SNORT(8)SNORT(8)NAME
Snort - open source network intrusion detection system
SYNOPSISsnort [-abCdDeGINoOpqsTUvVxXy?] [-A alert-mode ] [-c rules-file ] [-F
bpf-file ] [-g grpname ] [-h home-net ] [-i interface ] [-k checksum-
mode ] [-l log-dir ] [-L bin-log-file ] [-m umask ] [-M smb-hosts-file
] [-n packet-count ] [-P snap-length ] [-r tcpdump-file ] [-S vari‐
able=value ] [-t chroot_directory ] [-u usrname ] [-z connect-status ]
expression
DESCRIPTION
Snort is an oPen source network intrusion detection system, capable of
performing real-time traffic analysis and packet logging on IP net‐
works. It can perform protocol analysis, content searching/matching
and can be used to detect a variety of attacks and probes, such as buf‐
fer overflows, stealth port scans, CGI attacks, SMB probes, OS finger‐
printing attempts, and much more. Snort uses a flexible rules language
to describe traffic that it should collect or pass, as well as a detec‐
tion 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, WinPopup
messages to Windows clients using Samba's smbclient, database
(Mysql/PostgreSQL/Oracle/ODBC) or XML.
Snort has three primary uses. 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 in tcpdump(1) binary format, to a database 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.
OPTIONS-A alert-mode
Alert using the specified alert-mode. Valid alert modes include
fast, full, none, and unsock. Fast writes alerts to the default
"alert" file in a single-line, syslog style alert message. Full
writes the alert to the "alert" file with the full decoded
header as well as the alert message. None turns off alerting.
Unsock is an experimental mode that sends the alert information
out over a UNIX socket to another process that attaches to that
socket.
-a Display ARP packets when decoding packets.
-b Log packets in a 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.
-c config-file
Use the rules located in file config-file.
-C Print the character data from the packet payload only (no hex).
-d Dump the application layer data when displaying packets in ver‐
bose or packet logging mode.
-D Run Snort in daemon mode. Alerts are sent to
/var/log/snort/alert unless otherwise specified.
-e Display/log the link layer packet headers.
-F bpf-file
Read BPF filters from bpf-file. This is handy for people run‐
ning Snort as a SHADOW replacement or with a love Of super com‐
plex BPF filters. See the "expressions" section of this man
page for more info on writing BPF fileters.
-g groupname
Change the GID Snort runs under to grpname after initialization.
This switch allows Snort to drop root priveleges after it's ini‐
tialization phase has completed as a security measure.
-G ghetto-mode
Ghetto backwards compatibility switch, prints cross reference
info in the 1.7 format. Available modes are basic and url.
-h home-net
Set the "home network" to 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.
-i interface
Sniff packets on interface.
-I Print out the receiving interface name in alerts.
-k checksum-mode
Tune the internal checksum verification functionality with
alert-mode. Valid checksum modes include all, noip, notcp,
noudp, noicmp, and none. All activates checksum verification
for all supported protocols. Noip turns off IP checksum verifi‐
cation, which is handy if the gateway router is already dropping
packets that fail their IP checksum checks. Notcp turns off TCP
checksum verification, all other checksum modes are on. noudp
turns off UDP checksum verification. Noicmp turns off ICMP
checksum verification. None turns off the entire checksum veri‐
fication subsystem.
-l log-dir
Set the output logging directory to 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.
-L binary-log-file
Set the filename of the binary log file to 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".
-M smb-hosts-file
Send WinPopup messages to the list of workstations contained in
the smb-hosts-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.
-m umask
Set the file mode creation mask to umask
-n packet-count
Process packet-count packets and exit.
-N Turn off packet logging. The program still generates alerts
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.
-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 obfus‐
cated while non- homenet IPs will be left visible. Perfect for
posting to your favorite security mailing list!
-p Turn off promiscuous mode sniffing.
-P snap-length
Set the packet snaplen to snap-length
-q Quiet operation. Don't display banner and initialization infor‐
mation.
-r tcpdump-file
Read the tcpdump-formatted file 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 tcp‐
dump formatted file.
-s Send alert messages to syslog. On linux boxen, they will appear
in /var/log/secure, /var/log/messages on many other platforms.
-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.
-t chroot
Changes Snort's root directory to chroot after initialization.
Please note that all log/alert filenames are relative to the
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 initialization.
-U Changes the timestamp in all logs to be in UTC
-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, don't use the -v switch, you WILL drop packets.
-V Show the version number and exit.
-X Dump the raw packet data starting at the link layer. This
switch overrides the -d switch.
-y Include the year in alert and log files
-z The -z switch is used in concert with the stream4 preprocessor
code. It takes advantage of stream3's stateful inspection capa‐
bilities to reduce the amount of spoofing that may be done
against Snort. There are two arguments available for the -z
switch, all and est. All tells Snort not to worry about the TCP
state of a packet when it's about to issue an alert. Est tells
Snort to only allow alerts to be generated for packets that are
part of a known established session. This allows Snort to
greatly reduce the effect of anti-NIDS tools like stick and
snot.
-? Show the program usage statement and exit.
expression
selects which packets will be dumped. If no expression is
given, all packets on the net will be dumped. Otherwise, only
packets for which expression is `true' will be dumped.
The expression consists of one or more primitives. Primitives
usually consist of an id (name or number) preceded by one or
more qualifiers. There are three different kinds of qualifier:
type qualifiers say what kind of thing the id name or number
refers to. Possible types are host, net and port. E.g.,
`host foo', `net 128.3', `port 20'. If there is no type
qualifier, host is assumed.
dir qualifiers specify a particular transfer direction to
and/or from id. Possible directions are src, dst, src or
dst and src and dst. E.g., `src foo', `dst net 128.3',
`src or dst port ftp-data'. If there is no dir quali‐
fier, src or dst is assumed. For `null' link layers
(i.e. point to point protocols such as slip) the inbound
and outbound qualifiers can be used to specify a desired
direction.
proto qualifiers restrict the match to a particular protocol.
Possible protos are: ether, fddi, ip, arp, rarp, decnet,
lat, sca, moprc, mopdl, tcp and 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'.
[`fddi' is actually an alias for `ether'; the parser treats them
identically as meaning ``the data link level used on the speci‐
fied 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 con‐
tain other fields, but you cannot name them explicitly in a fil‐
ter expression.]
In addition to the above, there are some special `primitive'
keywords that don't follow the pattern: gateway, broadcast,
less, greater and arithmetic expressions. All of these are
described below.
More complex filter expressions are built up by using the words
and, or and 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'.
Allowable primitives are:
dst host host
True if the IP destination field of the packet is host,
which may be either an address or a name.
src host host
True if the IP source field of the packet is host.
host host
True if either the IP source or destination of the packet
is host. Any of the above host expressions can be
prepended with the keywords, ip, arp, or rarp as in:
ip host host
which is equivalent to:
ether proto \ip and host host
If host is a name with multiple IP addresses, each
address will be checked for a match.
ether dst ehost
True if the ethernet destination address is ehost. Ehost
may be either a name from /etc/ethers or a number (see
ethers(3N) for numeric format).
ether src ehost
True if the ethernet source address is ehost.
ether host ehost
True if either the ethernet source or destination address
is ehost.
gateway host
True if the packet used host as a gateway. I.e., the
ethernet source or destination address was host but nei‐
ther the IP source nor the IP destination was host. Host
must be a name and must be found in both /etc/hosts and
/etc/ethers. (An equivalent expression is
ether host ehost and not host host
which can be used with either names or numbers for host /
ehost.)
dst net net
True if the IP destination address of the packet has a
network number of net. Net may be either a name from
/etc/networks or a network number (see networks(4) for
details).
src net net
True if the IP source address of the packet has a network
number of net.
net net
True if either the IP source or destination address of
the packet has a network number of net.
net net mask mask
True if the IP address matches net with the specific net‐
mask. May be qualified with src or dst.
net net/len
True if the IP address matches net a netmask len bits
wide. May be qualified with src or dst.
dst port port
True if the packet is ip/tcp or ip/udp and has a destina‐
tion port value of port. The port can be a number or a
name used in /etc/services (see tcp(4P) and 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., dst port 513 will print
both tcp/login traffic and udp/who traffic, and port
domain will print both tcp/domain and udp/domain traf‐
fic).
src port port
True if the packet has a source port value of port.
port port
True if either the source or destination port of the
packet is port. Any of the above port expressions can be
prepended with the keywords, tcp or udp, as in:
tcp src port port
which matches only tcp packets whose source port is port.
less length
True if the packet has a length less than or equal to
length. This is equivalent to:
len <= length.
greater length
True if the packet has a length greater than or equal to
length. This is equivalent to:
len >= length.
ip proto protocol
True if the packet is an ip packet (see ip(4P)) of proto‐
col type protocol. Protocol can be a number or one of
the names icmp, igrp, udp, nd, or tcp. Note that the
identifiers tcp, udp, and icmp are also keywords and must
be escaped via backslash (\), which is \\ in the C-shell.
ether broadcast
True if the packet is an ethernet broadcast packet. The
ether keyword is optional.
ip broadcast
True if the packet is an IP broadcast packet. It checks
for both the all-zeroes and all-ones broadcast conven‐
tions, and looks up the local subnet mask.
ether multicast
True if the packet is an ethernet multicast packet. The
ether keyword is optional. This is shorthand for
`ether[0] & 1 != 0'.
ip multicast
True if the packet is an IP multicast packet.
ether proto protocol
True if the packet is of ether type protocol. Protocol
can be a number or a name like ip, arp, or rarp. Note
these identifiers are also keywords and must be escaped
via backslash (\). [In the case of FDDI (e.g., `fddi
protocol arp'), the protocol identification comes from
the 802.2 Logical Link Control (LLC) header, which is
usually layered on top of the FDDI header. Tcpdump
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.]
decnet src host
True if the DECNET source address is 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.]
decnet dst host
True if the DECNET destination address is host.
decnet host host
True if either the DECNET source or destination address
is host.
ip, arp, rarp, decnet
Abbreviations for:
ether proto p
where p is one of the above protocols.
lat, moprc, mopdl
Abbreviations for:
ether proto p
where p is one of the above protocols. Note that Snort
does not currently know how to parse these protocols.
tcp, udp, icmp
Abbreviations for:
ip proto p
where p is one of the above protocols.
expr relop expr
True if the relation holds, where relop is one of >, <,
>=, <=, =, !=, and expr is an arithmetic expression com‐
posed of integer constants (expressed in standard C syn‐
tax), the normal binary operators [+, -, *, /, &, |], a
length operator, and special packet data accessors. To
access data inside the packet, use the following syntax:
proto [ expr : size ]
Proto is one of ether, fddi, ip, arp, rarp, tcp, udp, or
icmp, and indicates the protocol layer for the index
operation. The byte offset, relative to the indicated
protocol layer, is given by expr. Size 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 len, gives
the length of the packet.
For example, `ether[0] & 1 != 0' catches all multicast
traffic. The expression `ip[0] & 0xf != 5' catches all
IP packets with options. The expression `ip[6:2] & 0x1fff
= 0' catches only unfragmented datagrams and frag zero of
fragmented datagrams. This check is implicitly applied
to the tcp and udp index operations. For instance,
tcp[0] always means the first byte of the TCP header, and
never means the first byte of an intervening fragment.
Primitives may be combined using:
A parenthesized group of primitives and operators (paren‐
theses are special to the Shell and must be escaped).
Negation (`!' or `not').
Concatenation (`&&' or `and').
Alternation (`||' or `or').
Negation has highest precedence. Alternation and concatenation
have equal precedence and associate left to right. Note that
explicit and tokens, not juxtaposition, are now required for
concatenation.
If an identifier is given without a keyword, the most recent
keyword is assumed. For example,
not host vs and ace
is short for
not host vs and host ace
which should not be confused with
not ( host vs or ace )
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.
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.html.
NOTES
The following signals have the specified effect when sent to the daemon
process using the kill(1) command:
SIGHUP Causes the daemon to close all opened files and restart. Please
note that this will only work if the full pathname is used to
invoke snort in daemon mode, otherwise snort will just exit with
an error message being sent to syslogd(8)
SIGUSR1
Causes the program to dump its current packet statistical infor‐
mation to the cosole or syslogd(8) if in daemon mode.
Any other signal causes the daemon to close all opened files and exit.
HISTORY
Snort has been freely available under the GPL license since 1998.
DIAGNOSTICS
Snort returns a 0 on a successful exit, 1 if it exits on an error.
BUGS
Send bug reports to roesch@clark.net, snort-devel@lists.sourceforge.net
AUTHOR
Martin Roesch <roesch@clark.net>
SEE ALSOtcpdump(1), pcap(3)
July 2001 SNORT(8)
