Tom Eastep 2004-07-15 2001 - 2004 Thomas M. Eastep Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.2 or any later version published by the Free Software Foundation; with no Invariant Sections, with no Front-Cover, and with no Back-Cover Texts. A copy of the license is included in the section entitled GNU Free Documentation License.

The disposition of packets entering a Shorewall firewall is determined by one of a number of Shorewall facilities. Only some of these facilities permit logging. The packet is part of an established connecection. The packet is accepted and cannot be logged. The packet represents a connection request that is related to an established connection (such as a data connection associated with an FTP control connection). These packets also cannot be logged. The packet is rejected because of an option in /etc/shorewall/shorewall.conf or /etc/shorewall/interfaces. These packets can be logged by setting the appropriate logging-related option in /etc/shorewall/shorewall.conf. The packet matches a rule in /etc/shorewall/rules. By including a syslog level (see below) in the ACTION column of a rule (e.g., ACCEPT:info net fw tcp 22), the connection attempt will be logged at that level. The packet doesn't match a rule so it is handled by a policy defined in /etc/shorewall/policy. These may be logged by specifying a syslog level in the LOG LEVEL column of the policy's entry (e.g., loc net ACCEPT info).

By default, Shorewall directs NetFilter to log using syslog (8). Syslog classifies log messages by a facility and a priority (using the notation facility.priority). The facilities defined by syslog are auth, authpriv, cron, daemon, kern, lpr, mail, mark, news, syslog, user, uucp and local0 through local7. Throughout the Shorewall documentation, I will use the term level rather than priority since level is the term used by NetFilter. The syslog documentation uses the term priority.

Syslog levels are a method of describing to syslog (8) the importance of a message. A number of Shorewall parameters have a syslog level as their value. Valid levels are: 7 - debug (Debug-level messages) 6 - info (Informational) 5 - notice (Normal but significant Condition) 4 - warning (Warning Condition) 3 - err (Error Condition) 2 - crit (Critical Conditions) 1 - alert (must be handled immediately) 0 - emerg (System is unusable) For most Shorewall logging, a level of 6 (info) is appropriate. Shorewall log messages are generated by NetFilter and are logged using the kern facility and the level that you specify. If you are unsure of the level to choose, 6 (info) is a safe bet. You may specify levels by name or by number. Syslogd writes log messages to files (typically in /var/log/*) based on their facility and level. The mapping of these facility/level pairs to log files is done in /etc/syslog.conf (5). If you make changes to this file, you must restart syslogd before the changes can take effect.

There are a couple of limitations to syslogd-based logging: If you give, for example, kern.info it's own log destination then that destination will also receive all kernel messages of levels 5 (notice) through 0 (emerg). All kernel.info messages will go to that destination and not just those from NetFilter. Beginning with Shorewall version 1.3.12, if your kernel has ULOG target support (and most vendor-supplied kernels do), you may also specify a log level of ULOG (must be all caps). When ULOG is used, Shorewall will direct netfilter to log the related messages via the ULOG target which will send them to a process called ulogd. The ulogd program is available from http://www.gnumonks.org/projects/ulogd and can be configured to log all Shorewall message to their own log file. The ULOG logging mechanism is completely separate from syslog. Once you switch to ULOG, the settings in /etc/syslog.conf have absolutely no effect on your Shorewall logging (except for Shorewall status messages which still go to syslog). You will need to have the kernel source available to compile ulogd. Download the ulog tar file and: Be sure that /usr/src/linux is linked to your kernel source tree cd /usr/local/src (or whereever you do your builds) tar -zxf source-tarball-that-you-downloaded cd ulod-version ./configure make make install If you are like me and don't have a development environment on your firewall, you can do the first six steps on another system then either NFS mount your /usr/local/src directory or tar up the /usr/local/src/ulogd-version directory and move it to your firewall system. Now on the firewall system, edit /usr/local/etc/ulogd.conf and set: syslogfile <the file that you wish to log to> syslogsync 1 Also on the firewall system: touch <the file that you wish to log to> I also copied the file /usr/local/src/ulogd-version/ulogd.init to /etc/init.d/ulogd. I had to edit the line that read daemon /usr/local/sbin/ulogd to read daemon /usr/local/sbin/ulogd -d. On a RedHat system, a simple chkconfig --level 3 ulogd on starts ulogd during boot up. Your init system may need something else done to activate the script. You will need to change all instances of log levels (usually info) in your configuration files to ULOG - this includes entries in the policy, rules and shorewall.conf files. Here's what I have: [root@gateway shorewall]# grep ULOG * policy:loc fw REJECT ULOG policy:net all DROP ULOG 10/sec:40 policy:all all REJECT ULOG rules:REJECT:ULOG loc net tcp 6667 shorewall.conf:TCP_FLAGS_LOG_LEVEL=ULOG shorewall.conf:RFC1918_LOG_LEVEL=ULOG [root@gateway shorewall]# Finally edit /etc/shorewall/shorewall.conf and set LOGFILE=<file that you wish to log to>. This tells the /sbin/shorewall program where to look for the log when processing its show log, logwatch and monitor commands.

Here is a post describing configuring syslog-ng to work with Shorewall.

For general information on the contents of Netfilter log messages, see http://logi.cc/linux/netfilter-log-format.php3. For Shorewall-specific information, see FAQ #17.