Add test for the proposed fix

This test asserts that option "deny-answer-aliases" works correctly
when forwarding requests.

As a matter of example, the behavior expected for a forwarder BIND
instance, having an option such as deny-answer-aliases { "domain"; }
is that when forwarding a request for *.anything-but-domain, it is
expected that it will return SERVFAIL if any answer received has a CNAME
for "*.domain".

bin/tests/system/forward/ns4/malicious.db

@@ -0,0 +1,13 @@
+$TTL    86400
+@       IN      SOA     malicious. admin.malicious. (
+                              1         ; Serial
+                         604800         ; Refresh
+                          86400         ; Retry
+                        2419200         ; Expire
+                          86400 )       ; Negative Cache TTL
+
+@           IN    NS      ns
+
+ns          IN    A       10.53.0.4
+
+target      IN    CNAME   subdomain.rebind.

bin/tests/system/forward/ns4/named.conf.in

@@ -55,3 +55,8 @@ zone "grafted" {
 	forward only;
 	forwarders { 10.53.0.2; };
 };
+
+zone "malicious." {
+	type master;
+	file "malicious.db";
+};

bin/tests/system/forward/ns5/named.conf.in

@@ -19,6 +19,7 @@ options {
 	listen-on-v6 { none; };
 	forward only;
 	forwarders { 10.53.0.4; };
+	deny-answer-aliases { "rebind"; };
 	dnssec-validation yes;
 };
 
@@ -26,3 +27,8 @@ zone "." {
 	type hint;
 	file "root.db";
 };
+
+zone "rebind" {
+	type master;
+	file "rebind.db";
+};

bin/tests/system/forward/ns5/rebind.db

@@ -0,0 +1,13 @@
+$TTL    86400
+@       IN      SOA     rebind. admin.rebind. (
+                              1         ; Serial
+                         604800         ; Refresh
+                          86400         ; Retry
+                        2419200         ; Expire
+                          86400 )       ; Negative Cache TTL
+
+@           IN    NS    ns
+
+ns          IN    A     10.53.0.5
+
+subdomain   IN    A     10.53.0.1

bin/tests/system/forward/tests.sh

@@ -217,5 +217,18 @@ grep "status: NOERROR" dig.out.$n.f8 > /dev/null || ret=1
 if [ $ret != 0 ]; then echo_i "failed"; fi
 status=$((status+ret))
 
+n=$((n+1))
+echo_i "checking that rebinding protection works in forward only mode ($n)"
+ret=0
+# 10.53.0.5 will forward target.malicious. query to 10.53.0.4
+# which in turn will return a CNAME for subdomain.rebind.
+# to honor the option deny-answer-aliases { "rebind"; };
+# ns5 should return a SERVFAIL to avoid potential rebinding attacks
+dig_with_opts +noadd +noauth @10.53.0.5 target.malicious. > dig.out.$n || ret=1
+grep "status: SERVFAIL" dig.out.$n > /dev/null || ret=1
+if [ $ret != 0 ]; then echo_i "failed"; fi
+status=$((status+ret))
+
+
 echo_i "exit status: $status"
 [ $status -eq 0 ] || exit 1