nixos/babeld: run as DynamicUser

The last bits to prevent babeld from running unprivileged was its
kernel_setup_interface routine, that wants to set per interface
rp_filter. This behaviour has been disabled in a patch that has been
submitted upstream at https://github.com/jech/babeld/pull/68 and reuses
the skip-kernel-setup config option.

→ Overall exposure level for babeld.service: 1.7 OK 🙂

nixos/doc/manual/release-notes/rl-2105.xml

@@ -680,6 +680,13 @@ environment.systemPackages = [
      All CUDA toolkit versions prior to CUDA 10 have been removed.
     </para>
    </listitem>
+   <listitem>
+     <para>
+       The <package>babeld</package> service is now being run as an unprivileged user. To achieve that the module configures
+       <literal>skip-kernel-setup true</literal> and takes care of setting forwarding and rp_filter sysctls by itself as well
+       as for each interface in <varname>services.babeld.interfaces</varname>.
+     </para>
+   </listitem>
   </itemizedlist>
  </section>
 

nixos/modules/services/networking/babeld.nix

@@ -19,7 +19,10 @@ let
     "interface ${name} ${paramsString interface}\n";
 
   configFile = with cfg; pkgs.writeText "babeld.conf" (
-    (optionalString (cfg.interfaceDefaults != null) ''
+    ''
+      skip-kernel-setup true
+    ''
+    + (optionalString (cfg.interfaceDefaults != null) ''
       default ${paramsString cfg.interfaceDefaults}
     '')
     + (concatMapStrings interfaceConfig (attrNames cfg.interfaces))
@@ -84,13 +87,22 @@ in
 
   config = mkIf config.services.babeld.enable {
 
+    boot.kernel.sysctl = {
+      "net.ipv6.conf.all.forwarding" = 1;
+      "net.ipv6.conf.all.accept_redirects" = 0;
+      "net.ipv4.conf.all.forwarding" = 1;
+      "net.ipv4.conf.all.rp_filter" = 0;
+    } // lib.mapAttrs' (ifname: _: lib.nameValuePair "net.ipv4.conf.${ifname}.rp_filter" (lib.mkDefault 0)) config.services.babeld.interfaces;
+
     systemd.services.babeld = {
       description = "Babel routing daemon";
       after = [ "network.target" ];
       wantedBy = [ "multi-user.target" ];
       serviceConfig = {
         ExecStart = "${pkgs.babeld}/bin/babeld -c ${configFile} -I /run/babeld/babeld.pid -S /var/lib/babeld/state";
+        AmbientCapabilities = [ "CAP_NET_ADMIN" ];
         CapabilityBoundingSet = [ "CAP_NET_ADMIN" ];
+        DynamicUser = true;
         IPAddressAllow = [ "fe80::/64" "ff00::/8" "::1/128" "127.0.0.0/8" ];
         IPAddressDeny = "any";
         LockPersonality = true;
@@ -98,7 +110,7 @@ in
         MemoryDenyWriteExecute = true;
         ProtectSystem = "strict";
         ProtectClock = true;
-        ProtectKernelTunables = false; # Couldn't write sysctl: Read-only file system
+        ProtectKernelTunables = true;
         ProtectKernelModules = true;
         ProtectKernelLogs = true;
         ProtectControlGroups = true;