Copyright 2001 Martin Roesch, All Rights Reserved Martin Roesch Sourcefire Inc.

Copyright 2001 Martin Roesch, All Rights Reserved Topics Background –What is Snort? Using Snort Snort Architecture The Future of Snort and Snort 2.0

Copyright 2001 Martin Roesch, All Rights Reserved Background – Intrusion Detection Intrusion Detection defined: “the problem of identifying individuals who are using a computer system without authorization” –Attempts to break in also have to be identified Intrusion detection is NOT intrusion prevention!

Copyright 2001 Martin Roesch, All Rights Reserved Background – Policy Successful intrusion detection depends on policy and management as much as technology –Security Policy (defining what is acceptable and what is being defended) is the first step –Notification Who, how fast? –Response Coordination

Copyright 2001 Martin Roesch, All Rights Reserved Intro to Snort What is Snort? –Snort is a multi-mode packet analysis tool Sniffer Packet Logger Forensic Data Analysis tool Network Intrusion Detection System Where did it come from? –Developed out of my evolving need to perform network traffic analysis in both real-time and for forensic post processing

Copyright 2001 Martin Roesch, All Rights Reserved Snort “Metrics” Small (~800k source download) Portable (Linux, Windows, MacOS X, Solaris, BSD, IRIX, Tru64, HP-UX, etc) Fast (High probability of detection for a given attack on 100Mbps networks) Configurable (Easy rules language, many reporting/logging options Free (GPL/Open Source Software)

Copyright 2001 Martin Roesch, All Rights Reserved Snort Design Packet sniffing “lightweight” network intrusion detection system Libpcap-based sniffing interface Rules-based detection engine Plug-in system allows endless flexibility

Copyright 2001 Martin Roesch, All Rights Reserved Detection Engine Rules form “signatures” Modular detection elements are combined to form these signatures Wide range of detection capabilities –Stealth scans, OS fingerprinting, buffer overflows, back doors, CGI exploits, etc. Rules system is very flexible, and creation of new rules is relatively simple

Copyright 2001 Martin Roesch, All Rights Reserved Plug-Ins Preprocessor –Packets are examined/manipulated before being handed to the detection engine Detection –Perform single, simple tests on a single aspect/field of the packet Output –Report results from the other plug-ins

Copyright 2001 Martin Roesch, All Rights Reserved Uses for Snort Standard packet sniffing NIDS Policy Enforcement Honeypot monitor Scan detection/traps

Copyright 2001 Martin Roesch, All Rights Reserved IDS Implementation Map Filtering Router (Perimeter Logs) Firewall (Perimeter Logs) Generic Server (Host-Based ID) (Snort 2.0) Network IDS (Snort) Internet Honeypot (Deception System) Statistical IDS (Snort)

Copyright 2001 Martin Roesch, All Rights Reserved Using Snort Three main operational modes –Sniffer Mode –Packet Logger Mode –NIDS Mode –(Forensic Data Analysis Mode) Operational modes are configured via command line switches –Snort automatically tries to go into NIDS mode if no command line switches are given, looks for snort.conf configuration file in /etc

Copyright 2001 Martin Roesch, All Rights Reserved Using Snort – Sniffer Mode Works much like tcpdump Decodes packets and dumps them to stdout BPF filtering interface available to shape displayed network traffic

Copyright 2001 Martin Roesch, All Rights Reserved What Do The Packet Dumps Look Like? =+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+ 11/09-11:12: :1032 -> :23 TCP TTL:128 TOS:0x0 ID:31237 IpLen:20 DgmLen:59 DF ***AP*** Seq: 0x16B6DA Ack: 0x1AF156C2 Win: 0x2217 TcpLen: 20 FF FC 23 FF FC 27 FF FC 24 FF FA E 53..#..'..$....ANS 49 FF F0 I.. =+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+ 11/09-11:12: :23 -> :1032 TCP TTL:255 TOS:0x0 ID:49900 IpLen:20 DgmLen:61 DF ***AP*** Seq: 0x1AF156C2 Ack: 0x16B6ED Win: 0x2238 TcpLen: 20 0D 0A 0D 0A E 4F E 37 0D 0A 0D....SunOS D 0A 0D =+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+

Copyright 2001 Martin Roesch, All Rights Reserved How is it different from tcpdump? 11:16: > : P 16:34(18) ack 16 win 8760 (DF) (ttl 255, id 49913) a c2f ff06 a2b4 0a a cf9 e7f6 001a e c fffe 1fff fe23 fffe 27ff fe24 fffa 11:16: > : P 16:19(3) ack 34 win 8727 (DF) (ttl 128, id 57861) b e b8 0a a a e050 1cf9 e f fffc 1f

Copyright 2001 Martin Roesch, All Rights Reserved Packet Logger Mode Gee, it sure would be nice if I could save those packets to disk… Multi-mode packet logging options available –Flat ASCII, tcpdump, XML, database, etc available Log all data and post-process to look for anomalous activity

Copyright 2001 Martin Roesch, All Rights Reserved NIDS Mode Uses all phases of Snort + plug-ins to analyze traffic for both misuse detection and anomalous activity Can perform portscan detection, IP defragmentation, TCP stream reassembly, application layer analysis and normalization, etc

Copyright 2001 Martin Roesch, All Rights Reserved NIDS Mode… Various output options available –Database (MySQL, PostgreSQL, Oracle, unixODBC, etc) –XML (snml DTD from CMU/CERT) –Tcpdump binary format –Unified (Snort specific) format –ASCII, syslog, WinPopup (SMB) –Etc.

Copyright 2001 Martin Roesch, All Rights Reserved NIDS Mode… Wide variety of rules available for signature engine (~1300 as of June 2001) Multiple detection modes available via rules and plug-ins –Rules/signature –Statistical anomaly –Protocol verification

Copyright 2001 Martin Roesch, All Rights Reserved Snort Architecture

Copyright 2001 Martin Roesch, All Rights Reserved Snort 1.x Data Flow Packet Decoder Preprocessor (Plug-ins) Detection Engine (Plug-ins) Output Stage (Plug-ins) Packet Stream Sniffing Snort Data Flow Alerts/Logs

Copyright 2001 Martin Roesch, All Rights Reserved Snort 1.x Architecture Snort’s existing architecture for the 1.x series of code is a study in organic software development Snort’s evolution –Sniffer->packet logger->NIDS Speed by subsystem –Decode = very fast –Detection engine = fast –Output/preprocessor modules = implementation dependent

Copyright 2001 Martin Roesch, All Rights Reserved Snort 1.x Detection Engine Implemented as a 3-dimensional linked list –Dimensions 1 & 2 contain data nodes to be tested against current packet –Dimension 3 contains linked lists of function pointers to test the node’s data against the packet –Entire engine is walked recursively –Very fast, very robust –“First exit” detection strategy First detect causes engine to perform rule action & then go on to next packet

Copyright 2001 Martin Roesch, All Rights Reserved Rule Header Alert tcp any -> any Rule Options (flags: SF; msg: “SYN-FIN Scan”;) Alert tcp any -> any (flags: S12; msg: “Queso Scan”;) (flags: F; msg: “FIN Scan”;) Detection Engine: Rules

Copyright 2001 Martin Roesch, All Rights Reserved Alert tcp any -> any Rule Node (flags: SF; msg: “SYN-FIN Scan”;) (flags: S12; msg: “Queso Scan”;) (flags: F; msg: “FIN Scan”;) Option Node Detection Engine: Internal Representation

Copyright 2001 Martin Roesch, All Rights Reserved Rule Node Rule Node Rule Node Rule Node Rule Node Option Node Option Node Option Node Option Node Option Node Option Node Option Node Option Node Option Node Option Node Option Node Detection Engine: Fully Populated

Copyright 2001 Martin Roesch, All Rights Reserved Snort 1.x Performance and Flexibility Development process lead to very high speed decoding and stateless intrusion detection How fast is it? –Configuration dependent, but 100Mbps is not too difficult for Snort to manage Flexibility made Snort the platform of choice for a number of applications in the R&D space –Govt and University researchers frequently use Snort as a rapid prototyping platform for new ideas in intrusion detection

Copyright 2001 Martin Roesch, All Rights Reserved Snort 1.x Limitations Snort is an IP-centric program Packet analysis –IP defragmentation and TCP stream reassembly are via the preprocessor interface –Internal data structures don’t scale well for addition of new protocols NOTE: Adding new protocol support is not hard, just a little clunky –Application layer is not decoded by packet decoder Left for pattern analysis in detection engine

Copyright 2001 Martin Roesch, All Rights Reserved Snort 1.x Limitations Detection Engine & Preprocessors –Revelation: Not everyone is as concerned with performance as I am! –Not all preprocessors are created equal –Adding additional protocol support to detection engine is not well modularized Adding “IP” rules support took about 7 lines of code, but knowing which 7 required me to do it –Rules description language is limited at the protocol level Easy to describe IP/TCP/UDP/ICMP/IGMP/Etc, hard to describe HTTP, RPC, SMTP, etc

Copyright 2001 Martin Roesch, All Rights Reserved Snort 1.x Limitations Output –People have a really nasty tendency to write slow output plug-ins! –Variable output formats mean performance is highly variable based on the selected output modes –No way to control Snort’s performance effectively, leading to negative reviews and user “Snort’s eating 90% of the CPU!?!”

Copyright 2001 Martin Roesch, All Rights Reserved Snort 2.0 Architecture Basic goals –Faster –More extensible –Better protocol support –Better able to analyze the full gestalt of network intrusion activity

Copyright 2001 Martin Roesch, All Rights Reserved Snort 2.0 Plug-Ins More of them for more flexibility –Data acquisition –Traffic decoders Full protocol analysis and verification Multi-path traffic flows, packet and stream –Multi-format rules input DB, XML, etc –Pluggable detection engines Standard NIDS, Target-based IDS, Statistical IDS, Host- based IDS

Copyright 2001 Martin Roesch, All Rights Reserved Snort 2.0 Improvements Improved detection & pattern matching capabilities –Aho-Corasick/Boyer-Moore implementation from Silicon Defense –LANL/RADIANT Team work on set-wise Boyer-Moore-Horspool algorithm –~500% in pattern matching performance improvement reported in research work!

Copyright 2001 Martin Roesch, All Rights Reserved Snort 2.0 Improvements Spooling output stage –Write Snort alert/log data to spool files, have a secondary process (‘barnyard’) read the spools and reformat for final output –Output plug-ins attach to barnyard instead of being directly linked to Snort main code Deterministic performance measurements and focused performance improvement will be possible through this method

Copyright 2001 Martin Roesch, All Rights Reserved Snort 2.0 Detection Engine Far more self-optimizing than 1.x –Rules will be “treed” to a greater extent –Most tests will be performed only once More rules can be loaded with less impact on the overall performance of the program Speed and structure of engine will allow “last-exit” detection strategy to be used

Copyright 2001 Martin Roesch, All Rights Reserved Snort 2.0 Detection Engine Comparison – V 1.x Sip: Dip: Dp: 80 (flags: A+; content: “”foo”;) (flags: A+; content: “bar”;) (flags: A+; content: “baz”;) alert tcp

Copyright 2001 Martin Roesch, All Rights Reserved Snort 2.0 Detection Engine Comparison – V 2.0 content: “”foo”; content: “bar”; content: “baz”; alerttcp Dip: Dip: /24 Flags: A+; Sip: Dp: 80

Copyright 2001 Martin Roesch, All Rights Reserved Acquisition Plugins Libpcap allows us to be very cross platform but is also a bottleneck Acquisition plugins allow arbitrary data input sources Interesting applications –Netfilter/divert socket input stream –Gateway IDS… –Host-based IDS… High speed platform specific acquistion capability

Copyright 2001 Martin Roesch, All Rights Reserved Decoder Plugins Arbitrary protocol support in Snort Snort is currently limited to… –Ethernet, FDDI, T/R, SLIP, PPP, ISDN, Raw –IP, ARP –TCP, UDP, ICMP With plug-ins, new decoders can be painlessly dropped into Snort, automatically making Snort “aware” of that protocol and capable of performing traffic analysis on it Additional support for “unknown” protocols will have to be added to the detection engine

Copyright 2001 Martin Roesch, All Rights Reserved Pluggable Detection Engines Current signature based engine isn’t necessarily the only way to do NID The current primary detection engine in Snort is really just a very involved preprocessor Other possibilities –Snort + Netfilter (or Divert Sockets) = Gateway IDS (or “packet scrubber”) –Snort + NMAP = Target-based IDS –Snort + SAS = Statistical Anomaly IDS (ok, just kidding)

Copyright 2001 Martin Roesch, All Rights Reserved Learning More –Writing Snort Rules –FAQ, USAGE file, README file, man page –Snort mailing lists Books –Intrusion Detection: An Analysts Handbook by Northcutt –Intrusion Signatures and Analysis by Northcutt –The Practical Intrusion Detection Handbook by Paul Proctor

Copyright 2001 Martin Roesch, All Rights Reserved FIN Martin Roesch Get Snort – –Win32 version Get Snort Rules – Commercial Snort Tech Support and Info – Commercial Snort Network Security Appliances – Security Info – –packetstorm.securify.com – – –Many more