systemd: disable NSCD when DNSSEC validation is disabled in timesyncd

When a system has a wrong date and time timesyncd is unable to synchronize it
because DNSSEC doesn't work. In order to break this chicken and egg problem
systemd-timesync disables DNSSEC validation by setting
SYSTEMD_NSS_RESOLVE_VALIDATE=0 in the unit file. However, it doesn't work in
NixOS because it uses NSCD. This patch disables NSCD in systemd-timesyncd when
SYSTEMD_NSS_RESOLVE_VALIDATE is set to 0 so that it uses NSS libraries
directly. In order for it to be able to find the libnss_resolve.so.2 library
this patch adds the systemd directory in the nix store to the LD_LIBRARY_PATH.

nixos/modules/system/boot/timesyncd.nix

@@ -46,6 +46,13 @@ with lib;
       wantedBy = [ "sysinit.target" ];
       aliases = [ "dbus-org.freedesktop.timesync1.service" ];
       restartTriggers = [ config.environment.etc."systemd/timesyncd.conf".source ];
+      # systemd-timesyncd disables DNSSEC validation in the nss-resolve module by setting SYSTEMD_NSS_RESOLVE_VALIDATE to 0 in the unit file.
+      # This is required in order to solve the chicken-and-egg problem when DNSSEC validation needs the correct time to work, but to set the
+      # correct time, we need to connect to an NTP server, which usually requires resolving its hostname.
+      # In order for nss-resolve to be able to read this environment variable we patch systemd-timesyncd to disable NSCD and use NSS modules directly.
+      # This means that systemd-timesyncd needs to have NSS modules path in LD_LIBRARY_PATH. When systemd-resolved is disabled we still need to set
+      # NSS module path so that systemd-timesyncd keeps using other NSS modules that are configured in the system.
+      environment.LD_LIBRARY_PATH = config.system.nssModules.path;
 
       preStart = (
         # Ensure that we have some stored time to prevent

nixos/tests/all-tests.nix

@@ -847,6 +847,7 @@ in {
   systemd-shutdown = handleTest ./systemd-shutdown.nix {};
   systemd-sysupdate = runTest ./systemd-sysupdate.nix;
   systemd-timesyncd = handleTest ./systemd-timesyncd.nix {};
+  systemd-timesyncd-nscd-dnssec = handleTest ./systemd-timesyncd-nscd-dnssec.nix {};
   systemd-user-tmpfiles-rules = handleTest ./systemd-user-tmpfiles-rules.nix {};
   systemd-misc = handleTest ./systemd-misc.nix {};
   systemd-userdbd = handleTest ./systemd-userdbd.nix {};

nixos/tests/systemd-timesyncd-nscd-dnssec.nix

@@ -0,0 +1,61 @@
+# This test verifies that systemd-timesyncd can resolve the NTP server hostname when DNSSEC validation
+# fails even though it is enforced in the systemd-resolved settings. It is required in order to solve
+# the chicken-and-egg problem when DNSSEC validation needs the correct time to work, but to set the
+# correct time, we need to connect to an NTP server, which usually requires resolving its hostname.
+#
+# This test does the following:
+# - Sets up a DNS server (tinydns) listening on the eth1 ip addess, serving .ntp and fake.ntp records.
+# - Configures that DNS server as a resolver and enables DNSSEC in systemd-resolved settings.
+# - Configures systemd-timesyncd to use fake.ntp hostname as an NTP server.
+# - Performs a regular DNS lookup, to ensure it fails due to broken DNSSEC.
+# - Waits until systemd-timesyncd resolves fake.ntp by checking its debug output.
+#   Here, we don't expect systemd-timesyncd to connect and synchronize time because there is no NTP
+#   server running. For this test to succeed, we only need to ensure that systemd-timesyncd
+#   resolves the IP address of the fake.ntp host.
+
+import ./make-test-python.nix ({ pkgs, ... }:
+
+let
+  ntpHostname = "fake.ntp";
+  ntpIP = "192.0.2.1";
+in
+{
+  name = "systemd-timesyncd";
+  nodes.machine = { pkgs, lib, config, ... }:
+    let
+      eth1IP = (lib.head config.networking.interfaces.eth1.ipv4.addresses).address;
+    in
+    {
+      # Setup a local DNS server for the NTP domain on the eth1 IP address
+      services.tinydns = {
+        enable = true;
+        ip = eth1IP;
+        data = ''
+          .ntp:${eth1IP}
+          +.${ntpHostname}:${ntpIP}
+        '';
+      };
+
+      # Enable systemd-resolved with DNSSEC and use the local DNS as a name server
+      services.resolved.enable = true;
+      services.resolved.dnssec = "true";
+      networking.nameservers = [ eth1IP ];
+
+      # Configure systemd-timesyncd to use our NTP hostname
+      services.timesyncd.enable = lib.mkForce true;
+      services.timesyncd.servers = [ ntpHostname ];
+      services.timesyncd.extraConfig = ''
+        FallbackNTP=${ntpHostname}
+      '';
+
+      # The debug output is necessary to determine whether systemd-timesyncd successfully resolves our NTP hostname or not
+      systemd.services.systemd-timesyncd.environment.SYSTEMD_LOG_LEVEL = "debug";
+    };
+
+  testScript = ''
+    machine.wait_for_unit("tinydns.service")
+    machine.wait_for_unit("systemd-timesyncd.service")
+    machine.fail("resolvectl query ${ntpHostname}")
+    machine.wait_until_succeeds("journalctl -u systemd-timesyncd.service --grep='Resolved address ${ntpIP}:123 for ${ntpHostname}'")
+  '';
+})

pkgs/os-specific/linux/systemd/0020-timesyncd-disable-NSCD-when-DNSSEC-validation-is-dis.patch

@@ -0,0 +1,46 @@
+From 7a27556920fe1feefd17096841c8f3ca1294a1b3 Mon Sep 17 00:00:00 2001
+From: Yuri Nesterov <yuriy.nesterov@unikie.com>
+Date: Wed, 21 Jun 2023 17:17:38 +0300
+Subject: [PATCH] timesyncd: disable NSCD when DNSSEC validation is disabled
+
+Systemd-timesyncd sets SYSTEMD_NSS_RESOLVE_VALIDATE=0 in the unit file
+to disable DNSSEC validation but it doesn't work when NSCD is used in
+the system. This patch disabes NSCD in systemd-timesyncd when
+SYSTEMD_NSS_RESOLVE_VALIDATE is set to 0 so that it uses NSS libraries
+directly.
+---
+ src/timesync/timesyncd.c | 11 +++++++++++
+ 1 file changed, 11 insertions(+)
+
+diff --git a/src/timesync/timesyncd.c b/src/timesync/timesyncd.c
+index 1d8ebecc91..2b0ae361ff 100644
+--- a/src/timesync/timesyncd.c
++++ b/src/timesync/timesyncd.c
+@@ -21,6 +21,11 @@
+ #include "timesyncd-conf.h"
+ #include "timesyncd-manager.h"
+ #include "user-util.h"
++#include "env-util.h"
++
++struct traced_file;
++extern void __nss_disable_nscd(void (*)(size_t, struct traced_file *));
++static void register_traced_file(size_t dbidx, struct traced_file *finfo) {}
+ 
+ static int advance_tstamp(int fd, const struct stat *st) {
+         assert_se(fd >= 0);
+@@ -198,6 +203,12 @@ static int run(int argc, char *argv[]) {
+         if (r < 0)
+                 return log_error_errno(r, "Failed to parse fallback server strings: %m");
+ 
++        r = getenv_bool_secure("SYSTEMD_NSS_RESOLVE_VALIDATE");
++        if (r == 0) {
++                log_info("Disabling NSCD because DNSSEC validation is turned off");
++                __nss_disable_nscd(register_traced_file);
++        }
++
+         log_debug("systemd-timesyncd running as pid " PID_FMT, getpid_cached());
+ 
+         notify_message = notify_start("READY=1\n"
+-- 
+2.34.1
+

pkgs/os-specific/linux/systemd/default.nix

@@ -206,6 +206,7 @@ stdenv.mkDerivation (finalAttrs: {
     ./0017-core-don-t-taint-on-unmerged-usr.patch
     ./0018-tpm2_context_init-fix-driver-name-checking.patch
     ./0019-systemctl-edit-suggest-systemdctl-edit-runtime-on-sy.patch
+    ./0020-timesyncd-disable-NSCD-when-DNSSEC-validation-is-dis.patch
   ] ++ lib.optional stdenv.hostPlatform.isMusl (
     let
       oe-core = fetchzip {