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Practical UNIX & Internet Security

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Previous: 10.3 Program-Specific Log Files Chapter 10
Auditing and Logging
Next: 10.5 The UNIX System Log (syslog) Facility

10.4 Per-User Trails in the Filesystem

Although not obvious, there are some files that are kept on a per-user basis that can be helpful in analyzing when something untoward has happened on your system. While not real log files, as such, they can be treated as a possible source of information on user behavior.

10.4.1 Shell History

Many of the standard user command shells, including csh , tcsh , and ksh , can keep a history file . When the user issues commands, the text of each command and its arguments are stored into the history file for later re-execution. If you are trying to recreate activity performed on an account, possibly by some intruder, the contents of this file can be quite helpful when coupled with system log information. You must check the modification time on the file to be sure that it was in use during the time the suspicious activity occurred. If it was created and modified during the intruder's activities, you should be able to determine the commands run, the programs compiled, and sometimes even the names of remote accounts or machines that might also be involved in the incident. Be sure of your target, however, because this is potentially a violation of privacy for the real user of this account.

Obviously, an aware intruder will delete the file before logging out. Thus, this mechanism may be of limited utility. However, there are two ways to increase your opportunity to get a useful file. The first way is to force the logout of the suspected intruder, perhaps by using a signal or shutting down the system. If a history file is being kept, this will leave the file on disk where it can be read. The second way to increase your chances of getting a usable file is to make a hard link to the existing history file, and to locate the link in a directory on the same disk that is normally inaccessible to the user (e.g., in a root-owned directory). Even if the intruder unlinks the file from the user's directory, it can still be accessed through the extra link.

Also note that this technique can come in handy if you suspect that an account is being used inappropriately. You can alter the system profile to create and keep a history file, if none was kept before. On some systems, you can even designate a named pipe ( FIFO ) as the history file, thus transmitting the material to a logging process in a manner that cannot be truncated or deleted.

Even if you were unable to preserve a copy of the history file, but one was created and then unlinked by the intruder, you can still gain some useful information if you act quickly enough. The first thing you must do is to either take the system to single-user mode, or umount the disk with the suspect account (we recommend going to single-user mode). Then, you can use disk-examination tools to look at the records on the free list. When a file is deleted, the contents are not immediately overwritten. Instead, the data records are added back into the freelist on disk. If they are not reused yet (which is why you umount the disk or shut the system down), you can still read the contents.

10.4.2 Mail

Some user accounts are configured to make a copy of all outgoing mail in a file. If an intruder sends mail from a user account where this feature is set (or where you set it), this feature can provide you with potentially useful information. In at least one case we know of, a person stealing confidential information by using a coworker's pirated password was exposed because of recorded email to his colleagues that he signed with his own name!

Some systems also record a log file of mail sent and received. This file can be kept per-user, or it may be part of the system-wide syslog audit trail. The contents of this log can be used to track what mail has come in and left the system. If nothing else, we have found this information to be useful when a disk error (or human error) wipes out a whole set of mailboxes - the people listed in the mail log file can be contacted to resend their mail.

10.4.3 Network Setup

Each user account can have several network configuration files that can be edited to provide shortcuts for commands, or to assert access rights. Sometimes, the information in these files will provide a clue as to the activities of a malefactor. Examples include the .rhosts file for remote logins, and the .netrc file for FTP . Examine these files carefully for clues, but remember: the presence of information in one of these files may have been there prior to the incident, or it may have been planted to throw you off.

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