I have:

  1. internal DNS server ns1.internal with IP
  2. external DNS server with an external TLD mydns.example.com and internal IP It's accessible both from the Internet (via a static NAT rule) and from the local network.

I'm trying to setup my external DNS server to forward zone subzone.mydns.example.com to the internal DNS server. The internal DNS server is authoritative for this zone.

Important: I can't modify the internal DNS server configuration. I can read it, however, if that's needed to diagnose the issue.

File /etc/named.conf on the external DNS server:

options {
  directory "/var/named";
  version "get lost";

  recursion yes;
  allow-transfer {"none";};
  allow-query { any; };
  allow-recursion { any; };

  channel example_log{
   file "/var/log/named/named.log" versions 3 size 2m;
   severity info;
   print-severity yes;
   print-time yes;
   print-category yes;
 category default{

// Zones:

zone "mydns.example.com" {
  type master;
  file "mydns.example.com.zone";

zone "subzone.mydns.example.com" {
  type forward;
  forwarders {; };

File /var/named/mydns.example.com.zone on the external DNS server:

$TTL 1
$ORIGIN mydns.example.com.
@             IN      SOA   mydns.example.com. root.mydns.example.com. (
                        2003080800 ; se = serial number
                        60         ; ref = refresh
                        60         ; ret = update retry
                        60         ; ex = expiry
                        60         ; min = minimum

@             IN      NS      mydns.example.com.

So, now I try to resolve some DNS records. The external server zone seems to work.

workstation$ dig mydns.example.com NS +tcp +short

But the forwarded zone does not work:

workstation$ dig subzone.mydns.example.com NS +tcp

; <<>> DiG 9.8.1-P1 <<>> subzone.mydns.example.com NS +tcp
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NXDOMAIN, id: 36887
;; flags: qr rd ra; QUERY: 1, ANSWER: 0, AUTHORITY: 1, ADDITIONAL: 0

;subzone.mydns.example.com.        IN      NS

mydns.example.com.    1       IN      SOA     mydns.example.com. root.mydns.example.com. 2003080800 60 60 60 60

;; Query time: 3 msec
;; WHEN: Thu Jul 19 17:27:54 2012
;; MSG SIZE  rcvd: 108

The results are identical when these commands are executed on remote Internet host and on an internal host.

If I try to resolve subzone.mydns.example.com. from external name server AND specify the internal server explicitly, I get:

mydns$ dig @ subzone.mydns.example.com NS

; <<>> DiG 9.3.6-P1-RedHat-9.3.6-16.P1.el5 <<>> @ subzone.mydns.example.com NS
; (1 server found)
;; global options:  printcmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 87
;; flags: qr aa rd; QUERY: 1, ANSWER: 3, AUTHORITY: 0, ADDITIONAL: 3

;subzone.mydns.example.com.        IN      NS

subzone.mydns.example.com. 3600 IN NS      ns1.internal.

ns1.internal.      3600    IN      A

;; Query time: 613 msec
;; WHEN: Thu Jul 19 18:20:55 2012
;; MSG SIZE  rcvd: 163

What's wrong? How do I configure the forwarding DNS zone to work as I expect?

  • My suspicion (I 'm not sure how to check it) is that the external DNS server fetches the records from the internal one and does not overwrite the fact that ns1.internal is responsible for the zone. So, the client's resolver tries to resolve that name (ns1.internal) and fails. – vadipp Jul 19 '12 at 12:14
  • 3
    try a dig +trace to see what exactly is happening. Also use nscd to enable query logging and check for errors. – coredump Jul 19 '12 at 12:38
  • First, increase the log level of the external bind to log individual requests. My speculation is that there has to be a delegation of the zone subzone.mydns.example.com to the DNS server mydns.example.com itself. Try to add this to the mydns.example.com zonefile: subzone IN NS mydns.example.com. (i assume the zonefile has somewhere also the A record for @=mydns.example.com, correct?) – Nils Toedtmann Sep 12 '14 at 8:07

Add a 'forward only;' statement to the forwarded zone:

zone "subzone.mydns.example.com" {
    type forward;
    forward only;
    forwarders {; };
| improve this answer | |

You have to configure the A RR for the NS "subzone.mydns.example.com." on your external DNS. It's named "glue record" and will correspond to the IP of your internal DNS. Currently, your external DNS is not capable to known the IP of the internal DNS. Regards

| improve this answer | |

I did and other step more, the first one mentioned by @brandon-xavier:

zone "subzone.mydns.example.com" {
    type forward;
    forward only;
    forwarders {; };

and the new one:

$ORIGIN subzone.mydns.example.com.
@             IN      NS      ns1.subzone.mydns.example.com.

But I don't know why it is neccesary...

| improve this answer | |

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