I have a BIND9 server. I can query it and telnet to it from one system. It will not respond at all when this is attempted from another system on the same subnet/VLAN. It behaves as if the packets are being dropped.

With just a simple telnet, I can see, with tcpdump, that the packets are being received from this system, but I do not understand why the server never responds. I have already restarted the non-working client system. To minimize interference, I have also disabled that client's local iptables.

For reference, the ACL of the BIND9 server is "any" and iptables is permitting UDP/TCP on 53 from any source. iptables is totally disabled on the non-working client system (by way of UFW).

The server is Ubuntu 16.04 . The working client system is 17.04 . The non-working client system is 14.04 .


  • Check your firewall. – Michael Hampton Dec 13 '17 at 17:41
  • It's already disabled. Forgot to mention. Updated. – Dustin Oprea Dec 13 '17 at 19:25
  • Not the client system, you need to check the firewall on the server. And any intermediate firewalls between them. – Michael Hampton Dec 13 '17 at 19:32
  • No firewalls/gateways in-between. I've mentioned that other clients work and that I can see the traffic from the non-working one on the server. – Dustin Oprea Dec 13 '17 at 21:37
  • OK, now actually go check the firewall on the server. The fact that you can see packets with tcpdump doesn't mean they aren't being dropped by the firewall. – Michael Hampton Dec 13 '17 at 22:51

Even though I had restarted my client system to see if that would help, which it didn't, I was suddenly able to resolve DNS queries against the server. Something must've been out of whack and fell into place on its own.

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