Snort - open source network intrusion detection system

snort [-bCdDeGINoOpqsTUvVxXyz?] [-A alert-mode ] [-B address-conversion-mask ] [-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 ] [-n packet-count ] [-P snap-length ] [-r tcpdump-file ] [-S variable=value ] [-t chroot_directory ] [-u usrname ] expression

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, 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.

-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.

-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.

-B address-conversion-mask

Convert all IP addresses in home-net to addresses specified by address-conversion-mask. Used to obfuscate IP addresses within binary logs. Specify home-net with the '-h' switch. Note this is not the same as $HOME_NET.

-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 verbose 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 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 fileters.

-g group

Change the group/GID Snort runs under to group after initialization. This switch allows Snort to drop root priveleges after it's initialization phase has completed as a security measure.

-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 verification, 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 verification 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 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 obfuscated 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 information.

-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 tcpdump 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. Note, Snort looks for either /etc/snort.conf or ./snort.conf. If your config lives elsewhere, use the -c option to specify a valid config-file.

-u user

Change the user/UID Snort runs under to user 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 stream4's stateful inspection capabilities to reduce the amount of spoofing that may be done against Snort. By default, snort doesn't worry about the TCP state of a packet when it's about to issue an alert. The -z switch 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 qualifier, 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 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.]

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 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 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 True if the ethernet source address is ehost.

"ether True if either the ethernet source or destination address is ehost.

"gateway

True if the packet used host as a gateway. I.e., the ethernet source or destination address was host but neither 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 netmask. 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 destination 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 traffic).

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 protocol 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 conventions, 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 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:

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 (parentheses 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

not host vs and host ace

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.

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.

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 information to the cosole or syslogd(8) if in daemon mode.

Any other signal causes the daemon to close all opened files and exit.

Snort has been freely available under the GPL license since 1998.

Snort returns a 0 on a successful exit, 1 if it exits on an error.

After consulting the BUGS file included with the source distribution, send bug reports to snort-devel@lists.sourceforge.net

Martin Roesch <roesch@snort.org>

tcpdump(1), pcap(3)