systemd: Allow setting the transient hostname via DHCP

This permits using method_set_hostname but still denies
method_set_static_hostname. As a result DHCP clients can now always set
the transient hostname via the SetHostname method of the D-Bus interface
of systemd-hostnamed (org.freedesktop.hostname1.set-hostname).
If the NixOS option networking.hostName is set to an empty string (or
"localhost") the static hostname (kernel.hostname but NOT /etc/hostname)
will additionally be updated (this is intended).

From "man hostnamectl": The transient hostname is a fallback value
received from network configuration. If a static hostname is set, and is
valid (something other than localhost), then the transient hostname is
not used.

Fix #74847.

Note: It's possible to restrict access to the org.freedesktop.hostname1
interface using Polkit rules.

1 files changed, 5 insertions, 0 deletions

diff --git a/nixos/tests/systemd.nix b/nixos/tests/systemd.nix
index ca2e36a443e..24949c4a9f6 100644
--- a/nixos/tests/systemd.nix
+++ b/nixos/tests/systemd.nix
@@ -97,6 +97,11 @@ import ./make-test-python.nix ({ pkgs, ... }: {
             re.search(r"^Filesystem state: *clean$", extinfo, re.MULTILINE) is not None
         ), ("File system was not cleanly unmounted: " + extinfo)
 
+    # Regression test for https://github.com/NixOS/nixpkgs/pull/91232
+    with subtest("setting transient hostnames works"):
+        machine.succeed("hostnamectl set-hostname --transient machine-transient")
+        machine.fail("hostnamectl set-hostname machine-all")
+
     with subtest("systemd-shutdown works"):
         machine.shutdown()
         machine.wait_for_unit("multi-user.target")