internal/apply/apply.go

@@ -812,6 +812,13 @@ func applyContainer(ctx context.Context, r *declaration.Container, run Runner) (
 	for _, v := range r.Volumes {
 		args = append(args, "--volume", v)
 	}
+	for _, n := range r.Nameservers {
+		// Per container rather than by changing the machine's resolver configuration. That file
+		// belongs to something else on most machines, and a host that edited it would be fighting
+		// whatever owns it on every boot — the fault this host exists to avoid, in the one place
+		// it would be hardest to see.
+		args = append(args, "--dns", n)
+	}
 	args = append(args, r.Image)
 	args = append(args, r.Args...)
 

internal/apply/apply_test.go

@@ -1224,3 +1224,75 @@ func TestAFailedActionStopsWhatFollows(t *testing.T) {
 		t.Errorf("the message does not say the rest was not tried: %v", err)
 	}
 }
+
+// A container is told which resolver to use, because it does not inherit the machine's names.
+//
+// A container gets its own `/etc/hosts` holding its own hostname, and a runtime rewrites
+// `resolv.conf` — so every internal name the mesh wrote for the machine is invisible to what the
+// machine is running. That was hit for real: a database client on one node could not resolve
+// another node, on a mesh where both names were correct and present on both machines.
+func TestAContainerIsToldWhichResolverToUse(t *testing.T) {
+	var ran []string
+	run := func(_ context.Context, name string, args ...string) (string, error) {
+		if name != "docker" {
+			return "", errors.New("not installed")
+		}
+		switch args[0] {
+		case "info":
+			return "29.0.0\n", nil
+		case "inspect":
+			return "false\t\n", errors.New("no such container")
+		case "run":
+			ran = args
+			return "deadbeef\n", nil
+		}
+		return "", nil
+	}
+	d := parseTrusted(t, `{"declaration":1,"resources":[
+	  {"id":"app","type":"container","name":"app","image":"`+pinned+`",
+	   "nameservers":["10.42.0.1"]}
+	]}`)
+	_, _, _ = Apply(context.Background(), archHost(t), d, store.State{}, store.OriginCarried, run, nil, nil)
+
+	var told bool
+	for i, a := range ran {
+		if a == "--dns" && i+1 < len(ran) && ran[i+1] == "10.42.0.1" {
+			told = true
+		}
+	}
+	if !told {
+		t.Fatalf("the container was not told where to resolve names: %v", ran)
+	}
+}
+
+// And a container that was told nothing is run exactly as before: most containers resolve
+// whatever the machine resolves, and passing an empty flag would be a change of behaviour
+// dressed as a default.
+func TestAContainerToldNothingIsRunAsBefore(t *testing.T) {
+	var ran []string
+	run := func(_ context.Context, name string, args ...string) (string, error) {
+		if name != "docker" {
+			return "", errors.New("not installed")
+		}
+		switch args[0] {
+		case "info":
+			return "29.0.0\n", nil
+		case "inspect":
+			return "false\t\n", errors.New("no such container")
+		case "run":
+			ran = args
+			return "deadbeef\n", nil
+		}
+		return "", nil
+	}
+	d := parseTrusted(t, `{"declaration":1,"resources":[
+	  {"id":"app","type":"container","name":"app","image":"`+pinned+`"}
+	]}`)
+	_, _, _ = Apply(context.Background(), archHost(t), d, store.State{}, store.OriginCarried, run, nil, nil)
+
+	for _, a := range ran {
+		if a == "--dns" {
+			t.Fatalf("a container that was told nothing was given a resolver anyway: %v", ran)
+		}
+	}
+}

internal/declaration/declaration.go

@@ -328,6 +328,19 @@ type Container struct {
 	Ports   []string          `json:"ports,omitempty"`
 	Volumes []string          `json:"volumes,omitempty"`
 	Args    []string          `json:"args,omitempty"`
+	// Nameservers this container resolves through.
+	//
+	// **Because a container does not inherit the machine's names.** It gets its own `/etc/hosts`
+	// holding its own hostname, and a runtime rewrites `resolv.conf` — so every internal name the
+	// mesh wrote for this machine is invisible to the thing the machine is running. That was hit
+	// for real: a database client on one node could not resolve another node, on a mesh where
+	// both names were correct and present.
+	//
+	// Set by the mesh rather than by a module: which resolver a machine has is a fact about the
+	// machine, and a module that named one would be a module that only runs where somebody put
+	// that resolver.
+	Nameservers []string `json:"nameservers,omitempty"`
+
 	// Network is the container's network, passed to the runtime unchanged.
 	//
 	// Needed because the control plane must reach the store and the broker on the machine it was