
 			Default: fs.Duration(60 * time.Second),
@@ -109,6 +120,7 @@ type Options struct {
 	User            string      `config:"user"`
 	Pass            string      `config:"pass"`
 	Domain          string      `config:"domain"`
+	SPN             string      `config:"spn"`
 	HideSpecial     bool        `config:"hide_special_share"`
 	CaseInsensitive bool        `config:"case_insensitive"`
 	IdleTimeout     fs.Duration `config:"idle_timeout"`