9

I've the following configuration: CISCO 881, IOS 15.2(4)M6. On WAN interface is configured the 86.34.156.48/29 subnet from our ISP. Behind the router is staying 2 webserver. NAT: 86.34.156.51 <> 10.10.10.100 (private IP, server1). If I configure the BIND DNS server A records with the external IP of the server(86.34.156.51) the outside world never get the response from my DNS server(I don't know what happens there, perhaps the router does DNS doctoring or something similar ( it is a question)), and it is sure the DNS server is configured and works properly(I captured the IP packets with TCPdump). But if I configure the DNS server A records with the server private IP (10.10.10.100) the packets is alwasy arriving to proper destination(with the server external IP, perhaps DNS doctoring too). The only one problem with this is the DNS packet TTL value disappearing, as you can see in the below digs output.

What could I do???

        +-----+
        |     |  an other host somewhere on internet(C)
        |     |
        +-----+
            |
            |
            :
            :  internet
            :

            |  

        +-----+
        |     |      ISP's router
        |     |       black box, without acces
        +-----+

            |
            | 86.34.456.48/29
        +-----+
        |     |      CISCO 881,
        |     |       IOS 15.2(4)M6
        +-----+
            |
            |
------------------------------------------------------------  local private network 10.10.10.0/24
    |                                           |
    | (86.34.156.51)                            | (Nat rule not yet attached)
    | 10.10.10.100                              | 10.10.10.101
    |                                           |
+-----+                                       +-----+
|     |                                       |     |
|     |                                       |     |
+-----+                                       +-----+
linux server (A)                                linux server (B)
BIND DNS server
 style2take.ro

Here is some dig (dns diagnostic tool under linux):

From host B : $dig style2take.ro

;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 42222
;; flags: qr rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 0, ADDITIONAL: 1

;; OPT PSEUDOSECTION:
; EDNS: version: 0, flags:; udp: 4096
;; QUESTION SECTION:
;style2take.ro.                 IN      A

;; ANSWER SECTION:
style2take.ro.          0       IN      A       10.10.10.100                                                <-- here you can see the TTL is 0

;; Query time: 52 msec
;; SERVER: 193.231.100.130#53(193.231.100.130)
;; WHEN: Fri Feb 20 10:27:25 EET 2015
;; MSG SIZE  rcvd: 58

From host B: $dig @10.10.10.100 style2take.ro

;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 65374
;; flags: qr aa rd; QUERY: 1, ANSWER: 1, AUTHORITY: 2, ADDITIONAL: 3
;; WARNING: recursion requested but not available

;; OPT PSEUDOSECTION:
; EDNS: version: 0, flags:; udp: 4096
;; QUESTION SECTION:
;style2take.ro.                 IN      A

;; ANSWER SECTION:
style2take.ro.          3600    IN      A       10.10.10.100                                                <-- here you can see the TTL is 3600

;; AUTHORITY SECTION:
style2take.ro.          3600    IN      NS      ns1.style2take.ro.
style2take.ro.          3600    IN      NS      ns2.style2take.ro.

;; ADDITIONAL SECTION:
ns1.style2take.ro.      3600    IN      A       10.10.10.100
ns2.style2take.ro.      3600    IN      A       10.10.10.100

;; Query time: 0 msec
;; SERVER: 10.10.10.100#53(10.10.10.100)
;; WHEN: Fri Feb 20 10:28:58 EET 2015
;; MSG SIZE  rcvd: 126

From host C : $dig style2take.ro

;; global options:  printcmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 32364
;; flags: qr rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 6, ADDITIONAL: 0

;; QUESTION SECTION:
;style2take.ro.                 IN      A

;; ANSWER SECTION:
style2take.ro.          0       IN      A       86.34.156.51                                                <-- here you can see the TTL is 0

;; AUTHORITY SECTION:
ro.                     106161  IN      NS      sns-pb.isc.org.
ro.                     106161  IN      NS      primary.rotld.ro.
ro.                     106161  IN      NS      sec-dns-a.rotld.ro.
ro.                     106161  IN      NS      sec-dns-b.rotld.ro.
ro.                     106161  IN      NS      dns-at.rotld.ro.
ro.                     106161  IN      NS      dns-ro.denic.de.

;; Query time: 149 msec
;; SERVER: 82.79.24.74#53(82.79.24.74)
;; WHEN: Fri Feb 20 10:29:52 2015
;; MSG SIZE  rcvd: 201

From host C: $dig @86.34.156.51 style2take.ro

; (1 server found)
;; global options:  printcmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 48385
;; flags: qr aa rd; QUERY: 1, ANSWER: 1, AUTHORITY: 2, ADDITIONAL: 2

;; QUESTION SECTION:
;style2take.ro.                 IN      A

;; ANSWER SECTION:
style2take.ro.          0       IN      A       86.34.156.51                                                <-- here you can see the TTL is 0

;; AUTHORITY SECTION:
style2take.ro.          0       IN      NS      ns2.style2take.ro.
style2take.ro.          0       IN      NS      ns1.style2take.ro.

;; ADDITIONAL SECTION:
ns1.style2take.ro.      0       IN      A       86.34.156.51
ns2.style2take.ro.      0       IN      A       86.34.156.51

;; Query time: 29 msec
;; SERVER: 86.34.156.51#53(86.34.156.51)
;; WHEN: Fri Feb 20 10:35:05 2015
;; MSG SIZE  rcvd: 115
Szilveszter Zsigmond
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2 Answers2

0

Well, if you don't get DNS resolved, problem might be two-fold:

  1. Improper DNS configuration - is the global DNS system able to tell what is the NS for your domain? This is something you need to set at your name provider. What is dig style2take.ro NS output?

  2. Your router doesn't allow DNS requests. Get on a server outside and try to telnet to port 53 of your DNS telnet 86.34.156.51 - DNS should work both over UDP and TCP, this tests at least TCP part.

StanTastic
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0

You do not need to have the external IP in any of your DNS settings (named) configuration. You can use the internal one (10.10.10.100) everywhere and NAT it from the router to 86.34.156.51 (ip nat inside source static 10.10.10.100 86.34.156.51). If you want to configure a secondary DNS, the same thing applies for it: use 10.10.10.101 (as NS2) NAT'ed to something else (like 86.34.156.51). Also, make sure the DNS ports are actually forwarded in the router and not blocked by the firewall in the linux server. Use a web-based port checker to make sure UDP 53 is open.

Overmind
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