LIBNIDS(3) LIBNIDS(3)
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);
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int
nids_getfd(void);
int
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;
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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
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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
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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;
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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:
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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.
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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>
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