====================
libnids-1.20
====================

1. Introduction
2. IP defragmentation
3. TCP stream assembly
4. A sample application
5. Basic libnids structures and functions
6. Misc useful hacks

Declarations of data structures and functions defined by libnids are gathered in include file "nids.h". An application which uses libnids must include this file and must be linked with libnids.a (or libnids.so.x.x).

An application's function main usually looks this way:

main()
{
	application private processing, not related to libnids
	optional modification of libnids parameters
	if (!nids_init() ) something's wrong, terminate;
	registration of callback functions
	nids_run();
	// not reached in normal situation
}

Another method is mentioned later.

In order to receive all IP packets seen by libnids (including fragmented ones, packets with invalid checksum et cetera) a programmer should define a callback function of the following type

After calling nids_init, this function should be registered with libnids:

Function ip_frag_func will be called from libnids; parameter a_packet will point to a received datagram, len is the packet length.

Analogically, in order to receive only packets, which will be accepted by a target host (that is, packets not fragmented or packets assembled from fragments; a header correctness is verified) one should define a callback function

and register it with

In order to receive data exchanged in a TCP stream, one must declare a callback function

Structure tcp_stream provides all info on a TCP connection. For instance, it contains two fields of type struct half_stream (named client and server), each of them describing one side of a connection. We'll explain all its fields later.

One of tcp_stream field is named nids_state. Behaviour of tcp_callback depends on value of this field.

•  ns->nids_state==NIDS_JUST_EST

  In this case, ns describes a connection which has just been established. Tcp_callback must decide if it wishes to be notified in future of arrival of data in this connection. All the connection parameters are available (IP addresses, ports numbers etc). If the connection is interesting, tcp_callback informs libnids which data it wishes to receive (data to client, to server, urgent data to client, urgent data to server). Then the function returns.

•  ns->nids_state==NIDS_DATA

  In this case, new data has arrived. Structures half_stream (members of tcp_stream) contain buffers with data.
• The following values of nids_state field :
  • NIDS_CLOSE
  • NIDS_RESET
  • NIDS_TIMED_OUT
  mean that the connection has been closed. Tcp_callback should free allocated resources, if any.

• ns->nids_state==NIDS_EXITING

  In this case, libnids is exiting. This is the applications last opportunity to make use of any data left stored in the half_stream buffers. When reading traffic from a capture file rather than the network, libnids may never see a close, reset, or timeout. If the application has unprocessed data (e.g., from using nids_discard(), this allows the application to process it.

Now let's have a look at a simple application, which displays on stderr data exchanged in all TCP connections seen by libnids.

#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <netinet/in_systm.h>
#include <arpa/inet.h>
#include <string.h>
#include <stdio.h>
#include "nids.h"

#define int_ntoa(x)	inet_ntoa(*((struct in_addr *)&x))

// struct tuple4 contains addresses and port numbers of the TCP connections
// the following auxiliary function produces a string looking like
// 10.0.0.1,1024,10.0.0.2,23
char *
adres (struct tuple4 addr)
{
  static char buf[256];
  strcpy (buf, int_ntoa (addr.saddr));
  sprintf (buf + strlen (buf), ",%i,", addr.source);
  strcat (buf, int_ntoa (addr.daddr));
  sprintf (buf + strlen (buf), ",%i", addr.dest);
  return buf;
}

void
tcp_callback (struct tcp_stream *a_tcp, void ** this_time_not_needed)
{
  char buf[1024];
  strcpy (buf, adres (a_tcp->addr)); // we put conn params into buf
  if (a_tcp->nids_state == NIDS_JUST_EST)
    {
    // connection described by a_tcp is established
    // here we decide, if we wish to follow this stream
    // sample condition: if (a_tcp->addr.dest!=23) return;
    // in this simple app we follow each stream, so..
      a_tcp->client.collect++; // we want data received by a client
      a_tcp->server.collect++; // and by a server, too
      a_tcp->server.collect_urg++; // we want urgent data received by a
                                   // server
#ifdef WE_WANT_URGENT_DATA_RECEIVED_BY_A_CLIENT
      a_tcp->client.collect_urg++; // if we don't increase this value,
                                   // we won't be notified of urgent data
                                   // arrival
#endif
      fprintf (stderr, "%s established\n", buf);
      return;
    }
  if (a_tcp->nids_state == NIDS_CLOSE)
    {
      // connection has been closed normally
      fprintf (stderr, "%s closing\n", buf);
      return;
    }
  if (a_tcp->nids_state == NIDS_RESET)
    {
      // connection has been closed by RST
      fprintf (stderr, "%s reset\n", buf);
      return;
    }

  if (a_tcp->nids_state == NIDS_DATA)
    {
      // new data has arrived; gotta determine in what direction
      // and if it's urgent or not

      struct half_stream *hlf;

      if (a_tcp->server.count_new_urg)
      {
        // new byte of urgent data has arrived 
        strcat(buf,"(urgent->)");
        buf[strlen(buf)+1]=0;
        buf[strlen(buf)]=a_tcp->server.urgdata;
        write(1,buf,strlen(buf));
        return;
      }
      // We don't have to check if urgent data to client has arrived,
      // because we haven't increased a_tcp->client.collect_urg variable.
      // So, we have some normal data to take care of.
      if (a_tcp->client.count_new)
	{
          // new data for the client
	  hlf = &a_tcp->client; // from now on, we will deal with hlf var,
                                // which will point to client side of conn
	  strcat (buf, "(<-)"); // symbolic direction of data
	}
      else
	{
	  hlf = &a_tcp->server; // analogical
	  strcat (buf, "(->)");
	}
    fprintf(stderr,"%s",buf); // we print the connection parameters
                              // (saddr, daddr, sport, dport) accompanied
                              // by data flow direction (-> or <-)

   write(2,hlf->data,hlf->count_new); // we print the newly arrived data
      
    }
  return ;
}

int 
main ()
{
  // here we can alter libnids params, for instance:
  // nids_params.n_hosts=256;
  if (!nids_init ())
  {
  	fprintf(stderr,"%s\n",nids_errbuf);
  	exit(1);
  }
  nids_register_tcp (tcp_callback);
  nids_run ();
  return 0;
}

Now it's time for more systematic description of libnids structures. As mentioned, they're all declared in nids.h

   struct tuple4 // TCP connection parameters
   {
   unsigned short source,dest; // client and server port numbers
   unsigned long saddr,daddr;  // client and server IP addresses
   };


   struct half_stream // structure describing one side of a TCP connection
   {
   char state;            // socket state (ie TCP_ESTABLISHED )
   char collect;          // if >0, then data should be stored in 
                          // "data" buffer; else
                          // data flowing in this direction will be ignored
                          // have a look at samples/sniff.c for an example
                          // how one can use this field
   char collect_urg;      // analogically, determines if to collect urgent 
                          // data
   char * data;           // buffer for normal data
   unsigned char urgdata; // one-byte buffer for urgent data
   int count;             // how many bytes has been appended to buffer "data"
                          // since the creation of a connection 
   int offset;            // offset (in data stream) of first byte stored in 
                          // the "data" buffer; additional explanations
                          // follow
   int count_new;         // how many bytes were appended to "data" buffer 
                          // last (this) time; if == 0, no new data arrived 
   char count_new_urg;    // if != 0, new urgent data arrived

   ... // other fields are auxiliary for libnids

   };


   struct tcp_stream
   {
   struct tuple4 addr;   // connections params (saddr, daddr, sport, dport)
   char nids_state;                  // logical state of the connection
   struct half_stream client,server; // structures describing client and
                                     // server side of the connection 
   ...                               // other fields are auxiliary for libnids
   };

In the above sample program function tcp_callback printed data from hlf->data buffer on stderr, and this data was no longer needed. After tcp_callback return, libnids by default frees space occupied by this data. Field hlf->offset will be increased by number of discarded bytes, and new data will be stored at the beginning of "data" buffer. If the above is not the desired behaviour (for instance, data processor needs at least N bytes of input to operate, and so far libnids received count_new<N bytes) one should call function

before tcp_callback returns. As a result, after tcp_callback return libnids will discard at most num_bytes first bytes from buffer "data" (updating "offset" field accordingly, and moving rest of the data to the beginning of the buffer). If nids_discard function is never called (like in above sample program), buffer hlf->data contains exactly hlf->count_new bytes. Generally, number of bytes in buffer hlf->data equals hlf->count-hlf->offset.

Thanks to nids_discard function, a programmer doesn't have to copy received bytes into a separate buffer - hlf->data will always contain as many bytes, as possible. However, often arises a need to maintain auxiliary data structures per each pair (libnids_callback, tcp stream). For instance, if we wish to detect an attack against wu-ftpd (this attack involves creating deep directory on the server), we need to store somewhere current directory of a ftpd daemon. It will be changed by "CWD" instructions sent by ftp client. That's what the second parameter of tcp_callback is for. It is a pointer to a pointer to data private for each (libnids_callback, tcp stream) pair. Typically, one should use it as follows:

   void
   tcp_callback_2 (struct tcp_stream * a_tcp, struct conn_param **ptr)
   {
   if (a_tcp->nids_state==NIDS_JUST_EST)
   {
        struct conn_param * a_conn;
   	if the connection is uninteresting, return;
        a_conn=malloc of some data structure
        init of a_conn
        *ptr=a_conn // this value will be passed to tcp_callback_2 in future
                    // calls
        increase some of "collect" fields
        return;
   }
   if (a_tcp->nids_state==NIDS_DATA)
   {
	struct conn_param *current_conn_param=*ptr;
        using current_conn_param and the newly received data from the net
        we search for attack signatures, possibly modyfying
        current_conn_param  
        return ;

   }

Functions nids_register_tcp and nids_register_ip* can be called arbitrary number of times. Two different functions (similar to tcp_callback) are allowed to follow the same TCP stream (with a certain non-default exception).

Libnids parameters can be changed by modification of fields of the global variable nids_params, declared as follows:

   struct nids_prm
   {
   int n_tcp_streams; // size of the hash table used for storing structures 
                      // tcp_stream; libnis will follow no more than 
                      // 3/4 * n_tcp_streams connections simultaneously
                      // default value: 1040. If set to 0, libnids will
                      // not assemble TCP streams.
   int n_hosts;       // size of the hash table used for storing info on
                      // IP defragmentation; default value: 256
   char * filename;   // capture filename from which to read packets; 
                      // file must be in libpcap format and device must
                      // be set to NULL; default value: NULL
   char * device;     // interface on which libnids will listen for packets;
                      // default value == NULL, in which case device will
                      // be determined by call to pcap_lookupdev; special
                      // value of "all" results in libnids trying to
                      // capture packets on all interfaces (this works only
                      // with Linux kernel > 2.2.0 and libpcap >= 0.6.0); 
                      // see also doc/LINUX 
   int sk_buff_size;  // size of struct sk_buff, a structure defined by
                      // Linux kernel, used by kernel for packets queuing. If 
                      // this parameter has different value from 
                      // sizeof(struct sk_buff), libnids can be bypassed
                      // by attacking resource managing of libnis (see TEST
                      // file). If you are paranoid, check sizeof(sk_buff)
                      // on the hosts on your network, and correct this 
                      // parameter. Default value: 168
   int dev_addon;     // how many bytes in structure sk_buff is reserved for
                      // information on net interface; if dev_addon==-1, it
                      // will be corrected during nids_init() according to
                      // type of the interface libnids will listen on.
                      // Default value: -1.
   void (*syslog)();  // see description below the nids_params definition
   int syslog_level;  // if nids_params.syslog==nids_syslog, then this field
                      // determines loglevel used by reporting events by
                      // system daemon syslogd; default value: LOG_ALERT
   int scan_num_hosts;// size of hash table used for storing info on port
                      // scanning; the number of simultaneuos port
		      // scan attempts libnids will detect. if set to 
		      // 0, port scanning detection will be turned
		      // off. Default value: 256.
   int scan_num_ports;// how many TCP ports has to be scanned from the same
                      // source. Default value: 10.
   int scan_delay;    // with no more than scan_delay milisecond pause
                      // between two ports, in order to make libnids report
                      // portscan attempt. Default value: 3000
   void (*no_mem)();  // called when libnids runs out of memory; it should
                      // terminate the current process
   int (*ip_filter)(struct ip*);  // this function is consulted when an IP
                      // packet arrives; if ip_filter returns non-zero, the
                      // packet is processed, else it is discarded. This way
                      // one can monitor traffic directed at selected hosts
                      // only, not entire subnet. Default function 
                      // (nids_ip_filter) always returns 1
   char *pcap_filter; // filter string to hand to pcap(3). Default is
		      // NULL. be aware that this applies to the
		      // link-layer, so filters like "tcp dst port 23"
		      // will NOT correctly handle fragmented traffic; one
                      // should add "or (ip[6:2] & 0x1fff != 0)" to process
                      // all fragmented packets
   int promisc;       // if non-zero, the device(s) libnids reads packets
                      // from will be put in promiscuous mode. Default: 1
   int one_loop_less; // disabled by default; see the explanation
   int pcap_timeout;  // the "timeout" parameter to pcap_open_live
                      // 1024 (ms) by default ; change to a lower value
                      // if you want a quick reaction to traffic; this
                      // is present starting with libnids-1.20
   } nids_params;

The field syslog of nids_params variable by default contains the address of function nids_syslog, declared as:

Function nids_params.syslog is used to report unusual condition, such as port scan attempts, invalid TCP header flags and other. This field should be assigned the address of a custom event logging function. Function nids_syslog (defined in libnids.c) can be an example on how to decode parameters passed to nids_params.syslog. Nids_syslog logs messages to system daemon syslogd, disregarding such things like message rate per second or free disk space (that is why it should be replaced).

If one is interested in UDP datagrams, one should declare

and register it with

Parameter addr contains address info, buf points to data carried by UDP packet, len is the data length, and iph points to the IP packet which contained the UDP packet. The checksum is verified.

As a nice toy :) function

is implemented. It terminates TCP connection described by a_tcp by sending RST segments.

It calls pcap_next() instead of pcap_loop, that is it processes only one packet. If no packet is available, the process will sleep. Nids_next() returns 1 on success, 0 on error (nids_errbuf contains appropriate message then).

Typically, when using nids_next(), an aplication will sleep in a select() function, with a snooping socket fd present in read fd_set. This fd can be obtained via a call to

It returns a file descriptor when succeeded and -1 on error ( nids_errbuf is filled then).
Similarly, function

1. Nowadays, some NIC drivers are capable of computing checksums of outgoing packets. In such case, outgoing packets passed to libpcap can have uncomputed checksums. So, you may want to not check checksums on outgoing packets.
2. In order to improve performance, you may wish to not compute checksums for hosts one trusts (or protects), e.g. one's server farm.

   struct nids_chksum_ctl
{       u_int netaddr;
        u_int mask;
        u_int action;
	/* reserved fields */
};