A useful name gives people and software a reference they can carry across an address change. That reference needs a defined scope and owner. A machine’s local hostname, a service’s DNS name, and an instance identifier may intentionally differ. Naming them clearly helps operators understand logs and deployments without pretending that one label automatically establishes every other identity in the system. ...
DNS supports both UDP and TCP. A server sets the TC flag when a response is truncated for the available message space. A client receiving that flag must handle the incomplete result, commonly by retrying over TCP. This is about the DNS response limit in use, not proof that the physical network cannot carry any larger UDP datagram. ...
Discovering A and AAAA records provides possible destinations. It does not select a successful connection by itself. Address ordering, source-address selection, routing, and the application’s connection algorithm all contribute. RFC 6724 describes default source and destination selection rules, while applications can impose their own choices. A program requesting only IPv4 has already narrowed the result before any dual-stack connection strategy can act. ...
EDNS extends DNS through an OPT pseudo-record. Among other information, a request can advertise the UDP payload size its sender is prepared to receive. This allows answers beyond the traditional 512-byte DNS limit. It is an advertisement within a DNS exchange, not a network path-MTU discovery protocol or a promise that every intervening device will deliver a datagram of that size. ...
DNS can return useful information by saying that data is absent. NXDOMAIN reports a name error; NODATA describes a successful response without the requested record type. In a direct query without aliases, these distinguish a nonexistent name from an existing name that lacks the requested data. Caching those results avoids repeating the same unsuccessful upstream lookup for every client request. ...
DNSSEC adds origin authentication and integrity checking for DNS data. RRSIG records sign RRsets, DNSKEY records publish verification keys, and a parent’s DS record identifies a child DNSKEY through a digest. The parent signs its DS RRset; it does not simply sign the child’s public key directly. Validation follows these relationships from a configured trust anchor to the requested data. ...
A hostname lookup starts with the program making it. On Linux, an application using glibc name-service interfaces can consult the services listed for hosts in /etc/nsswitch.conf. A program with its own DNS client may take a different path. There is no universal sequence in which every application checks a cache, then a hosts file, then the internet. ...
Prepare the TTL ahead of time, observe authoritative and recursive answers, and keep both endpoints valid through the cache window.