Allow the fallback hostname to be overriden using an environment variable

See https://bugzilla.redhat.com/show_bug.cgi?id=1893417 for the back story:
the fallback hostname matters a lot in certain environments. Right now the only
way to configure the fallback hostname is by recompiling systemd, which is
obviously problematic in case when the fallback hostname shall differ between
different editions of the same distro that share a single compiled rpm.

By making this overridable through an envvar, we're providing an escape hatch
without making this a top-level api. Later on a way to set this through
os-release is added, but I think the approach with the variable is still
useful. It it very convenient for testing, or to override settings only in a
particular service, etc.

7 files changed, 94 insertions, 32 deletions

diff --git a/docs/ENVIRONMENT.md b/docs/ENVIRONMENT.md
index 82bb52b344..07a65864bd 100644
--- a/docs/ENVIRONMENT.md
+++ b/docs/ENVIRONMENT.md
@@ -55,6 +55,10 @@ All tools:
 * `$SYSTEMD_EFI_OPTIONS` — if set, used instead of the string in the
   SystemdOptions EFI variable. Analogous to `$SYSTEMD_PROC_CMDLINE`.
 
+* `$SYSTEMD_DEFAULT_HOSTNAME` — override the compiled-in fallback hostname
+  (relevant in particular for the system manager and `systemd-hostnamed`).
+  Must be a valid hostname (either a single label or a FQDN).
+
 * `$SYSTEMD_IN_INITRD=[auto|lenient|0|1]` — if set, specifies initrd detection
   method. Defaults to `auto`. Behavior is defined as follows:
   `auto`: Checks if `/etc/initrd-release` exists, and a temporary fs is mounted
diff --git a/src/basic/hostname-util.c b/src/basic/hostname-util.c
index d7aba2c263..5109605e0c 100644
--- a/src/basic/hostname-util.c
+++ b/src/basic/hostname-util.c
@@ -3,14 +3,27 @@
 #include <errno.h>
 #include <limits.h>
 #include <stdio.h>
+#include <stdlib.h>
 #include <sys/utsname.h>
 #include <unistd.h>
 
 #include "alloc-util.h"
 #include "hostname-util.h"
+#include "os-util.h"
 #include "string-util.h"
 #include "strv.h"
 
+char* get_default_hostname(void) {
+        const char *e = secure_getenv("SYSTEMD_DEFAULT_HOSTNAME");
+        if (e) {
+                if (hostname_is_valid(e, 0))
+                        return strdup(e);
+                log_debug("Invalid hostname in $SYSTEMD_DEFAULT_HOSTNAME, ignoring: %s", e);
+        }
+
+        return strdup(FALLBACK_HOSTNAME);
+}
+
 char* gethostname_malloc(void) {
         struct utsname u;
         const char *s;
@@ -23,7 +36,7 @@ char* gethostname_malloc(void) {
 
         s = u.nodename;
         if (isempty(s) || streq(s, "(none)"))
-                s = FALLBACK_HOSTNAME;
+                return get_default_hostname();
 
         return strdup(s);
 }
@@ -31,6 +44,7 @@ char* gethostname_malloc(void) {
 char* gethostname_short_malloc(void) {
         struct utsname u;
         const char *s;
+        _cleanup_free_ char *f = NULL;
 
         /* Like above, but kills the FQDN part if present. */
 
@@ -38,7 +52,10 @@ char* gethostname_short_malloc(void) {
 
         s = u.nodename;
         if (isempty(s) || streq(s, "(none)") || s[0] == '.') {
-                s = FALLBACK_HOSTNAME;
+                s = f = get_default_hostname();
+                if (!s)
+                        return NULL;
+
                 assert(s[0] != '.');
         }
 
diff --git a/src/basic/hostname-util.h b/src/basic/hostname-util.h
index 6cff9c1d4c..576ca083e0 100644
--- a/src/basic/hostname-util.h
+++ b/src/basic/hostname-util.h
@@ -7,6 +7,7 @@
 #include "macro.h"
 #include "strv.h"
 
+char* get_default_hostname(void);
 char* gethostname_malloc(void);
 char* gethostname_short_malloc(void);
 int gethostname_strict(char **ret);
diff --git a/src/hostname/hostnamed.c b/src/hostname/hostnamed.c
index eb06957397..9937d159f4 100644
--- a/src/hostname/hostnamed.c
+++ b/src/hostname/hostnamed.c
@@ -322,6 +322,7 @@ static int context_update_kernel_hostname(
                 Context *c,
                 const char *transient_hn) {
 
+        _cleanup_free_ char *_hn_free = NULL;
         const char *hn;
         HostnameSource hns;
         int r;
@@ -340,7 +341,10 @@ static int context_update_kernel_hostname(
 
         /* ... and the ultimate fallback */
         } else {
-                hn = FALLBACK_HOSTNAME;
+                hn = _hn_free = get_default_hostname();
+                if (!hn)
+                        return log_oom();
+
                 hns = HOSTNAME_FALLBACK;
         }
 
@@ -502,16 +506,20 @@ static int property_get_hostname(
                 void *userdata,
                 sd_bus_error *error) {
 
-        _cleanup_free_ char *current = NULL;
+        _cleanup_free_ char *hn = NULL;
         int r;
 
-        r = gethostname_strict(&current);
-        if (r == -ENXIO)
-                return sd_bus_message_append(reply, "s", FALLBACK_HOSTNAME);
-        if (r < 0)
-                return r;
+        r = gethostname_strict(&hn);
+        if (r < 0) {
+                if (r != -ENXIO)
+                        return r;
 
-        return sd_bus_message_append(reply, "s", current);
+                hn = get_default_hostname();
+                if (!hn)
+                        return -ENOMEM;
+        }
+
+        return sd_bus_message_append(reply, "s", hn);
 }
 
 static int property_get_static_hostname(
@@ -531,7 +539,21 @@ static int property_get_static_hostname(
         return sd_bus_message_append(reply, "s", c->data[PROP_STATIC_HOSTNAME]);
 }
 
-static BUS_DEFINE_PROPERTY_GET_GLOBAL(property_get_fallback_hostname, "s", FALLBACK_HOSTNAME);
+static int property_get_fallback_hostname(
+                sd_bus *bus,
+                const char *path,
+                const char *interface,
+                const char *property,
+                sd_bus_message *reply,
+                void *userdata,
+                sd_bus_error *error) {
+
+        _cleanup_free_ char *hn = get_default_hostname();
+        if (!hn)
+                return log_oom();
+
+        return sd_bus_message_append(reply, "s", hn);
+}
 
 static int property_get_hostname_source(
                 sd_bus *bus,
diff --git a/src/resolve/resolved-manager.c b/src/resolve/resolved-manager.c
index 2ed9f2d47b..ae46d2dcd8 100644
--- a/src/resolve/resolved-manager.c
+++ b/src/resolve/resolved-manager.c
@@ -442,20 +442,25 @@ static int determine_hostname(char **full_hostname, char **llmnr_hostname, char
         return 0;
 }
 
-static const char *fallback_hostname(void) {
+static char* fallback_hostname(void) {
 
-        /* Determine the fall back hostname. For exposing this system to the outside world, we cannot have it to be
-         * "localhost" even if that's the compiled in hostname. In this case, let's revert to "linux" instead. */
+        /* Determine the fall back hostname. For exposing this system to the outside world, we cannot have it
+         * to be "localhost" even if that's the default hostname. In this case, let's revert to "linux"
+         * instead. */
 
-        if (is_localhost(FALLBACK_HOSTNAME))
-                return "linux";
+        _cleanup_free_ char *n = get_default_hostname();
+        if (!n)
+                return NULL;
+
+        if (is_localhost(n))
+                return strdup("linux");
 
-        return FALLBACK_HOSTNAME;
+        return TAKE_PTR(n);
 }
 
 static int make_fallback_hostnames(char **full_hostname, char **llmnr_hostname, char **mdns_hostname) {
-        _cleanup_free_ char *n = NULL, *m = NULL;
-        char label[DNS_LABEL_MAX], *h;
+        _cleanup_free_ char *h = NULL, *n = NULL, *m = NULL;
+        char label[DNS_LABEL_MAX];
         const char *p;
         int r;
 
@@ -463,7 +468,10 @@ static int make_fallback_hostnames(char **full_hostname, char **llmnr_hostname,
         assert(llmnr_hostname);
         assert(mdns_hostname);
 
-        p = fallback_hostname();
+        p = h = fallback_hostname();
+        if (!h)
+                return log_oom();
+
         r = dns_label_unescape(&p, label, sizeof label, 0);
         if (r < 0)
                 return log_error_errno(r, "Failed to unescape fallback hostname: %m");
@@ -478,14 +486,9 @@ static int make_fallback_hostnames(char **full_hostname, char **llmnr_hostname,
         if (r < 0)
                 return log_error_errno(r, "Failed to concatenate mDNS hostname: %m");
 
-        h = strdup(fallback_hostname());
-        if (!h)
-                return log_oom();
-
         *llmnr_hostname = TAKE_PTR(n);
         *mdns_hostname = TAKE_PTR(m);
-
-        *full_hostname = h;
+        *full_hostname = TAKE_PTR(h);
 
         return 0;
 }
diff --git a/src/shared/hostname-setup.c b/src/shared/hostname-setup.c
index 5cdcdbd93c..0cc1a268bd 100644
--- a/src/shared/hostname-setup.c
+++ b/src/shared/hostname-setup.c
@@ -194,7 +194,7 @@ int hostname_setup(bool really) {
                 }
         }
 
-        if (isempty(hn)) {
+        if (!hn) {
                 /* Don't override the hostname if it is already set and not explicitly configured */
 
                 char buf[HOST_NAME_MAX + 1] = {};
@@ -206,7 +206,10 @@ int hostname_setup(bool really) {
                 if (enoent)
                         log_info("No hostname configured, using fallback hostname.");
 
-                hn = FALLBACK_HOSTNAME;
+                hn = b = get_default_hostname();
+                if (!hn)
+                        return log_oom();
+
                 source = HOSTNAME_FALLBACK;
 
         }
diff --git a/src/test/test-hostname-util.c b/src/test/test-hostname-util.c
index 24c8ed9e3b..6d62958d67 100644
--- a/src/test/test-hostname-util.c
+++ b/src/test/test-hostname-util.c
@@ -10,6 +10,8 @@
 #include "tmpfile-util.h"
 
 static void test_hostname_is_valid(void) {
+        log_info("/* %s */", __func__);
+
         assert_se(hostname_is_valid("foobar", 0));
         assert_se(hostname_is_valid("foobar.com", 0));
         assert_se(!hostname_is_valid("foobar.com.", 0));
@@ -49,6 +51,8 @@ static void test_hostname_is_valid(void) {
 static void test_hostname_cleanup(void) {
         char *s;
 
+        log_info("/* %s */", __func__);
+
         s = strdupa("foobar");
         assert_se(streq(hostname_cleanup(s), "foobar"));
         s = strdupa("foobar.com");
@@ -94,6 +98,8 @@ static void test_hostname_cleanup(void) {
 static void test_hostname_malloc(void) {
         _cleanup_free_ char *h = NULL, *l = NULL;
 
+        log_info("/* %s */", __func__);
+
         assert_se(h = gethostname_malloc());
         log_info("hostname_malloc: \"%s\"", h);
 
@@ -101,21 +107,27 @@ static void test_hostname_malloc(void) {
         log_info("hostname_short_malloc: \"%s\"", l);
 }
 
-static void test_fallback_hostname(void) {
+static void test_default_hostname(void) {
+        log_info("/* %s */", __func__);
+
         if (!hostname_is_valid(FALLBACK_HOSTNAME, 0)) {
                 log_error("Configured fallback hostname \"%s\" is not valid.", FALLBACK_HOSTNAME);
                 exit(EXIT_FAILURE);
         }
+
+        _cleanup_free_ char *n = get_default_hostname();
+        assert_se(n);
+        log_info("get_default_hostname: \"%s\"", n);
+        assert_se(hostname_is_valid(n, 0));
 }
 
 int main(int argc, char *argv[]) {
-        test_setup_logging(LOG_INFO);
+        test_setup_logging(LOG_DEBUG);
 
         test_hostname_is_valid();
         test_hostname_cleanup();
         test_hostname_malloc();
-
-        test_fallback_hostname();
+        test_default_hostname();
 
         return 0;
 }