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
@@ -42,7 +42,6 @@ $CFG{"clat-v4-addr"} = "192.0.0.1";	# from RFC 7335
 $CFG{"clat-v6-addr"} = "shared";	# re-use primary address from host OS
 $CFG{"dns64-servers"} = undef;		# use system resolver by default
 $CFG{"cmd-ip"} = "ip";			# assume in $PATH
-$CFG{"cmd-ipxlat-ctl"} = "ipxlat-ctl";	# assume in $PATH
 $CFG{"cmd-networkctl"} = "networkctl";	# assume in $PATH
 $CFG{"cmd-nft"} = "nft";		# assume in $PATH
 $CFG{"cmd-tayga"} = "tayga";		# assume in $PATH
@@ -63,7 +62,6 @@ $CFG{"v4-defaultroute-replace"} = 0;	# replace existing v4 defaultroute?
 $CFG{"v4-defaultroute-metric"} = 2048;	# metric for the IPv4 defaultroute
 $CFG{"v4-defaultroute-mtu"} = 1260;	# MTU for the IPv4 defaultroute
 $CFG{"v4-defaultroute-advmss"} = 0;	# TCP MSS for the IPv4 defaultroute
-$CFG{"xlat-engine"} = undef;		# which translation engine to use
 
 
 #
@@ -359,8 +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) {
@@ -394,7 +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);
   }
@@ -668,7 +665,6 @@ sub get_clat_v6_addr {
 #
 my $cleanup_remove_tayga_clat_dev;	# true if having created it
 my $cleanup_remove_nat46_clat_dev;	# true if having created it
-my $cleanup_remove_ipxlat_clat_dev;	# true if having created it
 my $cleanup_delete_taygaconf;		# true if having made a temp confile
 my $cleanup_zero_forwarding_sysctl;	# zero forwarding sysctl if set
 my @cleanup_accept_ra_sysctls;		# accept_ra sysctls to be reset to '1'
@@ -700,9 +696,6 @@ sub cleanup_and_exit {
     print $nat46_control_fh "del ", cfg("clat-dev"), "\n";
     close($nat46_control_fh) or err("close($nat46_control_fh: $!");
   }
-  if(defined($cleanup_remove_ipxlat_clat_dev)) {
-    cmd(\&w, cfg("cmd-ip"), qw(link delete dev), cfg("clat-dev"));
-  }
   if(defined($cleanup_zero_forwarding_sysctl)) {
     d("Cleanup: Resetting forwarding sysctl to 0");
     sysctl("net/ipv6/conf/all/forwarding", 0);
@@ -913,31 +906,17 @@ if(cfgbool("v4-conncheck-enable") and !cfgbool("v4-defaultroute-replace")) {
   d("Skipping IPv4 connectivity check at user request");
 }
 
-
+# Let's figure out if there's nat46 kernel module loaded
 my $nat46_controlfile = "/proc/net/nat46/control";
+my $use_nat46 = (-e $nat46_controlfile);
 
-#
-# Auto-detect which translation engine to use if not specified in config
-if(!cfg("xlat-engine") and (-e $nat46_controlfile)) {
-  p("Using translation engine: nat46");
-  $CFG{"xlat-engine"} = "nat46";
-} elsif(!cfg("xlat-engine") and can_run(cfg("cmd-ipxlat-ctl"))) {
-  p("Using translation engine: ipxlat");
-  $CFG{"xlat-engine"} = "ipxlat";
-} elsif(!cfg("xlat-engine") and can_run(cfg("cmd-tayga"))) {
-  p("Using translation engine: TAYGA");
-  $CFG{"xlat-engine"} = "tayga";
-} elsif(!cfg("xlat-engine")) {
-  err("No supported translation engine available. Please install TAYGA or ensure ",
-      "either the nat46 or ipxlat kernel modules is loaded.");
-}
 
 
 #
 # Write out the TAYGA config file, either to the user-specified location,
 # or to a temporary file (which we'll delete later)
 #
-if(cfg("xlat-engine") eq "tayga") {
+unless($use_nat46) {
   my $tayga_conffile = cfg("tayga-conffile");
   my $tayga_conffile_fh;
   if(!$tayga_conffile) {
@@ -1047,17 +1026,14 @@ close($fd) or err("'ip -6 rule show prio 0 table local' failed");
 # route to the corresponding IPv6 address, and possibly an IPv4 default route
 #
 p("Creating and configuring up CLAT device '", cfg("clat-dev"), "'");
-if(cfg("xlat-engine") eq "nat46") {
+if($use_nat46) {
   my $nat46_control_fh;
   open($nat46_control_fh, ">$nat46_controlfile") or
       err("Could not open nat46 control socket for writing");
   print $nat46_control_fh "add ", cfg("clat-dev"), "\n";
   close($nat46_control_fh) or err("close($nat46_control_fh: $!");
   $cleanup_remove_nat46_clat_dev = 1;
-} elsif(cfg("xlat-engine") eq "ipxlat") {
-  cmd(\&err, cfg("cmd-ip"), qw(link add name), cfg("clat-dev"), qw(type ipxlat));
-  $cleanup_remove_ipxlat_clat_dev = 1;
-} elsif(cfg("xlat-engine") eq "tayga") {
+} else {
   cmd(\&err, cfg("cmd-tayga"), "--config", cfg("tayga-conffile"), "--mktun",
       cfgint("debug") ? "-d" : "");
   $cleanup_remove_tayga_clat_dev = 1;
@@ -1079,13 +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 $fd "add rule ip6 clatd prerouting",
             " iif ", cfg("plat-dev"),
@@ -1139,7 +1115,7 @@ if(cfg("script-up")) {
 # All preparation done! We can now start nat46 or TAYGA, which will handle the actual
 # translation of IP packets.
 #
-if(cfg("xlat-engine") eq "nat46") {
+if($use_nat46){
   p("Setting up nat46 kernel module");
   my $nat46_control_fh;
   open($nat46_control_fh, ">$nat46_controlfile") or
@@ -1155,13 +1131,7 @@ if(cfg("xlat-engine") eq "nat46") {
   $SIG{'INT'} = \&cleanup_handler;
   $SIG{'TERM'} = \&cleanup_handler;
   sleep();
-} elsif(cfg("xlat-engine") eq "ipxlat") {
-  cmd(\&err, cfg("cmd-ipxlat-ctl"), cfg("clat-dev"), "pool6", cfg("plat-prefix"));
-  # Nothing more to do here, we just set up a cleanup handler and sleep forever.
-  $SIG{'INT'} = \&cleanup_handler;
-  $SIG{'TERM'} = \&cleanup_handler;
-  sleep();
-} elsif(cfg("xlat-engine") eq "tayga") {
+} else {
   my $tayga_conffile = cfg("tayga-conffile");
   p("Starting up TAYGA, using config file '$tayga_conffile'");