Copyright © 2001 - 2007 Thomas M. Eastep
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2008/02/09
Table of Contents
This article applies to Shorewall 3.0 and later. If you are running a version of Shorewall earlier than Shorewall 3.0.0 then please see the documentation for that release.
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. While the packet can be logged using LOG rules in the ESTABLISHED section of /etc/shorewall/rules, that is not recommended because of the large amount of information that may 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 may be logged using LOG rules in the RELATED section of /etc/shorewall/rules.
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.
Syslog may also write to your system console. See Shorewall FAQ 16 for ways to avoid having Shorewall messages written to the console.
There are a couple of limitations to syslogd-based logging:
If you give, for example, kern.info its 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 included in most distributions and is also available from http://www.netfilter.org/projects/ulogd/index.html. Ulogd can be configured to log all Shorewall messages 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 change all instances of log levels (usually “info”) in your Shorewall configuration files to “ULOG” - this includes entries in the policy, rules and shorewall.conf files. Here's what I had at one time:
gateway:/etc/shorewall# grep -v ^\# * | egrep '\$LOG|ULOG|LOGFILE' params:LOG=ULOG policy:loc $FW REJECT $LOG policy:net all DROP $LOG 10/sec:40 policy:all all REJECT $LOG rules:REJECT:$LOG loc net tcp 25 rules:REJECT:$LOG loc net udp 1025:1031 rules:REJECT:$LOG dmz net udp 1025:1031 rules:ACCEPT:$LOG dmz net tcp 1024: 20 rules:REJECT:$LOG $FW net udp 1025:1031 shorewall.conf:LOGFILE=/var/log/shorewall shorewall.conf:LOGUNCLEAN=$LOG shorewall.conf:MACLIST_LOG_LEVEL=$LOG shorewall.conf:TCP_FLAGS_LOG_LEVEL=$LOG shorewall.conf:RFC1918_LOG_LEVEL=$LOG gateway:/etc/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
“dump” commands.
Beginning in Shorewall-perl 4.1, the NFLOG target is supported.
NFLOG is a successor to ULOG. When using Shorewall-perl 4.1 or later, both ULOG and NFLOG may be followed by a list of up to three numbers in parentheses.
The first number specifies the netlink group (1-32). If omitted (e.g., NFLOG(,0,10)) then a value of 1 is assumed.
The second number specifies the maximum number of bytes to copy. If omitted, 0 (no limit) is assumed.
The third number specifies the number of log messages that should be buffered in the kernel before they are sent to user space. The default is 1.
Examples:
/etc/shorewall/shorewall.conf:
MACLIST_LOG_LEVEL=NFLOG(1,0,1)
/etc/shorewall/rules:
#ACTION SOURCE DEST PROTO DEST # PORT(S) ACCEPT:NFLOG(1,0,1) vpn fw tcp ssh,time,631,8080
Here
is a post describing configuring syslog-ng to work with Shorewall. Recent
SuSE™ releases come preconfigured with syslog-ng
with Netfilter messages (including Shorewall's) are written to
/var/log/firewall.
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.