FreeFritzBox's DNS

README.md

Free choice of DNS in malign devices

Some cable providers are blocking the option to change the DNS in the provided routers. This villain-esque behaviour alone justifies the circumvention of this cyber-terrorism.

Why exactly would you want to change the DNS?

• It's blazingly fast! (Often times the provider's DNS is super slow)
• It's secure.
• No censorship.
• No user tracking.

I looked at the webpage of the CCC to get an overview of free (as in speech) DNS server.

But how are you going to change the DNS at a FritzBox, which has this very option blocked?

1. Export the settings.
2. Open the .export file in your favorite text editor. It's similar to JSON.
3. Look for the lines starting with overwrite_dns1 and overwrite_dns2.
4. Use the DNS you want.
5. Save the fi... Not so fast! Every FritzBox config has a checksum. The one in the file is now invalid.
6. To generate a new cheksum, run the perl script from above, like ./FritzBoxChecksum.pl myExportedSettings.export
   It worked fine in WSL 1. (BTW: the ./ is a Linux security feature.)
7. Copy & Paste the generated checksum. Save the file. Upload it to your FritzBox. Wait for it's restart.
8. Now you can finally surf with inner peace.

---

I do not claim any authorship on the code.
The code is from David Tonhofer. Thanks, David!

FritzBoxChecksum.pl

#!/usr/bin/perl

# Read Fritz!Box configuration dump file and compute its checksum using CRC32.
# The problem is only knowing what to checksum exactly, and in this case its not pretty.
# Inspired from the very compact Visual Basic script by Michael Engelke available at
# http://www.mengelke.de/Projekte/FritzBoxVBScript 

# The Fritz!Box accepts a modified file where the checksum has been changed 
# manually to the output of this program in the last line. I have no idea what 
# happens if there is a syntax error anywhere inside the config file, so beware.

# THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
# IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
# FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
# AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
# LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
# OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
# THE SOFTWARE.

use strict;
use warnings;

# ---
# Compute CRC start values
# ---

sub build_crc_table() {
   my @crctbl = (); # Accepts 256 values

   for (my $a = 0; $a < 256; $a++) {
      my $c = $a;
      for (my $b = 0; $b < 8; $b++) {
         my $d = $c & 0x01;
         $c = ($c >> 1) & 0x7FFFFFFF;
         if ($d) { 
            $c = $c ^ 0xEDB88320;
         }
      }
      push @crctbl, $c
   }

   my $print = 0;

   if ($print) {
      my $i = 0;
      foreach my $x (@crctbl) {
         print sprintf("CRC table value $i: %08x\n", $x);
         $i++
      }
   }

   return @crctbl
}

# ---
# Transform a string into a vector of its ASCII code points
# ---

sub numerize {
   my ($str) = @_;
   my @res = ();
   foreach my $ch (split('',$str)) {
      push @res, ord($ch)
   }
   return @res;
}

# ---
# Transform a hexstring into a vector of its 8-bit values
# ---

sub hexnumerize {
   my ($str) = @_;
   my @res = ();
   my $tmp;
   my $i = 0;
   foreach my $ch (split('',$str)) {
      if ($i == 0) { $tmp = $ch; $i++; }
      else         { $tmp.= $ch; $i=0; push @res,hex($tmp) } 
   }
   if ($i != 0) {
      die "Irregular hex string: $str\n";
   }
   return @res;
}

# ---
# Compute CRC-32 checksum
# See https://en.wikipedia.org/wiki/Cyclic_redundancy_check
# This must yield 414fa339:
# print sprintf("%08Xi\n", compute_crc32(numerize("The quick brown fox jumps over the lazy dog")));
# ---

sub compute_crc32 {
   my(@data)  = @_;
   my @crctbl = build_crc_table(); # not very efficient on multiple calls
   my $crc = 0xFFFFFFFF;
   my $i = 0;
   foreach my $x (@data) {
      my $index = ($crc ^ $x) & 0xFF;
      $crc = $crctbl[$index] ^ ($crc >> 8); # ">>" is zero-filling in Perl
      $crc = $crc & 0xFFFFFFFF;             # format to 32 bit
      #if ($i > 98700 && $i < 99000) {
      #   print sprintf("$i %c : $x => %08X\n",$x,$crc); 
      #}
      $i++
   }
   return $crc ^ 0xFFFFFFFF
}

# ---
# The name of the configuration file may have been given on the command line. 
# If so slurp the file.
# If nothing has been given, slurp STDIN.
# After that, the input can be found in "@lines" (line terminators are still in there)
# ---

my @lines;

if ($ARGV[0]) {
   open(my $fh,'<',$ARGV[0]) or die "Could not open file '$ARGV[0]': $!";
   @lines = <$fh>;
   close($fh)
}
else {
   @lines = <>;
}

# ---
# Stateful analysis.
# If there are lines after "END OF EXPORT" we will disregard them
# ---

my $firmware;       # will capture firmware version
my $fritzbox;       # will capture name string in header
my @data = ();      # will capture 8-bit values to be CRC-checksummed later
my $cursum;         # will capture the current CRC checksum in the text

# "state" indicates where we are in the file

my $state = 'NOTSTARTED';

# we consider the lines as a stack and push/pop to the stack

@lines = reverse @lines;

while (@lines && $state ne 'END') {

   my $line = pop @lines;

   if ($state eq 'NOTSTARTED') {
      if ($line =~ /^\*{4} (.*?) CONFIGURATION EXPORT/) {
         $fritzbox = $1;
         $state    = 'HEADER'
      }
      else {
         chomp $line;
         die "Expected 'CONFIGURATION EXPORT' in NOTSTARTED state, got '$line'"
      }
      next
   }

   if ($state eq 'HEADER') {
      chomp $line;
      if ($line =~ /(\w+)=([\w\$\.]+)$/) {
         my $key = $1;
         my $val = $2;
         if ($key eq 'FirmwareVersion') { $firmware = $val }
         push @data,numerize($key);
         push @data,numerize($val);
         push @data,0;
      }
      elsif ($line =~ /^\*{4}/) {
         $state = 'NEXTSECTION';
         push @lines,$line
      }
      else {
         die "Unexpected line in HEADER state: '$line'"
      }
      next
   }

   if ($state eq 'NEXTSECTION') {
      chomp $line;
      if ($line =~ /^\*{4} (?:CRYPTED)?(CFG|BIN)FILE:(\S+)/) {
         $state = "INSIDESECTION_$1";
         my $secname = $2;
         print "Section $line\n";
         push @data,numerize($secname);
         push @data,0
      }   
      elsif ($line =~ /^\*{4} END OF EXPORT (\w{8}) \*{4}/) {
         $state  = 'END';
         $cursum = $1
      }
      else {
          die "Unexpected line in NEXTSECTION state: '$line'"
      }
      next
   }

   if ($state eq 'INSIDESECTION_BIN') {
      chomp $line;
      if ($line =~ /^\*{4} END OF FILE \*{4}/) {
         $state = 'NEXTSECTION'
      } 
      else {
         push @data,hexnumerize($line)
      } 
      next
   }

   if ($state eq 'INSIDESECTION_CFG') {
      if ($line =~ /^\*{4} END OF FILE \*{4}/) {
         # Something unbelievably dirty: All section-internal linefeeds (and carriage returns)
         # are kept as is for checksumming, except for the last one before the "END OF FILE"
         if ($data[-1] == ord("\n")) {
            pop @data;
            if ($data[-1] == ord("\r")) {
               pop @data;
            }
         }
         $state = 'NEXTSECTION'
      }
      else {
         # More dirty stuff: Double backspaces are replaced by single ones for some reason.
         $line =~ s/\\\\/\\/g;
         push @data,numerize($line)
      }
      next
   } 

   die "Unexpected state $state"

}

if ($state ne 'END') {
   die 'Did not find proper end of configuration'
}
if (@lines) {
   my $n = @lines;
   print "There are still $n lines left that will be disregarded"
}

my $crc = compute_crc32(@data);
my $newsum = sprintf("%08X", $crc);

print "$fritzbox with firmware $firmware\n";

if ($newsum eq $cursum) {
   print "Checksum is OK: $cursum\n"
}
else {
   print "Found new checksum: $newsum\n";
   print "Checksum embedded in file is $cursum\n"
}