Provided by: libnids-dev_1.26-2.1build2_amd64 
NAME
libnids - network intrusion detection system E-box library
SYNOPSIS
#include <nids.h>
extern struct nids_prm nids_params;
extern char nids_errbuf[];
int
nids_init(void);
void
nids_register_ip_frag(void (*ip_frag_func)(struct ip *pkt, int len));
void
nids_unregister_ip_frag(void (*ip_frag_func)(struct ip *pkt, int len));
void
nids_register_ip(void (*ip_func)(struct ip *pkt, int len));
void
nids_unregister_ip(void (*ip_func)(struct ip *pkt, int len));
void
nids_register_udp(void (*udp_func)(struct tuple4 *addr, u_char *data, int len, struct ip *pkt));
void
nids_unregister_udp(void (*udp_func)(struct tuple4 *addr, u_char *data, int len, struct ip *pkt));
void
nids_register_tcp(void (*tcp_func)(struct tcp_stream *ts, void **param));
void
nids_unregister_tcp(void (*tcp_func)(struct tcp_stream *ts, void **param));
void
nids_killtcp(struct tcp_stream *ts);
void
nids_discard(struct tcp_stream *ts, int numbytes);
void
nids_run(void);
int
nids_dispatch(int cnt);
int
nids_next(void);
int
nids_getfd(void);
void
nids_register_chksum_ctl(struct nids_chksum_ctl *, int);
void
nids_pcap_handler(u_char *par, struct pcap_pkthdr *hdr, u_char *data);
struct tcp_stream *
nids_find_tcp_stream(struct tuple4 *addr);
DESCRIPTION
libnids provides the functionality of a network intrusion detection system (NIDS) E-box component. It
currently performs:
1. IP defragmentation
2. TCP stream reassembly
3. TCP port scan detection
libnids performs TCP/IP reassembly in exactly the same way as Linux 2.0.36 kernels, and correctly handles
all of the attacks implemented in fragrouter(8) (plus many other attacks as well).
ROUTINES
nids_init() initializes the application for sniffing, based on the values set in the global variable
nids_params, declared as follows:
struct nids_prm {
int n_tcp_streams;
int n_hosts;
char *device;
char *filename;
int sk_buff_size;
int dev_addon;
void (*syslog)(int type, int err, struct ip *iph, void *data);
int syslog_level;
int scan_num_hosts;
int scan_num_ports;
int scan_delay;
void (*no_mem)(void);
int (*ip_filter)(struct ip *iph);
char *pcap_filter;
int promisc;
int one_loop_less;
int pcap_timeout;
int multiproc;
int queue_limit;
int tcp_workarounds;
pcap_t *pcap_desc;
} nids_params;
The members of this structure are:
n_tcp_streams
Size of the hash table used for storing TCP connection information ( a maximum of 3/4 *
n_tcp_streams TCP connections will be followed simultaneously). Default value: 1024
n_hosts
Size of the hash table used for storing IP defragmentation information. Default value: 256
filename
It this variable is set, libnids will call pcap_open_offline with this variable as the argument
(instead of pcap_open_live()). Default value: NULL
device Interface to monitor. Default value: NULL (in which case an appropriate device is determined
automatically). If this variable is assigned value all, libnids will attempt to capture packets on
all interfaces (which works on Linux only)
sk_buff_size
Size of struct sk_buff (used for queuing packets), which should be set to match the value on the
hosts being monitored. Default value: 168
dev_addon
Number of bytes in struct sk_buff reserved for link-layer information. Default value: -1 (in which
case an appropriate offset if determined automatically based on link-layer type)
syslog Syslog callback function, used to report unusual conditions, such as port scan attempts, invalid
TCP header flags, etc. Default value: nids_syslog (which logs messages via syslog(3) without
regard for message rate per second or free disk space)
syslog_level
Log level used by nids_syslog for reporting events via syslog(3). Default value: LOG_ALERT
scan_num_hosts
Size of hash table used for storing portscan information (the maximum number portscans that will
be detected simultaneously). If set to 0, portscan detection will be disabled. Default value: 256
scan_num_ports
Minimum number of ports that must be scanned from the same source host before it is identifed as a
portscan. Default value: 10
scan_delay
Maximum delay (in milliseconds) between connections to different ports for them to be identified
as part of a portscan. Default value: 3000
no_mem Out-of-memory callback function, used to terminate the calling process gracefully.
ip_filter
IP filtering callback function, used to selectively discard IP packets, inspected after
reassembly. If the function returns a non-zero value, the packet is processed; otherwise, it is
discarded. Default value: nids_ip_filter (which always returns 1)
pcap_filter
pcap(3) filter string applied to the link-layer (raw, unassembled) packets. Note: filters like
``tcp dst port 23'' will NOT correctly handle appropriately fragmented traffic, e.g. 8-byte IP
fragments; one should add "or (ip[6:2] & 0x1fff != 0)" at the end of the filter to process
reassembled packets. Default value: NULL
promisc
If non-zero, libnids will set the interface(s) it listens on to promiscuous mode. Default value: 1
one_loop_less
Disabled by default; see comments in API.html file
pcap_timeout
Sets the pcap read timeout, which may or may not be supported by your platform. Default value:
1024.
multiproc
If nonzero, creates a separate thread for packets processing. See API.html. Default value: 0.
queue_limit
If multiproc is nonzero, this is the maximum number of packets queued in the thread which reads
packets from libpcap. Default value: 20000
tcp_workarounds
Enables 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 (as stated
by RFC 793, section 3.5). If non-zero, libnids will set the NIDS_TIMED_OUT state for savagely
closed connections. Default value: 0
pcap_desc
It this variable is set, libnids will call neither pcap_open_live nor pcap_open_offline, but will
use a pre-opened PCAP descriptor; use this with nids_pcap_handler() in order to interactively feed
packets to libnids. Default value: NULL
Returns 1 on success, 0 on failure (in which case nids_errbuf contains an appropriate error message).
nids_register_ip_frag() registers a user-defined callback function to process all incoming IP packets
(including IP fragments, packets with invalid checksums, etc.).
nids_unregister_ip_frag() unregisters a user-defined callback function to process all incoming IP
packets.
nids_register_ip() registers a user-defined callback function to process IP packets validated and
reassembled by libnids.
nids_unregister_ip() unregisters a user-defined callback function to process IP packets.
nids_register_udp() registers a user-defined callback function to process UDP packets validated and
reassembled by libnids.
nids_unregister_udp() unregisters a user-defined callback function to process UDP packets.
nids_register_tcp() registers a user-defined callback function to process TCP streams validated and
reassembled by libnids. The tcp_stream structure is defined as follows:
struct tcp_stream {
struct tuple4 {
u_short source;
u_short dest;
u_int saddr;
u_int daddr;
} addr;
char nids_state;
struct half_stream {
char state;
char collect;
char collect_urg;
char *data;
u_char urgdata;
int count;
int offset;
int count_new;
char count_new_urg;
...
} client;
struct half_stream server;
...
void *user;
};
The members of the tuple4 structure identify a unique TCP connection:
source, dest
Client and server port numbers
saddr, daddr
Client and server IP addresses
The members of the half_stream structure describe each half of a TCP connection (client and server):
state Socket state (e.g. TCP_ESTABLISHED).
collect
A boolean which specifies whether to collect data for this half of the connection in the data
buffer.
collect_urg
A boolean which specifies whether to collect urgent data pointed to by the TCP urgent pointer for
this half of the connection in the urgdata buffer.
data Buffer for normal data.
urgdata
One-byte buffer for urgent data.
count The number of bytes appended to data since the creation of the connection.
offset The current offset from the first byte stored in the data buffer, identifying the start of newly
received data.
count_new
The number of bytes appended to data since the last invocation of the TCP callback function (if 0,
no new data arrived).
count_new_urg
The number of bytes appended to urgdata since the last invocation of the TCP callback function (if
0, no new urgent data arrived).
The value of the nids_state field provides information about the state of the TCP connection, to be used
by the TCP callback function:
NIDS_JUST_EST
Connection just established. Connection parameters in the addr structure are available for
inspection. If the connection is interesting, the TCP callback function may specify which data it
wishes to receive in the future by setting non-zero values for the collect or collect_urg
variables in the appropriate client or server half_stream structure members.
NIDS_DATA
New data has arrived on a connection. The half_stream structures contain buffers of data.
NIDS_CLOSE, NIDS_RESET, NIDS_TIMED_OUT
Connection has closed. The TCP callback function should free any resources it may have allocated
for this connection.
The param pointer passed by libnids as argument to the TCP callback function may be set to save a pointer
to user-defined connection-specific data to pass to subsequent invocations of the TCP callback function
(ex. the current working directory for an FTP control connection, etc.).
The user pointer in the tcp_stream structure has the same purpose except it is global to the stream,
whereas the param pointer is different from one callback function to the other even though they were
called for the same stream.
nids_unregister_tcp() unregisters a user-defined callback function to process TCP streams.
nids_killtcp() tears down the specified TCP connection with symmetric RST packets between client and
server.
nids_discard() may be called from the TCP callback function to specify the number of bytes to discard
from the beginning of the data buffer (updating the offset value accordingly) after the TCP callback
function exits. Otherwise, the new data (totalling count_new bytes) will be discarded by default.
nids_run() starts the packet-driven application, reading packets in an endless loop, and invoking
registered callback functions to handle new data as it arrives. This function does not return.
nids_dispatch() attempts to process cnt packets before returning, with a cnt of -1 understood as all
packets available in one pcap buffer, or all packets in a file when reading offline. On success, returns
the count of packets processed, which may be zero upon EOF (offline read) or upon hitting pcap_timeout
(if supported by your platform). On failure, returns -1, putting an appropriate error message in
nids_errbuf.
nids_next() process the next available packet before returning. Returns 1 on success, 0 if no packet was
processed, setting nids_effbuf appropriately if an error prevented packet processing.
nids_getfd() may be used by an application sleeping in select(2) to snoop for a socket file descriptor
present in the read fd_set. Returns the file descriptor on success, -1 on failure (in which case
nids_errbuf contains an appropriate error message).
nids_register_chksum_ctl() takes as arguments an array of struct nids_chksum_ctl elements and the number
of elements in the array. A nids_chksum_ctl element is defined as follows:
struct nids_chksum_ctl {
u_int netaddr;
u_int mask;
u_int action;
/* private members */
};
Internal 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.
nids_pcap_handler() may be used by an application already running a capture with libpcap, in order to
pass frames to libnids interactively (frame per frame) instead of having libnids itself do the capture.
nids_find_tcp_stream() returns a pointer to the tcp_stream structure corresponding to the tuple passed as
argument if libnids knows about this TCP connection already, otherwise it returns NULL.
nids_free_tcp_stream() removes the given tcp_stream from the list of streams tracked by libnids.
Warning: its usage can result in crashes! See comments in the API.html file.
SEE ALSO
pcap(3), libnet(3), fragrouter(8)
AUTHOR
Rafal Wojtczuk <nergal@icm.edu.pl>
Manpage by Dug Song <dugsong@monkey.org>, minor updates by Michael Pomraning <mjp@pilcrow.madison.wi.us>
LIBNIDS(3)