No Reverse DNS Lookup in RED Networks

Hello everyone,

we have a customer with a UTM v9 and two RED 15 appliances in transparent/split mode which he uses in his branch offices. Everything is working except that clients fail to nslookup the internal IP address of the DNS server (Windows SBS 2011). This seems to cause some problems (Synology Diskstation can't join the domain, Clients don't register with the DNS server). Results look like this:

nslookup 192.168.88.100
Server: 192.168.88.100
Address: 192.168.88.100#53

** server can't find 100.88.168.192.in-addr.arpa: NXDOMAIN

Forward Lookup works though:

nslookup domain.local
Server: 192.168.88.100
Address: 192.168.88.100#53

Name: domain.local
Address: 192.168.88.100

Reverse DNS is set up correctly on the DC. I've tested joining the domain from our own diskstation (we're connected through Site-to-site IPsec), it worked perfectly fine.

Any help is appreciated here.

Regards, Stefano