Fixed rebinding protection bug when using forwarder setups

BIND wasn't honoring option "deny-answer-aliases" when configured to
forward queries.

Before the fix it was possible for nameservers listed in "forwarders"
option to return CNAME answers pointing to unrelated domains of the
original query, which could be used as a vector for rebinding attacks.

The fix ensures that BIND apply filters even if configured as a forwarder
instance.

diff --git a/lib/dns/resolver.c b/lib/dns/resolver.c
index 1ccc04311770cf7715329ffe4091cd46b532b7cf..73fc5763dcb3f63f497ed858a185143ba9a94dcd 100644 (file)
--- a/lib/dns/resolver.c
+++ b/lib/dns/resolver.c
@@ -6985,9 +6985,15 @@ is_answertarget_allowed(fetchctx_t *fctx, dns_name_t *qname, dns_name_t *rname,
 
        /*
         * If the target name is a subdomain of the search domain, allow it.
+        *
+        * Note that if BIND is configured as a forwarding DNS server, the
+        * search domain will always match the root domain ("."), so we
+        * must also check whether forwarding is enabled so that filters
+        * can be applied; see GL #1574.
         */
-       if (dns_name_issubdomain(tname, &fctx->domain))
+       if (!fctx->forwarding && dns_name_issubdomain(tname, &fctx->domain)) {
                return (true);
+       }
 
        /*
         * Otherwise, apply filters.