policy/v2: name ssh-behavior-v1 a deliberate difference

SaaS stamps it on every node. Sending it would change existing users' login
environment, so it stays a nodeAttr. Document that and pin it.

docs/ref/policy.md

@@ -214,6 +214,9 @@ configuration and attributes. At least the following node attributes are current
   port 41641.
 - `disable-captive-portal-detection`: [Disable automatic captive portal
   detection](https://tailscale.com/docs/integrations/captive-portals#disable-captive-portal-detection).
+- `ssh-behavior-v1`: Run Tailscale SSH sessions on a Linux node without `su`, PAM or a login environment, so `ssh -tt`
+  keeps job control but `~/.profile` is not read. Tailscale's own control plane sets it on every node; Headscale leaves
+  it off.
 
 ```json title="policy.json"
 {

hscontrol/policy/v2/tailnet_state_caps.go

@@ -128,9 +128,12 @@ var unmodelledTailnetStateCaps = []nodecap.Cap{
 	// [tailscale.com/control/controlknobs.Knobs.UpdateFromNodeAttributes].
 	nodecap.ProbeUDPLifetime,
 
-	// [tailcfg.NodeAttrSSHBehaviorV1]: configures the embedded SSH
-	// server (no su, in-process SFTP). Internal tuning; the embedded
-	// server picks Tailscale-vendored defaults without the cap.
+	// [tailcfg.NodeAttrSSHBehaviorV1]: picks the embedded SSH server's
+	// older path on a root-run Linux daemon: no su, in-process SFTP, no
+	// PAM session or login environment. SaaS stamps it on every node;
+	// headscale leaves it to a nodeAttr on purpose, as switching changes
+	// existing users' login environment. It is more than tuning, so
+	// TestSSHBehaviorIsAnAdminChoice pins it apart from this strip.
 	nodecap.SSHBehaviorV1,
 }
 

hscontrol/servertest/nodeattrs_test.go

@@ -226,6 +226,36 @@ func TestTaildropDisabledWithholdsFileSharingCap(t *testing.T) {
 		})
 }
 
+// TestSSHBehaviorIsAnAdminChoice pins a deliberate difference from SaaS, which
+// stamps ssh-behavior-v1 on every node: headscale sends it only through a
+// nodeAttr, so sessions keep their login environment and PAM by default.
+func TestSSHBehaviorIsAnAdminChoice(t *testing.T) {
+	t.Parallel()
+
+	srv := servertest.NewServer(t)
+	user := srv.CreateUser(t, "ssh-behavior-user")
+
+	c := servertest.NewClient(t, srv, "ssh-behavior-node", servertest.WithUser(user))
+	c.WaitForCondition(t, "baseline without ssh-behavior-v1", 10*time.Second,
+		func(nm *netmap.NetworkMap) bool {
+			if nm == nil || !nm.SelfNode.Valid() {
+				return false
+			}
+
+			return hasCap(nm, nodecap.SSH) && !hasCap(nm, nodecap.SSHBehaviorV1)
+		})
+
+	reloadPolicy(t, srv, `{
+		"acls": [{"action": "accept", "src": ["*"], "dst": ["*:*"]}],
+		"nodeAttrs": [{"target": ["*"], "attr": ["ssh-behavior-v1"]}]
+	}`)
+
+	c.WaitForCondition(t, "ssh-behavior-v1 through a nodeAttr", 10*time.Second,
+		func(nm *netmap.NetworkMap) bool {
+			return hasCap(nm, nodecap.SSHBehaviorV1)
+		})
+}
+
 // TestNodeAttrsAddsToBaseline verifies that policy nodeAttrs caps land on
 // nodes alongside the always-on baseline. The baseline caps remain
 // regardless of policy contents.