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1.  Introduction

Extra logging should be added to catch warnings or errors that might indicate an ongoing attack or a successful compromise. Attackers often scan or probe before attacking.

It's also vital that your log files are easily readable and manageable. Gentoo Linux lets you choose between 3 different loggers when installing.

1.  Logging: Syslogd

Syslogd is the most common logger for Linux and Unix in general. It has some log rotation facilities, but using /usr/sbin/logrotate in a cron job (logrotate is configured in /etc/logrotate.conf) might prove to be more powerful as logrotate has many features. How often log rotation should be done depends on the system load.

Below is the standard syslog.conf with some added features. We have uncommented the cron and tty lines and added a remote logging server. To further enhance security you could add logging to two places.

Code Listing 1.1: /etc/syslog.conf

#  /etc/syslog.conf      Configuration file for syslogd.
#
#                       For more information see syslog.conf(5)
#                       manpage.
#                       This is from Debian, we are using it for now
#                       Daniel Robbins, 5/15/99

#
# First some standard logfiles.  Log by facility.
#

auth,authpriv.*                 /var/log/auth.log
*.*;auth,authpriv.none          -/var/log/syslog
cron.*                         /var/log/cron.log
daemon.*                        -/var/log/daemon.log
kern.*                          -/var/log/kern.log
lpr.*                           -/var/log/lpr.log
mail.*                          /var/log/mail.log
user.*                          -/var/log/user.log
uucp.*                          -/var/log/uucp.log
local6.debug                    /var/log/imapd.log

#
# Logging for the mail system. Split it up so that
# it is easy to write scripts to parse these files.
#
mail.info                       -/var/log/mail.info
mail.warn                       -/var/log/mail.warn
mail.err                        /var/log/mail.err

# Logging for INN news system
#
news.crit                       /var/log/news/news.crit
news.err                        /var/log/news/news.err
news.notice                     -/var/log/news/news.notice

#
# Some `catch-all' logfiles.
#
*.=debug;\
        auth,authpriv.none;\
        news.none;mail.none     -/var/log/debug
*.=info;*.=notice;*.=warn;\
        auth,authpriv.none;\
        cron,daemon.none;\
        mail,news.none          -/var/log/messages

#
# Emergencies and alerts are sent to everybody logged in.
#
*.emerg                         *
*.=alert                        *

#
# I like to have messages displayed on the console, but only on a virtual
# console I usually leave idle.
#
daemon,mail.*;\
       news.=crit;news.=err;news.=notice;\
       *.=debug;*.=info;\
       *.=notice;*.=warn       /dev/tty8

#Setup a remote logging server
*.*                        @logserver

# The named pipe /dev/xconsole is for the `xconsole' utility.  To use it,
# you must invoke `xconsole' with the `-file' option:
#
#    $ xconsole -file /dev/xconsole [...]
#
# NOTE: adjust the list below, or you'll go crazy if you have a reasonably
#      busy site..
#
#daemon.*,mail.*;\
#       news.crit;news.err;news.notice;\
#       *.=debug;*.=info;\
#       *.=notice;*.=warn       |/dev/xconsole

local2.*                --/var/log/ppp.log

Attackers will most likely try to erase their tracks by editing or deleting log files. You can make it harder for them by logging to one or more remote logging servers on other machines. Get more info about syslogd by executing man syslog.

1.  Metalog

Metalog by Frank Dennis is not able to log to a remote server, but it does have advantages when it comes to performance and logging flexibility. It can log by program name, urgency, facility (like syslogd), and comes with regular expression matching with which you can launch external scripts when specific patterns are found. It is very good at taking action when needed.

The standard configuration is usually enough. If you want to be notified by email whenever a password failure occurs use one of the following scripts.

For postfix:

Code Listing 1.1: /usr/local/sbin/mail_pwd_failures.sh for postfix

#! /bin/sh
echo "$3" | mail -s "Warning (program : $2)" root

For netqmail:

Code Listing 1.1: /usr/local/sbin/mail_pwd_failures.sh for netqmail

#!/bin/sh
echo "To: root
Subject:Failure (Warning: $2)
$3
" | /var/qmail/bin/qmail-inject -f root

Remember to make the script executable by issuing /bin/chmod +x /usr/local/sbin/mail_pwd_failures.sh

Then uncomment the command line under "Password failures" in /etc/metalog/metalog.conf like:

Code Listing 1.1: /etc/metalog/metalog.conf

command  = "/usr/local/sbin/mail_pwd_failures.sh"

1.  Syslog-ng

Syslog-ng provides some of the same features as syslog and metalog with a small difference. It can filter messages based on level and content (like metalog), provide remote logging like syslog, handle logs from syslogd (even streams from Solaris), write to a TTY, execute programs, and it can act as a logging server. Basically it is the best of both loggers combined with advanced configuration.

Below is a classic configuration file slightly modified.

Code Listing 1.1: /etc/syslog-ng/syslog-ng.conf

options {
        chain_hostnames(no);

        # The default action of syslog-ng is to log a STATS line
        # to the file every 10 minutes.  That's pretty ugly after a while.
        # Change it to every 12 hours so you get a nice daily update of
        # how many messages syslog-ng missed (0).
        stats_freq(43200);
};

source src {
    unix-stream("/dev/log" max-connections(256));
    internal();
};

source kernsrc { file("/proc/kmsg"); };

# define destinations
destination authlog { file("/var/log/auth.log"); };
destination syslog { file("/var/log/syslog"); };
destination cron { file("/var/log/cron.log"); };
destination daemon { file("/var/log/daemon.log"); };
destination kern { file("/var/log/kern.log"); };
destination lpr { file("/var/log/lpr.log"); };
destination user { file("/var/log/user.log"); };
destination mail { file("/var/log/mail.log"); };

destination mailinfo { file("/var/log/mail.info"); };
destination mailwarn { file("/var/log/mail.warn"); };
destination mailerr { file("/var/log/mail.err"); };

destination newscrit { file("/var/log/news/news.crit"); };
destination newserr { file("/var/log/news/news.err"); };
destination newsnotice { file("/var/log/news/news.notice"); };

destination debug { file("/var/log/debug"); };
destination messages { file("/var/log/messages"); };
destination console { usertty("root"); };

# By default messages are logged to tty12...
destination console_all { file("/dev/tty12"); };

# ...if you intend to use /dev/console for programs like xconsole
# you can comment out the destination line above that references /dev/tty12
# and uncomment the line below.
#destination console_all { file("/dev/console"); };

# create filters
filter f_authpriv { facility(auth, authpriv); };
filter f_syslog { not facility(authpriv, mail); };
filter f_cron { facility(cron); };
filter f_daemon { facility(daemon); };
filter f_kern { facility(kern); };
filter f_lpr { facility(lpr); };
filter f_mail { facility(mail); };
filter f_user { facility(user); };
filter f_debug { not facility(auth, authpriv, news, mail); };
filter f_messages { level(info..warn)
        and not facility(auth, authpriv, mail, news); };
filter f_emergency { level(emerg); };

filter f_info { level(info); };
filter f_notice { level(notice); };
filter f_warn { level(warn); };
filter f_crit { level(crit); };
filter f_err { level(err); };
filter f_failed { message("failed"); };
filter f_denied { message("denied"); };

# connect filter and destination
log { source(src); filter(f_authpriv); destination(authlog); };
log { source(src); filter(f_syslog); destination(syslog); };
log { source(src); filter(f_cron); destination(cron); };
log { source(src); filter(f_daemon); destination(daemon); };
log { source(kernsrc); filter(f_kern); destination(kern); };
log { source(src); filter(f_lpr); destination(lpr); };
log { source(src); filter(f_mail); destination(mail); };
log { source(src); filter(f_user); destination(user); };
log { source(src); filter(f_mail); filter(f_info); destination(mailinfo); };
log { source(src); filter(f_mail); filter(f_warn); destination(mailwarn); };
log { source(src); filter(f_mail); filter(f_err); destination(mailerr); };

log { source(src); filter(f_debug); destination(debug); };
log { source(src); filter(f_messages); destination(messages); };
log { source(src); filter(f_emergency); destination(console); };

# default log
log { source(src); destination(console_all); };

Syslog-ng is very easy to configure, but it is also very easy to miss something in the configuration file since it is huge. The author still promises some extra features like encryption, authentication, compression and MAC (Mandatory Access Control) control. With these options it will be a perfect for network logging, since the attacker cannot spy on the log.

And syslog-ng does have one other advantage: it does not have to run as root!

1.  Log analysis with Logcheck

Of course, keeping logs alone is only half the battle. An application such as Logcheck can make regular log analysis much easier. Logcheck is a script, accompanied by a binary called logtail, that runs from your cron daemon and checks your logs against a set of rules for suspicious activity. It then mails the output to root's mailbox.

Logcheck and logtail are part of the app-admin/logsentry package.

Logcheck uses four files to filter important log entries from the unimportant. These files are logcheck.hacking, which contains known hacking attack messages, logcheck.violations, which contains patterns indicating security violations, logcheck.violations.ignore, which contains keywords likely to be matched by the violations file, allowing normal entries to be ignored, and logcheck.ignore, which matches those entries to be ignored.

Warning: Do not leave logcheck.violations.ignore empty. Logcheck uses grep to parse logs, some versions of which will take an empty file to mean wildcard. All violations would thus be ignored.

Page updated April 2, 2010

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