Validate settings

[Martin Kealey]

Validate the fields and give warnings when validations fail.

In particular:

• validate the hostname field by looking in the DNS for an A or AAAA record.
  • give a warning
  • on a device with no routable IPv6 address, give a warning if the hostname has no A record,
  • on a device with no routable IPv4 address, give a warning if the hostname has no AAAA record
• validate the port number field as being a positive 16-bit integer (between 1 and 65535).

Among other things, this will detect the case where people try to put a URI instead of a hostname (as exemplified by #328).

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Future enhancement: also permit a name that (when prefixed by _irc._tcp. or _ircs._tcp) has a DNS SRV record. (This would require additional logic when connecting.)

These give a list of priority, weight, hostname, port tuples:

$ dig +short srv _irc._tcp.example.chat.
 1 100 6667 loadbalancer.irc.example.chat.
 2  20  194 host1.irc.example.chat.
 2  80  194 bighost.irc.example.chat.
 3 100 7000 fallback.irc.example.chat.
$ dig +short srv _ircs._tcp.example.chat.
 1 100 6697 loadbalancer.irc.example.chat.
 2  40 6697 host1.irc.example.chat.
 2  20 6697 host2.irc.example.chat.
 2  20 6697 host3.irc.example.chat.
 2  20 6697 host4.irc.example.chat.
 3 100 7001 fallback.irc.example.chat.
$ dig +short srv _irc._tcp.libera.example.lookaside.
 1   1 6667 irc.libera.chat.
$ dig +short srv _ircs._tcp.libera.example.lookaside.
 1   1 6697 irc.libera.chat.