fwsnort(8) Firewall Snort


fwsnort [options]


fwsnort translates SNORT rules into iptables rules on Linux systems and generates a corresponding iptables policy in iptables-save format. This ruleset allows network traffic that matches Snort signatures (i.e. attacks and other suspicious network behavior) to be logged and/or dropped by iptables directly without putting an interface into promiscuous mode or queuing packets from kernel to user space. Note that fwsnort can also build an iptables policy that combines the string match extension with the NFQUEUE or QUEUE targets to allow the kernel to perform preliminary string matches that are defined within Snort rules before queuing matching packets to a userspace snort_inline instance. Because the bulk of network communications are not generallly malicious, this should provide a speedup for snort_inline since the majority of packets do not then have to be copied from kernel memory into user memory and subsequently inspected by snort_inline. There is a tradeoff here in terms of signature detection however because snort_inline when deployed in this way does not have the opportunity to see all packets associated with a session, so stream reassembly and signature comparisons against a reassembled buffer do not take place (the stream preprocessor should be disabled in the userspace snort_inline instance).

As of fwsnort-1.5 all iptables rules built by fwsnort are written out to the /var/lib/fwsnort/fwsnort.save file in iptables-save format. This allows a long fwsnort policy (which may contain thousands of iptables rules translated from a large Snort signature set) to be quickly instantiated via the "iptables-restore" command. A wrapper script /var/lib/fwsnort/fwsnort.sh is also written out to make this easy. Hence, the typical work flow for fwsnort is to: 1) run fwsnort, 2) note the Snort rules that fwsnort was able to successfully translate (the number of such rules is printed to stdout), and then 3) execute the /var/lib/fwsnort/fwsnort.sh wrapper script to instantiate the policy in the running kernel.

fwsnort (optionally) uses the IPTables::Parse CPAN module to parse the iptables ruleset on the machine to determine which Snort rules are applicable to the specific iptables policy. After all, if iptables is blocking all inbound http traffic from external addresses for example, it is probably not of much use to try detecting inbound attacks against against tcp/80. By default fwsnort generates iptables rules that log Snort sid's within a --log-prefix to syslog where the messages can be analyzed with a log analyzer such as psad (see http://www.cipherdyne.org/psad/). fwsnort relies on the iptables string match module to match Snort content fields in the application portion of ip traffic. Since Snort rules can contain hex data in content fields (specified between pipe "|" characters), fwsnort implements a patch against iptables (which has been accepted by the Netfilter project as of iptables-1.2.7a) which adds a "--hex-string" option. This allow iptables to accept content fields from Snort rules such as "|0d0a5b52504c5d3030320d0a|" without any modification. fwsnort is able to translate approximately 60% of all rules from the Snort-2.3.3 IDS into equivalent iptables rules. For more information about the translation strategy as well as advantages/disadvantages of the method used by fwsnort to obtain intrusion detection data, see the README included with the fwsnort sources or browse to: http://www.cipherdyne.org/fwsnort/

fwsnort is able to apply Snort rules to IPv6 traffic by building an ip6tables policy (see the "--ip6tables" command line argument).


-c, --config <configuration file>
By default fwsnort makes use of the configuration file /etc/fwsnort/fwsnort.conf for almost all configuration parameters. fwsnort can be made to override this path by specifying a different file on the command line with the --config option. When fwsnort is not executed as root, then a path to a readable fwsnort.conf file is required.
Download the latest Emerging Threats rules from http://www.emergingthreats.net This will overwrite the emerging-all.rules file in the /etc/fwsnort/snort_rules/ directory. Note that the automatic downloading of Snort rules from http://www.snort.org/ as of March, 2005 is only offered as a pay service.
--rules-url  <url>
Specify the URL to use when updating the Emerging Threats rule set (or any other rule set). The default URL is: http://rules.emergingthreats.net/open/snort-2.9.0/emerging-all.rules
-6, --ip6tables
Enable ip6tables mode so that the fwsnort rule set is built into an ip6tables policy instead of the iptables policy. This allows fwsnort controls to apply to IPv6 traffic.
--include-type  <rules type>
Restrict to processing snort rules of <rules type>. Example rule types would include "ddos", "backdoor", and "web-attacks". This option also supports a comma-separated list of types, e.g. "ddos,backdoor".
--exclude-type  <rules type>
Exclude all Snort rules from of type <rules type> from the translation process. For example, if you don't want any rules from the file emerging-all.rules to be translated, then use "emerging-all" as the argument to this option. A comma-separated list of types to exclude can be specified.
--include-regex  <regex>
Only translate Snort rules that match the specified regular expression. This is useful to build fwsnort policies for Snort rules that have a common characteristic (such as a string match on the word "Storm" for the Storm worm for example).
--exclude-regex  <regex>
Translate all Snort rules except those that match the specified regular expression. This is useful to omit Snort rules from fwsnort policies that have a common characteristic (such as a string match on "HTTP_PORTS" for example).
Make the rule matchine regular expression specified with --include-regex match case insensitively.
Make the rule matchine regular expression specified with --exclude-regex match case insensitively.
--snort-rdir <snort-rules-directory>
Manually specify the directory where the snort rules files are located. The default is /etc/fwsnort/snort_rules. Multiple directories are supported as a comma-separated list.
--snort-rfile <snort-rules-file>
Manually specify a Snort rules file to translated into iptables rules. Multiple files are also supported as a comma-separated list.
--snort-sid <sid>
Generate an iptables ruleset for a single snort rule specified by <sid>. A comma-separated list of sids can be specified, e.g. "2001842,1834".
--exclude-sid <sid>
Provide a list of Snort ID's to be excluded from the translation process.
Include 'perl -e print ...' commands as comments in the fwsnort.sh script. These commands allow payloads that are designed to trigger snort rules to easily be built, and when combined with netcat (or other software that can send bytes over the wire) it becomes possible to test whether an fwsnort policy appropriately triggers on matching traffic.
--ipt-script <script file>
Specify the path to the iptables script generated by fwsnort. The default location is /var/lib/fwsnort/fwsnort.sh.
Check iptables capabilities and exit.
Run fwsnort with the same command line arguments as the previous execution. This is a convenient way of rebuilding the /var/lib/fwsnort/fwsnort.sh script without having to remember what the last command line args were.
Build an fwsnort policy that sends packets that match Snort content or uricontent fields to userspace via the iptables NFQUEUE target for further analysis. This is a mechanism for reducing the signature inspection load placed on snort_inline. A parallel set of Snort rules that are successfully translated are placed in the /etc/fwsnort/snort_rules_queue directory. This requires CONFIG_NETFILTER_XT_TARGET_NFQUEUE support in the Linux kernel.
Same as the --NFQUEUE command line argument except that the older QUEUE target is used instead of the NFQUEUE target. This requires CONFIG_IP_NF_QUEUE support in the Linux kernel.
--queue-num <num>
Specify a queue number in --NFQUEUE mode.
--queue-pre-match-max <num>
In --QUEUE or --NFQUEUE mode, limit the number of content matches that are performed within the kernel before sending a matching packet to a userspace Snort instance. This allows a level of tuning with respect to how much work the kernel does to qualify a packet based on a signature match before having Snort do the same thing. The default is to perform all specified content matches in the signature before queuing the packet to userspace because the multiple in-kernel content matches is probably less expensive than sending a packet to userspace by default.
--string-match-alg <alg>
Specify the string matching algorithm to use with the kernel. By default, this is 'bm' for the 'Boyer-Moore' string matching algorithm, but 'kmp' may also be specified (short for the 'Knuth–Morris–Pratt' algorithm).
Execute the iptables script generated by fwsnort.
Synonym for --ipt-apply.
Revert to a version of the iptables policy without any fwsnort rules. Note that this reverts to the iptables policy as it was when fwsnort was originally executed. So, it is not recommended to use this option if there is a large amount of time between when fwsnort is run to translate Snort rules vs. running it with this option. For most purposes it is better to use the --ipt-flush option below.
Flush all fwsnort currently active iptables rules (flushes the fwsnort chains).
List all fwsnort currently active iptables rules (lists the fwsnort chains).
For each logging rule generated by fwsnort add a corresponding DROP rule. Note that for TCP sessions using this option will cause retransmissions as packets that are part of established sessions selectively dropped. Remember that false positives are common occurrences for intrusion detection systems, and so using this or the --ipt-reject option may break things on your network! You have been warned.
For each logging rule generated by fwsnort add a corresponding REJECT rule. Reset packets will be generated for TCP sessions through the use of the "--reject-with tcp-reset" option, and ICMP port unreachable messages will be generated for UDP packets through the use of the "--reject-with icmp-port-unreachable" option.
-C, --Conntrack-state <state>
Specify a conntrack state in place of the "established" state that commonly accompanies the Snort "flow" keyword. By default, fwsnort uses the conntrack state of "ESTABLISHED" for this. In certain corner cases, it might be useful to use "ESTABLISHED,RELATED" instead to apply application layer inspection to things like ICMP port unreachable messages that are responses to real attempted communications.
By default fwsnort generates an iptables script that implements a logging rule for each successfully translated snort rule. This can be disabled with the --no-ipt-log option, but --ipt-drop must also be specified.
This is a deprecated option since the default behavior is to translate as many Snort rules into iptables rules as possible. With fwsnort able to produce iptables rules in iptables-save format, it is extremely fast to instantiate a large set of translated Snort rules into an iptables policy. A new --ipt-sync option has been added to reverse this behavior (not recommended).
Consult the iptables policy currently running on the machine for applicable snort rules.
Do not test the iptables build for existence of support for the LOG and REJECT targets, and ascii and hex string matching.
Do not jump packets from the built-in iptables INPUT, OUTPUT, and FORWARD chains to the custom fwsnort chains. This options is mostly useful to make it easy to manually alter the placement of the jump rules in the iptables ruleset.
By default fwsnort includes the rule number within the logging prefix for each of the rules it adds to the fwsnort chains. E.g. the logging prefix for rule 34 would look something like "[34] SID1242 ESTAB". Use this option to not include the rule number.
If the iptables "comment" match exists, then fwsnort puts the Snort "msg", "classtype", "reference", "priority", and "rev" fields within a comment for each iptables rule. Use this option to disable this.
Do not jump packets from the iptables INPUT chain to the fwsnort chains.
Do not jump packets from the iptables OUTPUT chain to the fwsnort chains.
Do not jump packets from the iptables FORWARD chain to the fwsnort chains.
Cause fwsnort to not try to reorder pattern matches to process the longest pattern first. The Snort fast_pattern keyword is also ignored if this option is specified.
-H, --Home-net <network/mask>
Specify the internal network instead of having fwsnort derive it from the HOME_NET keyword in the fwsnort.conf configuration file.
-E, --External-net <network/mask>
Specify the external network instead of having fwsnort derive it from the EXTERNAL_NET keyword in the fwsnort.conf configuration file.
Disable all checks against the output of ifconfig for proper IP addresses. This is useful if fwsnort is running on a bridging firewall.
Print the fwsnort configuration on STDOUT and exit.
Run in debug mode. This will cause all parse errors which are normally written to the fwsnort logfile /var/log/fwsnort.log to be written to STDOUT instead.
Run fwsnort in "strict" mode. This will prevent fwsnort from translating snort rules that contain the keywords "offset", "uricontent", and "depth".
-U, --Ulog
Force the usage of the ULOG target for all log messages instead of the default LOG target.
Specify the netlink group for ULOG rules. Such rules are only added for Snort rules that have an action of "log", or when fwsnort is run in --Ulog mode.
-l, --logfile <logfile>
By default fwsnort logs all parse errors to the logfile /var/log/fwsnort.log. This path can be manually changed with the --logfile option.
-v, --verbose
Run fwsnort in verbose mode. This will cause fwsnort to add the original snort rule as a comment to the fwsnort.sh script for each successfully translated rule.
-V, --Version
Print the fwsnort version and exit.
-h, --help
Print usage information on STDOUT and exit.


The fwsnort configuration file. The path to this file can be changed on the command line with --config.


The iptables script generated by fwsnort. The path can be manually specified on the command line with the --ipt-script option.


This section describes what each of the more important fwsnort configuration variables do and how they can be tuned to meet your needs. These variables are located in the fwsnort configuration file /etc/fwsnort/fwsnort.conf
fwsnort uses the same HOME_NET and EXTERNAL_NET variables as defined in Snort rules, and the same semantics are supported. I.e., individual IP addresses or networks in standard dotted-quad or CIDR notation can be specified, and comma separated lists are also supported.
Defines the external network. See the HOME_NET variable for more information.


The following examples illustrate the command line arguments that could be supplied to fwsnort in a few situations:

Script generation in logging mode, parse errors written to the fwsnort logfile, and iptables policy checking are enabled by default without having to specify any command line arguments:

# fwsnort

Generate ip6tables rules for attacks delivered over IPv6:

# fwsnort -6

Generate iptables rules for ddos and backdoor Snort rules only:

# fwsnort --include-type ddos,backdoor

Generate iptables rules for Snort ID's 2008475 and 2003268 (from emerging-all.rules):

fwsnort --snort-sid 2008475,2003268

Generate iptables rules for Snort ID's 1834 and 2001842 but queue them to userspace via the NFQUEUE target and restrict exclude the INPUT and OUTPUT chains:

fwsnort --snort-sid 1834,2001842 --NFQUEUE --no-ipt-INPUT --no-ipt-OUTPUT

Instruct fwsnort to only inspect traffic that traverses the eth0 and eth1 interfaces:

# fwsnort --restrict-intf eth0,eth1

Generate iptables rules for Snort rules that appear to be allowed by the local iptables policy, and write original snort rules to the iptables script as a comment:

# fwsnort --ipt-sync --verbose


fwsnort requires that the iptables string match module be compiled into the kernel (or as a loadable kernel module) in order to be able to match snort signatures that make use of the "content" keyword. Note that the --no-opt-test option can be specified to have fwsnort generate an iptables script even if the string match module is not compiled in.

fwsnort also requires the IPTables::Parse CPAN module in order to parse iptables policies. This module is bundled with the fwsnort sources in the deps/ directory for convenience.


The --debug option can be used to display on STDOUT any errors that are generated as fwsnort parses each snort rule. Normally these errors are written to the fwsnort logfile /var/log/fwsnort.log


Michael Rash <[email protected]>


Many people who are active in the open source community have contributed to fwsnort; see the CREDITS file in the fwsnort sources, or visit http://www.cipherdyne.org/fwsnort/docs/contributors.html to view the online list of contributors.

fwsnort is based on the original snort2iptables script written by William Stearns.


Send bug reports to [email protected]. Suggestions and/or comments are always welcome as well.


fwsnort is distributed under the GNU General Public License (GPLv2), and the latest version may be downloaded from http://www.cipherdyne.org/ Snort is a registered trademark of Sourcefire, Inc.