coen 252 computer forensics network protocols. network protocols: layering tcp/ip stack has four...

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. Application TCP IP Link

Link Layer

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

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

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

Internet Control Message Protocol Created to deal with non-transient

problems. 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.

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)

Reliable Connection-Oriented. Slow

User Datagram Protocol (UDP) Unreliable Connectionless. Fast.

TCP

Only supports unicasting. Full duplex connection. Message numbers to prevent loss

of messages.

TCP:Three Way Handshake

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

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 Party 1 to Party 2: Fin Party 2 to Party 1: Ack Party 2 to Party 1: Fin Party 1 to Party 2: Ack

Abrupt shutdown Party 1 to Party 2: Res

TCP:Shutting down a connection

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)

20:48:45.235303 IP server8.engr.scu.edu.23 > Bobadilla.scu.edu.1570: F 23:23(0) ack 5 win 32768 (DF)

20:48:45.235331 IP Bobadilla.scu.edu.1570 > server8.engr.scu.edu.23: . ack 24 win 16940 (DF)

20:48:45.235494 IP Bobadilla.scu.edu.1570 > server8.engr.scu.edu.23: F 5:5(0) ack 24 win 16940 (DF)

20:48:45.236027 IP server8.engr.scu.edu.23 > Bobadilla.scu.edu.1570: . ack 6 win 32767 (DF)

TCPExchanging Data

Each packet has a sequence number. (One for each direction.)

Initial sequence numbers are created during initial three way handshake. 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

Party that receives packet sends an acknowledgement.

Acknowledgement consists in Ack flag. Sequence number of the next

package to be expected.

TCP Exchanging Data

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 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 Part of TCP header

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

“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

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 Fragments are reassembled at the

destination. Fragments carry:

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

Fragmentationping –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

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

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) Fragments with overlapping offset fields. Many contemporary OS crash, hang,

reboot. Jolt2

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. Trusting OS tries to allocate memory and

dies. Ping of Death

Win95 allowed to send a ping that was just a tad too long. Receiving host would crash.

Unnamed Attacks Missing fragments lead to resource

allocation.

ICMP ICMP has no port numbers. No acks, no message delivery

guarantee http://www.iana.org/assignments/icm

p-parameters 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

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

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) 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

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

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

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: Command channel between server8.engr.scu.edu.21 and

dhcp-19-211.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)

SYN flood 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 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. 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)

Victim trusts B

B

Attacker

Malicious TCP Use: Mitnick Attack (obsolete)

Attacker can predict the sequence number that victim expects.

Victim trusts B

B

Attacker

Malicious TCP Use: Mitnick Attack (obsolete)

Attacker SYN floods B. B cannot respond.

Victim trusts B

B

Attacker

Malicious TCP Use: Mitnick Attack (obsolete)

Attacker takes over B’s identity. Spoofs packet from B to Victim.

Victim trusts B

B

AttackerSYN

Malicious TCP Use: Mitnick Attack (obsolete)

Victim responds with SYN / ACK to B.

B does not respond.

Victim trusts B

B

Attacker

ACK / SYN

Malicious TCP Use: Mitnick Attack (obsolete)

Attacker sends the ACK with the guessed sequence number to victim

Victim trusts B

B

Attacker

ACK

Malicious TCP Use: Mitnick Attack (obsolete) Attacker sends another TCP packet with

payload: rsh victim “echo ++ >> .rhosts”

Victim trusts B

B

AttackerBad stuff

Malicious TCP Use: Mitnick Attack (obsolete)

Now victim trusts everyone.

Victim trusts everyone.

B

Attacker

Malicious TCP Use: Mitnick Attack (obsolete)

Attacker terminates connection with a FIN exchange

Victim trusts everyone

B

Attacker

FIN ACK FIN ACK

Malicious TCP Use: Mitnick Attack (obsolete)

To wake up B, attacker sends it a bunch of RES to free B from the SYN flood.

Victim trusts everyone

B

Attacker

RES

RES

RES

Malicious TCP Use: Mitnick Attack (obsolete)

Attacker now starts a new connection with the victim.

Victim trusts everyone

B

Attacker

Yak yak yak

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. Network mapping. SYN flooding. Access to dangerous ports.

Host-based firewall blocks 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 protocol1. Client to local DNS server gethostbyname2. Local DNS server sends forwards request to root server.3. Root server returns with name of remote DNS server.4. Local DNS server queries remote DNS server.5. Remote DNS server answers with IP address.6. 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.in-addr.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.

DNSZone 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

e-commerce 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 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

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 Address resolution protocol associates

MAC addresses with IP addresses. Four Messages

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

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

Denial of Service: Spoofed ARP message can associate

the default gateway address with a non-existing MAC.

Traffic to the outside is no longer picked up.

ARP Poisoning Man in the Middle

Intercept traffic between devices A and B. 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 MAC flooding

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 Small networks:

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

Large Networks 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

Monitor ARP traffic (ARP monitoring tool)

IP Options

IP options enhance the IP protocol. Security Stream Identification Internet Timestamp Loose Source Routing Strict Source Routing Record Route

These are security risks

IP Route Options 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

IP header is larger than 20B IP option field has a hex value of

83: loose source routing 89: strict source routing

ip[0] & 0x0f > 5 and (ip[20] = 0x83 or ip[20] = 89)

Source Route Exploit

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)

Defined by RFC 1112. IGMP messages use IP Protocol 2 IGMP are used to join and leave

multicast groups.