From c7837840f5747694838a6b36b6eee8e82b221889 Mon Sep 17 00:00:00 2001
From: Arian van Putten <arian@arianvp.me>
Date: Sat, 18 Jul 2026 22:19:18 +0200
Subject: [PATCH] firstboot: apply hostname via hostnamed varlink API

Previously we just wrote /etc/hostname and returned. The kernel hostname
was left at whatever PID 1 sethostname()'d very early in boot (before
/etc/hostname existed), which meant hostnamectl reported the transient
hostname instead of the static one until the next reboot.

Call io.systemd.Hostname.SetStaticHostname on the running system so
hostnamed writes /etc/hostname and updates the kernel hostname in one
step. If hostnamed is unreachable (masked or absent on minimal images)
fall back to writing /etc/hostname directly. --root=/--image= also keeps
writing the file directly, since hostnamed only manages the running
system.
---
 src/firstboot/firstboot.c       | 35 +++++++++++++++++++++++++++++----
 units/systemd-firstboot.service |  1 +
 2 files changed, 32 insertions(+), 4 deletions(-)

diff --git a/src/firstboot/firstboot.c b/src/firstboot/firstboot.c
index b7d30baf3c..cfadbea289 100644
--- a/src/firstboot/firstboot.c
+++ b/src/firstboot/firstboot.c
@@ -777,19 +777,46 @@ static int process_hostname(int rfd, sd_varlink **mute_console_link) {
          * and let it be resolved on each first boot. */
         const char *hostname = arg_hostname;
         _cleanup_free_ char *resolved = NULL;
+        _cleanup_(sd_varlink_unrefp) sd_varlink *vl = NULL;
         if (!arg_root) {
                 r = hostname_substitute_wildcards(arg_hostname, &resolved);
                 if (r < 0)
                         log_warning_errno(r, "Failed to resolve wildcards in hostname '%s', writing it verbatim: %m", arg_hostname);
                 else if (!hostname_is_valid(resolved, VALID_HOSTNAME_TRAILING_DOT))
                         log_warning("Resolved hostname '%s' is invalid, writing template '%s' verbatim instead.", resolved, arg_hostname);
-                else
+                else {
                         hostname = resolved;
+
+                        r = sd_varlink_connect_address(&vl, "/run/systemd/io.systemd.Hostname");
+                        if (r < 0)
+                                log_warning_errno(r, "Failed to connect to systemd-hostnamed, writing /etc/hostname directly: %m");
+                }
         }
 
-        r = write_string_file_full_label(pfd, f, hostname,
-                                   WRITE_STRING_FILE_CREATE|WRITE_STRING_FILE_SYNC|WRITE_STRING_FILE_ATOMIC|WRITE_STRING_FILE_LABEL,
-                                   /* ts= */ NULL, /* label_fn= */ NULL, arg_label_context);
+        if (vl) {
+                _cleanup_(sd_json_variant_unrefp) sd_json_variant *reply = NULL;
+                const char *error_id = NULL;
+                r = sd_varlink_callbo(
+                                vl,
+                                "io.systemd.Hostname.SetStaticHostname",
+                                &reply,
+                                &error_id,
+                                SD_JSON_BUILD_PAIR_STRING("newValue", hostname));
+                if (r < 0)
+                        log_warning_errno(r, "Failed to call io.systemd.Hostname.SetStaticHostname, writing /etc/hostname directly: %m");
+                else if (error_id)
+                        log_warning_errno(sd_varlink_error_to_errno(error_id, reply),
+                                          "Failed to set static hostname, writing /etc/hostname directly: %s", error_id);
+                else {
+                        log_info("Static hostname configured via systemd-hostnamed.");
+                        return 0;
+                }
+        }
+
+        r = write_string_file_full_label(
+                        pfd, f, hostname,
+                        WRITE_STRING_FILE_CREATE|WRITE_STRING_FILE_SYNC|WRITE_STRING_FILE_ATOMIC|WRITE_STRING_FILE_LABEL,
+                        /* ts= */ NULL, /* label_fn= */ NULL, arg_label_context);
         if (r < 0)
                 return log_error_errno(r, "Failed to write /etc/hostname: %m");
 
diff --git a/units/systemd-firstboot.service b/units/systemd-firstboot.service
index fb1c59ad6f..9fb0314f6f 100644
--- a/units/systemd-firstboot.service
+++ b/units/systemd-firstboot.service
@@ -23,6 +23,7 @@ After=systemd-sysusers.service systemd-tmpfiles-setup.service
 # Let vconsole-setup do its setup before starting user interaction:
 After=systemd-vconsole-setup.service
 After=systemd-mute-console.socket
+After=systemd-hostnamed.socket
 
 Wants=first-boot-complete.target
 Before=first-boot-complete.target sysinit.target