How to verify hosts are being denied access to nameserver?

Question

[root@workshop1 /]# ifconfig
eth0      Link encap:Ethernet  HWaddr 08:00:27:AF:A3:28  
          inet addr:10.0.2.15  Bcast:10.0.2.255  Mask:255.255.255.0
          inet6 addr: fe80::a00:27ff:feaf:a328/64 Scope:Link
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
          RX packets:35311 errors:0 dropped:0 overruns:0 frame:0
          TX packets:20174 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000 
          RX bytes:50544153 (48.2 MiB)  TX bytes:1108333 (1.0 MiB)

eth1      Link encap:Ethernet  HWaddr 08:00:27:C4:A8:B6  
          inet addr:192.168.56.101  Bcast:192.168.56.255  Mask:255.255.255.0
          inet6 addr: fe80::a00:27ff:fec4:a8b6/64 Scope:Link
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
          RX packets:3634 errors:0 dropped:0 overruns:0 frame:0
          TX packets:213 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000 
          RX bytes:498977 (487.2 KiB)  TX bytes:32394 (31.6 KiB)

eth2      Link encap:Ethernet  HWaddr 08:00:27:DB:B6:AB  
          inet addr:10.0.4.16  Bcast:10.0.4.255  Mask:255.255.255.0
          inet6 addr: fe80::a00:27ff:fedb:b6ab/64 Scope:Link
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
          RX packets:0 errors:0 dropped:0 overruns:0 frame:0
          TX packets:28 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000 
          RX bytes:0 (0.0 b)  TX bytes:4147 (4.0 KiB)

In my /etc/named.conf I want to configure an ACL to disallow the 10.0.4.0/24 network access to my BIND nameserver.

https://access.redhat.com/documentation/en-US/Red_Hat_Enterprise_Linux/3/html/Reference_Guide/s1-bind-namedconf.html

How can I verify that hosts from 10.0.4.0/24 cannot access the nameserver using network tools? Because I will still leave Google's nameserver in /etc/named.conf, so I am assuming if they can't use the BIND nameserver, they will use 8.8.8.8 for name resolution..

Answer 1

Assuming that 10.0.2.15 is your DNS, from any of the servers in 10.0.4.0/24 network, use the following command to verify this:

dig www.google.com @10.0.2.15

This command will try to resolve google.com (or any hostnames you want to try) using the nameserver 10.0.2.15. If the server inside the 10.0.4.0/24 network is unable to resolve the hostname with this command, it means they are blocked from using this particular nameserver.

P.S: In the output of the dig, the ANSWER SECTION section will be empty or the ANSWER SECTION itself won't be seen in the output if 10.0.4.0/24 is not allowed to use that nameserver.