open source network intrusion detection system
Capture packets with verbose output
Capture packets and dump application layer data with verbose output
Capture packets and display link layer packet headers with verbose output
Capture packets and save them in the specified directory
Capture packets according to rules and save offending packets along with alerts
snort [-bCdDeEfHIMNOpqQsTUvVwWxXy?] [-A alert-mode ] [-B address-conversion-mask ] [-c rules-file ] [-F bpf-file ] [-g group-name ] [-G id ] [-h home-net ] [-i interface ] [-k checksum-mode ] [-K logging-mode ] [-l log-dir ] [-L bin-log-file ] [-m umask ] [-n packet-count ] [-P snap-length ] [-r tcpdump-file ] [-R name ] [-S variable=value ] [-t chroot_directory ] [-u user-name ] [-Z pathname ] [--logid id ] [--perfmon-file pathname ] [--pid-path pathname ] [--snaplen snap-length ] [--help ] [--version ] [--dynamic-engine-lib file ] [--dynamic-engine-lib-dir directory ] [--dynamic-detection-lib file ] [--dynamic-detection-lib-dir directory ] [--dump-dynamic-rules directory ] [--dynamic-preprocessor-lib file ] [--dynamic-preprocessor-lib-dir directory ] [--dynamic-output-lib file ] [--dynamic-output-lib-dir directory ] [--alert-before-pass ] [--treat-drop-as-alert ] [--treat-drop-as-ignore ] [--process-all-events ] [--enable-inline-test ] [--create-pidfile ] [--nolock-pidfile ] [--no-interface-pidfile ] [--disable-attribute-reload-thread ] [--pcap-single= tcpdump-file ] [--pcap-filter= filter ] [--pcap-list= list ] [--pcap-dir= directory ] [--pcap-file= file ] [--pcap-no-filter ] [--pcap-reset ] [--pcap-reload ] [--pcap-show ] [--exit-check count ] [--conf-error-out ] [--enable-mpls-multicast ] [--enable-mpls-overlapping-ip ] [--max-mpls-labelchain-len ] [--mpls-payload-type ] [--require-rule-sid ] [--daq type ] [--daq-mode mode ] [--daq-var name=value ] [--daq-dir dir ] [--daq-list [dir] ] [--dirty-pig ] [--cs-dir dir ] [--ha-peer ] [--ha-out file ] [--ha-in file ] 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 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 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.
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.
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.
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 $HOME_NET.
Use the rules located in file config-file.
Print the character data from the packet payload only (no hex).
Dump the application layer data when displaying packets in verbose or packet logging mode.
Run Snort in daemon mode. Alerts are sent to /var/log/snort/alert unless otherwise specified.
Display/log the link layer packet headers.
*WIN32 ONLY* Log alerts to the Windows Event Log.
Activate PCAP line buffering
Read BPF filters from bpf-file. This is handy for people running Snort as a SHADOW replacement or with a love Of super complex BPF filters. See the "expressions" section of this man page for more info on writing BPF filters.
Change the group/GID Snort runs under to group after initialization. This switch allows Snort to drop root privileges after it’s initialization phase has completed as a security measure.
Use id as a base event ID when logging events.
Set the "home network" to home-net. The format of this address variable is a network prefix plus a CIDR block, such as 192.168.1.0/24. Once this variable is set, all decoded packet logging will be done relative to the home network address space. This is useful because of the way that Snort formats its ASCII log data. With this value set to the local network, all decoded output will be logged into decode directories with the address of the foreign computer as the directory name, which is very useful during traffic analysis. This option does not change "$HOME_NET" in IDS mode.
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.
Sniff packets on interface.
Print out the receiving interface name in alerts.
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.
Select a packet logging mode. The default is pcap. logging-mode. Valid logging modes include pcap, ascii, and none. Pcap logs packets through the pcap library into pcap (tcpdump) format. Ascii logs packets in the old "directories and files" format with packet printouts in each file. None Turns off packet logging.
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.
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".
Set the file mode creation mask to umask
Log console messages to syslog when not running daemon mode. This switch has no impact on logging of alerts.
Process packet-count packets and exit.
Turn off packet logging. The program still generates alerts normally.
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!
Turn off promiscuous mode sniffing.
Set the packet snaplen to snap-length. By default, this is set to 1514.
Quiet operation. Don’t display banner and initialization information.
Enable inline mode operation.
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.
Use name as a suffix to the snort pidfile.
Send alert messages to syslog. On linux boxen, they will appear in /var/log/secure, /var/log/messages on many other platforms.
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.
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.
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.
Change the user/UID Snort runs under to user after initialization.
Changes the timestamp in all logs to be in UTC
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.
Show the version number and exit.
Show management frames if running on an 802.11 (wireless) network.
*WIN32 ONLY* Enumerate the network interfaces available.
Exit if Snort configuration problems occur such as duplicate gid/sid or flowbits without Stream5.
Dump the raw packet data starting at the link layer. This switch overrides the ’-d’ switch.
Include the year in alert and log files
Set the perfmonitor preprocessor path/filename to pathname.
Show the program usage statement and exit.
Same as -G.
Same as -Z.
Specify the directory for the Snort PID file.
Same as -P.
Same as -?
Same as -V
Load a dynamic detection engine shared library specified by file.
Load all dynamic detection engine shared libraries specified from directory.
Load a dynamic detection rules shared library specified by file.
Load all dynamic detection rules shared libraries specified from 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.
Load a dynamic preprocessor shared library specified by file.
Load all dynamic preprocessor shared libraries specified from directory.
Process alert, drop, sdrop, or reject before pass. Default is pass before alert, drop, etc.
Converts drop, sdrop, and reject rules into alert rules during startup.
Use drop, sdrop, and reject rules to ignore session traffic when not inline.
Process all triggered events in group order, per Rule Ordering configuration. Default stops after first group.
Enable Inline-Test Mode Operation.
Specify the path for Snort’s PID file.
Create PID file, even when not in Daemon mode.
Do not try to lock Snort PID file.
Do not include the interface name in Snort PID file
Same as -r. Added for completeness.
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.
A space separated list of pcaps to read.
A directory to recurse to look for pcaps. Sorted in ascii order.
File that contains a list of pcaps to read. Can specify path to pcap or directory to recurse to get pcaps.
Reset to use no filter when getting pcaps from file or directory.
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.
Print a line saying what pcap is currently being read.
Signal termination after <count> callbacks from DAQ_Acquire(), showing the time it takes from signaling until DAQ_Stop() is called.
Same as -x.
Require an SID for every rule to be correctly threshold all rules.
Select packet acquisition module (default is pcap).
Select the DAQ operating mode.
Specify extra DAQ configuration variable.
Tell Snort where to find desired DAQ.
List packet acquisition modules available in dir.
Tell Snort to use control socket and create the socket in dir.
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:
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.
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.
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.
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.
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.
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.
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.
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.
True if the packet has a length less than or equal to length. This is equivalent to:
len <= 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.
True if the packet is an ethernet broadcast packet. The ether keyword is optional.
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.
True if the packet is an ethernet multicast packet. The ether keyword is optional. This is shorthand for ’ether & 1 != 0’.
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
ether proto p
where p is one of the above protocols.
lat, moprc, mopdl
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
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 & 1 != 0’ catches all multicast traffic. The expression ’ip & 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 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
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.
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.
Read a single pcap
$ snort -r foo.pcap
$ snort --pcap-single=foo.pcap
Read pcaps from a file
$ cat foo.txt
$ 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.
Read pcaps from a command line list
$ snort --pcap-list="foo1.pcap foo2.pcap foo3.pcap"
This will read foo1.pcap, foo2.pcap and foo3.pcap.
Read pcaps under a directory
$ snort --pcap-dir="/home/foo/pcaps"
This will include all of the files under /home/foo/pcaps.
$ cat foo.txt
$ snort --pcap-filter="*.pcap" --pcap-file=foo.txt
$ 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 \
> --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 \
> --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 \
> --pcap-no-filter --pcap-dir=/home/foo/pcaps \
> --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.
$ 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.
Printing the pcap
$ 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.
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.
The following signals have the specified effect when sent to the daemon process using the kill(1) command:
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).
Causes the program to dump its current packet statistical information to the console or syslogd(8) if in daemon mode.
Causes the program to rotate Perfmonitor statistical information to the console or syslogd(8) if in daemon mode.
Causes the program to reload attribute table.
Please refer to manual for more details. Any other signal might cause 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 firstname.lastname@example.org
Martin Roesch <email@example.com>