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dnsa1.c - Exploit code for a denial of service attack using DNS (updated version)
From: zelea () IBM COM (Zelea)
Date: Mon, 28 Feb 2000 09:49:01 -0800


This program works by sending spoofed UDP packets to a list of
nameservers. The packets contain valid type ANY class IN queries.
The amplification effect comes from the fact that the query packet
is much smaller in size than the answer.
A list of queries is already included in this source. These queries
have been selected for the highest amplification ratio (currently
between 20-25) but you can provide your own list of queries in a
separate file. ICMP (udp port unreachable) packets are sent back
to the nameservers reducing even more the victim's bandwidth.

This program has a short delay at the start due to the time required
to resolve the nameserver's IPs. Then it sends each UDP query at full
speed to each nameserver from the list.

The nameservers file has one server per line.

To quickly build yourself a list of nameservers use the following
script after you have downloaded the domain inaddr.zone.gz file.

       #!/bin/sh
       ZONE=inaddr.zone;
       if [ "x$1x" == "xx"  ]; then
         echo "Usage: print_ns aprox_nr_of_servers";
         exit
       fi
       if [ ! -f $ZONE ]; then
         echo "Zone file $ZONE not found";
         exit
       fi
       NR=`wc -l $ZONE`;
       awk --assign=TOT="$NR" --assign=DES="$1" '
         BEGIN { srand(); th=DES/TOT; };
         /NS/  { if( rand() < th )
                 {
                   fi = split( $0, entry );
                   if( entry[fi-1] == "NS" )
                     serv[entry[fi]] = 1;
                 }};
         END   { for( ns in serv )
                   printf "%s\n", ns;
               }' $ZONE

If you plan to use high bandwidth you should select the nameservers
by their own bandwidth (they should have at least 50 times the bandwidth
of the incoming queries otherwise the packets send to them will be just wasted)
[ a way of checking a server's bandwidth capacity is using 'bing' ]

This kind of DoS attack has several advantages over a smurf attack.
There are more than 25000 nameservers which will always be out there.
It is also more difficult to block because UDP queries are valid
packets which pass through almost all firewalls.
Since every computer has to have some kind of name resolver the only
way to block this is by denying all UDP packets with source port 53
except those coming from your local nameserver.
[ There is still your own nameserver that can flood you in this case ]

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<LI>text/plain attachment: dnsa1.c
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