Provided by: natlog_1.00.2-1_amd64 
NAME
natlog - source-nat logging tool
SYNOPSIS
natlog [OPTIONS] command
DESCRIPTION
Firewalls like iptables(1) usually offer POSTROUTING (source network address translation, snat)
facilities changing the source address of a host behind the firewall to the address of the host before
the firewall. With snat the following combinations of IP addresses and port numbers are encountered:
o the IP address and port number used by the host behind the firewall (in this manual page referred
to as IPsrc, sport);
o the IP address and port number of the host IPsrc connects to (in this manual page referred to as
IPdst, dport);
o the IP address and port number used by the firewalling host when source natting IPsrc and sport
(in this manual page referred to as IPfw, fwport).
Source natting usually uses sport for fwport, but fwport may already be in use, in which case the
firewalling host must use another, available port to forward communication from IPsrc, sport to IPdst,
dport.
The general scheme that applies to source natting, therefore, looks like this:
IPsrc:sport is translated by the firewall to IPfw:fwport;
IPfw:fwport is used when communicating with IPdst:dport.
From the perspective of the destination host the communication originates at IPfw::fwport and
consequently all communication (e.g., and incident report) sent by the systems administrator maintaining
IPdst to IPfw’s systems administrator will refer to IPfw:fwport, rather than to IPsrc::sport.
Using the standard log facilities provided by iptables do not easily allow us to relate IPfw:fwport to
IPsrc:sport, and natlog was developed to fill in that particular niche.
When running natlog, messages are sent to the syslog daemon (e.g., rsyslogd(1)) and/or the standard
output stream showing the essential characteristics of the connection using source natting. Here is an
example:
NATLOG: From 1338990672:55588 until 1338990747:807100:
192.168.19.72:4467 (via: 129.125.90.132:4467) to 200.49.219.180:443
In this example the values 1338990672:55588 and 1338990747:807100 represent time stamps showing the
begin- and end-times in seconds:microseconds of the connection since Jan 1, 1970, 0:00 UTC. Natlog offers
the --datetime option if time representations like Nov 2 13:29:11 are preferred.
The next value (192.168.19.72:4467) represents IPsrc::sport. This is followed by 129.125.90.132:4467,
representing IPfw:fwport. The third pair of values (200.49.219.180:443) represents IPdst:dport.
In this example, host 192.168.19.72, using port 4467, connected to host 200.49.219.180, port 443. To this
latter host the connection appeared to have originated from 129.125.90.132 port 4467. The provided log
message easily allows us to related this to the `real’ host and port from which the connection
originated: 192.168.19.72:4467.
When natlog terminates natlog can no longer track connections that are still open. If natlog was
terminated by a SIGTERM signal, then it sends a `terminating’ line to syslog, followed by an overview of
all still open connections. The end-microseconds values of connections that are no longer tracked are
shown as 0000.
COMMANDS
o conntrack: this command can only be used on platforms using iptables(1) on which conntrack(1) has
also been installed. Information about source-natted connections is obtained from conntrack(1)’s
output.
o indevice outdevice: indevice is the name of the device behind the firewall. Addresses living
behind the indevice are source-natted to the firewall’s IP address when passed on to the
outdevice; outdevice is the name of the device to which source-natted packets are forwarded, c.q.
from where replies for source-natted hosts living behind the indevice are received
OPTIONS
o --config=config-path (-c)
The argument config-path defines the path to the configuration file to be used by natlog. By
default the configuration file is expected in /etc/natlog.conf. All configuration options have
defaults, which are used when no configuration file and no command-line options are provided.
Command-line options override configuration file options.
o --conntrack-path=path
The path to the conntrack(1) program. By default this is /usr/sbin/conntrack. The conntrack
program must be available when requesting natlog’s conntrack command.
o --no-daemon
By default, natlog runs in the background (a daemon). Natlog runs as an ordinary program (i.e., in
the foreground when the option no-daemon is provided). When running as a daemon, --stdout (see
below) is suppressed, and --verbose messages (see below) are sent to the sylog daemon, unless
--no-syslog was specified.
o --help (-h)
Write basic usage information to the standard output stream and terminate.
o --no-syslog
By default natlog writes syslog messages to the DAEMON facility with priority NOTICE. No messages
are sent to the syslog daemon when this option is specified.
o --pid-file=path (-p)
When natlog runs in the background, then path is the name of the path of the file holding the
daemon’s process-id. By default this is /var/run/natlog.pid.
o --stdout (-s)
Syslog-equivalent messages are sent to the standard output. This option is implied by --verbose,
but is suppressed when natlog runs as a daemon..
o --syslog-tag=tag
When syslog messages are generated they can be provided with a tag, which can be used to filter
natlog’s syslog messages from the log-files. By default the tag NATLOG is used. See also section
RSYSLOG FILTERING below.
o --syslog-facility=facility
The facility that is used to write the syslog messages to. By default this is DAEMON. For an
overview of facilities and their meanings, see, e.g., syslog(3). With natlog the facilities
DAEMON, LOCAL0, LOCAL1, LOCAL2, LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7, and USER can be used.
o --syslog-priority=priority
The priority that is used to write the syslog messages to. By default this is NOTICE. For an
overview of priorities and their meanings, see, e.g., syslog(3). With natlog all defined
priorities can be used. E.g., EMERG, ALERT, CRIT, ERR, WARNING, NOTICE, INFO and DEBUG.
o --time=spec (-t)
By default time stamps written by natlog are in raw, numeric form. E.g.,
NATLOG: From 1338990672:55588 until 1338990747:807100
These time stamps indicate times in seconds:microseconds since the beginning of the epoch, January
1, 1970, 0:00 UTC. This option can be used to change the seconds part of the time stamps to more
conventional representations.
Specify raw (the default) for the default representation in seconds since the epoch;
specify utc for a representation like Jun 6 13:29:11, using Universal Time Coordinated;
specify local for a representation like Jun 6 13:29:11, using the local time zone defined by the
computer running natlog.
o --verbose
Additional messages about natlog’s mode of operation are sent to the standard output stream. When
natlog runs as a daemon these messages are sent to the syslog daemon, unless --no-syslog was
specified.
o --version (-v)
Write natlog’s version number to the standard output stream and terminate.
o --warn (-w)
Warn about terminating connections not yet registered in natlog’s database. This normally only
happens during a short period after starting natlog, when existing connections haven’t yet been
noticed.
RSYSLOG FILTERING
When using rsyslogd(1) property based filters may be used to filter syslog messages and write them to a
file of your choice. E.g., to filter messages starting with the syslog message tag (e.g., NATLOG) use
:syslogtag, isequal, "NATLOG:" /var/log/natlog.log
:syslogtag, isequal, "NATLOG:" ~
Note that the colon is part of the tag, but is not specified with the syslog-tag option.
This causes all messages having the NATLOG: tag to be written on /var/log/natlog.log after which they are
discarded. More extensive filtering is also supported, see, e.g.,
http://www.rsyslog.com/doc/rsyslog_conf_filter.html and http://www.rsyslog.com/doc/property_replacer.html
EXAMPLES
Examples of natlog activations:
o natlog --no-daemon --no-syslog -s tun0 eth0
Natlog remains active as a foreground process, no syslog messages are written, syslog-equivalent
message are written to the standard output. Natlog uses the pcap library to capture packets from
the tun0 device (e.g., an openvpn(1) device), which is active behind the firewall, and to capture
packets from the eth0 device, which is the device to where source-natted packages are sent.
o natlog conntrack
Depending on the options specified in /etc/natlog.conf (or, if not available, natlog’s default
options) source-natted connections are obtained from conntrack(1). By default natlog continues as
a daemon process, generating syslog messages using syslog tags NATLOG:, and containing information
about source-natted connections.
Here is natlog’s default configuration file. Empty lines and lines starting with hash-marks (#) are
ignored. Options adhere to the
option value
syntax. Option and value are separated by white space, Option values may consist of multiple words.
# This configuration file shows the default option values.
# Each option could also be omitted without affecting natlog’s default
# behavior.
# all options and values are case sensitive
# see `man natlog’ for further details
# the path to the conntrack program:
conntrack-path /usr/sbin/conntrack
# the used syslog tag:
syslog-tag NATLOG
# the used syslog facility:
syslog-facility DAEMON
# the used syslog priority:
syslog-priority NOTICE
# the time specification:
time raw
# end of the cofiguration file
FILES
o /etc/natlog.conf: default configuration file.
SEE ALSO
conntrack(1), iptables(1), rsyslogd(1), syslog(3)
BUGS
None reported
AUTHOR
Frank B. Brokken (f.b.brokken@rug.nl).
natlog.1.00.2.tar.gz 2012-2013 natlog(1)