ipn/ipnlocal: return usernames when Tailscale SSH is enabled

It was checking if the sshServer was initialized as a proxy, but that
could either not have been initialized yet or Tailscale SSH could have
been disabled after intialized.

Also bump tailcfg.CurrentCapabilityVersion

Signed-off-by: Maisem Ali <maisem@tailscale.com>

ipn/ipnlocal/local.go

@@ -3228,6 +3228,17 @@ func (b *LocalBackend) setNetMapLocked(nm *netmap.NetworkMap) {
 	}
 }
 
+// operatorUserName returns the current pref's OperatorUser's name, or the
+// empty string if none.
+func (b *LocalBackend) operatorUserName() string {
+	b.mu.Lock()
+	defer b.mu.Unlock()
+	if b.prefs == nil {
+		return ""
+	}
+	return b.prefs.OperatorUser
+}
+
 // OperatorUserID returns the current pref's OperatorUser's ID (in
 // os/user.User.Uid string form), or the empty string if none.
 func (b *LocalBackend) OperatorUserID() string {
@@ -3596,6 +3607,17 @@ func (b *LocalBackend) DoNoiseRequest(req *http.Request) (*http.Response, error)
 	return cc.DoNoiseRequest(req)
 }
 
+// tailscaleSSHEnabled reports whether Tailscale SSH is currently enabled based
+// on prefs. It returns false if there are no prefs set.
+func (b *LocalBackend) tailscaleSSHEnabled() bool {
+	b.mu.Lock()
+	defer b.mu.Unlock()
+	if b.prefs == nil {
+		return false
+	}
+	return b.prefs.RunSSH
+}
+
 func (b *LocalBackend) sshServerOrInit() (_ SSHServer, err error) {
 	b.mu.Lock()
 	defer b.mu.Unlock()

ipn/ipnlocal/ssh.go

@@ -38,15 +38,16 @@ import (
 // running as root.
 var keyTypes = []string{"rsa", "ecdsa", "ed25519"}
 
+// getSSHUsernames discovers and returns the list of usernames that are
+// potential Tailscale SSH user targets.
+//
+// Invariant: must not be called with b.mu held.
 func (b *LocalBackend) getSSHUsernames(req *tailcfg.C2NSSHUsernamesRequest) (*tailcfg.C2NSSHUsernamesResponse, error) {
 	res := new(tailcfg.C2NSSHUsernamesResponse)
-
+	if !b.tailscaleSSHEnabled() {
-	b.mu.Lock()
-	defer b.mu.Unlock()
-
-	if b.sshServer == nil {
 		return res, nil
 	}
+
 	max := 10
 	if req != nil && req.Max != 0 {
 		max = req.Max
@@ -70,8 +71,8 @@ func (b *LocalBackend) getSSHUsernames(req *tailcfg.C2NSSHUsernamesRequest) (*ta
 		res.Usernames = append(res.Usernames, u)
 	}
 
-	if b.prefs != nil && b.prefs.OperatorUser != "" {
+	if opUser := b.operatorUserName(); opUser != "" {
-		add(b.prefs.OperatorUser)
+		add(opUser)
 	}
 
 	// Check popular usernames and see if they exist with a real shell.

tailcfg/tailcfg.go

@@ -79,7 +79,8 @@ type CapabilityVersion int
 //   - 40: 2022-08-22: added Node.KeySignature, PeersChangedPatch.KeySignature
 //   - 41: 2022-08-30: uses 100.100.100.100 for route-less ExtraRecords if global nameservers is set
 //   - 42: 2022-09-06: NextDNS DoH support; see https://github.com/tailscale/tailscale/pull/5556
-const CurrentCapabilityVersion CapabilityVersion = 42
+//   - 43: 2022-09-21: clients can return usernames for SSH
+const CurrentCapabilityVersion CapabilityVersion = 43
 
 type StableID string