Introduction to Network IDS
Linux User Group Singapore
Friday 7th May 2004
What we will cover:
What to expect from a Network IDS
How to physically connect a Network IDS
Where to connect the Network IDS
Different types of Network IDS
Interoperability between different vendors NIDS
What false positives / false negatives are
Classify network events using severity ratings
Q & A
Why Network Intrusion Detection?
Prevention is ideal, but detection is a must.
Provides forensic capabilities of network traffic
Compare with CCTV camera and recording equipment.
Side effect: You learn more about your network and discover protocols, services and other resource stealing objects.
Think about this before installing a Network IDS
Keep your system up-to-date with patches?
Remove unneeded services?
Configured IPTables to protect your host(s)?
Actually read (and understand) the IPTables generated log files?
If not, you are not ready to do Network IDS.
NIDS will not protect you against malicious traffic.
NIDS will generate even more logs.
NIDS will take a considerable amount of time to configure properly, and is several times more complicated compared to IPTables.
How to connect your
A NIDS can be connected to a network in 3 ways
Using a switch' SPAN port
Using a hub
Using a network TAP
Replicates cable signals for TX pair to two new cables
Have additional power to boost network signal
Expensive (can cost over S$1000 per unit)
Requires 2 NIC on the NIDS
Can only monitor one link
( TAP = Test Administrative Port)
Inside a Network TAP
Device ANetwork IDSDevice B
Can anyone spot the problem?
Can anyone spot the problem with this TAP design?
Did you figure it out?
The problem with this tap is:
If the sum of the streams collected from the two inputs exceeds the capacity of the single output, packets are dropped. Whoops!
Using a switch SPAN port
SPAN: Switched Port Analyzer (Cisco)
Known as mirror port (port mirror) by other vendors
Mirrors one or more port data to a 2nd port.
Can monitor several ports at the same time
No extra cost if hardware is already available
Not all switches supports SPAN ports
Creates additional load on switch (decreased switch performance)
Drop packets if you try to push too much traffic to a single port
Using a HUB
Ideal for home network / ADSL / cable connection for playing around
Packet collisions (packet drop)
10/100 hubs requires extra care (10 and 100 Mbit links does not propagate)
Where to connect your NIDS
Different types of NIDS
Looks for fingerprints of vulnerabilities or exploits
Signature database needs to be kept up-to-date
Creates a profile of normal network traffic
Suspicious events can be defined in various ways
RFC compliance checking
Traffic doesn't comply with normal traffic criteria.
The fact that protocols are well defined makes the use of Protocol Analysis a strong contender, but many implementations of protocols fail to follow their respective RFC.
False Positives / False Negatives
False Positives / False Negatives Explained
False positive: Alert generated for non-malicious traffic
The biggest published drawback with NIDS
Having too many false positives and the analyst(s) will be tired looking at them.
Can be reduced with tuning
False negative: Alert not generated for malicious traffic
Even more dangerous then false positives, as you don't get alerted on it.
Can be reduced with tuning
Result in it's absence
The level of certainty provided by detector when receive warning of possible event
The capability detector has for extensive and complex analysis in locating possible attacks
Network IDS Interoperability
Network IDS has been, and in large extent still is, vendor proprietary technology.
Signatures are written in different ways for different vendors.
This is starting to change, more and more NIDS products are at least incorporating part of the Snort signature language.
Alerts are sent and stored in vendor proprietary formats
IDMEF / IDXP
Intrusion Detection Message Exchange Format
Internet draft (proposed RFC) by IDWG
Uses IDXP (Intrusion Detection Exchange Protocol) for transport, also a proposed RFC, by IDWG
IDWG = Intrusion Detection Working Group, appointed by IETF
Released by ICSA Labs in February 2004
Cisco Systems, ISS, Sourcefire and TruSecure Corporation co-developed the SDEE transport protocol specification format
Is not really free... See next slide..
What thing particularly made me looking quite negatively at the SDEE spec is the ICSAlab involvement. I contacted the iscalab people on the day when SDEE was officially out, with a question of joining ids forum and contributing to the SDEE review. The response to my mail included an invoice for 9,000+ USD (5k for general forum membership, and 4k for the IDS cntm).
Snort discussion forum on Orkut,
21 March 2004
SANS Institute has developed the following formula to classify how bad an attack effects the target:
Where each item has a value between 1 and 5 assigned to it.
SANS Severity Ratings
System Countermeasures+Network Countermeasures
SANS Severity Ratings (cont'd)
Criticality: How critical is the target to the rest of the network or operations?
Lethality: How dangerous is the attack?
System Countermeasures: What countermeasures has been been implemented on the system to defend against this threat?
Network Countermeasures: What countermeasures has been implemented on the network to defend against this threat?
What have we learned?
Network IDS is resource intensive
Placement of the NIDS depends on what you want to monitor
The difference between a signature based and a abnormality based NIDS
IDMEF, SDEE and proprietary alert and storage formats
Calculate network event's severity level
Got any questions? Now is the time to ask them!
Recommended reading material
TCP/IP Illustrated Vol. 1
W. Richard Stevens; ISBN: 0201633469
Network Intrusion Detection (3rd ed)
Stephen Northcutt, Judy Novak; ISBN: 0735712654
Intrusion Signatures and Analysis
Stephen Northcutt, Mark Cooper, Matt Fearnow, Karen Frederick; ISBN: 0735710635
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Copyright 2004 Michael Boman. All Rights Reserved.