samhain - check file integrity

INITIALIZING, UPDATING, AND CHECKING

LISTING THE DATABASE

VERIFYING AN AUDIT TRAIL

MISCELLANEOUS

SERVER STARTUP

SERVER MISCELLANEOUS

LOG OPTIONS

The information in this man page is not always up to date. The authoritative documentation is the user manual.

samhain is a file integrity / intrusion detection system both for single hosts and networks. It consists of a monitoring application (samhain) running on individual hosts, and (optionally) a central log server (yule). Currently, samhain can monitor the integrity of files/directories, and (optionally) also check for kernel rootkits (Linux and FreeBSD only), search the disk for SUID/SGID, and watch for login/logout events.

samhain/yule can log by email, to a tamper-resistant, signed log file, to syslog, to the Prelude IDS, to a MySQL/PostgreSQL/Oracle database, and/or to stdout (/dev/console if run as daemon). samhain/yule can run as a daemon, and can use a time server instead of the host's system clock. Most of the functionality is defined by a configuration file that is read at startup.

Most options of these usually would be set in the configuration file. Options given on the command line will override those in the configuration file.

OPTIONS FOR INITIALIZING, UPDATING, AND CHECKING

OPTIONS FOR LISTING THE DATABASE

OPTIONS TO VERIFY AN AUDIT TRAIL

MISCELLANEOUS OPTIONS

SERVER STARTUP OPTIONS

MISCELLANEOUS SERVER OPTIONS

LOGGING OPTIONS

SIGUSR1

Switch on/off maximum verbosity for console output.

SIGUSR2

Suspend/continue the process, and (on suspend) send a message to the server. This message has the same priority as timestamps. This signal allows to run samhain -t init -e none on the client to regenerate the database, with download of the configuration file from the server, while the daemon is suspended (normally you would get errors because of concurrent access to the server by two processes from the same host).

SIGHUP

Reread the configuration file.

SIGTERM

Terminate.

SIGQUIT

Terminate after processing all pending requests from clients.

SIGABRT

Unlock the log file, pause for three seconds, then proceed, eventually re-locking the log file and starting a fresh audit trail on next access.

SIGTTOU

Force a file check (only client/standalone, and only in daemon mode).

The database (default name samhain_file) is a binary file, which can be created or updated using the -t init or the -t update option. If you use -t init, you need to remove the old database first, otherwise the new version will be appended to the old one. The file may be (clear text) signed by PGP/GnuPG.

It is recommended to use GnuPG with the options gpg -a --clearsign --not-dash-escaped

samhain will check the signature, if compiled with support for that.

At startup samhain will compute the checksum of the database, and verify it for each further access. This checksum is not stored on disk (i.e. is lost after program termination), as there is no secure way to store it.

Each entry in the log file has the format Severity : [Timestamp] Message, where the timestamp may be obtained from a time server rather than from the system clock, if samhain has been compiled with support for this. Each entry is followed by a signature, which is computed as Hash(Entry Key_N), and Key_N is computed as Hash(Key_N-1), i.e. only knowledge of the first signature key in this chain allows to verify the integrity of the log file. This first key is autogenerated and e-mailed to the designated recipient.

The default name of the log file is samhain_log. To prevent multiple instances of samhain from writing to the same log file, the log file is locked by creating a lock file, which is normally deleted at program termination. The default name of the lock file is samhain.lock. If samhain is terminated abnormally, i.e. with kill -9, a stale lock file might remain, but usually samhain will be able to recognize that and remove the stale lock file on the next startup.

E-mails are sent (using built-in SMTP code) to one recipient only. The subject line contains timestamp and hostname, which are repeated in the message body. The body of the mail contains a line with a signature similar to that in the log file, computed from the message and a key. The key is iterated by a hash chain, and the initial key is revealed in the first email sent. Obviously, you have to believe that this first e-mail is authentical ...

To monitor several machines, and collecting data by a central log server, samhain may be compiled as a client/server application. The log server (yule) will accept connection requests from registered clients only. With each client, the server will first engage in a challenge/response protocol for authentication of the client and establishing a session key.

This protocol requires on the client side a password, and on the server side a verifier that is computed from the password.

To register a client, simply do the following:

First, with the included utility program samhain_setpwd re-set the compiled-in default password of the client executable to your preferred value (with no option, a short usage help is printed). To allow for non-printable chars, the new value must be given as a 16-digit hexadecimal string (only 0123456789ABCDEF in string), corresponding to an 8-byte password.

Second, after re-setting the password in the client executable, you can use the server's convenience function yule -P password that will take as input the (16-digit hex) password, compute the corresponding verifier, and outputs a default configuration file entry to register the client.

Third, in the configuration file for the server, under the [Clients] section, enter the suggested registration entry of the form Client=hostname@salt@verifier, where hostname must be the (fully qualified) hostname of the machine on which the client will run. Don't forget to reload the server configuration thereafter.

If a connection attempt is made, the server will lookup the entry for the connecting host, and use the corresponding value for the verifier to engage in the session key exchange. Failure to verify the client's response(s) will result in aborting the connection.

samhain may be compiled with support for a stealth mode of operation, meaning that the program can be run without any obvious trace of its presence on disk. The supplied facilities are simple - they are more sophisticated than just running the program under a different name, and might thwart efforts using 'standard' Unix commands, but they will not resist a search using dedicated utilities.

In this mode, the runtime executable will hold no printable strings, and the configuration file is expected to be a postscript file with uncompressed image data, wherein the configuration data are hidden by steganography. To create such a file from an existing image, you may use e.g. the program convert(1), which is part of the ImageMagick(1) package, such as: convert +compress ima.jpg ima.ps.

To hide/extract the configuration data within/from the postscript file, a utility program samhain_stealth is provided. Use it without options to get help.

Database and log file may be e.g. existing image files, to which data are appended, xor'ed with some constant to mask them as binary data.

The user is responsible by herself for re-naming the compiled executable(s) to unsuspicious names, and choosing (at compile time) likewise unsuspicious names for config file, database, and log (+lock) file.

For security reasons, samhain will not write log or data files in a directory, remove the lock file, or read the configuration file, if any element in the path is owned or writeable by an untrusted user (including group-writeable files with untrusted users in the group, and world-writeable files).

root and the effective user are always trusted. You can add more users in the configuration file.

Using a numerical host address in the e-mail address is more secure than using the hostname (does not require DNS lookup).

If you use a precompiled samhain executable (e.g. from a binary distribution), in principle a prospective intruder could easily obtain a copy of the executable and analyze it in advance. This will enable her/him to generate fake audit trails and/or generate a trojan for this particular binary distribution.

For this reason, it is possible for the user to add more key material into the binary executable. This is done with the command:

samhain --add-key=key@/path/to/executable

This will read the file /path/to/executable, add the key key, which should not contain a '@' (because it has a special meaning, separating key from path), overwrite any key previously set by this command, and write the new binary to the location /path/to/executable.out (i.e. with .out appended). You should then copy the new binary to the location of the old one (i.e. overwrite the old one).

Note that using a precompiled samhain executable from a binary package distribution is not recommended unless you add in key material as described here.

For initializing the key(s), /dev/random is used, if available. This is a device supplying cryptographically strong (non-deterministic) random noise. Because it is slow, samhain might appear to hang at startup. Doing some random things (performing rain dances, spilling coffee, hunting the mouse) might speed up things. If you do not have /dev/random, lots of statistics from vmstat(8) and the like will be pooled and mixed by a hash function.

Some hosts might check whether the sender of the mail is valid. Use only login names for the sender.

For sending mails, you may need to set a relay host for the sender domain in the configuration file.

Whoever has the original signature key may change the log file and send fake e-mails. The signature keys are e-mailed at program startup with a one-time pad encryption. This should be safe against an eavesdropper on the network, but not against someone with read access to the binary, if she has caught the e-mail.

/etc/samhainrc /usr/local/man/man8/samhain.8 /usr/local/man/man5/samhainrc.5 /var/log/samhain_log /var/lib/samhain/samhain_file /var/lib/samhain/samhain.html /var/run/samhain.pid

samhainrc(5)

Rainer Wichmann (http://la-samhna.de)

If you find a bug in samhain, please send electronic mail to support@la-samhna.de. Please include your operating system and its revision, the version of samhain, what C compiler you used to compile it, your 'configure' options, and any information that you deem helpful.

Copyright (©) 1999, 2004 Rainer Wichmann

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