Creator: Jed Haile <firstname.lastname@example.org>
Current Maintainer: Rob McMillen <email@example.com>
William Metcalf <firstname.lastname@example.org>
Victor Julien <email@example.com>
Snort-Inline takes packets from iptables instead of libpcap. It then uses
new rule types to help iptables make pass or drop decisions based on
In order for snort_inline to work properly, you must download and compile
the iptables code to include "make install-devel" (www.iptables.org).
This will install the libipq library that allows snort_inline to interface
with iptables. Also, you must build and install LibNet, which is available
The snort_inline FAQ can be found at http://snort-inline.com/FAQ.html.
NEW RULE TYPES AND WHAT THEY DO:
drop - The drop rule type will tell iptables to drop the packet and log it
via usual snort means.
reject - The reject rule type will tell iptables to drop the packet, log it
via usual snort means, and send a TCP reset if the protocol is
TCP or an icmp port unreachable if the protocol is UDP.
sdrop - The sdrop rule type will tell iptables to drop the packet. Nothing
RESETS: We now have two options on how to send out resets. We can still use
a RAW socket which is the default behavior for snort_inline in which case
we need to have an interface that has an ip assigned. If there is not an
interface with an ip address assigned with access to the source of the packet,
the packet will be logged and the reset packet will never make it onto the
We can also now perform resets via a physical device when using iptables.
We take the indev name from ip_queue and use this as the interface on which
to send resets. We no longer need a ip loaded on the bridge, and can remain
pretty stealthy as the config layer2_resets in snort_inline.conf takes a source
mac address which we substitue for the mac of the bridge. For example:
tells snort_inline to use layer2 rests and uses the mac address of the bridge
as the source mac in the packet.
config layer2resets: 00:06:76:DD:5F:E3
will tell snort_inline to use layer2 resets and uses the src mac of
00:06:76:DD:5F:E3 in the rest packet.
There are two additional stream4 options:
inline_state (no args)
This option causes snort to drop TCP packets that are not associated with
an existing TCP session, and is not a valid TCP initiator.
midstream_drop_alerts (no args)
By default, when running in inline mode, snort will silently drop any
packets that were picked up in midstream and would have caused an alert
to be generated, if not for the 'flow: established' option. This is to
mitigate stick/snot type attacks when the user hasn't enabled
inline_state. If the user wants to see the alerts that are silently
dropped, then enable this keyword. Note that by enabling this keyword,
the user has opened themselves up to stick/snot type attacks.
Additionally, Jed's content replace code allows you to modify packets
before they leave the network. For example:
alert tcp any any <> any 80 (msg: "tcp replace"; content:"GET"; replace:"BET";)
alert udp any any <> any 53 (msg: "udp replace"; \
content: "yahoo"; replace: "xxxxx";)
these rules will comb tcp port 80 traffic looking for GET, and udp port 53
traffic looking for yahoo. Once they are found, they are replaced with BET
and xxxxx, respectively. The only catch is that the replace must be the same
length as the content.
RULE APPLICATION ORDER:
The current Rule application order:
This will ensure that a drop rule has precedence over an alert or log rule.
Also, the -o flag now changes the rule application order to:
Mike Clark <firstname.lastname@example.org> has taken the time and energy to go through
the snort ruleset to identify and consolidate rules that would meet the
drop criteria. These rules are located in the rules directory in
drop.rules, and should be considered "alpha". There is also a sample
drop.conf file located in the etc directory of the snort_inline.tgz. The
latest and greatest set of drop rules can always be found at:
Additionally, we have included a convert.sh script in the etc/ directory
that will convert all alert rules to drop rules. All you have to do is
copy it to the location of your snort rules and run it. It will convert
all alert rules to drop rules; change HOME_NET to HONEYNET; and reverse
the flow of the rules. Why reverse the flow of the rules? Because the
original purpose of snort_inline is to control a compromised network;
therefore, we need to drop packets leaving the Honeynet not entering it.
HONEYNET SNORT_INLINE TOOLKIT
This is a statically compiled snort_inline binary put together by the
Honeynet Project for the Linux Operating System. It comes with a set
of drop.rules, the snort_inline binary, a snort-inline rotation shell
script, and a good README. It can be found at:
First, you need to ensure that the ip_queue module is loaded. Then,
you need to send traffic to snort_inline using the QUEUE target. For
iptables -A OUTPUT -p tcp --dport 80 -j QUEUE
sends all tcp traffic leaving the firewall going to port 80 to the QUEUE
target. This is what sends the packet from kernel space to user space
(snort_inline). A quick way to get all outbound traffic going to the
QUEUE is to use the rc.firewall script created and maintained by the
Honeynet Project (http://www.honeynet.org/papers/honeynet/tools/)
This script is well documented and allows you to direct packets
to snort_inline by simply changing the QUEUE variable to yes.
Finally, start snort_inline.
snort_inline -QDc ../etc/drop.conf -l /var/log/snort
-Q => get packets from iptables.
-D => run in daemon mode. pid in /var/run/snort_inline.pid
-c => read the following configuration file.
-l => log to the following directory.
Ideally, snort_inline will be run using only its own drop.rules. If
you want to use Snort for just alerting, a separate process should be
running with its own ruleset.
If you execute snort_inline, and you see something like this:
Initializing Output Plugins!
Reading from iptables
Log directory = /var/log/snort
Initializing Inline mode
InlineInit: : Failed to send netlink message: Connection refused
more than likely, the ip_queue module is not loaded or ip_queue
support is not compiled into your kernel. Either recompile
your kernel to support ip_queue, or load the module.
The ip_queue module is loaded by executing:
Also, if you want to ensure snort_inline is getting packets, you can
start it in the following manner:
snort_inline -Qvc <configuration file>
This will display the header of every packet that snort_inline sees.