Snort Version 2.0.0
by Martin Roesch (email@example.com)
Copyright (C)1998-2003 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.
Snort is an open source network intrusion detection system, capable of
performing real-time traffic analysis and packet logging on IP networks.
It can perform protocol analysis and content searching/matching in order to
detect a variety of attacks and probes, such as buffer overflows, stealth port
scans, CGI attacks, SMB probes, OS fingerprinting attempts, and much more.
Snort uses a flexible rules language to describe traffic that it should collect
or pass, as well as a detection engine that utilizes a modular plugin
architecture. Snort has a real- time alerting capability as well,
incorporating alerting mechanisms for syslog, user specified files, or a
Snort has three primary functional modes. It can be used as a straight
packet sniffer like tcpdump(1), a packet logger (useful for network traffic
debugging, etc), or as a full blown network intrusion detection system.
Snort logs packets to many formats, including tcpdump(1) binary format or
Snort's decoded ASCII format to a hierarchical set of directories that are
named based on the IP address of the remote host.
Plugins allow the detection and reporting subsystems to be extended. Available
plugins include database or XML logging, small fragment detection, portscan
detection, and HTTP URI normalization, IP defragmentation, TCP stream
reassembly and statistical anomaly detection.
snort -[options] <filters>
-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.
-b 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.
-c <cf> Use configuration file <cf>. This is the rules file
which tells the system what to log, alert on, or pass!
-C Dump the ASCII characters in packet payloads only, no
-d Dump the application layer data
-D Run Snort in daemon mode. Alerts are sent to
/var/log/snort/alert unless otherwise specified.
-e Display/log the layer 2 packet header data.
-F <bpf> 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.
-g <gname> 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
-G Ghetto backwards compatibility switch, prints cross reference info
in the 1.7 format. Available modes are basic and url.
-h <hn> 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.
-i <if> Sniff on network interface <if>.
-I Add the interface name to alert printouts (first interface only)
-k <checksum mode>
Set <checksum mode> 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.
-l <ld> 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
-L <fn> Set the binary output file's filename to <fn>.
-m <mask> Set the umask for all of Snort's output files to the indicated
-M <wkstn> Send WinPopup messages to the list of workstations
contained in the <wkstn> file. This option requires
Samba to be resident and in the path of the machine
running Snort. The workstation file is simple: each
line of the file contains the SMB name of the box to
send the message to (no \\'s needed).
-n <num> Exit after processing <num> packets.
-N Turn off logging. Alerts still function normally.
-o 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
-O 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!
-p Turn off promiscuous mode sniffing. Useful for places
where that can screw up your host severely.
-P <snaplen> 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.
-q Quiet. Don't show banner and status report.
-r <tf> Read the tcpdump-generated file <tf>. This will cause
Snort to read and process the file fed to it. This is
useful if, for instance, you've got a bunch of Shadow
files that you want to process for content, or even if
you've got a bunch of reassembled packet fragments
which have been written into a tcpdump formatted file.
-s 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).
-S <n=v> 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.
-t <chroot> 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.
-T 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.
-u <uname> Change the UID Snort runs under to <uname> after
-U Turn on UTC timestamps.
-v 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
-V Show the version number and exit.
-X Dump the raw packet data starting at the link layer.
-y Turn on the year field in packet timestamps.
-z Set the assurance mode for Snort alerts. If the argument is set
to "all", all alerts come out of Snort as normal. If it is set to
"est" and the stream4 preprocessor is performing stateful
inspection (i.e. it's default mode), alerts will only be generated
for TCP packets that are part of an established session, greatly
reducing the noise generated by tools like stick and making Snort
more useful in general.
-? Show the usage summary and exit.
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
[zeus ~]# ./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.
[zeus ~]# ./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.
NOTE: The "official" rules document these days is available at:
and is also usually distributed as snort_manual.pdf in the distro. If
you don't have this file in your distribution of Snort, you can get it from
Snort has three primary run-time modes: sniffer, packet logger, and network
Sniffer Mode: When in this mode, Snort reads and decodes all packets from
the network and dumps them to the stdout. To put Snort into straight sniffing
mode, use the "-v" verbose switch. This will dump the packet headers only.
You can see the headers + the packet payloads by specifying the "-v" and "-d"
switch. To print a dump of the raw bytes in the entire packet, specify the
"-X" switch. If you specify the "-X" switch, the -d switch is overridden. You
can filter the traffic that shows up in this mode by using BPF filters.
Packet Logger Mode: This mode logs the packets to the disk in their decoded
ASCII format. This mode is activated merely by specifying a directory to log
packets to with the "-l" switch. This will log packets into the specified
logging directory in a hierarchy of directories based upon the IP addresses of
the packets on the wire. To log the packets in terms of the network being
monitored (i.e. the directories created under the logging directory are the
IP addresses of the remote/non-home hosts) use the "-h" switch. To log the
packets in their raw binary format to the disk, use the "-b" switch. Logging
the packets in this format will allow them to be run through other tools like
Ethereal, tcpdump, etc. Packet logger mode can be mixed with sniffer mode
switches with no ill effects, however logging performance may be impacted by
the slowness of the terminal.
Intrusion Detection Mode: Snort enters IDS mode when a configuration file is
specified with the "-c" switch. Output formats, rules, preprocessor
configuration, etc are all specified in the configuration file. Logger mode
is essentially disabled when in IDS mode, but that's ok because you specify
which packets you want to log when in IDS mode. See the rule document (above)
for how to write your own rules. When an alert rule goes off the alert data is
logged to the alerting mechanism (be default a file called "alert" in the
logging directory) in addition to being logged to the logging mechanism. The
default logging directory is /var/log/snort, which can be changed using the
You can use something like "rt" or just "tail -f" it to give a running display
of system alerts. Alerts can also be sent to syslog (and monitored with
something like swatch). There are a variety of other alerting and logging
mechanisms available, check out the snort.conf file for information on enabling
Note that the system requires the use of the "-l" flag to redirect rules based
logging to a specific directory. If you don't specify a place for it to go, it
defaults to /var/log/snort.
Please read the USAGE file or the snort_manual.pdf for more info.
/* $Id: README,v 1.23 2004/01/15 20:38:07 jh8 Exp $ */