resolved: if ipv6 is off patch hostname→address query with unspec family to ipv4

If the client never specified the address family, and IPv6 is off in the
kernel, then look only for A, never for AAAA RRs.

Fixes: #23951

src/resolve/resolved-dns-question.c

@@ -4,6 +4,7 @@
 #include "dns-domain.h"
 #include "dns-type.h"
 #include "resolved-dns-question.h"
+#include "socket-util.h"
 
 DnsQuestion *dns_question_new(size_t n) {
         DnsQuestion *q;
@@ -330,6 +331,11 @@ int dns_question_new_address(DnsQuestion **ret, int family, const char *name, bo
         if (!IN_SET(family, AF_INET, AF_INET6, AF_UNSPEC))
                 return -EAFNOSUPPORT;
 
+        /* If IPv6 is off and the request has an unspecified lookup family, restrict it automatically to
+         * IPv4. */
+        if (family == AF_UNSPEC && !socket_ipv6_is_enabled())
+                family = AF_INET;
+
         if (convert_idna) {
                 r = dns_name_apply_idna(name, &buf);
                 if (r < 0)