#! /usr/bin/perl
#
#	$Id$

use strict;
use warnings;
use Getopt::Long;
require IO::Socket::INET;
require Net::DNS::Packet;
use Fcntl;
use Carp qw(cluck confess);
use Socket;


my $D;
my $opt_addr;
my $opt_port=5353;
my $opt_forward_addr="localhost";
my $opt_forward_port=53;
my $opt_spoof_ip="192.196.60.1";
die if !GetOptions(
		"a|addr=s",\$opt_addr,
		"p|port=s",\$opt_port,
		  "forward-addr=s",\$opt_forward_addr,
		  "forward-port=s",\$opt_forward_port,
		"i|spoof-ip=s",\$opt_spoof_ip,
		"d|debug+",\$D,
		);
my $forward_host=gethostbyname($opt_forward_addr) or die "resolving $opt_forward_addr: $!";
my $forward_addr=sockaddr_in($opt_forward_port,$forward_host) or die "assembling $opt_forward_addr:$opt_forward_port";

my $sock_udp=IO::Socket::INET->new(
		LocalAddr=>$opt_addr,
		LocalPort=>$opt_port,
		Proto=>"udp",
		) or die "socket(): $!";
my $sock_udp_priv=IO::Socket::INET->new(
		Proto=>"udp",
		) or die "socket(): $!";
###fcntl($sock_udp,F_SETFL,O_NONBLOCK) or die "fnctl(sock_udp,F_SETFL,O_NONBLOCK)";

sub id_next()
{
	our $id;
	$id=int(rand(0x10000)) if !defined $id;
	$id++;
	$id&=0xFFFF;
	return $id;
}

my %sent;

sub got_query($$)
{
my($msg,$from_addr)=@_;

	my $query=Net::DNS::Packet->new(\$msg);
	my $query_id_orig=$query->header()->id();
	my $query_id=id_next();
	$query->header()->id($query_id);
	my $msg_forward=$query->data();
	$query->header()->id($query_id_orig);	# fix-up back
	$sent{$query_id}={
		"msg"=>$msg,
		"from_addr"=>$from_addr,
		"query"=>$query,
		};
	warn "registered pending query id $query_id..." if $D;
	warn "sending forwarded DNS query (mapped id $query_id_orig -> $query_id)..." if $D;
	send $sock_udp_priv,$msg_forward,0,$forward_addr or cluck "send(): $!";
}

sub check_spoofable($$)
{
my($reply,$query)=@_;

	warn "checking packet spoofability from the reply... (non-spoofable if silent)" if $D;
	my @questions=$query->question();
	return 0 if 1!=@questions;
	my $question=$questions[0];
	return 0 if $question->qtype() ne "A"
	         && $question->qtype() ne "ANY";
	return 0 if $reply->header()->rcode() ne "NXDOMAIN";
	warn "packet considered as spoofable." if $D;
	return 1;
}

sub reply_spoof($$)
{
my($query_from_addr,$query)=@_;

	warn "assembling spoof reply..." if $D;
	my $question=($query->question())[0];
	my $spoof=Net::DNS::Packet->new($question->qname(),$question->qclass(),$question->qtype());
	$spoof->push("answer",Net::DNS::RR->new(join(" ",
			$question->qname(),
			3600,	# ttl
			$question->qclass(),
			"A",	# qtype
			$opt_spoof_ip,	# rdata
			)));
	$spoof->header()->rcode("NOERROR");
	$spoof->header()->aa(0);
	$spoof->header()->qr(1);
	$spoof->header()->ra(1);
	$spoof->header()->rd($query->header()->rd());
	$spoof->header()->id($query->header()->id());
	my $msg_spoof=$spoof->data();
	warn "sending spoof reply..." if $D;
	send $sock_udp,$msg_spoof,0,$query_from_addr or cluck "send(): $!";
}

sub got_forward_reply($)
{
my($msg)=@_;

	warn "parsing reply from our forwarded DNS..." if $D;
	my $reply=Net::DNS::Packet->new(\$msg);
	my $reply_id=$reply->header()->id();
	my($query_msg,$query_from_addr,$query);
	if (my $hash=$sent{$reply_id}) {
		delete $sent{$reply_id};
		$query_msg      =$hash->{"msg"};
		$query_from_addr=$hash->{"from_addr"};
		$query          =$hash->{"query"};
		warn "deleted pending record with id $reply_id." if $D;
		}
	else {
		warn "Got DNS reply for unknown packet id $reply_id";
		return;
		}
	if (check_spoofable($reply,$query)) {
		reply_spoof $query_from_addr,$query;
		return;
		}
	$reply->header()->id($query->header()->id());
	my $reply_back=$reply->data();
	warn "parsing reply back to the original query host..." if $D;
	send $sock_udp,$reply_back,0,$query_from_addr or cluck "send(): $!";
}

for (;;) {
	my $rfds="";
	vec($rfds,fileno($sock_udp),1)=1;
	vec($rfds,fileno($sock_udp_priv),1)=1;
	warn "select(2)..." if $D;
	my $got=select $rfds,undef(),undef(),undef();
	warn "got from select." if $D;
	die "Invalid select(2): ".Dumper($got) if !defined $got || $got<0;

	for my $sock ($sock_udp,$sock_udp_priv) {
		my $msg;
		defined(my $from_addr=recv $sock,$msg,0x1000,0) or do { cluck "recv(): $!"; next; };
		warn "got packet." if $D;
		my($from_addr_port,$from_addr_host)=sockaddr_in($from_addr);

		if ($from_addr_host eq $forward_host && $from_addr_port eq $opt_forward_port) {
			warn "packet returned from forwarding DNS..." if $D;
			if ($sock eq $sock_udp) {
				warn "packet returned from forwarding DNS forbidden from the main listening socket";
				next;
				}
			got_forward_reply $msg;
			}
		else {
			warn "original query..." if $D;
			if ($sock eq $sock_udp_priv) {
				warn "original query forbidden from the private forwarding socket";
				next;
				}
			got_query $msg,$from_addr;
			}
		}
	}