#!/usr/bin/perl

# usage in squid.conf :
# url_rewrite_program /usr/local/bin/squidredirect.pl

# maybe you have to allow this in apparmour

# choose your fighter, e.g. this one: http://www.surbl.org/lists#multi 

# define the $redir 

use strict;
use File::Temp qw/ tempfile tempdir /;
use Net::DNS;
use Sys::Syslog;

my $bl = '.multi.surbl.org';                                            # DNSBL to use
my $ns = '127.0.0.1';                                                   # NameServer to use, preferably your local caching named
my $redir = your replacement URL ;                                      # Block-URL to use, maybe a PNG just in case its just a pic that's blocked
my $res = Net::DNS::Resolver->new;                                      # resolver

my $url;                                                                # URL to check, later stripped down to hostname
my $rest;                                                               # not needed other infos we get from Squid
my $question;                                                           # question to DNS
my $query;                                                              # query-result from DNS
my $answer;                                                             # answer to Squid

openlog('squidredir', 'pid', 'local0');                                 # incude PID because Squid starts a bunch of us 
$res->nameservers($ns);                                                 # contact NameServer
$| = 1;                                                                 # line buffered
syslog("info", "starting");                                                             

while (<>){                                                             # for evey request from Squid
        chomp;
        syslog("debug", $_);
        ($url, $rest) = split(/ /, $_);                                 # first column is the requested target URL

        $url =~ s/^https?:// ;                                          # drop scheme
        $url =~ s:^[/]{2}:: ;                                           # drop / slashes 
        $url =~ s:/.*$:: ;                                              # drop path
        $url =~ s/:.*$// ;                                              # drop port

        $answer = "OK";                                                 # set default answer to Squid
        $question = $url . $bl;                                         # define the questions to DNS
        $query = $res->search($question);                               # ask DNSBL

        if ($query) {                                                   # we got an answer
                foreach my $rr ($query->answer) {                       # go through all parts
                        next unless $rr->type eq "A";                   # skip all but A-entris (normal IPv4)
                        $answer = "OK status=301 url=\"$redir\"";       # all other answers mean 'is a bad host' -> Redirect
                }
        }
        
        syslog("info", "$question $answer");
        print "$answer\n";                                              # send answer back to Squid
}

syslog("info", "stopping");