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<articleinfo>
<title>Shorewall Logging</title>
<authorgroup>
<author>
<firstname>Tom</firstname>
<surname>Eastep</surname>
</author>
<author>
<surname>Bill Shirley</surname>
</author>
</authorgroup>
<pubdate><?dbtimestamp format="Y/m/d"?></pubdate>
<copyright>
<year>2001 - 2016</year>
<holder>Thomas M. Eastep</holder>
<holder>Bill Shirley</holder>
</copyright>
<legalnotice>
<para>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
<quote><ulink url="GnuCopyright.htm">GNU Free Documentation
License</ulink></quote>.</para>
</legalnotice>
</articleinfo>
<caution>
<para><emphasis role="bold">This article applies to Shorewall 4.3 and
later. If you are running a version of Shorewall earlier than Shorewall
4.3.5 then please see the documentation for that
release.</emphasis></para>
</caution>
<section id="Log">
<title>How to Log Traffic Through a Shorewall Firewall</title>
<para>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.</para>
<orderedlist>
<listitem>
<para>The packet is part of an established connection. While the
packet can be logged using LOG rules in the ESTABLISHED section of
<ulink
url="manpages/shorewall-rules.html">/etc/shorewall/rules</ulink>, that
is not recommended because of the large amount of information that may
be logged.</para>
</listitem>
<listitem>
<para>The packet represents a connection request that is related to an
established connection (such as a <ulink url="FTP.html">data
connection associated with an FTP control connection</ulink>). These
packets may be logged using LOG rules in the RELATED section of <ulink
url="manpages/shorewall-rules.html">/etc/shorewall/rules</ulink>.</para>
</listitem>
<listitem>
<para>The packet is rejected because of an option in <ulink
url="manpages/shorewall.conf.html">/etc/shorewall/shorewall.conf</ulink>
or <ulink
url="manpages/shorewall-interfaces.html">/etc/shorewall/interfaces</ulink>.
These packets can be logged by setting the appropriate logging-related
option in <ulink
url="manpages/shorewall.conf.html">/etc/shorewall/shorewall.conf</ulink>.</para>
</listitem>
<listitem>
<para>The packet matches a rule in <ulink
url="manpages/shorewall-rules.html">/etc/shorewall/rules</ulink>. By
including a syslog level (see below) in the ACTION column of a rule
(e.g., <quote>ACCEPT<emphasis role="bold">:info</emphasis> net $FW tcp
22</quote>), the connection attempt will be logged at that
level.</para>
</listitem>
<listitem>
<para>The packet doesn't match a rule so it is handled by a policy
defined in <ulink
url="manpages/shorewall-policy.html">/etc/shorewall/policy</ulink>.
These may be logged by specifying a syslog level in the LOG LEVEL
column of the policy's entry (e.g., <quote>loc net ACCEPT <emphasis
role="bold">info</emphasis></quote>).</para>
</listitem>
</orderedlist>
</section>
<section id="Where">
<title>Where the Traffic is Logged and How to Change the
Destination</title>
<para>By default, Shorewall directs Netfilter to log using syslog (8).
Syslog classifies log messages by a <emphasis>facility</emphasis> and a
<emphasis>priority</emphasis> (using the notation
<emphasis>facility.priority</emphasis>).</para>
<para>The facilities defined by syslog are <emphasis>auth, authpriv, cron,
daemon, kern, lpr, mail, mark, news, syslog, user, uucp</emphasis> and
<emphasis>local0</emphasis> through <emphasis>local7.</emphasis></para>
<para>Throughout the Shorewall documentation, I will use the term
<emphasis>level</emphasis> rather than <emphasis>priority </emphasis>since
<emphasis>level</emphasis> is the term used by Netfilter. The syslog
documentation uses the term <emphasis>priority</emphasis>.</para>
<section id="Levels">
<title>Syslog Levels</title>
<para>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.</para>
<para>Valid levels are:</para>
<simplelist>
<member>7 - <emphasis role="bold">debug</emphasis> (Debug-level
messages)</member>
<member>6 - <emphasis role="bold">info</emphasis>
(Informational)</member>
<member>5 - <emphasis role="bold">notice</emphasis> (Normal but
significant Condition)</member>
<member>4 - <emphasis role="bold">warning</emphasis> (Warning
Condition)</member>
<member>3 - <emphasis role="bold">err</emphasis> (Error
Condition)</member>
<member>2 - <emphasis role="bold">crit</emphasis> (Critical
Conditions)</member>
<member>1 - <emphasis role="bold">alert</emphasis> (must be handled
immediately)</member>
<member>0 - <emphasis role="bold">emerg</emphasis> (System is
unusable)</member>
</simplelist>
<para>For most Shorewall logging, a level of 6 (info) is appropriate.
Shorewall log messages are generated by Netfilter and are logged using
the <emphasis>kern</emphasis> 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.</para>
<para>Beginning with Shorewall 4.5.5, the
<replaceable>level</replaceable> name or number may be optionally
followed by a comma-separated list of one or more<replaceable> log
options</replaceable>. The list is enclosed in parentheses. Log options
cause additional information to be included in each log message.</para>
<para>Valid log options are:</para>
<variablelist>
<varlistentry>
<term><emphasis role="bold">ip_options</emphasis></term>
<listitem>
<para>Log messages will include the option settings from the IP
header.</para>
</listitem>
</varlistentry>
<varlistentry>
<term><emphasis role="bold">macdecode</emphasis></term>
<listitem>
<para>Decode the MAC address and protocol.</para>
</listitem>
</varlistentry>
<varlistentry>
<term><emphasis role="bold">tcp_sequence</emphasis></term>
<listitem>
<para>Include TCP sequence numbers.</para>
</listitem>
</varlistentry>
<varlistentry>
<term><emphasis role="bold">tcp_options</emphasis></term>
<listitem>
<para>Include options from the TCP header.</para>
</listitem>
</varlistentry>
<varlistentry>
<term><emphasis role="bold">uid</emphasis></term>
<listitem>
<para>Include the UID of the sending program; only valid for
packets originating on the firewall itself.</para>
</listitem>
</varlistentry>
</variablelist>
<para>Example: <emphasis
role="bold">info(tcp_options,tcp_sequence)</emphasis></para>
<para>Syslogd writes log messages to files (typically in <filename
class="directory">/var/log/</filename>*) 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.</para>
<para>Syslog may also write to your system console. See <ulink
url="FAQ.htm#faq16">Shorewall FAQ 16</ulink> for ways to avoid having
Shorewall messages written to the console.</para>
</section>
<section id="ULOG">
<title>Configuring a Separate Log for Shorewall Messages (ulogd)</title>
<para>There are a couple of limitations to syslogd-based logging:</para>
<orderedlist>
<listitem>
<para>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).</para>
</listitem>
<listitem>
<para>All kernel.info messages will go to that destination and not
just those from Netfilter.</para>
</listitem>
</orderedlist>
<para>If your kernel has NFLOG target support (and most vendor-supplied
kernels do), you may also specify a log level of NFLOG (must be all
caps). When NFLOG is used, Shorewall will direct Netfilter to log the
related messages via the NFLOG target which will send them to a process
called <quote>ulogd</quote>. The ulogd program is included in most
distributions and is also available from <ulink
url="http://www.netfilter.org/projects/ulogd/index.html">http://www.netfilter.org/projects/ulogd/index.html</ulink>.
Ulogd can be configured to log all Shorewall messages to their own log
file.</para>
<note>
<para>The NFLOG logging mechanism is <emphasis
role="underline">completely separate</emphasis> from syslog. Once you
switch to NFLOG, the settings in <filename>/etc/syslog.conf</filename>
have absolutely no effect on your Shorewall logging (except for
Shorewall status messages which still go to syslog).</para>
</note>
<para>You will need to change all instances of log levels (usually
<quote>info</quote>) in your Shorewall configuration files to
<quote>NFLOG</quote> - this includes entries in the policy, rules and
shorewall.conf files. If you initially installed using Shorewall 5.1.2
or later, you can simply change the setting of LOG_LEVEL in
shorewall.conf.</para>
<para>Otherwise, you must search for all instances of log levels in your
configuration and change them accordingly. If you currently run
Shorewall 5.1.2 or later, then change them to $LOG_LEVEL and set
LOG_LEVEL accordingly. If you are running an earlier release, using a
shell variable simplifies future changes. Here's what I had at one
time:</para>
<programlisting>gateway:/etc/shorewall# grep -v ^\# * | egrep '\$LOG|ULOG|LOGFILE'
params:LOG=NFLOG
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# </programlisting>
<para>Finally edit <filename>/etc/shorewall/shorewall.conf</filename>
and set LOGFILE=<<emphasis>file that you wish to log
to</emphasis>>. This tells the <filename>/sbin/shorewall</filename>
program where to look for the log when processing its
<quote><command>show log</command></quote>,
<quote><command>logwatch</command></quote> and
<quote><command>dump</command></quote> commands.</para>
<para>The NFLOG target is a successor to ULOG. Both ULOG and NFLOG may
be followed by a list of up to three numbers in parentheses.</para>
<itemizedlist>
<listitem>
<para>The first number specifies the netlink group (0-65535). If
omitted (e.g., NFLOG(,0,10)) then a value of 0 is assumed.</para>
</listitem>
<listitem>
<para>The second number specifies the maximum number of bytes to
copy. If omitted, 0 (no limit) is assumed.</para>
</listitem>
<listitem>
<para>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.</para>
</listitem>
</itemizedlist>
<para>Examples:</para>
<para><filename>/etc/shorewall/shorewall.conf</filename>:
<programlisting>MACLIST_LOG_LEVEL=NFLOG(1,0,1)</programlisting></para>
<para><filename>/etc/shorewall/rules</filename>:<programlisting>#ACTION SOURCE DEST PROTO DPORT
ACCEPT:NFLOG(1,0,1) vpn fw tcp ssh,time,631,8080 </programlisting><important>
<para>Shorewall considers <emphasis role="bold">ULOG(...)</emphasis>
and <emphasis role="bold">NFLOG(...)</emphasis> to be <emphasis
role="bold">log levels</emphasis>, just like info, debug, etc. even
though they are not defined by syslog.</para>
</important></para>
<para>Here is a copy of a ulogd.conf file that logs to
/var/log/firewall. It was contributed by a Shorewall user on IRC:</para>
<programlisting>[global]
user="ulogd"
logfile="/var/log/ulogd/ulogd.log"
loglevel=7
plugin="/usr/lib64/ulogd/ulogd_inppkt_NFLOG.so"
plugin="/usr/lib64/ulogd/ulogd_filter_IFINDEX.so"
plugin="/usr/lib64/ulogd/ulogd_filter_IP2STR.so"
plugin="/usr/lib64/ulogd/ulogd_filter_PRINTPKT.so"
plugin="/usr/lib64/ulogd/ulogd_output_LOGEMU.so"
plugin="/usr/lib64/ulogd/ulogd_raw2packet_BASE.so"
stack=log:NFLOG,base1:BASE,ifi1:IFINDEX,ip2str1:IP2STR,print1:PRINTPKT,firewall:LOGEMU
[firewall]
file="/var/log/firewall"
sync=1</programlisting>
<note>
<para>This sample config file assumes that NFLOG is being used in
logging rules and policies.</para>
</note>
</section>
</section>
<section>
<title id="Backends">Log Backends</title>
<para>Netfilter logging allows configuration of multiple backends. Logging
backends provide the The low-level forward of log messages. There are
currently three backends:</para>
<variablelist>
<varlistentry>
<term>LOG (ipt_LOG and ip6t_LOG).</term>
<listitem>
<para>Normal kernel-based logging to a syslog daemon.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>ULOG (ipt_ULOG)</term>
<listitem>
<para>ULOG logging as described ablve. Only available for
IPv4.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>netlink (nfnetlink_log)</term>
<listitem>
<para>The logging backend behind NFLOG, defined above.</para>
</listitem>
</varlistentry>
</variablelist>
<para>The currently-available and currently-selected IPv4 and IPv6
backends are shown in /proc/sys/net/netfilter/nf_log:</para>
<programlisting>cat /proc/net/netfilter/nf_log
0 NONE (nfnetlink_log)
1 NONE (nfnetlink_log)
2 ipt_ULOG (ipt_ULOG,ipt_LOG,nfnetlink_log)
3 NONE (nfnetlink_log)
4 NONE (nfnetlink_log)
5 NONE (nfnetlink_log)
6 NONE (nfnetlink_log)
7 NONE (nfnetlink_log)
8 NONE (nfnetlink_log)
9 NONE (nfnetlink_log)
10 ip6t_LOG (ip6t_LOG,nfnetlink_log)
11 NONE (nfnetlink_log)
12 NONE (nfnetlink_log)</programlisting>
<para>The magic numbers (0-12) are Linux address family numbers (AF_INET
is 2 and AF_INET6 is 10).</para>
<para>The name immediately following the number is the currently-selected
backend, and the ones in parantheses are the ones that are available. You
can change the currently selected backend by echoing it's name into
/proc/net/netfilter/nf_log.<replaceable>number</replaceable>.</para>
<para>Example - change the IPv4 backend to LOG:</para>
<programlisting>sysctl net.netfilter.nf_log.2=ipt_LOG</programlisting>
<para>Beginning with Shorewall 4.6.4, you can configure the backend using
the LOG_BACKEND option in <ulink
url="manpages/shorewall.conf.html">shorewall.conf(5)</ulink> and <ulink
url="manpages/shorewall.conf.html">shorewall6.conf(5)</ulink>.</para>
</section>
<section id="Syslog-ng">
<title>Syslog-ng</title>
<para><ulink
url="http://marc.info/?l=gentoo-security&amp;m=106040714910563&amp;w=2">Here</ulink>
is a post describing configuring syslog-ng to work with Shorewall. Recent
<trademark>SUSE</trademark> releases come preconfigured with syslog-ng
with Netfilter messages (including Shorewall's) are written to
<filename>/var/log/firewall</filename>.</para>
</section>
<section id="Contents">
<title>Understanding the Contents of Shorewall Log Messages</title>
<para>For general information on the contents of Netfilter log messages,
see <ulink
url="http://logi.cc/en/2010/07/netfilter-log-format/">http://logi.cc/en/2010/07/netfilter-log-format/</ulink>.</para>
<para>For Shorewall-specific information, see <ulink
url="FAQ.htm#faq17">FAQ #17</ulink>.</para>
</section>
<section>
<title>Customizing the Content of Shorewall Log Messages</title>
<para>In addition to the <link linkend="Levels">options</link> mentioned
above, a certain amount of customization of the Netfilter-generated
messages is allowed.</para>
<section id="LogTags">
<title>Log Tags</title>
<para>In a Shorewall logging rule, the log level can be followed by
a<firstterm> log tag</firstterm> as in "DROP:NFLOG:junk". The generated
log message will include "<emphasis>chain-name</emphasis> junk
DROP".</para>
</section>
<section>
<title>LOGTAGONLY</title>
<para>By setting the LOGTAGONLY option to Yes in <ulink
url="manpages/shorewall.conf.html">shorewall.conf(5)</ulink> or <ulink
url="manpages/shorewall.conf.html">shorewall6.conf(5)</ulink>, the
disposition ('DROP' in the above example) will be omitted. Consider the
following rule:</para>
<programlisting>#ACTION SOURCE DEST PROTO
REJECT(icmp-proto-unreachable):notice:IPv6 loc net 41 # who's using IPv6 tunneling</programlisting>
<para>This rule generates the following warning at compile time:</para>
<simplelist>
<member>WARNING: Log Prefix shortened to "Shorewall:IPv6:REJECT(icmp-p
" /etc/shorewall/rules (line 212)</member>
</simplelist>
<para>and produces the rather ugly prefix "Shorewall:IPv6:REJECT(icmp-p
".</para>
<para>Now consider this similar rule:</para>
<programlisting>#ACTION SOURCE DEST PROTO
REJECT(icmp-proto-unreachable):notice:IPv6,tunneling loc net 41 # who's using IPv6 tunneling</programlisting>
<para>With LOGTAGONLY=Yes, no warning is generated and the prefix
becomes "Shorewall:IPv6:tunneling:"</para>
<para>See the shorewall[6].conf man page for further information about
how LOGTAGONLY=Yes can be used.</para>
</section>
<section>
<title>Log Levels in shorewall[6].conf</title>
<para><ulink
url="manpages/shorewall.conf.html">shorewall.conf(5)</ulink> and <ulink
url="manpages/shorewall.conf.html">shorewall6.conf(5)</ulink> have a
number of options whose values are log levels. Beginning with Shorewall
5.0.0, these specifcations may include a log tag as described <link
linkend="LogTags">above</link>.</para>
</section>
</section>
<section>
<title>Some Additional Thoughts on Logging (by Bill Shirley)</title>
<para>As a side note to the LOGTAGONLY example above, i recommend blocking
all tunneling because it bypasses the firewall rules:</para>
<programlisting>#ACTION SOURCE DEST PROTO DPORT
?COMMENT tunneling
REJECT(icmp-proto-unreachable):notice:IPv6,tunneling loc net 41 # who's using IPv6 tunneling
REJECT(icmp-port-unreachable) loc net tcp,udp teredo
REJECT(icmp-port-unreachable) loc net tcp,udp isakmp,ipsec-nat-t</programlisting>
<para>Here is an example of logging traffic only once:</para>
<para><filename>/etc/shorewall/init:</filename></para>
<programlisting>ipset -exist create IPv4 hash:ip timeout 86400
ipset -exist create IPv4-port hash:ip,port timeout 14400</programlisting>
<para><filename>/etc/shorewall/rules</filename> (at the top):</para>
<programlisting>#ACTION SOURCE DEST PROTO
?SECTION NEW
# ------------------
?COMMENT drop previously flagged
DROP net:+IPv4[src] fw
DROP net:+IPv4-port[src,dst] fw</programlisting>
<para>After all the rules have been checked, at the bottom of
<filename>/etc/shorewall/rules</filename>:</para>
<programlisting># =============================================================================
# =============================== H@ck0rz =====================================
# =============================================================================
?COMMENT dont whack myself
REJECT:notice inet:$ME_NET fw
?COMMENT not public
ADD(+IPv4-port:src,dst) net fw tcp,udp domain
ADD(+IPv4-port:src,dst) net fw tcp ldap,ldaps
ADD(+IPv4-port:src,dst) net fw tcp,udp ipp
?COMMENT H@ck0rz
ADD(+IPv4:src) net fw tcp ssh
ADD(+IPv4:src) net fw tcp ftp,ftps,sftp,telnet,telnets,exec,login,shell,sunrpc
ADD(+IPv4:src) net fw tcp,udp ms-sql-s,ms-sql-m
?COMMENT drop if added
DROP:info:BAN,IPv4 net:+IPv4[src] fw
DROP:info:BAN,IPv4-port net:+IPv4-port[src,dst] fw</programlisting>
<para>One final note: I wanted less firewall messages in /var/log/messages
so I added to rsyslog.conf:</para>
<programlisting>#### RULES #### <-- find this
if $msg contains 'Shorewall' then {
action(type="omfile" file="/var/log/shorewall.log")
# if ($syslogfacility == 0 and $syslogseverity >= 4) then stop # warning
# if ($syslogfacility == 0 and $syslogseverity >= 5) then stop # notice
if ($syslogfacility == 0 and $syslogseverity >= 6) then stop # info
}</programlisting>
<para>I log at 'notice' log level if I want the message in
<filename>/var/log/messages</filename> and everything goes to
<filename>/var/log/shorewall.log</filename>. Don't forget to add
/var/log/shorewall.log to logrotate.</para>
</section>
</article>