BIND DNS won't resolve CNAME to external domain

Question

I'm trying to setup my own DNS server on my server 192.168.1.70.

I want to be able to resolve my own names in my LAN but I want all other zones to be resolved by my router (192.168.1.1).

Here is my named.conf (I simplified it with only the relevant entries):

options {

    allow-query                 { 192.168.1.0/24; };
    allow-query-cache           { 192.168.1.0/24; };

    recursion                   yes;
    forwarders                  { 192.168.1.1; };
    forward                     only;

    dnssec-enable               yes;
    dnssec-validation           yes;
};

include "/etc/named/named.root.key";
include "/etc/named/named.rfc1912.zones";

zone "." IN {
    type hint;
    file "named.ca";
};

zone "example.com" IN {
    type master;
    file "named.example.com";
    allow-update { none; };
};

The named.example.com:

$ORIGIN example.com.
$TTL 10D
@               IN      SOA     server   root.example.com. (2017031301 1D 1H 1W 3H )
@               IN      NS      server
@               IN      A       192.168.1.70
server          IN      A       192.168.1.70
work            IN      A       192.168.1.71
tv              IN      A       192.168.1.72
backups         IN      CNAME   external.com
test            IN      CNAME   server

With this configuration, almost everything works.

I can:

• Resolve external names (like google.com);
• Resolve internal names (like server, work, tv and test).

I can not:

• Resolve backups, which should be a CNAME for external.com.

Do I have to specify a zone file for external.com? I wanted it to be resolved by my router.

Thank you