Make lxc-docker a privileged container: unprivileged can't NFS-mount at all

The kernel's NFS client filesystem doesn't set FS_USERNS_MOUNT, so mounting
NFS from inside any non-init user namespace -- exactly what an unprivileged
LXC container's UID-mapped root runs in -- is rejected at the VFS layer
with EPERM, regardless of Proxmox's mount=nfs;nfs4 container feature (which
only patches the AppArmor layer). Confirmed live on the redeployed lxc-docker
container: TCP to the NFS server's port 2049 succeeds, the server's export
table matches the container's IP, and mount.nfs: Operation not permitted
still fires immediately with no corresponding denial anywhere in the
server's own logs -- a kernel-level rejection that no amount of DNS/
automount/export tweaking (this branch's earlier commits) could ever fix.

modules/platforms/lxc.nix now keys proxmoxLXC.privileged off hostName
("docker" -> true) rather than a blanket false, since build-types/docker.nix
is also composed for linode-docker/proxmox-docker, which don't import
proxmox-lxc.nix at all -- setting this option there would break their eval.
create-proxmox-resource.sh reads the value back via a new
flake_target_lxc_privileged helper instead of hardcoding --unprivileged 1,
so the two stay in sync automatically for every lxc-* target.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01T48qgH3VTvs8wvwj44FEbE

modules/common/configuration.nix

@@ -13,24 +13,6 @@
 
   networking.networkmanager.enable = true; # Easiest to use and most distros use this by default.
 
-  # No host declares a DNS search domain anywhere else, and cross-host
-  # references throughout this repo (vars.nfsServerHost, vars.nixCacheHost,
-  # vars.dockerHost, ...) are bare short names, not FQDNs -- resolving them
-  # depends entirely on whatever network stack happens to be in play
-  # picking up the DHCP-advertised domain as a search suffix. NetworkManager
-  # does that by default, which is why this went unnoticed on
-  # NetworkManager-managed hosts, but LXC containers (modules/platforms/lxc.nix
-  # force-disables NetworkManager and Proxmox writes their systemd-networkd
-  # config itself) never get one. Confirmed live on lxc-docker: systemd-resolved
-  # had no search domain for eth0, "server" failed to resolve
-  # ("Name or service not known") while "server.sweet.home" resolved fine via
-  # the same DNS server, so every NFS mount in modules/docker/mount-data.nix
-  # failed even after fixing the automount/mount=nfs bugs. This applies the
-  # search domain globally via systemd-resolved's own config rather than the
-  # per-link DHCP path, so it isn't at the mercy of whichever component owns
-  # a given host's interface file.
-  networking.search = [ vars.homeDomain ];
-
   # Recommended over the true default (bypasses ZFS's own import safeguards)
   # per the option's own docs; matches hosts/docker/host.nix and
   # modules/services/zfs/enable-service.nix, which already set this

modules/docker/mount-data.nix

@@ -9,11 +9,25 @@ let
   # (the VM platforms rely on automount itself to get that same
   # non-blocking behavior, so they don't need `nofail` too).
   automountOpts = if config.boot.isContainer then [ "nofail" ] else [ "x-systemd.automount" ];
+
+  # A bare hostname here never resolves reliably: systemd-resolved only
+  # ever tries LLMNR for single-label names (never DNS, regardless of any
+  # configured search domain), and a *global* search domain (the first fix
+  # attempted here) backfires worse -- confirmed live on lxc-docker, adding
+  # `networking.search` made systemd-resolved prioritize its domain-matched
+  # but server-less global scope over eth0's correctly-configured one for
+  # every "*.sweet.home" query, silently sending them to public fallback
+  # DNS instead. `resolvectl query --interface=eth0 server.sweet.home`
+  # resolved fine throughout, proving the LAN DNS server was never the
+  # problem -- only the ambient, unqualified device string was. Using the
+  # FQDN directly sidesteps all of that, matching the pattern
+  # ../raspi/mount-data.nix already uses for the same reason.
+  nfsServer = "${vars.nfsServerHost}.${vars.homeDomain}";
 in
 {
   fileSystems = {
     ${vars.nfsShares.dockerConfig.mountpoint} = {
-      device = "${vars.nfsServerHost}:${vars.storageRoot}/${vars.nfsShares.dockerConfig.subpath}";
+      device = "${nfsServer}:${vars.storageRoot}/${vars.nfsShares.dockerConfig.subpath}";
       fsType = "nfs";
 
       options = [
@@ -24,7 +38,7 @@ in
     };
 
     ${vars.nfsShares.dockerDatabases.mountpoint} = {
-      device = "${vars.nfsServerHost}:${vars.storageRoot}/${vars.nfsShares.dockerDatabases.subpath}";
+      device = "${nfsServer}:${vars.storageRoot}/${vars.nfsShares.dockerDatabases.subpath}";
       fsType = "nfs";
 
       options = [
@@ -35,7 +49,7 @@ in
     };
 
     ${vars.nfsShares.dockerVolumes.mountpoint} = {
-      device = "${vars.nfsServerHost}:${vars.storageRoot}/${vars.nfsShares.dockerVolumes.subpath}";
+      device = "${nfsServer}:${vars.storageRoot}/${vars.nfsShares.dockerVolumes.subpath}";
       fsType = "nfs";
 
       options = [
@@ -46,7 +60,7 @@ in
     };
 
     ${vars.nfsShares.nextcloudData.mountpoint} = {
-      device = "${vars.nfsServerHost}:${vars.storageRoot}/${vars.nfsShares.nextcloudData.subpath}";
+      device = "${nfsServer}:${vars.storageRoot}/${vars.nfsShares.nextcloudData.subpath}";
       fsType = "nfs";
 
       options = [
@@ -57,7 +71,7 @@ in
     };
 
     ${vars.nfsShares.raspiVolumes.mountpoint} = {
-      device = "${vars.nfsServerHost}:${vars.storageRoot}/${vars.nfsShares.raspiVolumes.subpath}";
+      device = "${nfsServer}:${vars.storageRoot}/${vars.nfsShares.raspiVolumes.subpath}";
       fsType = "nfs";
 
       options = [

modules/platforms/lxc.nix

@@ -1,4 +1,4 @@
-{ lib, modulesPath, flakeTarget, ... }:
+{ config, lib, modulesPath, flakeTarget, ... }:
 
 let
   # Bakes this exact flake target's pre-generated SSH host key straight
@@ -58,8 +58,28 @@ in
     # host.nix declares each host's real hostname (networking.hostName);
     # keep that instead of letting Proxmox's ambient container config win.
     manageHostName = true;
-    # Unprivileged matches how these containers are actually created.
-    privileged = false;
+    # Unprivileged by default -- matches how these containers are actually
+    # created (scripts/proxmox/create-proxmox-resource.sh reads this value
+    # back to decide `pct create`'s --unprivileged flag, so the two stay
+    # in sync).
+    #
+    # lxc-docker is the one exception: the kernel's NFS client doesn't set
+    # FS_USERNS_MOUNT, so mounting NFS from inside *any* non-init user
+    # namespace -- which is exactly what an unprivileged container's
+    # UID-mapped root runs in -- is rejected at the VFS layer with EPERM,
+    # no matter what Proxmox's own `mount=nfs;nfs4` container feature
+    # allows at the AppArmor layer (confirmed live: TCP to the NFS server
+    # succeeds, the server's export table matches the container's IP, and
+    # `mount.nfs: Operation not permitted` still fires immediately with no
+    # corresponding denial anywhere in the server's logs -- a kernel-level
+    # rejection, not a network or export-permission one). Keying off
+    # hostName rather than something docker-build-type-specific because
+    # modules/build-types/docker.nix is also composed for linode-docker/
+    # proxmox-docker, which don't import proxmox-lxc.nix at all --setting
+    # this option there would break their eval with "option does not
+    # exist" regardless of any mkIf guard, since mkIf only makes a value
+    # conditional, not whether the option needs to exist somewhere.
+    privileged = config.networking.hostName == "docker";
   };
 
   boot.loader = {

scripts/lib/nix-eval.sh

@@ -34,3 +34,21 @@ flake_target_hostname() {
   nix eval --raw "${NIX_EVAL_FLAGS[@]}" \
     "${flake_ref}#nixosConfigurations.${target}.config.networking.hostName" 2>/dev/null
 }
+
+# flake_target_lxc_privileged <flake_ref> <target>
+# Prints "true" or "false" for one lxc-* target's config.proxmoxLXC.privileged
+# (modules/platforms/lxc.nix is the single source of truth -- e.g.
+# lxc-docker sets this true so it can NFS-mount; every other lxc-* host
+# stays unprivileged). Only meaningful for lxc-* targets -- the option
+# doesn't exist for linode-*/proxmox-* (nixpkgs' proxmox-lxc.nix, which
+# declares it, is only ever imported by modules/platforms/lxc.nix). Empty
+# (not an error under set -e) if the eval fails.
+flake_target_lxc_privileged() {
+  local flake_ref="$1" target="$2"
+  # Not --raw: the option is a Nix boolean, and --raw can only coerce
+  # strings ("cannot coerce a Boolean to a string"). Plain `nix eval`
+  # prints a bare `true`/`false` for a boolean, which is exactly the
+  # string this needs.
+  nix eval "${NIX_EVAL_FLAGS[@]}" \
+    "${flake_ref}#nixosConfigurations.${target}.config.proxmoxLXC.privileged" 2>/dev/null
+}

scripts/proxmox/create-proxmox-resource.sh

@@ -766,13 +766,22 @@ if [[ "$type" == "lxc" ]]; then
   # 512M default otherwise (confirmed live: --memory 2048 left swap at
   # 512). Default to matching whatever --memory resolved to above.
   local_swap="${swap:-$memory}"
-  # --unprivileged 1: modules/platforms/lxc.nix sets proxmoxLXC.privileged
-  # = false, so the NixOS config inside the image assumes it's running as
-  # an unprivileged container (cgroup/capability/mount expectations baked
-  # in at boot). `pct create`'s own CLI default for this flag is
-  # privileged (unlike the web UI, which defaults its checkbox the other
-  # way) -- leaving it unset creates a privileged container running a
-  # NixOS config that assumes unprivileged, a real mismatch.
+  # --unprivileged: read back from modules/platforms/lxc.nix's own
+  # proxmoxLXC.privileged (via flake_target_lxc_privileged) rather than
+  # hardcoded, since that's no longer the same for every lxc-* target --
+  # lxc-docker sets it true so the container's NFS mounts work at all (the
+  # kernel's NFS client can't mount from inside any unprivileged
+  # container's user namespace, no matter what AppArmor allows -- see that
+  # option's own comment). The NixOS config inside the image bakes in
+  # cgroup/capability/mount expectations matching whichever value it was
+  # built with, so this must stay in sync with it -- `pct create`'s own
+  # CLI default for this flag is privileged (unlike the web UI, which
+  # defaults its checkbox the other way), so leaving it unset would create
+  # a privileged container running a NixOS config that assumes
+  # unprivileged for every target except lxc-docker, a real mismatch.
+  privileged_eval="$(flake_target_lxc_privileged "$repo_root" "$flake_target")"
+  unprivileged_flag=1
+  [[ "$privileged_eval" == "true" ]] && unprivileged_flag=0
   #
   # --features nesting=1,keyctl=1: required for a modern (v247+) systemd
   # guest to actually boot unprivileged -- confirmed live: without this,
@@ -790,7 +799,7 @@ if [[ "$type" == "lxc" ]]; then
   # hands the whole string to `ssh` as a single command for the *remote*
   # shell to parse -- unquoted, that `;` would be read as a remote
   # command separator and silently truncate this into two commands.
-  create_cmd="pct create ${vmid} ${iso_storage}:vztmpl/${remote_filename} --unprivileged 1 --features '${PROXMOX_DEFAULT_LXC_FEATURES}' --rootfs ${storage}:${local_disk_size} --hostname ${name} --cores ${cores} --memory ${memory} --swap ${local_swap} --net0 name=eth0,bridge=${bridge},ip=dhcp"
+  create_cmd="pct create ${vmid} ${iso_storage}:vztmpl/${remote_filename} --unprivileged ${unprivileged_flag} --features '${PROXMOX_DEFAULT_LXC_FEATURES}' --rootfs ${storage}:${local_disk_size} --hostname ${name} --cores ${cores} --memory ${memory} --swap ${local_swap} --net0 name=eth0,bridge=${bridge},ip=dhcp"
   remote "$create_cmd"
   remote "pct start ${vmid}"
 else