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3	<title>HOWTO Write Samhain Modules</title>
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118
119	<body>
120	<div class="body">
121	<p style="text-align: center; background: #ccc; border: 1px solid #2d5588;"><a
122	style="text-decoration: none;"
123	href="http://www.la-samhna.de/samhain/">samhain file integrity
124	scanner</a> \| <a style="text-decoration: none;"
125	href="http://www.la-samhna.de/samhain/s_documentation.html">online
126	documentation</a></p>
127	<br><center>
128	<h1>Writing modules for samhain</h1>
129	</center>
130	<br>
131	<hr>
132	<p>
133	This document should help anyone who is sitting down to write a module
134	for the samhain host intrusion detection system. We give an overview
135	of samhain's structure from the point of view of the module author,
136	and describe some of the samhain utility and interface functions
137	available. Lastly, we explain how to integrate your module into the
138	samhain autoconf build tools.
139	</p>
140	<h2>Introduction</h2>
141	<p>
142	Samhain is a rather useful file integrity and host intrusion detection
143	system. It is written entirely in C, and much care has been given to
144	making it robust and secure. Additionally, it has been written with
145	extensibility in mind, and so interfaces for adding user-contributed
146	modules have been provided. A module author can easily extend the
147	configuration file syntax and have his checking code run on a regular
148	basis as one of samhain's internal checks.
149	</p>
150
151	<h2>Prerequisites</h2>
152	<p>
153	You'll need to know how to read and write C. You'll need the latest
154	source for samhain. You'll need to have read all of samhain's other
155	documentation. Finally, if you want to make your module build as part
156	of the samhain tree (you do), you'll need GNU's autoconf package.
157	</p>
158	<h2>An overview of samhain's execution</h2>
159	<p>
160	Here's what happens when samhain starts:
161	<ul>
162	<li>
163	Check if samhain has been called with one of the "init.d" type
164	commands - start, stop, reload, status. If so, these are handled
165	as you might expect. Nice feature.
166	</li>
167	<li>
168	Initialise all global structures and parse command-line options.
169	</li>
170	<li>
171	Read the configuration file. This is handled in sh_readconf_read().
172	This includes attempting to download the file if samhain has been
173	compiled to do so.
174	</li>
175	<li>
176	Drop privileges if server.
177	</li>
178	<li>
179	Test the checksum on the database if client or standalone.
180	</li>
181	<li>
182	Now test if samhain has been compiled as a client or a server.
183	<ul>
184	<li>
185	If server, enter server main loop sh_receive() in
186	sh_forward.c. This is simple enough; apart from checks for
187	signals received, the server just accepts incoming
188	connections, verifies that they are from an authorised
189	client, and logs the message received.
190	</li>
191	<li>
192	If client or standalone, we run the rest of main() in
193	samhain.c, which follows:
194	</li>
195	</ul>
196	</li>
197	<li>
198	Initialise modules - that is, call the mod_init() function on each
199	module. Note that if the module intialisation routine returns a
200	nonzero value, you should also have it free anything that's been
201	allocated by the configuration file reading functions, since this
202	method is always called after an sh_readconf_read(), i.e. when the
203	configuration file is re-read after a SIGHUP.
204	</li>
205	<li>
206	Test the setup that's been read from the configuration - for example,
207	check if any files or directories have been defined twice.
208	</li>
209	<li>
210	Enter the main loop (which runs just once if samhain is not
211	configured as a daemon). Test if any signals have been received,
212	and handle them appropriately:
213	<ul>
214	<li>
215	On reconfiguration (SIGHUP), clear internal file lists etc.
216	and call the mod_reconf() function on each module. This should
217	clean up anything internal to the module before the
218	configuration file is re-read. Then read the configuration
219	file again and set things up as before, including a new call
220	to mod_init().
221	</li>
222	<li>
223	On SIGIOT (SIGABRT), shut down the log-file for a moment
224	to allow for rotation.
225	</li>
226	<li>
227	On SIGQUIT, terminate. Note that any call to exit() will
228	invoke the exit_handler() defined in samhain.c; the first
229	thing this does is to call mod_cleanup() on all modules.
230	Then it cleans up everything else in samhain and exits.
231	</li>
232	<li>
233	On SIGUSR1 turn toggle debugging on/off.
234	</li>
235	<li>
236	On SIGUSR2 suspend the daemon an notify the server to
237	allow a second instance of samhain downloading its
238	configuration file without triggering an alert (restart
239	without exit) on the server.
240	</li>
241	</ul>
242	</li>
243	<li>
244	If it's time to check files, check directories and then files, and
245	then flush the mail queue.
246	</li>
247	<li>
248	Execute modules. For each module, if mod_timer(tcurrent) returns a
249	nonzero value, then execute mod_check().
250	</li>
251	<li>
252	Do various maintenance operations such as logging a timestamp/sending
253	some mail if it's time, seeding/re-seeding the PRNG, etc.
254	</li>
255	</ul>
256	You'll note that in the text above I refer to a couple of module
257	functions - mod_init(), mod_check(), etc. These are function pointers
258	that act as hooks for attaching modules to samhain. Next we'll
259	describe how they are used.
260	</p>
261
262	<h2>Samhain's module interface</h2>
263	<p>
264	Here we'll describe the interface samhain provides to module authors.
265	</p>
266
267	<h3>The module list</h3>
268	<p>
269	In sh_modules.h, the following structure is defined:
270	</p>
271	<pre>
272
273	typedef struct mod_type
274	{
275	/* The name of the module */
276	char * name;
277
278	/* Set by samhain to 1 on successful initialization, else 0 */
279	int initval;
280
281	/* The initialization function. Return 0 on success. */
282	int (* mod_init) (void);
283
284	/* The timer function. Return 0 if NOT time to check. */
285	int (* mod_timer) (unsigned long tcurrent);
286
287	/* The check function. Return 0 on success. */
288	int (* mod_check) (void);
289
290	/* The cleanup function. Return 0 on success. */
291	int (* mod_cleanup) (void);
292
293	/* The preparation for reconfiguration. Return 0 on success. */
294	int (* mod_reconf) (void);
295
296	/* Section header in config file */
297	char * conf_section;
298
299	/* A table of key/handler_function for config file entries */
300	sh_rconf * conf_table;
301
302	} sh_mtype;
303	</pre>
304
305	<p>
306	This is the structure used to hook modules into samhain. There is a
307	list of these structures (modList), defined in sh_modules.c,
308	containing pointers to the functions to be used for each module
309	compiled into samhain. For example,
310	</p>
311
312	<pre>
313
314	sh_mtype modList[] = {
315	#ifdef SH_USE_UTMP
316	{
317	N_("UTMP"),
318	0,
319	sh_utmp_init,
320	sh_utmp_timer,
321	sh_utmp_check,
322	sh_utmp_end,
323	sh_utmp_null,
324
325	N_("[Utmp]"),
326	sh_utmp_table,
327	},
328	#endif
329	</pre>
330	<p>
331	is the beginning of that table. The author of the sh_utmp module has
332	initialised the structure with the name of the module (note that N_()
333	is just a macro used to delimit strings here), a 0 to signify that the
334	module has not yet been initialised, and then pointers to _init(),
335	_timer(), _check(), _cleanup() and _reconf() functions for the
336	module. Finally, the last two structure elements are for configuration
337	file parsing: the first is the section heading in the configuration
338	file for this module, and the second is a table of type
339	</p>
340
341	<pre>
342
343	typedef struct rconf
344	{
345	char * the_opt;
346	int (func)(char opt);
347	} sh_rconf;
348	</pre>
349
350	<p>
351	(also defined in sh_modules.h). This structure is for storing options
352	for this module to be found in the configuration file, as well as the
353	functions that will be used to parse them when found. In the sh_utmp
354	example above, we can see that this table has been set to
355	sh_utmp_table - this is a reference to a list of the Utmp module's
356	configuration options declared in sh_utmp.h. It should be clear now
357	that one of the changes you will need to make to samhain's source
358	files is to include your header file in sh_modules.c and add a modList
359	entry like the above.
360	</p>
361
362	<p>
363	For a description of when during samhain's execution these various
364	module hooks are called, see the overview above. It would likely be
365	helpful to you now to read through the source for one of the modules
366	provided with samhain and see the above actually implemented. You
367	should also be able to use one of these modules as a template for your
368	own.
369	</p>
370
371	<h3>The message catalogue</h3>
372
373	<p>
374	Most module authors will want to log messages in their own specified
375	format; samhain stores all of its message formats in a "messages
376	catalogue" found in sh_cat.h and sh_cat.c. For example, for the
377	sh_suidchk module we find the following entries in sh_cat.h, as part
378	of an enum:
379	</p>
380
381	<pre>
382
383	#ifdef SH_USE_SUIDCHK
384	MSG_SUID_POLICY,
385	MSG_SUID_FOUND,
386	MSG_SUID_STAT,
387	MSG_SUID_SUMMARY,
388	#endif
389	</pre>
390
391	<p>
392	Correspondingly in sh_cat.c we find
393	</p>
394
395	<pre>
396
397	#ifdef SH_USE_SUIDCHK
398	{ MSG_SUID_POLICY, SH_ERR_SEVERE, RUN, N_("msg=\"POLICY SUIDCHK %s\" path=\"%s\"") },
399	{ MSG_SUID_FOUND, SH_ERR_INFO, RUN, N_("msg=\"Found suid/sgid file\" path=\"%s\"") },
400	{ MSG_SUID_STAT, SH_ERR_ERR, ERR, N_("msg=\"stat: %s\" path=\"%s\"") },
401	{ MSG_SUID_SUMMARY,SH_ERR_INFO, RUN, N_("msg=\"Checked for SUID programs: %ld files, %ld seconds\"") },
402	#endif
403	</pre>
404	<p>
405	as part of the table msg_cat[] of type cat_entry:
406	</p>
407	<pre>
408
409	typedef struct foo_cat_entry {
410	unsigned long id;
411	unsigned long priority;
412	unsigned long class;
413	char * format;
414	} cat_entry;
415	</pre>
416	<p>
417	The first member of this structure is the message type's ID, as
418	defined in the enum in sh_cat.h. The second is the default priority of
419	such messages, defined as in the samhain documentation. The third is
420	the class of the message, again defined as in the samhain
421	documentation. Finally we have the message format itself, which is a
422	printf() style format string.
423	</p>
424	<p>
425	This catalogue is used by the logging functions in samhain; you will
426	need to add your own message types and formats to sh_cat.h and
427	sh_cat.c. Note that because samhain can be compiled for XML style
428	logging, you will actually need to make two entries in sh_cat.c for
429	each message; see the file itself for details.
430	</p>
431	<p>
432	Note that there is a generic message format with the ID 'MSG_E_SUBGEN'
433	and the default priority 'SH_ERR_ERR'. If you are using this message
434	format, then you can log (a) a string, and (b) the name of the subroutine.
435	</p>
436	<p>
437	This completes our description of samhain's module interface.
438	</p>
439
440	<h2>Samhain's utility functions</h2>
441	<p>
442	Here we'll describe the main utility functions available to samhain module
443	authors.
444	</p>
445
446	<h3>String wrapping macros</h3>
447
448	<p>
449	Constant strings should be wrapped in the _(string) macro. Initialisation
450	strings that cannot be replaced with a function should be wrapped
451	in a N_(string) macro, and the variable thus initialized should be
452	wrapped in a _(var) macro whereever used. This is important for the
453	'stealth' functionality of samhain.
454	</p>
455
456	<h3>Logging messages</h3>
457
458	<pre>
459	#include "sh_error.h"
460
461	void sh_error_handle(int severity, char * file, long line, long status,
462	unsigned long msg_id, ...)
463	</pre>
464	<p>
465	This is samhain's logging/reporting function, so the name is a little
466	misleading - errors are not the only thing we should handle with
467	this. The first four arguments are simple enough: severity is the
468	logging severity, defined in the enum ShErrLevel from sh_error.h; file
469	and line are the current file and line - usually you'll be using FIL__
470	and __LINE__ for these; status is not very important - for module
471	authors it'll do to always pass 0 to this. The final named argument is
472	msg_id, which should be one of the message IDs defined in sh_cat.h;
473	these correspond to message format strings in printf() format, which
474	will be interpolated with the following arguments to form the log
475	message.
476	</p>
477	<p>
478	The '__LINE__' macro is provided by the C preprocessor. The FIL__ macro
479	should be #defined to '_("sourcefile_name")' (see 'String wrapping macros'
480	above).
481	<p>
482	Example of use:
483	</p>
484	<pre>
485	#undef
486	#define FIL__ _("sh_mounts.c")
487
488	sh_error_handle(ShMountsSevMnt, FIL__, __LINE__, 0, MSG_MNT_MNTMISS,
489	cfgmnt->path);
490	</pre>
491	<p>
492	See cat.c for the definition of MSG_MNT_MNTMISS:
493	</p>
494	<pre>
495
496	{ MSG_MNT_MNTMISS, SH_ERR_WARN, RUN, N_("msg=\"Mount missing\" path=\"%s\"")},
497	</pre>
498	<p>
499	So we print this out at severity ShMountsSevMnt, which in this case is
500	a configured value read from the samhain configuration file (see
501	sh_mounts.c). If we wanted to print it at the default severity
502	(SH_ERR_WARN), we could pass -1 as the severity.
503	</p>
504
505	<h3>Checking files for modification</h3>
506
507	<pre>
508	#include "sh_files.h"
509
510	int sh_files_pushdir_?? (char * dirName);
511	int sh_files_pushfile_?? (char * fileName);
512	</pre>
513	<p>
514	These functions push directories and files onto the stack of those to check
515	for the specified policy (see the samhain documentation for further
516	information):
517	<table>
518	<tr><td> sh_files_pushdir_user0 </td><td>pushes the directory at USER0 </td></tr>
519	<tr><td align=right> ... _user1</td><td align=right>USER1</td></tr>
520	<tr><td align=right> ... _attr</td><td align=right>ATTR</td></tr>
521	<tr><td align=right> ... _ro</td><td align=right>READONLY</td></tr>
522	<tr><td align=right> ... _log</td><td align=right>LOGFILE</td></tr>
523	<tr><td align=right> ... _glog</td><td align=right>GROWING LOGFILE</td></tr>
524	<tr><td align=right> ... _noig</td><td align=right>IGNORE NONE</td></tr>
525	<tr><td align=right> ... _allig</td><td align=right>IGNORE ALL</td></tr>
526	</table>
527	So if you're writing a module that adds particular files to check, like the
528	sh_userfiles module for example, these are the functions to use.
529	</p>
530
531	<h3>Managing memory</h3>
532
533	<pre>
534	#include "sh_mem.h"
535
536	#define SH_FREE(a) ...
537	#define SH_ALLOC(a) ...
538	</pre>
539	<p>
540	These are the macros to use when you're allocating/freeing memory in
541	samhain. They do all the error checking/reporting you need, so when
542	you get memory from SH_ALLOC you can just get to using it right away.
543	</p>
544
545	<h3>Parsing strings</h3>
546
547	<pre>
548	#include "sh_utils.h"
549
550	char * sh_util_strdup (const char * str);
551	char * sh_util_strsep (char *str, const char delim);
552	char * sh_util_strconcat (const char * arg1, ...);
553
554	int sh_util_flagval(char * c, int * fval);
555	int sh_util_isnum (char *str);
556
557	#include "slib.h"
558
559	int sl_strlcpy (char * dst, const char * src, size_t siz);
560	int sl_strlcat (char * dst, const char * src, size_t siz);
561	int sl_snprintf(char str, size_t n, const char format, ... );
562	</pre>
563	<p>
564	These functions are the samhain internal functions for string
565	handling. The first three act like their C library counterparts,
566	except using samhain's memory management functions and error
567	checking. sh_util_flagval converts the passed string into a truth
568	value - the value is stored in <b>fval</b> as 1 or 0 - and returns 0
569	on success, -1 on failure. sh_util_isnum just checks if the passed
570	string is all numeric.
571	</p>
572	<p>
573	The functions sl_strlcpy and sl_strlcat work similar to the C library
574	strncpy/strncat functions, except that the destination string is always
575	null terminated, and the third argument must be the full length of the
576	destination buffer, <i>not</i> the remaining space. On success, the
577	return value is 0.
578	</p>
579	<p>
580	The function sl_snprintf provides either the system snprintf, or a replacement,
581	if the system has no or a buggy snprintf.
582	</p>
583
584	<h3>Tracing execution</h3>
585
586	<pre>
587	#include "slib.h"
588
589	#define SL_ENTER(s) ...
590	#define SL_RETURN(retval, s) ...
591	</pre>
592	<p>
593	These macros are for tracing execution through samhain functions. You
594	should use SL_ENTER with the name of the function for each function
595	entered, and SL_RETURN with the return value and the name of the
596	function for each exit if you want to maintain compatibility with the
597	rest of samhain.
598	</p>
599
600	<h3>Executing external programs (popen)</h3>
601
602	<pre>
603	#include "sh_extern.h"
604
605
606	sh_tas_t task;
607	/* Prepare task */
608	sh_ext_tas_init(&task);
609	sh_ext_tas_command(&task, char * command);
610	sh_ext_tas_add_argv(&task, char * val);
611	sh_ext_tas_add_envv (&task, char * environment_variable, char * value);
612
613	int sh_ext_popen(&task);
614	int sh_ext_pclose(&task);
615	</pre>
616	<p>
617	To prepare a task to run, use 'sh_ext_tas_init' to initialise the task
618	structure. With 'sh_ext_tas_command' the command (absolute path) is set,
619	with 'sh_ext_tas_add_argv' command line options are added. Environment
620	variables can be set with 'sh_ext_tas_add_envv'.
621	</p>
622	<p>
623	To open for read, set "task.rw = 'r';", to open for write
624	use "task.rw = 'w';".
625	</p>
626	<p>
627	To run the task with privileges dropped to another UID, set
628	"task.privileged = 0;" and task.run_user_uid, task.run_user_gid
629	to the desired UID/GID.
630	</p>
631	<p>
632	To verify the checksum of the called executable, set
633	task.checksum[KEY_LEN+1] to the TIGER192 checksum of the executable.
634	</p>
635	<p>
636	After successful execution of sh_ext_popen (return status 0),
637	task.pipe is the stream opened for read or write, and task.pipeFD
638	its associated file descriptor.
639	</p>
640
641	<h3>Inserting arbitrary data into the baseline database</h3>
642	<pre>
643
644	#include "sh_hash.c"
645
646	void sh_hash_push2db (char * key, unsigned long val1,
647	unsigned long val2, unsigned long val3,
648	unsigned char * str, int size);
649
650	char * sh_hash_db2pop (char * key, unsigned long * val1,
651	unsigned long * val2, unsigned long * val3,
652	int * size);
653	</pre>
654	<p>
655	The baseline database has a fixed record format. To enter data, these need
656	to be prepared in the required format. To retrieve the data, the
657	'filepath' is used as key (if your data is not a file, you would provide
658	a dummy pathname as key). For convenience, the two functions noted below
659	are provided.
660	</p>
661	<p>
662	When checking files, samhain will walk the database to find files that
663	are in the database, but have been deleted from the disk. If you enter
664	data, you need to mark it as such by using a key that
665	starts with something else but '/', otherwise samhain will complain
666	if it has not been checked during the file check.
667	</p>
668	<pre>
669
670	#include "sh_hash.c"
671
672	void sh_hash_push2db (char * key, unsigned long val1,
673	unsigned long val2, unsigned long val3,
674	unsigned char * str, int size);
675
676	char * sh_hash_db2pop (char * key, unsigned long * val1,
677	unsigned long * val2, unsigned long * val3,
678	int * size);
679	</pre>
680	<p>
681	To insert data, use 'sh_hash_push2db'. You can insert up to three long
682	integers (val1, val2, val3) and/or a binary string of length size
683	(max. (PATH_MAX-1)/2). As noted
684	above, you need to supply a key (stored as the 'filepath', which should
685	start with a character different from '/'). To retrieve data, you can use
686	'sh_hash_db2pop'. The return value is either NULL (if no string was
687	stored under this key), or the stored string (length returned in 'size').
688	</p>
689	<p>
690	A string to store may consist of any characters, including NULLs, and
691	need not be NULL terminated. The returned string is
692	always NULL terminated (the terminating NULL is not included in 'size'),
693	and should be freed with SH_FREE() if not required anymore.
694	</p>
695	<p>
696	If the key is not found in the database, <b>size</b> is set to -1.
697	</p>
698
699	<h2>Incorporating modules into the samhain build</h2>
700	<p>
701	This is a somewhat secondary but important part of writing a module
702	for samhain:
703	how to incorporate it into the samhain configuration and build process.
704	This just involves hacking the autoconf and makefile setup to include your
705	module. We'll present this file-by-file.
706	</p>
707	<h3>Makefile.in</h3>
708	<p>
709	You need to add a few bits to this file. First, add your header,
710	source and object filenames to the HEADERS, SOURCES and OBJECTS
711	variables. Then add your header to the dependencies for sh_modules.o
712	and ./sh_modules.o. Finally add dependency lines for your module
713	object file sh_whatever.o and ./sh_whatever.o, modelling them on the
714	other module object dependency lines.
715	</p>
716	<h3>acconfig.h</h3>
717	<p>
718	The config.h.in will be generated from this file by 'autoheader'.
719	You just need to add a line like
720	<pre>
721	#undef SH_USE_MOUNTS
722	</pre>
723	that will be defined by the ./configure code if the user specifies
724	the module as enabled.
725	</p>
726	<h3>aclocal.m4</h3>
727	<p>
728	This file is used by 'autoconf' to help generate ./configure. You need to add
729	your module's ./configure option to the SH_ENABLE_OPTS variable; for example,
730	to add the option --enable-mounts-check, we added the string 'mounts-check' to
731	this variable.
732	</p>
733	<h3>configure.ac</h3>
734	This is the other file used by 'autoconf' to generate ./configure. You need to
735	add an AC_ARG_ENABLE call to this file, along the lines of those for other
736	modules. For example, we added
737	<pre>
738	AC_ARG_ENABLE(mounts-check,
739	[ --enable-mounts-check check mount options on filesystems [[no]
740	]],
741	[
742	if test "x${enable_mounts_check}" = xyes; then
743	AC_DEFINE(SH_USE_MOUNTS)
744	fi
745	]
746	)
747	</pre>
748	for the sh_mounts module. This causes the #undef from acconfig.h above
749	to be defined when ./configure is run with the --enable-mounts-check argument.
750	</p>
751	This is all that you need. Once you've done the above, you'll need to
752	run 'autoheader' and 'autoconfig' to generate config.h.in and the
753	./configure script. Then your module will build as part of the samhain
754	source.
755	</p>
756
757	<h2>Conclusion</h2>
758	<p>
759	Armed with the above information, any proficient C programmer should
760	be able to adapt and extend samhain to do whatever it is they need. We
761	hope that this document has been reasonably clear, easy to follow and
762	useful; please feel free to update it for clarity, accuracy and
763	completeness and resubmit it to the samhain project.
764	</p>
765	<p>
766	This document was written by the eircom.net Computer Incident Response Team.
767	Updated with CSS by Rainer Wichmann.
768	</p>
769	</div>
770	</body>
771	</html>

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