resolvectl:tldr:f2ca3

The command resolvectl tlsa tcp ${domain}:443 is used to query some DNS (Domain Name System) information related to a specified domain over a secure connection using TCP.

Let's break it down:

• resolvectl: It is a command-line utility tool used for managing and troubleshooting DNS settings in systemd-resolved, which is a system service that handles DNS lookups on Linux systems.
• tlsa: It is an option or argument used with the resolvectl command, specifying that the query should be performed using the DNS-based Authentication of Named Entities (DANE) Transport Layer Security (TLS) certificate association.
• tcp: It is another option or argument used with resolvectl, specifying that the query should be done over TCP (Transmission Control Protocol). TCP is a reliable and connection-oriented protocol, commonly used for sending and receiving data over a network.
• ${domain}:443: It represents a placeholder for the domain name and port number to be queried. The $ denotes a variable, and domain is the variable name. The :443 is the port number that typically corresponds to HTTPS (Hypertext Transfer Protocol Secure) communication.

So, the complete command is essentially asking resolvectl to perform a TLSA query over TCP for the specified domain on port 443 (usually associated with HTTPS).