BSDCon 2002 Paper
[BSDCon '02 Tech Program Index]
|Pp. 131142 of the Proceedings|
Log Monitors in BSD UNIX
Brett Glass, Glassware
P.O. Box 1693
Laramie, WY 82073-1693
Presented at BSDCon 2002 San Francisco
Slides at https://www.brettglass.com/logmonitors/
A log monitor is a process, or daemon, which monitors log messages produced
by a computer system and the programs which run on it. A properly designed
log monitor can recognize unusual activity (or inactivity), alert an administrator
to problems, gather statistics about system activity, and/or take automatic
action to contain a threat. It can even "learn," over time, what is normal
and identify message traffic that may betray an abnormal situation.
On BSD UNIX systems, most applications and daemons log events via the
system logging daemon (syslogd). Because Berkeley syslogd
allows log messages to be piped to a program as well as written to one
or more files, implementation of a BSD log monitor often begins with the
creation of a program which accepts formatted log messages via standard
input, parses them, and extracts necessary information. Utilities which
do not log events via syslogd (e.g. the Apache Web server) may require
a separate log monitor process. In all cases, data validation, robust input
handling, and careful parsing of log messages are especially important
considerations in log monitor design, lest the monitor itself compromise
the security of the system it is intended to monitor. Languages which facilitate
string processing and pattern matching, such as Perl and SNOBOL4, are good
choices for the implementation of log monitors for this reason.
Log monitors can work in tandem with firewall software to block traffic
from a would-be intruder, spammer, or "mail bomber." For example, it can
tally the number of outgoing e-mail messages sent by users on an internal
network and detect activity which appears to constitute spamming or mail
bombing. A log monitor can also recognize efforts to exploit security holes,
such as attempted infection by the Code Red worm or attempts to exploit
CGI scripts included with older versions of the Apache Web server. Included
in this paper is source code for a log monitor that identifies and blocks
attacks from Code Red, sadmin/IIS, Nimda, and similar worms. Policy issues
-- including the usefulness of "amnesty" to prevent inadvertent blocking
of innocent third parties -- are discussed. The work described in this
paper --which is still progressing at this writing -- will hopefully culminate
in the release of a general purpose log monitoring facility.
[Note: Because the camera-ready copy of this paper had to be submitted
two months before the BSDCon 2002 conference to allow time for printing,
and USENIX enforces strict limits on size, the version of this paper which
appears online may contain more detail as well as new information from
ongoing work. For the latest version, and/or to follow up the many references
via HTML links, access the master copy at https://www.brettglass.com/logmonitors/paper.html
on the World Wide Web.]
System administrators' time is valuable. Few, if any, can afford to spend
many hours each day poring over the voluminous log files generated by systems
and network applications. Yet, if an administrator fails to quickly recognize
and respond to anomalous events chronicled in log messages, systems or
entire networks can be abused, "hijacked" for use in illicit activities,
and/or removed from service by malicious parties. Log monitors aid the
administrator by automatically filtering and digesting the flood of log
information -- and, in some cases, responding on his or her behalf.
2 What is a Log Monitor?
A log monitor is an agent which responds automatically to conditions
revealed by one or more system log messages. The response may consist of
autonomous action to handle a situation and/or the notification of a human
A stateful log monitor is a log monitor that infers the presence
of a condition requiring attention by compiling data from more than one
log message. It may simply note the number and/or frequency of log messages
related to a particular type of activity or may generate more sophisticated
cumulative statistics from those messages.
2.1 Capabilities of Log Monitors
What can a log monitor do? Among other things, it can:
While it is useful for a log monitor to be able to recognize failures or
threats for which it has been given a human-crafted "signature," it can
also incorporate more subtle heuristics. By accumulating statistics about
what constitutes "normal" activity, log monitors may be able to recognize
anomalous behaviors that a human system administrator might at first overlook,
such as the cessation of events which normally happen with a given frequency.
Detect abnormal usage patterns
Recognize system or network abuse (e.g. spamming and mail bombing)
Catch worms and other malware
Detect vulnerability scans (e.g. port scans)
Detect intruders (or attempted intrusions)
Detect resource shortages (e.g. slow response times, out-of-memory conditions,
out-of-disk conditions, inadequate swap space)
Detect imminent or actual system failures
Compile statistics in real time (including running averages, etc.)
React to conditions by notifying an administrator and/or taking immediate
2.2 Examples of Existing Log Monitors
("Simple Watchdog") ,
developed by Stephen Hansen and Todd Atkins of Stanford University, and
an enhanced version of swatch developed by the Pacific Institute
for Computer Security (PICS) research group at the San Diego Supercomputer
Center, are two very simple general-purpose log monitors. swatch
accepts a configuration file whose entries consist of patterns and lists
of actions. (A sample pattern/action block is shown in Listing 1.)
Listing 1: Sample pattern/action block for
watchfor /ANONYMOUS FTP LOGIN REFUSED FROM (\S*)/
mail addresses=admin,subject=Attempted anonymous FTP from $1
exec blackhole $1
It then "tails" a system log file looking for the regular expression
specified in the "watchfor" line of the block. If the expression is matched,
swatch takes the series of actions that follow. Possible actions include
producing a beep, writing a message in a specified color to the console,
sending an e-mail message including the log entry that was matched, or
executing an arbitrary command. (In the example above, the $1 causes the
text matched by the subexpression within the first set of parentheses to
be interpolated.) swatch incorporates features to allow throttling
and to recognize patterns only during certain hours of the day and/or days
of the week.
2swatch extends swatch by offering the possibility
of deferred as well as immediate action. 2swatch can accumulate
a series of messages matching a pattern into a report that is e-mailed
as a single message. This prevents mail systems from being flooded with
e-mail messages that each contain a single log entry. Unfortunately, neither
swatch nor 2swatch can perform stateful monitoring beyond
their respective throttling and accumulation functions, though it is of
course possible to implement statefulness via cleverly designed external
programs or scripts. Nor can these two programs, by themselves, correlate
entries from more than one log. A final drawback of these two programs
is that they have been released under licensing terms (the GPL and a unique
"no commercial use" license, respectively) which hamper or prohibit their
reuse in commercial products.
The primary objective of the work described in this paper is to develop
a generalized and portable framework which eases the creation of customized
log monitors and overcomes the limitations described above. It is intended
that the results of this work, which is initially being performed on FreeBSD,
be released under "truly free" (i.e. MIT- or BSD-style) licensing terms
so as to permit adaptation to other operating systems and commercial as
well as non-commercial reuse.
2.3 Log Monitors vs. Log Analyzers
A log analyzer differs from a log monitor in that it does not operate
in real time (or nearly real time) but is run against system logs after
the fact. Many such utilities exist, especially for Web and mail servers.
At this writing, log analyzers are more common than log monitors. Most
implementations of BSD UNIX come with primitive but helpful log analysis
scripts that report significant log events to an administrator via e-mail.
These scripts are often found at /etc/daily, /etc/weekly,
and /etc/monthly and are usually run at appropriate intervals
by the cron(8) 
daemon. (In recent versions of FreeBSD, the default /etc/crontab
file instead activates Trainer and Somers' periodic(8) 
utility, which in turn runs these scripts from the /etc/periodic
directory.) Apache's logresolve program performs a simple analysis
of Apache log files that consists primarily of efficient reverse domain
name resolution. Tom Boutell's Wusage 
web log analysis utility, released as Shareware, is a much more sophisticated
log analyzer for Apache; it can generate daily, weekly, monthly, and annual
Some log analyzers are designed to "wake up" periodically and scan the
latest entries in a system log, acting, if appropriate, upon what they
"see." (Kai's Spamshield ,
which detects incoming spam and blocks the sender via a "blackhole" route,
is an example.) If the interval between scans is sufficiently short, such
log analyzers can perform some of the functions of a log monitor, detecting
and handling conditions which require timely but not instantaneous attention.
3 Log Monitors and BSD syslogd
In BSD, the kernel and most daemons traditionally log their activities
via syslogd, the system logging daemon, originally written by
Eric Allman. Messages from the kernel are received via the pseudo-device
/dev/klog, while messages from applications are received via the
UNIX domain socket /var/run/log or via UDP socket 514. In the
early, more trusting days of the Internet, syslogd listened on
this socket and by default accepted any message that came in. But in most
modern UNIX implementations it does not do this, because -- as stated in
many versions of the syslogd manual pages -- it was "equivalent
to an unauthenticated remote disk-filling service." 
3.1 syslogd Facilities, Priorities,
The standard Berkeley syslogd(8) is
configured via the file /etc/syslog.conf, which specifies the
disposition of log messages. In traditional Berkeley UNIX, the messages
are sorted according to a facility code and a priority or
severity level. These parameters are used to route each log message
to one or more destinations, including the system console, the terminals
of specific users, other machines, files, and/or programs. The facility
codes used by 4.4BSD Lite2 are listed in Table 1, while the severity levels
are listed in Table 2. Each code corresponds to an integer constant defined
within the C header file syslog.h:
Table 1: Standard facility codes used by Berkeley
Modern UNIX and UNIX-like systems have many more facilities than were contemplated
in the original UNIX logging scheme. Nonetheless, most implementations
have hewn to the traditional list of facilities defined in 4.4BSD for the
sake of compatibility and tradition. A few (such as FreeBSD) have added
additional facility codes, including console, ntp,
and security, and DEC ULTRIX uses facility number 10 (authpriv
in many versions of BSD) to log events for its AdvFS filesystem. 
Unfortunately, these additions and conflicts may hinder software portability.
What's more, the facilities which were added frequently do not reflect
a consistent design philosophy. It is unclear, for example, why the authors
of FreeBSD's syslogd implementation felt that NTP was deserving
of its own facility code while DNS, DHCP, PPP, and HTTP (or, for that matter,
any of the many other protocols listed in /etc/services) were
Table 2: Severity levels (priorities) used
by Berkeley syslogd
Further confusing the issue of how to classify log messages is the fact
that many recent versions of syslogd have added the ability to
sort messages via strings called "tags," which are transmitted to the logging
daemon along with each message. Tags usually (but not always) contain the
name of the program that generated the message. (At this writing, the ability
to sort messages by tag is available in OpenBSD and FreeBSD but not in
NetBSD.) To facilitate logging across networks, some implementations of
syslogd add yet another sorting criterion: they allow messages
to be dispatched according to the name of the host from which they originated.
As if all of this weren't enough, there's yet another fly in the ointment.
While Berkeley syslogd itself is able to sort messages by facility,
severity, tag, and originating host, it does not record the facility and
severity level in each log message, and in most implementations there is
no way to make it do so. This makes it difficult for a log monitor (and,
in some cases, for humans) to use these same criteria to sort messages
that syslogd has aggregated into a single log file. The version of syslogd
found in recent versions of FreeBSD is one of the few that provides a solution
to this problem. If the daemon is started with the -v ("verbose") command
line option, the facility and priority level are logged as numbers. Specifying
-v twice causes them to be shown by name between angle brackets (e.g. <auth.notice>).
The latter choice, while it consumes a bit more disk space, makes the logs
much easier both to read and to monitor. It would be a relatively simple
matter to add similar capabilities to other syslogd implementations.
3.2 Monitoring Techniques for
Use With syslogd
A log monitor can monitor the output of syslogd in one of three
ways. The most common technique is to "tail" the log, either by accepting
the output of the "tail -f" as input or by incorporating a module that
provides equivalent functionality (e.g. Matija Grabnar's File::Tail 
for Perl). The downsides of this technique are twofold. First, the application
(or the tail(1) utility) must "poll" the file at regular intervals
to see if it has grown. This consumes resources even if there is nothing
to be done. Secondly, if the log file is "rotated" (most often done by
renaming it and creating a new file for future input), the application
may be left "watching" the old file (which is no longer growing) and miss
all subsequent messages. (This is a problem of some, but not all, "tail"
Another technique is to open the log file every so often, collect the
last n lines, and then close the file. This technique avoids problems
when logs are rotated, but may cause the log monitor to miss important
messages if the log has grown more quickly than expected (as can happen
during an unusual situation). Repeated opening and closing of the file
also creates substantial overhead.
The best technique, when it is possible to use it, is to instruct syslogd
to pipe messages directly to a log monitor process. This option is not
available in all implementations of syslogd; however, it is present
in FreeBSD's syslogd and BalaBit's syslog-ng 
and can be easily added to other logging daemons. Some versions of syslogd,
including the Berkeley-derived Linux syslogd and Core-SDI's modular
syslog (msyslog) ,
cannot pipe directly to an arbitrary program but can send output to a named
pipe where an application is listening.
FreeBSD's syslogd makes piping to applications especially easy
by handling nearly all of the logistics for the programmer. The lines shown
in Listing 2, when placed in in /etc/syslog.conf will cause messages
regarding mail to be appended to /var/mail/maillog in the usual
way and also piped to a monitoring program written in Perl.
Listing 2: Sample piping configuration for
# Log to /var/log/maillog, as usual
# Also pipe the same messages to mailmon.pl
mail.info |exec perl /usr/local/bin/mailmon.pl
Because syslogd handles the logistics of distributing each
log message to the required destinations (including the log monitor), the
log monitor process is unaffected by log file rotation. It merely has to
be prepared to save its state and exit if syslogd restarts (see
One feature of syslogd which may actually defeat the purpose
of a log monitor is automatic output compression. When syslogd
sees two or more identical messages bound for the same destination, it
outputs the first and then counts (but does not output) the duplicates.
After a predetermined delay, it outputs a message of the form "Last message
repeated n times" indicating the number of copies received during
the delay period. If still more copies of the same message arrive, the
process is repeated with a longer delay between reports. Ironically, this
feature -- which cannot be turned off in any version of syslogd
known to the author -- is just the opposite of what is needed for effective
log monitoring. (It is precisely when an unexpected flood of repeated messages
arrives that it is most useful for a log monitor to take prompt, autonomous
action. But if notice of those messages are delayed, it cannot do so.)
The author has submitted a patch to the FreeBSD Project which disables
repeat counting on messages piped to a program. (With the patch in place,
if the same messages also go to a file or terminal, compression will still
occur on those outputs.) It may also be desirable to disable compression
when logging to a remote host, since -- while this would cause an increase
in network traffic -- it would facilitate the implementation of remote
log monitors. Similar modifications should be made to other logging daemons
that direct output either to programs or to named pipes, to facilitate
the use of log monitors.
3.3 Processing Piped Output from FreeBSD's
syslogd: Caveats and Tricks
While implementing his first group of trial log monitors under FreeBSD,
the author learned by experience how to deal with the quirks of this particular
logging daemon and operating environment. FreeBSD's syslogd does
not start a program to which messages are "piped" until it has output for
it. When it does start such a program, it executes it via sh(1)
so that command line processing may performed. To avoid the overhead of
a vestigial shell process, it is best to use the "exec" command (as shown
above) to launch the log monitor script or program. The log monitor should
expect to receive messages via standard input; its standard output
and standard error file handles will be redirected to /dev/null.
Secure programming practices are of the utmost importance when one is
creating log monitors. Piped applications started by FreeBSD's syslogd
run with the same uid as syslogd itself -- normally root. Because
a key function of log monitors is to take adminstrative action as a result
of what they observe in a stream of log messages, they often must run as
the superuser and may not be able to accomplish their intended functions
if they "drop" privileges for safety. (In a capabilities-based system,
it may be possible for a log monitor to drop capabilities that the author
knows it will never use.) It is therefore especially important to
avoid potential buffer overflows, format string vulnerabilities, and security
holes that might arise when unfiltered input is passed directly to system
APIs or programs. "Tainting" and/or extremely careful validation of input
is strongly recommended.
Should an application which receives piped output from syslogd
terminate of its own accord, it will be restarted when there is more input
for it. However, syslogd may itself need to request that a log monitor
terminate. (The most common situation in which this will occur is when
syslogd receives a "hangup" signal -- SIGHUP -- indicating that
it must restart and reread its configuration file.) syslogd indicates
its desire to shut down the log monitor by closing the pipe it has created
to the log monitor's standard input. The log monitor then has a predetermined
amount of time -- 60 seconds in most implementations -- to save any state
it wishes to preserve and terminate. If it takes longer, syslogd will attempt
to kill it by sending it a "terminate" signal -- SIGTERM. To ensure that
syslogd is able to kill a log monitor that is frozen, it is advisable
for a log monitor not to catch SIGTERM unless it may need a very
long time to save its state.
4 Log Monitors and Apache
The Apache Web server 
is perhaps most widely deployed application for UNIX-like operating systems
which does not normally log via syslogd. (Apache can be easily
configured to log errors via syslogd, but there is no built-in
option that allows the logging of all traffic this way.) Fortunately, Apache's
own logging facilities
are more powerful and flexible than those of syslogd itself, so
logging output via syslogd is rarely necessary.
Apache can pipe log messages to external programs on all UNIX and UNIX-like
platforms, making the implementation of log monitors, log rotators, and
other such utilities very straightforward even on systems whose syslogd
implementation does not support piped commands (See Listing 3).
Listing 3: Examples of piped logs in Apache
# Piped logs can be used for log rotation as well as monitoring
CustomLog "|rotatelogs /var/log/www_log 86400" combined
ErrorLog "|rotatelogs /var/log/www_errors 86400"
# Apache can maintain more than one access log, so you can feed
# access information to a monitor and also to a log file
CustomLog "|exec perl /usr/local/bin/webmon.pl" combined
# Piping error log messages to a monitor is especially useful
# when one wants to detect abuse and/or attacks. Alas, there can
# only be one error log, so you must either rely on access logs
# for error information (usually an acceptable solution) or
# design your monitor to "tee" the messages to a file for you.
ErrorLog "|exec snobol4 -b /usr/local/bin/wormwatch.sno"
4.1 Creating Primitive Log Monitors Within
Apache's logging modules also provide conditional output, pattern matching,
and custom formatting. These features not only facilitate the use of external
log monitors but in some cases allow primitive log monitors to be implemented
directly within Apache itself. The fragment in Listing 4, when added to
the httpd.conf server configuration file (either in the main body
or within the <VirtualHost> directives), will automatically block traffic
from worms such as Nimda ,
Code Red , and sadmind/IIS .
Listing 4: Worm blocker implemented entirely
within Apache's httpd.conf
# Flag requests for URIs containing common strings from Nimda-like worms
# (including Code Red, sadmind/IIS). Note that the patterns below are regexes;
# remember to escape dots and other characters with special significance!
SetEnvIf Request_URI "/winnt/system32/cmd\.exe" worm
SetEnvIf Request_URI "/scripts/root\.exe" worm
SetEnvIf Request_URI "/MSADC/root\.exe" worm
# Don't use the following patterns if you use "upreferences" in URIs
SetEnvIf Request_URI "/\.\." worm
SetEnvIf Request_URI "\.\./" worm
# Block attackers who send the patterns above within URIs. The command below
# uses a blackhole route. It's more efficient to firewall (the command
# will vary depending upon the firewall in use) or to use SSH to add rules to
# an upstream firewall to block the attacker, but this method has the
# advantage that it is relatively independent of configuration. If several
# commands must be executed, or if postprocessing of output is desired, it
# is best to invoke a script or compiled program rather than doing all the
# work from within httpd.conf.
CustomLog "|exec sh" "route -nq add -host %a 127.0.0.1 -blackhole" env=worm
# Note that no input from the client is used in the shell command, so this
# set of directives is not subject to exploits via crafted strings. If strings
# from the client were used, stronger input validation would be in order.
While BSD and Apache cannot be "infected" by the worms targeted by the
directives in Listing 4, a worm can nonetheless tax a Web server by consuming
processes in the Apache process pool, glutting system logs with error messages,
and infecting susceptible machines elsewhere on the network.
The configuration shown in Listing 4 uses Apache's SetEnvIf 
directive (implemented by the module mod_setenvif) to perform regular expression
matching on incoming URIs. It then uses the CustomLog 
directive (implemented in mod_log_config) to do conditional logging based
on the results of the match. When a worm is detected, Apache pipes a specially
formatted "log message" -- actually a command -- to a shell for execution.
(The "exec sh" may at first glance seem redundant. However, it is necessary
to restart the shell -- which is initially invoked so as to accept a command
as an argument -- so that it will accept commands via standard input instead.)
The command creates a "blackhole" route on the host machine and locks out
the attacker. If the Web server does double duty as the gateway between
the Internet and an office LAN (as is often the case in small office/home
office networks), blackholing the attacker will also protect the machines
that sit "behind" the server. If there is a firewall upstream of the Web
server, it may be desirable to replace the command that creates a blackhole
route with one that causes the firewall to block the attacker.
While the author had great success with this simple log monitor (which
he crafted during the early morning hours of 18 September, 2001 when Nimda
began to spread), it clearly has many deficiencies. For example, it does
not check to see whether an attack is coming from the Web server's own
address (which can easily happen if the machine is doing double duty as
a NAT router or dial-up server.) to prevent it from blackholing itself!
Nonetheless, this example is a valuable proof of concept. It demonstrates
that the ability to apply a general pattern matching facility to log messages,
and then execute commands based on those messages, are sufficient to allow
the creation of a useful, if not perfect, log monitor. More sophisticated
tools -- such as languages with built-in pattern matching -- make it even
easier to write quite sophisticated agents.
4.2 Monitoring Techniques for
Use With Apache
To construct effective log monitors for use with Apache, one must understand
the conventions it uses when piping log output to applications. Like syslogd,
Apache expects piped applications to be trustworthy. It runs them with
the permissions accorded to Apache's master process, which usually runs
as the superuser. (Apache, in its recommended configuration, maintains
a single privileged "master" process which forks a pool of unprivileged
processes to handle incoming requests.) As mentioned earlier, a log monitor
often requires this high privilege level if it is to take autonomous administrative
action. Thus, good programming practices are of paramount importance.
Unlike syslogd, Apache starts piped applications as soon as
it has finished reading its configuration file. This gives them time to
start up before receiving the first message, improving response time at
the expense of overhead. If a piped application terminates, Apache restarts
it the next time a message is to be delivered to it. Like syslogd,
Apache normally uses sh(1) to parse command lines and set up file
redirection for piped commands.
It is important to remember, when writing log monitors for Apache, that
users' log formats may vary. It is therefore best to use a custom log format
that the log monitor expects -- or, alternatively, to monitor the error
log, whose format cannot be customized and is therefore almost fixed.
(One aspect of the error log format can be changed via configuration: the
way hosts are identified. If HostNameLookups 
is on, domain names are output instead of numerical IP addresses.) Note
that Apache allows error log messages to be sent only go to one destination.
(If there is more than one ErrorLog directive, each overrides the previous
ones rather than supplying an additional destination.) Fortunately, Apache
also records errors in the access logs, so using ErrorLog to feed messages
to a log monitor still allows a human to review messages denoting errors.
4.3 An Extensible Worm Blocker/IDS for Apache
After implementing the "quick and dirty" Apache worm blocker described
above, the author decided to create a more powerful, extensible, and maintainable
one. SNOBOL4  
was chosen because of this language's prodigious pattern matching, text
handling, and parsing abilities.
The 28 executable lines of SNOBOL4 in Listing 5 detect infection attempts
from worms such as Code Red, Nimda, and sadmind/IIS and "blackhole" the
attacking machine. Unlike the earlier example, however, this SNOBOL program
completely parses, and validates the fields of, each Apache ErrorLog message
before taking action. This eliminates any chance of a "false positive,"
which might occur if a regular expression in the earlier example happens
to match part of a legitimate request.
Listing 5: Extensible worm blocker/IDS for
Apache in 28 lines of SNOBOL4
* An Extensible worm blocker/IDS for Apache in SNOBOL4
* Copyright (c) 2001 by Brett Glass
* Licensing terms are at the end of this file.
* This program accepts the piped error output from the
* Apache Web server and spots lines indicating an attack
* from Nimda.A or similar worms, including Code Red,
* sadmind/IIS, and Nimda.E. It can then firewall or
* blackhole the attacking host. Add it to your Apache
* configuration by inserting a line such as
* ErrorLog "|exec snobol4 -b /usr/local/bin/wormblock.sno"
* Also, make sure that HostNameLookups is off so that the
* log messages contain numeric IP addresses.
* This program is designed to be easily extensible to catch
* a wide variety of potential exploits.
* An Apache error log message generated by the Nimda worm
* might look like this (wrapped for readability):
* [Thu Nov 1 12:46:07 2001] [error] [client 126.96.36.199]
* File does not exist: /usr/local/www/data/textorics/scripts/
* Build up SNOBOL patterns for Apache ErrorLog messages.
* Use the pattern "WS" for whitespace (tabs or blanks)
WS = SPAN(' ' CHAR(9))
DIGITS = '0123456789'
WEEKDAY = 'Mon' | 'Tue' | 'Wed' | 'Thu' | 'Fri' | 'Sat' | 'Sun'
MONTH = 'Jan' | 'Feb' | 'Mar' | 'Apr' | 'May' | 'Jun' |
+ 'Jul' | 'Aug' | 'Sep' | 'Oct' | 'Nov' | 'Dec'
DAYOFMONTH = (SPAN(DIGITS) $ NUMBER) *LE(NUMBER,31) *GE(NUMBER,1)
HOUR = (SPAN(DIGITS) $ NUMBER) *LE(NUMBER,23)
MINUTE = (SPAN(DIGITS) $ NUMBER) *LE(NUMBER,59)
SECOND = MINUTE
DAYTIME = HOUR ':' MINUTE ':' SECOND
YEAR = SPAN(DIGITS)
DATEANDTIME = '[' WEEKDAY WS MONTH WS DAYOFMONTH WS DAYTIME WS YEAR ']'
OCTET = (SPAN(DIGITS) $ NUMBER) *LE(NUMBER,255)
IPADDRESS = OCTET '.' OCTET '.' OCTET '.' OCTET
ERRSTR = '[error]'
CLIENTINFO = '[client' WS (IPADDRESS . CLIENTIP) ']'
FILEERR = 'File does not exist:'
FILENOTFOUNDERROR = DATEANDTIME WS ERRSTR WS CLIENTINFO WS
+ FILEERR WS REM . PATH
DANGEROUSPATH = '/winnt/system32/cmd.exe' | '/scripts/root.exe' |
+ '/MSADC/root.exe' | "/.." | "../"
LOOP LOGLINE = INPUT* Anchor the matching of the error message for efficiency
&ANCHOR = 1
* We're using unevaluated expressions ("thunks") and so must do full scans
&FULLSCAN = 1
LOGLINE FILENOTFOUNDERROR :F(LOOP)
* Scan the path, using an unanchored match, for strings betraying a worm
&ANCHOR = 0
&FULLSCAN = 0
PATH DANGEROUSPATH :F(LOOP)
HOST(1,'logger -t wormblock -pauth.notice Nimda or similar attack detected!'
+ 'Blocking IP address ' CLIENTIP)
HOST(1,'route -nq add -host ' CLIENTIP ' 127.0.0.1 -blackhole')
* Note that a blackhole route is a brute force blocking method. It
* allows the first SYN to arrive but blocks the outgoing SYN-ACK,
* causing the TCP three-way handshake to fail. Its advantage is
* that it works on nearly any system regardless of configuration.
* If you're running a firewall, you can replace the route command
* above with ones that add firewall rules. Here are samples for
* FreeBSD's ipfw:
* HOST(1,'/sbin/ipfw -q add deny all from any to ' CLIENTIP)
* HOST(1,'/sbin/ipfw -q add deny all from ' CLIENTIP ' to any')
* The commands for ipf and pf are similar.
* If you want to block attacks at a different machine (say, the
* firewall that guards your entire network), you can use SSH to send
* the firewall similar commands. The exact commands required will
* depend upon your network and firewall configurations.
* Copyright (c) 2001 by Brett Glass
* Permission is hereby granted, free of charge, to any person obtaining a copy
* of this software and associated documentation files (the "Software"), to deal
* in the Software without restriction, including without limitation the rights
* to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
* copies of the Software, and to permit persons to whom the Software is
* furnished to do so, subject to the following conditions:
* The above copyright notice and this permission notice shall be included in
* all copies or substantial portions of the Software.
* THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
* IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
* FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
* AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
* LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM
* OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
The example in Listing 5 was written for Philip Budne's free Macro
SNOBOL4 for UNIX ,
which compiles on most BSD UNIX implementations and is present in the NetBSD
and FreeBSD ports collections. It is readily portable to other implementations
of the language including Catspaw SPITBOL .
The author has found SNOBOL4 to be extremely useful for log monitoring
-- even more so than Perl -- because its extremely powerful recursive pattern
matching allows it to completely parse a log message by executing a single
line of code. SNOBOL4 also allows more extensive input validation to be
done within a pattern than can easily be done within a Perl regular expression.
For example, in the patterns DAYOFMONTH, HOURS, MINUTES, and SECONDS patterns,
the GE() and LE() predicates are applied to strings of digits during pattern
matching to ensure that the numbers they represent are within allowable
limits. SNOBOL's pattern matching engine can backtrack (or indicate failure)
if these conditions are not met.
4.4 Refinements to the Initial Design
Note that the code in Listing 5, like that in Listing 4, immediately and
unconditionally blocks any host which attacks the server on which it is
running. As noted earlier, this could be an unfortunate administrative
decision under certain circumstances. For example, if an infected dial-up
user is blocked, subsequent users of that dial-up line will not be able
to reach the site. If an attacking machine is behind a NAT router or a
proxy, every other user arriving from the same site may be blocked. (This
is a particular concern in the case of AOL, which passes all HTTP requests
through caching proxies to conserve bandwidth and anonymize users.) Also,
a malicious third party could post (or e-mail to users) links which, when
followed, caused a block to be put in place.
Fortunately, it is relatively simple to make refinements to the log
monitor shown above to handle these problems. A "do not block" list can
be added to ensure that the machine does not block itself. By requiring
two or more hits from an IP address before blocking it, the monitor can
reduce the chances of an accidental block or of a block caused by a maliciously
distributed link. Use of the MAPS
Dial-up List (DUL) 
to recognize dial-ups, plus an "amnesty" policy, can protect against long
term blocking of dial-ups, though at the expense of a few more hits from
Other refinements suggested during previous presentations of this work
All of these refinements are easy to add due to the flexibility of the
SNOBOL4 language, which provides associative arrays, record types, indirect
references, and other features typically found in high level interpreted
The ability to notify an administrator of the current block list (and/or
"repeat offenders") so that s/he can notify administrators by phone or
The ability to mail or page an administrator when an attack is detected
from a host on the "do not block" list (or under other conditions);
The ability to gather statistics (such as the number of hits received from
a particular Web address or subnet per hour) and automatically notice anomalies;
The ability to place the log monitor on a separate machine, so as to preserve
both copies of logs and information about attacks or malfunctions in the
event of catastrophic system failure or tampering; and
The ability to view a display and/or reports detailing the log monitor's
5 Creating a Generalized Logging and
Log Monitoring Facility
The goal of the author's ongoing research, however, is not to perfect any
one special-purpose log monitor. It is, rather, to learn, via the creation
of a diverse collection of log monitors, what features are desirable in
a generalized logging and log monitoring facility. The long term goal is
to create a single facility -- much more powerful than the swatch
and 2swatch scripts mentioned earlier -- which can subsume
the functions of simple log monitors and facilitate the generation of more
sophisticated ones. Such a facility should be usable across a wide range
of operating system platforms, and should allow the creation of monitors
via a process which consists as much as possible of configuration rather
than programming. Features of this facility are expected to include:
Input from system architects and administrators regarding suggested features
and functionality is welcome.
Compatibility with the "legacy" logging facilities and facility/severity
codes of current UNIX implementations;
The ability to apply pre-written message parsing templates to messages
(akin to the "distillation" process used by Lire 
but performed in real time) so that rules can refer to message field by
The ability to identify and report messages which were not parsed (possibly
indicating an obsolete template and/or a software problem);
The ability to access all information associated with a log message and
the process that generated it -- including the identity of the program,
effective user and group ids, facility and severity codes, point of origin
(if not on the local system), etc.;
Accumulation of statistics (e.g. number of e-mail messages received from
a specific user or IP address) for use in rules;
The ability to correlate log messages and statistics produced by different
applications, e.g. a POP server and an SMTP server;
The ability to generate one or more periodically refreshed displays (e.g.
bar graphs) based on log statistics;
The ability to query external databases such as DNS blacklists;
The ability to maintain, save, and restore internal databases (e.g. of
blocked hosts and times at which they were blocked) and report their contents
The ability to "fire" rules at specific times or intervals as well as in
response to messages;
The ability to send log messages to, and accept them on or from, arbitrary
UDP or TCP ports;
The ability to log to another machine via an encrypted connection (e.g.
through SSH or SSL);
Stronger authentication than that implemented in current versions of syslogd
(most of which use source IP address and port number);
Flexible notification facilities, including the ability to send notices
via e-mail, pager, IRC, and instant messaging systems;
The ability to issue commands to firewalls, routers, bridges, managed hubs,
and remote power controllers; and
The ability to allow or deny users access to facilities (e.g. by changing
group memberships, changing a user's login shell to /etc/nologin,
or removing and restoring passwords).
Any computer system which is connected to the Internet, and/or subject
to misuse by its users, requires constant vigilance to fend off attacks
and thwart abuse. Because system administrators cannot be expected to monitor
system activity 24x7, intelligent agents -- or log monitors -- can alert
them to situations that require attention and "hold the fort" until help
arrives. Log monitors can also perform highly routine chores -- such as
blocking worms and spammers -- automatically.
While a handful of "plug and play" log monitors now exist, none contain
the features necessary to allow them to perform sophisticated stateful
monitoring. The author's goal is to fill this gap by implementing a collection
of complex log monitors and then creating a generalized facility which
can subsume all of their functions.
To ease the implementation of log monitors, the logging facilities in
different UNIX implementations -- which have diverged in subtle ways and
often hide useful information from administrators and intelligent agents
alike -- should be updated or replaced with a more modern scheme that is
backward-compatible with what exists today. It will then be much easier
to implement a generalized log monitoring facility that runs on a wide
variety of platforms.
Of course, no log monitoring system can completely replace the insight
or talents of a human administrator. As Bruce Schneier, founder of Counterpane
Network Security, writes in a white paper posted at https://www.counterpane.com/msm.html:
Network attacks can be much more subtle than a broken window.
Much depends on context. Software can filter the tens of megabytes of audit
information a medium-sized network can generate in a day, but software
is too easy for an attacker to fool. Intelligent alert requires people.
People to analyze what the software finds suspicious. People to delve deeper
into suspicious events, determining what is really going on. People to
separate false alarms from real attacks. People who understand context. 
The correct approach, therefore, is not one that eliminates people but
one that uses intelligent agents -- log monitors -- as a first line of
defense. This frees skilled administrators from the tedium of reviewing
logs, so that they may focus on the bona fide anomalies detected by log
monitors and on other problems more worthy of their talents.
Thanks to the many attendees of BSDCon Europe 2001, who provided many useful
suggestions regarding this ongoing work, and to "shepherd" Gregory Neil
Shapiro who guided the preparation of this paper. Thanks also to the authors
and maintainers of the BSDs and of the other utilities mentioned in this
document for their contributions to the state of the art. Trademarks mentioned
in this document are the property of their respective owners.
 Stephen E. Hansen and E. Todd
Atkins. Centralized System Monitoring With Swatch. In Proceedings of
the Seventh Systems Administration Conference (LISA), Monterey, CA,
Nov. 1993. Paper URL: https://www.stanford.edu/~atkins/swatch/lisa93.html.
Software at URL: ftp://ftp.stanford.edu/general/security-tools/swatch.
 Pacific Institute for Computer
Security (PICS) research group, San Diego Supercomputer Center (SDSC).
2swatch. Software at URL: ftp://ftp.sdsc.edu/pub/sdsc/security/PICS/2swatch/.
 Paul Vixie and contributors.
cron. FreeBSD 5.0-current version documented at URL: https://www.FreeBSD.org/cgi/man.cgi?query=cron&apropos=0&sektion=8&manpath=FreeBSD+5.0-current&format=html.
 Paul Traina and Brian Somers.
periodic. FreeBSD 5.0-current version documented at URL: https://www.FreeBSD.org/cgi/man.cgi?query=periodic&apropos=0&sektion=8&manpath=FreeBSD+5.0-current&format=html.
 Tom Boutell. Wusage. Software
and documentation at URL: https://www.boutell.com/wusage/.
 Kai Schlichting. Kai's Spamshield.
Software and documentation at URL: https://spamshield.conti.nu/.
 Eric Allman, The University
of Califonia at Berkeley, The FreeBSD Project and contributors. syslogd.
FreeBSD 5.0-current version documented at URL: https://www.FreeBSD.org/cgi/man.cgi?query=syslogd&apropos=0&sektion=8&manpath=FreeBSD+5.0-current&format=html.
 Eric Allman, University
of California at Berkeley and contributors. syslogd. 4.4BSD Lite2 version
documented at URL: https://www.FreeBSD.org/cgi/man.cgi?query=syslogd&apropos=0&sektion=8&manpath=4.4BSD+Lite2&format=html.
Eric Allman, The University of Califonia at Berkeley, The FreeBSD Project
and contributors. syslogd.conf. FreeBSD 5.0-current version documented
at URL: https://www.FreeBSD.org/cgi/man.cgi?query=syslog.conf&apropos=0&sektion=5&manpath=FreeBSD+5.0-current&format=html.
 Matija Grabnar. File::Tail.
Software at URL: https://www.cpan.org/modules/by-module/File/File-Tail-0.98.tar.gz.
BalaBit IT Ltd. syslog-ng. Software and documentation at URL: https://www.balabit.hu/en/downloads/syslog-ng/.
Core-SDI. msyslog. Software and documentation at URL: https://community.corest.com/pub/msyslog/.
 The Apache Software Foundation.
Apache HTTPD Server Project. Software and documentation at URL: https://www.apache.org/.
 Computer Emergency Response
Team (CERT). Advisory CA-2001-26: Nimda Worm. At URL: https://www.cert.org/advisories/CA-2001-26.html.
 Computer Emergency Response
Team (CERT). Advisory CA-2001-19: "Code Red" Worm Exploiting Buffer Overflow
In IIS Indexing Service DLL. At URL: https://www.cert.org/advisories/CA-2001-19.html.
 Computer Emergency Response
Team (CERT). Advisory CA-2001-11: sadmind/IIS Worm. At URL: https://www.cert.org/advisories/CA-2001-11.html.
 The Apache Software Foundation.
mod_setenvif. Documentation at URL: https://httpd.apache.org/docs/mod/mod_setenvif.html#setenvif.
 The Apache Software Foundation.
mod_log_config. Documentation at URL: https://httpd.apache.org/docs/mod/mod_log_config.html#customlog
 The Apache Software Foundation.
Apache Core Features. HostNameLookups directive. At URL: https://httpd.apache.org/docs/mod/core.html#hostnamelookups.
 R. E. Griswold, J. F.
Poage, I. P. Polonsky. The SNOBOL4 Programming Language, 2nd Edition. Bell
Telephone Laboratories/Prentice-Hall, 1971.
 Phil Budne. Phil's SNOBOL
Resources Page. At URL: https://people.ne.mediaone.net/philbudne/snobol.html.
 Phil Budne. Macro Implementation
of SNOBOL4 in C (C-MAINBOL). Software and documentation at URL: https://people.ne.mediaone.net/philbudne/src.html#snobol
 Mark Emmer. Catspaw SPITBOL.
Information at URL: ftp://ftp.snobol4.com/specshet.pdf
 Mail Abuse Prevention
System (mail-abuse.org). MAPS DUL Introduction. At URL: https://www.mail-abuse.org/dul/intro.htm
 LogReport Foundation.
Report Production Line. At URL: https://www.logreport.org/documentation/about=architecture.
 Bruce Schneier. Managed
Security Monitoring: Network Security for the 21st Century . At URL: https://www.counterpane.com/msm.html.