If you use any DNS masking/caching service that is run on your local machine, it will hide the real DNS servers. – karatedog Sep 7 '15 at 9:12 7 Ubuntu 18.04 just shows the local dns cache: SERVER: 127.0.0.53#53(127.0.0.53) – wisbucky Nov 14 '18 at 2:16
Regardless of the distribution of Linux that you are using (such as Ubuntu, CentOS, Debian, Arch, Redhat, etc), the process of determining what DNS servers are currently being used for domain name resolution is the same. To determine what DNS servers are being used, you simply need to view the contents of the “/etc/resolv.conf” file. Linux for freshers: DNS Server Interview Questions And DNS servers use port 53 by default. Incoming and outgoing packets should be allowed onport 53. Also allow connections on port 921 if you configure a lightweight resolver server.The DNS control utility, rndc, connects to the DNS server with TCP port 953 by default. How to Set DNS Nameservers on Ubuntu 18.04 - Linux
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resolv.conf isn't really used anymore, unless you implement it yourself. The network manager does it now. I created an alias to list the DNS servers on my system, as I sometimes switch from OpenDNS to Google's open DNS. Best free and public DNS servers in 2020 | TechRadar Jun 18, 2020 DNS Servers: What Are They and Why Are They Used?