samhain offers several pre-defined monitoring policies. Each of these policies has its own section in the configuration file. Placing a file in one of these sections will select the respective policy for that file.

The available policies (section headings) are:

Note

Each policy can be modified in the config file section Misc with entries like RedefReadOnly= +XXX[,...] or RedefReadOnly= -XXX[,...] to add (+XXX) or remove (-XXX) a (a comma-separated list of) tests XXX, where XXX can be any of CHK (checksum), TXT (store file content in database), LNK (link), HLN (hardlink), INO (inode), USR (user), GRP (group), MTM (mtime), ATM (atime), CTM (ctime), SIZ (size), RDEV (device numbers), MOD (file mode), PRE (Linux; prelinked binary), SGROW (file size is allowed to grow), and/or AUDIT (Linux; report who changed the file) This must come before any file policies are used in the config file.

Entries for files have the following syntax:

file= /full/path/to/the/file

Entries for directories have the following syntax:

dir= [recursion depth]/full/path/to/the/directory

The specification of a (numerical) recursion depth is optional (see Section 4.5 ). (Do not put the recursion depth in brackets — they just indicate that this is an optional argument!).

Wildcard patterns ('*', '?', '[...]') as in shell globbing are supported for paths. The leading '/' is mandatory. Since version 2.7.1, it is allowed to enclose the value of the directive (i.e. the path for files, the optional recursion depth and the path for directories) within matching single or double quotes, which allows to have trailing blanks ( note: it is not neccessary to escape quotes in between - the algorithm does not scan forward to find the matching quote, rather it uses the last character). Also since version 2.7.1, C quoting style is supported ('\a' [bell], '\b' [backspace], '\f' [form feed], '\n' [newline], '\r' [carriage return], '\t' [horizontal tab], '\v' [vertical tab], '\\' [backslash], '\'' [single quote], '\"' [double quote], '\nnn' [dree digit octal value], \xNN' [two digit hexadecimal value]). Example:

	  [ReadOnly]
	  # valid examples
	  dir = /u01/oracle/archive00
	  dir = 7/u01/oracle/archive02
	  dir = "7/u01/oracle/archive03  "
	  dir = "7/u01/oracle/archi"ve"
	  dir = /u01/oracle/archive\v04
	  dir = /u01/oracle/archive\\04
	  dir = /u01/oracle/archive\076
	  file = '/u01/oracle/archive\x0a'
	  
	  # valid (no quote at start, thus quote at end
	  #        is considered part of filename)
	  file = /u01/oracle/archive_0"
	  
	  #invalid (no matching quote at end)
	  file = "/u01/oracle/archive_0
	  
	  #invalid (\03 is bad, must be 3 digits [octal]
	  #         or \x03 for hexadecimal)
	  file = /u01/oracle/archive_\03
	  
	  #invalid (\g is undefined escape sequence)
	  file = /u01/oracle/archive_\g
	

Note on directories

A directory is (a) a collection of files, with (b) a directory special file where a listing of all files in the directories is kept. This directory special file will be modified in case of a file addition, removal, or renaming. Depending on the chosen policy, samhain will report on such modifications of the directory special file. The addition and/or deletion of files from a directory modifies the directory special file (mtime/ctime). The addition/deletion of subdirectories will also modify the number of hardlinks of the directory special file. A modification of a file may modify a directory special file (mtime/ctime), if this modification is done by first creating a temporary file, followed by renaming this temporary file to the original one.

	    [ReadOnly]
	    dir = /u01/oracle/archive00
	    [IgnoreAll]
	    file = /u01/oracle/archive00
	    
	    # Note: /u01/oracle/archive00/archive01.dbf -> archive99.dbf *should* be
	    # mounted in the DB as a read-only tablespace and should never be
	    # changed, however, the DBA thinks he's God and does not need to consult
	    # with the Admin, so he may be adding new, deleting, or renaming the
	    # DBFs using SQLPlus without consulting with the admin, so tell me about
	    # changes to the files inside that we know about at Samhain INIT but
	    # such as when he adds a file.
	  

	    [Attributes] 
	    file = /var/spool/mqueue 
	    file = /tmp 
	    [IgnoreAll]
	    dir=-1/var/spool/mqueue 
	    dir=-1/tmp
	  

	    [Attributes] 
	    file = /root 
	    [IgnoreAll] 
	    dir=0/root
	  

If you want to suppress messages about the creation of certain files (e.g. rotated log files), you can use the options IgnoreAdded=/fullpath/with_some_regex_inside and/or IgnoreMissing=/fullpath/with_some_regex_inside (to be placed in the [Misc] section of the configuration files. If you want to add more regular expressions, you can use these options multiple times. Since transient files might get modified during their lifetime, there is also the option IgnoreModified=/fullpath/with_some_regex_inside

Note

The argument to IgnoreAdded , IgnoreMissing , and IgnoreModified must be a regular expression that matches the full path. In particular, it has to start with a forward slash.. To test your regex before putting in samhain, you do something like this: # This regex matches all files added by logrotate # (e.g: messages.1 or messages.2.gz, etc.) cd /var/log for file in *; do echo $file| \ egrep "(cron|messages|rpmpkgs|secure|up2date|wmtp)\.[0-9](\.gz)?$"; done Once it's work this way, you can add it to your samhainrc file, but don't forget to add the full path. e.g: IgnoreAdded = your_tested_regex This tip has been provided by jim at aegis hyphen corp dot org.

Alternative

If a directory is added to [Attributes] as a file=/dir, then only the directory special file is monitored for permissions/ownership. The advantage is that additions/removals of files to that subdirectory can happen without recourse, but the integrity of that directory is defended. Assuming the administrator doesn't want to get granular level of detail. Good for such directories as: /var/mail /var/cron/tabs /var/tmp /tmp This tip has been provided by Brian A. Seklecki

samhain reads the file signature database at startup and creates an in-memory copy. This in-memory copy is then dynamically updated to reflect changes in the file system.

I.e. for each modified/disappeared/new file you will receive an alarm, then the in-memory copy of the file signature database is updated, and you will only receive another alarm for that file if it is modified again (or disappears/appears again).

Note that the on-disk file signature database is not updated (if you have signed it, the daemon could not do that anyway). However, as long as the machine is not rebooted, there should be no need to update the on-disk file signature database.

If files disappear after initialization, you will get an error message with the severity specified for file access errors ( except if the file is placed under the IgnoreAll policy, in which case a message of SeverityIgnoreAll— see Section 1.1 — is generated).

If new files appear in a monitored directory after initialization, you will get an error message with the severity specified for that directory's file policy ( except if the file is placed under the IgnoreAll policy, in which case a message of SeverityIgnoreAll— see Section 1.1 — is generated).

The special treatment of files under the IgnoreAll policy allows to handle cases where a file might be deleted and/or recreated by the system more or less frequently.

Directories can be monitored up to a maximum recursion depth of 99 (i.e. 99 levels of subdirectories. The recursion depth actually used is defined in the following order of priority:

As of version 1.8.4, samhain will by default compare the number of hardlinks of a directory to the number of its subdirectories (including "." and ".."). Normally, these numbers should be equal. The idea here is that a (kernel) rootkit may hide a directory, but fail to "fix" the parent directory hardlink count (actually, I am not aware of any kernel rootkit that would care to fix the hardlink count of the parent directory). This is an experimental feature; if there are any problems, it can be disabled with the option UseHardlinkCheck= no in the [Misc] section of the configuration file.

Errors will be reported at the same severity as directory access errors option SeverityDirs= severity in section [EventSeverity]).

	MacOS X
	This feature is not supported on MacOS X (because the resource fork is implemented as an invisible directory, it modifies the parent directory hardlink count.)

4.6.1. Specify exceptions for the hardlink check

Some filesystems do not always follow the rule mentioned above (directory hardlink equals number of subdirectories). E.g. the root directory of reiserfs partitions generally seems to have two additional hardlinks. To account for such exceptions, you can specify exceptions with the option HardlinkOffset= N: /path in the [Misc] section of the configuration file. Here, N is the numerical offset (actual - expected hardlinks) for /path. For multiple exceptions, use this options multiple times (note that '/path N:/path2' would itself be a valid path, so using the option only once with multiple exceptions on the same line would be ambiguous).

	Note
	Please note that samhain will not check for an exception if the standard rule (offset = 0) is true for a directory. Thus it will not warn if a directory that once was exceptional is not anymore.

Samhain checks for weird filenames (containing control/nonprintable characters, newlines or tabs) and warns about them at a severity level that is set with SeverityNames= severity in section [EventSeverity]. The rationale is: most of the time, such names are either the result of user errors, buggy scripts, or questionable activity.

If you want to add characters to the set of 'good' ones, you can do so with the option: AddOKChars= N1, N2, ... in the [Misc] section of the configuration file. Nn should be the unsigned byte value of the character(s) in hex (leading '0x': 0xNN), octal (leading zero: 0NNN), or decimal.

	UTF-8 filenames
	To specify that filenames are UTF-8 rather than ASCII, use FileNamesAreUTF8=yes . Samhain will check for invalid UTF-8 sequences, and for filenames ending with invisible characters.

	Tip
	This check will not be performed for files under the IgnoreAll policy. To completely disable this check, use AddOKChars=all .

prelink is a tool available on modern Linux systems that can significantly reduce the startup time of applications. It does this by performing some of the work of the dynamic linker in advance. As this changes both executables and shared libraries, file integrity verification will fail unless prelink is supported, in particular as prelinking has to be redone if libraries are updated (so initializing the checksum database after prelinking may not be good enough).

The disadvantage is that prelinking modifies libraries and executables, and may need to be redone (potentially modifying all or many executables again) if a library is updated. This is a major problem for file integrity checkers.

Version 2.0 of samhain and later support prelink. To use this support, you need to place prelinked executables and libraries (or directories holding them) under the [Prelink] policy rather than under the (e.g.) [ReadOnly] policy. For all files under the [Prelink] policy, inode, size, and timestamps will be ignored (prelinking changes them). In addition, for ELF binaries under the [Prelink] policy, /usr/sbin/prelink --verify will be used to compute checksums (i.e. the checksum will be computed on the output of this command). For other files, checksums are computed as usual.

	Speed
	Obviously, invoking prelink results in a significant overhead, and slows down file integrity checking (tests indicate a factor of three - your mileage may vary).

	Verification failures (zero checksum)
	It seems that prelink --verify fails if the dependencies of a prelinked binary have changed. This results in a zero checksum, and can be fixed by re-prelinking the affected binary.

There are two configuration options in the [Misc] section that can are relevant for prelink support:

SetPrelinkPath= fullpath sets the path to the prelink executable. The default is /usr/sbin/prelink.

SetPrelinkChecksum= checksum sets the TIGER192 checksum for the prelink executable. You can compute this with samhain -H /usr/sbin/prelink (remove whitespace from the computed checksum). If the checksum is set, samhain will verify the prelink executable immediately before using it, otherwise prelink will be used without this special precaution.

	Note for users of SQL database logging
	You need to update the database scheme before using this feature, if you are upgrading from a version below 2.3.0. See Section 12.1 for details.

As of version 2.3, samhain supports checking and verifying of SELinux attributes and/or Posix ACLs, if the operating system supports these features. SELinux attributes are a Linux-specific feature, while Posix ACLs are supported by multiple operating systems.

These features will only get compiled if the required development environment is available on the host where samhain is compiled (e.g. on Debian Linux, packages libattr1-dev and libacl1-dev).

For backward compatibility, these features are disabled by default, even if they are compiled in. To enable them, use the configuration directives:

	  [Misc] 
	  UseACLCheck = yes 
	  UseSelinuxCheck = yes
	

As of version 1.8.2, reports about modified files include a short code in the message field to describe which properties have been modified. The codes are: 'C' for 'checksum', 'L' for (soft) 'link', 'D' for 'device number', 'I' for 'inode', 'H' for (number of) 'hardlinks', 'M' for 'mode', 'U' for 'user' (owner), 'G' for 'group' (owner), 'T' for 'time' (any), and finally 'S' for 'size'.

As an example, 'C--I----TS' would indicate that a file has been replaced by one with different checksum, inode, timestamp, and size, but (e.g.) same mode (type and access permissions) and same ownership.

If files are added to, or removed from a directory, or modified by writing a temporary file and renaming it to the original, samhain will report the changed file as well as the changed directory inode. If you regard the report on the directory inode as redundant, you can suppress it with the option: LooseDirCheck= true in the [Misc] section of the configuration file. This will cause samhain to ignore modified directory inodes if nothing else but size and timestamps has changed.

This is discussed in Section 20 .

First of all, the UID of the user who changed or created a file is not stored in the file metadata, and hence in general not available. However, some operating systems may have non-standard security enhancements to log such information.

In particular, sufficiently recent versions of the Linux kernel provide an audit subsystem that can be used to gather such information if the required userspace tools are installed and the system is properly configured.

It should be noted that the Linux audit subsystem does NOT audit every file access by default. Rather, files are only audited if a watch is placed on them. What Samhain can do is making sure that watches are indeed placed on all files of interest for you (as defined in the Samhain configuration file), and collecting relevant information in case of an event.

Samhain supports the Linux audit system insofar as it can automatically mark files of interest for logging, and automatically collects and reports the log information after a change has been detected. This implies that you don't need to maintain two separate configurations (one for Samhain and one for the audit daemon).

4.13.1. Compiling

If you compile a Samhain standalone or client executable on a Linux system, support for the Linux audit system will be compiled in automatically if the required development packages for libaudit and libauparse are present (Debian: libaudit-dev, RedHat: audit-libs-devel).

To verify that the compiled executable has audit support, use samhain --version, which should list "optionally report auditd record of changed file".

	Not supported with static compiling
	This extension is incompatible with static compiling, i.e. it is not available if '--enable-static' has been used for compiling.

4.13.3. Usage

In order to activate this feature for some particular file or directory, you have to add the AUDIT flag to the policy under which you place the file or directory (see Section 4.1 ). Note that placing an audit rule on a directory will cause the whole file hierarchy under that directory to be audited. Also note that you cannot place an audit rule on the root directory itself. This is a limitation of the Linux audit system itself, not of Samhain. You have to place audit rules on the individual directories in the root directory.

	    [Misc]
	    RedefReadOnly = +AUDIT 
	    # 
	    [ReadOnly] 
	    file = /etc/login.defs 
	    dir = /bin 
	  

By default, Samhain will set the flags "wa" (write|attribute change) on which the watch will trigger. If you want to use other flags (e.g. "r" for read), you can do so with the SetAuditdFlags= r|w|x|a (i.e. any combination of these four characters, see also man 8 auditctl). Please note that order matters (as with Redef... directives), i.e. this will affect only files/directories specified afterwards in the config file.

The rules set by Samhain are flagged with samhain, i.e. you can check them with auditctl -l -k samhain, and delete them manually with auditctl -D -k samhain. See the man page of the auditctl command for further reference.

If you want to verify that the audit system works properly, you can e.g. use ausearch -k samhain to see all audit log entries generated by rules flagged with 'samhain', i.e. inserted by Samhain.

	Persistance of audit rules
	If Samhain runs in the foreground, it will not delete the inserted rules upon exit. This is to ensure that file changes are still audited as desired when Samhain is run only occasionally or at fixed intervals (e.g. as a cron job). If you want to delete the rules, use the command auditctl -D -k samhain .

Checksumming can put a high I/O load on a machine, and in some cases one might want to skip this for particular files. As of version 2.8.2, Samhain allows to specify certain conditions for which checksumming of a file should be skipped. These are:

Files that should not be checksummed are specified with SkipChecksum= list of conditions in the [Misc] section of the configuration file. The following rules apply:

Growing log files should be placed under the [GrowingLogFiles] policy which (as of 3.0.11+) uses the following logic to handle log rotation:

If the size has shrunk (e.g. because of log rotation), samhain will look for a file with the same inode as before and check whether it has a size >= previous_size, and checksum(file start up to previous size) equal to the previous checksum. No report will be issued if this check succeeds. Thus log rotation will be handled gracefully as long as the inode is kept (i.e. the old file is moved rather than copied) and the first rotated file is not compressed (the logrotate tool can be told to compress only after the second cycle, and on Debian this seems to be standard anyway).

Samhain will report on files that are unowned (i.e. belong to a UID/GID that is not associated with an existing user or group. The rationale is that this is a security risk. An administrator (or administrative script) may get tricked into creating a user/group for that UID/GID, thus allowing the freshly created user/group to hijack the unowned files.

As of version 4.3.0, on Linux Samhain will ignore unowned files if the UID/GID is mapped in a user namespace via the /etc/subuid and /etc/subgid files respectively.