Presentation is loading. Please wait.

Presentation is loading. Please wait.

Network Defense COEN 250.

Similar presentations

Presentation on theme: "Network Defense COEN 250."— Presentation transcript:

1 Network Defense COEN 250

2 Network Protocols: Layering
Complexity of networking leads to layered architectures. TCP/IP stack has four levels. OSI has seven.

3 Network Protocols: Layering

4 Network Protocols: Layering
Each layer adds a header. Application TCP IP Link

5 Data Link Layer Sits on top of physical layer, which provides
Hardware specification Encoding and signaling Data transmission and reception Topology and physical network design Example Data Link Layers: Ethernet Token Ring FDDI Wi-Fi (802.11) Divided into two sublayers Logical Link Control Media Access Control (MAC)

6 Link Layer Address Resolution
Network Interface Cards (NIC) Unique Medium Access Control (MAC) number Now typically changeable In order to accommodate device change when using authentication through MAC address Format 48b written as twelve hex bytes. First 6 identify vendor. Last 6 serial number. NICs either select based on MAC address or are in promiscuous mode (capture every packet).

7 Link Layer Address Resolution
Address Resolution Protocol (ARP) Resolves IP addresses to MAC addresses RFC 826

8 Link Layer: ARP Resolution Protocol
Assume node A with IP address and MAC 00:01:02:03:04:05 wants to talk to IP address Sends out a broadcast who-has request: 00:01:02:03:04:05; ff:ff:ff:ff:ff:ff; arp 42 who-has All devices on the link capture the packet and pass it to the IP layer. is the only one to answer: a0:a0:a0:a0:a0:a0; 00:01:02:03:04:05; arp 64; arp reply is-at a0:a0:a0:a0:a0:a0 A caches the value in its arp cache.

9 Link Layer: ARP Resolution Protocol
ARP requests:

10 Link Layer: ARP Resolution Protocol

11 Link Layer Intrusion Detection
Network monitoring tools such as Argus or Ethereal log MAC addresses.

12 Link Layer Forensics Example:
Spike in network traffic comes from a computer with a certain IP address. However, Argus logs reveal that the traffic comes from a computer with a different MAC then the computer assigned that IP. (Spoofing) Finally, intrusion response finds the computer with that MAC, a Linux laptop that has been compromised and is used for a Denial of Service attack.

13 Link Layer Intrusion Detection
ARP cache can be viewed on Windows NT/2000/XP with arp –a command.

14 Link Layer Intrusion Detection
Some organizations log ARP information. Routers keep ARP tables. show ip arp All hosts keep ARP tables. DHCP often assigns addresses only to computers with known MAC.

15 Link Layer Intrusion Detection
An employee received harassing from a host on the employer’s network with IP address DHCP server database showed that this IP was assigned to a computer with MAC address 00:00:48:5c:3a:6c. This MAC belonged to a network printer. The router’s ARP table showed that the IP address was used by a computer with MAC 00:30:65:4b:2a:5c. (IP-spoofing) Although this MAC was not on the organization’s list, there were only a few Apple computers on the network and the culprit was soon found.

16 Link Layer Intrusion Detection
Analyze and filter log files: Keyword searches E.g. for USER, PASS, login Nicknames, channel names Filters Reconstruction E.g. contents of web-mail inbox.

17 Link Layer Intrusion Detection
NetIntercept Screenshot An example for a Network Forensics / Network Intrusion Detection commercial tool that reveals link layer evidence

18 ARP Package RFC 826 ARP package :
0-1: Hardware type (0x0001 – Ethernet) 2-3: Protocol type (0x0800 – IP) 4: Number of bytes in hardware address (6 for MAC) 5: Number of bytes in protocol address (4 for IP) 6-7: Opcode: 1 for ARP request, 2 for an ARP reply 8-13: Source MAC 14-17: Source IP 18-23: Target MAC 24-27: Target IP

19 ARP Package Ethereal deassembly of ARP package

20 Monitoring Tools Arpwatch
monitors ethernet activity and keeps a database of ethernet/ip address pairings.

21 Attacks on ARP Package Generators for various OS.
Allow an attacker to subvert a chosen protocol hping2 for Windows. *NIX, XWindows: packit IP Sorcery and many, many more. Use to create arbitrary packages

22 Attacks on ARP Switch Flooding
Switches contain a switch address table. Switch address table associates ports with MAC addresses. Switch flooding creates many false entries. Switches fail in two different modes: Fail open: Switch converts into a hub. This allows to monitor traffic through the switch from any port. Fail closed: Switch stops functioning. Denial of Service (DoS) attack

23 Attacks on ARP ARP Poisoning: attacker switch victim Outside world

24 Attacks on ARP ARP Poisoning: Attacker configures IP forwarding to send packets to the default router for the LAN attacker switch victim Outside world router

25 Attacks on ARP ARP Poisoning: Attacker sends fake ARP to remap default router IP address to his MAC address attacker switch victim Outside world router

26 Attacks on ARP ARP Poisoning: Switch now takes packet from victim and forwards it to attacker. attacker switch victim Outside world router

27 Attacks on ARP ARP Poisoning: Attackers machine intercepts message for sniffing and sends it back to the switch with the MAC address of router. attacker switch victim Outside world router

28 Attacks on ARP

29 RARP RARP (Reverse Address Resolution Protocol)
Used to allow diskless systems to obtain a static IP address. System requests an IP address from another machine (with its MAC-address). Responder either uses DNS with name-to-Ethernet address or looks up a MAC to IP ARP table. Administrator needs to place table in a gateway. RARP-daemon (RARP-d) responds to RARP requests.

30 RARP RARP vulnerability
Use RARP together with ARP spoofing to request an IP address and take part in communications over the network.

31 RARP Package Package Format as in ARP:
0-1: Hardware type (0x0001 – Ethernet) 2-3: Protocol type (0x0800 – IP) 4: Number of bytes in hardware address (6 for MAC) 5: Number of bytes in protocol address (4 for IP) 6-7: Opcode: 1 for ARP request, 2 for an ARP reply 8-13: Source MAC 14-17: Source IP 18-23: Target MAC 24-27: Target IP

32 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.

33 IP Addresses contain IP address and port number.
IPv4 addresses are 32 bit longs IPv6 addresses are 8*16 bits long.

34 DHCP Dynamic Host Configuration Protocol
Evolved from TCP/IP Boot Protocol BOOTP Solves problem of disk-less workstations Boot process: First obtain IP address Then download OS etc. BOOTP client sends broadcast to UDP port 67 (BOOTREQUEST) BOOTP server listens on that port Replies to client by either Use client’s hardware address to create ARP entry Use broadcast Client downloads OS (using e.g. TFTP)

35 DHCP Assigns addresses Manual allocation (just as BOOTP)
Single point of administration Automatic Allocation DHCP assigns address to a given device automatically from a pool of addresses Dynamic Allocation DHCP assigns an address from a pool of addresses for the length of a lease Addresses are reused and shared Clients need to renew a lease periodically If clients are rebooting, but still have an active lease, they reconfirm their lease during reboot. If renewal fails, clients will rebind to any active DHCP server Clients can release a DHCP assigned IP address

36 DHCP Attacks Denial of Service
Attacker sends DHCP requests, using up all IP address in pool Attacker uses random MAC addresses Switches can limit the number of MAC addresses used on a given link and prevent this attack

37 DHCP Attacks Man in the Middle Attack: Default Gateway
Attacker assigns DHCP addresses by Attacker disables DHCP server and then operates own DHCP server Attacker runs faster DHCP server Attacker specifies itself as default gateway Attacker redirects traffic from victim through itself

38 DHCP Attacks Man in the Middle Attack: DNS Redirection
Attacker assigns DHCP addresses Attacker specifies itself as the DNS server Attacker only redirects traffic to selected IP addresses Banking, Shopping, …

39 IP: ICMP Internet Control Message Protocol
Created to deal with non-transient problems. For example Fragmentation is necessary, but the No Frag flag is set. UPD datagram sent to a non-listening port. Ping. Used to detect network connectivity before it became too useful for attack reconnaissance. Does not use ports. Allows broadcasting. More on ICMP later

40 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. Otherwise: ICMP messages could proliferate and throttle a network

41 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-pong. A destination broadcast address. Don’t answer with destination unreachable for a broadcast. Otherwise, this makes it trivial to scan a network.

42 Transport Layer: TCP and UDP
Transmission Control Protocol (TCP) Reliable Connection-Oriented. Slow User Datagram Protocol (UDP) Unreliable Connectionless. Fast.

43 TCP Only supports unicasting. Full duplex connection.
Message numbers to prevent loss of messages.

44 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.

45 TCP: Three Way Handshake
20:13: IP > S : (0) win <mss 1460,nop,nop,sackOK> (DF) 20:13: IP > S : (0) ack win <mss 1460> (DF) 20:13: IP > . ack 1 win (DF) Sequence number Flag Window: number of bytes accepted

46 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

47 TCP: Shutting down a connection
20:48: IP > P 4:5(1) ack 5 win (DF) 20:48: IP > P 5:7(2) ack 5 win (DF) 20:48: IP > P 7:23(16) ack 5 win (DF) 20:48: IP > . ack 23 win (DF) 20:48: IP > F 23:23(0) ack 5 win (DF) 20:48: IP > . ack 24 win (DF) 20:48: IP > F 5:5(0) ack 24 win (DF) 20:48: IP > . ack 6 win (DF)

48 TCP Exchanging 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.

49 TCP Exchanging Data Party that receives packet sends an acknowledgement. Acknowledgement consists in Ack flag. Sequence number of the next package to be expected. (TCPDump shows number of bytes acknowledged).

50 TCP Exchanging Data If a package is lost, then the ack sequence number will not change: “Duplicate acknowledgement” Depending on settings, sender will resend, after at most three stationary ack numbers. Also, senders resend after timeout.

51 TCP Exchanging Data 20:48: IP > . ack 4 win (DF) 20:48: IP > P 3:4(1) ack 4 win (DF) 20:48: IP > P 4:5(1) ack 4 win (DF) 20:48: IP > P 4:5(1) ack 5 win (DF) 20:48: IP > P 5:7(2) ack 5 win (DF) 20:48: IP > P 7:23(16) ack 5 win (DF) 20:48: IP > . ack 23 win (DF)

52 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

53 TCP Example with Ethereal

54 TCP Example with Ethereal
First Syn message

55 TCP Example with Ethereal
This is the Syn-ack packet with sequence number 68 8d 5c ad and ack number 10 3f 21 1e

56 TCP Example with Ethereal
Syn number 10 3f 21 1e Ack number 68 8d 5c ae

57 TCP Example with Ethereal

58 TCP Example with Ethereal

59 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

60 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.

61 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.

62 Fragmentation Example Large Echo Request ping -l 1480
Assume MTU is 1500

63 Fragmentation

64 Fragmentation: First Fragment

65 Fragmentation: Second Fragment

66 Fragmentation: Last Fragment

67 Fragmentation ping –l 12:02: IP dhcp > icmp 1472: echo request seq 6400 (frag 12:02: IP dhcp > icmp (frag 12:02: IP dhcp > icmp (frag 12:02: IP dhcp > : udp 50 12:02: IP dhcp > icmp (frag 12:02: IP dhcp > icmp (frag 12:02: IP dhcp > icmp (frag 12:02: IP dhcp > icmp (frag 12:02: IP dhcp > icmp (frag 12:02: IP dhcp > icmp (frag 12:02: IP dhcp > icmp (frag 12:02: IP dhcp > icmp (frag

68 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.

69 Fragmentation Fragmentation has security implications
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.

70 Fragments: Teardrop and Friends
Fragments with overlapping offset fields. Many contemporary OS crashed, hang, rebooted. Jolt2 Single fragment with non-zero offset. Receiving system allocates resources to reconstruct a datagram that never arrives.

71 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.

72 ICMP Protocols like TCP can send error messages themselves.
Stateless protocols like UDP need another mechanism to send error messages. Host uses ICMP for Simple replies and requests Inform other hosts of some kind of error condition. E.g.: To throttle delivery rate, receiving host can use the ICMP source quench message. E.g.: Router can send “admin prohibited” ICMP message.

73 ICMP ICMP has no port numbers. No acks, no message delivery guarantee
Allows broadcasting ICMP types at assignments/icmp-parameters First Byte of package is Type Second Byte of package is Code

74 ICMP Attackers can use ICMP for scanning: Mapping a network.
Detect availability of target. Detect OS through the way that host responds.

75 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. Therefore: Firewalls should filter incoming ping requests.

76 ICMP Efficient Mapper Use the ICMP echo request with a broadcast address. Ping

77 ICMP Clever Mapper Use a different ICMP message such as ICMP address mask. Determines the class of the network

78 ICMP: Normal activity Normal messages: Host unreachable
Port unreachable Admin prohibited Need to fragment Time exceeded in transit

79 ICMP: Normal activity Host unreachable
Router at target host’s network sends such a message. This gives out info to an attacker. Some routers (Cisco) allow an access control list entry: no ip unreachable

80 ICMP: Normal activity Port unreachable > icmp: udp port ntp unreachable (DF) Used for UDP TCP has the RESET message to inform sender.

81 ICMP: Normal activity Unreachable - Admin Prohibited
Router informs sender that this type of message cannot be forwarded. Router decision based on access control list. Message leaks information to outside scanner.

82 ICMP: Normal activity Need to Frag
Router informs sender that DF is set, but that the package is larger than the MTU.

83 ICMP: Normal activity Time Exceeded In-Transit
Packages contain Time To Live (TTL) value. Each router handling a package decrements the TTL value. If TTL is zero, router discards package and sends the Time Exceeded In-Transit message to the sender.

84 ICMP: Normal activity ICMP messages contain additional date in the package. In particular: IP header followed by eight bytes of protocol header and data of the original datagram. Not all OS implementations do this in exactly the same way. Nmap used this for OS fingerprinting. Lately, all TCP/IP stack implementations have been fixed to remove OS idiosyncracies.

85 Malicious ICMP: Smurf Attack
Smurf attack on victim Step 1: Send ICMP echo request to a broadcast address with spoofed IP of Step 2: Router allows in ICMP echo request to broadcast address Step 3: All live hosts respond with ICMP echo reply to real machine with source IP

86 Malicious ICMP: Smurf Attack
ISMP 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.

87 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

88 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.

89 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.

90 Malicious ICMP: Simple Counter-Measures
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.

91 Harmless Behavior: TCP
Destination Host not Listening on Requested Port Receiver acknowledges and resets at the same time. Destination Host does not Exist Router sends with the ICMP: Host xxx.yyy unreachable

92 Harmless Behavior: TCP
Destination Port Blocked Router responds with an icmp message: icmp: xxx.yyy unreachable – admin prohibited filter Router does not respond. Sender retries up to a protocol dependent maximum number of retries time

93 Harmless Behavior: UDP
Destination Host not Listening on Requested Port Destination host sends icmp message: icmp: xxx.yyy port domain unreachable Or: destination host does not respond. Sender will possibly retry several times

94 Harmless Behavior: Windows Tracert
tracert (traceroute) uses ICMP pings Tracing host sends ICMP echo request with TTL = 1. Then tracing host sends ICMP echo request with TTL = 2, etc. First router responds to first request. If not destination, then with icmp: time exceeded in transit message Second router responds to second request, etc.

95 Harmless Behavior: Unix Tracert
traceroute uses UDP to random ephemeral port. Tracing host sends UDP package with TTL = 1. Then tracing host sends UDP package with TTL = 2, etc. First router responds to first request. If not destination, then with icmp: time exceeded in transit message Second router responds to second request, etc. Target responds with a port unreachable message.

96 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

97 FTP: Active FTP Example:
Command channel between and Bobadilla.1628 Dir command creates a new connection between and Bobadilla.5001

98 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.

99 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.

100 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

101 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?

102 Malicious TCP Use: Mitnick Attack (obsolete)
Victim trusts B Attacker

103 Malicious TCP Use: Mitnick Attack (obsolete)
Attacker can predict the sequence number that victim expects. B Victim trusts B Attacker

104 Malicious TCP Use: Mitnick Attack (obsolete)
Attacker SYN floods B. B cannot respond. B Victim trusts B Attacker

105 Malicious TCP Use: Mitnick Attack (obsolete)
Attacker takes over B’s identity. Spoofs packet from B to Victim. B SYN Victim trusts B Attacker

106 Malicious TCP Use: Mitnick Attack (obsolete)
Victim responds with SYN / ACK to B. B cannot respond. ACK / SYN B Victim trusts B Attacker

107 Malicious TCP Use: Mitnick Attack (obsolete)
Attacker sends the ACK with the guessed sequence number to victim B ACK Victim trusts B Attacker

108 Malicious TCP Use: Mitnick Attack (obsolete)
Attacker sends another TCP packet with payload: rsh victim “echo ++ >> .rhosts” B Bad stuff Victim trusts B Attacker

109 Malicious TCP Use: Mitnick Attack (obsolete)
Now victim trusts everyone. B Victim trusts everyone. Attacker

110 Malicious TCP Use: Mitnick Attack (obsolete)
Attacker terminates connection with a FIN exchange B FIN ACK FIN ACK Victim trusts everyone Attacker

111 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 Victim trusts everyone Attacker

112 Malicious TCP Use: Mitnick Attack (obsolete)
Attacker now starts a new connection with the victim. B Yak yak yak Victim trusts everyone Attacker

113 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.

114 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.

115 Malicious TCP Use: Mitnick Attack Detection
Computer Forensics can detect the attack by Logging network traffic. Examining MAC of important files (.rhosts)

116 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 Security policy Disallows reconnaissance tools. Enforces better authentication.

117 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)

118 DNS: Resolution protocol
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.

119 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.

120 DNS: Reverse Lookup IP-address to host-name
Query for send to

121 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.

122 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.

123 DNS: Abuse for Reconnaissance
nslookup: Get name servers.

124 DNS: Abuse for Reconnaissance
HINFO: host information.

125 DNS: Abuse for Reconnaissance
List the zone map information. > ls –d in nslookup

126 DNS: Abuses and Problems
DNS cache poisoning Affects BIND versions before Based on lack of authentication Some BIND versions cache every DNS data they see.

127 DNS Cache Poisoning Attack on Hillary Clinton’s Run for Senate Website
Traffic to (IP address ) redirected to (IP address )

128 DNS Cache Poisoning Step 1: Evil sends a bogus query to the victim’s name server that contains data at

129 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 and is directed to Vulnerability arises from lack of authentication and of using queries to update entries at the queried server.

130 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.

131 DNS: The Bind Birthday Attack

132 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.

133 Routing Local Routing Table: netstat -r

134 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

135 Routing Static routes are typically added during the boot process.
Administrative changes with a “routing” command. ICMP routing discovery messages

136 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.

137 Routing Changes A B C D ICMP Redirect Message A sends message to D.
Routing table says to send to B first. A B C D

138 Routing Changes A B C D ICMP Redirect Message B forwards to C
B informs A that there is a direct route to C A B C D

139 Routing Changes A B C D ICMP Redirect Message
C forwards package to target. A updates routing table. A B C D

140 IRDP DoS Exploit Attacker (E) sends spoofed IRDP message to A A updates routing table to reflect bogus default value. A looses connectivity E ? A B D

141 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.

142 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.

143 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

144 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.

145 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)

146 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.

147 Network Address Translation
Allows many internal IP addresses appearing to be few external IP addresses Local hosts have typically non-routable addresses Function: Local machine connects to NAT box as gateway NAT box assigns connection a routable IP address and port Outside host answers to latter address. NAT box forwards requests to local machine From: /whitepaper/vxworks.html

148 Internet Group Management Protocol (IGMP)
Defined by RFC 1112. IGMP messages use IP Protocol 2 IGMP are used to join and leave multicast groups.

149 IPSec Security layer based on IPv6
Implemented as Bump In The Stack Architecture Upper layer protocols TCP/UDP IP IPSec Data link layer Implemented in the IP layer

150 IPSec Provides authentication of source IP address
Provides message integrity and encryption Take COEN 350

151 SNMP: Simple Network Management Protocol
Allows remote managing and managing TCP/IP devices Example Vulnerability SNMP default accounts public and private When queried, will return SNMP information Can be used for network mapping Might spell out passwords

152 Network Authentication Threats
Passive Sniffing Malicious Mallory can read messages between Alice and Bob. Spoofing Malicious Mallory can create messages that seem to come from either Alice or Bob. Standard Attack Modes: Breaking Cryptography Man-in-the-Middle Replay Attacks Reflection Attack (Open several connections)

153 Man In the Middle Attack Bucket Brigade Attack
Attacker reroutes traffic through itself. Example: Victim connects to attacker:80, thinking that attacker is Attacker displays login screen from to victim Attacker goes to

154 Man In the Middle Attack Bucket Brigade Attack
Bank to Black Hat Password Please to black hat Login Please Black Hat to Bank Password is “fiddlesticks” Black hat to bank Login sue user Black hat to victim Login Please Black Hat to Victim Password Please Victim to black hat Password is “fiddlesticks” Black Hat to Victim to (intercepted by black hat) Victim: Login sue user

155 Man In the Middle Attack Bucket Brigade Attack
Could be prevented with SSL But only if victim’s browser ascertains certificate of bank

156 Replay Attack Remote authentication protocol
Instead of sending password, user sends password encrypted Attacker sniffs password exchange and now knows what to send.

157 Reflection Attack Simple, mutual authentication protocol based on capability to encrypt a challenge Session 1 Trudy: I am Alice. RA. Session 1 Bob: RB. EK(RA). Session 2 Trudy: I am Alice. RB. Session 2 Bob: RB’. EK(RB). Session 1 Trudy: Hi Bob. EK(RB). Session 1 Bob: Hi Alice. Alice: I am Alice. RA Bob: RB. EK(RA). Alice: Hi Bob. EK(RB). Bob: Hi Alice.

158 Reflection Attack Reflection Attack: Session 1 Trudy: I am Alice. RA.
Session 1 Bob: RB. EK(RA). Session 2 Trudy: I am Alice. RB. Session 2 Bob: RB’. EK(RB). Session 1 Trudy: Hi Bob. EK(RB). Session 1 Bob: Hi Alice.

159 Protecting Networks Terms of Trade
Border Router First / last router under control of system administration. DMZ Demilitarized zone. Security is low, since not protected by firewall. Locate webservers and other services there that generate potentially unsafe traffic. Firewall Filters packages based on a variety of rules.

160 Protecting Networks Terms of Trade
IDS Intrusion Detection System. NIDS: glean intrusion signatures from traffic. HIDS: monitor activity at a host on which they are located. VPN Virtual private network Screened subnet Area protected by an internal firewall.

161 Protecting Networks Terms of Trade
Configuration Management Known vulnerabilities account for most of actually perpetrated exploits. For most of them, patches were available, but not installed. CM tries to enforce uniform security policies. Backdoors An entrance into the system that avoids perimeter defenses.

162 Defense in Depth Rule 1: Multitude of security measures.
Do not relay on one security mechanism.

163 Defense in Depth Example: External tcp packet passes:
Internet Perimeter Router Internet perimeter firewall DMZ firewall Network IPS NetFlow Analyzes connections on network Antivirus Scanner on host Host IPS

164 Firewalls Firewalls are perimeter defense:
Keep the bad stuff outside, enjoy life inside.

165 Filtering Signature Any distinctive characteristic that identifies something (with a high degree of probability) Signature Types Atomic Signatures Single packet, single event, single activity is examined. Stateful Signatures State: Needed when analyzing multiple pieces of information that are not available at the same time.

166 Filtering Atomic vs. Stateful Signatures LAND attack
Attacker sends TCP-SYN packet with same source and destination address. Caused TCP stacks to crash. Can be discovered looking at a single packet. Search for string “etc/password” in a URL Attacker fragments the packet so that the string is not in either fragment. State is needed in order to recognize the attack.

167 Filtering Signature Triggers Pattern Detection Anomaly Detection
Simple string search Search for string “etc/passwords” ARP Protocol decoders search for string only in protocol fields. ARP request with source address FF:FF:FF:FF:FF:FF Anomaly Detection Traffic going to an unusual port. Protocol compliance for http traffic Behavior Detection Abnormally large / small fragmented packets Search for RPC requests that do not initially utilize the PortMapper

168 Filtering Signature Actions Generating an alert
Dropping / preventing an activity Logging the activity Resetting a TCP connection Blocking future activity Allow activity

169 Packet Filtering Static Packet Filtering
Allow or deny access to packets based on internal characteristics. access list 111 deny ip host any access list 111 permit tcp host any access list 111 deny icmp any any echo-request access list 111 permit icmp any any packet-to-big access list 111 deny icmp any any Cisco extended ACL

170 Static Packet Filtering
Difficult to design efficient rules. Easy to get the rules tables wrong and allow bad traffic. Security risks People can piggy-back bad messages in harmless ones. http traffic is known to be used as a backdoor. Loki uses unused fields in normal TCP packets. Fragmentation allows the filter to look only at a fragment Most only look at the first fragment

171 Static Packet Filtering
Configuring a packet filter: Security Policy: what is allowed, what is not allowed. Allowable types of packets must be specified logically, in terms of logical expression on packet fields. Expressions need to be rewritten in the firewall vendor’s language.

172 Static Packet Filtering
Example Security Policy: Allow inbound mail messages (SMTP, port 25), but only to gateway. Block host faucet. action Our host port Their host comment block * faucet We don’t trust these people. allow OUR-GW 25 Connection to our SMTP server

173 Static Packet Filtering
Example If no rule applies, then the packet is dropped. Without additional rules, our rule set would drop all non-mail packets. There would also be no replies. Beware of a rule like this (intended to allow acks) Based solely on outside host’s port number. Port 25 is usually the mail port. But there is no guarantee. action Our host port Their host comment allow * 25 Connection to their SMTP port

174 Static Packet Filtering
Example Expand rule set to allow connection with the outside: action Our host port Their host Flag comment block * faucet allow OUR-GW 25 (our host) Our packets to their port ACK Their replies Specify the names of all machines allowed to send mail to the outside here.

175 Static Packet Filtering
Combating Address Spoofing At a minimum: Don’t allow inside source addresses coming in. Don’t allow outside source addresses going out. Block source routing at the border routers.

176 Static Packet Filtering
Routing Information If a node is unreachable from the outside then the node is almost (but not quite) as safe as a node disconnected from the net. Internal routers should not advertise paths to such nodes to the outside. Filter routes learned from the outside: Protects against subversion by route confusion. Route squatting: Use internal addresses that belong to a different domain. The nodes are de facto unreachable from the outside. Use non-announced addresses. (e.g. 10.x.x.x) But beware, when companies merge, these addresses tend to be incompatible. So pick addresses in unpopular address ranges.

177 Static Packet Filtering
Performance Packet filtering is done at the border. No degradation for the internal network. Typically, connection to ISP is the bottleneck. However: Degradation depends on the number of rules applied. Can be mitigated by careful ordering of rules.

178 Application Level Filtering
Packet filters only look at The source address The destination address TCP / UDP port numbers TCP / UDP flags. Application filters deals with the details of the service they are checking. E.g. a mail application filter looks at RFC 822 headers. MIME attachments. Might identify virus infected attachments.

179 Application Level Filtering
Snort: Allows to set up rules that pass a packet on to another service. Commercial firewalls Include application level filters for many products. Use non-disclosure agreement to obtain proprietary protocols

180 Dynamic Packet Filtering
Stateful Firewall Still look at each packet. Maintains a state of each connection. Implements connection filtering. Dynamically adjust a filtering table of current connections. Implementation Adjust the filtering rules dynamically. E.g.: We started an HTTP connection to a given host. Now HTTP packages from that host are allowed. OR: Terminate the connection at the firewall and then have the firewall call the ultimate destination (proxying).

181 Proxy Firewalls Proxies act on behalf of a client. Proxy firewall
Reverse Proxy Receives packages on one card. Processes requests. Translates them into internal requests on other card. Receives answers from inside and translates to the outside.

182 Proxy Firewalls Proxy firewall Forward Proxy
Receives requests from the inside. Processes requests. Translates them into requests to the outside on other card. Receives answers from outside and translates to the inside. Acts on behalf of inside machine that is protected from the vagaries of the internet.

183 Proxy Firewalls Application level proxies work at the level of application. Circuit-level proxies does not understand the application makes filtering decisions by validating and monitoring sessions.

184 Possible Configurations Dual Homed Host
Internet Dual-homed host acting as firewall Internal Network

185 Possible Configurations Screened Host Architecture
Internet Router only allows traffic to bastion host (screening router) Bastion host sits on internal network Bastion host works as proxy Internal Network

186 Possible Configurations Screened Subnet
Internet Exterior Router a.k.a. access router Bastion host sits on perimeter network Perimeter Network Interior Router a.k.a. choke router Internal Network

187 Possible Configurations
Attach bastion host(s) to perimeter network (DMZ) Two possibilities to allow access to internet for internal hosts Use exterior and interior router to filter packages Use bastion host as proxy

188 Possible Configurations
O.K. to have many bastion hosts O.K. to merge interior and exterior router O.K. to merge bastion host and exterior router Performance of bastion host might not be sufficient O.K. to have many interior subnetworks. O.K. to have many exterior routers O.K. to have multiple perimeter networks NOT O.K. to merge bastion host and interior router Bastion host becomes single point of failure NOT O.K. to use multiple interior routers Need to maintain same policy on all interior routers

189 Securing Public Web Servers
Isolate the web server internet firewall internal network Only SQL Protocol permitted webserver SQL server firewall

190 Firewall Settings for DNS
Use a bastion host to host fake DNS server True DNS server on the interior network DNS query proceeds with DNS proxying: Local DNS client goes to local DNS server (interior network) Local DNS server sends query to bastion host (perimeter network) Bastion host forwards query to internet DNS system Internet DNS system answers question to bastion host Bastion host forwards to real DNS server Real DNS server forwards to local DNS client

191 Hiding DNS Server Internet Exterior Router a.k.a. access router
Fake DNS server Perimeter Network Interior Router a.k.a. choke router True DNS server Internal Network Local DNS client

192 Firewall Settings for DNS
Fake DNS server provides basic hostname and IP addresses for Machines in the perimeter network Machines in the interior network that someone on the outside needs to connect to. Fake information on machines that can contact the outside world directly.

193 Firewall Settings for DNS
Packet filtering on internal router needs to allow: DNS queries from the internal server to the bastion host server USP packets from port 53 from an internal host to port 53 bastion host TCP packets from ephemeral port on internal host to port 53 on bastion host Responses from bastion host to the internal server UDP packets from port 53 on bastion host to port 53 on internal server TCP packets with ACK bit set from port 53 on the bastion host to ephemeral ports on internal server DNS queries from bastion host DNS clients to internal server UDP and TCP packets from ephemeral ports on bastion host to port 53 on internal server Responses from internal server to bastion host DNS clients UDP and TCP packets with ACK bit from port 53 on the internal server to ephemeral ports on bastion host

194 Hiding DNS Server Internet Exterior Router a.k.a. access router
Fake DNS server Perimeter Network Interior Router a.k.a. choke router True DNS server Internal Network Local DNS client

195 Application Inspection
Dynamic Firewalls allow selective inspection of applications: http ftp dns icmp

196 Application Inspection
DNS example (Cisco ASA DNS inspection) Guarantees that the ID of the DNS machine matches the ID of the DNS query Allows translation of DNS packets using NAT Reassembles DNS packets to verify its length.

197 Application Inspection
SMTP (Cisco ASA protection) Protects against SMTP-based attacks by restricting the types of SMTP commands. Illegal command is modified and forwarded. Typically, receiver replies with an SMTP error 500 (command not recognized) Checks size, …

198 Virtual Private Networks

199 Virtual Private Networks
VPN uses connections over an existing public network Connection secured with encryption Host to Host Host to Gateway Gateway to Gateway

200 Virtual Private Networks

201 Virtual Private Networks
Encryption can be done at Application level. Transport level. Network level. Data link level.

202 Virtual Private Networks VPN Technologies
Application Level Pretty Good Privacy Secure Shell (SSH) Transport Level Secure Socket Layer Does not protect the package, but its content. Typically runs at the application level of the OS, so OS does not need to be changed. Network Level IPSec Encrypts package itself. Encrypted package receives a new package header. IPSec protects port address, but not destination address. OS need to be changed (but only once: Win2000, WinXP) Data Link Layer 2 Tunneling Protocol addition to Point-to-Point protocol (PPP) Encrypts packets on the data layer. L2TP (Layer 2 Tunneling)

203 Virtual Private Networks
Alternatives are dedicated point-to-point connections such as a private T1 line. Most secure. Most expensive. Takes time to set-up.

Download ppt "Network Defense COEN 250."

Similar presentations

Ads by Google