Distribution Site: http://www.snort.org

COPYRIGHT

Copyright (C)2001-2005 Sourcefire Inc.
Copyright (C)1998-2001 Martin Roesch

This program is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation; either version 2 of the License, or (at your option) any later version.

This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details.

You should have received a copy of the GNU General Public License along with this program; if not, write to the Free Software Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.

Some of this code has been taken from tcpdump, which was developed by the Network Research Group at Lawrence Berkeley National Lab, and is copyrighted by the University of California Regents.

Description:

Snort is an open source network intrusion detection and prevention system. It is capable of performing real-time traffic analysis, alerting, blocking and packet logging on IP networks. It utilizes a combination of protocol analysis and pattern matching in order to detect a anomalies, misuse and attacks. Snort uses a flexible rules language to describe activity that can be considered malicious or anomalous as well as an analysis engine that incorporates a modular plugin architecture. Snort is capable of detecting and responding in real-time, sending alerts, performing session sniping, logging packets, or dropping sessions/packets when deployed in-line.

Snort has three primary functional modes. It can be used as a packet sniffer like tcpdump(1), a packet logger (useful for network traffic debugging, etc), or as a full blown network intrusion detection and prevention system.

Please read the snort_manual.pdf file that should be included with this distribution for full documentation on the program as well as a guide to getting started.

Command line:

     snort -[options] <filters>

Options:

-A <alert>

     Set <alert> mode to full, fast or none. Full mode does normal "classic Snort"-style alerts to the alert file. Fast mode just writes the timestamp, message, IP's, and ports to the file. None turns off alerting. There is experimental support for UnixSock alerts that allow alerting to a separate process. Use the unsock argument to activate this feature. There's also the cmg option that prints out the full packet dump with the alert information. The console option prints fast mode alerts to stdout, great for testing new rules and debugging preprocessor anomaly detectors.

     Log packets in tcpdump format. All packets are logged in their native binary state to a tcpdump formatted log file called snort.log. This option results in much faster operation of the program since it doesn't have to spend time in the packet binary->text converters. Snort can keep up pretty well with 100Mbps networks in -b mode.

     Obfuscate IP addresses in alerts and packet dumps using the provided CIDR mask as a substitution for the destination IP addresses in events.

     Use configuration file <cf>. This is puts Snort into IDS mode and it reads the runtime configuration from <cf>.

     Dump the ASCII characters in packet payloads only, no hex dump.

     Dump the application layer data.

     Run Snort in daemon mode. Alerts are sent to /var/log/snort/alert unless otherwise specified.

     Display/log the layer 2 packet header data.

     WIN32 ONLY Log alerts to the Windows Event Log.

     Activate PCAP line buffering.

     Read BPF filters from file <bpf>. Handy for those of you running Snort as a SHADOW replacement or with a love of super complex BPF filters.

     Run Snort as group ID <gname> after initialization. This switch allows Snort to drop root privileges after it's initialization phase has completed as a security measure.

     Set a base event_id value for event generation, useful for unified logging and alerting primarily.

     Set the "home network" to <hn>, which is a class C IP address something like 192.168.1.0 or whatever. If you use this switch, traffic coming from external networks will be formatted with the directional arrow of the packet dump pointing right for incoming external traffic, and left for outgoing internal traffic. Kind of silly, but it looks nice.

     Sniff on network interface <if>.

     Add the interface name to alert printouts (first interface only)

     When running in in-line mode on a system with divert sockets this switch will select which <port> to read packets from.

     Set to all, noip, notcp, noudp, noicmp, or none. Setting this switch modifies the checksum verification subsystem of Snort to tune for maximum performance. For example, in many situations Snort is behind a router or firewall that doesn't allow packets with bad checksums to pass, in which case it wouldn't make sense to have Snort re-verify checksums that have already been checked. Turning off specific checksum verification subsystems can improve performance by reducing the amount of time required to inspect a packet.

     Set the packet output mode for logging. There are three modes available, pcap, ascii and none. Pcap mode is the default, if you don't specify a logging mode pcap is used now. Pcap format is the same as the -b switch, tcpdump format. Ascii format is the old default, it logs in the text-based "directories and files" format. Be careful using ascii mode on uncontrolled networks, it can exhaust your filesystem's inodes. None mode turns off packet logging.

     Log packets to directory <ld>. Sets up a hierarchical directory structure with the log directory as the base starting directory, and the IP address of the remote peer generating traffic as the directory which packets packets from that address are stored in. If you do not use the -l switch, the default logging directory is /var/log/snort.

     Set the binary output file's filename to <fn>.

     Set the umask for all of Snort's output files to the indicated mask.

     Exit after processing <num> packets.

     Turn off logging. Alerts still function normally.

     Change the order in which the rules are applied to packets. Instead of being applied in the standard Alert->Pass->Log order, this will apply them in Pass->Alert->Log order, allowing people to avoid having to make huge BPF command line arguments to filter their alert rules.

     Obfuscate the IP addresses when in ASCII packet dump mode. This switch changes the IP addresses that get printed to the screen/log file to "xxx.xxx.xxx.xxx". If the homenet address switch is set (-h), only addresses on the homenet will be obfuscated while non-homenet IP's will be left visible. Perfect for posting to your favorite security mailing list!

     Turn off promiscuous mode sniffing. Useful for places where that can screw up your host severely.

     Set the snaplen of Snort to <snaplen>. This filters how much of each packet gets into Snort, the default is the MTU for the interface that Snort is currently listening on.

     Quiet. Don't show banner and status report.

     When running in-line, read packets from iptables/IPQ (on Linux).

     Read packets from the pcap formatted file <tf>. This will cause Snort to read and process the file fed to it as if the file was the network. This is essentially the same as tcpdump's readback mode.

     Add a custom sufffix to the snort pidfile.

     Log alert messages to the syslog. On Linux boxen, they will appear in /var/log/secure, /var/log/messages on many other platforms. You can change the logging facility by using the syslog output plugin, at which point the -s switch should not be used (command line alert/log switches override any config file output variables).

     Set variable name "n" to value "v". This is useful for setting the value of a defined variable name in a Snort rules file to a command line specified value. For instance, if you define a HOME_NET variable name inside of a Snort rules file, you can set this value from it's predefined value at the command line.

     Changes Snort's root directory to <chroot> after initialization. Please note that all log/alert filenames are relevant to chroot directory, if chroot is used.

     Snort will start up in self-test mode, checking all the supplied command line switches and rules files that are handed to it and indicating that everything is ready to proceed. This is a good switch to use if daemon mode is going to be used, it verifies that the Snort configuration that is about to be used is valid and won't fail at run time.

     Change the UID Snort runs under to <uname> after initialization.

     Turn on UTC timestamps.

     Be verbose. Prints packets out to the console. There is one big problem with verbose mode: it's still kind of slow. If you are doing IDS work with Snort, don't use the -v switch, you WILL drop packets (not many, but some).

     Show the version number and exit.

     If running on a 802.11 network, show management frames.

     Enumerate the network interfaces available on a win32 system.

     Dump the raw packet data starting at the link layer.

     Turn on the year field in packet timestamps.

     The -z switch is deprecated.

     Set the perfmon path/filename to <path>.

     Show the usage summary and exit.

Filters:

The "filters" are standard BPF style filters as seen in tcpdump. Look at the man page for snort for docs on how to use it properly. In general,you can give it a host, net or protocol to filter on and some logical statements to tie it together and get the specific traffic you're interested in. For example:

  # ./snort -h 192.168.1.0/24 -d -v host 192.168.1.1

records the traffic to and from host 192.168.1.1.

  # ./snort -h 192.168.1.0/24 -d -v net 192.168.1 and not host 192.168.1.1

records all traffic on the 192.168.1.0/24 class C subnet, but not traffic to/from 192.168.1.1. Notice that the command line data specified after the -h switch is formated differently from the BPF commands provided at the end of the command line. Sorry for the confusion, but I like the CIDR notation and I'm not rewriting libpcap to make it consistent! Anyway, you get the picture. Mail me if you have trouble with it.

You can use the -F switch to read your BPF filters in from a file.