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	my $e_nexthop_s = inet_ntoa($e_nexthop);
	
	# Validate the route
	my($result, $reason) = validate_route($e_net_i, $e_net_s, $e_netmask_i, $e_netmask_s, $e_nexthop_s);
	if (!$result) {
		warn "entry ignored ($reason): af $e_af rtag $e_rtag $e_net_s/$e_netmask_s via $e_nexthop_s metric $e_metric\n" if ($verbose);
		return 0;
	}
	
	warn "entry: af $e_af rtag $e_rtag $e_net_s/$e_netmask_s via $e_nexthop_s metric $e_metric\n" if ($verbose > 1);
	
	# Ok, we have a valid route, consider adding it in the kernel's routing table
	consider_route($e_net_s, $e_netmask_s, $e_nexthop_s, $e_rtag);
	
	return 1;
}

# process a RIP message

sub process_msg($$$)
{
	my($addr_s, $perr_port, $msg) = @_;
	
	# validate packet's length
	if (length($msg) < RIP_HDR_LEN + RIP_ENTRY_LEN) {
		warn "$me: ignored too short packet from $addr_s: " . length($msg) . "\n";
		return -1;
	}
	
	if (length($msg) > RIP_HDR_LEN + RIP_ENTRY_LEN*25) {
		warn "$me: ignored too long packet from $addr_s: " . length($msg) . "\n";
		return -1;
	}
	
	# packet's length must be divisible by the length of an entry
	if ((length($msg) - RIP_HDR_LEN) % RIP_ENTRY_LEN != 0) {
		warn "$me: ignored invalid length packet from $addr_s: " . length($msg) . "\n";
		return -1;
	}
	
	# validate RIP packet header
	my $hdr = substr($msg, 0, 4);
	my $entries = substr($msg, 4);
	
	my($rip_command, $rip_version, $zero1, $zero2) = unpack('C*', $hdr);
	if ($rip_command != RIP_CMD_RESPONSE) {
		warn "$me: ignored non-response RIP packet from $addr_s\n";
		return -1;
	}
	if ($rip_version != 2) {
		warn "$me: ignored RIP version $rip_version packet from $addr_s (only accept v2)\n";
		return -1;
	}
	if ($zero1 != 0 || $zero2 != 0) {
		warn "$me: ignored RIP packet from $addr_s: zero bytes are not zero in header\n";
		return -1;
	}
	
	my $init_msg = 0;
	
	# if password auth is required, require it!
	if (defined $rip_passwd) {
		return -1 if (!process_rip_auth_entry(substr($entries, 0, RIP_ENTRY_LEN)));
		$init_msg += RIP_ENTRY_LEN;
	}
	
	# Ok, process the actual route entries
	my $routes = 0;
	for (my $i = $init_msg; $i < length($entries); $i += RIP_ENTRY_LEN) {
		my $entry = substr($entries, $i, RIP_ENTRY_LEN);
		my $n = process_rip_route_entry($entry);
		return -1 if ($n < 0);
		$routes += $n;
	}
	
	return $routes;
}

#
####### main #############################################
#

# command line parsing
my %opts;
getopts('i:p:a:vd', \%opts);

if (defined $opts{'i'}) {
	$tunnel_if = $opts{'i'};
}
if (defined $opts{'p'}) {
	$rip_passwd = $opts{'p'};
}
if ($opts{'v'} && !$verbose) {
	$verbose = 1;
}
if ($opts{'d'}) {
	$verbose = 2;		
}
if ($opts{'a'}) {
	foreach my $a (split(',', $opts{'a'})) {
		$my_addresses{$a} = 1;
	}
}

fill_local_ifs();

# Enable multicast on the tunnel interface, the flag is
# not set by default
system($ifconfig, $tunnel_if, 'multicast') == 0 or die "ifconfig $tunnel_if multicast failed: $?\n";

# Create the UDP multicast socket to receive RIP broadcasts
warn "opening UDP socket...\n" if ($verbose);
my $socket = IO::Socket::Multicast->new(
	LocalPort => 520,
	ReuseAddr => 1,
) or die $!;

$socket->mcast_add('224.0.0.9', $tunnel_if) or die $!;

my $expire_interval = 60*60;
my $next_expire = time() + $expire_interval;