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LIBNIDS(3) Library Functions Manual 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); 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 val- ues 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 informa- tion ( 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 in- formation. 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 ap- propriate 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 infor- mation. 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. De- fault 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 sys- log(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 dis- carded. Default value: nids_ip_filter (which always returns 1) pcap_filter pcap(3) filter string applied to the link-layer (raw, unassem- bled) 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. De- fault 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 invoca- tion of the TCP callback function (if 0, no new urgent data ar- rived). 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 in- teresting, 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-spe- cific data to pass to subsequent invocations of the TCP callback func- tion (ex. the current working directory for an FTP control connection, etc.). The user pointer in the tcp_stream structure has the same purpose ex- cept 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 dis- carded 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 fail- ure, returns -1, putting an appropriate error message in nids_errbuf. nids_next() process the next available packet before returning. Re- turns 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_er- rbuf 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 check- summed; if the action field is NIDS_DONT_CHKSUM, the packet will not be checksummed. If the packet matches none of the array elements, the de- fault 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 Pom- raning <mjp@pilcrow.madison.wi.us> LIBNIDS(3)
NAME | SYNOPSIS | DESCRIPTION | ROUTINES | SEE ALSO | AUTHOR
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