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 .