Get PLAT prefix from systemd-networkd, if possible

Makes clatd check if systemd-networkd is aware of any PLAT prefix (which
it may have learned from the PREF64 Router Advertisement option, cf. RFC
8781).

If a prefix is obtained from systemd-network, DNS64-based PLAT prefix
discovery is skipped, as mandated by
https://datatracker.ietf.org/doc/draft-ietf-v6ops-prefer8781/.

However, if the dns64-servers config option is set, clatd will use
DNS64-based PLAT prefix discovery towards the specified servers, and it
will not query systemd-networkd at all.

Closes #32

Makefile

@@ -7,11 +7,9 @@ DNF_OR_YUM:=$(shell which dnf || which yum)
 SYSTEMCTL:=$(shell which systemctl)
 TAYGA:=$(shell which tayga)
 
-all:
-
 install:
 	# Install the main script
-	install -D -m0755 clatd $(DESTDIR)$(PREFIX)/sbin/clatd
+	install -m0755 clatd $(DESTDIR)$(PREFIX)/sbin/clatd
 	# Install manual page if pod2man is installed
 	pod2man --name clatd --center "clatd - a CLAT implementation for Linux" --section 8 README.pod $(DESTDIR)$(PREFIX)/share/man/man8/clatd.8 && gzip -f9 $(DESTDIR)$(PREFIX)/share/man/man8/clatd.8 || echo "pod2man is required to generate manual page"
 	# Install systemd service file if applicable for this system

clatd

@@ -27,7 +27,7 @@ use strict;
 use IPC::Cmd qw(can_run);
 use Net::IP;
 
-my $VERSION = "2.1.0";
+my $VERSION = "2.0.0";
 
 #
 # Populate the global config hash with the default values
@@ -357,8 +357,7 @@ sub get_plat_prefix_from_dns64 {
     $res->nameservers(map {
       Net::IP->new($_)->version() == 4 ? "::ffff:$_" : $_;
     } $res->nameservers);
-    d2("Nameservers after Net::DNS bug workaround: ",
-       join(" ", $res->nameservers));
+    d2("Nameservers after Net::DNS bug workaround: ", $res->nameservers);
 
     my $pkt = $res->query('ipv4only.arpa', 'AAAA');
     if(!$pkt) {
@@ -392,7 +391,7 @@ sub get_plat_prefix_from_dns64 {
     p("No PLAT prefix could be discovered, using fallback");
     return $CFG{"plat-fallback-prefix"};
   } else {
-    p("No PLAT prefix could be discovered. Your connection probably doesn't provide",
+    p("No PLAT prefix could be discovered. Your ISP probably doesn't provide",
       " NAT64/DNS64 PLAT service. Exiting.");
     cleanup_and_exit(0);
   }
@@ -1056,13 +1055,13 @@ if(cfgint("ctmark")) {
     or err("'nft -f-' failed to execute");
   print $fd "add table ip6 clatd\n";
   print $fd "add chain ip6 clatd prerouting ",
-            "{ type filter hook prerouting priority mangle; }\n";
+            "{ type filter hook prerouting priority 0; }\n";
   print $fd "add rule ip6 clatd prerouting",
             " iif ", cfg("clat-dev"),
             " ip6 saddr ", cfg("clat-v6-addr"),
             " ip6 daddr ", cfg("plat-prefix"),
             " ct mark set ", cfgint("ctmark"),
-            # set meta mark as well, to placate firewalld's IPv6_rpfilter and NixOS' rpfilter rules
+            # set meta mark as well, to placate firewalld's IPv6_rpfilter
             " meta mark set ", cfgint("ctmark"), " counter\n";
   print $fd "add rule ip6 clatd prerouting",
             " iif ", cfg("plat-dev"),