An Introduction to Snort

Richard BejtlichTaoSecurity

Houston ISSA Meeting 11 Apr 02

Outline Introduction to Intrusion Detection What is Snort? Installing Snort Snort Rules Snort in Action Third-Party Enhancements Conclusion

About Me Bejtlich = “bate-lik” Senior engineer for managed network

security operations, BATC (2001-) Former captain at US Air Force Computer

Emergency Response Team (1998-2001) Student of intrusion detection, incident

response, and digital forensics www.taosecurity.com

Introduction to Intrusion Detection Network defense or “protection” model

Planning Prevention Detection Response

Intrusion detection is the process of discovering, analyzing, and reporting unauthorized or damaging network or computer activities

Introduction to Intrusion Detection All network and computer activities fall in

one of three categories: Normal Abnormal but not malicious Malicious

Intrusion detection operates in two arenas: What a computer says: network traffic What a computer thinks: computer processes

Introduction to Intrusion Detection Network traffic is monitored by network-

based intrusion detection systems (NIDS) Computer processes are monitored by host-

based intrusion detection systems (HIDS) So-called “hybrid” systems examine

network traffic to or from a host, as well as processes on that host

NIDS are easier to deploy and manage, but HIDS may give greater visibility to events

What is Snort? Snort is a fast, flexible, small-footprint,

open-source NIDS developed by the security community and a “benevolent dictator”

Lead coder: Marty Roesch, now founder of Sourcefire (www.sourcefire.com)

Initially developed in late 1998 as a sniffer with consistent output, unlike protocol-dependent output of TCPDump

Licensed under GPL, but version 2.0 may change to a different license

Installing Snort As of 16 Mar 02, version 1.8.4 available at

www.snort.org/dl/ in these forms: UNIX source Solaris, FreeBSD, Windows binary packages Red Hat .rpm

Requires installation of libpcap first! UNIX:

www.tcpdump.org/release/libpcap-0.7.1.tar.gz or newest available source

Windows: netgroup-serv.polito.it/winpcap/install/ offers winpcap 2.2 and 2.3 beta (XP requires 2.3)

Installing Snort On Red Hat Linux 7.2, as root:

Download and install libpcap Download and install these three .rpm:

libnet-1.0.2a-1snort.i386.rpm snort-1.8.4-1snort.i386.rpm snort-postgresql+flexresp-1.8.4-1snort.i386.rpm

Create /var/log/snort directory Files installed:

/etc/snort contains conf and rule files /var/log/snort will contain logs /usr/sbin/snort contains snort binary

Installing Snort For a quick test, execute this command

within the /etc/snort directory: snort –A console

From a separate machine, use nmap to generate events for Snort to detect: nmap –sP <snort_machine_IP_address>

You should see an alert like this:

03/27-15:18:06.911226 [**] [1:469:1] ICMP PING NMAP [**] [Classification: Attempted Information Leak] [Priority: 2] {ICMP} 192.168.1.20 -> 192.168.1.237

Installing Snort On Windows 2000, as administrator:

Download and install winpcap Download and execute Snort184Win32.exe,

and select “typical” installation mkdir “c:\Program Files\Sourcefire\Snort\log”

Files installed in c:\Program Files\Sourcefire\Snort: snort.conf \rules directory contains rules snort.exe executable

Installing Snort To test, execute this command within the

c:\Program Files\Sourcefire\Snort directory: snort –A console You may need to specify and interface with ‘-i 2’

From a separate machine, use nmap to generate events for Snort to detect: nmap –sP <snort_machine_IP_address>

You should see an alert like this:03/27-15:18:06.911226 [**] [1:469:1] ICMP PING NMAP [**]

[Classification: Attempted Information Leak] [Priority: 2] {ICMP} 192.168.1.20 -> 192.168.1.237

Installing Snort While not an element of Snort, Ethereal

is the best open source GUI-based packet viewer

www.ethereal.com offers: Windows:

www.ethereal.com/distribution/win32/ethereal-setup-0.9.2.exe

UNIX: www.ethereal.com/download.html Red Hat Linux RPMs:

ftp.ethereal.com/pub/ethereal/rpms/

Installing Snort Also not an element of Snort, tcpdump is

a well-established CLI packet capture tool www.tcpdump.org offers UNIX source netgroup-serv.polito.it/windump/install/ offers

windump, a Windows port of tcpdump windump is helpful because it will help you see the

different interfaces available on your sensor Next slide shows two available interfaces; note only

the second is capable of sniffing snort –A console –i 2

Snort Rules Snort rules are extremely flexible and are

easy to modify, unlike many commercial NIDS

Sample rule to detect SubSeven trojan:

alert tcp $EXTERNAL_NET 27374 -> $HOME_NET any (msg:"BACKDOOR subseven 22"; flags: A+; content: "|0d0a5b52504c5d3030320d0a|"; reference:arachnids,485; reference:url,www.hackfix.org/subseven/; sid:103; classtype:misc-activity; rev:4;)

Elements before parentheses comprise ‘rule header’

Elements in parentheses are ‘rule options’

Snort Rulesalert tcp $EXTERNAL_NET 27374 -> $HOME_NET any (msg:"BACKDOOR

subseven 22"; flags: A+; content: "|0d0a5b52504c5d3030320d0a|"; reference:arachnids,485; reference:url,www.hackfix.org/subseven/; sid:103; classtype:misc-activity; rev:4;)

alert action to take; also log, pass, activate, dynamic tcp protocol; also udp, icmp, ip $EXTERNAL_NET source address; this is a variable – specific IP is

ok 27374 source port; also any, negation (!21), range (1:1024) -> direction; best not to change this, although <> is allowed $HOME_NET destination address; this is also a variable here any destination port

Snort Rulesalert tcp $EXTERNAL_NET 27374 -> $HOME_NET any (msg:"BACKDOOR

subseven 22"; flags: A+; content: "|0d0a5b52504c5d3030320d0a|"; reference:arachnids,485; reference:url,www.hackfix.org/subseven/; sid:103; classtype:misc-activity; rev:4;)

msg:”BACKDOOR subseven 22”; message to appear in logs flags: A+; tcp flags; many options, like SA, SA+, !R, SF* content: “|0d0…0a|”; binary data to check in packet;

content without | (pipe) characters do simple content matches reference…; where to go to look for background on this rule sid:103; rule identifier classtype: misc-activity; rule type; many others rev:4; rule revision number other rule options possible, like offset, depth, nocase

Snort Rules

bad-traffic.rules exploit.rules scan.rules finger.rules ftp.rules telnet.rules smtp.rules rpc.rules rservices.rules dos.rules ddos.rules dns.rules tftp.rules web-cgi.rules web-coldfusion.rules web-frontpage.rules web-iis.rules web-misc.rules web-attacks.rules sql.rules x11.rules icmp.rules netbios.rules misc.rules backdoor.rules shellcode.rules policy.rules porn.rules info.rules icmp-info.rules virus.rules local.rules attack-responses.rules

Snort Rules Rules which actually caught intrusions

alert tcp $EXTERNAL_NET any -> $SQL_SERVERS 1433 (msg:"MS-SQL xp_cmdshell - program execution"; content: "x|00|p|00|_|00|c|00|m|00|d|00|s|00|h|00|e|00|l|00|l|00|"; nocase; flags:A+; classtype:attempted-user; sid:687; rev:3;) caught compromise of Microsoft SQL Server

alert tcp $EXTERNAL_NET any -> $HTTP_SERVERS 80 (msg:"WEB-IIS cmd.exe access"; flags: A+; content:"cmd.exe"; nocase; classtype:web-application-attack; sid:1002; rev:2;) caught Code Red infection

alert tcp $EXTERNAL_NET any -> $HOME_NET 21 (msg:"INFO FTP \"MKD / \" possible warez site"; flags: A+; content:"MKD / "; nocase; depth: 6; classtype:misc-activity; sid:554; rev:3;) caught anonymous ftp server

Snort Rules More rules that caught intrusions

alert tcp $EXTERNAL_NET any -> $HTTP_SERVERS 80 (msg:"WEB-IIS multiple decode attempt"; flags:A+; uricontent:"%5c"; uricontent:".."; reference:cve,CAN-2001-0333; classtype:web-application-attack; sid:970; rev:2;) caught NIMDA infection

alert tcp $HOME_NET 23 -> $EXTERNAL_NET any (msg:"TELNET Bad Login"; content: "Login incorrect"; nocase; flags:A+; classtype:bad-unknown; sid:1251; rev:2;) caught telnet username brute-force

Consider how a rule with “Login incorrect” might trigger on many non-malicious events

Snort in Action Three operational modes:

Sniffer: snort –dvae will be display payloads, be verbose, display arp traffic, and display link layer data

Packet logger: snort –b –l /var/log/snort will log binary data to the /var/log/snort directory

NIDS: snort –b –l /var/log/snort –A full –c /etc/snort/snort.conf will log binary data in the /var/log/snort directory, with full alerts in /var/log/snort/alert, reading the configuration file in /etc/snort

Snort in Action Three ways to tell Snort how to act

snort.conf configures variables, preprocessors, output plugins, and active rule sets

.rules files define actual signatures Command line options start snort and will

override options in snort.conf file Snort will run with default snort.conf, but

you will have less ‘chaff’ to sort through if you spend time configuring snort.conf properly

Snort in Action Snort as NIDS (third mode) captures recon Contents of /var/log/snort/alert

[**] [1:469:1] ICMP PING NMAP [**][Classification: Attempted Information Leak] [Priority: 2]03/28-09:48:40.739935 192.168.1.2 -> 192.168.1.3ICMP TTL:46 TOS:0x0 ID:61443 IpLen:20 DgmLen:28Type:8 Code:0 ID:10629 Seq:0 ECHO[Xref => http://www.whitehats.com/info/IDS162]

[**] [1:469:1] spp_portscan: PORTSCAN DETECTED from 192.168.1.2 (THRESHOLD 4 connections exceeded in 0 seconds) [**]03/28-09:48:41.052635

[**] [100:2:1] spp_portscan: portscan status from 192.168.1.2: 183 connections across 1 hosts: TCP(183), UDP(0) [**]03/2809:48:45.007501

Snort in Action Partial contents of

/var/log/snort/portscan.logMar 28 09:48:41 192.168.1.2:45061 -> 192.168.1.3:106 SYN ******S* Mar 28 09:48:41 192.168.1.2:45061 -> 192.168.1.3:193 SYN ******S* Mar 28 09:48:41 192.168.1.2:45061 -> 192.168.1.3:138 SYN ******S* Mar 28 09:48:41 192.168.1.2:45061 -> 192.168.1.3:128 SYN ******S* Mar 28 09:48:41 192.168.1.2:45061 -> 192.168.1.3:156 SYN ******S* Mar 28 09:48:41 192.168.1.2:45061 -> 192.168.1.3:35 SYN ******S* Mar 28 09:48:41 192.168.1.2:45061 -> 192.168.1.3:48 SYN ******S* Mar 28 09:48:41 192.168.1.2:45061 -> 192.168.1.3:16 SYN ******S* Mar 28 09:48:41 192.168.1.2:45061 -> 192.168.1.3:173 SYN ******S* Mar 28 09:48:41 192.168.1.2:45061 -> 192.168.1.3:72 SYN ******S* Mar 28 09:48:41 192.168.1.2:45061 -> 192.168.1.3:65 SYN ******S* Mar 28 09:48:41 192.168.1.2:45061 -> 192.168.1.3:36 SYN ******S* Mar 28 09:48:41 192.168.1.2:45061 -> 192.168.1.3:149 SYN ******S* Mar 28 09:48:41 192.168.1.2:45061 -> 192.168.1.3:17 SYN ******S* Mar 28 09:48:41 192.168.1.2:45061 -> 192.168.1.3:218 SYN ******S*

Snort in Action /var/log/snort/snort-0328\@0948.log is

in binary format; read with Ethereal or tcpdump

Sample tcpdump output of this log file

09:48:40.739935 192.168.1.2 > 192.168.1.3: icmp: echo request

09:48:40.743705 192.168.1.2 > 192.168.1.3: icmp: echo request

As configured, Snort will only log the packets which triggered an alert

Packets triggering portscans are not logged by default

Third-Party Enhancements SnortSnarf

www.silicondefense.com/software/snortsnarf/

SnortSnarf is a Perl program to take files of alerts from the Snort to produce HTML reports

Output intended for diagnostic inspection Silicon Defense also supplies sensors with

commercial support Description and screenshot taken from SnortSnarf

web

Third-Party Enhancements Analysis Console for Intrusion

Databases (ACID) acidlab.sourceforge.net/ PHP-based analysis engine to search and

process a database of security events generated by various IDSes, firewalls, and network monitoring tools

Query-builder and search interface, packet viewer (decoder), alert management, chart and statistics generation

Description and screenshots taken from ACID web

Demarc www.demarc.com NIDS management console, integrating Snort

with the convenience and power of a centralized interface for all network sensors

Monitor all servers / hosts to make sure network services such as a mail or web servers remain accessible at all times

Monitor system logs for anomalous log entries that may indicate intruders or system malfunctions

Description and screenshots taken from demarc web

Conclusion Snort is a powerful tool, but maximizing its

usefulness requires a trained operator Becoming proficient with network intrusion

detection takes 12 months; “expert” 24-36? Snort is considered a superior NIDS when

compared to most commercial systems Managed network security providers should

collect enough information to make decisions without calling clients to ask what happened