Transcript Network Protocols Pre.

COEN 252 Computer Forensics
Network Protocols
Network Protocols: Layering


TCP/IP stack has four levels.
OSI has seven.
Network Protocols: Layering
Network Protocols: Layering

Each layer adds a header.
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Application
TCP
IP
Link
Link Layer
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Network Interface Cards (NIC)
Unique Medium Access Control (MAC)
number
Format 48b written as 6B in hex.
NICs either select based on MAC
address or are in promiscuous mode
(capture every packet).
Link Layer

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
Address Resolution Protocol (ARP)
Resolves IP addresses to MAC
addresses
RFC 826
Link Layer:
ARP Resolution Protocol


Assume node A with IP address 10.10.10.100 and MAC
00:01:02:03:04:05 wants to talk to IP address
10.10.10.101.
Sends out a broadcast who-has request:
00:01:02:03:04:05; ff:ff:ff:ff:ff:ff; arp 42 who-has 10.10.10.101


All devices on the link capture the packet and pass it to
the IP layer.
10.10.10.101 is the only one to answer:
a0:a0:a0:a0:a0:a0; 00:01:02:03:04:05; arp 64; arp reply
10.10.10.101 is-at a0:a0:a0:a0:a0:a0

A caches the value in its arp cache.
IP

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Uses IP addresses of source and
destination.
IP datagrams are moved from hop to
hop.
“Best Effort” service.
Corrupted datagrams are detected and
dropped.
IP



Addresses contain IP address and port
number.
IPv4 addresses are 32 bit longs
IPv6 addresses are longer.
IP: ICMP
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
Internet Control Message Protocol
Created to deal with non-transient
problems.

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
Fragmentation is necessary, but the No Frag
flag is set.
UPD datagram sent to a non-listening port.
Ping.
IP: ICMP

ICMP error messages should not be
sent,


For any but the first fragment.
A source address of broadcast or loopback
address.

Are probably malicious, anyway.
IP: ICMP

ICMP errors are not sent,

In response to an ICMP error message.
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
Otherwise, craft a message with invalid UDP
source and destination port. Then watch ICMP
ping-ponging.
A destination broadcast address.

Don’t answer with destination unreachable for
a broadcast. Otherwise, this makes it trivial to
scan a network.
Transport Layer: TCP and UDP

Transmission Control Protocol (TCP)
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Reliable
Connection-Oriented.
Slow
User Datagram Protocol (UDP)
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Unreliable
Connectionless.
Fast.
TCP
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
Only supports unicasting.
Full duplex connection.
Message numbers to prevent loss of
messages.
TCP:
Three Way Handshake
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Initiator to responder: Syns
Responder to initator: Acks, Synt
Initiator to responder: Ackt
Sets up two connections with initial
message numbers s and t.
TCP:
Three Way Handshake
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20:13:34.972069 IP Bobadilla.scu.edu.1316 >
server8.engr.scu.edu.23: S 2882650416:2882650416(0) win
16384 <mss 1460,nop,nop,sackOK> (DF)
20:13:34.972487 IP server8.engr.scu.edu.23 >
Bobadilla.scu.edu.1316: S 1012352000:1012352000(0) ack
2882650417 win 32768 <mss 1460> (DF)
20:13:34.972500 IP Bobadilla.scu.edu.1316 >
server8.engr.scu.edu.23: . ack 1 win 17520 (DF)
TCP:
Terminating Connections

Graceful shutdown
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Party
Party
Party
Party
1
2
2
1
to
to
to
to
Party
Party
Party
Party
2:
1:
1:
2:
Fin
Ack
Fin
Ack
Abrupt shutdown

Party 1 to Party 2: Res
TCP:
Shutting down a connection
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20:48:45.221851
16958 (DF)
20:48:45.226300
32768 (DF)
20:48:45.231650
win 32768 (DF)
20:48:45.231666
16940 (DF)
20:48:45.235303
win 32768 (DF)
20:48:45.235331
16940 (DF)
20:48:45.235494
win 16940 (DF)
20:48:45.236027
(DF)
IP Bobadilla.scu.edu.1570 > server8.engr.scu.edu.23: P 4:5(1) ack 5 win
IP server8.engr.scu.edu.23 > Bobadilla.scu.edu.1570: P 5:7(2) ack 5 win
IP server8.engr.scu.edu.23 > Bobadilla.scu.edu.1570: P 7:23(16) ack 5
IP Bobadilla.scu.edu.1570 > server8.engr.scu.edu.23: . ack 23 win
IP server8.engr.scu.edu.23 > Bobadilla.scu.edu.1570: F 23:23(0) ack 5
IP Bobadilla.scu.edu.1570 > server8.engr.scu.edu.23: . ack 24 win
IP Bobadilla.scu.edu.1570 > server8.engr.scu.edu.23: F 5:5(0) ack 24
IP server8.engr.scu.edu.23 > Bobadilla.scu.edu.1570: . ack 6 win 32767
TCP
Exchanging Data

Each packet has a sequence number.


(One for each direction.)
Initial sequence numbers are created
during initial three way handshake.
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NMap uses the creation of these sequence
numbers to determine the OS.
OS are now much better with truly random
sequence numbers.
TCP
Exchanging Data
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
Party that receives packet sends an
acknowledgement.
Acknowledgement consists in


Ack flag.
Sequence number of the next package to
be expected.
TCP
Exchanging Data


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If a package is lost, then the ack
number will not change:
“Duplicate acknowledgement”
Depending on settings, sender will
resend, after at most three stationary
ack numbers.
Also, resend after timeout.
TCP
Exchanging Data
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20:48:45.087563 IP Bobadilla.scu.edu.1570 >
server8.engr.scu.edu.23: . ack 4 win 16959 (DF)
20:48:45.087583 IP Bobadilla.scu.edu.1570 >
server8.engr.scu.edu.23: P 3:4(1) ack 4 win 16959 (DF)
20:48:45.096443 IP server8.engr.scu.edu.23 >
Bobadilla.scu.edu.1570: P 4:5(1) ack 4 win 32768 (DF)
20:48:45.221851 IP Bobadilla.scu.edu.1570 >
server8.engr.scu.edu.23: P 4:5(1) ack 5 win 16958 (DF)
20:48:45.226300 IP server8.engr.scu.edu.23 >
Bobadilla.scu.edu.1570: P 5:7(2) ack 5 win 32768 (DF)
20:48:45.231650 IP server8.engr.scu.edu.23 >
Bobadilla.scu.edu.1570: P 7:23(16) ack 5 win 32768 (DF)
20:48:45.231666 IP Bobadilla.scu.edu.1570 >
server8.engr.scu.edu.23: . ack 23 win 16940 (DF)
TCP flags
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Part of TCP header
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F : FIN - Finish; end of session
S : SYN - Synchronize; indicates request to start session
R : RST - Reset; drop a connection
P : PUSH - Push; packet is sent immediately
A : ACK - Acknowledgement
U : URG - Urgent
E : ECE - Explicit Congestion Notification Echo
W : CWR - Congestion Window Reduced
UDP
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“Send and pray”
No connection.
No special header like TCP.
Protocol field in the IP header is 0x11
Another field in the IP header contains
UDP specific header information
Fragmentation
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
IP datagram can come across smaller
maximum transmission units than its
own size.
Resender chops up the IP datagram
into many IP datagrams, the fragments.
Fragmentation
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Fragments are reassembled at the
destination.
Fragments carry:

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
Fragment identifier
Offset in original data portion
Length of data payload in fragment
Flag that indicates whether or not this is
the final fragment.
Fragmentation
Example
 Large Echo Request
 ping -l 1480 129.218.19.198
 Assume MTU is 1500
Fragmentation
Fragmentation:
First Fragment
Fragmentation:
Second Fragment
Fragmentation:
Last Fragment
Fragmentation
ping –l 65500 129.218.19.198
12:02:18.256066 IP dhcp-19-211.engr.scu.edu > Bobadilla.scu.edu: icmp 1472: echo request seq 6400
(frag 10712:1472@0+)
12:02:18.257282 IP dhcp-19-211.engr.scu.edu > Bobadilla.scu.edu: icmp (frag 10712:1472@1472+)
12:02:18.258498 IP dhcp-19-211.engr.scu.edu > Bobadilla.scu.edu: icmp (frag 10712:1472@2944+)
12:02:18.258502 IP dhcp-19-115.engr.scu.edu.137 > 129.210.19.255.137: udp 50
12:02:18.259714 IP dhcp-19-211.engr.scu.edu > Bobadilla.scu.edu: icmp (frag 10712:1472@4416+)
12:02:18.261177 IP dhcp-19-211.engr.scu.edu > Bobadilla.scu.edu: icmp (frag 10712:1472@5888+)
12:02:18.262389 IP dhcp-19-211.engr.scu.edu > Bobadilla.scu.edu: icmp (frag 10712:1472@7360+)
12:02:18.263604 IP dhcp-19-211.engr.scu.edu > Bobadilla.scu.edu: icmp (frag 10712:1472@8832+)
12:02:18.264820 IP dhcp-19-211.engr.scu.edu > Bobadilla.scu.edu: icmp (frag 10712:1472@10304+)
12:02:18.266037 IP dhcp-19-211.engr.scu.edu > Bobadilla.scu.edu: icmp (frag 10712:1472@11776+)
12:02:18.267495 IP dhcp-19-211.engr.scu.edu > Bobadilla.scu.edu: icmp (frag 10712:1472@13248+)
12:02:18.268712 IP dhcp-19-211.engr.scu.edu > Bobadilla.scu.edu: icmp (frag 10712:1472@14720+)
Fragmentation
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DF (Don’t Fragment) Flag
If forwarding node finds that the
datagram needs to be fragmented but
that the DF flag is set, it should respond
with ICMP host unreachable – need to
fragment.
Useful to find minimum MTU on a link.
Fragmentation
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

Stateless firewalls look only at individual
packages.
Protocol header is only in the first
fragment.
“Stealth attacks / scans” have evil
payload only in the second and
following fragments.
Fragments:
Teardrop and Friends

Teardrop (1997)
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Fragments with overlapping offset fields.
Many contemporary OS crash, hang,
reboot.
Jolt2
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Single fragment with non-zero offset.
Receiving system allocates resources to
reconstruct a datagram that never arrives.
Fragments:
Teardrop and Friends

Create fragments that seem to come from
a GB datagram.
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Ping of Death
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Trusting OS tries to allocate memory and dies.
Win95 allowed to send a ping that was just a
tad too long. Receiving host would crash.
Unnamed Attacks
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Missing fragments lead to resource allocation.
ICMP
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ICMP has no port numbers.
No acks, no message delivery guarantee
http://www.iana.org/assignments/icmpparameters
First Byte Type
Second Byte Code
ICMP



Mapping Techniques.
Detect up host.
Detect OS through responses.
ICMP
Tireless Mapper
 Sends ICMP echo requests messages to
all possible IP addresses
 Many IDS might not capture this scan if
the number of packages per hour is
small.
 Firewalls should filter incoming ping
requests.
ICMP
Efficient Mapper
 Use the ICMP echo request with a
broadcast address.
 Ping 129.210.19.255
ICMP
Clever Mapper
 Use a different ICMP message such as
ICMP address mask.
 Determines the class of the network
ICMP
Normal messages
 Host unreachable
 Port unreachable
 Admin prohibited
 Need to fragment
 Time exceeded in transit
Malicious ICMP: Smurf Attack
Smurf attack on victim 129.219.19.198
 Step 1: Send ICMP echo request to a
broadcast address with spoofed IP of
129.219.19.198
 Step 2: Router allows in ICMP echo
request to broadcast address
 Step 3: All live hosts respond with ICMP
echo reply to real source IP
Malicious ICMP: Smurf Attack
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Denial of Service Attack.
Effort of Attacker << Effort of Victim.
Uses ICMP replies from network as an
amplifier.
Works well if victim has a slow
connection.
Malicious ICMP:
Tribal Flood Network
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Based on Smurf
Creates zombies out of compromised
machines
Compromised machines use a trigger to
start bombarding a victim with requests
Many variations on this theme
Malicious ICMP:
Winfreeze (obsolete)
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Uses the ICMP redirect message.
Legal use is to update routing
information.
Flood of redirect message causes the
victim (Win95 / Win98) to redirect
traffic to itself via random hosts.
Victim spends too much time updating
routing table.
Malicious ICMP: Loki

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Uses ICMP packages for covert channel
A compromised host with a Loki server
responds to requests from a Loki client.
Requests are sent via ping messages
with data embedded in ICMP pings.
Originally used bytes 6 and 7.
Malicious ICMP: Conclusions
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
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Limit ICMP messages at the firewall.
Leads to inefficiencies, such as trying a
TCP connection to a host that is down.
Need to admit path MTU discovery.
Log those that are let through.
FTP
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

Uses TCP
Active / Passive FTP
Both use port 21 to issue FTP
commands.
Active FTP:


Uses port 20 for data.
FTP server establishes connection to client
FTP: Active FTP Example:
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
Command channel between server8.engr.scu.edu.21 and dhcp-19211.engr.scu.edu.3268
Dir command creates a new connection between
server8.engr.scu.edu.20 and dhcp-19-211.engr.scu.edu.5003
FTP


The opening of a connection from the
outside to an ephemeral port is
dangerous.
Passive FTP: The client initiates the
data connection to port 20.
Malicious TCP Use:
Mitnick Attack (obsolete)
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SYN flood
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Goal is to disconnect victim from the net.
Throws hundreds / thousands of SYN
packets
Return address is spoofed.
Recipient’s stack of connections waiting to
be established is flooded.
Still works with DDoS attack.
Malicious TCP Use:
Mitnick Attack (obsolete)

Identify Trust Relationships
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
Extensive network mapping.
Nbtstat/finger, showmount, rpcinfo -r, …
Rpcinfo provides information about the
remote procedure call services and their
ports
Malicious TCP Use:
Mitnick Attack (obsolete)

Initiate a number of TCP connections to
the host.

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Send SYN packet. Receive SYN/ACK
packet. Send RES so that victim is not
flooded.
Observe the sequence number values
between different connections.
Can they be predicted?
Malicious TCP Use:
Mitnick Attack (obsolete)
B
Victim trusts B
Attacker
Malicious TCP Use:
Mitnick Attack (obsolete)

Attacker can predict the sequence
number that victim expects.
B
Victim trusts B
Attacker
Malicious TCP Use:
Mitnick Attack (obsolete)


Attacker SYN floods B.
B cannot respond.
B
Victim trusts B
Attacker
Malicious TCP Use:
Mitnick Attack (obsolete)


Attacker takes over B’s identity.
Spoofs packet from B to Victim.
B
SYN
Victim trusts B
Attacker
Malicious TCP Use:
Mitnick Attack (obsolete)


Victim responds with SYN / ACK to B.
B does not respond.
ACK / SYN
Victim trusts B
B
Attacker
Malicious TCP Use:
Mitnick Attack (obsolete)

Attacker sends the ACK with the
guessed sequence number to victim
B
ACK
Victim trusts B
Attacker
Malicious TCP Use:
Mitnick Attack (obsolete)

Attacker sends another TCP packet with
payload: rsh victim “echo ++ >> .rhosts”
B
Bad stuff
Victim trusts B
Attacker
Malicious TCP Use:
Mitnick Attack (obsolete)

Now victim trusts everyone.
B
Victim trusts
everyone.
Attacker
Malicious TCP Use:
Mitnick Attack (obsolete)

Attacker terminates connection with a
FIN exchange
B
FIN ACK FIN ACK
Victim trusts
everyone
Attacker
Malicious TCP Use:
Mitnick Attack (obsolete)

To wake up B, attacker sends it a bunch
of RES to free B from the SYN flood.
B
RES
RES
RES
Victim trusts
everyone
Attacker
Malicious TCP Use:
Mitnick Attack (obsolete)

Attacker now starts a new connection
with the victim.
B
Yak yak yak
Victim trusts
everyone
Attacker
Malicious TCP Use:
Mitnick Attack Detection


Network based intrusion detection (NID) can
find the original site mapping.
NID can find the reconnaissance by finding
“finger” “showmount” etc. commands.



Directed to the same port (111).
This is a dangerous port.
Frequent.
Malicious TCP Use:
Mitnick Attack Detection



Host scans log instances where a single
system accesses multiple hosts at the
same time.
Host-based Intrusion Detection (HID)
can find access to a single port.
HID / Tripwire could find changes to
.rhosts.
Malicious TCP Use:
Mitnick Attack Detection
Computer Forensics can detect the attack
by
 Logging network traffic.
 Examining MAC of important files
(.rhosts)
Malicious TCP Use:
Mitnick Attack Prevention

Router-based Firewall blocks certain type of
traffic.




Host-based firewall blocks


Network mapping.
SYN flooding.
Access to dangerous ports.
Access to dangerous ports.
Security policy


Disallows reconnaissance tools.
Enforces better authentication.
Domain Name Servers


Provide mapping from host names to IP
addresses.
DNS resolution process



Client sends a gethostbyname message to
the local domain name server.
Local domain name server sends back ip
address.
Uses UDP (almost exclusively)
DNS: Resolution protocol
1.
2.
3.
4.
5.
6.
Client to local DNS server gethostbyname
Local DNS server sends forwards request to root server.
Root server returns with name of remote DNS server.
Local DNS server queries remote DNS server.
Remote DNS server answers with IP address.
Local DNS server gives data to client.
DNS


Use caching to prevent overload by root
servers.
DNS records have a TTL


Responding DNS server sets TTL.
Receiving DNS server caches record for
TTL time.
DNS: Reverse Lookup


IP-address to host-name
Query for 1.2.3.4 send to 4.3.2.1.inaddr.arpa
DNS:
Master - Slave Name Servers




Each domain has a single master DNS
server.
Add slaves for redundancy.
Slave server periodically contacts
master to see whether there are
changes.
Older BIND download all data from
domain, even if only one record has
changed.
DNS
Zone Transfer



Slave server restarts  zone transfer
from master to slave
Uses TCP, port 53.
Attackers like zone transfer


Gives all IP addresses and names in
subnet.
Newer versions of BIND limit transfers
based on IP address.
DNS:
Abuse for Reconnaissance

nslookup: Get name servers.
DNS:
Abuse for Reconnaissance

HINFO: host information.
DNS:
Abuse for Reconnaissance


List the zone map information.
> ls –d engr.scu.edu in nslookup
DNS:
Abuses and Problems




DNS cache poisoning
Affects BIND versions before 8.1.1.
Based on lack of authentication
Some BIND versions cache every DNS
data they see.
DNS Cache Poisoning


Attack on Hillary Clinton’s Run for
Senate Website
Traffic to www.hillary2000.org (IP
address 206.245.150.74) redirected to
www.hillaryno.com (IP address
206.245.150.74.)
DNS Cache Poisoning

Step 1: Evil sends a bogus query to the victim’s
name server that contains data
www.hillary2000.org at 206.245.150.74
DNS Cache Poisoning



Step 2: Name server accepts the bogus
information (even though it is contained
in a query).
Step 3: Victim requests IP address of
hillary2000.org and is directed to
hillaryno.com.
Vulnerability arises from lack of
authentication and of using queries to
update entries at the queried server.
DNS Cache Poisoning





Birthday Attack
Attacker sends large number of queries to a
vulnerable name server asking for hillary2000.
Attacker sends an equal number of phony replies
(with the poisoned data).
Name server will generate requests to resolve
hillary2000.
With high probability, one of the phony answers will
have the same transaction number as the name
server’s query.
DNS: The Bind Birthday Attack
DNS Cache Poisoning



Redirect traffic to a fake Pay-Pal or other ecommerce site.
Set-up Man in the Middle Attacks
Defenses:


Domain Owner has to rely on the DNS system.
ISP name server admin needs to protect by



Updating BIND or replacing it with djbdns
Two name servers, one for the public domain information
to the outside, another for internal use.
End user has to rely on the DNS system.
Routing

Local Routing Table: netstat -r
Static Routing

IP Layer searches the routing table in
the following order



Search for a matching destination host
address
Search for a matching destination network
address
Search for a default entry
Routing



Static routes are typically added during
the boot process.
Administrative changes with a “routing”
command.
ICMP routing discovery messages
Routing Changes


A host might have inefficient entries in
the routing table.
ICMP Router Discovery Protocol (IRDP)



ICMP redirect messages
ICMP routing discovery messages
IRDP needs to be enabled.
Routing Changes

ICMP Redirect Message


A sends message to D.
Routing table says to send to B first.
Routing Changes

ICMP Redirect Message


B forwards to C
B informs A that there is a direct route to C

ICMP Redirect Message
Routing Changes

ICMP Redirect Message


C forwards package to target.
A updates routing table.
IRDP DoS Exploit



Attacker (E) sends spoofed IRDP message to A
A updates routing table to reflect bogus default
value.
A looses connectivity
IRDP Windows Exploit




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Windows (95, 98, 2000) and some Solaris systems
are vulnerable.
If a Windows hosts runs a Dynamic Host
Configuration Protocol (DHCP) client, it obtains its
default route from the DHCP server.
ICMP router advertisement can be spoofed.
First router advertisement is checked for correct IP
address.
Second router advertisement is erroneously not.
IRDP Windows Exploit
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Attacker sends two ICMP router
advertisements to victim.
Victim updates its default gateway to IP
determined by attacker.
Use for man in the middle attacks or
DoS.
ARP Poisoning
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Address resolution protocol associates MAC
addresses with IP addresses.
Four Messages
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ARP Request: “Who has this IP?”
ARP Reply: “I have this IP. My MAC is …”
Reverse ARP Request: “Who has that MAC?”
Reverse ARP Request Reply: “I have that MAC, my
IP is …”
ARP Poisoning
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ARP is very efficient, but does not do
any authentication.
Many OS still accept ARP replies even
without making an ARP request.
ARP poisoning: Spoofing an ARP
package with false ARP data.
ARP Poisoning
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Denial of Service:
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Spoofed ARP message can associate the
default gateway address with a nonexisting MAC.
Traffic to the outside is no longer picked
up.
ARP Poisoning
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Man in the Middle
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Intercept traffic between devices A and B.
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A has IP IA and MAC MA.
B has IP IB and MAC MB.
Attacker has machine C with MAC MC.
Attacker sends an ARP reply to B: IA is at MC.
B updates its ARP cache entry: IA is at MC.
Attacker sends an ARP reply to A: IB is at MC.
A updates its ARP cache entry: IB is at MC.
A sends traffic to IB on a level 1 frame to MC.
C intercepts the package and forwards it to MB.
Traffic from A to B (and vice versa) now flows through C.
ARP Poisoning
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MAC flooding
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Switches maintain a MAC to port table.
Traffic only flows to destination.
Attacker sends lots of bogus ARP data to switch.
Switch’s ARP table is flooded.
Switches either stop functioning (DoS attack) or
drop to hub mode.
Switch in hub mode forwards a package to all
ports.
Allows traffic to be sniffed.
ARP Poisoning
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Small networks:
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Could use a static ARP table.
Disables ARP messaging.
All ARP entries need to be put in by hand and
maintained.
Will not work with DHCP.
Maintenance becomes quickly impossible with
larger size of network.
Some Win OS will still accept and use dynamic
ARP updates, even if all routes are statically
encoded.
ARP Poisoning
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Large Networks
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Use Port Security features on higher-end
switches.
Allow only one MAC address.
Prevents hackers from embedding their
MAC address more than once.
All networks
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Monitor ARP traffic (ARP monitoring tool)
IP Options
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IP options enhance the IP protocol.
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Security
Stream Identification
Internet Timestamp
Loose Source Routing
Strict Source Routing
Record Route
These are
security risks
IP Route Options
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Loose Source Routing specifies a route
that includes a list of required nodes.
Strict Source Routing specifies the
beginning of a route (up to 9 nodes)
completely.
Record Route: does not alter the
routing but requires that all nodes are
recorded.
Detecting IP Source Routing
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IP header is larger than 20B
IP option field has a hex value of
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83: loose source routing
89: strict source routing
ip[0] & 0x0f > 5 and (ip[20] = 0x83 or
ip[20] = 89)
Source Route Exploit
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Spoofing host requires source routing
through a host trusted by the victim.
Victim decides that the traffic comes
from a trusted host.
Therefore: firewalls need to disable
source-routing or network admin needs
to disable trust relationships.
Internet Group Management
Protocol (IGMP)
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Defined by RFC 1112.
IGMP messages use IP Protocol 2
IGMP are used to join and leave
multicast groups.