This document takes you through the basics of intrusion detection, the steps necessary to configure a host to run the snort network intrusion detection system, testing its operation, and alerting you to possible intrusion events.
Snort is a software-based real-time network intrusion detection system developed by Martin Roesch that can be used to notify an administrator of a potential intrusion attempt. The ever-increasing amount of Internet crackers, armed with “ready-to-run” exploits, as well as the sophisticated attacker that’s intent on defacing your web page necessitates the use of a method to track their activity and alert you to this.
Until now, intrusion detection devices were either dedicated-use commercial products, or not real-time and difficult to install. Snort is the solution for monitoring small TCP/IP networks where it is not cost-effective to deploy commercial products. Snort is an easy-to-use, “lightweight”, and very functional alternative.
What is Network Intrusion Detection?
A Network Intrusion Detection System (NIDS) is a system that is responsible for detecting anamolous, inappropriate, or other data that may be considered unauthorized occuring on a network. Unlike a firewall, which is configured to allow or deny access to a particular service or host based on a set of rules. If the traffic matches an acceptible pattern, it is permitted regardless of what the packet contains. However, an NIDS captures and inspects all traffic, regardless of whether it’s permitted or not. Based on the contents, at either the IP or application level, an alert is generated.
Snort is a “lightweight” NIDS in that it is non-intrusive, easily configured, utilizes familiar methods for rule development, and takes only a few minutes to install. Snort currently includes the ability to detect more than 1100 potential vulnerabilities. Keep in mind that Intrusion Detection devices work in conjunction with other security measures, and are not a replacement for other good security practices.
It is also quite feature-packed right out of the box. Among its features include the ability to:
- Detect and alert based on pattern matching for threats including buffer overflows, stealth port scans, CGI attacks, SMB probes and NetBIOS queries, NMAP and other portscanners, well-known backdoors and system vulnerabilities, DDoS clients, and many more;
- Use syslog, SMB “WinPopUp” messages, or a file to alert an administrator;
- Develop new rules quickly once the pattern (attack signature) is known for the vulnerability;
- Record packets in their human-readable form from the offending IP address in a hierarchial directory structure.
- Used as a “passive trap” to record the presence of traffic that should not be found on a network, such as NFS or Napster connections;
- Used on an existing workstation to monitor a home DSL connection, or on a dedicated server to monitor a corporate web site
Snort uses the popular libpcap library, the same library that tcpdump uses to perform its packet sniffing. Snort decodes all the packets passing by on the network to which it’s attached by entering promiscous mode. Based upon the content of the individual packets and the rules defined in the configuration file, an alert is generated.
Why Use Intrusion Detection?
Intrusion detection devices are an integral part of any network. The Internet is constantly evolving, and new vulnerabilities and exploits are found regularly. They provide an additional level of protection to detect the presence of an intruder, and help to provide accoutability for the attacker’s actions.
Before Installing Snort
There are a few things you should determine before you install snort.
The device may be placed outside an organization’s firewall between the firewall and the external untrusted network. This allows snort to detect not only the attacks that may make it through the firewall, but also those that are blocked by the firewall.
The presence of switches, routers and firewalls will all have an effect on the correct placement of the box. A decision must be made as to which network segment will catch the traffic you actually want to monitor. Placement of the NIDS on the local side of the firewall will allow the NIDS to monitor traffic that the firewall has already determined to be permissable, but not necessarily benign. This will, of course, not catch traffic that the firewall has already blocked, potentially masquerading port scans, probes and other types of attack.
The easiest configuration is a box with a single interface. The same interface that listens to the network traffic is the same one from which administration is done.
Images Courtesy Network Flight Recorder
This will be the typical configuration for home network users and administrators monitoring internal networks.
In a dual-interface configuration, one interface is used to listen to network traffic in promiscuous mode while the other is used for remote administration. This type of configuration is used in environments where it is not possible to administrate the box from the same interface that is listening to the network traffic.
In this configuration, the external interface should be well-protected and the box designed explictly for this purpose. The box should not be offering any network services except for ssh on the internal interface only.
Images Courtesy Network Flight Recorder
Installation of Snort
INSTALL included with the distribution contains information on installing and configuration snort from the downloadable source code. It is very easy to compile, configure, and install.
If you are a Red Hat or compatible user, a precompiled RPM file is available from https://www.linuxsecurity.com/programs/snort-22.214.171.124-1.i386.rpm. You will also need the
libpcap-0.4 package which is included with all Linux distributions. The source code and other information are downloadable from ftp://ftp.ee.lbl.gov/libpcap.tar.Z.
Installing the Snort Ruleset
After snort is installed, you’ll want to download the latest rules file. Currently there are two different rulesets that people use. A ruleset developed by Jim Forster can be downloaded from https://www.snort.org/snort-files.htm#Rules
Another ruleset, developed as part of Max Vision’s ArachNIDS work, is available from https://dev.whitehats.com/ids/vision.conf and updated hourly.
The Max Vision ruleset is particularly nice because it follows the Common Vulnerabilities and Exposures (CVE) database, allowing people to refer to a particular vulnerability using a consistent name. From the CVE Frequently Asked Questions:
“CVE is a list of information security vulnerabilities and exposures that aims to provide common names for publicly known problems. The goal of CVE is to make it easier to share data across separate vulnerability databases and security tools with this “common enumeration.” “
If you’ve installed the snort RPM, the
/usr/sbin/snort-update script written by Dave Dittrich can be used to download the latest Max Vision ruleset from cron:
00 00 * * * root /usr/sbin/snort-update -q
It can also be downloaded from https://www.linuxsecurity.com/programs/snort-update
This assumes the
wget package is installed, the local box can download the file at https://dev.whitehats.com/ids/vision.conf, and it can successfully deliver mail to an administrator.
snort-update script will place the
vision.conf file in
/etc/snort/vision.conf.new and an email notification will be sent to the local
root account with the differences from the previous version, if any. You must then rename the
You might also consider using the snort.org ruleset in addition to the Max Vision ruleset. This can be achieved by downloading the snort.org ruleset. Information on combining these rulesets together is included below.
overflow-lib and other similar files are included with the source code, but are pretty dated and not typically used.
Before snort can be started, a few variables must be defined. Also included in the snort RPM is a file called
rules.base which was derived from Max Vision’s
vision.conf file. It is a short file that contains a few variables that define your internal and external networks, hosts that snort should ignore portscans from, and on which networks snort should watch for portscans. It can also be downloaded from https://www.linuxsecurity.com/programs/rules.base.
A portscan is defined as TCP connection attempts to more than P ports in T seconds or UDP packets sent to more than P ports in T sections. Read Martin’s “Writing Snort Rules” for a full description of portscan.
You’ll need to supply the information for your INTERNAL and EXTERNAL networks, and from DNS servers which tend to trigger the portscan detection.
rules.base file looks as follows:
# # Taken and modified from "vision.conf", part of Max Vision's # ArachNIDs work. See /usr/doc/snort-1.6/README.snort-stuff for more # information on how to use this file. var INTERNAL 192.168.1.0/24 var EXTERNAL 126.96.36.199/24 var DNSSERVERS 188.8.131.52/32 184.108.40.206/32 preprocessor http_decode: 80 443 8080 preprocessor minfrag: 128 preprocessor portscan-ignorehosts: $DNSSERVERS preprocessor portscan: $EXTERNAL 3 5 /var/log/snort/portscan.log # | # Log file (path/name) ----------------------------------+ # Ruleset, available (updated hourly) from: # # https://dev.whitehats.com/ids/vision.conf # Include the latest copy of Max Vision's ruleset include /etc/snort/vision.conf # # Uncomment the next line if you wish to include the latest # copy of the snort.org ruleset. Be sure to download the latest # one from https://www.snort.org/snort-files.htm#Rules # # include /etc/snort/06082k.rules # # If you wish to monitor multiple INTERNAL networks, you can include # another variable that defines the additional network, then include # the snort ruleset again. Uncomment the two following lines. # # var INTERNAL 192.168.2.0/24 # include /etc/snort/vision.conf # include other rules here if you wish.
If you are on a dialup machine, the INTERNAL setting would be your dialup interface with a
/32 subnet mask, indicating the host itself.
Additionally, you may need to configure
syslogd to log snort and other security events to a specific log file. Edit
/etc/syslog.conf to log snort alerts, then signal
syslogd to re-read the
authpriv.* /var/log/secure.log [[email protected] ~]# /usr/bin/killall -HUP syslogd
preprocessor directives listed above are used to examine the data flow before the intrusion detection engine applies the ruleset to the packets. This can be used to modify the contents of the packet, or to signal the detection engine to not process a particular packet.
preprocessor http_decode: directive instructs the detection engine to convert the data within HTTP URI strings into a format that defeats attempts at eluding the content analysis strings used to examine HTTP traffic for suspicious activity. Supply the ports on which a web server is running.
preprocessor portscan: directive is used to define the host or network for which snort should watch for a portscan. The <>/32 appearing after the IP addresses indicates CIDR notation for a 32-bit subnet mask (the host itself). For a Class C network, use
It may also be necessary to exclude some hosts from triggering the portscan detection module. Boxes that generate legitimately generate traffic on a large number of ports in a short amount of time, including NFS or DNS servers are typical candidates. More on Patrick Mullen’s portscan preprocessor can be found at https://www.linuxrc.org/projects/snort/.
portscan-ignorehosts preprocessor is used for this, and includes the hosts to ignore as its argument seperated by spaces:
preprocessor portscan-ignorehosts: 220.127.116.11/32 18.104.22.168/32
You might consider trying it without the
portscan-ignorehosts directive first, as you don’t want to unnecessarily disable the ability to detect portscans if it’s not necessary.
minfrag preprocessor checks for fragmented packets which can be used to disguise TCP packets from IP filters used in routers and hosts. Packets less than 512 bytes do not need to be fragmented with modern networks, and packets found smaller than that typically indicate attempts to subvert a firewall or intrusion detection.
You should now be able to test your configuration file and the normal operation of snort:
Press ctrl-C to stop snort for now. At this point you should be able to start snort using the normal startup procedure. If you need to start snort manually, use the following:
[[email protected] ~]# /usr/sbin/snort -s -d -D -i eth0 -l /var/log/snort -c /etc/snort/rules.base [[email protected] ~]#
-D switch is used to have snort run in the background as a daemon. You may need to supply the correct interface name if you are not using eth0. Edit the
INTERFACE variable in the
/etc/rc.d/init.d/snortd file if you have installed the snort RPM. If you’ve installed the snort RPM, you can start snort with the following:
[[email protected] ~]# /etc/rc.d/init.d/snortd Starting snort: [ OK ] [[email protected] ~]# ps awx | grep snort 9131 ? S 0:04 /usr/sbin/snort -s -d -D -i eth0 -l /var/log/snort -c /etc/snort/rules.base 9149 pts/1 S 0:00 grep snort [[email protected] ~]#
After you believe snort is working correctly, you should do what an intruder might do, and attack your own network. Notice that qualifier — your own network!. My attack host (
192.168.100.189 and the victim host is
192.168.200.189. With nmap in hand, using the decoy feature, you might run the following as root:
192.168.200.189 with the IP address of a host on your network. The
/var/log/auth.log file would then show something similiar to:
Jun 18 15:57:52 krypton snort: spp_portscan: PORTSCAN DETECTED from 22.214.171.124 Jun 18 15:57:52 krypton snort: spp_portscan: PORTSCAN DETECTED from 126.96.36.199 Jun 18 15:57:58 krypton snort: spp_portscan: portscan status from 192.168.1.100: 2 connections across 1 hosts: TCP(2), UDP(0) STEALTH Jun 18 15:57:58 krypton snort: spp_portscan: portscan status from 188.8.131.52: 2 connections across 1 hosts: TCP(2), UDP(0) STEALTH Jun 18 15:57:58 krypton snort: spp_portscan: portscan status from 184.108.40.206: 2 connections across 1 hosts: TCP(2), UDP(0) STEALTH
/var/log/snort directory contains a hierarchial listing with each host having it’s own directory beneth which contains a file detailing the information that makes up the intrusion attempt. For example:
[[email protected] ~]# cd /var/log/snort [[email protected] snort]# find 192.168.200.189 192.168.100.189 192.168.100.189/ICMP_ECHO 192.168.100.189/ICMP_PORT_UNRCH 192.168.100.189/TCP:57554-32771 192.168.100.189/TCP:57555-32771 [[email protected] ~]#
Looking within the
TCP:57554-32771 file reveals:
[[email protected] 192.168.100.189]# cat TCP:57554-32771 ** MISC-Attempted Sun RPC high port access ** 06/18-00:48:31.928357 192.168.100.189:57554 -> 192.168.200.189:32771 TCP TTL:42 TOS:0x0 ID:5410 ***F*P*U Seq: 0x0 Ack: 0x0 Win: 0x400 00 00 00 00 00 00 ......
syslog entry looks like:
Jun 18 00:48:31 krypton snort: MISC-Attempted Sun RPC high port access: 192.168.100.189:57554 -> 192.168.200.189:32771
Several perl scripts exist to process this information into an HTML page which can be accessed periodically to determine what is occuring on the network. The snort2html is a perl script written by Dan Swan that does a nice job of producing useable information in HTML format. If you decide to save this in a directory accessable by a web server, be sure to apply the proper authentication mechanism to prevent outsiders viewing this file. It contains a great deal of helpful information to a potential intruder. It can be used as follows:
[[email protected] ~]# /usr/bin/snort2html /var/log/auth.log
This will produce an HTML file in
/var/log/snort-log.html that is for use with the Max Vision ruleset and creates links to the CVE definition of each intrusion attempt. This version has been modified from the original to accept the filename from the command-line. You can edit this file to save the resulting HTML file in another location.
- Lance Spitzner shows snort in action as he tracks an intruder and performs forensic analysis on the trails of the intruder.
- Fyodor has put together a document that describes the snort internals
- The snort download page includes pointers to source code, plugins to send snort output data directly to a database, the Win32 port, and the latest rulesets.
- The snort.org has a wealth of helpful information and documentation.
Have questions or comments about this document? We’d love to discuss them with you- please leave a comment below!