psad - Intrusion Detection with iptables Logs

Introduction

The Port Scan Attack Detector psad is a lightweight system daemon written in
is designed to work with Linux iptables/ip6tables/firewalld firewalling code to
detect suspicious traffic such as port scans and sweeps, backdoors, botnet
command and control communications, and more. It features a set of highly
configurable danger thresholds (with sensible defaults provided), verbose alert
messages that include the source, destination, scanned port range, begin and
end times, TCP flags and corresponding nmap options, reverse DNS info, email
and syslog alerting, automatic blocking of offending IP addresses via dynamic
configuration of iptables rulesets, passive operating system fingerprinting,
and DShield reporting. In addition, psad incorporates many of the TCP, UDP,
and ICMP signatures included in the Snort intrusion detection system.
to detect highly suspect scans for various backdoor programs (e.g. EvilFTP,
GirlFriend, SubSeven), DDoS tools (Mstream, Shaft), and advanced port scans
(SYN, FIN, XMAS) which are easily leveraged against a machine via nmap. psad
can also alert on Snort signatures that are logged via
fwsnort, which makes use of the iptables
string match extension to detect traffic that matches application layer
signatures. As of the 2.4.4 release, psad can also detect the IoT default
credentials scanning phase of the Mirai botnet.

The complete feature list is below.

Features

• 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.

Visualizing Malicious Traffic

psad offers integration with gnuplot and afterglow to produce graphs of
malicious traffic. The following two graphs are of the Nachi worm from the
Honeynet Scan30 challenge. First, a
link graph produced by afterglow after analysis of the iptables log data by
psad:

"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:

apt-get install psad

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:

iptables -A INPUT -j LOG
iptables -A FORWARD -j LOG

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 github

License

psad is released as open source software under the terms of
the GNU General Public License (GPL v2+). The latest release can be found
at https://github.com/mrash/psad/releases

psad makes use of many of the TCP, UDP, and ICMP signatures available in
Snort (written by Marty Roesch, see http://www.snort.org). Snort is a
registered trademark of Sourcefire, Inc.

Contact

All feature requests and bug fixes are managed through github issues tracking.
However, you can email me (michael.rash_AT_gmail.com), or reach me through
Twitter (@michaelrash).