A mesh on both address families renders both

nftables matches ip and ip6 separately and one set holding both is a syntax
error, so the file would not load: the service reports a configuration fault
and the machine filters nothing. Also a make target for the builder image,
which the lab now stocks.

internal/catalogue/filtering.go

@@ -168,8 +168,15 @@ func AsNftables(rules []Rule, mesh []string) string {
 					rule.Protocol, rule.Port))
 				break
 			}
-			b.WriteString(fmt.Sprintf("\t\tip saddr { %s } %s dport %d accept\n",
-				strings.Join(mesh, ", "), rule.Protocol, rule.Port))
+			four, six := byFamily(mesh)
+			if len(four) > 0 {
+				b.WriteString(fmt.Sprintf("\t\tip saddr { %s } %s dport %d accept\n",
+					strings.Join(four, ", "), rule.Protocol, rule.Port))
+			}
+			if len(six) > 0 {
+				b.WriteString(fmt.Sprintf("\t\tip6 saddr { %s } %s dport %d accept\n",
+					strings.Join(six, ", "), rule.Protocol, rule.Port))
+			}
 		case FromEverywhere:
 			b.WriteString(fmt.Sprintf("\t\t%s dport %d accept\n", rule.Protocol, rule.Port))
 		}
@@ -190,3 +197,20 @@ func AsNftables(rules []Rule, mesh []string) string {
 	b.WriteString("}\n")
 	return b.String()
 }
+
+// byFamily splits addresses into the two nftables understands separately.
+//
+// `ip saddr` and `ip6 saddr` are different matches, and one set holding both families is a syntax
+// error — which means the whole file fails to load and the machine filters nothing while the
+// service reports a configuration fault. A mesh that is entirely one family renders one line, and
+// a mixed one renders both rather than dropping half its nodes.
+func byFamily(addresses []string) (four []string, six []string) {
+	for _, a := range addresses {
+		if strings.Contains(a, ":") {
+			six = append(six, a)
+			continue
+		}
+		four = append(four, a)
+	}
+	return four, six
+}

internal/catalogue/filtering_test.go

@@ -219,3 +219,21 @@ func TestAskingForTheRuleSetWithNowhereToPutItIsRefused(t *testing.T) {
 		t.Fatal("a module asked for the rule set and named no path; it would receive nothing")
 	}
 }
+
+// nftables matches the two address families separately, and one set holding both is a syntax
+// error — so the file fails to load, the service reports a fault, and the machine filters nothing.
+func TestAMeshOnBothAddressFamiliesRendersBoth(t *testing.T) {
+	nft := AsNftables((Resolution{Modules: []Manifest{
+		{Module: "store", Listens: []Listening{{Port: 5432, From: FromMesh}}},
+	}}).Filtering(), []string{"198.51.100.2", "2001:db8::2"})
+	if !strings.Contains(nft, "ip saddr { 198.51.100.2 } tcp dport 5432 accept") {
+		t.Fatalf("the machines with v4 addresses were dropped:\n%s", nft)
+	}
+	if !strings.Contains(nft, "ip6 saddr { 2001:db8::2 } tcp dport 5432 accept") {
+		t.Fatalf("the machines with v6 addresses were dropped:\n%s", nft)
+	}
+	// And never in one set, which is the syntax error this exists to avoid.
+	if strings.Contains(nft, "198.51.100.2, 2001:db8::2") {
+		t.Fatalf("both families are in one set, so the file will not load:\n%s", nft)
+	}
+}