disables fallback to less specific routes explained in RFC 4191 3.2 and 3.6; patch --ignore-whitespace net/ipv6/route.c route.patch; tested on Linux 6.12.38

route.patch

--- net/ipv6/route.c~   2025-08-08 13:44:04.928000000 +0300
+++ net/ipv6/route.c    2025-08-08 13:48:01.448000000 +0300
@@ -2221,24 +2221,13 @@
 int fib6_table_lookup(struct net *net, struct fib6_table *table, int oif,
                      struct flowi6 *fl6, struct fib6_result *res, int strict)
 {
-       struct fib6_node *fn, *saved_fn;
+       struct fib6_node *fn;

        fn = fib6_node_lookup(&table->tb6_root, &fl6->daddr, &fl6->saddr);
-       saved_fn = fn;
-
-redo_rt6_select:
        rt6_select(net, fn, oif, res, strict);
-       if (res->f6i == net->ipv6.fib6_null_entry) {
-               fn = fib6_backtrack(fn, &fl6->saddr);
-               if (fn)
-                       goto redo_rt6_select;
-               else if (strict & RT6_LOOKUP_F_REACHABLE) {
-                       /* also consider unreachable route */
-                       strict &= ~RT6_LOOKUP_F_REACHABLE;
-                       fn = saved_fn;
-                       goto redo_rt6_select;
-               }
-       }
+
+       if (res->f6i == net->ipv6.fib6_null_entry)
+               return -ENOENT;

        trace_fib6_table_lookup(net, res, table, fl6);