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Log files and other forms of monitoring


One integral part of any UNIX system are the logging facilities. The majority of logging in Linux is provided by two main programs, sysklogd and klogd, the first providing logging services to programs and applications, the second providing logging capability to the Linux kernel. Klogd actually sends most messages to the syslogd facility but will on occasion pop up messages at the console (i.e. kernel panics). Sysklogd actually handles the task of processing most messages and sending them to the appropriate file or device, this is configured from within /etc/syslog.conf. By default most logging to files takes place in /var/log/, and generally speaking programs that handle their own logging (most httpd servers handle their logging internally) log to /var/log/program-name/, which allows you to centralize the log files and makes it easier to place them on a separate partition (some attacks can fill your logs quite quickly, and a full / partition is no fun). Additionally there are programs that handle their own interval logging, one of the more interesting being the bash command shell. By default bash keeps a history file of commands executed in ~username/.bash_history, this file can make for extremely interesting reading, as oftentimes many admins will accidentally type their passwords in at the command line. Apache handles all of its logging internally, configurable from httpd.conf and extremely flexible with the release of Apache 1.3.6 (it supports conditional logging). Sendmail handles its logging requirements via syslogd but also has the option (via the command line -X switch) of logging all SMTP transactions straight to a file. This is highly inadvisable as the file will grow enormous in a short span of time, but is useful for debugging. See the sections in network security on Apache and Sendmail for more information.

General log security

Generally speaking you do not want to allow users to see the log files of a server, and you especially don't want them to be able to modify or delete them. Generally speaking most log files are owned by the root user and group, and have no permissions assigned for other, so in most cases the only user able to modify the logs will be the root user (and if someone cracks the root account all bets are off). There are a few extra security precautions you can take however, the simplest being to use the "chattr" (CHange ATTTRibutes command) to set the log files to append only. This way in the event of a problem like a /tmp race that allows people to overwrite files on the system they cannot significantly damage the log files. To set a file to append only use: 

chattr +a filename 

only the superuser has access to this function of chattr. If you set all your log files to append only you must remember that log rotation programs will fail as they will not be able to zero the log file. Add a line to the script to unset the append only attribute:

chattr -a filename

and add a line after the log rotation script to reset the append only flag. If you keep log files on the system you may also wish to set them immutable so they cannot be tampered with as easily, to set the file immutable simply:

chattr +i filename

and this will prevent any changes (due to /tmp races, etc.) to the file unless the attacker has root access (in which case you're already in a world of hurt). 

chattr -i filename

only the root user has access to the immutable flag.

sysklogd / klogd

In a nutshell klogd handles kernel messages, depending on your setup this can range from almost none to a great deal if for example you turn on process accounting. It then passes most messages to syslogd for actual handling (that is it places the data in a physical file). The man pages for sysklogd, klogd and syslog.conf are pretty good with clear examples. One exceedingly powerful and often overlooked ability of syslog is to log messages to a remote host running syslog. Since you can define multiple locations for syslog messages (i.e. send all kern messages to the /var/log/messages file, and to console, and to a remote host or multiple remote hosts) this allows you to centralize logging to a single host and easily check log files for security violations and other strangeness. There are several problems with syslogd and klogd however, the primary ones being the ease of which once an attacker has gained root access to deleting/modifying log files, there is no authentication built into the standard logging facilities. 

The standard log files that are usually defined in syslog.conf are:


The first one (messages) gets the majority of information typically; user logins, TCP_WRAPPERS dumps information here, IP firewall packet logging typically dumps information here and so on. The second typically records entries for events like users changing their UID/GID (via su, sudo, etc.), failed attempts when passwords are required and so on. The maillog file typically holds entries for every pop/imap connection (user login and logout), and the header of each piece of email that goes in or out of the system (from whom, to where, msgid, status, and so on). The spooler file is not often used anymore as the number of people running usenet or uucp has plummeted, uucp has been basically replaced with ftp and email, and most usenet servers are typically extremely powerful machines to handle a full, or even partial newsfeed, meaning there aren't many of them (typically one per ISP or more depending on size). Most home users and small/medium sized business will not (and should not in my opinion) run a usenet server, the amount of bandwidth and machine power required is phenomenal, let alone the security risks. 

You can also define additional log files, for example you could add:

kern.* /var/log/kernel-log

And you can selectively log to a separate log host:

*.emerg @syslog-host
mail.* @mail-log-host

Which would result in all kernel messages being logged to /var/log/kernel-log, this is useful on headless servers since by default kernel messages go to /dev/console (i.e. someone logged in at the machines). In the second case all emergency messages would be logged to the host "syslog-host", and all the mail log files would be sent to the "mail-log-host" server, allowing you to easily maintain centralized log files of various services. The default syslogd that ships with most Linux distributions is horribly insecure, log files are easily tampered with (or outright destroyed), and logging across the network is completely insecure as well as dangerous for the servers involved. I do not advise using syslog if you actually have a need for reliable logging (i.e. the ability to later view log files in the event of a break-in). 

The default file permissions on the log files are usually read / write for root, and nothing for anyone else. In addition to this you can (and should) set the files append only (remember to take logrotate into account though, it needs to zero the files). This will prevent any deletion / modifications to the log files unless root unsets the append only attribute first. 


The major problem with syslog however is that tampering with log files is trivial (setting the log files append only with "chattr +a" helps, but if an attacker gains root, they can unset the attribute). There is however a secure version of syslogd, available at http://www.core-sdi.com/english/freesoft.html (these guys generally make good tools and have a good reputation, in any case it is open source software for those of you who are truly paranoid). This allows you to cryptographically sign logs to ensure they haven't been tampered with. Ultimately, however, an attacker can still delete the log files so it is a good idea to send them to another host, especially in the case of a firewall to prevent the hard drive being filled up. 

next generation syslog

Another alternative is "syslog-ng" (Next Generation Syslog), which seems much more customizable then either syslog or secure-syslog, it supports digital signatures to prevent log tampering, and can filter based on content of the message, not just the facility it comes from or priority (something that is very useful for cutting down on volume). Syslog-ng is available at: http://www.balabit.hu/products/syslog-ng.html


Nsyslogd supports tcp, and SSL for logging to remote systems. It runs on a variety of UNIX platforms and you can download it from: http://coombs.anu.edu.au/~avalon/nsyslog.html.

Log monitoring


Psionic Logcheck

Psionic Logcheck will go through the messages file (and others) on a regular basis (invoked via crontab usually) and email out a report of any suspicious activity. It is easily configurable with several 'classes' of items, active penetration attempts which is screams about immediately, bad activity, and activity to be ignored (for example DNS server statistics or SSH rekeying). Psionic Logcheck is available from: http://www.psionic.com/abacus/logcheck/.


colorlogs will color code log files allowing you to easily spot suspicious activity. Based on a config file it looks for keywords and colors the lines (red, cyan, etc.), it takes input from STDIN so you can use it to review log files quickly (by using "cat", "tail" or other utilities to feed the log file through the program). You can get it at: http://www.resentment.org/projects/colorlogs/.


WOTS collects log files from multiple sources and will generate reports or take action based on what you tell it to do. WOTS looks for regular expressions you define and then executes the commands you list (mail a report, sound an alert, etc.). WOTS requires you have Perl installed and is available from: http://www.vcpc.univie.ac.at/~tc/tools/.


swatch is very similar to WOTS, and the log files configuration is very similar. You can download swatch from: ftp://ftp.stanford.edu/general/security-tools/swatch/.

Kernel logging



auditd allows you to use the kernel logging facilities (a very powerful tool). You can log mail messages, system events and the normal items that syslog would cover, but in addition to this you can cover events such as specific users opening files, the execution of programs, of setuid programs, and so on. If you need a solid audit trail then this is the tool for you, you can get it at: ftp://ftp.hert.org/pub/linux/auditd/.

Shell logging



I will also cover bash since it is the default shell in most Linux installations, and thus its logging facilities are generally used. bash has a large number of variables you can configure at run time or during it's use that modify how it behaves. Everything from the command prompt style to how many lines to keep in the log file.

name of the history file, by default it is ~username/.bash_history

maximum number of commands to keep in the file, it rotates them as needed.

the number of commands to remember (i.e. when you use the up arrow key).

The variables are typically set in /etc/profile, which configures bash globally for all users, however, the values can be over-ridden by users with the ~username/.bash_profile file, and/or by manually using the export command to set variables such as export EDITOR=emacs. This is one of the reasons that user directories should not be world readable; the .bash_history file can contain a lot of valuable information to a hostile party. You can also set the file itself non world readable, set your .bash_profile not to log, set the file non writeable (thus denying bash the ability to write and log to it) or link it to /dev/null (this is almost always a sure sign of suspicious user activity, or a paranoid user). For the root account I would highly recommend setting the HISTFILESIZE and HISTSIZE to a low value such as 10. On the other hand if you want to log users shell history and otherwise tighten up security I would recommend setting the configuration files in the user's home directory to immutable using the chattr command, and set the log files (such as .bash_history) to append only. Doing this however opens up some legal issues, so make sure your users are aware they are being logged and have agreed to it, otherwise you could get into trouble.


Security Portal

Written by Kurt Seifried