snort.8

.\" Process this file with
.\" groff -man -Tascii snort.8
.\"
.\" $Id$
.TH SNORT 8 "December 2011"
.SH NAME
Snort \- open source network intrusion detection system
.SH SYNOPSIS
.B snort [-bCdDeEfHIMNOpqQsTUvVwWxXy?] [-A
.I alert-mode
.B ] [-B
.I address-conversion-mask
.B ] [-c
.I rules-file
.B ] [-F
.I bpf-file
.B ] [-g
.I group-name
.B ] [-G
.I id
.B ] [-h
.I home-net
.B ] [-i
.I interface
.B ] [-k
.I checksum-mode
.B ] [-K
.I logging-mode
.B ] [-l
.I log-dir
.B ] [-L
.I bin-log-file
.B ] [-m
.I umask
.B ] [-n
.I packet-count
.B ] [-P
.I snap-length
.B ] [-r
.I tcpdump-file
.B ] [-R
.I name
.B ] [-S
.I variable=value
.B ] [-t
.I chroot_directory
.B ] [-u
.I user-name
.B ] [-Z
.I pathname
.B ] [--logid
.I id
.B ] [--perfmon-file
.I pathname
.B ] [--pid-path
.I pathname
.B ] [--snaplen
.I snap-length
.B ] [--help
.B ] [--version
.B ] [--dynamic-engine-lib
.I file
.B ] [--dynamic-engine-lib-dir
.I directory
.B ] [--dynamic-detection-lib
.I file
.B ] [--dynamic-detection-lib-dir
.I directory
.B ] [--dump-dynamic-rules
.I directory
.B ] [--dynamic-preprocessor-lib
.I file
.B ] [--dynamic-preprocessor-lib-dir
.I directory
.B ] [--alert-before-pass
.B ] [--treat-drop-as-alert
.B ] [--treat-drop-as-ignore
.B ] [--process-all-events
.B ] [--enable-inline-test
.B ] [--create-pidfile
.B ] [--nolock-pidfile
.B ] [--pcap-single=
.I tcpdump-file
.B ] [--pcap-filter=
.I filter
.B ] [--pcap-list=
.I list
.B ] [--pcap-dir=
.I directory
.B ] [--pcap-file=
.I file
.B ] [--pcap-no-filter
.B ] [--pcap-reset
.B ] [--pcap-show
.B ] [--exit-check
.I count
.B ] [--conf-error-out
.B ] [--require-rule-sid
.B ] [--daq
.I type
.B ] [--daq-mode
.I mode
.B ] [--daq-var
.I name=value
.B ] [--daq-dir
.I dir
.B ] [--daq-list
.I [dir]
.B ] [--cs-dir
.I dir
.B ]
.I expression
.SH DESCRIPTION
.B 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, content searching/matching and can be used 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 also has a modular real-time alerting capability, incorporating alerting
and logging plugins for syslog, a ASCII text files, UNIX sockets or XML.
.PP
Snort has three primary uses.  It can be used as a straight packet sniffer like
.BR tcpdump (1),
a packet logger (useful for network traffic debugging, etc), or as a full
blown network intrusion detection system.
.PP
Snort logs packets in
.BR tcpdump (1)
binary format or in Snort's decoded ASCII format to a hierarchy
of logging directories that are named based on the IP address of the "foreign"
host.
.SH OPTIONS
.IP "-A alert-mode"
Alert using the specified
.I alert-mode.
Valid alert modes include
.B fast, full, none,
and
.B unsock.
.B Fast
writes alerts to the default "alert" file in a single-line, syslog style alert
message.
.B Full
writes the alert to the "alert" file with the full decoded header as well as
the alert message.
.B None
turns off alerting.
.B Unsock
is an experimental mode that sends the alert information out over a UNIX socket
to another process that attaches to that socket.
.IP -b
Log packets in a
.BR tcpdump (1)
formatted file.   All packets are logged in their native binary state to a
tcpdump formatted log file named with the snort start timestamp and
"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.  To choose
an alternate name for the binary log file, use the '-L' switch.
.IP "-B address-conversion-mask"
Convert all IP addresses in
.I home-net
to addresses specified by
.I address-conversion-mask.
Used to obfuscate IP addresses within binary logs. Specify
.I home-net
with the '-h' switch.  Note this is
.B not
the same as $HOME_NET.
.IP "-c config-file"
Use the rules located in file
.I config-file.
.IP -C
Print the character data from the packet payload only (no hex).
.IP -d
Dump the application layer data when displaying packets in verbose or packet
logging mode.
.IP -D
Run Snort in daemon mode.  Alerts are sent to /var/log/snort/alert unless
otherwise specified.
.IP -e
Display/log the link layer packet headers.
.IP -E
.B *WIN32 ONLY*
Log alerts to the Windows Event Log.
.IP -f
Activate PCAP line buffering
.IP "-F bpf-file"
Read BPF filters from
.I bpf-file.
This is handy for people running Snort as a SHADOW replacement or with a love
Of super complex BPF filters.  See the "expressions" section of this man page
for more info on writing BPF filters.
.IP "-g group"
Change the group/GID Snort runs under to
.I group
after initialization.  This switch allows Snort to drop root privileges after
it's initialization phase has completed as a security measure.
.IP "-G id"
Use id as a base event ID when logging events.
.IP "-h home-net"
Set the "home network" to
.I home-net.
The format of this address variable is a network prefix plus a CIDR block, such
as 192.168.1.0/24.  Once this variable is set, all decoded packet logging will
be done relative to the home network address space.  This is useful because of
the way that Snort formats its ASCII log data.  With this value set to the
local network, all decoded output will be logged into decode directories
with the address of the foreign computer as the directory name, which is
very useful during traffic analysis. This option does not change "$HOME_NET" in
IDS mode.
.IP "-H"
Force hash tables to be deterministic instead of using a random number
generator for the seed & scale.  Useful for testing and generating repeatable
results with the same traffic.
.IP "-i interface"
Sniff packets on
.I interface.
.IP "-I"
Print out the receiving interface name in alerts.
.IP "-k checksum-mode"
Tune the internal checksum verification functionality with
.I alert-mode.
Valid checksum modes include
.B all, noip, notcp, noudp, noicmp,
and
.B none.
.B All
activates checksum verification for all supported protocols.
.B Noip
turns off IP checksum verification, which is handy if the gateway router is
already dropping packets that fail their IP checksum checks.
.B Notcp
turns off TCP checksum verification, all other checksum modes are
.B on.
.B noudp
turns off UDP checksum verification.
.B Noicmp
turns off ICMP checksum verification.
.B None
turns off the entire checksum verification subsystem.
.IP "-K logging-mode"
Select a packet logging mode.  The default is pcap.
.I logging-mode.
Valid logging modes include
.B pcap, ascii,
and
.B none.
.B Pcap
logs packets through the pcap library into pcap (tcpdump) format.
.B Ascii
logs packets in the old "directories and files" format with packet printouts in each file.
.B None
Turns off packet logging.
.IP "-l log-dir"
Set the output logging directory to
.I log-dir.
All plain text alerts and packet logs go into this directory.  If this option
is not specified, the default logging directory is set to /var/log/snort.
.IP "-L binary-log-file"
Set the filename of the binary log file to
.I binary-log-file.
If this switch is not used, the default name is a timestamp for the time that
the file is created plus "snort.log".
.IP "-m umask"
Set the file mode creation mask to
.I umask
.IP "-M"
Log console messages to syslog when not running daemon mode.  This switch
has no impact on logging of alerts.
.IP "-n packet-count"
Process
.I packet-count
packets and exit.
.IP -N
Turn off packet logging.  The program still generates alerts normally.
.IP -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 IPs will be left visible.  Perfect for posting
to your favorite security mailing list!
.IP -p
Turn off promiscuous mode sniffing.
.IP "-P snap-length"
Set the packet snaplen to
.I snap-length.
By default, this is set to 1514.
.IP "-q"
Quiet operation.  Don't display banner and initialization information.
.IP "-Q"
Enable inline mode operation.
.IP "-r tcpdump-file"
Read the tcpdump-formatted file
.I tcpdump-file.
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.
.IP "-R name"
Use name as a suffix to the snort pidfile.
.IP -s
Send alert messages to syslog.  On linux boxen, they will appear in
/var/log/secure, /var/log/messages on many other platforms.
.IP "-S variable=value"
Set variable name "variable" to value "value".  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.
.IP "-t chroot"
Changes Snort's root directory to
.I chroot
after initialization.  Please note that all log/alert filenames are relative
to the chroot directory if chroot is used.
.IP -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. Note, Snort looks for either /etc/snort.conf or ./snort.conf.
If your config lives elsewhere, use the -c option to specify a valid
.I config-file.
.IP "-u user"
Change the user/UID Snort runs under to
.I user
after initialization.
.IP -U
Changes the timestamp in all logs to be in UTC
.IP -v
Be verbose.  Prints packets out to the console.  There is one big problem with
verbose mode: it's slow.  If you are doing IDS work with Snort,
.B don't
use the '-v' switch, you
.B WILL
drop packets.
.IP -V
Show the version number and exit.
.IP "-w"
Show management frames if running on an 802.11 (wireless) network.
.IP "-W"
.B *WIN32 ONLY*
Enumerate the network interfaces available.
.IP "-x"
Exit if Snort configuration problems occur such as duplicate gid/sid or flowbits without Stream5.
.IP "-X"
Dump the raw packet data starting at the link layer.  This switch overrides the '-d' switch.
.IP "-y"
Include the year in alert and log files
.IP "-Z pathname"
Set the perfmonitor preprocessor path/filename to pathname.
.IP -?
Show the program usage statement and exit.
.IP "--logid id"
Same as -G.
.IP "--perfmon-file pathname"
Same as -Z.
.IP "--pid-path directory"
Specify the directory for the Snort PID file.
.IP "--snaplen snap-length"
Same as -P.
.IP "--help"
Same as -?
.IP "--version"
Same as -V
.IP "--dynamic-engine-lib file"
Load a dynamic detection engine shared library specified by file.
.IP "--dynamic-engine-lib-dir directory"
Load all dynamic detection engine shared libraries specified from directory.
.IP "--dynamic-detection-lib file"
Load a dynamic detection rules shared library specified by file.
.IP "--dynamic-detection-lib-dir directory"
Load all dynamic detection rules shared libraries specified from directory.
.IP "--dump-dynamic-rules directory"
Create stub rule files from all loaded dynamic detection rules libraries.
Files will be created in directory.  This is required to be done prior
to running snort using those detection rules and the generated rules files
must be included in snort.conf.
.IP "--dynamic-preprocessor-lib file"
Load a dynamic preprocessor shared library specified by file.
.IP "--dynamic-preprocessor-lib-dir directory"
Load all dynamic preprocessor shared libraries specified from directory.
.IP "--alert-before-pass"
Process alert, drop, sdrop, or reject before pass.
Default is pass before alert, drop, etc.
.IP "--treat-drop-as-alert"
Converts drop, sdrop, and reject rules into alert rules during startup.
.IP "--treat-drop-as-ignore"
Use drop, sdrop, and reject rules to ignore session traffic when not inline.
.IP "--process-all-events"
Process all triggered events in group order, per Rule Ordering
configuration.  Default stops after first group.
.IP "--enable-inline-test"
Enable Inline-Test Mode Operation.
.IP "--pid-path directory"
Specify the path for Snort's PID file.
.IP "--create-pidfile"
Create PID file, even when not in Daemon mode.
.IP "--nolock-pidfile"
Do not try to lock Snort PID file.
.IP "--pcap-single=\fItcpdump-file\fP"
Same as -r.  Added for completeness.
.IP "--pcap-filter=\fIfilter\fP"
Shell style filter to apply when getting pcaps from
file or directory.  This filter will apply to any
--pcap-file or --pcap-dir arguments following.  Use
--pcap-no-filter to delete filter for following
--pcap-file or --pcap-dir arguments or specify
--pcap-filter again to forget previous filter and
to apply to following --pcap-file or --pcap-dir arguments.
.IP "--pcap-list=\fI""list""\fP"
A space separated list of pcaps to read.
.IP "--pcap-dir=\fIdirectory\fP"
A directory to recurse to look for pcaps.  Sorted in ascii order.
.IP "--pcap-file=\fIfile\fP"
File that contains a list of pcaps to read.  Can specify path to
pcap or directory to recurse to get pcaps.
.IP "--pcap-no-filter"
Reset to use no filter when getting pcaps from file or directory.
.IP "--pcap-reset"
If reading multiple pcaps, reset snort to post-configuration state
before reading next pcap.  The default, i.e. without this option,
is not to reset state.
.IP "--pcap-show"
Print a line saying what pcap is currently being read.
.IP "--exit-check=\fIcount\fP"
Signal termination after <count> callbacks from DAQ_Acquire(), showing the
time it takes from signaling until DAQ_Stop() is called.
.IP "--conf-error-out"
Same as -x.
.IP "--require-rule-sid"
Require an SID for every rule to be correctly threshold all rules.
.IP "--daq <type>"
Select packet acquisition module (default is pcap).
.IP "--daq-mode <mode>"
Select the DAQ operating mode.
.IP "--daq-var <name=value>"
Specify extra DAQ configuration variable.
.IP "--daq-dir <dir>"
Tell Snort where to find desired DAQ.
.IP "--daq-list [<dir>]"
List packet acquisition modules available in dir.
.IP "--cs-dir <dir>"
Tell Snort to use control socket and create the socket in dir.

.IP "\fI expression\fP"
.RS
selects which packets will be dumped.  If no \fIexpression\fP
is given, all packets on the net will be dumped.  Otherwise,
only packets for which \fIexpression\fP is `true' will be dumped.
.LP
The \fIexpression\fP consists of one or more
.I primitives.
Primitives usually consist of an
.I id
(name or number) preceded by one or more qualifiers.  There are three
different kinds of qualifier:
.IP \fItype\fP
qualifiers say what kind of thing the id name or number refers to.
Possible types are
.BR host ,
.B net
and
.BR port .
E.g., `host foo', `net 128.3', `port 20'.  If there is no type
qualifier,
.B host
is assumed.
.IP \fIdir\fP
qualifiers specify a particular transfer direction to and/or from
.I id.
Possible directions are
.BR src ,
.BR dst ,
.B "src or dst"
and
.B "src and"
.BR dst .
E.g., `src foo', `dst net 128.3', `src or dst port ftp-data'.  If
there is no dir qualifier,
.B "src or dst"
is assumed.
For `null' link layers (i.e. point to point protocols such as slip) the
.B inbound
and
.B outbound
qualifiers can be used to specify a desired direction.
.IP \fIproto\fP
qualifiers restrict the match to a particular protocol.  Possible
protos are:
.BR ether ,
.BR fddi ,
.BR ip ,
.BR arp ,
.BR rarp ,
.BR decnet ,
.BR lat ,
.BR sca ,
.BR moprc ,
.BR mopdl ,
.B tcp
and
.BR udp .
E.g., `ether src foo', `arp net 128.3', `tcp port 21'.  If there is
no proto qualifier, all protocols consistent with the type are
assumed.  E.g., `src foo' means `(ip or arp or rarp) src foo'
(except the latter is not legal syntax), `net bar' means `(ip or
arp or rarp) net bar' and `port 53' means `(tcp or udp) port 53'.
.LP
[`fddi' is actually an alias for `ether'; the parser treats them
identically as meaning ``the data link level used on the specified
network interface.''  FDDI headers contain Ethernet-like source
and destination addresses, and often contain Ethernet-like packet
types, so you can filter on these FDDI fields just as with the
analogous Ethernet fields.  FDDI headers also contain other fields,
but you cannot name them explicitly in a filter expression.]
.LP
In addition to the above, there are some special `primitive' keywords
that don't follow the pattern:
.BR gateway ,
.BR broadcast ,
.BR less ,
.B greater
and arithmetic expressions.  All of these are described below.
.LP
More complex filter expressions are built up by using the words
.BR and ,
.B or
and
.B not
to combine primitives.  E.g., `host foo and not port ftp and not port ftp-data'.
To save typing, identical qualifier lists can be omitted.  E.g.,
`tcp dst port ftp or ftp-data or domain' is exactly the same as
`tcp dst port ftp or tcp dst port ftp-data or tcp dst port domain'.
.LP
Allowable primitives are:
.IP "\fBdst host \fIhost\fR"
True if the IP destination field of the packet is \fIhost\fP,
which may be either an address or a name.
.IP "\fBsrc host \fIhost\fR"
True if the IP source field of the packet is \fIhost\fP.
.IP "\fBhost \fIhost\fP"
True if either the IP source or destination of the packet is \fIhost\fP.
Any of the above host expressions can be prepended with the keywords,
\fBip\fP, \fBarp\fP, or \fBrarp\fP as in:
.in +.5i
.nf
\fBip host \fIhost\fR
.fi
.in -.5i
which is equivalent to:
.in +.5i
.nf
\fBether proto \fI\\ip\fB and host \fIhost\fR
.fi
.in -.5i
If \fIhost\fR is a name with multiple IP addresses, each address will
be checked for a match.
.IP "\fBether dst \fIehost\fP"
True if the ethernet destination address is \fIehost\fP.  \fIEhost\fP
may be either a name from /etc/ethers or a number (see
.IR ethers (3N)
for numeric format).
.IP "\fBether src \fIehost\fP"
True if the ethernet source address is \fIehost\fP.
.IP "\fBether host \fIehost\fP"
True if either the ethernet source or destination address is \fIehost\fP.
.IP "\fBgateway\fP \fIhost\fP"
True if the packet used \fIhost\fP as a gateway.  I.e., the ethernet
source or destination address was \fIhost\fP but neither the IP source
nor the IP destination was \fIhost\fP.  \fIHost\fP must be a name and
must be found in both /etc/hosts and /etc/ethers.  (An equivalent
expression is
.in +.5i
.nf
\fBether host \fIehost \fBand not host \fIhost\fR
.fi
.in -.5i
which can be used with either names or numbers for \fIhost / ehost\fP.)
.IP "\fBdst net \fInet\fR"
True if the IP destination address of the packet has a network
number of \fInet\fP. \fINet\fP may be either a name from /etc/networks
or a network number (see \fInetworks(4)\fP for details).
.IP "\fBsrc net \fInet\fR"
True if the IP source address of the packet has a network
number of \fInet\fP.
.IP "\fBnet \fInet\fR"
True if either the IP source or destination address of the packet has a network
number of \fInet\fP.
.IP "\fBnet \fInet\fR \fBmask \fImask\fR"
True if the IP address matches \fInet\fR with the specific netmask.
May be qualified with \fBsrc\fR or \fBdst\fR.
.IP "\fBnet \fInet\fR/\fIlen\fR"
True if the IP address matches \fInet\fR a netmask \fIlen\fR bits wide.
May be qualified with \fBsrc\fR or \fBdst\fR.
.IP "\fBdst port \fIport\fR"
True if the packet is ip/tcp or ip/udp and has a
destination port value of \fIport\fP.
The \fIport\fP can be a number or a name used in /etc/services (see
.IR tcp (4P)
and
.IR udp (4P)).
If a name is used, both the port
number and protocol are checked.  If a number or ambiguous name is used,
only the port number is checked (e.g., \fBdst port 513\fR will print both
tcp/login traffic and udp/who traffic, and \fBport domain\fR will print
both tcp/domain and udp/domain traffic).
.IP "\fBsrc port \fIport\fR"
True if the packet has a source port value of \fIport\fP.
.IP "\fBport \fIport\fR"
True if either the source or destination port of the packet is \fIport\fP.
Any of the above port expressions can be prepended with the keywords,
\fBtcp\fP or \fBudp\fP, as in:
.in +.5i
.nf
\fBtcp src port \fIport\fR
.fi
.in -.5i
which matches only tcp packets whose source port is \fIport\fP.
.IP "\fBless \fIlength\fR"
True if the packet has a length less than or equal to \fIlength\fP.
This is equivalent to:
.in +.5i
.nf
\fBlen <= \fIlength\fP.
.fi
.in -.5i
.IP "\fBgreater \fIlength\fR"
True if the packet has a length greater than or equal to \fIlength\fP.
This is equivalent to:
.in +.5i
.nf
\fBlen >= \fIlength\fP.
.fi
.in -.5i
.IP "\fBip proto \fIprotocol\fR"
True if the packet is an ip packet (see
.IR ip (4P))
of protocol type \fIprotocol\fP.
\fIProtocol\fP can be a number or one of the names
\fIicmp\fP, \fIigrp\fP, \fIudp\fP, \fInd\fP, or \fItcp\fP.
Note that the identifiers \fItcp\fP, \fIudp\fP, and \fIicmp\fP are also
keywords and must be escaped via backslash (\\), which is \\\\ in the C-shell.
.IP "\fBether broadcast\fR"
True if the packet is an ethernet broadcast packet.  The \fIether\fP
keyword is optional.
.IP "\fBip broadcast\fR"
True if the packet is an IP broadcast packet.  It checks for both
the all-zeroes and all-ones broadcast conventions, and looks up
the local subnet mask.
.IP "\fBether multicast\fR"
True if the packet is an ethernet multicast packet.  The \fIether\fP
keyword is optional.
This is shorthand for `\fBether[0] & 1 != 0\fP'.
.IP "\fBip multicast\fR"
True if the packet is an IP multicast packet.
.IP  "\fBether proto \fIprotocol\fR"
True if the packet is of ether type \fIprotocol\fR.
\fIProtocol\fP can be a number or a name like
\fIip\fP, \fIarp\fP, or \fIrarp\fP.
Note these identifiers are also keywords
and must be escaped via backslash (\\).
[In the case of FDDI (e.g., `\fBfddi protocol arp\fR'), the
protocol identification comes from the 802.2 Logical Link Control
(LLC) header, which is usually layered on top of the FDDI header.
\fITcpdump\fP assumes, when filtering on the protocol identifier,
that all FDDI packets include an LLC header, and that the LLC header
is in so-called SNAP format.]
.IP "\fBdecnet src \fIhost\fR"
True if the DECNET source address is
.IR host ,
which may be an address of the form ``10.123'', or a DECNET host
name.  [DECNET host name support is only available on Ultrix systems
that are configured to run DECNET.]
.IP "\fBdecnet dst \fIhost\fR"
True if the DECNET destination address is
.IR host .
.IP "\fBdecnet host \fIhost\fR"
True if either the DECNET source or destination address is
.IR host .
.IP "\fBip\fR, \fBarp\fR, \fBrarp\fR, \fBdecnet\fR"
Abbreviations for:
.in +.5i
.nf
\fBether proto \fIp\fR
.fi
.in -.5i
where \fIp\fR is one of the above protocols.
.IP "\fBlat\fR, \fBmoprc\fR, \fBmopdl\fR"
Abbreviations for:
.in +.5i
.nf
\fBether proto \fIp\fR
.fi
.in -.5i
where \fIp\fR is one of the above protocols.
Note that
\fISnort\fP does not currently know how to parse these protocols.
.IP  "\fBtcp\fR, \fBudp\fR, \fBicmp\fR"
Abbreviations for:
.in +.5i
.nf
\fBip proto \fIp\fR
.fi
.in -.5i
where \fIp\fR is one of the above protocols.
.IP  "\fIexpr relop expr\fR"
True if the relation holds, where \fIrelop\fR is one of >, <, >=, <=, =, !=,
and \fIexpr\fR is an arithmetic expression composed of integer constants
(expressed in standard C syntax), the normal binary operators
[+, -, *, /, &, |], a length operator, and special packet data accessors.
To access
data inside the packet, use the following syntax:
.in +.5i
.nf
\fIproto\fB [ \fIexpr\fB : \fIsize\fB ]\fR
.fi
.in -.5i
\fIProto\fR is one of \fBether, fddi,
ip, arp, rarp, tcp, udp, \fRor \fBicmp\fR, and
indicates the protocol layer for the index operation.
The byte offset, relative to the indicated protocol layer, is
given by \fIexpr\fR.
\fISize\fR is optional and indicates the number of bytes in the
field of interest; it can be either one, two, or four, and defaults to one.
The length operator, indicated by the keyword \fBlen\fP, gives the
length of the packet.

For example, `\fBether[0] & 1 != 0\fP' catches all multicast traffic.
The expression `\fBip[0] & 0xf != 5\fP'
catches all IP packets with options. The expression
`\fBip[6:2] & 0x1fff = 0\fP'
catches only unfragmented datagrams and frag zero of fragmented datagrams.
This check is implicitly applied to the \fBtcp\fP and \fBudp\fP
index operations.
For instance, \fBtcp[0]\fP always means the first
byte of the TCP \fIheader\fP, and never means the first byte of an
intervening fragment.
.LP
Primitives may be combined using:
.IP
A parenthesized group of primitives and operators
(parentheses are special to the Shell and must be escaped).
.IP
Negation (`\fB!\fP' or `\fBnot\fP').
.IP
Concatenation (`\fB&&\fP' or `\fBand\fP').
.IP
Alternation (`\fB||\fP' or `\fBor\fP').
.LP
Negation has highest precedence.
Alternation and concatenation have equal precedence and associate
left to right.  Note that explicit \fBand\fR tokens, not juxtaposition,
are now required for concatenation.
.LP
If an identifier is given without a keyword, the most recent keyword
is assumed.
For example,
.in +.5i
.nf
\fBnot host vs and ace\fR
.fi
.in -.5i
is short for
.in +.5i
.nf
\fBnot host vs and host ace\fR
.fi
.in -.5i
which should not be confused with
.in +.5i
.nf
\fBnot ( host vs or ace )\fR
.fi
.in -.5i
.LP
Expression arguments can be passed to Snort as either a single argument
or as multiple arguments, whichever is more convenient.
Generally, if the expression contains Shell metacharacters, it is
easier to pass it as a single, quoted argument.
Multiple arguments are concatenated with spaces before being parsed.
.SH READING PCAPS
Instead of having Snort listen on an interface, you can give it a packet
capture to read.  Snort will read and analyze the packets as if they came
off the wire.  This can be useful for testing and debugging Snort.

\fBRead a single pcap\fR

.RS 5
.PD 0
$ snort -r foo.pcap
.PP
$ snort --pcap-single=foo.pcap

.RE 0
\fBRead pcaps from a file\fR

.RS 5
$ cat foo.txt
.PP
foo1.pcap
.PP
foo2.pcap
.PP
/home/foo/pcaps

$ snort --pcap-file=foo.txt

This will read foo1.pcap, foo2.pcap and all files under /home/foo/pcaps.
Note that Snort will not try to determine whether the files under that
directory are really pcap files or not.

.RE 0
\fBRead pcaps from a command line list\fR

.RS 5
$ snort --pcap-list="foo1.pcap foo2.pcap foo3.pcap"

This will read foo1.pcap, foo2.pcap and foo3.pcap.

.RE 0
\fBRead pcaps under a directory\fR

.RS 5
$ snort --pcap-dir="/home/foo/pcaps"

This will include all of the files under /home/foo/pcaps.

.RE 0
\fBUsing filters\fR

.RS 5
$ cat foo.txt
.PP
foo1.pcap
.PP
foo2.pcap
.PP
/home/foo/pcaps

$ snort --pcap-filter="*.pcap" --pcap-file=foo.txt
.PP
$ snort --pcap-filter="*.pcap" --pcap-dir=/home/foo/pcaps

The above will only include files that match the shell pattern "*.pcap",
in other words, any file ending in ".pcap".

$ snort --pcap-filter="*.pcap --pcap-file=foo.txt \\
.PP
> --pcap-filter="*.cap" --pcap-dir=/home/foo/pcaps

In the above, the first filter "*.pcap" will only be applied to the
pcaps in the file "foo.txt" (and any directories that are recursed
in that file).  The addition of the second filter "*.cap" will cause
the first filter to be forgotten and then applied to the directory
/home/foo/pcaps, so only files ending in ".cap" will be included from
that directory.

$ snort --pcap-filter="*.pcap --pcap-file=foo.txt \\
.PP
> --pcap-no-filter --pcap-dir=/home/foo/pcaps

In this example, the first filter will be applied to foo.txt, then
no filter will be applied to the files found under /home/foo/pcaps,
so all files found under /home/foo/pcaps will be included.

$ snort --pcap-filter="*.pcap --pcap-file=foo.txt \\
.PP
> --pcap-no-filter --pcap-dir=/home/foo/pcaps \\
.PP
> --pcap-filter="*.cap" --pcap-dir=/home/foo/pcaps2

In this example, the first filter will be applied to foo.txt, then
no filter will be applied to the files found under /home/foo/pcaps,
so all files found under /home/foo/pcaps will be included, then the
filter "*.cap" will be applied to files found under /home/foo/pcaps2.

.RE 0
\fBResetting state\fR

.RS 5
$ snort --pcap-dir=/home/foo/pcaps --pcap-reset

The above example will read all of the files under /home/foo/pcaps, but
after each pcap is read, Snort will be reset to a post-configuration
state, meaning all buffers will be flushed, statistics reset, etc.  For
each pcap, it will be like Snort is seeing traffic for the first time.

.RE 0
\fBPrinting the pcap\fR

.RS 5
$ snort --pcap-dir=/home/foo/pcaps --pcap-show

The above example will read all of the files under /home/foo/pcaps and
will print a line indicating which pcap is currently being read.
.RE 0
.PD
.SH RULES
Snort uses a simple but flexible rules language to describe network packet
signatures and associate them with actions.  The current rules document can
be found at http://www.snort.org/snort-rules.
.SH NOTES
The following signals have the specified effect when sent to the daemon process using the \fBkill(1)\fR command:
.PP
.IP SIGHUP
Causes the daemon to close all opened files and restart.
Please \fBnote\fR that this will only work if the \fBfull\fR pathname is
used to invoke snort in daemon mode, otherwise snort will just exit with an
error message being sent to \fBsyslogd(8)\fR.
.PP
.IP SIGUSR1
Causes the program to dump its current packet statistical information to the
console or \fBsyslogd(8)\fR if in daemon mode.
.PP
.IP SIGUSR2
Causes the program to rotate Perfmonitor statistical information to the 
console or \fBsyslogd(8)\fR if in daemon mode.
.PP
.IP SIGURG
Causes the program to reload attribute table.
.PP
.IP SIGCHLD
Used internally.
.PP
Please refer to manual for more details. Any other signal might cause the 
daemon to close all opened files and exit.

.SH HISTORY
.B Snort
has been freely available under the GPL license since 1998.
.SH DIAGNOSTICS
.B Snort
returns a 0 on a successful exit, 1 if it exits on an error.
.SH BUGS
After consulting the BUGS file included with the source distribution, send bug
reports to snort-devel@lists.sourceforge.net
.SH AUTHOR
Martin Roesch <roesch@snort.org>
.SH "SEE ALSO"
.BR tcpdump (1),
.BR pcap (3)