====================
libnids-1.23
====================
1. Introduction
2. IP defragmentation
3. TCP stream assembly
4. A sample application
5. Basic libnids structures and functions
6. Misc useful hacks
7. New features in version 1.21
1. Introduction
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.
2. IP defragmentation
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
void ip_frag_func(struct ip * a_packet, int len)
After calling nids_init, this function should be registered with libnids:
nids_register_ip_frag(ip_frag_func);
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
void ip_func(struct ip * a_packet, int len)
and register it with
nids_register_ip(ip_func);
3. TCP stream assembly
In order to receive data exchanged in a TCP stream, one must declare a
callback function
void tcp_callback(struct tcp_stream * ns, void ** param)
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.
4. A sample application
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;
}
5. Basic libnids structures and functions
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
void nids_discard(struct tcp_stream * a_tcp, int num_bytes)
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
int multiproc; // start ip defragmentation and tcp stream assembly in a
// different thread parameter to a nonzero value and
// compiling libnids in an environment where glib-2.0 is
// available enables libnids to use two different threads
// - one for receiving IP fragments from libpcap,
// and one, with lower priority, to process fragments,
// streams and to notify callbacks. Preferrably using
// nids_run() this behavior is invisible to the user.
// Using this functionality with nids_next() is quite
// useless since the thread must be started and stopped
// for every packet received.
// Also, if it is enabled, global variables (nids_last_pc
ap_header
// and nids_last_pcap_data) may not point to the
// packet currently processed by a callback
int queue_limit; // limit on the number of packets to be queued;
// used only when multiproc=true; 20000 by default
int tcp_workarounds; // enable (hopefully harmless) workarounds for some
// non-rfc-compliant TCP/IP stacks
pcap_t *pcap_desc; // pcap descriptor
} nids_params;
The field syslog of nids_params variable by default contains the address of
function nids_syslog, declared as:
void nids_syslog (int type, int errnum, struct ip *iph, void *data);
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
void udp_callback(struct tuple4 * addr, char * buf, int len, struct ip *
iph);
and register it with
nids_register_udp(udp_callback)
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.
6. Misc useful hacks
As a nice toy :) function
void nids_killtcp(struct tcp_stream * a_tcp)
is implemented. It terminates TCP connection described by a_tcp by sending
RST segments.
Originally the RST segments sent by libnids were given a sequence number in
the half of the TCP window of the destination. MS Windows systems with
MS05-019 patch applied do not seem to tear down a connection upon receiving
such RSTs, so now libnids sends two RSTs in each direction - additional one
has the lowest (expected) seq. Unfortunately, it is somewhat unreliable: if
due to traffic burst, your application is a few miliseconds delayed behind
the current traffic, its view of what the current/expected seq is may be
incorrect.
Naturaly, sending a RST as a defensive measure is unreliable by design,
unless deployed on an "inline NIDS", or NIPS, as a few call it; therefore
the "toy" label.
_________________________________________________________________
Using nids_run() has one disadvantage - the application becomes totally
packets driven. Sometimes it is necessary to perform some task even when no
packets arrive. Instead of nids_run(), one can use function
int nids_next()
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
int nids_getfd()
It returns a file descriptor when succeeded and -1 on error ( nids_errbuf is
filled then).
Similarly, function
int nids_dispatch(int cnt)
is a wrapper around pcap_dispatch. It maybe advantageous to use it instead
of nids_next() when we want to distinguish between return values (ie
end-of-file vs error).
_________________________________________________________________
There are a few reasons why you may want to skip checksum processing on
certain packets:
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.
In order to let libnids know which packets should not be checksummed, you
should allocate an array of struct nids_chksum_ctl (defined in nids.h):
struct nids_chksum_ctl
{ u_int netaddr;
u_int mask;
u_int action;
/* reserved fields */
};
and register it with
nids_register_chksum_ctl(struct nids_chksum_ctl *, int);
where the second parameter indicates the number of elements in the array.
Checksumming functions will first check elements of this array one by one,
and if the source ip SRCIP of the current packet satisfies condition
(SRCIP&chksum_ctl_array[i].mask)==chksum_ctl_array[i].netaddr
then if the "action" field is NIDS_DO_CHKSUM, the packet will be
checksummed; if the "action" field is NIDS_DONT_CHKSUM, the packet will not
be checksummed. If the packet matches none of the array elements, the
default action is to perform checksumming.
The example of usage is available in the samples/chksum_ctl.c file.
_________________________________________________________________
The include file nids.h defines the constants NIDS_MAJOR (1) and NIDS_MINOR
(21), which can be used to determine in runtime the version of libnids.
Nids.h used to define HAVE_NEW_PCAP as well, but since 1.19 it is
nonsupported as obsolete.
_________________________________________________________________
Typically, data carried by a tcp stream can be divided into
protocol-dependent records (say, lines of input). A tcp callback can receive
an amount of data, which contains more then one record. Therefore, a tcp
callback should iterate its protocol parsing routine over the whole amount
of data received. This adds complexity to the code.
If nids_params.one_loop_less is non-zero, libnids behaviour changes
slightly. If a callback consumes some (but not all) of newly arrived data,
libnids calls it immediately again. Only non-processed data remain in the
buffer, and rcv->count_new is decreased appropriately. Thus, a callback can
process only one record at the time - libnids will call it again, until no
new data remain or no data can be processed. Unfortunately, this behaviour
introduces horrible semantics problems in case of 2+ callbacks reading the
same half of a tcp stream. Therefore, if nids_params.one_loop_less is
non-zero, you are not allowed to attach two or more callbacks to the same
half of tcp stream. Unfortunately, the existing interface is unable to
propagate the error to the callback - therefore, you must watch it yourself.
You have been warned.
_________________________________________________________________
The pcap header of the last seen packet is exported as
extern struct pcap_pkthdr *nids_last_pcap_header;
It is wise to use it to get timestamp, to get a better accuracy and save a
syscall.
_________________________________________________________________
Other applications using libnids can be found in "samples" directory.
6. New features in version 1.21
Version 1.21 brings several bugfixes, optimizations and a few new features,
but mostly extra external variables and functions to access libnids'
intrinsics from the outside.
nids_last_pcap_data is a new external variable to get the data of the last
PCAP frame, like it was already possible to use nids_last_pcap_header in
order to get the header of the last PCAP frame.
nids_linkoffset is a new external variable to get the computed offset
between the link layer and the network layer for the current PCAP device. It
is useful to reconstruct PCAP frames from IP defragmented packets which you
get in your ip_func (see chapter on IP defragmentation) by copying the same
amount of bytes from the beginning of nids_last_pcap_data representing the
link layer, like this:
void ip_callback(struct ip *pkt, int len)
{
u_char *frame;
struct pcap_pkthdr ph;
frame = malloc(len + nids_linkoffset);
memcpy(frame, nids_last_pcap_data, nids_linkoffset);
memcpy(frame + nids_linkoffset, pkt, len);
ph.ts = nids_last_pcap_header->ts;
ph.caplen = ph.len = len + nids_linkoffset;
pcap_dump(nids_params.pcap_desc, &ph, frame);
free(frame);
}
In versions prior to 1.21 it was only possible to give libnids a device or
file name and have it take total control over libpcap operations when using
nids_run() or nids_next(). Now, with nids_params.pcap_desc it is possible to
have your pcap_handler outside libnids and choose which frames you want to
be processed by libnids (e.g. only TCP packets to keep track of TCP
connections whilst this is not your only objective); all you have to do is
copy your pointer to the pcap_t structure (returned by pcap_open_live(),
pcap_open_dead() or pcap_open_offline()) to nids_params.pcap_desc and call
nids_pcap_handler(), normally with the same parameters as your own
pcap_handler (the one you registered with pcap_dispatch() or pcap_loop())
was called with. NOTE: since libnids cannot know when you are finished if
you interactively pass packets to it with nids_pcap_handler(), you must tell
it when to free the allocated resources by calling nids_exit().
nids_params.tcp_workarounds is a new libnids runtime option which can be
used to enable extra checks for faulty implementations of TCP such as the
ones which allow connections to be closed despite the fact that there should
be retransmissions for lost packets first, thus violating section 3.5 of RFC
793. In those cases, and if this option is non-zero, libnids will set the
NIDS_TIMED_OUT state for TCP connections that were savagely closed.
nids_find_tcp_stream() is a new external function that can be used to find
the corresponding tcp_stream structure for a given pointer to a tuple4
structure.
nids_free_tcp_stream() is a new external function that can be used for
example to force libnids into not following a TCP stream anymore. BEWARE!
Calling nids_free_tcp_stream() from inside one of your registered
tcp_callbacks on a TCP stream that is already in a closing state
(NIDS_CLOSE, NIDS_TIMED_OUT, NIDS_RESET or NIDS_EXITING) will result in a
double free (because libnids will call nids_free_tcp_stream() internally
when your tcp_callback returns) and your program will crash.
nids_unregister_ip_frag(), nids_unregister_ip(), nids_unregister_udp() and
nids_unregister_tcp() are new external functions that can be used to
unregister callbacks previous registed with the corresponding
nids_register_*(), at any time.
tcp_stream.user is a new field in the structure passed to TCP callbacks. It
is similar to their void **param argument, except that it is global to all
the TCP callbacks for the same stream, whereas param is specific to each
callback.
