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Submitted By sunilkhan
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CS 487

Firewalls and Network Defense

slide 1

 Idea: separate local network from the Internet
Trusted hosts and networks Firewall

Intranet DMZ

Router Demilitarized Zone: publicly accessible servers and networks

slide 2

Castle and Moat Analogy
 More like the moat around a castle than a firewall
• Restricts access from the outside • Restricts outbound connections, too (!!)
– Important: filter out undesirable activity from internal hosts!

slide 3

• System to enhance of protection a local system or network of systems from networkbased security threats • All this while affording access to the outside world via WAN`s or the Internet • Deployed because of assumptions about different networks

slide 4

• A policy that dictates what traffic to allow, what to block, why this is done, and what to do if things break • Log connections and refused attempts • Periodic Auditing necessary for any successful deployment

slide 5

Firewall Locations in the Network
 Between internal LAN and external network  At the gateways of sensitive subnetworks within the organizational LAN
• Payroll’s network must be protected separately within the corporate network

 On end-user machines
• “Personal firewall” • Microsoft’s Internet Connection Firewall (ICF) comes standard with Windows XP slide 6

Firewall Types
 Packet- or session-filtering router (filter)  Proxy gateway
• All incoming traffic is directed to firewall, all outgoing traffic appears to come from firewall • Application-level: separate proxy for each application
– Different proxies for SMTP (email), HTTP, FTP, etc. – Filtering rules are application-specific

• Circuit-level: application-independent, “transparent”
– Only generic IP traffic filtering (example: SOCKS)

 Personal firewall with application-specific rules
• E.g., no outbound telnet connections from email client slide 7

Firewall Types: Illustration

slide 8

Packet Filtering
 For each packet, firewall decides whether to allow it to proceed
• Decision must be made on per-packet basis
– Stateless; cannot examine packet’s context (TCP connection details – state of the connection.)

 To decide, use information available in the packet
• • • • IP source and destination addresses, ports Protocol identifier (TCP, UDP, ICMP, etc.) TCP flags (SYN, ACK, RST, PSH, FIN) ICMP message type slide 9

 Filtering rules are based on pattern-matching

Packet Filtering Examples

slide 10

Example: FTP
FTP server
20 Data 21 Command

(borrowed from Wenke Lee)
FTP client

 Client opens command channel to server; tells server second port number  Server acknowledges  Server opens data channel to client’s second port  Client acknowledges

Connection from a random port on an external host



151” PORT 5






slide 11

FTP Packet Filter
The following filtering rules allow a user to FTP from any IP address to the FTP server at access-list 100 permit tcp any gt 1023 host eq 21 access-list 100 permit tcp any gt 1023 host eq 20 ! Allows packets from any client to the FTP control and data ports access-list 101 permit tcp host eq 21 any gt 1023 access-list 101 permit tcp host eq 20 any gt 1023 ! Allows the FTP server to send packets back to any IP address with TCP ports > 1023 interface Ethernet 0 access-list 100 in ! Apply the first rule to inbound traffic access-list 101 out ! Apply the second rule to outbound traffic ! Anything not explicitly permitted by the access list is denied!

slide 12

Weaknesses of Packet Filters
 Do not prevent application-specific attacks
• For example, if there is a buffer overflow in URL decoding routine, firewall will not block an attack string

 No user authentication mechanisms
• … except (spoofable) address-based authentication • Firewalls don’t have any upper-level functionality

 Vulnerable to TCP/IP attacks such as spoofing
• Solution: list of addresses for each interface (packets with internal addresses shouldn’t come from outside)

 Security breaches due to misconfiguration slide 13

Stateless Filtering Is Not Enough
 In TCP connections, ports with numbers less than 1024 are permanently assigned to servers
• 20,21 for FTP, 23 for telnet, 25 for SMTP, 80 for HTTP…

 Clients use ports numbered from 1024 to 16383
• They must be available for clients to receive responses

 What should a firewall do if it sees, say, an incoming request to some client’s port 5612?
• It must allow it: this could be a server’s response in a previously established connection… • …OR it could be malicious traffic • Can’t tell without keeping state for each connection slide 14

Example: Variable Port Use

Inbound SMTP

Outbound SMTP

slide 15

Session Filtering
 Decision is still made separately for each packet, but in the context of a connection
• If new connection, then check against security policy • If existing connection, then look it up in the table and update the table, if necessary
– Only allow incoming traffic to a high-numbered port if there is an established connection to that port

 Hard to filter stateless protocols (UDP) and ICMP  Typical filter: deny everything that’s not allowed
• Must be careful filtering out service traffic such as ICMP slide 16

Example: Connection State Table

slide 17

Application-Level Gateway

 Splices and relays two application-specific connections
• Example: Web browser proxy • Daemon spawns proxy process when communication is detected • Big processing overhead, but can log and audit all activity

 Can support high-level user-to-gateway authentication
• Log into the proxy server with your name and password

 Simpler filtering rules than for arbitrary TCP/IP traffic  Each application requires implementing its own proxy slide 18

Circuit-Level Gateway

 Splices two TCP connections, relays TCP segments  Less control over data than application-level gateway
• Does not examine the contents of TCP segment

 Client’s TCP stack must be aware of the gateway  Often used when internal users are trusted
• Application-level proxy on inbound connections, circuit-level proxy on outbound connections (lower overhead) slide 19

Performance Modify client application Defends against fragm. attacks

 Packet filter  Session filter  Circuit-level gateway  Application-level gateway


No No Yes

No Maybe Yes Yes



slide 20

Bastion Host
 Bastion host is a hardened system implementing application-level gateway behind packet filter
• All non-essential services are turned off • Application-specific proxies for supported services
– Each proxy supports only a subset of application’s commands, is logged and audited, disk access restricted, runs as a nonprivileged user in a separate directory (independent of others)

• Support for user authentication

 All traffic flows through bastion host
• Packet router allows external packets to enter only if their destination is bastion host, and internal packets to leave only if their origin is bastion host

slide 21

Single-Homed Bastion Host

If packet filter is compromised, traffic can flow to interrnal network

slide 22

Dual-Homed Bastion Host

No physical connection between internal and external networks

slide 23

General Problems with Firewalls
 Interfere with networked applications  Don’t solve the real problems
• Buggy software (think buffer overflow exploits) • Bad protocol design

 Generally don’t prevent denial of service  Don’t prevent insider attacks  Increasing complexity and potential for misconfiguration

slide 24

Protecting Addresses and Routes
 Hide IP addresses of hosts on internal network
• Only services that are intended to be accessed from outside need to reveal their IP addresses • Keep other addresses secret to make spoofing harder

 Use NAT (network address translation) to map addresses in packet headers to internal addresses

slide 25

Reading Assignment
 “Firewall Gateways” (chapter 3 of “Firewalls and Internet Security” by Cheswick and Bellovin)
• Linked from the course website

slide 26

 process of breaking down an IP datagram into smaller packets to be transmitted over different types of network media  reassembling them at the other end  is necessary in order for traffic, which is being sent across different types of network media to reach successfully  reason for this is that different types of network media and protocols have different rules involving the maximum size allowed for datagrams (MTU) slide 27

Fragmentation rules
 Rules for packet to be successfully reassembled at the destination each fragment  Must share a common fragment identification number (fragment Id.)  Each fragment must say what its place or offset is in the original unfragmented packet.  Each fragment must tell the length of the data carried in the fragment.  Finally the fragment must know whether more fragments follow this one. slide 28

Abnormal Fragmentation

For example, ACK bit is set in both fragments, but when reassembled, SYN bit is set (can stage SYN flooding through firewall)

slide 29

Fragmentation Attack
Telnet Server

(borrowed from Wenke Lee)

Telnet Client

, Send 2 fragments with the ACK bit set; fragment offsets are chosen so that the full datagram re-assembled by server forms a packet with the SYN bit set (the fragment offset of the second packet overlaps into the space of the first packet)  All following packets will have the ACK bit set

Allow only if ACK bit set
23 1234

(with FRAG1 (wit FRAG2
SYN packet (no ACK)


" "

h ACK)


slide 30

More Fragmentation Attacks
 Split ICMP message into two fragments, the assembled message is too large
• Buffer overflow, OS crash

 Fragment a URL or FTP “put” command
• Firewall needs to understand application-specific commands to catch this

 chargen attacks
• “Character generation” debugging tool: connect to a certain port and receive a stream of data • If attacker fools it into connecting to itself, CPU locks slide 31…...

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... Networks, Telecommunications, and Wireless Computing | | | Telecommunication systems enable the transmission of data over public or private networks. A network is a communications, data exchange, and resource-sharing system created by linking two or more computers and establishing standards, or protocols, so that they can work together. Telecommunication systems and networks are traditionally complicated and historically ineffi cient. However, businesses can benefi t from today’s modern network infrastructures that provide reliable global reach to employees and customers. Businesses around the world are moving to network infrastructure solutions that allow greater choice in how they go to market—solutions with global reach. These alternatives include wireless, voice-over internet protocol (VoIP), and radio-frequency identification (RFID). | | | | | Knowledge Areas | Business Dilemma | | | Business Dilemma Personal sensing devices are becoming more commonplace in everyday life. Unfortunately, radio transmissions from these devices can create unexpected privacy concerns if not carefully designed. We demonstrate these issues with a widely-available commercial product, the Nike+iPod Sport Kit, which contains a sensor that users put in one of their shoes and a receiver that users attach to their iPod Nanos. Students and researchers from the University of Washington found out that the transmitter in a sneaker can be read up to 60 feet away. Through the use of......

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