Distributed Port Scan Detection

Distributed Port Scan Detection

Himanshu Singh and Robert Chun

12

Contents

12.1 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 221

12.2 Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22212.2.1 Port scanning . . . . . . . . . . . . . . . . . . . . . . . 22212.2.2 Classification of Scans . . . . . . . . . . . . . . . 223

12.3 Motivation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22312.3.1 Design Considerations . . . . . . . . . . . . . . . 22312.3.2 Related Work . . . . . . . . . . . . . . . . . . . . . . . 224

12.4 Approach . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22512.4.1 Simulation Environment . . . . . . . . . . . . . 22512.4.2 TCP Scanner . . . . . . . . . . . . . . . . . . . . . . . 22512.4.3 Packet Sniffer . . . . . . . . . . . . . . . . . . . . . . . 22712.4.4 Detector . . . . . . . . . . . . . . . . . . . . . . . . . . . 22712.4.5 Network Topology . . . . . . . . . . . . . . . . . . 230

12.5 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 230

12.6 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 231

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 233

The Authors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 234

Conventional network intrusion detection systems(NIDS) have heavyweight processing and memoryrequirements as they maintain per flow state usingdata structures such as linked lists or trees. This isrequired for some specialized jobs such as statefulpacket inspection (SPI) where the network commu-nications between entities are recreated in their en-tirety to inspect application-level data. The down-side to this approach is that the NIDS must be ina position to view all inbound and outbound traf-fic of the protected network. The NIDS can be over-whelmed by a distributed denial of service attacksince most such attacks try and exhaust the avail-able state of network entities. For some applications,

such as port scan detection, we do not need to recon-struct the complete network traffic.Wepropose inte-grating a detector into all routers so that a more dis-tributed detection approach can be achieved. Sincerouters are devices with limited memory and pro-cessing capabilities, conventional NIDS approachesdo notworkwhile integrating a detector in them.Wedescribe a method to detect port scans using aggre-gation. A data structure called a partial completionfilter (PCF) or a counting Bloom filter is used to re-duce the per flow state.

12.1 Overview

Scanning activity is regarded to be a threat by thesecurity community – an indicator of an imminentattack. Panjwani et al. found that 50% of all scanningactivity was followed by an attack [12.1].

Incidents of computer break-in and sensitive in-formation being compromised are fairly common.Utility providers using information technology forefficient management of resources across increas-ingly greater regions are vulnerable to service dis-ruption by electronic sabotage of their centralizedsystems [12.2].

Attack programs search for openings in a network,much as a thief tests locks on doors. Once inside,these programs and their human controllers can ac-quire the same access and powers as a systems ad-ministrator [12.3].

There are substantial financial gains to be madefrom electronic theft of data. Government comput-ers were the target of an espionage network whichcompromised thousands of official systems world-wide [12.4]. The attacker with the greatest techni-cal sophistication is the professional criminal or the

221© Springer 2010

, Handbook of Information and Communication Security(Eds.)Peter Stavroulakis, Mark Stamp

222 12 Distributed Port Scan Detection

cyber terrorist. Sophisticated adversaries are risk-averse and may go to great lengths to hide theirtracks [12.5].This is because detection may provokea response by the defender – either retaliation orupgrading of the defenses. One of the tactics usedin warfare is reconnaissance or information gath-ering. Reconnaissance can be nontechnical – socialengineering and dumpster diving – or technical –scanning the target’s network and monitoring traf-fic [12.6].

The method of determining the services avail-able on a computer by sending packets to severalports is called port scanning [12.7]. Further com-munication on the ports that services are availablecan determine the vulnerability to any available ex-ploit and is termed vulnerability scanning.The scan-ning packets traverse the target network and so arevisible to any network application such as an intru-sion detection system (IDS). This may cause themto be detected. Avoiding detection by the IDS can beas simple as insertion of a time delay between scan-ning packets, thereby defeating most thresholding-based IDS algorithms. However, this is not efficientas it slows down the scanning activity. For a more ef-ficient approach, other methods have evolved, suchas coordinated/distributed port scans. These dividethe target space amongmultiple source IPs such thateach source IP scans a portion of the target. The IDSmay not detect this activity owing to the small num-ber of connection attempts, or if it does, then it maynot be able to detect the collaboration between thesource machines.

Early detection and reaction to potential intrud-ers is made possible by the detection of port scans,stealthy or coordinated port scans. Cohen [12.8] de-termines optimal defender strategies by simulatingcomputer attacks and defenses. He finds that re-sponding quickly to an attack is the best strategy thata defender can employ. A quick response is betterthan having a highly skilled and multilevel defensein place, but an increased response time to an attack.

Problem statement Scalable port scan detection –in a nutshell, we would like to use aggregation tech-niques to scalably detect distributed port scanningactivity by fast-spreading Internet worms and vali-date the detector using a simulator [12.9]

Organization Section 12.2 is a primer on typesof scans and detectors. In Sect. 12.3 we present ourmotivation and related work in port scan detection.

Section 12.4 introduces the detector that we havebuilt. Section 12.5 is an analysis of the data gener-ated by the simulation of the detection algorithm.Our conclusion is presented in Sect. 12.6.

12.2 Background

Port scanning is a method of determining the avail-able services on a computer by sending packets. Itis generally viewed as a reconnaissance activity orinformation gathering phase distinct from the at-tack phase. This implies that there will be a gap be-tween the scan and the attack. But there are no tech-nical reasons for separating the reconnaissance ac-tivity from the attack phase when fast propagationis a key consideration. This can be achieved with anintegrated scan and exploit tool. There is a trade-offbetween between the speed and stealth of the scan-ning activity.The motivation of the attacker dictatesthe choice between speed and stealth. Fast propaga-tion is a kind of brute force scan/attack and is easilydetected by the target network security personnel.Some scanning activity is immediately followed byan attack.This is probably to take advantage of zero-day exploits.

12.2.1 Port scanning

A listening service on anetworkhost is referencedbythe combination of its host IP address and the boundport number. A port is a logical address on a ma-chine. There are 65,536 TCP and 65,536 UDP portson amachine.These are split into three ranges by theInternet Assigned Numbers Authority [12.10]:

1. Well-known ports, from 0 through 1,0232. Registered ports, from 1,024 through 49,1513. Dynamic and/or private ports, from 49,152

through 65,535

Port Scanning is the process of identifying some orall open ports (listening services) on one or morehosts [12.11].

A port scanmay be the precursor to an actual attack,so it is essential for the network administrator to beable to detect it when it occurs.

A simple port scan by itself does not harm thehost as it concentrates on the well-known ports, andis done in a sequentialmanner. If, on the other hand,

12.3 Motivation 223

enough such simultaneous connection attempts aremade, the host’s resourcesmay get exhausted and itsperformance may be adversely affected, as the con-nection state has to be maintained. Clearly, this canbe used as a denial Of service attack.

To detect and prevent port scanning, various ID-S/intrusion prevention systems (IPS) are used. TheIDS/IPS identifies multiple connection requests ondifferent ports from a single host and automaticallyblocks the corresponding IP address. The best ex-ample of this kind of IDS/IPS is Snort [12.11]. Dis-tributed port scanning is used to evade detectionand avoid the corresponding black listing of thesource machine by the target host/network.

A conventional port scan targets a single or a fewchosen hosts,with a limited subset of carefully cho-sen ports. This type of scan is slow and is generallyused on prechosen targets, so its IP coverage focusis narrow. A specific type of port scan called a sweeptargets whole IP ranges, but only one or two ports.Here the objective is to quickly cover as many hostsas possible, so its IP coverage focus is broad. Thissweep behavior is generally exhibited by a worm oran attacker looking for a specific vulnerable service.

12.2.2 Classification of Scans

Scans can be classified by their footprint (Fig. 12.1),which is nothing but the set of IP/port combinationsthat is the focus of the attacker. The footprint is in-dependent of how the scan was conducted or the

a.b.0.0 a.b.255.255Destination IP addresses

65,535

0

Destinationports

Strobe scan Verticalscan

Horizontal scan

Fig. 12.1 Conceptual geomet-ric pattern of common scanfootprints [12.13]

script of the scan [12.12]. Staniford et al. note that themost common footprint is a horizontal scan. Theyinfer that this is due to the attacker being in posses-sion of an exploit and interested in any hosts whichexpose that service. This footprint results in a scanwhich covers the port of interest across all IP ad-dresses within a range. Horizontal scans may alsobe indicative of a network mapping attempt to findavailable hosts in a range of IP addresses. Scans onsome or all ports of a single host are termed verticalscans. The target is more specific here and the pur-pose is to findout if the host exposes any service withan existing exploit. A combination of horizontal andvertical scans is termed a block scan of multiple ser-vices on multiple hosts [12.12].

12.3 Motivation

Wedeveloped a distributed port scannerwhichusedproxy response fingerprinting based on a presenta-tion at the RSA 2006 conference [12.11]. We usedthe free open application proxy Squid [12.14] as theintermediary and implemented the scanner in Perl.

12.3.1 Design Considerations

There are a lot of variables that require careful con-sideration while designing a detector. We make thefollowing assumptions about the operating condi-tions of the detector:

224 12 Distributed Port Scan Detection

• A medium-sized to large network with multiplegateways and quite possibly delegated adminis-trative authority.

• The core network administrators require fast de-tection and logging of any distributed scanningactivity. However, there will be no automated re-sponse to any flagged scanning activity (no autoban or blacklisting). The flagged activity detailswill be handed over to the administrators of theaffected networks. This will avoid issues such asblocking traffic from legitimate IP addresses ow-ing to spoofing of their IP addresses by the scan-ners. This kind of denial of service can theoreti-cally be prevented by a whitelist, but it requiresa substantial administrative overhead to main-tain the whitelist.

• The amount of network data captured or storedfor consumption by the detector must be sub-stantially smaller than the original amount.

Considering the above operating conditions, onecan obtain the detector characteristics:

• It operates on packet-level summaries.• It operates in real time as it has access to all the

required packet summaries immediately. Flow-level data can only be obtained when the flow isfinished and the information is purged to stor-age. This can take a long time as the flow dura-tion varies greatly.This forces any detector basedon flow-level data to be non-real-time.

• It is stateless in nature. Inspecting application-level data requires the storage of complete pack-ets and their reassembly, requiring the detectorto maintain state. We do not require storage orreassembly of packets as we just need the sum-maries. We can see that the storage requirementsfor these summaries is based on the volume ofpackets. A way to decouple the storage require-ments with the traffic volume is to use aggrega-tion.

12.3.2 RelatedWork

The network security monitor [12.15] was the pi-oneering NIDS. Its scan detection rules detectedany source IP address which attempted to connectto more than 15 hosts. A time window is not ex-plicitly mentioned in the paper. Since then, mostNIDS have used a variant of this thresholding al-

gorithm – N scans over M hosts in T seconds intheir scan detection engine. A detector using a fixedthreshold is easy to circumvent once the threshold isknown.

Snort has a preprocessor for detecting port scansbased on invalid flag combinations or exceedinga preset threshold. Scans which abuse the TCP pro-tocol such as NULL scans, Xmas tree scans and syn-chronize (SYN)–finish (FIN) scans can be detectedby their invalid TCP flag combinations. Scans whichuse valid flags can be detected by a threshold mech-anism. Snort is configured by default to generate analarm only if it detects a single host sending SYNpackets to four different ports in less than 3 s [12.16].

Bro also uses thresholding to detectscans [12.17]. A single source attempting to contactmultiple destination IP addresses is considereda scanner if the number of destinations exceedsa preset threshold. A vertical scan is flagged bya single source contacting more than the thresh-old number of destination ports. Paxson indicatesthat this method generates false positives, such asa single source client contacting multiple internalWeb servers. To reduce the number of false posi-tives, Bro uses packet and payload information forapplication-level analysis.

Staniford et al. use simulated annealing to de-tect stealthy and distributed port scans [12.12].Packets are initially preprocessed by Spade, whichflags packets as normal or anomalous. Spice usesthe packets flagged as anomalous and places themin a graph, with connections formed using simu-lated annealing. Packets which are most similar toeach other are grouped together. This approach isused in the detection of port scans. Scans whichproduce highly anomalous packets are consideredstraightforward to detect by a simple rule-basedengine and are ignored. The focus of researchersis on full connect scans, SYN scans, and UDPscans where the individual packets could mas-querade as normal traffic. Techniques such asslowing down and randomizing scan order, in-terprobe timing, nonessential fields, and theireffects on the detection algorithm are discussed.The algorithm is run off-line on network tracesand is designed to detect stealth or low-rate scan-ning.

Threshold random walk developed by Jung etal. requires information if a particular host andservice are available on the target network [12.18].This information is obtained by analysis of return

12.4 Approach 225

traffic or through an oracle. A sequential hypothesistesting is applied on new connection requests thatarrive to determine whether a source is performinga scan. The assumption is that a destination is morelikely to respond with a SYN-acknowledgement(ACK) to a benign source (legitimate connectionrequests are generally from clients who are awareof the services that exist on the destination) thanto a scanner source. The threshold random walkalgorithm requires only five connection attemptsto distinct IP addresses by a scanner for a suc-cessful detection, compared with 13 for Snort.Scalability is an issue as the algorithm needs to keeptrack of all the distinct connections on a per hostbasis.

Kompella et al. focus on scalable TCP flood at-tack detection by aggregating the per flow state intoa data structure they call a partial completion filter(PCF) [12.19]. The PCF data structure is similar tothat of a counting Bloom filter [12.20, 21]. State canbe evicted from the PCF, unlike with Bloom filters,where this is not possible.A smaller filter can be usedas a result of state eviction.

12.4 Approach

12.4.1 Simulation Environment

We selected OMNeT++ [12.9, 22] as the simulationenvironment. OMNeT++ is a discrete event simu-lator with support for network simulation using theINET framework [12.23].

There is a distinct separation of form/structureand function/behavior in the OMNeT++ simulator.Simulations are made up of modules. There are twotypes of modules: simple and compound (Fig. 12.2).A simple module is composed of its structure (de-fined in the NED programming language), which isnothing but a container with gates or connectionswith which it communicates with other modules.The behavior of a simple module is defined by itsC++ implementation.

Simple modulesNetwork

Compound module

Fig. 12.2 Simple and compound modules

12.4.2 TCP Scanner

TCP has a very complex state diagram (seeFig. 12.3). The setup of a TCP connection requiresa three-way handshake. The listening applicationis informed only when the handshake is success-ful [12.7].

Several types of TCP scanning methods are usedin the field [12.7]:

• TCP connect() scanning• TCP SYN (half-open) scanning• TCP FIN (stealth) scanning• Xmas and NULL scans• ACK andWindow scans• Reset (RST) scans.

A TCP connect() scan completes the three-wayhandshake and is logged as a connection attemptby the application. This scan is easy to implementand does not require root privileges. The port isconsidered open when the connection is establishedand closed if the connection attempt fails. Thescanner sends a SYN packet, receives a SYN-ACK toacknowledge the connection, followed by an ACKby the scanner to complete the connection setup.The connection is then torn down by a FIN from thescanner. This method is only used in port scanningwhen the scan is run with user privileges. The moretypical usage is to probe the application-level serviceversion as part of a vulnerability scan.

A TCP SYN (half-open) scan is the most popu-lar type of port scan when root privileges are possi-ble. The scan does not show up in the application-level logs since the three-way TCP handshake isnot completed. It stops the TCP connection openprocess midway after the first response from theserver, so is known as the half-open scan. The scan-ner sends a SYN packet to the target. If the re-sponse is a SYN-ACK, the port is open. A closedport causes the target operating system to respondwith a RST-ACK. If the response received was SYN-ACK, the scanner responds with a RST to abort theconnection.The advantage of this method is that thescan leaves no trace in the application-level servicelogs.

If there is no response from the target port, theport could be filtered, which means that a firewall isdropping all SYN-ACK packets to the closed port.If that is the case, then the FIN scan can be used.The firewall rule set will generally allow all inboundpackets with a FIN to pass through without ex-

226 12 Distributed Port Scan Detection

CLOSED

LISTEN

ESTABLISHED

CLOSE_WAIT

CLOSE_ACK

CLOSEDTIME_WAIT

CLOSINGFIN_WAIT_2

FIN_WAIT_1

SYN_WAIT_1 SYN_SENT

Timeout after twosegment lifetimes

ACK

ACK

ACK

ACKFIN/ACK

FIN/ACK

Close/FIN

Close/FIN

Close/FIN

Send/SYN

Active open/SYN

SYN/SYN+ACK

FIN/ACK

SYN+ACK/ACK

SYN/SYN+ACK

CloseClosePassive open

Fig. 12.3 TCP state diagram [12.24]

ception. When the scanner sends a FIN packet toa closed port, then the response will be a RST. If theport is open, then no response will be received.

There are several variations of the FIN scan. Ina Xmas scan, the URG, PSH, and FIN flags are set.In a NULL scan, none of the flags are set. In bothcases the sequence number is 0.

ACK scans are used to determinewhichports arefiltered by the firewall by sending a packet to a portwith only theACKflag set. ARST response indicatesthat the port is unfiltered and is accessible remotely.If no response is received or if an ICMP unreachableresponse is received, then the port is filtered by thefirewall.

We implemented a distributed TCP port scan-ner in the OMNeT++ simulation environment.Thescanner supports the TCP SYN (half-open) type ofscan. The algorithm of the scanner is shown in Al-gorithm 12.1.

Algorithm 12.1 TCP scanner

Input : Number o f s c a nn e r s nInput : L i s t o f IP / po r t p a i r s P

f o r e v e r y s c anne rp o r t s P e r S c ann e r = | P | / | n |wh i l e p o r t s P e r S c a nn e r > 0 do

send SYNi f r e c v (SYN+ACK) then

po r t OPENsend RST

end i fi f r e c v (SYN+RST ) then

po r t CLOSEDend i fi f r e c v (TIMEOUT) then

po r t FILTEREDend i fp o r t s P e r S c ann e r

= p o r t s P e r S c ann e r − 1end wh i l e

12.4 Approach 227

12.4.3 Packet Sniffer

Specific packet fields serve as an input to the IDS forgeneration of the packet summary information. Werequire the following fields from every incoming IPpacket on all the router interfaces:

1. Source IP2. Destination IP3. Source port4. Destination port5. SYN6. FIN7. ACK8. RST.

We can extract the source IP and the destination IPfrom the IP packet header (see Fig. 12.4). The otherfields are from the encapsulated TCP packet header(see Fig. 12.5).

The packet sniffer is notifiedwhenever there is anincoming packet on any interface. It is programmed

0

4

8

12

16

20

1 20 3Byteoffset

10 1 2 3 4 5 6 7 8 9

20 1 2 3 4 5 6 7 8 9

30 10 1 2 3 4 5 6 7 8 9

Nibble Byte Word

Version IHL (header length) Type of service (TOS) Total length

Identification Fragment offset

Time to live (TTL) Protocol Header checksum

Source address

Destination address

IP option (optional, not common)

Bit

IP flagsx D M

IP header (version 4)

Header checksum

Checksum of entire IPheader

Header length

Number of 32-bit words in TCP header, minimum value of 5. Multiply by 4 to get byte count.

Fragment offset

Fragment offset from start of IP datagram. Measured in 8 B (2 words, 64 bit) increments. If IP datagram is fragmented, fragment size (total length) must be a multiple of 8 B.

Version

Version of IP protocol. 4 and 6 are valid. This diagram represents version 4 structure only.

Total length

Total length of IP datagram, or IP fragment if fragmented.Measured in byte.

Protocol

ICMPIGMPTCPIGRP

1269

UDPGREESPAH

17475051

SKIPEIGRPOSPFL2TP

578889

115

IP protocol ID. Including (but not limited to):

xDM

0x800x400x20

Reserved (evil bit)Do not fragmentMore fragments follow

IP flags

x D M

RFC 791

Please refer to RFC 791 for the complete internet protocol (IP) specification.

20 B

IHL(internetheaderlength)

Fig. 12.4 IPv4 header [12.25]

only to extract the required header fields (see Ta-ble 12.1) even though the sniffer has complete accessto the packet header and payload information (thesniffer operates in privileged or rootmode, which al-lows it to hook into the operating system TCP/IPstack).

The TCP information is encapsulated within theIPv4 payload. We just peek at the required fields bymaking a temporary copy of the original IPv4 packetand deencapsulating it to extract the required TCPfields. The fields are then converted to a text formatready to be pushed to the detector mechanism.

12.4.4 Detector

The detector is designed to be strapped onto routerfirmware. This design choice dictates that the detec-tor must have the following characteristics:

1. Should not be processor-intensive.2. Very low and predictable memory require-

ments.

228 12 Distributed Port Scan Detection

0

4

8

12

16

20

20 B

Offset

0 1 2 3Byteoffset

Source port Destination port

Sequence number

Acknowledgment number

Offset Reserved Window

TCP options (optional)

Checksum Urgent pointer

Nibble Byte Word

10 1 2 3 4 5 6 7 8 9

20 1 2 3 4 5 6 7 8 9

30 10 1 2 3 4 5 6 7 8 9Bit

TCP flagsFSRPAUEC

FSRPAUEC

CEUAPRSF

0x800x400x200x100x080x040x020x01

Congestion windowReduced (CWR)ECN Echo (ECE)UrgentAckPushResetSynFin

TCP header

TCP options

0 End of options list1 No operation (NOP, Pad)2 Maximum segment size3 Window scale4 Selective ACK ok8 Timestamp

Checksum

Checksum of entire TCPsegment and pseudoheader (parts of IP header)

Offset

Number of 32-bit words inTCP header, minimumvalue of 5. Multiply by 4 toget byte count.

TCP flags Congestion notification

Packet stateSyn

Syn-AckAck

No congestionNo congestion

CongestionReciever response

Sender response

DSB0 00 00 1

0 11 0

1 11 11 1

ECN bits1 10 10 0

0 00 0

0 00 11 1

ECN (explicit congestionnotification). See RFC3,168 for full details, validstates below.

RFC 793

Please refer to RFC 793 forthe complete transmissioncontrol protocol (TCP)specification.

Fig. 12.5 TCP header [12.25]

Table 12.1 Fields extracted by the packet sniffer

Type Range Field Abbreviation Header

IP address 0.0.0.0–255.255.255.255 Source IP SIP IPIP address 0.0.0.0–255.255.255.255 Destination IP DIP IPNumeric 0–65,535 Source port SP TCPNumeric 0–65,535 Destination port DP TCPFlag Boolean Synchronize SYN TCPFlag Boolean Acknowledgement ACK TCPFlag Boolean Finish FIN TCPFlag Boolean Reset RST TCP

In other words, the prime function of a router ispacket forwarding and any IDS functionality in-cluded should scale gracefully and not cause the pri-mary functionality to fail. The emphasis is on real-time detection, which means that processing speedis one of the design goals. We are willing to sacrificeaccuracy to some extent to achieve this goal.

The IDS integrated within a router is shown inFig. 12.6. The packet sniffer and the detector canbe seen in the router. Whenever a packet arrives ona router interface, a lookup of the routing table isperformed to determine the next hop if the desti-nation is not local. After the route lookup, the timeto live is decremented and the packet is forwarded

12.4 Approach 229

Fig. 12.6 Prototype intru-sion detection system withinrouter r3

to the corresponding interface for the particularroute.

Patterns in TCP Packet Traffic

The patterns of benign and TCP scan traffic are dif-ferent. Our scan detection algorithm uses these dif-ferences to flag a particular set of packets as beingscanners or benign.

Symmetry in benign TCP connections TCP hasan elaborate setup and a tear-down process. A be-nign connection will look like the following to anobserver of the communication between the clientand the server:

TCP(Setup) ��������������Session Established �TCP(TEARDOWN)

We can see that there are three different stages:

1. Setup: This is the TCP three-way handshake:

(a) SYN(b) SYN-ACK(c) ACK.

2. Session established: The period during whichthe client will communicate with the server. Anexample would be to fetch a page from a Webserver.

3. Tear-down:This is when the FIN packet is usedto bring down the connection

Asymmetry in TCP scan traffic We take theTCP SYN (half-open) scanning into consideration.The traffic between a scanner and a server willlook like the following to an observer who is ina position to observe both sides of the communica-tion:

TCP(OPEN) ��������������Handshake Aborted � TCP(ABORT)

1. Open: This is the standard TCP three-wayhandshake till 1b.Then in 1c the scanner abortsthe handshake:

(a) SYN(b) SYN-ACK(c) RST.

2. Handshake aborted:The session was not able tobe set up as the RST from the scanner abortedthe TCP three-way handshake.

3. Abort: This is when the RST packet aborts thehandshake. There is no FIN packet associatedwith the abort process.

Partial Completion Filter

The PCF was introduced by Kompella et al. [12.19].It is similar to a counting Bloomfilter.There aremul-tiple parallel stages in aPCF,with each stage contain-ing hash buckets that hold a counter (see Fig. 12.7).

230 12 Distributed Port Scan Detection

Comparator

Comparator

Comparator

Greater than threshold

Hashfunctions

Fieldextraction

Extraction of various fieldsfor hash generation

Increment for a SYNDecrement for a FIN

All stages indicatecounter value greater

Than threshold

MULTI STAGE PCFsMAINTAIN PARTIAL COMPLETION COUNT

PER HASH BUCKET

Fig. 12.7 Multiple-stage partial completion filter (PCF) [12.19]

The hash bucket counter in scope is incremented fora SYN and decremented for a FIN. For benign TCPconnections, the symmetry between the SYNs andFINs will ensure that the counter will tend towards0. If an IP address hashes into buckets which havelarge counter values in all stages, then we can assertwith a high degree of confidence that the IP addressis involved in a scan.

12.4.5 Network Topology

Theprototype IDS is deployed on a/16CIDR [12.26]within the OMNeT++ simulator. The numbers ofscanners and target servers are variable.There is alsoa provision to add other hosts which can generatebackground traffic.

12.5 Results

We used an experimental setup with the followingconfigurations:

• Two scanners, two regular routers, one routerwith the IDS, and two targets (Fig. 12.8). Thethreshold chosen was 3.

The results are shown in Table 12.2.

• Four scanners, two regular routers, one routerwith the IDS system, and two targets (Fig. 12.9).The threshold chosen was 3.

The results are shown in Table 12.3.

We measure the detection rate as the number ofscanner IPs that the detector could identify. The re-sults of both these setups are unusual in that theyare constant for a wide variation of parameters. Theonly parameter which has a significant effect is thethreshold. Any scanner that operates below the cur-rently set threshold is mislabeled. Since the amountof traffic generated in the network is limited, it re-mains to be seen whether this behavior manifestsitself in scaled-up simulations or actual networktraces.

12.6 Conclusion 231

srv[0]

r1 r3

r3

cli[1]

cli[0] srv[1]

Fig. 12.8 Experimental setup with two scanners and two targets

Table 12.2 Results of two scanners and two targets

Ports PCF stages Buckets/PCF stage Bucket size (bit) Memory for PCF (kb) Threshold Detection rate (%)

4 1 3 32 0.012 3 � 9010 1 3 32 0.012 3 � 9020 1 3 32 0.012 3 � 904 3 1,000 32 12 3 � 9010 3 1,000 32 12 3 � 9020 3 1,000 32 12 3 � 904 1 3 32 0.012 1 � 9010 1 3 32 0.012 1 � 9020 1 3 32 0.012 1 � 904 3 1,000 32 12 2 � 9010 3 1,000 32 12 2 � 9020 3 1,000 32 12 2 � 90

PCF partial completion filter

12.6 Conclusion

Conventional NIDS have heavyweight processingand memory requirements as they maintain perflow state using data structures such as linked listsor trees. This is required for some specialized jobssuch as stateful packet inspectionwhere the networkcommunications between entities are recreated intheir entirety to inspect application-level data. Thedownside to this approach is that:

• The NIDS must be in a position to view all in-bound and outbound traffic of the protected net-work.

• The NIDS can be overwhelmed by a distributeddenial of service attack since most such attackstry and exhaust the available state of network en-tities.

For some applications, such as port scan detection,we do not need to reconstruct the complete networktraffic. We can see that the aggregation approachworks well, somewhat like a set lookup with a verycompact storage mechanism. The data structure isunique in the following respects:

1. The values stored cannot be retrieved verbatimor enumerated.

2. An input value can be tested for prior existenceamong the set of values stored.

These properties listed above are used in reducingthe detector state to a constant value. Since routersare devices with limited memory and processing ca-pabilities, these properties fit in exceedingly wellwith our requirements of fitting a detection mech-anism into them.

232 12 Distributed Port Scan Detection

srv[0]

r1 r3

r3

cli[1]

cli[0]

srv[1]

cli[2]

cli[3]

channelinstaller configurator nam

9 IP nodes5 non-IP nodes

NClientsScan

Fig. 12.9 Experimental setup with four scanners and two targets

Table 12.3 Results of four scanners and two targets

Ports PCF stages Buckets/PCF stage Bucket size (bit) Memory for PCF (kb) Threshold Detection rate (%)

4 1 3 32 0.012 3 � 9010 1 3 32 0.012 3 � 9020 1 3 32 0.012 3 � 904 3 1,000 32 12 3 � 9010 3 1,000 32 12 3 � 9020 3 1,000 32 12 3 � 904 1 3 32 0.012 1 � 9010 1 3 32 0.012 1 � 9020 1 3 32 0.012 1 � 904 3 1,000 32 12 2 � 9010 3 1,000 32 12 2 � 9020 3 1,000 32 12 2 � 90

Gaming the detector system can be attemptedin the forward path by sending spurious client gen-erated FINs. This can be countered by eliminat-ing client FINs from the equation. The spuriousFIN technique is not possible in the reverse pathas the server would have to terminate the connec-tion.

Future work includes incorporating the detectorinto multiple routers and formulating a peer to peeror client server distributed detector communicationnetwork. A distributed set lookup is then possiblefrom any point in the network. So routers in vari-ous segments can be queried like a directory to checkwhether a particular packet was forwarded by them.

References 233

References

12.1. S. Panjwani, S. Tan, K. Jarrin, M. Cukier: An exper-imental evaluation to determine if port scans areprecursors to an attack, Proc. 2005 InternationalConference on Dependable Systems and Networks(2005) pp. 602–611

12.2. E. Mills: Just how vulnerable is the electricalgrid? available at http://news.cnet.com/-_--.html (last accessed April 2009)

12.3. S. Gorman: Electricity grid in U.S. penetratedby spies, available at http://online.wsj.com/article/SB.html (last accessed April2009)

12.4. R. Deibert, R. Rohozinski: Tracking GhostNet:Investigating a cyber espionage network, online(March 2009)

12.5. M. Allman, V. Paxson, J. Terrell: A brief history ofscanning, ACM InternetMeasurement Conference2007 (2007)

12.6. E. Skoudis, T. Liston: Counter Hack Reloaded:a Step-by-Step Guide to Computer Attacks andEffective Defenses, 2nd edn. (Prentice Hall, UpperSaddle River, NJ 2005)

12.7. Fyodor:The art of port scanning, PhrackMagazine7(51) (1997), available at http://www.phrack.com/issues.html?issue=&id= (last accessed January2009)

12.8. F. Cohen: Simulating cyber attacks, defenses,and consequences, available at http://www.all.net/journal/ntb/simulate/simulate.html (last accessedApril 2009)

12.9. A. Varga et al.: OMNeT++ (2009), availableat http://www.omnetpp.org (last accessed March2009)

12.10. J. Postel: IANA – Internet Assigned NumbersAuthority Port Number Assignment, available athttp://www.iana.org/assignments/port-numbers(last accessed April 2009)

12.11. O. Maor: Divide and conquer: real world dis-tributed port scanning, RSA Conference, Feb2006, available at http://www.hacktics.com/frpresentations.html (last accessed March 2008)

12.12. S. Staniford, J.A. Hoagland, J.M. McAlerney: Prac-tical automated detection of stealthy portscans, J.Comput. Secur. 10(1/2), 105–136 (2002)

12.13. C. Gates, J. McNutt, J. Kadane, M. Kellner:Detecting scans at the ISP level, Tech. Rep.CMU/SEI-2006-TR-005 (Software Engineering In-stitute, Carnegie Mellon University Pittsburgh, PA15213, 2006)

12.14. Various contributors: Squid: optimizing web deliv-ery, available at http://www.squid-cache.org/ (lastaccessed March 2008)

12.15. L. Heberlein, G. Dias, K. Levitt, B. Mukherjee,J. Wood, D. Wolber: A network security monitor(May 1990) pp. 296–304

12.16. M. Roesch: Snort – lightweight intrusion detectionfor networks, LISA’99: Proc. 13th USENIX confer-ence on System administration (USENIX Associa-tion, Berkeley, CA 1999) pp. 229–238

12.17. V. Paxson: Bro: a system for detecting networkintruders in real-time, Comput. Netw. 31, 23–24(1999)

12.18. J. Jung, V. Paxson, A.W. Berger, H. Balakrishnan:Fast portscan detection using sequential hypothe-sis testing, Proc. IEEE Symposium on Security andPrivacy (2004)

12.19. R.R. Kompella, S. Singh, G. Varghese: On scalableattack detection in the network. In: IMC 04: Proc.4th ACM SIGCOMM Conference on Internet Mea-surement, ed. by A. Lombardo, J.F. Kurose (ACMPress, Taormina, Sicily, Italy 2004) pp. 187–200

12.20. B. Bloom: Space/time trade-offs in hash codingwith allowable errors, Commun. ACM 13, 422–426(1970)

12.21. A. Broder,M.Mitzenmacher:Network applicationsof bloomfilters: a survey, InternetMath. 1, 636–646(2002)

12.22. A.Varga, R.Hornig:Anoverviewof theOMNeT++simulation environment, Simutools ’08: Proc. 1stInt. Conference on Simulation Tools and Tech-niques for Communications, Networks and Sys-tems and Workshops, ICST, Brussels, Belgium,Belgium (Institute for Computer Sciences, Social-Informatics andTelecommunications Engineering,2008) pp. 1–10

12.23. A.Varga et al.: INET framework forOMNeT++4.0,available at http://inet.omnetpp.org/ (last accessedMarch 2009)

12.24. S. Sinha: TCP state transition diagram, available athttp://www.winlab.rutgers.edu/hongbol/tcpWeb/tcpTutorialNotes.html (last accessed April 2009)

12.25. M. Baxter: Header drawings, available at http://www.fatpipe.org/mjb/Drawings/ (last accessedApril 2009)

12.26. Wikipedia: Classless inter-domain routing –Wikipedia, the free encyclopedia, available athttp://en.wikipedia.org/w/index.php?title=Classless_Inter-Domain_Routing&oldid= (last accessed April 2009)

234 12 Distributed Port Scan Detection

The Authors

Himanshu Singh received his MS degree in computer science from San Jose State Universityand his BE degree in electronics engineering from Pune University. In 2005, he spearheadedan effort which brought together Reliance, Qualcomm, and OEMs such as LG, Samsung, andMotorola to prototype and develop a thin client rich content delivery platform for resource-constrained wireless devices. It enabled wireless data services for millions of rural consumers.At present, he is working at IBM on enhancing the Tivoli Storage Manager.

Himanshu SinghSan Jose State UniversityDepartment of Computer ScienceSan Jose, [email protected]

Robert Chun received his BS degree in electrical engineering and his MS and PhD degrees incomputer science from the University of California at Los Angeles. He is currently a professorin the Computer Science Department at San Jose State University, where he teaches classesmainly in computer architecture and operating systems. His research interests include high-performance fault-tolerant computer design, parallel programming, computer-aided VLSIand software design, and cloud computing.

Robert ChunSan Jose State UniversityDepartment of Computer ScienceSan Jose, California, [email protected]