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.

1 files changed, 18 insertions, 15 deletions

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;
 }