Merge pull request #50316 from arianvp/fix-dynamic-user

Disable nscd caching

nixos/doc/manual/release-notes/rl-1903.xml

@@ -245,6 +245,65 @@
      options.
    </para>
   </listitem>
+  <listitem>
+   <para>
+     The <literal>nscd</literal> service now disables all caching of
+     <literal>passwd</literal> and <literal>group</literal> databases by
+     default. This was interferring with the correct functioning of the
+     <literal>libnss_systemd.so</literal> module which is used by
+     <literal>systemd</literal> to manage uids and usernames in the presence of
+     <literal>DynamicUser=</literal> in systemd services.  This was already the
+     default behaviour in presence of <literal>services.sssd.enable =
+       true</literal> because nscd caching would interfere with
+     <literal>sssd</literal> in unpredictable ways as well.  Because we're
+     using nscd not for caching, but for convincing glibc to find NSS modules
+     in the nix store instead of an absolute path, we have decided to disable
+     caching globally now, as it's usually not the behaviour the user wants and
+     can lead to surprising behaviour.  Furthermore, negative caching of host
+     lookups is also disabled now by default. This should fix the issue of dns
+     lookups failing in the presence of an unreliable network.
+   </para>
+   <para>
+     If the old behaviour is desired, this can be restored by setting
+     the <literal>services.nscd.config</literal> option
+     with the desired caching parameters.
+     <programlisting>
+     services.nscd.config =
+     ''
+     server-user             nscd
+     threads                 1
+     paranoia                no
+     debug-level             0
+
+     enable-cache            passwd          yes
+     positive-time-to-live   passwd          600
+     negative-time-to-live   passwd          20
+     suggested-size          passwd          211
+     check-files             passwd          yes
+     persistent              passwd          no
+     shared                  passwd          yes
+
+     enable-cache            group           yes
+     positive-time-to-live   group           3600
+     negative-time-to-live   group           60
+     suggested-size          group           211
+     check-files             group           yes
+     persistent              group           no
+     shared                  group           yes
+
+     enable-cache            hosts           yes
+     positive-time-to-live   hosts           600
+     negative-time-to-live   hosts           5
+     suggested-size          hosts           211
+     check-files             hosts           yes
+     persistent              hosts           no
+     shared                  hosts           yes
+     '';
+     </programlisting>
+     See <link xlink:href="https://github.com/NixOS/nixpkgs/pull/50316">#50316</link>
+     for details.
+   </para>
+  </listitem>
   <listitem>
    <para>
      GitLab Shell previously used the nix store paths for the

nixos/modules/services/misc/nscd-sssd.conf

@@ -1,36 +0,0 @@
-server-user             nscd
-threads                 1
-paranoia                no
-debug-level             0
-
-enable-cache            passwd          yes
-positive-time-to-live   passwd          0
-negative-time-to-live   passwd          0
-suggested-size          passwd          211
-check-files             passwd          yes
-persistent              passwd          no
-shared                  passwd          yes
-
-enable-cache            group           yes
-positive-time-to-live   group           0
-negative-time-to-live   group           0
-suggested-size          group           211
-check-files             group           yes
-persistent              group           no
-shared                  group           yes
-
-enable-cache            hosts           yes
-positive-time-to-live   hosts           600
-negative-time-to-live   hosts           5
-suggested-size          hosts           211
-check-files             hosts           yes
-persistent              hosts           no
-shared                  hosts           yes
-
-enable-cache            services        yes
-positive-time-to-live   services        0
-negative-time-to-live   services        0
-suggested-size          services        211
-check-files             services        yes
-persistent              services        no
-shared                  services        yes

nixos/modules/services/misc/sssd.nix

@@ -75,7 +75,6 @@ in {
       };
 
       system.nssModules = optional cfg.enable pkgs.sssd;
-      services.nscd.config = builtins.readFile ./nscd-sssd.conf;
       services.dbus.packages = [ pkgs.sssd ];
     })
 

nixos/modules/services/system/nscd.conf

@@ -1,28 +1,52 @@
+# We basically use nscd as a proxy for forwarding nss requests to appropriate
+# nss modules, as we run nscd with LD_LIBRARY_PATH set to the directory
+# containing all such modules
+# Note that we can not use `enable-cache no` As this will actually cause nscd
+# to just reject the nss requests it receives, which then causes glibc to
+# fallback to trying to handle the request by itself. Which won't work as glibc
+# is not aware of the path in which the nss modules live.  As a workaround, we
+# have `enable-cache yes` with an explicit ttl of 0
 server-user             nscd
 threads                 1
 paranoia                no
 debug-level             0
 
 enable-cache            passwd          yes
-positive-time-to-live   passwd          600
+positive-time-to-live   passwd          0
-negative-time-to-live   passwd          20
+negative-time-to-live   passwd          0
 suggested-size          passwd          211
 check-files             passwd          yes
 persistent              passwd          no
 shared                  passwd          yes
 
 enable-cache            group           yes
-positive-time-to-live   group           3600
+positive-time-to-live   group           0
-negative-time-to-live   group           60
+negative-time-to-live   group           0
 suggested-size          group           211
 check-files             group           yes
 persistent              group           no
 shared                  group           yes
 
+enable-cache            netgroup        yes
+positive-time-to-live   netgroup        0
+negative-time-to-live   netgroup        0
+suggested-size          netgroup        211
+check-files             netgroup        yes
+persistent              netgroup        no
+shared                  netgroup        yes
+
 enable-cache            hosts           yes
 positive-time-to-live   hosts           600
-negative-time-to-live   hosts           5
+negative-time-to-live   hosts           0
 suggested-size          hosts           211
 check-files             hosts           yes
 persistent              hosts           no
 shared                  hosts           yes
+
+enable-cache            services        yes
+positive-time-to-live   services        0
+negative-time-to-live   services        0
+suggested-size          services        211
+check-files             services        yes
+persistent              services        no
+shared                  services        yes

nixos/tests/systemd.nix

@@ -56,6 +56,11 @@ import ./make-test.nix {
       $machine->succeed('test -z $(ls -1 /var/log/journal)');
     };
 
+    # Regression test for https://github.com/NixOS/nixpkgs/issues/50273
+    subtest "DynamicUser actually allocates a user", sub {
+        $machine->succeed('systemd-run --pty --property=Type=oneshot --property=DynamicUser=yes --property=User=iamatest whoami | grep iamatest');
+    };
+
     # Regression test for https://github.com/NixOS/nixpkgs/issues/35268
     subtest "file system with x-initrd.mount is not unmounted", sub {
       $machine->shutdown;