1. Firewalls
firewall
Isolates organization’s internal net from larger Internet, allowing some packets to pass, blocking others.
administered
network
public
Internet
firewall
Network Security

2. Firewalls: Why Prevent denial of service attacks:
Denial-of-Service (DoS) attack:
Send many fake requests to congest link or consume server resource (CPU, memory)
SYN flooding:
attacker sends many SYNs to victim; victim has to allocate connection resource; victim has no resource left for real connection requests any more.
Usually with spoofed source IP address
Prevent illegal modification/access of internal data.
e.g., attacker replaces CIA’s homepage with something else
Network Security

3. Firewalls: Why Allow only authorized access to inside network
Set of authenticated users
Set of authorized IP addresses
Two types of firewalls:
application-level
Checking application level data
packet-filtering
Checking TCP or IP packets only
Network Security

4. Should arriving packet be allowed in? Departing packet let out?
Packet Filtering
Should arriving packet be allowed in? Departing packet let out?
internal network connected to Internet via router firewall
router filters packet-by-packet, decision to forward/drop packet based on:
source IP address, destination IP address
TCP/UDP source and destination port numbers
ICMP message type
TCP SYN and ACK bits
Network Security

5. (control panelsecurity centerwindows firewall)
Packet Filtering
Example 1: block incoming and outgoing datagrams with IP protocol field = 17 and with either source or dest port = 23.
All incoming and outgoing UDP flows and telnet connections are blocked.
Example 2: Block inbound TCP segments with SYN=1.
Prevents external clients from making TCP connections with internal clients, but allows internal clients to connect to outside.
Example of Windows XP
service pack 2 firewall
(control panelsecurity centerwindows firewall)
Network Security

6. Stateless packet filtering: more examples
Policy
Firewall Setting
No outside Web access.
Drop all outgoing packets to any IP address, port 80, 443
No incoming TCP connections, except those for institution’s public Web server only.
Drop all incoming TCP SYN packets to any IP except , port 80,443
Prevent Web-radios (UDP based) from eating up the available bandwidth.
Drop all incoming UDP packets - except DNS and router broadcasts.
Prevent your network from being used for a smurf DoS attack.
Drop all ICMP packets going to a “broadcast” address (eg ).
Prevent your network from being tracerouted
Drop all outgoing ICMP TTL expired traffic
Network Security

7. A Real Example: Lab setup when I was a PhD student
DMZ
Gateway: a cheap Linux machine runs Iptables
Web server: Linux machine runs Apache web server
Main server: Linux machine runs SSH, Sendmail (SMTP and IMAPS)
Allow a specific machine outside to print to main server’s printer
Network Security

8. Traffic Allowance Policy:
DMZ
Traffic Allowance Policy:
(1). Internet ==> LAN
   To Main Server: SSH, SMTP,IMAP+SSL, LPD request from a trusted IP.
(2). Internet ==> DMZ Web Server SSH, Web request.
(3). Internet ==> Gateway ----    SSH only (for admin)
(4). DMZ Web Server==> LAN, Internet
    SSH to Internet and only to Main Server in LAN;
DNS request; WWW/FTP request to Internet.
(5). LAN ==> Internet, DMZ WebServer SSH, DNS,
WWW/FTP, SMTP, RealPlay.
Network Security

9. HTTP forwarding from Internet to DMZ web server (port 80)
iptables -t nat -A PREROUTING -p tcp --sport $UNPRIVPORTS -d $out.IP --dport 80 -j DNAT --to-destination $DMZ_SERVER
iptables -A FORWARD -o $eth2 -p tcp --sport $UNPRIVPORTS -d $DMZ_SERVER --dport 80 -m state --state NEW -j ACCEPT
SMTP from internal mail server to external Mail Server
iptables -A FORWARD -i $eth1 -o $eth0 -p tcp -s $LAN_MAINSERVER --sport $UNPRIVPORTS --dport 25 -m state --state NEW -j ACCEPT
Allow Telnet to outside from LAN
iptables -A FORWARD -i $LAN_IF -o $EXT_IF -p tcp --sport $UNPRIVPORTS --dport 23 -m state --state NEW -j ACCEPT
Network Security

10. Application gateways
gateway-to-remote
host telnet session
host-to-gateway
telnet session
Filters packets on application data as well as on IP/TCP/UDP fields.
Example: allow select internal users to telnet outside.
application
gateway
router and filter
1. Require all telnet users to telnet through gateway.
2. For authorized users, gateway sets up telnet connection to dest host. Gateway relays data between 2 connections
3. Router filter blocks all telnet connections not originating from gateway.
Example: block user access to know porn websites
Check if the Web URL is in a “black-list”
Network Security

11. Limitations of firewalls and gateways
IP spoofing: router can’t know if data “really” comes from claimed source
SYN flood attack
UDP traffic
client software must know how to contact application gateway.
e.g., must set IP address of proxy in Web browser
Speed constraint on high-bandwidth link
Application-level firewall is time consuming
filters often use all or nothing policy for UDP
Usually most incoming UDP ports are blocked
The trouble caused to real-time Internet video
Network Security

12. Limitations of firewalls and gateways
tradeoff: degree of communication with outside world, level of security
Trend --- remote office
Blurred boundary between inside <-> outside
Employee laptop threat
many highly protected sites still suffer from attacks
Network Security

13. Internet security threats
Mapping:
before attacking: “case the joint” – find out what services are implemented on network
Use ping to determine what hosts have addresses on network
Port-scanning: try to establish TCP connection to each port in sequence (see what happens)
nmap ( mapper: “network exploration and security auditing”
Countermeasures?
Network Security

14. Internet security threats
Mapping: countermeasures
record traffic entering network
look for suspicious activity (IP addresses, pots being scanned sequentially)
Firewall to block incoming TCP/SYN to ports or computers not providing the services
Block ping traffic
Network Security

15. Internet security threats
Packet sniffing:
broadcast media
promiscuous NIC reads all packets passing by
can read all unencrypted data (e.g. passwords)
e.g.: C sniffs B’s packets A C
src:B dest:A payload B
Countermeasures?
Network Security

16. Internet security threats
Packet sniffing: countermeasures
all hosts in orgnization run software that checks periodically if host interface in promiscuous mode.
one host per segment of broadcast media (switched Ethernet at hub) A C
src:B dest:A payload B
Network Security

17. Internet security threats
IP Spoofing:
can generate “raw” IP packets directly from application, putting any value into IP source address field
receiver can’t tell if source is spoofed
e.g.: C pretends to be B A C
src:B dest:A payload B
Countermeasures?
Network Security

18. Internet security threats
IP Spoofing: egress filtering
routers should not forward outgoing packets with invalid source addresses (e.g., datagram source address not in router’s network)
great, but egress filtering can not be mandated for all networks A C
src:B dest:A payload B
Network Security

19. Internet security threats
Denial of service (DOS):
flood of maliciously generated packets “swamp” receiver
Distributed DOS (DDOS): multiple coordinated sources swamp receiver
e.g., C and remote host SYN-attack A A C
SYN
SYN
SYN
SYN
SYN B
SYN
Countermeasures?
SYN
Network Security

20. Internet security threats
Denial of service (DOS): countermeasures
filter out flooded packets (e.g., SYN) before reaaching host
Cooperation with source routers
Detect spoofed SYN based on TTL values
traceback to source of floods (most likely an innocent, compromised machine) A C
SYN
SYN
SYN
SYN
SYN B
SYN
SYN
Network Security

21. Pretty good privacy (PGP)
Internet encryption scheme, de-facto standard.
uses symmetric key cryptography, public key cryptography, hash function, and digital signature as described.
provides secrecy, sender authentication, integrity.
inventor, Phil Zimmerman, was target of 3-year federal investigation.
A PGP signed message:
---BEGIN PGP SIGNED MESSAGE---
Hash: SHA1
Bob:My husband is out of town tonight.Passionately yours, Alice
---BEGIN PGP SIGNATURE---
Version: PGP 5.0
Charset: noconv
yhHJRHhGJGhgg/12EpJ+lo8gE4vB3mqJhFEvZP9t6n7G6m5Gw2
---END PGP SIGNATURE---
Network Security

22. Secure sockets layer (SSL)
provides transport layer security to any TCP-based application using SSL services.
e.g., between Web browsers, servers for e-commerce (shttp)
security services:
server authentication, data encryption, client authentication (optional)
Application
Application
SSL sublayer
SSL
socket
TCP
TCP
TCP
socket IP IP
TCP API
TCP enhanced with SSL
Network Security

23. Secure sockets layer (SSL)
server authentication:
SSL-enabled browser includes public keys for trusted CAs.
Browser requests server certificate, issued by trusted CA.
Browser uses CA’s public key to extract server’s public key from certificate.
check your browser’s security menu to see its trusted CAs.
transport layer security to any TCP-based app using SSL services.
used between Web browsers, servers for e-commerce (shttp).
security services:
server authentication
data encryption
client authentication (optional)
Network Security

24. SSL (continued) Encrypted SSL session:
Browser generates symmetric session key, encrypts it with server’s public key, sends encrypted key to server.
Using private key, server decrypts session key.
Browser, server know session key
All data sent into TCP socket (by client or server) encrypted with session key.
SSL: basis of IETF Transport Layer Security (TLS).
SSL can be used for non-Web applications, e.g., IMAP.
Client authentication can be done with client certificates.
Not widely used since too many clients
Network Security

25. SSL: three phases 1. Handshake:
TCP SYN
1. Handshake:
Bob establishes TCP connection to Alice
authenticates Alice via CA signed certificate
creates, encrypts (using Alice’s public key), sends master secret key to Alice
nonce exchange not shown
TCP SYNACK
TCP ACK
SSL hello
certificate
create
Master
Secret
(MS)
KA+(MS)
decrypt using KA-
to get MS
Network Security

26. SSL: three phases 2. Key Derivation:
Alice, Bob use shared secret (MS) to generate 4 keys:
EB: Bob->Alice data encryption key
EA: Alice->Bob data encryption key
MB: Bob->Alice MAC key (message authentication code)
MA: Alice->Bob MAC key
encryption and MAC algorithms negotiable between Bob, Alice
why 4 keys?
Different keys by each person
Different keys for encryption and integrity checking
Network Security

27. . . SSL: three phases 3. Data transfer TCP byte stream MB
b1b2b3 … bn d MB
block n bytes together
H( ) .
compute MAC EB d
H(d)
H( ) .
encrypt d, MAC, SSL seq. #
(why ssl#?)
SSL
seq. # d
H(d)
SSL record
format
Type Ver Len d
H(d)
unencrypted
encrypted using EB
Network Security

28. IPsec: Network Layer Security
network-layer secrecy:
sending host encrypts the data in IP datagram
TCP and UDP segments; ICMP and SNMP messages.
network-layer authentication
destination host can authenticate source IP address
two principal protocols:
authentication header (AH) protocol
encapsulation security payload (ESP) protocol
for both AH and ESP, source, destination handshake:
create network-layer logical channel called a security association (SA)
each SA unidirectional.
uniquely determined by:
security protocol (AH or ESP)
source IP address
32-bit connection ID
Network Security

29. Authentication Header (AH) Protocol
provides source authentication, data integrity, no confidentiality
AH header inserted between IP header, data field.
protocol field: 51
intermediate routers process datagrams as usual
AH header includes:
connection identifier
authentication data: source- signed message digest calculated over original IP datagram.
next header field: specifies type of data (e.g., TCP, UDP, ICMP)
IP header
data (e.g., TCP, UDP segment)
AH header
Network Security

30. IEEE 802.11 security Packet sniffing is unavoidable
War-driving: drive around Bay area, see what networks available?
More than 9000 accessible from public roadways
85% use no encryption/authentication
packet-sniffing and various attacks easy!
Wired Equivalent Privacy (WEP): authentication as in protocol ap4.0 (require shared symmetric key)
host requests authentication from access point
access point sends 128 bit nonce
host encrypts nonce using shared symmetric key
access point decrypts nonce, authenticates host
Network Security

31. IEEE 802.11 security Wired Equivalent Privacy (WEP): data encryption
Host/AP share 40 bit symmetric key (semi-permanent)
Host appends 24-bit initialization vector (IV) to create 64-bit key
IV is equivalent to a “nonce”
64 bit key used to generate stream of keys, kiIV (using RC4 stream cipher)
kiIV used to encrypt i-th byte, di, in frame:
ci = di XOR kiIV
IV and encrypted bytes, ci sent in frame
Network Security

32. Sender-side WEP encryption
Network Security

33. 802.11 WEP Security Concern 40 bits in encryption is too short
RC4 is not properly used in
A more secure protocol is standard i
Network Security

34. Breaking 802.11 WEP encryption
security hole:
24-bit IV, one IV per frame, -> IV’s eventually reused
IV transmitted in plaintext -> IV reuse detected
attack:
Trudy causes Alice to encrypt known plaintext d1 d2 d3 d4 …
Trudy sees: ci = di XOR kiIV
Trudy knows ci di, so can compute kiIV
Trudy knows encrypting key sequence k1IV k2IV k3IV …
Next time IV is used, Trudy can decrypt!
Network Security

35. Network Security (summary)
Basic techniques…...
cryptography (symmetric and public)
authentication
message integrity
key distribution
…. used in many different security scenarios
secure
secure transport (SSL)
IP sec
security
Network Security