DNS connects human-readable names to services. An A record points a name to an IPv4 address, while AAAA does the same for IPv6. A CNAME makes one name an alias of another.
MX selects mail servers and TXT carries verification or policy text. Keep TTL values short during a migration, then increase them after the destination is stable.
Before changing a record, capture its current value and check whether a proxy or CDN is changing the answer seen by clients.
REFERENCE
HTTP status codes worth remembering
200 means the request succeeded. 301 and 308 are permanent redirects; 302 and 307 are temporary. The difference between 302 and 307 is whether the request method is preserved.
401 means authentication is required, 403 means the server understood but refused, and 404 means no matching resource. 429 signals rate limiting. A 502 often points to an upstream problem, while 504 means that upstream took too long.
LINUX
A calm service-checking routine
Start with three questions: is the process running, is the expected port listening, and can a local client complete a request? Then inspect recent logs, restart counts and memory pressure.
Avoid changing configuration while the failure boundary is still unclear. Record the original listener, process ID and health response so the post-change check has a real baseline.
systemctl status service-name
ss -lntp
journalctl -u service-name --since "30 min ago"