util: generalize is_localhost() and use it everywhere where applicable

src/hostname/hostnamed.c

@@ -258,7 +258,7 @@ static char* context_fallback_icon_name(Context *c) {
 }
 
 static bool hostname_is_useful(const char *hn) {
-        return !isempty(hn) && !streq(hn, "localhost");
+        return !isempty(hn) && !is_localhost(hn);
 }
 
 static int context_update_kernel_hostname(Context *c) {

src/login/pam_systemd.c

@@ -357,9 +357,7 @@ _public_ PAM_EXTERN int pam_sm_open_session(
         if (isempty(class))
                 class = streq(type, "unspecified") ? "background" : "user";
 
-        remote = !isempty(remote_host) &&
-                !streq_ptr(remote_host, "localhost") &&
-                !streq_ptr(remote_host, "localhost.localdomain");
+        remote = !isempty(remote_host) && !is_localhost(remote_host);
 
         /* Talk to logind over the message bus */
 

src/network/networkd-link.c

@@ -1928,18 +1928,6 @@ static int link_enter_enslave(Link *link) {
         return 0;
 }
 
-/* make sure the hostname is not "localhost" */
-static bool is_localhost(const char *hostname) {
-        assert(hostname);
-
-        return streq(hostname, "localhost") ||
-               streq(hostname, "localhost.") ||
-               endswith(hostname, ".localhost") ||
-               endswith(hostname, ".localhost.") ||
-               endswith(hostname, ".localdomain") ||
-               endswith(hostname, ".localdomain.");
-}
-
 static int link_configure(Link *link) {
         int r;
 

src/shared/util.c

@@ -6732,3 +6732,18 @@ char *tempfn_random(const char *p) {
 
         return t;
 }
+
+/* make sure the hostname is not "localhost" */
+bool is_localhost(const char *hostname) {
+        assert(hostname);
+
+        /* This tries to identify local hostnames described in RFC6761
+         * plus the redhatism of .localdomain */
+
+        return streq(hostname, "localhost") ||
+               streq(hostname, "localhost.") ||
+               endswith(hostname, ".localhost") ||
+               endswith(hostname, ".localhost.") ||
+               endswith(hostname, ".localdomain") ||
+               endswith(hostname, ".localdomain.");
+}

src/shared/util.h

@@ -959,3 +959,5 @@ int fflush_and_check(FILE *f);
 
 char *tempfn_xxxxxx(const char *p);
 char *tempfn_random(const char *p);
+
+bool is_localhost(const char *hostname);