Transcript spoofed IP packets

DoS Seminar 2
Spoofed Packet Attacks and
Detection Methods
By
Prateek Arora
Introduction
• When a denial of service (DoS) attack
occurs, a computer or a network user is
unable to access resources like e-mail and
the Internet. An attack can be directed at
an operating system or at the network.
Types of DoS attacks
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Ping Flood Attack (ICMP echo)
SYN Flood Attack (DoS attack)
DDoS Attack (Distributed SYN Flood)
UDP Flood Attacks
Smurf Attack
DNS name server Attack
Land Attack
Ping of Death Attack
Fragmentation / Teardrop Attack
Connection Spoofing
Bounce Scanning
Stealth Communication
What is a “Spoofed Packet”?
• Packets sent by an attacker such that the
true source is not authentic
– MAC spoofing
– IP packet spoofing
– Email spoofing
• This is not same as routing attacks
– These cause packets to be redirected
• e.g. DNS cache poisoning; router table attacks;
ARP spoofing
Significance of “Spoofed Packets”
in DoS attacks
• Spoofed packets are a part of many
attacks
– SYN Flood Attack
– Smurf Attack
– Connection Spoofing
– Bounce Scanning
– Stealth Communication
IP/TCP Header Review
IP Header Format
version
header
length
TOS
identification
TTL
total length
flags
protocol
fragment offset
header checksum
source IP address
destination IP address
options (if any)
data
20 bytes
IP/TCP Header Review
TCP Header Format
source port number
destination port number
sequence number
acknowledgement number
header
length
reserved
U A P R S F
R C S S Y I
G K H T N N
TCP checksum
window size
urgent pointer
options (if any)
data (if any)
20 bytes
Smurf Attack
• In this attack, spoofed IP packets containing
ICMP Echo-Request with a source address
equal to that of the attacked system and a
broadcast destination address are sent to the
intermediate network.
• Sending a ICMP Echo Request to a broadcast
address triggers all hosts included in the
network to respond with an ICMP response
packet, thus creating a large mass of packets
which are routed to the victim's spoofed
address.
Smurf Attack (contd.)
ICMP echo (spoofed source address of victim)
Sent to IP broadcast address
ICMP echo reply
ICMP = Internet Control
Message Protocol
INTERNET
1 SYN
PERPETRATOR
VICTIM
Simultaneous10,000 SYN/ACKs - VICTIM IS DEAD
INNOCENT
REFLECTOR SITES
BANDWIDTH MULTIPLICATION:
A T1 (1.54 Mbps) can easily
yield 100 MBbps of attack
SOURCE: CISCO
SYN Flood Attack
• TCP Handshake Review
– client
SYN
• sends SYN packet to server
• waits for SYN-ACK from server
– server
• responds with SYN-ACK packet
• waits for ACK packet from client
SYN-ACK
– client
• sends ACK to server
ACK
SYN Flood Attack
• Attacker causes TCP
buffer to be exhausted
with half-open connections
• No reply from target
needed, so source may be
spoofed.
• Claimed source must not
be an active host.
TCP
Buffers
169.237.5.23
168.150.241.155
169.237.7.114
Half-open
connection;
Waiting for
ACK
Completed
handshake;
connection
open
empty
buffer
SYN Flood Attack
• Attacker causes TCP
buffer to be exhausted
with half-open connections
• No reply from target
needed, so source may be
spoofed.
• Claimed source must not
be an active host.
TCP
Buffers
128.120.254.1
128.120.254.2
128.120.254.3
128.120.254.4
128.120.254.5
128.120.254.6
128.120.254.7
128.120.254.8
128.120.254.9
128.120.254.10
128.120.254.11
128.120.254.12
128.120.254.13
128.120.254.14
169.237.7.114
128.120.254.15
Half-open
connection;
Waiting for
ACK
Completed
handshake;
connection
open
empty
buffer
Summary of attack methods
Attack packets
Reply packets
Smurf
ICMP echo queries to
broadcast address
ICMP echo replies
SYN flooding
TCP SYN packets
TCP SYN ACK
packets
RST flooding
TCP packets to closed ports
TCP RST packets
ICMP flooding
•ICMP queries
•UDP packets to closed ports
•IP packets with low TTL
•ICMP replies
•Port unreachable
•Time exceeded
DNS reply
flooding
DNS queries (recursive) to
DNS servers
DNS replies
Detection Methods
• Routing-based
• Active
– Proactive
– Reactive
• Passive
Routing-based Method
• For a given network topology
certain source IP addresses
should never be seen
– Internal addresses arriving on
external interface
– External addresses arriving on
internal interface
– IANA non-routable addresses
on external interface
– Other special addresses
External NIC
Internal NIC
Special Addresses
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0.0.0.0/8
10.0.0.0/8
127.0.0.0/8
169.254.0.0/16
172.16.0.0/12
192.0.2.0/24
192.168.0.0/16
240.0.0.0/5
248.0.0.0/5
255.255.255.255/32
- Historical Broadcast
- RFC 1918 Private Network
- Loopback
- Link Local Networks
- RFC 1918 Private Network
- TEST-NET
- RFC 1918 Private Network
- Class E Reserved
- Unallocated
- Broadcast
Routing-based Methods
• Most commonly used method
– firewalls, filtering routers
• Relies on knowledge of network topology and
routing specs.
• Primarily used at organizational border.
• Cannot detect many examples of spoofing
– Externally spoofed external addresses
– Internally spoofed internal addresses
Proactive methods
• Looks for behavior that would not occur if
client actually processed packet from
client.
• Method: change in IP stack behavior
• Can observe suspicious activity
• Examples –
– TCP window games
– SYN-Cookies (block with out detection)
TCP Window Games
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Modified TCP Handshake
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client
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server
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sends SYN packet and ACK number to server
waits for SYN-ACK from server w/ matching
ACK number
responds with SYN-ACK packet w/ initial
“random” sequence number
Sets window size to zero
waits for ACK packet from client with
matching sequence number
client
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sends ACK to server with matching sequence
number, but no data
Waits for ACK with window > 0
After receiving larger window, client sends
data.
Spoofer will not see 0-len window and will
send data without waiting.
SYN
ack-number
SYN-ACK
seq-number, ack-number
window = 0
ACK
seq_number, ack-number
(no data)
ACK
seq-number, ack-number
window = 4096
ACK
seq_number, ack-number
w/ data
SYN-Cookies
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Modified TCP Handshake
Example of “stateless” handshake
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client
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responds with SYN-ACK packet with initial SYN-cookie
sequence number
Sequence number is cryptographically generated value
based on client address, port, and time.
No TCP buffers are allocated
SYN-ACK
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NO BUFFER ALLOCATED
sends ACK to server with matching sequence number
server
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seq-number as SYN-cookie,
ack-number
ACK
client
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ack-number
server
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sends SYN packet and ACK number to server
waits for SYN-ACK from server with matching ACK
number
SYN
If ACK is to an unopened socket, server validates
returned sequence number as SYN-cookie
If value is reasonable, a buffer is allocated and socket
is opened.
seq_number
ack-number+data
SYN-ACK
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Spoofed packets will not consume TCP buffers
seq-number, ack-number
TCP BUFFER ALLOCATED
Reactive methods
• When a suspicious packet is received, a probe
of the source is conducted to verify if the packet
was spoofed
• May use same techniques as proactive methods
• Example probes
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Is TTL appropriate?
Is ID appropriate?
Is host up?
Change window size
Passive Methods
• Learn expected values for observed
packets
• When an anomalous packet is received,
treat it as suspicious
• Example values –
– Expected TTL
– Expected client port
– Expected client OS idiosyncrasies
Experiments
• Determine the validity of various spoofedpacket detection methods
• Predictability of TTL
• Predictability of TTL (active)
• Predictability of ID (active)
Experiment Description Passive
• Monitor network traffic
• Record
– Source IP address
– TTL
– Protocol
• Count occurrences of all unique combinations
• Statistically analyze predictability of the data
Results - Passive
• Data collected over 2 week periods at
University of California, Davis
• 23,000,000 IP packets observed
– 23461 source IP addresses
• 110 internal
• 23351 external
Results - Passive
• Predictability measure
– Conditional Entropy (unpredictability)
H (Y | X )   P( x, y ) log P( x | y )
x, y
• Values closer to zero indicate higher
predictability
Results - Passive
All packets
Protocol
H mean
H variance
Number
Addresses
Number
Packets
All
0.055759
0.029728
23461
22999999
ICMP
0.027458
0.023726
801
223341
IGMP
0
0
23
297
TCP
0.046149
0.023114
15891
20925893
UDP
0.065164
0.040655
7397
1850468
Results - Passive
External addresses only
Protocol
H mean
H variance
Number
Addresses
Number
Packets
All
0.055505
0.029731
23351
9229608
ICMP
0.026159
0.023271
780
88371
IGMP
0
0
3
26
TCP
0.046324
0.023201
15825
8857983
UDP
0.065537
0.041015
7306
283228
Results - Passive
Internal Addresses Only
Protocol
H mean
H variance
Number
Addresses
Number
Packets
0.109633
0.026097
110
13770391
0.075714
0.03822
21
134970
0
0
20
271
0.004189
0.000321
66
12067910
0.035207
0.010859
91
1567240
All
ICMP
IGMP
TCP
UDP
Results - Passive
Only Addresses with more than 250 packets
Protocol
H mean
H variance
Number
Addresses
Number
Packets
All
0.060041
0.035521
2876
22338795
ICMP
0.035778
0.020212
33
219605
IGMP
0
0
1
0
TCP
0.051132
0.027288
2713
20332940
UDP
0.165818
0.175238
148
1779896
Results - Passive
Only Addresses with more than 500 packets
Protocol
H mean
H variance
Number
Addresses
Number
Packets
All
0.050635
0.031506
2306
22140140
ICMP
0.022401
0.014516
30
218560
IGMP
0
0
1
0
TCP
0.042716
0.022273
2190
20150197
UDP
0.164326
0.209436
104
1764716
Results - Passive
• TTL differs by protocol
• UDP most unreliable
– traceroute is major contributor (can be
filtered)
– certain programs set TTL anomalously
– ToS may be useful in reducing inconsistencies
• TTL on local network highly regular
– must filter traceroute traffic
Experiment Description Reactive
• Monitor network traffic
• Record IP address, Protocol, TTL and ID
• Send probe packet(s)
– ICMP echo reply packet
– TCP syn packet
– UDP packet
• Note the differences between the stored TTL/ID
to that of the returning probes.
Results - Reactive
• Evaluate –
– initial vs. probe reply TTL
– Initial vs. probe reply ID (delta from original)
• Predictability measure
– Conditional Entropy (unpredictability)
• Values closer to zero indicate higher
predictability
Results - Reactive
• Preliminary only
– Ran for 18 hours
– 8058 probes sent
– 218 unique addresses
• 173 external
• 45 internal
Results - Reactive
• TTL off by:
– Total # probes
– +/- 2 or less
– +/-1 or less
–0
8058
6467
6096
5110
1591
371
986
80%
75%
63%
Results - Reactive
• ID off by:
– Total # probes 8058
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Offset
1
2
4
6
5
7
8
Count
601
57
21
16
14
11
9
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Offset
256
512
768
1280
Count
73
5
22
10
Conclusion
• Spoofed-packets used in many different
attacks
• Spoofed-packets can be detected by a
number of methods
• High predictability in TTL and ID allow use
of passive and active methods
References
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www.google.co.in
http://seclab.cs.ucdavis.edu/
www.cert.org
www.caida.com
http://www.uspto.gov/
www.cisco.com