hostname-util: introduce new is_gateway_hostname() call

This moves is_gateway() from nss-myhostname into the basic APIs, and
makes it more like is_localhost(). Also, we rename it to
is_gateway_hostname() to make it more expressive.

Sharing this function in src/basic/ allows us to reuse the function for
routing name requests in resolved (in a later commit).

src/basic/hostname-util.c

@@ -151,6 +151,17 @@ bool is_localhost(const char *hostname) {
                endswith_no_case(hostname, ".localdomain.");
 }
 
+bool is_gateway_hostname(const char *hostname) {
+        assert(hostname);
+
+        /* This tries to identify the valid syntaxes for the our
+         * synthetic "gateway" host. */
+
+        return
+                strcaseeq(hostname, "gateway") ||
+                strcaseeq(hostname, "gateway.");
+}
+
 int sethostname_idempotent(const char *s) {
         char buf[HOST_NAME_MAX + 1] = {};
 

src/basic/hostname-util.h

@@ -33,6 +33,7 @@ bool hostname_is_valid(const char *s, bool relax) _pure_;
 char* hostname_cleanup(char *s);
 
 bool is_localhost(const char *hostname);
+bool is_gateway_hostname(const char *hostname);
 
 int sethostname_idempotent(const char *s);