PCAP(3)

NAME
libnids - network intrusion detection system E-box library

SYNOPSIS
#include <nids.h>

extern struct nids_prm nids_params;

extern char *nids_warnings[];

extern char nids_errbuf[];
int
nids_init(void);
void
nids_register_ip_frag(void (*ip_frag_func)(struct ip *pkt));
void
nids_register_ip(void (*ip_func)(struct ip *pkt));
void
nids_register_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_killtcp(struct tcp_stream *ts);
void
nids_discard(struct tcp_stream *ts, int numbytes);
void
nids_run(void);
int
nids_dispatch(int);
int
nids_next(void);
int
nids_getfd(void);
void
nids_register_chksum_ctl(struct nids_chksum_ctl *, int);

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;
    	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 pcap_timeout;
    } 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 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.

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_register_ip() registers a user-defined callback function to process IP packets validated and reassembled by libnids.

nids_register_udp() registers a user-defined callback function to process UDP packets validated and reassembled by libnids.

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;
    	...
    };

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 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 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.).

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 exists. 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.

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>