Support complete replacement of metric calculation

This adds a new key, `:route_metric_fun`, that let's users get rid of the default metric calculator and use their own. The default calculator probably works for most users, but once you find yourself outside of what works with the defaults, it's probably easiest to code a solution that exactly works with your setup rather than try to encourage the default calculator to work for you.
nerves-networking · Aug 19, 2021 · d380400 · d380400
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diff --git a/README.md b/README.md
@@ -140,6 +140,7 @@ persistence_secret | A 16-byte secret or an MFA for getting a secret
 internet_host_list | IP address/ports to try to connect to for checking Internet connectivity. Defaults to a list of large public DNS providers. E.g., `[{{1, 1, 1, 1}, 53}]`.
 regulatory_domain  | ISO 3166-1 alpha-2 country (`00` for global, `US`, etc.)
 additional_name_servers     | List of DNS servers to be used in addition to any supplied by an interface. E.g., `[{1, 1, 1, 1}, {8, 8, 8, 8}]`
+route_metric_fun   | Customize how network interfaces are prioritized. See `VintageNet.Route.DefaultMetric.compute_metric/2`
 
 ## Network interface configuration
 

diff --git a/lib/vintage_net/application.ex b/lib/vintage_net/application.ex
@@ -27,7 +27,7 @@ defmodule VintageNet.Application do
        dispatcher: &VintageNet.OSEventDispatcher.dispatch/2},
       VintageNet.InterfacesMonitor,
       {VintageNet.NameResolver, args},
-      VintageNet.RouteManager,
+      {VintageNet.RouteManager, args},
       {Registry, keys: :unique, name: VintageNet.Interface.Registry},
       VintageNet.InterfacesSupervisor
     ]

diff --git a/lib/vintage_net/route.ex b/lib/vintage_net/route.ex
@@ -3,6 +3,8 @@ defmodule VintageNet.Route do
   Types for handling routing tables
   """
 
+  alias VintageNet.Route.InterfaceInfo
+
   @typedoc """
   Metric (priority) for a routing table entry
   """
@@ -50,4 +52,12 @@ defmodule VintageNet.Route do
   A list of routing table entries
   """
   @type entries :: [entry()]
+
+  @typedoc """
+  Compute a route metric value from information about the interface
+
+  See `VintageNet.Route.DefaultMetric.compute_metric/2` for an example. This can be
+  set using the `:route_metric_fun` application environment key.
+  """
+  @type route_metric_fun() :: (VintageNet.ifname(), InterfaceInfo.t() -> metric())
 end
diff --git a/lib/vintage_net/route/calculator.ex b/lib/vintage_net/route/calculator.ex
@@ -12,7 +12,7 @@ defmodule VintageNet.Route.Calculator do
   """
 
   alias VintageNet.Route
-  alias VintageNet.Route.{DefaultMetric, InterfaceInfo}
+  alias VintageNet.Route.InterfaceInfo
 
   @type table_indices :: %{VintageNet.ifname() => Route.table_index()}
 
@@ -41,9 +41,11 @@ defmodule VintageNet.Route.Calculator do
   The entries are ordered so that List.myers_difference/2 can be used to
   minimize the routing table changes.
   """
-  @spec compute(table_indices(), interface_infos()) :: {table_indices(), Route.entries()}
-  def compute(table_indices, infos) do
-    {new_table_indices, entries} = Enum.reduce(infos, {table_indices, []}, &make_entries(&1, &2))
+  @spec compute(table_indices(), interface_infos(), Route.route_metric_fun()) ::
+          {table_indices(), Route.entries()}
+  def compute(table_indices, infos, route_metric_fun) do
+    {new_table_indices, entries} =
+      Enum.reduce(infos, {table_indices, []}, &make_entries(&1, &2, route_metric_fun))
 
     sorted_entries = Enum.sort(entries, &sort/2)
 
@@ -73,9 +75,9 @@ defmodule VintageNet.Route.Calculator do
     priority_a <= priority_b
   end
 
-  defp make_entries({ifname, info}, {table_indices, entries}) do
+  defp make_entries({ifname, info}, {table_indices, entries}, route_metric_fun) do
     {new_table_indices, table_index} = get_table_index(ifname, table_indices)
-    metric = DefaultMetric.compute_metric(ifname, info)
+    metric = route_metric_fun.(ifname, info)
 
     new_entries = routing_table_entries(metric, ifname, table_index, info)
 

diff --git a/lib/vintage_net/route_manager.ex b/lib/vintage_net/route_manager.ex
@@ -3,7 +3,8 @@ defmodule VintageNet.RouteManager do
   require Logger
 
   alias VintageNet.Interface.NameUtilities
-  alias VintageNet.Route.{Calculator, InterfaceInfo, IPRoute, Properties}
+  alias VintageNet.Route
+  alias VintageNet.Route.{Calculator, DefaultMetric, InterfaceInfo, IPRoute, Properties}
 
   @moduledoc """
   This module manages the default route.
@@ -35,14 +36,21 @@ defmodule VintageNet.RouteManager do
   ```
   """
 
-  defmodule State do
-    @moduledoc false
-
-    defstruct interfaces: nil, route_state: nil, routes: []
-  end
+  @typedoc false
+  @type state() :: %{
+          interfaces: %{VintageNet.ifname() => InterfaceInfo.t()},
+          route_state: Calculator.table_indices(),
+          routes: Route.entries(),
+          route_metric_fun: Route.route_metric_fun()
+        }
 
   @doc """
-  Start the route manager.
+  Start the route manager
+
+  Options:
+
+  * `:route_metric_fun` - a 2-arity function that takes a ifname and `VintageNet.Route.InterfaceInfo`
+    and returns `VintageNet.Route.metric()`
   """
   @spec start_link(keyword) :: GenServer.on_start()
   def start_link(args) do
@@ -107,21 +115,32 @@ defmodule VintageNet.RouteManager do
   ## GenServer
 
   @impl GenServer
-  def init(_args) do
+  def init(args) do
+    route_metric_fun = args[:route_metric_fun] |> check_compute_metric()
+
     # Fresh slate
     IPRoute.clear_all_routes()
     IPRoute.clear_all_rules(Calculator.rule_table_index_range())
 
     state =
-      %State{
+      %{
         interfaces: %{},
-        route_state: Calculator.init()
+        route_state: Calculator.init(),
+        route_metric_fun: route_metric_fun,
+        routes: []
       }
       |> update_route_tables()
 
     {:ok, state}
   end
 
+  defp check_compute_metric(fun) when is_function(fun, 2), do: fun
+
+  defp check_compute_metric(_other) do
+    Logger.error("RouteManager: Expecting :route_metric_fun to be a 2-arity function")
+    &DefaultMetric.compute_metric/2
+  end
+
   @impl GenServer
   def handle_call({:set_route, ifname, ip_subnets, default_gateway}, _from, state) do
     if interface_info_changed?(state, ifname, ip_subnets, default_gateway) do
@@ -206,12 +225,8 @@ defmodule VintageNet.RouteManager do
   end
 
   # Only process routes if the status changes
-  defp update_connection_status(
-         %State{interfaces: interfaces} = state,
-         ifname,
-         new_status
-       ) do
-    case interfaces[ifname] do
+  defp update_connection_status(state, ifname, new_status) do
+    case state.interfaces[ifname] do
       nil ->
         Logger.warn(
           "RouteManager: set_connection_status to #{inspect(new_status)} on unknown ifname: #{ifname}"
@@ -236,7 +251,8 @@ defmodule VintageNet.RouteManager do
 
   defp update_route_tables(state) do
     # See what changed and then run it.
-    {new_route_state, new_routes} = Calculator.compute(state.route_state, state.interfaces)
+    {new_route_state, new_routes} =
+      Calculator.compute(state.route_state, state.interfaces, state.route_metric_fun)
 
     route_delta = List.myers_difference(state.routes, new_routes)
 

diff --git a/mix.exs b/mix.exs
@@ -64,7 +64,8 @@ defmodule VintageNet.MixProject do
         # Contain processes in cgroups by setting to:
         #   [cgroup_base: "vintage_net", cgroup_controllers: ["cpu"]]
         muontrap_options: [],
-        power_managers: []
+        power_managers: [],
+        route_metric_fun: &VintageNet.Route.DefaultMetric.compute_metric/2
       ],
       extra_applications: [:logger, :crypto],
       mod: {VintageNet.Application, []}

diff --git a/test/vintage_net/route/calculator_test.exs b/test/vintage_net/route/calculator_test.exs
@@ -1,14 +1,18 @@
 defmodule VintageNet.Route.CalculatorTest do
   use ExUnit.Case
 
-  alias VintageNet.Route.{Calculator, InterfaceInfo}
+  alias VintageNet.Route.{Calculator, DefaultMetric, InterfaceInfo}
 
   doctest Calculator
 
+  defp compute(state, interfaces) do
+    Calculator.compute(state, interfaces, &DefaultMetric.compute_metric/2)
+  end
+
   test "no interfaces" do
     state = Calculator.init()
 
-    assert {%{}, []} == Calculator.compute(state, %{})
+    assert {%{}, []} == compute(state, %{})
   end
 
   test "one interface" do
@@ -31,7 +35,7 @@ defmodule VintageNet.Route.CalculatorTest do
               {:local_route, "eth0", {192, 168, 1, 50}, 24, 10, :main},
               {:default_route, "eth0", {192, 168, 1, 1}, 0, 100},
               {:default_route, "eth0", {192, 168, 1, 1}, 10, :main}
-            ]} == Calculator.compute(state, interfaces)
+            ]} == compute(state, interfaces)
   end
 
   test "a disconnected interface" do
@@ -49,7 +53,7 @@ defmodule VintageNet.Route.CalculatorTest do
       }
     }
 
-    assert {%{"eth0" => 100}, []} == Calculator.compute(state, interfaces)
+    assert {%{"eth0" => 100}, []} == compute(state, interfaces)
   end
 
   test "interface w/o addresses" do
@@ -65,7 +69,7 @@ defmodule VintageNet.Route.CalculatorTest do
       }
     }
 
-    assert {%{"eth0" => 100}, []} == Calculator.compute(state, interfaces)
+    assert {%{"eth0" => 100}, []} == compute(state, interfaces)
   end
 
   test "interface w/o default gateway" do
@@ -87,7 +91,7 @@ defmodule VintageNet.Route.CalculatorTest do
               {:local_route, "eth0", {192, 168, 1, 50}, 24, 0, 100},
               {:local_route, "eth0", {192, 168, 1, 50}, 24, 50, :main}
             ]} ==
-             Calculator.compute(state, interfaces)
+             compute(state, interfaces)
   end
 
   test "two interfaces, both internet" do
@@ -122,7 +126,7 @@ defmodule VintageNet.Route.CalculatorTest do
               {:default_route, "eth0", {192, 168, 1, 1}, 10, :main},
               {:default_route, "wlan0", {192, 168, 1, 1}, 0, 101},
               {:default_route, "wlan0", {192, 168, 1, 1}, 20, :main}
-            ]} == Calculator.compute(state, interfaces)
+            ]} == compute(state, interfaces)
   end
 
   test "two interfaces, bad ethernet" do
@@ -157,7 +161,7 @@ defmodule VintageNet.Route.CalculatorTest do
               {:default_route, "eth0", {192, 168, 1, 1}, 50, :main},
               {:default_route, "wlan0", {192, 168, 1, 1}, 0, 101},
               {:default_route, "wlan0", {192, 168, 1, 1}, 20, :main}
-            ]} == Calculator.compute(state, interfaces)
+            ]} == compute(state, interfaces)
   end
 
   test "one interface and many addresses" do
@@ -186,7 +190,7 @@ defmodule VintageNet.Route.CalculatorTest do
               {:local_route, "eth0", {192, 168, 1, 50}, 24, 10, :main},
               {:default_route, "eth0", {192, 168, 1, 1}, 0, 100},
               {:default_route, "eth0", {192, 168, 1, 1}, 10, :main}
-            ]} == Calculator.compute(state, interfaces)
+            ]} == compute(state, interfaces)
   end
 
   test "rule table index range is as expected" do
@@ -237,6 +241,6 @@ defmodule VintageNet.Route.CalculatorTest do
               {:default_route, "eth1", {192, 168, 1, 1}, 11, :main},
               {:default_route, "eth2", {192, 168, 2, 1}, 0, 102},
               {:default_route, "eth2", {192, 168, 2, 1}, 12, :main}
-            ]} == Calculator.compute(state, interfaces)
+            ]} == compute(state, interfaces)
   end
 end