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.
This commit is contained in:
Yuri Nesterov 2023-06-22 15:17:42 +03:00
parent 688991ba84
commit 6008246790
5 changed files with 116 additions and 0 deletions

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@ -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

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@ -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 {};

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@ -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}'")
'';
})

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@ -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

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@ -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 {