DNS
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... port" attribute and an "fqdn"
attribute but no "srv" attribute, then "fqdn" is looked up as an A
record via DNS for translation to an IP number. An "ip4" attribute
is interpreted as the dotted-quad representation of an IPv4 address ...
... has a "srv" attribute, the concatenation of the "srv" attribute and
the "fqdn" attribute (with a period between) is looked up in the DNS
for a SRV record [6 ...
...
450 Requested action not taken
(E.g., DNS lookup failed or connection could not
be established. See too 550.)
...
... Gulbrandsen, A., Vixie, P. and L. Esibov, "A DNS RR for specifying the location of services (DNS SRV ...