• Detection for TCP SYN, FIN, NULL, and XMAS scans as well as UDP scans.
• Support for both IPv4 and IPv6 logs generated by iptables and ip6tables respectively.
• Detection of many signature rules from the Snort intrusion detection system.
• Forensics mode iptables/ip6tables logfile analysis (useful as a forensics tool for extracting scan information from old iptables/ip6tables logfiles).
• Passive operating system fingerprinting via TCP syn packets. Two different fingerprinting strategies are supported; a re-implementation of p0f that strictly uses iptables/ip6tables log messages (requires the --log-tcp-options command line switch), and a TOS-based strategy.
• Email alerts that contain TCP/UDP/ICMP scan characteristics, reverse dns and whois information, snort rule matches, remote OS guess information, and more.
• When combined with fwsnort and the iptables string match extension, psad can generate alerts for application layer buffer overflow attacks, suspicious application commands, and other suspect layer 7 traffic.
• Icmp type and code header field validation.
• Configurable scan thresholds and danger level assignments.
• Iptables ruleset parsing to verify "default drop" policy stance.
• IP/network danger level auto-assignment (can be used to ignore or automatically escalate danger levels for certain networks).
• DShield alerts.
• Auto-blocking of scanning IP addresses via iptables/ip6tables and/or tcpwrappers based on scan danger level. (This feature is NOT enabled by default.)
• Parsing of iptables/ip6tables log messages and generation of CSV output that can be used as input to AfterGlow. This allows iptables/ip6tables logs to be visualized. Gnuplot is also supported.
• Status mode that displays a summary of current scan information with associated packet counts, iptables/ip6tables chains, and danger levels.

"Nachi Worm Link Graph"

The second shows Nachi worm traffic on an hourly basis from the Scan30 iptables data:
"Nachi Worm Hourly Graph"

Configuration Information
Information on config keywords referenced by psad may be found both in the psad(8) man page, and also here:
http://www.cipherdyne.org/psad/docs/config.html

Methodology
All information psad analyzes is gathered from iptables log messages. psad by default reads the /var/log/messages file for new iptables messages and optionally writes them out to a dedicated file (/var/log/psad/fwdata). psad is then responsible for applying the danger threshold and signature logic in order to determine whether or not a port scan has taken place, send appropriate alert emails, and (optionally) block offending ip addresses. psad includes a signal handler such that if a USR1 signal is received, psad will dump the contents of the current scan hash data structure to /var/log/psad/scan_hash.$$ where "$$" represents the pid of the running psad daemon.
NOTE: Since psad relies on iptables to generate appropriate log messages for unauthorized packets, psad is only as good as the logging rules included in the iptables ruleset. Hence if your firewall is not configured to log packets, then psad will NOT detect port scans or anything else. Usually the best way setup the firewall is with default "drop and log" rules at the end of the ruleset, and include rules above this last rule that only allow traffic that should be allowed through. Upon execution, the psad daemon will attempt to ascertain whether or not such a default deny rule exists, and will warn the user if not. See the FW_EXAMPLE_RULES file for example firewall rulesets that are compatible with psad.
Additionally, extensive coverage of psad is included in the book "Linux Firewalls: Attack Detection and Response" published by No Starch Press, and a supporting script in this book is compatible with psad. This script can be found here:
http://www.cipherdyne.org/LinuxFirewalls/ch01/

Installation
Depending on the Linux distribution, psad may already be available in the default package repository. For example, on Debian or Ubuntu systems, installation is done with a simple:
If psad is not available in the package repository, it can be installed with the install.pl script bundled in the psad sources. The install.pl script also handles upgrades if psad is already installed. psad requires several perl modules that may or may not already be installed on your Linux system. These modules are included in the deps/ directory in the psad sources, and are automatically installed by the install.pl script. The list of modules is:

• Bit::Vector
• Date::Calc
• IPTables::ChainMgr
• IPTables::Parse
• NetAddr::IP
• Storable
• Unix::Syslog

psad also includes a whois client written by Marco d'Itri (see the deps/whois directory). This client does better than others at collecting the correct whois information for a given IP address.

Firewall Setup
The main requirement for an iptables configuration to be compatible with psad is simply that iptables logs packets. This is commonly accomplished by adding rules to the INPUT and FORWARD chains like so:
The rules above should be added at the end of the INPUT and FORWARD chains after all ACCEPT rules for legitimate traffic and just before a corresponding DROP rule for traffic that is not to be allowed through the policy. Note that iptables policies can be quite complex with protocol, network, port, and interface restrictions, user defined chains, connection tracking rules, and much more. There are many pieces of software such as Shorewall and Firewall Builder, that build iptables policies and take advantage of the advanced filtering and logging capabilities offered by iptables. Generally the policies built by such pieces of software are compatible with psad since they specifically add rules that instruct iptables to log packets that are not part of legitimate traffic. Psad can be configured to only analyze those iptables messages that contain specific log prefixes (which are added via the --log-prefix option), but the default is for psad to analyze all iptables log messages for evidence of port scans, probes for backdoor programs, and other suspect traffic.

Platforms
psad generally runs on Linux systems, and is available in the package repositories of many major Linux distributions. If there are any operational issues with psad, please open an issue on psad