Basic Packet-Sniffer Construction
                from the Ground Up

                                       Part 1
                                         by

                                     Chad Renfro
         raw_sock@hotmail.com


   Packet sniffers are applications used by network administrators to monitor and
validate network traffic. Sniffers are programs used to read packets that travel across 
the network at various levels of the OSI layer. And like most security tools sniffers too
can be used for both good and destructive purposes. On the light-side of network
administration sniffers help quickly track down problems such as bottlenecks and
misplaced filters. However on the dark-side sniffers can be used to reap tremendous
amounts of havoc by gathering legitimate user names and passwords so that other
machines can be quickly compromised. Hopefully this paper will be used to help
administrators gain control of their networks by being able to analyze network traffic 
not only by using preconstructed  sniffers but by being able to create their own. This
paper will look at the packet sniffer from the bottem up, looking in depth at the sniffer
core and then gradualy adding functionality to the application. The example included
here will help illustrate some rather cumbersome issues when dealing with network
programing. In no way will this single paper teach a person to write a complete sniffing
application like tcpdump or sniffit. It will however teach some very fundamental issues
that are inherent to all packet sniffers. Like how the packets are accessed on the network
and how to work with the packets at different layers.








The most basic sniffer...

Sniffer #1.

   This sniffer will illustrate the use of the  SOCK_RAW device and show how to gather
packets from the network and print out some simple header  information to std_out.
Although the basic premise is that packet sniffers operate  in a promiscuous mode which
listens to all packets weather or not the packet is destined  for the machines mac address,
this example will collect packets in a non-promiscuous mode . This will let usconcentrate
on the SOCK_RAW device for the first example. To operate this same  code  in a
promiscous mode  the network card may be put in a promiscous mode manually. To do
this type this in after the log in :


   > su -
   Password : ********
   # ifconfig eth0 promisc

   This will now set the network interface eth0 in promiscous mode. 


/************************simple_Tcp_sniff.c********************/

1.  #include <stdio.h>
2.  #include <sys/socket.h>
3.  #include <netinet/in.h>
4.  #include <arpa/inet.h>

5.  #include "headers.h"

6.  int main()
7.  {
8.      int sock, bytes_recieved, fromlen;
9.      char buffer[65535];
10.      struct sockaddr_in from;
11.      struct ip  *ip;
12.      struct tcp *tcp;
13.

14.      sock = socket(AF_INET, SOCK_RAW, IPPROTO_TCP);

15.  while(1)
16.   {
17.    fromlen = sizeof from;
18.    bytes_recieved = recvfrom(sock, buffer, sizeof buffer, 0,
                                           (struct sockaddr *)&from, &fromlen);
19.    printf("\nBytes received ::: %5d\n",bytes_recieved);
20.    printf("Source address ::: %s\n",inet_ntoa(from.sin_addr));
21.    ip = (struct ip *)buffer;
22.    printf("IP header length ::: %d\n",ip->ip_length);
23.    printf("Protocol ::: %d\n",ip->ip_protocol);
24.    tcp = (struct tcp *)(buffer + (4*ip->ip_length));
25.    printf("Source port ::: %d\n",ntohs(tcp->tcp_source_port);
26.    printf("Dest port  ::: %d\n",ntohs(tcp->tcp_dest_port));

27.           }
28. }
/***********************EOF**********************************/

What this means :

Line 1-4 :
   These are the header files required to use some needed c functions we will use later

  <stdio.h>      =     functions like printf and std_out
  <sys/socket.h> =     this will give access to the SOCK_RAW and the 
                       IPPROTO_TCP defines   
  <netinet/in.h> =     structs like the sockaddr_in 
  <arpa/inet.h>  =     lets us use the functions to do network to host byte 
                       order conversions
line 5 :
   This is the header file headers.h that is also included with this program to give standard
   structures to access the ip and tcp fields. The structures identify each field in the ip and
   tcp header for instance :
 
  struct ip {
         unsigned int        ip_length:4;         /* length of ip-header in 32-bit
                                                           words*/
          unsigned int        ip_version:4;        /* set to "4", for Ipv4 */
         unsigned char       ip_tos;              /* type of service*/
         unsigned short      ip_total_length;     /* Total length of ip datagram in
                                                           bytes */
         unsigned short      ip_id;               /*identification field*/
         unsigned short      ip_flags;
         unsigned char       ip_ttl;              /*time-to-live, sets upper limit
                                                          for max number of routers to 
                                                          go through before the packet is
                                                          discarded*/

         unsigned char       ip_protocol;         /*identifies the correct transport
                                        protocol */
         unsigned short      ip_cksum;            /*calculated for the ip header ONLY*/
               unsigned int        ip_source;           /*source ip */
               unsigned int        ip_dest;             /*dest ip*/
  };



  struct tcp {
                 unsigned short     tcp_source_port; /*tcp source port*/
           unsigned short     tcp_dest_port;   /*tcp dest port*/
           unsigned int       tcp_seqno;       /*tcp sequence number,
                                                       identifies the byte in the 
                                                       stream of data*/
           unsigned int       tcp_ackno;       /*contains the next seq num that
                                                       the sender expects to recieve*/
           unsigned int       tcp_res1:4,      /*little-endian*/
                                    tcp_hlen:4,      /*length of tcp header in 32-bit
                                                       words*/ 
                              tcp_fin:1,       /*Finish flag "fin"*/
                                    tcp_syn:1,       /*Synchronize sequence
                                                       numbers to start a connection
                        tcp_rst:1,       /*Reset flag */
                                    tcp_psh:1,       /*Push, sends data to the
                                                       application*/
                                    tcp_ack:1,       /*acknowledge*/
                                    tcp_urg:1,       /*urgent pointer*/
                                    tcp_res2:2;
                 unsigned short     tcp_winsize;     /*maxinum number of bytes able
                                           to recieve*/
           unsigned short     tcp_cksum;       /*checksum to cover the tcp
                                                       header and data portion of the
                                                       packet*/

           unsigned short     tcp_urgent;     /*vaild only if the urgent flag is
                                    set, used to transmit
                                                      emergency data */
  };


line 8-13 :
   This is the variable declaration section
      
  integers :
       sock                 = socket file descriptor 
       bytes_recieved       = bytes read from the open socket "sock" 
       fromlen              = the size of the from structure char :
             buffer               = where the ip packet that is read off the 
                        wire will be held buffer will hold a datagram 
                        of 65535 bytes which is the maximum length 
                        of an ip datagram.

       Struct sockaddr_in :

     struct sockaddr_in {
    short int          sin_family;  /* Address family   */
    unsigned short int sin_port;    /* Port number      */
    struct in_addr     sin_addr;    /* Internet address */
    unsigned char      sin_zero[8]; /* Same size as struct sockaddr */
      };

      Before we go any further two topics should be covered,byte-ordering and sockaddr
   structures.  Byte-ordering,is the way that the operating system stores bytes in memory.
   There are two ways that this is done first with the low-order byte at the starting address
   this is known as "little-endian" or host-byte order. Next bytes can be stored with the
   high order byte at the starting address, this is called "big-endian" or network byte order.
   The Internet protocol uses >>>>>> network byte order.
    
       This is important because if you are working on an intel based linux box you will be
   programming on a little-endian machine and to send data via ip you must convert the
   bytes to network-byte order. For examle lets say we are going to store a 2-byte number
   in memory say the value is (in hex) 0x0203


   First this is how the value is stored on a big-endian machine:

                    ___________
                   | 02  | 03  |
                   |_____|_____| 
        address:    0       1


   And here is the same value on a little-endian machine:

                   ___________
                  |03   | 02  |
                  |_____|_____|
       address:    1       0



   The same value is being represented in both examples it is just how we order the bytes
   that changes.

   The next topic that you must understand is the sockaddr vs. the sockaddr_in structures.
   The struct sockaddr is used to hold information about the socket such as the family type
   and other address information it looks like :

  struct sockaddr {
            unsigned short sa_family;         /*address family*/ 
                  char           sa_data[14];       /*address data*/
  };
   
  
      The first element in the structure "sa_family" will be used to reference what the family
   type is for the socket, in our sniffer it will be AF_INET. Next the "sa_data" element
   holds the destination port and address for the socket. To make it easier to deal with the
   sockaddr struct the use of the sockaddr_in structure is commonly used. Sockaddr_in 
   makes it easier to reference all of the elements that are contained by sockaddr.


   Sockaddr_in looks like:



   struct sockaddr_in {
             short int          sin_family;    /* Address family               */
             unsigned short int sin_port;      /* Port number                  */
             struct in_addr     sin_addr;      /* Internet address             */
             unsigned char      sin_zero[8];   /* Same size as struct sockaddr */
   };







      We will use this struct and declare a variable "from" which will give us the information
   on the packet that we will collect from the raw socket. For instance the var
   "from.sin_addr" will give access to the packets source address (in 
   network byte order). The thing to mention here is that all items in the sockaddr_in
   structure must be in network-byte order. When we receive the data in the sockaddr_in
   struct we must then convert it back to Host-byte order. To do this we can use some
   predefined functions to convert back and forth between  host and network byteorder.

   Here are the functions we will use:

  ntohs       : this function converts  network byte order to host byte order
                      for a 16-bit short

  ntohl       : same as above but for a 32-bit long

  inet_ntoa   : this function converts a 32-bit network binary value to a
                      dotted decimal ip address

  inet_aton   : converts a character string  address to the 32-bit network
                      binary value

  inet_addr   : takes a char string dotted decimal addr and returns a 32-bit
                      network binary value

   To further illustrate ,say I want to know the port number that this packet originated from:

  int packet_port; packet_port  =ntohs(from.sin_port);
                             ^^^^^  

   If I want the source IP address of the packet we will use a special function to get it to the
   123.123.123.123 format:

  char *ip_addr; ip_addr  =inet_ntoa(from.sin_addr)
                     ^^^^^^^^^

line 11-12:

   struct ip *ip :
   struct tcp *tcp :
  
      This is a structure that we defined in our header file "headers.h". This structure is
   declared so that we can access individual fields of the ip/tcp header. The structure is like
   a transparent slide with predefined fields drawn on it. When a packet is taken off 
   the wire it is a stream of bits, to make sense of it the "transparency" (or cast) is laid on
   top of or over the bits so the individual fields can be referenced.

Line 14 :

   sock = socket(AF_INET, SOCK_RAW, IPPROTO_TCP);

   This is the most important line in the entire program. Socket() takes three arguments in
   this form:

  sockfd = socket(int family, int type, int protocol);



  
    The first argument is the family. This could be either AF_UNIX which is used so a process
   can communicate with another process on the same host or AF_INET which is used for
   internet communication between remote hosts. In this case it will be  AF_INET . Next  
   is the type, the type is usually between 1 of 4 choices (there are others that we will not
   discuss here) the main four are :

   1.  SOCK_DRAM      : used for udp datagrams
   2.  SOCK_STREAM    : used for tcp packets
   3.  SOCK_RAW       : used to bypass the transport layer
                   and directly access the IP layer

   4.  SOCK_PACKET    : this is linux specific, it is similuar to
                         SOCK_RAW except it accesses the DATA LINK Layer

      For our needs we will use the SOCK_RAW type. You must have root acces to open a
    raw socket. The last parameter  is the protocol,the protocol value specifies what type of
    traffic the socket should receive , for normal sockets this value is usally set to "0"
    because the socket can figure out if for instance the "type" of SOCK_DGRAM is
    specified then the protocol should be UDP.In our case we just want to look at tcp 
    traffic so we will specify IPPROTO_TCP.

  
line 15 :
   while (1)

  The while (1) puts the program into an infinite loop this is necessary so that after the
 first packet is processed we will loop around and grab the next. 


Line 18:
   bytes_recieved = recvfrom(sock, buffer, sizeof buffer, 0, (struct sockaddr *)&from, &fromlen);
  
   Now here is where we are actually reading data from the open socket "sock".The from
   struct is also filled in but notice that we are casting "from" from a "sockaddr_in" struct
   to a "sockaddr" struct. We do this because the recvfrom() requires a sockaddr type but
   to access the separate fields we will continue to use the sockaddr_in structure. The 
   length of the "from" struct must also be present and passed by address. The recvfrom()
   call will return the number of bytes on success and a -1 on error and fill the global var
   errno.

   This is what we call "blocking-I/O" the recvfrom() will wait here forever until a
   datagram on the open socket is ready to be processed. This is opposed to 
   Non-blocking I/O which is like running a process in the background and move on to
   other tasks.


Line 20:
   printf("Source address ::: %s\n",inet_ntoa(from.sin_addr));

   This printf uses the special function inet_ntoa() to take the value of "from.sin_addr"
   which is stored in Network-byte order and outputs a value in a readable ip form such
   as 192.168.1.XXX.

Line 21:
   ip = (struct ip *)buffer;

   This is where we will overlay a predefined structure that will help us to individually
   identify the fields in the packet that we pick up from the open socket.


Line 22:
   printf("IP header length ::: %d\n",ip->ip_length);

   The thing to notice on this line is the "ip->ip_length" this will access a pointer in
   memory to the ip header length the important thing to remember is that the length 
   will be represented in 4-byte words this will be more important later when trying to
   access items past the ip header such as the tcp header or the data portion of the packet.



Line 23:
   printf("Protocol ::: %d\n",ip->ip_protocol);
  
   This gives access to the type of protocol such as 6 for tcp or 17 for udp.

Line 24:
   tcp = (struct tcp *)(buffer + (4*ip->ip_length));

       Remember earlier it was mentioned that the ip header length is stored in 4 byte words,
   this is where that bit of information becomes important. Here we are trying to get access
   to the tcp header fields, to do this we must overlay a structure that has the fields
   predefined just as we did with ip. There is one key difference here the ip header fields
   were easy to access due to the fact that the beginning of the buffer was also the beginning
   of the ip header as so :

  
  |----------------- buffer ----------------|
   _________________________________________
  | ip header          |                    |
         |____________________|____________________|
                             ^
                             *ip
                             ^
                             *buffer

      So to get access to the ip header we just set a pointer casted as an ip structure to the
   beginning of the buffer like "ip = (struct ip *)buffer;". To get access to the tcp header 
   is a little more difficult due to the fact that we must set a pointer and cast it as a tcp
   structure at the beginning of the tcp header which follows the ip header in the buffer 
   as so :

  
   |----------------- buffer ---------------|
          ________________________________________
         | ip header | tcp header |               |
         |___________|____________|_______________|
                     ^
                     *tcp
                      
This is why we use 4*ip->ip_length to find the start of the tcp header.

Line 25-26:
    printf("Source port ::: %d\n",ntohs(tcp->tcp_source_port);
    printf("Dest port  ::: %d\n",ntohs(tcp->tcp_dest_port));

   We can now access the source and dest ports which are located in the tcp header via 
   the structure as defined above.




     This will conclude  our  first very simple tcp sniffer. This was a very basic application
   that should help define how  to access packets passing on the network and how  to use
   sockets to access the packets. Hopefully this will be the first of many papers to come,
   which each proceeding paper we will add a new or more complex feature to the sniffer. I
   should also mention that there a number of great resources on the net that should aid you
   in further research in this area :

  1. Beej's Guide to Network Programming
       This is an awesome paper that really helps 
       clear up any misconceptions about network programming.
                [http://www.ecst.csuchico.edu/~beej/guide/net]

        2. TCP/IP Illustrated Vol 1,2,3
           W.Richard Stevens

To use the above program, cut out the above code and strip off all 
of the line numbers. Save the edited file as sniff.c. Next cut 
out the header file headers.h (below) and save it to a file headers.h
in the same directory. Now just compile: gcc -o sniff sniff.c
You should now have the executable "sniff", to run it type
#./sniff

/*************************headers.h**************************/
/*structure of an ip header             */
struct ip {
  unsigned int        ip_length:4;    /*little-endian*/
  unsigned int        ip_version:4;
  unsigned char       ip_tos;
  unsigned short      ip_total_length;
  unsigned short      ip_id;
  unsigned short      ip_flags;
  unsigned char       ip_ttl;
  unsigned char       ip_protocol;
  unsigned short      ip_cksum;
  unsigned int        ip_source;
  unsigned int        ip_dest;
};

/* Structure of a TCP header */
struct tcp {
  unsigned short      tcp_source_port;
  unsigned short      tcp_dest_port;
  unsigned int        tcp_seqno;
  unsigned int        tcp_ackno;
  unsigned int        tcp_res1:4,     /*little-endian*/
  tcp_hlen:4,
  tcp_fin:1,
   tcp_syn:1,
  tcp_rst:1,
  tcp_psh:1,
  tcp_ack:1,
  tcp_urg:1,
  tcp_res2:2;
  unsigned short      tcp_winsize;
  unsigned short      tcp_cksum;
  unsigned short      tcp_urgent;
};
/*********************EOF***********************************/