--- name: "psad" section: "8" platform: "linux" package: "psad" version: "2.4.6-3+b1" release: "Debian 13 (Trixie)" platforms: ["linux"] url: "https://new.carta.tech/psad" --- # psad(8) The Port Scan Attack Detector ## NAME **psad** - The Port Scan Attack Detector ## SYNOPSIS **psad \[options\]** ## DESCRIPTION **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 http://www.snort.org/) 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 **fwsnort** (http://www.cipherdyne.org/fwsnort/). This enables psad to send alerts for application layer attacks. **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 information. **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 **p0f** fingerprinter) from which scans originate. This requires the use of the **--log-tcp-options** argument for iptables logging rules; if this option is not used, **psad** will fall back to a fingerprinting method that makes use of packet length, TTL and TOS values, IP ID, and TCP window sizes. **psad** reads all iptables log data by default from the */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. **psad** consists of three daemons: psad, kmsgsd, and psadwatchd. **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. **kmsgsd** **(deprecated)** reads all messages that have been written to the */var/lib/psad/psadfifo* named pipe and writes any message that matches a particular regular expression (or string) to */var/log/psad/fwdata.* kmsgsd is only used if the **ENABLE\_SYSLOG\_FILE** variable is disabled in psad.conf. **psadwatchd** is a software watchdog that will restart any of the other two daemons should a daemon die for any reason. ## OPTIONS - **-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 */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 */var/log/messages* file. - **--analysis-fields \** In --Analyze mode restrict analysis to iptables log messages that have specific values for particular fields. Examples include "SRC:1.2.3.4", "DST:10.0.0.0/24, and "TTL:64", and multiple fields are supported as a comma-separated list like "SRC:1.2.3.4, LEN:44, DST:10.0.0.0/24". - **-i**, **--interface \** Specify the interface that **psad** will examine for iptables log messages. This interface will be the **IN=** interface for packets that are logged in the **INPUT** and **FORWARD** chains, and the **OUT=** interface for packets logged in the **OUTPUT** chain. - **--sig-update** Instruct **psad** to download the latest set of modified Snort signatures from http://www.cipherdyne.org/psad/signatures so that psad can take advantage of signature updates before a new release is made. - **-O**, **--Override-config \** 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. - **-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. - **-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). - **-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. - **--status-ip \** Display status information associated with *ip* such as the protocol packet counters as well as the last 10 packets logged by iptables. - **--status-dl \** Display status information only for scans that have reached a danger level of at least *dl* - **--status-summary** Instruct **psad** to omit detailed IP information from *--Status* and *--Analyze* modes. - **-m**, **--messages-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 */var/log/psad/fwdata.* - **--CSV** Instruct **psad** to parse iptables log messages out of */var/log/messages* (by default, but this path can be changed with the *-m* option), and print the packet fields on STDOUT in comma-separate value format. This is useful for graphing iptables log data with AfterGlow (see http://afterglow.sourceforge.net/index.html). - **--stdin** Acquire iptables log data from STDIN instead of the default */var/log/messages* file. - **--CSV-fields \** Instruct **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. - **-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. - **-R**, **--Restart** Restart the currently running psad processes. This option will preserve the command line options that were supplied to the original psad process. - **-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. - **-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. - **-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. - **-p**, **--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. - **-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. - **-c**, **--config \** By default all of the psad makes use of the configuration file */etc/psad/psad.conf* for almost all configuration parameters. **psad** can be made to override this path by specifying a different file on the command line with the --config option. - **--signatures \** 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. **psad** makes use of this logging capability to detect several types of TCP scan signatures included within */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. **psad** also detects many UDP and ICMP signatures that were originally included within snort. - **-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. - **-w**, **--whois-analysis** By default **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. - **--analysis-auto-block** Enable auto-blocking responses when running in --Analyze-msgs mode. This is mostly useful only for the **psad** test suite when auto-blocking responses are tested and verified. - **--snort-type \** Restrict the type of snort sids to *type.* Allowed types match the file names given to snort rules files such as "ddos", "backdoor", and "web-attacks". - **--snort-rdir \** Manually specify the directory where the snort rules files are located. The default is */etc/psad/snort\_rules.* - **--passive-os-sigs \** Manually specify the path to the passive operating system fingerprinting signatures file. The default is */etc/psad/posf.* - **--auto-dl \** 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 127.0.0.1 might be a good candidate for example). */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. - **--fw-search \** By default all of the psad makes use of the firewall search configuration file */etc/psad/fw\_search.conf* for firewall search mode and search strings. **psad** can be made to override this path by specifying a different file on the command line with the --fw-search option. - **--fw-list-auto** List all rules in iptables chains that are used by **psad** in auto-blocking mode. - **--fw-analyze** Analyze the local iptables ruleset, send any alerts if errors are discovered, and then exit. - **--fw-del-chains** By default, if ENABLE\_AUTO\_IDS is set to "Y" **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. - **--fw-dump** Instruct **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. - **--fw-block-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. - **--fw-rm-block-ip \** Specify an IP address or network to remove from the iptables controls that are auto-generated by psad. - **--fw-file \** Analyze the iptables ruleset contained within **policy-file** instead of the ruleset currently loaded on the local system. - **--CSV-regex \** Instruct **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. - **--CSV-neg-regex \** Instruct **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. - **--CSV-uniq-lines** Instruct **psad** to only print unique CSV data. That is, each line printed in *--CSV* mode will be unique. - **--CSV-max-lines \** Limit the number of CSV-formatted lines that **psad** generates on STDOUT. This is useful to allow AfterGlow graphs to be created that are not too cluttered. - **--CSV-start-line \** Specify the beginning line number to start parsing out of the iptables log file in *--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. - **--CSV-end-line \** Specify the ending line number to stop parsing the iptables log file in *--CSV* output mode. This is useful for when the log file is extremely large, and you do not want **psad** to process the entire thing. - **--gnuplot** Enter into Gnuplot mode whereby **psad** parses an iptables logfile and creates .gnu and .dat files that are suitable for graphing with Gnuplot. The various *--CSV* command line arguments apply to plotting iptables log with Gnuplot. - **--gnuplot-template \** Use a template file for all Gnuplot graphing directives (this is usually a .gnu file by convention). Normally **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. - **--gnuplot-file-prefix \** Specify a prefix for the .gnu, .dat, and .png files that are generated in *--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 **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. - **--gnuplot-x-label \** Set the label associated with the x-axis. - **--gnuplot-x-range \** Set the x-axis range. - **--gnuplot-y-label \** Set the label associated with the y-axis. - **--gnuplot-y-range \** Set the y-axis range. - **--gnuplot-z-label \** Set the label associated with the z-axis (only if *--gnuplot-3D* is used). - **--gnuplot-z-range \** Set the z-axis range. (only if *--gnuplot-3D* is used). - **--gnuplot-3D** Generate a Gnuplot **splot** graph. This produces a three-dimensional graph. - **--gnuplot-view** Set the viewing angle when graphing data in *--gnuplot-3D* mode. - **--gnuplot-title \** Set the graph title for the Gnuplot graph. - **-I**, **--Interval \** Specify the interval (in seconds) that psad should use to check whether or not packets have been logged by the firewall. **psad** will use the default of 15 seconds unless a different value is specified. - **-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. - **-V**, **--Version** Print the psad version and exit. - **--no-daemon** Do not run psad as a daemon. This option will display scan alerts on STDOUT instead of emailing them out. - **--no-ipt-errors** Occasionally iptables messages written by syslog to */var/log/messages* seem to not conform to the normal firewall logging format if the kernel ring buffer used by klogd becomes full. **psad** will write these message to */var/log/psad/errs/fwerrorlog* by default. Passing the --no-ipt-errors option will make psad ignore all such erroneous firewall messages. - **--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. - **--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. - **--no-auto-dl** Disable auto danger level assignments. This will instruct to not import any IP addresses or networks from the file */etc/psad/auto\_dl.* - **--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 **fwsnort** ). - **--no-signatures** Disable psad signature processing. Note that this is independent of snort SID matching in iptables messages generated by **fwsnort** and also from the ICMP type/code validation routines. - **--no-icmp-types** Disable ICMP type and code field validation. - **--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. - **--no-rdns** **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. - **--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 */var/log/psad/fwdata* file. - **--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. - **-h**, **--help** Print a page of usage information for psad and exit. ## FILES */etc/psad/psad.conf* The main psad configuration file which contains configuration variables mentioned in the section below. */etc/psad/fw\_search.conf* Used to configure the strategy both **psad** and **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). */etc/psad/signatures* Contains the signatures **psad** uses to recognize nasty traffic. The signatures are written in a manner similar to the \*lib signature files used in the snort IDS. */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. */etc/psad/snort\_rules/\*.rules* Snort rules files that are consulted by default unless the --no-snort-sids command line argument is given. */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 "\ \" where \ is an integer from 0 to 5, with 0 meaning to ignore all traffic from \, and 5 is to assign the highest danger level to \. */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. ## PSAD CONFIGURATION VARIABLES This section describes what each of the more important **psad** configuration variables do and how they can be tuned to meet your needs. Most of the variables are located in the **psad** configuration file */etc/psad/psad.conf* but the FW\_SEARCH\_ALL and FW\_MSG\_SEARCH variables are located in the file */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: **http://www.cipherdyne.org/psad/config.html** - **EMAIL\_ADDRESSES** Contains a comma-separated list of email addresses to which email alerts will be sent. The default is "root@localhost". - **HOSTNAME** Defines the hostname of the machine on which **psad is running. This will be** used in the email alerts generated by psad. - **HOME\_NET** 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. - **IMPORT\_OLD\_SCANS** Preserve scan data across restarts of **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 */var/log/psad.* - **FW\_SEARCH\_ALL** Defines the search mode **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 */etc/psad/fw\_search.conf* since it is referenced by both psad and kmsgsd. - **FW\_MSG\_SEARCH** Defines a set of search strings that **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 **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 **psad** and **kmsgsd** so it lives in the file */etc/psad/fw\_search.conf.* - **SYSLOG\_DAEMON** Define the specific syslog daemon that **psad** should interface with. Psad supports three syslog daemons: **syslogd,** **syslog-ng,** and **metalog.** The default value of SYSLOG\_DAEMON is **syslogd.** - **IGNORE\_PORTS** Specify a list of port ranges and/or individual ports and corresponding protocols that **psad** should complete ignore. This is particularly useful for ignore ports that are used as a part of a port knocking scheme (such as **fwknop** http://www.cipherdyne.org/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". - **ENABLE\_PERSISTENCE** 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. - **SCAN\_TIMEOUT** 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). - **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 **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 */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 */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 */etc/psad/auto\_dl,* then the corresponding danger level is automatically assigned to the scan. - **PORT\_RANGE\_SCAN\_THRESHOLD** 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 **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). - **SHOW\_ALL\_SIGNATURES** 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 */etc/psad/signatures.* - **SNORT\_SID\_STR** 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 **fwsnort** (http://www.cipherdyne.org/fwsnort/). - **ENABLE\_DSHIELD\_ALERTS** Enable dshield alerting mode. This will send a parsed version of iptables log messages to dshield.org which is a (free) distributed intrusion detection service. For more information, see http://www.dshield.org/ - **IGNORE\_CONNTRACK\_BUG\_PKTS** 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. - **ALERT\_ALL** 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. - **PSAD\_EMAIL\_LIMIT** 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. **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. - **EMAIL\_ALERT\_DANGER\_LEVEL** Defines the danger level a scan must reach before any alert is sent. This variable is set to 1 by default. - **ENABLE\_AUTO\_IDS** **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. **IMPORTANT:** 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 **--Flush** option). Your mileage will vary. - **AUTO\_IDS\_DANGER\_LEVEL** Defines the danger level a scan must reach before psad will automatically block the IP (ENABLE\_AUTO\_IDS must be set to "Y"). ## EXAMPLES 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): **# psad** Same as above, but this time we use the init script to start psad: **# /etc/init.d/psad start** Use psad as a forensics tool to analyze an old iptables logfile (psad defaults to analyzing the */var/log/messages* file if the -m option is not specified): **# psad -A -m \** Run psad in forensics mode, but limit its operations to a specific IP address "10.1.1.1": **# psad -A -m \ --analysis-fields src:10.1.1.1** Generate graphs of scan data using AfterGlow: **# psad --CSV --CSV-fields src dst dp --CSV-max 1000 -m \ \| perl afterglow.pl -c color.properties \| neato -Tgif -o iptables\_graph.gif** The **psad.conf,** **signatures,** and **auto\_dl** files are normally located within the /etc/psad/ directory, but the paths to each of these files can be changed: **# psad -c \ -s \ -a \** Disable the firewall check and the local port lookup subroutines; most useful if psad is deployed on a syslog logging server: **# 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: **# psad --no-rdns --no-whois** ## DEPENDENCIES **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 **FW\_EXAMPLE\_RULES** file that is bundled with the psad source distribution. **psad** by default parses the */var/log/messages* file for all iptables log data. ## DIAGNOSTICS 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 */var/log/psad/scan\_hash.$$* where **$$** represents the pid of the psad process. ## SEE ALSO [**iptables**(8)](/iptables/8.md), [**kmsgsd**(8)](/kmsgsd/8.md), [**psadwatchd**(8)](/psadwatchd/8.md), [**fwsnort**(8)](/fwsnort/8.md), [**snort**(8)](/snort/8.md), [**nmap**(1)](/nmap/1.md), [**p0f**(1)](/p0f/1.md), [**gnuplot**(1)](/gnuplot/1.md) ## AUTHOR Michael Rash \ ## CONTRIBUTORS Many people who are active in the open source community have contributed to psad. See the **CREDITS** file in the psad sources, or visit **http://www.cipherdyne.org/psad/docs/contributors.html** to view the online list of contributors. ## BUGS Send bug reports to mbr@cipherdyne.org. 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 TCP\_CONNTRACK\_CLOSE\_WAIT timeout value in 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. ## DISTRIBUTION **psad** is distributed under the GNU General Public License (GPL), and the latest version may be downloaded from: **http://www.cipherdyne.org/** Snort is a registered trademark of Sourcefire, Inc.