Support domain names and port numbers in dns_nameservers

[yadij]

Upgrade the DNS nameserver configuration parser
to better validate IP/hostname labels.

Add support for IPv6 with brackets and optional
port values. These are not valid for /etc/resolv.conf
content, but may be used by admin in squid.conf
dns_nameservers and Windows registry settings.

Add support for named servers. OS resolver is used
to lookup the configured host name. Currently only
one IP address per host is supported.

Also, polish cache.log output to avoid false claims
of adding a nameserver which is then ignored.
Configuration errors are now displayed as "ERROR"
and each added entry has its origin name/label
logged on the same output line.

[yadij]

Old cache.log would show:

2026/02/18 22:08:12 kid1| DNS IPv6 socket created at [::], FD 6
2026/02/18 22:08:12 kid1| DNS IPv4 socket created at 0.0.0.0, FD 7
2026/02/18 22:08:12 kid1| Adding nameserver ::1 from squid.conf
2026/02/18 22:08:12.271 kid1| 78,3| /dns_internal.cc(349) idnsAddNameserver: Added nameserver #0 ([::1]:53)
2026/02/18 22:08:12 kid1| Adding nameserver [::2] from squid.conf
2026/02/18 22:08:12 kid1| WARNING: rejecting '[::2]' as a name server, because it is not a numeric IP address
2026/02/18 22:08:12 kid1| Adding nameserver 127.0.0.1:52 from squid.conf
2026/02/18 22:08:12 kid1| WARNING: rejecting '127.0.0.1:52' as a name server, because it is not a numeric IP address
2026/02/18 22:08:12 kid1| Adding nameserver [::3]:81 from squid.conf
2026/02/18 22:08:12 kid1| WARNING: rejecting '[::3]:81' as a name server, because it is not a numeric IP address
2026/02/18 22:08:12 kid1| Adding nameserver example.com:56 from squid.conf
2026/02/18 22:08:12 kid1| WARNING: rejecting 'example.com:56' as a name server, because it is not a numeric IP address

new cache.log looks like:

2026/02/18 21:43:57 kid1| DNS IPv6 socket created at [::], FD 6
2026/02/18 21:43:57 kid1| DNS IPv4 socket created at 0.0.0.0, FD 7
2026/02/18 21:43:57 kid1| Added nameserver #0 ([::1]:53) from squid.conf dns_nameservers ::1
2026/02/18 21:43:57 kid1| Added nameserver #1 ([::2]:53) from squid.conf dns_nameservers  [::2]
2026/02/18 21:43:57 kid1| Added nameserver #2 (127.0.0.1:52) from squid.conf dns_nameservers 127.0.0.1:52
2026/02/18 21:43:57 kid1| Added nameserver #3 ([::3]:81) from squid.conf dns_nameservers [::3]:81
2026/02/18 21:43:57 kid1| Added nameserver #4 (104.18.26.120:56) from squid.conf dns_nameservers example.com:56

[rousskov]

I adjusted PR title to more prominently disclose that this PR adds domain name support and to mention the affected directive name for context.

I plan to come back to this PR once the backlog is dealt with.

[rousskov]

Before we make this code more complex, I recommend changing/fixing this code error handling so that configuration errors trigger an exception (and, hence, become fatal at startup). That change should be done in a dedicated prerequisite PR. It will affect how this code is structured and called/used.

[rousskov]

Please add a test for multiple dns_nameservers parameters on the same directive line.

[rousskov]

Please adjust dns_nameservers description in cf.data.pre to avoid the current possible implication that port numbers are not supported and to disclose that domain names are supported (in addition to IP addresses).

It is probably a good idea to also note

• that domain names specified in dns_nameservers are resolved without using (previously configured) dns_nameservers;
• how repeated/duplicate dns_nameservers directive entries are interpreted;
• how multiple dns_nameservers directives are interpreted; and
• what affect does the order of dns_nameservers` directive entries have.

Edit: GitHub hit this change request while I was writing my review, so I added another, similar one. I deleted the latter after I noticed that this change request was resurrected.

[rousskov]

Please do not add a yet another code snippet parsing host[:port].

[rousskov]

This code is not performance-sensitive enough for this XXX. I recommend removing it:

Suggested change

// XXX: performance regression. c_str() copies

[rousskov]

Please use std::optional<AnyP::KnownPort> instead of a magic number to handle unspecified ports.