Provided by: natlog_2.04.01-1_amd64 
NAME
natlog - source-nat logging tool
SYNOPSIS
natlog [OPTIONS] command
DESCRIPTION
Firewalls like iptables(1) may offer POSTROUTING (source network address translation,
snat) facilities changing the source address of a host behind the firewall to the address
of the host connected to the outer world. With snat the following combinations of IP
addresses and port numbers are encountered:
o the IP address and port number used by the host protected by (i.e., behind) the
firewall initiates a connection to the outer world (the source host, in this manual
page referred to as IPsrc, sport);
o the IP address and port number of the host outside (i.e., before) the firewall that
IPsrc connects to (the destination host, in this manual page referred to as IPdst,
dport);
o the IP address and port number of the host where the firewall has been installed.
This host performs the source natting, and its IP-address and the port it uses when
forwarding IPsrc, sport’s requests to IPdst, dport are 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., incident reports) sent by the systems
administrator maintaining IPdst to IPfw’s systems administrator will refer to IPfw:fwport,
rather than to IPsrc::sport.
Relating IPfw:fwport to IPsrc:sport is difficult when merely using the standard log
facilities provided by iptables and natlog was developed to fill in that particular niche.
Natlog provides data about source natting in various forms. The standard logging mode
consists of messages sent to the syslog daemon (cf., rsyslogd(8)) and/or to the standard
output stream showing the essential characteristics of connections using source natting.
Here is an example of a logged message (log-entries occupy single lines; the line-breaks
below are to enhance readability):
NATLOG: from 1338990672:55588 thru 1338990747:807100 (UTC): tcp
192.168.19.72:4467 (via: 129.125.90.132:4467) to
to 200.49.219.180:443; sent: 802, received: 7669
The values 1338990672:55588 and 1338990747:807100 are time stamps showing the begin- and
end-times in seconds:microseconds of a tcp connection since the beginning of the epoch
(Jan 1, 1970, 0:00 UTC). Natlog offers the --time option for requesting human-readable
time specifications like Nov 2 13:29:11 rather than time representations using seconds and
micro seconds.
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 appears to have originated from
129.125.90.132 port 4467. The log message allows us to associate this with the `real’ host
and port from which the connection originated: 192.168.19.72:4467.
The final entries show the number of bytes that were sent by the source-host (IPsrc) and
received from the destination-host (IPdst).
When natlog is terminated it can no longer track connections that are still open. If
natlog was terminated (by a SIGINT or SIGTERM signal), then it logs a `terminating’ line,
followed by an overview of all (potentially) still open connections. Those connections are
flagged with a trailing ’(EOP)’ (end of program) log-element, and their end-times show
natlog’s termination time. Incomplete connections show (EXPIRED).
In addition to the standard logs the option --log-data is available. This option requires
the path to a file where information is logged in tabular form, which can easily be
processed by statistical software like R(1). When specifying this option information will
be appended to an existing file. When the log file does not yet exist it is created. The
first line of the thus written log files names the columns of the table. The column names
are (all on one line):
type, srcNr, srcIP, srcPort, dstNr, dstIP, dstPort,
sent, recvd, begin, end, beginTime, endTime, status
Most column labels will be self-explanatory. Type indicates the connection type, logged as
icmp, tcp, or udp; srcNr and dstNr are the 32 bit numeric values of, respectively, the
source host’s IP address and the destination host’s IP address (decimal representations);
begin and end are the times in seconds since the beginning of the epoch, corresponding to
the times displayed at, respectively, beginTime and endTime; status indicates the status
of the logged connection information: ok indicates a connection that was normally
completed; expired indicates that the connection was recognized, but was not normally
completed; eop is used for connections that were still active by the time natlog
terminates. When the status equals expired, the time entries show the times of receiving
the first and last packets of that connection; when eop, then the end and endTime entries
show natlog’s termination time.
Log entries look like this (each entry occupies one line, header line and logged data
lines are right-aligned):
tcp, 101820608, 192.168.17.6, 48886,
4012145084, 188.121.36.239, 80,
430, 2266, 1517387644, 1517387644,
Jan 31 08:34:04:318340, Jan 31 08:34:04:383170, ok
MODES AND COMMANDS
o conntrack: the `conntrack’-mode. This command can only be used on platforms using
iptables(1) where conntrack(1) has also been installed. Information about snat
connections is obtained from conntrack(1)’s output. In this mode all, or one of the
tcp (the protocol used by default), udp, and icmp layer four protocols can be
monitored.
By default conntrack does not report byte counts. To have conntrack report byte
counts issue the command
$ echo 1 > /proc/sys/net/netfilter/nf_conntrack_acct
before starting conntrack. To omit byte counts from log entries specify option
no-bytes.
Conntrack includes the sizes of the IP headers (usually 20 bytes) in reported byte
counts. Thus, icmp packets are usually reported as having size 84, even though
ping(1) reports a payload of 64 bytes. Since the actual sizes of IP headers cannot
be determined from conntrack’s output, the sizes reported when using natlog’s
conntrack mode are as reported by conntrack, and are therefore not corrected for IP
header lengths. The option --conntrack-ip-header-size can be used to correct for
the (assumed) IP header sizes.
Conntrack can also be used to track all connections, not just the snat connections.
If that’s required omit conntrack’s option -n, and optionally specify option
no-via.
See also the conntrack-command option.
o indevice outdevice: the `devices’-mode. Here, indevice is the name of the device
behind the firewall: addresses living behind the indevice are source-natted to the
firewall host’s IP address when passed on to the outdevice.
Outdevice is the name of the device where source-natted packets are forwarded to,
and from where replies for source-natted hosts living behind the indevice are
received. With this command all, or any combination of the tcp (the protocol
monitored by default), udp, and icmp layer four protocols can be monitored.
For example, when specifying the arguments
eth1 eth0
thene eth1 is the device behind the firewall, and eth0 is the device to where
source-natted packets are forwared.
This command can also be used to track all connections using a single device,
instead of merely tracking snat connections. In that case specify the same devices
for indevice and outdevice, and optionally specify option no-via. E.g.,
eth0 eth0
o infile in-address in-mask outfile out-address out-mask: the `tcpdump’-mode. This
command can be used to process tcpdump(1) generated binary files, generated on the
source-natting host. If a source natting host uses interface eth1 behind the
firewall and eth0 to connect to the outside world, then the follow tcpdump commands
produce the required binary files (these commands will normally be run in the
background, hence the trailing &):
tcpdump -wi eth0 /tmp/eth0 &
tcpdump -wi eth1 /tmp/eth1 &
To have natlog process these files, end the tcpdump processes, and transfer the
files /tmp/eth0 and /tmp/eth1 to the host where natlog has been installed. The
required addresses and masks are shown by the ifconfig(1) command. E.g.,
eth0: flags=4163<UP,BROADCAST,RUNNING,MULTICAST> mtu 1500
inet 129.125.1.123 netmask 255.255.0.0
broadcast 129.125.255.255
eth1: flags=4163<UP,BROADCAST,RUNNING,MULTICAST> mtu 1500
inet 192.168.1.1 netmask 255.255.255.0
broadcast 192.168.1255
The relevant info is shown in the lines following the interface’s name: the value
following inet is the interface’s IP address, and the value following netmask is
the network’s mask.
Combining files and addresses, natlog is run as follows (all on one line):
natlog /tmp/eth0 129.125.1.123 255.255.0.0
/tmp/eth1 192.168.1.1 255.255.255.0
Instead of fully specifying the netmask, netmaks specifications like /24 are also
accepted. In that case the number following the slash indicates the number of
non-zero bits of the netmask. In practice, each value of the netmask is either 255
(8 bits are set) or 0 (0 bits are set), and so 255.255.0.0 can also be specified as
/16, while 255.255.255.0 can be specified like /24.
OPTIONS
See also section SYSTEMD.
o --config=config-path (-c)
The argument config-path defines the path to natlog’s configuration file. By
default it is /etc/natlog.conf. All configuration options have defaults, which are
used when no configuration file and no command-line options were provided.
All options, except for config, help, verbose and version can also be specified in
the configuration file. The configuration file ignores empty lines and all
information on lines beginning with a hash-mark (#). In the configuration file
initial hyphens should be omitted, and option names may immediately be followed by
a colon. Do not surround option values with quotes. Examples:
stdout
syslog-facility: LOCAL0
Command-line options override configuration file options.
o --conntrack-command=path [options]
The path and options to the conntrack(1) program. By default this is
/usr/sbin/conntrack -p tcp -E -n -o timestamp -e NEW,DESTROY
resulting in:
- Monitoring the tcp layer four protocol;
- Displaying real-time event logs (-E);
- Only use snat connections (-n);
- Displaying time stamps (-o timestamp);
- Logging all new and destroyed (ended) events (-e NEW,DESTROY);
By default tcp is monitored. Other protocols can be configured using the --protocol
option.
The conntrack program must be available when requesting natlog’s conntrack command.
Layer four protocols other than tcp, udp and icmp are currently not supported. A
subset of the supported protocols may be requested using conntrack’s -p tcp, -p udp
or -p icmp options.
When all connections should be logged (not just snat connections) then omit
conntrack’s -n option. See also option --no-via below.
Note: when specifying the conntrack-command option in the configuration file do not
sourround the command with quotes.
o --conntrack-device=dev
By default conntrack monitors the information made available at the
/proc/net/nf_conntrack device. When another device is used, it can be specified
using this option.
o --conntrack-ip-header-size=size
This option can be used to correct for the IP header sizes. By default, conntrack
includes these sizes in reported byte counts. By specifying this option packet
sizes reported by conntrack are reduced by size. Commonly IP headers consist of 20
bytes (so, to correct for this specify --conntrack-ip-header-size 20).
o --conntrack-restart=max
If the conntrack process prematurely ends it is restarted at most max times (these
are pure restarts: conntrack’s initial startup is not counted for this option). By
default 10 restarts are allowed.
o --debug
Write additional info to the log file. Suppressed when --log off has been
specified. Currently, --debug writes information about memory consumption to the
log file.
o --help (-h)
Write basic usage information to the standard output stream and terminate.
o --log=argument
By default natlog forwards log messages about natlog and connection information to
the syslog daemon using the DAEMON facility with priority NOTICE (see below at the
syslog* options). This is identical to specifying the argument syslog.
Alternatively, specify the argument off to suppress writing log messages. Any other
argument is interpreted as a path-specification to a file to receive the log
messages: log-messages are appended to existing files. If the log file does not yet
exist it is first created.
The stdout option is handled independently from the log option: log messages will
appear to the standard output stream if stdout and log: off are both specified.
o --log-data=path
Path specifies the pathname of the file where information about observed
connections is written in tabular form. Information is appended to existing files.
If path does not yet exist it is first created. Refer to the DESCRIPTION section
for information about the format of the generated table. Specify "" as command-line
option if the configuration file specifies a log data file but you don’t want the
data to be logged in a particular run of natlog.
o --log-rotate=spec
This option is used to specify the frequency and the number of log-files that are
rotated. By default log-files are not rotated.
Specify time[mhd] or time[mhd]nFiles. A time unit specification must follow time
and is m for minutes, h for hours, and d for days. nFiles specifies the max. number
of rotated files. If only time[mhd] is specified, then nFiles is set to 1. By
default (or if time or nfiles are specified as zero) log files are not rotated.
Note that when using rsyslogd(1) for logging (cf. option syslog-facility below) the
syslog daemon itself usually takes care of rotating its log files.
o --no-bytes
By default log-entries show numbers of sent and received bytes. Specify this option
to omit these statistics from log-entries.
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 syslog daemon, unless --no-syslog was specified. When
using the tcpdump-mode natlog does not run in the background. In this case, if
no-daemon is omitted a warning message is logged, and natlog continues as an
ordinary program.
o --no-via
Normally, when snat connections are logged the host handling the address
translations are logged as ’via’ entries in log-files. If the ’via’ entries can be
omitted activate no-via as configuration parameter or as option.
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 /run/natlog.pid. To end the
daemon, send a SIGINT or SIGTERM signal to the process id mentioned in the
pid-file. Natlog ignores SIGHUP signals (but writes a log message when receiving a
SIGHUP interrupt).
o --protocol=specification (-P)
The protocol(s) to monitor. By default the tcp layer four protocol is monitored.
Currently natlog’s conntrack command can monitor the tcp, udp, and icmp layer four
protocols. Using the protocol option (note: singular!) any subset of these
protocols can be selected by specifying a colon-separated subset of tcp, udp, and
icmp (e.g., --protocol udp:tcp). The specification all can be used to monitor all
three protocols (tcp, udp, and icmp). When using the conntrack-mode, only a single
protocol (including all) can be specified.
o -S
Use this option as first option, immediately following the program name, when
starting natlog from a systemd(1) natlog.service file. See also section SYSTEMD
below.
o --stdout (-s)
Syslog-equivalent messages are sent to the standard output. This option is implied
by --verbose, but is suppressed once natlog runs as a daemon.
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.
When rsyslog filtering is used (see that section below) then rsyslogd(8) uses that
instead of the specified facility.
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 --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 --terminate
When natlog is running as a daemon, a separate command natlog --terminate can be
issued to terminate the daemon. This assumes that the default pid-file
(/run/natlog.pid) was used or that a non-default pid-file was specified in the
default configuration file. If not, then the location of the pid-file or the
location of the non-default configuration file must also be specified using a
command like
natlog --terminate --pid-file=/path/to/the/pid-file
When the daemon could be terminated 0 is returned. Otherwise, an error message is
displayed and 1 is returned.
o --time=spec (-t)
By default time stamps written by natlog are in raw, numeric form. E.g.,
NATLOG: From 1338990672:55588 thru 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 --ttl=secs[ui] (-T)
time-to-live for received connections. At most two time-to-live specifications can
be provided: for udp/icmp connections a letter u must be appended to the specified
seconds. By default 60u is used. For tcp connections a letter t must be appended to
the specified seconds. By default 3000t is used. Both time-to-live specifications
may be combined: --ttl 120u1800t specifies a time-to-live of two minutes for
udp/icmp connections and a time-to-live of half an hour for tcp connections.
Time-to-live is not used in conntrack-mode.
o --verbose (-V)
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.
When --verbose is specified twice then all actual configuration parameters are
shown just before natlog starts.
When --verbose is specified more often then natlog ends after reporting the
configuration parameters.
o --version (-v)
Write natlog’s version number to the standard output stream and terminate.
SYSTEMD
An annoying characteristic of systemd(1) is that environment variables containing blanks
are passed as single arguments to the program being called by their .service files. As a
consequence, it is very hard to provide an environment variable in, e.g.,
/etc/default/natlog specifying natlog’s arguments: in practice the number of arguments
varies, and so even constructions like ARG1=value1, ARG2=value2, etc. are awkward at best.
As a stopgap for this unwelcome characteristic of systemd the option -S is provided. When
used it must be specified as natlog’s first argument. Natlog will then inspect all
remaining arguments, splitting arguments containing blanks into separate arguments, which
are then processed by natlog as intended. Be aware that, to limit the complexity of the
splitting-procedure, it is not full-proof: double- or single-quote delimited
string-arguments will also be split into separate arguments. Unless filenames themselves
containing blanks are passed as arguments to natlog this limitation is probably not very
serious.
As an example, here is an example of systemd’s ExecStart specification:
ExecStart=/usr/bin/natlog -S -p ${PIDFILE} ${DAEMON_ARGS}
where DAEMON_ARGS might have been specified in /etc/default/natlog as
DAEMON_ARGS=--log /tmp/natlog.log --log-data /dev/null conntrack
RSYSLOG FILTERING
When using rsyslogd(8) 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:" stop
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 br0 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 br0 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 following syntax:
option value
Option and value are separated by white space, a colon may be appended to option names:
# This configuration file shows the default option values.
# all options and values are case sensitive
# see `man natlog’ for further details
# the path and options of the conntrack program:
# when no filtering options are specified, the tcp
# protocol is monitored
# the default command is shown.
# Note: do not surround the conntrack command specification with quotes
#conntrack-command: /usr/sbin/conntrack -E -n -o timestamp -e NEW,DESTROY
# the device used by conntrack
#conntrack-device: /proc/net/nf_conntrack
# correction for the IP header size
# (standard IP header size is 20 bytes)
#conntrack-ip-header-size: 0
# max. number of conntrack restarts
#conntrack-restart: 10
# data file for tabular logs (specify path, default:
# data file is not used)
#log-data:
# flush the log-data file after writing log-data-flush lines
#log-data-flush: 32
# do not log the sent/received byte counts (default: counts are logged)
#no-bytes
# do not run as a daemon (by default: natlog runs as a daemon)
#no-daemon
# do not write messages handled by syslog
#no-syslog
# do not log the via: entries (by default: via-entries are logged)
#no-via
# the path to the pid-file of natlog’s daemon process
#pid-file: /run/natlog.pid
# the protocols that are scanned with the ’conntrack’ command:
# protocol: all - monitors tcp, udp, icmp
# protocol: udp:tcp - monitors upd and tcp (any non-empty subset,
# possibly including icmp is OK)
# ignored when conntrack-command is specified
#protocol: tcp
# write messages to stdout (ignored by daemons)
#stdout
# the default syslog facility:
#syslog-facility: DAEMON
# the default syslog priority:
#syslog-priority: NOTICE
# the default syslog tag:
#syslog-tag: NATLOG
# the time specification:
#time: raw
# ttl: time to live (seconds) for udp/icmp connections
#ttl: 60
# end of the configuration file
FILES
o /etc/natlog.conf: default configuration file location;
o /etc/default/natlog: arguments for startup scripts;
o /etc/init.d/natlog: SysV startup script;
o /etc/systemd/system/natlog.service: systemd startup script (calling
/etc/init.d/natlog).
SEE ALSO
conntrack(1), ifconfig(1), iptables(1), pcap-filter(7), ping(1), R(1), rsyslogd(8),
syslog(3), systemd(1), tcpdump(1)
BUGS
Natlog currently may process tcp, udp and icmp layer four protocols.
AUTHOR
Frank B. Brokken (f.b.brokken@rug.nl).