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.\" Process this file with
.\" groff -man -Tascii foo.1
.TH PSAD 8 "March 2009" Linux
.B psad
\- The Port Scan Attack Detector
.B psad [options]
.B psad
makes use of iptables log messages to detect, alert, and (optionally) block
port scans and other suspect traffic. For TCP scans psad analyzes TCP
flags to determine the scan type (syn, fin, xmas, etc.) and corresponding
command line options that could be supplied to nmap to generate such a scan.
In addition, psad makes use of many TCP, UDP, and ICMP signatures contained
within the Snort intrusion detection system (see to
detect suspicious network traffic such as probes for common backdoors, DDoS
tools, OS fingerprinting attempts, and more. By default psad also provides
alerts for snort rules that are detected directly by iptables through the
use of a ruleset generated by
.B fwsnort
( This enables psad to send alerts for
application layer attacks.
.B psad
features a set of highly configurable danger thresholds (with sensible
defaults provided) that allow the administrator to define what constitutes
a port scan or other suspect traffic. Email alerts sent by psad contain the
scanning ip, number of packets sent to each port, any TCP, UDP, or ICMP
signatures that have been matched (e.g. "NMAP XMAS scan"), the scanned port
range, the current danger level (from 1 to 5), reverse dns info, and whois
.B psad
also makes use of various packet header fields associated with TCP SYN packets
to passively fingerprint remote operating systems (in a manner similar to the
.B p0f
fingerprinter) from which scans originate. This requires the use of the
.B --log-tcp-options
argument for iptables logging rules; if this option is not used,
.B psad
will fall back to a fingerprinting method that makes use of packet length,
TTL and TOS values, IP ID, and TCP window sizes.
.B psad
reads all iptables log data by default from the
.I /var/log/messages
file. By parsing firewall log messages, psad is provided with data that
represents packets that have been logged (and possibly dropped) by the
running iptables policy. In this sense, psad is supplied with a pure data stream
that exclusively contains packets that the firewall has deemed unfit to
enter the network.
.B psad
consists of three daemons: psad, kmsgsd, and psadwatchd.
.B psad
is responsible for processing all packets that have been logged by the
firewall and applying the signature logic in order to determine what type
of scan has been leveraged against the machine and/or network.
.B kmsgsd
.B (deprecated)
reads all messages that have been written to the
.I /var/lib/psad/psadfifo
named pipe and writes any message that matches a particular regular
expression (or string) to
.I /var/log/psad/fwdata.
kmsgsd is only used if the
variable is disabled in psad.conf.
.B psadwatchd
is a software watchdog that will restart any of the other two daemons should
a daemon die for any reason.
.BR \-A ", " \-\^\-Analyze-msgs
Analyze an iptables logfile for scans and exit. This will generate email alerts
just as a normal running psad process would have for all logged scans. By
default the psad data file
.I /var/log/psad/fwdata
is parsed for old scans, but any file can be specified through the use
of the \-\-messages-file command line option. For example it might be useful
to point psad at your
.I /var/log/messages
.BR \-\^\-analysis-fields\ \<search\ fields>
In \-\-Analyze mode restrict analysis to iptables log messages that have specific
values for particular fields. Examples include "SRC:", "DST:,
and "TTL:64", and multiple fields are supported as a comma-separated list like
"SRC:, LEN:44, DST:".
.BR \-i "\fR,\fP " \-\^\-interface\ \<interface>
Specify the interface that
.B psad
will examine for iptables log messages. This interface will be the
.B IN=
interface for packets that are logged in the
chains, and the
interface for packets logged in the
.BR \-\^\-sig-update
.B psad
to download the latest set of modified Snort signatures from so that psad can take advantage of
signature updates before a new release is made.
.BR \-O "\fR,\fP " \-\^\-Override-config\ \<file>
Override config variable values that are normally read from the
/etc/psad/psad.conf file with values from the specified file. Multiple
override config files can be given as a comma separated list.
.BR \-D ", " \-\^\-Dump-conf
Dump the current psad config to STDOUT and exit. Various pieces of information
such as the home network, alert email addresses, and DShield user id are removed
from the resulting output so it is safe to send to others.
.BR \-F ", " \-\^\-Flush
Remove any auto-generated firewall block rules if psad was configured
to automatically respond to scans (see the ENABLE_AUTO_IDS variable
in psad.conf).
.BR \-S ", " \-\^\-Status
Display the status of any psad processes that may or not be running.
The status output contains a listing of the number of packets that
have been processed by psad, along with all IP addresses and
corresponding danger levels that have scanned the network.
.BR \-\^\-status-ip\ \<ip>
Display status information associated with
.I ip
such as the protocol packet counters as well as the last 10 packets
logged by iptables.
.BR \-\^\-status-dl\ \<dl>
Display status information only for scans that have reached a danger
level of at least
.I dl
.BR \-\^\-status-summary
.B psad
to omit detailed IP information from
.I --Status
.I --Analyze
.BR \-m "\fR,\fP " \-\^\-messages-file\ \<file>
This option is used to specify the file that will be parsed in analysis
mode (see the \-\-Analyze-msgs option). The default path is the psad
data file
.I /var/log/psad/fwdata.
.BR \-\^\-CSV
.B psad
to parse iptables log messages out of
.I /var/log/messages
(by default, but this path can be changed with the
.I -m
option), and print the packet fields on STDOUT in comma-separate value
format. This is useful for graphing iptables log data with AfterGlow
.BR \-\^\-stdin
Acquire iptables log data from STDIN instead of the default
.I /var/log/messages
.BR \-\^\-CSV-fields\ \<tokens>
.B psad
to only include a specific set of iptables log message fields within the CSV
output. AfterGlow accepts up to three fields for its graph data, so the most
common usage of this option is "src dst dp" to print the source and destination
IP addresses, and the destination port number.
.BR \-K ", " \-\^\-Kill
Kill the current psad process along with psadwatchd and kmsgsd.
This provides a quick and easy way to kill all psad processes without
having to look in the process table or appeal to the psad-init script.
.BR \-R ", " \-\^\-Restart
Restart the currently running psad processes. This option will
preserve the command line options that were supplied to the original
psad process.
.BR \-U ", " \-\^\-USR1
Send a running psad process a USR1 signal. This will cause psad to
dump the contents of the %Scan hash to the file "/var/log/psad/scan_hash.$$"
where "$$" represents the pid of the psad process. This is mostly
useful for debugging purposes, but it also allows the administrator to
peer into the %Scan hash, which is the primary data structure used to
store scan data within system memory.
.BR \-H ", " \-\^\-HUP
Send all running psad daemons a HUP signal. This will instruct the
daemons to re-read their respective configuration files without causing
scan data to be lost in the process.
.BR \-B ", " \-\^\-Benchmark
Run psad in benchmark mode. By default benchmark mode will simulate
a scan of 10,000 packets (see the \-\-packets option) and then report
the elapsed time. This is useful to see how fast psad can process
packets on a specific machine.
.BR \-p "\fR,\fP " \-\^\-packets\ \<packets>
Specify the number of packets to analyze in \-\-Analyze mode or use in \-\-Benchmark mode.
The default is 10,000 packets in \-\-Benchmark mode, and unlimited in \-\-Analyze mode.
.BR \-d ", " \-\^\-debug
Run psad in debugging mode. This will automatically prevent
psad from running as a daemon, and will print the contents
of the %Scan hash and a few other things on STDOUT at crucial
points as psad executes.
.BR \-c "\fR,\fP " \-\^\-config\ \<configuration-file>
By default all of the psad makes use of the configuration file
.I /etc/psad/psad.conf
for almost all configuration parameters.
.B psad
can be made to
override this path by specifying a different file on the command
line with the \-\-config option.
.BR \-\^\-signatures\ \<signatures-file>
The iptables firewalling code included within the linux 2.4.x kernel
series has the ability to distinguish and log any of the TCP flags
present within TCP packets that traverse the firewall interfaces.
.B psad
makes use of this logging capability to detect several types of TCP scan
signatures included within
.I /etc/psad/signatures.
The signatures were
originally included within the snort intrusion detection
system. New signatures can be included and modifications to existing
signatures can be made to the signature file and psad will import
the changes upon receiving a HUP signal (see the \-\-HUP command line
option) without having to restart the psad process.
.B psad
also detects
many UDP and ICMP signatures that were originally included within snort.
.BR \-e ", " \-\^\-email-analysis
Send alert emails when run in \-\-Analyze-msgs mode. Depending on the size of
the iptables logfile, using the \-\-email-analysis option could extend the runtime
of psad by quite a bit since normally both DNS and whois lookups will be issued
against each scanning IP address. As usual these lookups can be disabled with
the \-\-no-rdns and \-\-no-whois options respectively.
.BR \-w ", " \-\^\-whois-analysis
By default
.B psad
does not issue whois lookups when running in \-\-Analyze-msgs mode. The
\-\-whois-analysis option will override this behavior (when run in analysis mode)
and instruct psad to issue whois lookups against IP addresses from which scans
or other suspect traffic has originated.
.BR \-\^\-analysis-auto-block
Enable auto-blocking responses when running in \-\-Analyze-msgs mode. This is
mostly useful only for the
.B psad
test suite when auto-blocking responses are tested and verified.
.BR \-\^\-snort-type\ \<type>
Restrict the type of snort sids to
.I type.
Allowed types match the file names given to snort rules files such as
"ddos", "backdoor", and "web-attacks".
.BR \-\^\-snort-rdir\ \<snort-rules-directory>
Manually specify the directory where the snort rules files are located.
The default is
.I /etc/psad/snort_rules.
.BR \-\^\-passive-os-sigs\ \<passive-os-sigs-file>
Manually specify the path to the passive operating system fingerprinting
signatures file. The default is
.I /etc/psad/posf.
.BR \-\^\-auto-dl\ \<auto-dl-file>
Occasionally certain IP addresses are repeat offenders and
should automatically be given a higher danger level than
would normally be assigned. Additionally, some IP addresses
can always be ignored depending on your network configuration
(the loopback interface might be a good candidate
for example).
.I /etc/psad/auto_dl
provides an interface for psad to automatically
increase/decrease/ignore scanning IP danger levels. Modifications
can be made to auto_dl (installed by default in /etc/psad)
and psad will import them with 'psad \-H' or by restarting the psad process.
.BR \-\^\-fw-search\ \<fw_search-file>
By default all of the psad makes use of the firewall search configuration
.I /etc/psad/fw_search.conf
for firewall search mode and search strings.
.B psad
can be made to
override this path by specifying a different file on the command
line with the \-\-fw-search option.
.BR \-\^\-fw-list-auto
List all rules in iptables chains that are used by
.B psad
in auto-blocking mode.
.BR \-\^\-fw-analyze
Analyze the local iptables ruleset, send any alerts if errors are
discovered, and then exit.
.BR \-\^\-fw-del-chains
By default, if ENABLE_AUTO_IDS is set to "Y"
.B psad
will not delete the auto-generated iptables chains (see the IPT_AUTO_CHAIN
keywords in psad.conf) if the \-\-Flush option is given. The \-\-fw-del-chains
option overrides this behavior and deletes the auto-blocking chains from a
running iptables firewall.
.BR \-\^\-fw-dump
.B psad
to dump the contents of the iptables policy that is running on the local
system. All IP addresses are removed from the resulting output, so it is
safe to post to the psad list, or communicate to others. This option is
most often used with \-\-Dump-conf.
.BR \-\^\-fw-block-ip\ \<ip>
Specify an IP address or network to add to the iptables controls that are
auto-generated by psad. This allows psad to manage the rule timeouts.
.BR \-\^\-fw-rm-block-ip\ \<ip>
Specify an IP address or network to remove from the iptables controls that
are auto-generated by psad.
.BR \-\^\-fw-file\ \<policy-file>
Analyze the iptables ruleset contained within
.B policy-file
instead of the ruleset currently loaded on the local system.
.BR \-\^\-CSV-regex\ \<regex>
.B psad
to only print CSV data that matches the supplied regex. This regex is
used to match against each of the entire iptables log messages.
.BR \-\^\-CSV-neg-regex\ \<regex>
.B psad
to only print CSV data that does not match the supplied regex. This regex is
used to negatively match against each of the entire iptables log messages.
.BR \-\^\-CSV-uniq-lines
.B psad
to only print unique CSV data. That is, each line printed in
.I --CSV
mode will be unique.
.BR \-\^\-CSV-max-lines\ \<num>
Limit the number of CSV-formatted lines that
.B psad
generates on STDOUT. This is useful to allow AfterGlow graphs to be
created that are not too cluttered.
.BR \-\^\-CSV-start-line\ \<num>
Specify the beginning line number to start parsing out of the iptables log
file in
.I --CSV
output mode. This is useful for when the log file is extremely large, and you
want to begin parsing a specific place within the file. The default is begin
parsing at the beginning of the file.
.BR \-\^\-CSV-end-line\ \<num>
Specify the ending line number to stop parsing the iptables log
file in
.I --CSV
output mode. This is useful for when the log file is extremely large, and you
do not want
.B psad
to process the entire thing.
.BR \-\^\-gnuplot
Enter into Gnuplot mode whereby
.B psad
parses an iptables logfile and creates .gnu and .dat files that are suitable
for graphing with Gnuplot. The various
.I --CSV
command line arguments apply to plotting iptables log with Gnuplot.
.BR \-\^\-gnuplot-template\ \<file>
Use a template file for all Gnuplot graphing directives (this is usually
a .gnu file by convention). Normally
.B psad builds all of the graphing directives based on various --gnuplot
command line arguments, but the \-\-gnuplot-template switch allows you to
override this behavior.
.BR \-\^\-gnuplot-file-prefix\ \<file>
Specify a prefix for the .gnu, .dat, and .png files that are generated in
.I --gnuplot
mode. So, when visualizing attacks captured in an iptables logfile (let's
say you are interested in port scans), you could use this option to have
.B psad
create the two files portscan.dat, portscan.gnu, and Gnuplot will create
an additional file portscan.png when the portscan.gnu file is loaded.
.BR \-\^\-gnuplot-x-label\ \<label>
Set the label associated with the x-axis.
.BR \-\^\-gnuplot-x-range\ \<range>
Set the x-axis range.
.BR \-\^\-gnuplot-y-label\ \<label>
Set the label associated with the y-axis.
.BR \-\^\-gnuplot-y-range\ \<range>
Set the y-axis range.
.BR \-\^\-gnuplot-z-label\ \<label>
Set the label associated with the z-axis (only if
.I --gnuplot-3D
is used).
.BR \-\^\-gnuplot-z-range\ \<range>
Set the z-axis range. (only if
.I --gnuplot-3D
is used).
.BR \-\^\-gnuplot-3D
Generate a Gnuplot
.B splot
graph. This produces a three-dimensional graph.
.BR \-\^\-gnuplot-view
Set the viewing angle when graphing data in
.I --gnuplot-3D
.BR \-\^\-gnuplot-title\ \<title>
Set the graph title for the Gnuplot graph.
.BR \-I "\fR,\fP " \-\^\-Interval\ \<seconds>
Specify the interval (in seconds) that psad should use to
check whether or not packets have been logged by the
.B psad
will use the default of 15 seconds unless a
different value is specified.
.BR \-l ", " \-\^\-log-server
This option should be used if psad is being executed on a syslog
logging server. Running psad on a logging server requires that
check_firewall_rules() and auto_psad_response() not be executed
since the firewall is probably not being run locally.
.BR \-V ", " \-\^\-Version
Print the psad version and exit.
.BR \-\^\-no-daemon
Do not run psad as a daemon. This option will display scan
alerts on STDOUT instead of emailing them out.
.BR \-\^\-no-ipt-errors
Occasionally iptables messages written by syslog to
.I /var/log/messages
seem to not conform to the normal firewall logging format if the kernel
ring buffer used by klogd becomes full.
will write these message to
.I /var/log/psad/errs/fwerrorlog
by default. Passing the \-\-no-ipt-errors option will make psad ignore
all such erroneous firewall messages.
.BR \-\^\-no-whois
By default psad will issue a whois query against any IP from which
a scan has originated, but this can be disabled with the \-\-no-whois
command line argument.
.BR \-\^\-no-fwcheck
psad performs a rudimentary check of the firewall ruleset that
exists on the machine on which psad is deployed to determine
whether or not the firewall has a compatible configuration (i.e.
iptables has been configured to log packets). Passing the
\-\-no-fwcheck or \-\-log-server options will disable this check.
.BR \-\^\-no-auto-dl
Disable auto danger level assignments. This will instruct to not import
any IP addresses or networks from the file
.I /etc/psad/auto_dl.
.BR \-\^\-no-snort-sids
Disable snort sid processing mode. This will instruct psad to not import
snort rules (for snort SID matching in a policy generated by
.B fwsnort
.BR \-\^\-no-signatures
Disable psad signature processing. Note that this is independent of
snort SID matching in iptables messages generated by
.B fwsnort
and also from the ICMP type/code validation routines.
.BR \-\^\-no-icmp-types
Disable ICMP type and code field validation.
.BR \-\^\-no-passive-os
By default psad will attempt to passively (i.e. without sending
any packets) fingerprint the remote operating system from which
a scan originates. Passing the \-\-no-passive-os option will
disable this feature.
.BR \-\^\-no-rdns
.B psad
normally attempts to find the name associated with a
scanning IP address, but this feature can be disabled with
the \-\-no-rdns command line argument.
.BR \-\^\-no-kmsgsd
Disable startup of kmsgsd. This option is most useful for debugging
with individual iptables messages so that new messages are not appended
to the
.I /var/log/psad/fwdata
.BR \-\^\-no-netstat
By default for iptables firewalls psad will determine whether
or not your machine is listening on a port for which a TCP
signature has been matched. Specifying \-\-no-netstat
disables this feature.
.BR \-h ", " \-\^\-help
Print a page of usage information for psad and exit.
.I /etc/psad/psad.conf
The main psad configuration file which contains configuration variables
mentioned in the section below.
.I /etc/psad/fw_search.conf
Used to configure the strategy both
.B psad
.B kmsgsd
employ to parse iptables messages. Using configuration directive within
this file, psad can be configured to parse all iptables messages or only
those that match specific log prefix strings (see the \-\-log-prefix option
to iptables).
.I /etc/psad/signatures
Contains the signatures
.B psad
uses to recognize nasty traffic. The
signatures are written in a manner similar to the *lib signature
files used in the snort IDS.
.I /etc/psad/icmp_types
Contains all valid ICMP types and corresponding codes as defined by RFC 792.
By default, ICMP packets are validated against these values and an alert
will be generated if a non-matching ICMP packet is logged by iptables.
.I /etc/psad/snort_rules/*.rules
Snort rules files that are consulted by default unless the \-\-no-snort-sids
command line argument is given.
.I /etc/psad/auto_dl
Contains a listing of any IP addresses that should be assigned
a danger level based on any traffic that is logged by the
firewall. The syntax is "<IP address> <danger level>" where
<danger level> is an integer from 0 to 5, with 0 meaning to ignore
all traffic from <IP address>, and 5 is to assign the highest danger
level to <IP address>.
.I /etc/psad/posf
Contains a listing of all passive operating system fingerprinting
signatures. These signatures include packet lengths, ttl, tos,
IP ID, and TCP window size values that are specific to various
operating systems.
This section describes what each of the more important
.B psad
configuration variables do and how they can be tuned to meet your
needs. Most of the variables are located in the
.B psad
configuration file
.I /etc/psad/psad.conf
but the FW_SEARCH_ALL and FW_MSG_SEARCH variables are located in the
.I /etc/psad/fw_search.conf.
Each variable is assigned sensible defaults for most network
architectures during the install process. More information on psad config
keywords may be found at:
Contains a comma-separated list of email addresses to which email alerts
will be sent. The default is "root@localhost".
Defines the hostname of the machine on which
.B psad is running. This will be
used in the email alerts generated by psad.
Define the internal network(s) that are connected to the local system.
This will be used in the signature matching code to determine whether traffic
matches snort rules, which invariably contain a source and destination
network. Multiple networks are supported as a comma separated list, and
each network should be specified in CIDR notation. Normally the network(s)
contained in the HOME_NET variable should be directly connected to the
machine that is running psad.
Preserve scan data across restarts of
.B psad
or even across reboots of the machine. This is accomplished by importing
the data contained in the filesystem cache psad writes to during normal
operation back into memory as psad is started. The filesystem cache data
in contained within the directory
.I /var/log/psad.
Defines the search mode
.B psad
uses to parse iptables messages. By default FW_SEARCH_ALL is set to "Y"
since normally most people want all iptables log messages to be parsed for
scan activity. However, if FW_SEARCH_ALL is set to "N", psad
will only parse those iptables log messages that match certain search
strings that appear in iptables logs with the \-\-log-prefix option. This is
useful for restricting psad to only operate on specific iptables chains or
rules. The strings that will be searched for are defined with the FW_MSG_SEARCH
variable (see below). The FW_SEARCH_ALL variable is defined in the file
.I /etc/psad/fw_search.conf
since it is referenced by both psad and kmsgsd.
Defines a set of search strings that
.B psad
uses to identify iptables messages that should be parsed for scan activity.
These search strings should match the log prefix strings specified
in the iptables ruleset with the \-\-log-prefix option, and the default value
for FW_MSG_SEARCH is "DROP". Note that
.B psad
normally parses all iptables messages, and so the FW_MSG_SEARCH variable
is only needed if FW_SEARCH_ALL (see above) is set to "N". The FW_MSG_SEARCH
variable is referenced by both
.B psad
.B kmsgsd
so it lives in the file
.I /etc/psad/fw_search.conf.
Define the specific syslog daemon that
.B psad
should interface with. Psad supports three syslog daemons:
.B syslogd,
.B syslog-ng,
.B metalog.
The default value of SYSLOG_DAEMON is
.B syslogd.
Specify a list of port ranges and/or individual ports and corresponding protocols
.B psad
should complete ignore. This is particularly useful for ignore ports that are
used as a part of a port knocking scheme (such as
.B fwknop for network authentication since such log
messages generated by the knock sequence may otherwise be interpreted as a scan.
Multiple ports and/or port ranges may be specified as a comma-separated list, e.g.
"tcp/22, tcp/61000-61356, udp/53".
If "Y", psad will keep all scans in memory and not let them timeout.
This can help discover stealthy scans where an attacker tries to slip beneath
IDS thresholds by only scanning a few ports over a long period of time.
ENABLE_PERSISTENCE is set to "Y" by default.
If ENABLE_PERSISTENCE is "N" then psad will use the value set by SCAN_TIMEOUT
to remove packets from the scan threshold calculation. The default is 3600
seconds (1 hour).
.BR DANGER_LEVEL{1,2,3,4,5}
psad uses a scoring system to keep track of the severity a scans reaches
(represented as a "danger level") over time. The DANGER_LEVEL{n} variables
define the number of packets that must be dropped by the firewall before psad
will assign the respective danger level to the scan. A scan may also be
assigned a danger level if the scan matches a particular signature contained
in the
.B signatures
file. There are five
possible danger levels with one being the lowest and five the highest.
Note there are several factors that can influence how danger levels are
calculated: whether or not a scan matches a signature listed in
.I /etc/psad/signatures,
the value of PORT_RANGE_SCAN_THRESHOLD (see below), whether or not a scan comes
from an IP that is listed in the
.I /etc/psad/auto_dl
file, and finally whether or not scans are allowed to timeout
as determined by SCAN_TIMEOUT above. If a signature is matched or the scanning
IP is listed in
.I /etc/psad/auto_dl,
then the corresponding danger level is automatically assigned to the scan.
Defines the minimum difference between the lowest port and the highest port
scanned before an alert is sent (the default is 1 which means that at least
two ports must be scanned to generate an alert). For example, suppose an ip
repeatedly scans a single port for which there is no special signature in
.B signatures.
Then if PORT_RANGE_SCAN_THRESHOLD=1, psad will never send
an alert for this "scan" no matter how many packets are sent to the port (i.e.
no matter what the value of DANGER_LEVEL1 is). The reason for the default of
1 is that a "scan" usually means that at least two ports are probed, but if
you want psad to be extra paranoid you can set PORT_RANGE_SCAN_THRESHOLD=0
to alert on scans to single ports (as long as the number of packets also
exceeds DANGER_LEVEL1).
If "Y", psad will display all signatures detected from a single scanning
IP since a scan was first detected instead of just displaying newly-detected
signatures. SHOW_ALL_SIGNATURES is set to "N" by default. All signatures are
listed in the file
.I /etc/psad/signatures.
Defines the string kmsgsd will search for in iptables log messages that are
generated by iptables rules designed to detect snort rules. The default is
"SID". See
.B fwsnort
Enable dshield alerting mode. This will send a parsed version of iptables log
messages to which is a (free) distributed intrusion detection service.
For more information, see
If "Y", all TCP packets that have the ACK or RST flag bits set will be ignored
by psad since usually we see such packets being blocked as a result of the
iptables connection tracking bug. Note there are no signatures that make use
of the RST flag and very few that use ACK flag.
If "Y", send email for all new bad packets instead of just when a danger
level increases. ALERT_ALL is set to "Y" by default.
Defines the maximum number of emails that will be sent for a single scanning
IP (default is 50). This variable gives you some protection from psad
sending countless alerts if an IP scans your machine constantly.
.B psad
will send a special alert if an IP has exceeded the email limit. If
PSAD_EMAIL_LIMIT is set to zero, then psad will ignore the limit and send
alert emails indefinitely for any scanning ip.
Defines the danger level a scan must reach before any alert is sent.
This variable is set to 1 by default.
.B psad
has the capability of dynamically blocking all traffic from an IP that
has reached a (configurable) danger level through modification of iptables
or tcpwrapper rulesets.
This feature is disabled by default since it is possible for an attacker
to spoof packets from a well known (web)site in an effort to make it
look as though the site is scanning your machine, and then psad will
consequently block all access to it. Also, psad works by parsing firewall
messages for packets the firewall has already dropped, so the "scans" are
unsuccessful anyway. However, some administrators prefer to take this risk
anyway reasoning that they can always review which sites are being blocked
and manually remove the block if necessary (see the
.B --Flush
option). Your mileage will vary.
Defines the danger level a scan must reach before psad will automatically
block the IP (ENABLE_AUTO_IDS must be set to "Y").
The following examples illustrate the command line arguments that could
be supplied to psad in a few situations:
Signature checking, passive OS fingerprinting, and automatic IP danger
level assignments are enabled by default without having to specify any
command line arguments (best for most situations):
.B # psad
Same as above, but this time we use the init script to start psad:
.B # /etc/init.d/psad start
Use psad as a forensics tool to analyze an old iptables logfile (psad defaults
to analyzing the
.I /var/log/messages
file if the \-m option is not specified):
.B # psad -A -m <iptables logfile>
Run psad in forensics mode, but limit its operations to a specific IP
address "":
.B # psad -A -m <iptables logfile> --analysis-fields "src:"
Generate graphs of scan data using AfterGlow:
.B # psad --CSV --CSV-fields "src dst dp" --CSV-max 1000 -m <iptables logfile> | perl -c | neato -Tgif -o iptables_graph.gif
.B psad.conf,
.B signatures,
.B auto_dl
files are normally
located within the /etc/psad/ directory, but the paths to each of these
files can be changed:
.B # psad -c <config file> -s <signatures file> -a <auto ips file>
Disable the firewall check and the local port lookup subroutines; most useful
if psad is deployed on a syslog logging server:
.B # psad --log-server --no-netstat
Disable reverse dns and whois lookups of scanning IP addresses; most useful
if speed of psad is the main concern:
.B # psad --no-rdns --no-whois
.B psad
requires that iptables is configured with a "drop and log" policy for any
traffic that is not explicitly allowed through. This is consistent with a
secure network configuration since all traffic that has not been explicitly
allowed should be blocked by the firewall ruleset. By default, psad attempts
to determine whether or not the firewall has been configured in this way. This
feature can be disabled with the \-\-no-fwcheck or \-\-log-server options. The
\-\-log-server option is useful if psad is running on a syslog logging server
that is separate from the firewall. For more information on compatible iptables
rulesets, see the
file that is bundled with the psad source distribution.
.B psad
by default parses the
.I /var/log/messages
file for all iptables log data.
The \-\-debug option can be used to display crucial information
about the psad data structures on STDOUT as a scan generates firewall
log messages. \-\-debug disables daemon mode execution.
Another more effective way to peer into the runtime execution of psad
is to send (as root) a USR1 signal to the psad process which will
cause psad to dump the contents of the %Scan hash to
.I /var/log/psad/scan_hash.$$
.B $$
represents the pid of the psad process.
.BR iptables (8),
.BR kmsgsd (8),
.BR psadwatchd (8),
.BR fwsnort (8),
.BR snort (8),
.BR nmap (1),
.BR p0f (1),
.BR gnuplot (1)
Michael Rash <>
Many people who are active in the open source community have contributed to psad.
See the
file in the psad sources, or visit
to view the online list of contributors.
Send bug reports to Suggestions and/or comments are
always welcome as well.
For iptables firewalls as of Linux kernel version 2.4.26, if the ip_conntrack
module is loaded (or compiled into the kernel) and the firewall has been
configured to keep state of connections, occasionally packets that are supposed
to be part of normal TCP traffic will not be correctly identified due to a bug
in the firewall state timeouts and hence dropped. Such packets will then be
interpreted as a scan by psad even though they are not part of any malicious
activity. Fortunately, an interim fix for this problem is to simply extend the
linux/net/ipv4/netfilter/ip_conntrack_proto_tcp.c from 60 seconds to 2 minutes,
and a set of kernel patches is included within the patches/ directory in the
psad sources to change this. (Requires a kernel recompile of course; see the
Kernel-HOWTO.) Also, by default the IGNORE_CONNTRACK_BUG_PKTS variable is set
to "Y" in psad.conf which causes psad to ignore all TCP packets that have the
ACK bit set unless the packets match a specific signature.
.B psad
is distributed under the GNU General Public License (GPL), and the latest
version may be downloaded from:
Snort is a registered trademark of Sourcefire, Inc.