internal/catalogue/filtering.go

@@ -261,8 +261,21 @@ func AsNftables(rules []Rule, mesh []string, outward bool) string {
 				strings.Join(six, ", "), SSHPort))
 		}
 	}
-	if outward {
-		b.WriteString("\t\t# and from outside, because this machine faces it\n")
+	// From everywhere when this machine faces outward — and also when the mesh has no addresses to
+	// name yet.
+	//
+	// **A machine early in being adopted is the one this matters most for, and the one where the
+	// rule above emits nothing.** Before the private network exists there is no mesh address to
+	// allow from, so restricting to it restricts to nothing: the chain drops by default and ssh is
+	// simply shut. That is the first machine anybody adopts, reached over the network, with the
+	// port needed to fix it closed by the act of adopting it. Never leaving this chain without an
+	// ssh rule is worth more than the narrower rule it would have had.
+	if four, six := byFamily(mesh); outward || (len(four) == 0 && len(six) == 0) {
+		why := "and from outside, because this machine faces it"
+		if !outward {
+			why = "and from anywhere, because this mesh has no addresses to narrow it to yet"
+		}
+		b.WriteString("\t\t# " + why + "\n")
 		b.WriteString(fmt.Sprintf("\t\ttcp dport %d accept\n", SSHPort))
 	}
 

internal/catalogue/filtering_test.go

@@ -690,3 +690,15 @@ func TestSSHIsOpenFromOutsideOnAMachineThatFacesIt(t *testing.T) {
 		t.Fatalf("a machine reachable from outside does not answer ssh there:\n%s", nft)
 	}
 }
+
+// A machine the mesh knows no addresses for still answers ssh.
+//
+// **This is the machine it matters most for.** Before the private network exists there is nothing
+// to narrow the rule to, so narrowing it shuts the port entirely — on the first machine anybody
+// adopts, reached over the network, closed by the act of adopting it.
+func TestSSHIsNeverLeftWithoutARule(t *testing.T) {
+	nft := AsNftables(nil, nil, false)
+	if !strings.Contains(nft, "tcp dport 22 accept") {
+		t.Fatalf("a machine with no mesh addresses has no ssh rule, so adopting it locks it:\n%s", nft)
+	}
+}

internal/catalogue/print_rehearsal_test.go

@@ -0,0 +1,16 @@
+package catalogue
+
+import (
+	"os"
+	"testing"
+)
+
+func TestPrintRehearsalRuleset(t *testing.T) {
+	if os.Getenv("PRINT_RULESET") == "" {
+		t.Skip("set PRINT_RULESET")
+	}
+	rules := mustFilter(t, Resolution{Modules: []Manifest{
+		{Module: "pub", Listens: []Listening{{Port: 8099, From: FromMesh, Why: "the thing it serves"}}},
+	}}, nil)
+	t.Log("\n" + AsNftables(rules, []string{"192.0.2.20"}, true))
+}