dns: deal with --dhcp-options when --dns is active

dns: deal with --dhcp-options when --dns is active

Since --dns settings overrule DNS related --dhcp-options,
remove the latter when values were defined via --dns.

To stay as backward compatible as possible, we add foreign_options to
the script hook environment from the --dns values when a --up script
is defined. In that case the default --dns-updown is not run, even
when --dns values are present, to prevent double DNS configuration.
This way an existing --up script that deals with DNS can run, without
the immediate need to change after an openvpn upgrade and a server
pushing --dns options.

If you specify a custom --dns-updown, or force running the default
dns-updown that comes with openvpn, those compat env vars are not set
for --up scripts and the dns-updown command is run, even when there's
an --up script present.

Since Android uses the DNS values from tuntap_options, we always
override those with --dns stuff unconditionally. Also on Windows when
--ip-win32 is dynamic or adaptive, since DHCP relies on these as well.

Change-Id: I635c4018fb43b5976a39b6a90cb2e9cb2570cd6a
Signed-off-by: Heiko Hund <heiko@ist.eigentlich.net>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <20250618124835.24737-1-gert@greenie.muc.de>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg31922.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>