1. email or call for office visit, or call Kathy Cheek, 404 894-5696
ECE-6612
Prof. John A. Copeland
fax
Office: Centergy 5177
or call for office visit, or call Kathy Cheek,
Chapter 10a - Firewalls

2. Computer System Evolution
Central Data Processing System: - with directly attached peripherals (card reader, magnetic tapes, line printer).
Local Area Networks: - connects PC’s (in “terminal emulation” mode), remote terminals (next building) and mini-computers.
Premises Network: - connects LANs and LAN-attached devices to each other.
Enterprise-wide Network: - leased data lines (T1, DS-3) connect various offices.
Internet Connectivity: - initially for , now for Web access, e-commerce, Makes the world accessible, but now the world also has access to you. 2

3. Connectivity Provided by the Georgia Backbone Network•
Citizens •
Schools
WWW •
Contractors •
Libraries •
City & County •
Kiosks
Governments
Firewalls
State WWW Gateway
State Internet
Agency Gateway &
Other Agencies
Web Server
Agency Virtual
Private Network
Private Virtual
Connection
Agency
Server
LANs at Agency
Offices across Georgia
Non-Agency
State Server 3

4. Agency Firewall - Protects Agency Subnets
from Unwanted Connections
from Unwanted Connections
Subnet 1
Subnet 2
Gate-
WAN
Gate-
way
way
Firewalls (and many routers) can reject: •
Packets with certain source and destination addresses •
Packets with certain high-level protocols (UDP, Telnet)
Proxy Servers - for specific applications •
messages assembled and inspected, then passed to
internal server machine.
Prevent Cyber Loafing - Exploring the Internet for fun. 4

5. Router-Firewall can drop packets based on source or destination,
Web Server
Browser
Application
Application
Router-Firewall
can drop packets
based on
source or destination,
ip address and/or port
Layer
Layer
(HTTP)
(HTTP)
Port 80
Port 31337
Transport
Transport
Layer
Layer
(TCP,UDP)
(TCP,UDP)
Segment No.
Segment No.
Network
Network
Layer (IP)
Layer (IP)
IP Address
Network
Network
IP Address
Layer
Layer
Token Ring
E'net Data
Token Ring
E'net Data
Link Layer
Link Layer
Data-Link Layer
Data Link Layer
Ethernet
Token Ring
E'net Phys.
Token Ring
Phys. Layer
Phys. Layer
Layer
Phys. Layer 5

6. Transport or App.-Layer
Process
Process
Application
Application
Layer (HTTP,
Gateway, or Proxy
Layer
FTP, TELNET,
(HTTP(HTTP,
SMTP)
FTP, TELNET,
SMTP)
Transport
Transport
Transport
Transport
Layer
Layer
Layer
Layer
(TCP, UDP)
(TCP, UDP)
(TCP, UDP)
(TCP,UDP)
Network
Network
Network
Network
Layer (IP)
Layer (IP)
Layer (IP)
Layer (IP)
E'net Data
TR Data
Link
Link
E'net Data
TR Data
Layer
Link Layer
Layer
Link Layer
E'net Phys.
E'net Phys.
TR Phys.
TR Phys.
Layer
Layer
Layer
Layer 6

7. Policy
No outside Web access.
Outside connections to Public Web Server Only.
Prevent Web-Radios from eating up the available bandwidth.
Prevent your network from being used for a Smuft DoS attack.
Prevent your network from being tracerouted or Ping scanned.
Firewall Setting
Drop all outgoing packets to any IP, Port 80
Drop all incoming TCP SYN packets to any IP except 130:207: , port 80
Drop all incoming UDP packets - except DNS and Router Broadcasts.
Drop all ICMP packets going to a “broadcast” address ( or ).
Drop all incoming ICMP, UDP, or TCP echo-request packets, drop all packets with TTL < 5. 7

8. Firewall Attacks
IP Internal-Address Spoofing
Source Routing (External Spoof)
Tiny Fragment Attacks
2nd-Fragment Probes
SYN-ACK Probes
Internal Hacking
Firewall Defense
Drop all incoming packets with local address.
Drop all IP packets with Source-Routing Option.
Drop all incoming packet fragments with small size.
Assemble IP fragments (hard work), or at least *.
Be “Stateful” -keep track of TCP outgoing SYN
packets (start of all TCP connections) (hard work).
Drop all outgoing packets which do not have an "internal" source IP address.
* Fragments after the first one have no transport header (no way to tell if it is TCP, UDP, ICMP, ... , or determine port numbers. Firewall must at least keep a temporary list of approved IP ID-Numbers based on the first fragment decision. 8

9. A Firewall is a single point that a Network Administrator can control, even if individual computers are managed by workers or departments.
Over half of corporate computer misfeasance is caused by employees who are already behind the main firewall.
Solution 1 - isolate subnets with firewalls (usually routers or Ethernet switches with “filter” capabilities). Protect Finance department from Engineering department [Problem: Internal network is much higher bit rate, firewalls more expensive].
Solution 2 - implement /etc/hosts.allow, “IP Chains”, or “IP Tables” (PC “Personal Firewalls”) to limit access except from individual computers on certain ports for specific hosts and subnets. 9

10. "inetd" and "xinetd" hosts.allow
#cat /etc/hosts.deny
ALL:ALL
#cat /etc/hosts.allow
in.telnetd:
in.ftpd:
UNIX and Linux computers allow network contact to be limited to individual hosts or subnets ( means any).
Above, telnet connection is available to all on the subnet, and a single off-subnet host, FTP service is available to only to two local hosts, .19 and .102.
The format for each line is “daemon:host-list” 10

11. * replaced by IP Tables ("iptables”)
IP Chains*
The kernel starts with three lists of rules; these lists are called firewall chains or just chains. The three chains are called input, output and forward. When a packet comes in (say, through the Ethernet card) the kernel uses the input chain to decide its fate. If it survives that step, then the kernel decides where to send the packet next (this is called routing). If it is destined for another machine, it consults the forward chain. Finally, just before a packet is to go out, the kernel consults the output chain.
A chain is a checklist of rules. Each rule says `if the packet header looks like this, then here's what to do with the packet'. If the rule doesn't match the packet, then the next rule in the chain is consulted. Finally, if there are no more rules to consult, then the kernel looks at the chain policy to decide what to do. In a security-conscious system, this policy usually tells the kernel to reject or deny the packet.
* replaced by IP Tables ("iptables”) 11

12. IPchains - Linux Firewall and Router utility program
| ACCEPT/ "lo" (local) interface |
v REDIRECT _______ |
C --> S --> ______ --> D --> ~~~~~~~~ -->|forward|----> _______ -->
h a |input | e {Routing } |Chain | |output |ACCEPT
e n |Chain | m {Decision} |_______| --->|Chain |
c i |______| a ~~~~~~~~ | | ->|_______|
k t | s | | | | |
s y | q | v | | |
u | v e v DENY/ | | v
m | DENY/ r Local Process REJECT | | DENY/
| v REJECT a | | | REJECT
| DENY d |
v e
DENY
not “lo” interface
ipchains -A good-if -i ! eth1 -j DENY
ipchains -A good-if -p ICMP --icmp-type ping -j ACCEPT
ipchains -A good-if -p ICMP --icmp-type pong -j ACCEPT
ipchains -A good-if -j icmp-acc
IPchains - Linux Firewall and Router utility program 12

13. IP Tables and Netfilter
Whereas ipchains and ipfwadm combine packet filtering and NAT (particularly three specific kinds of NAT, called masquerading, port forwarding and redirection), netfilter makes it possible to separate packet operations into three parts: packet filtering, connection tracking, and Network Address Translation. Each part connects to the netfilter hooks at different points to access packets. The connection tracking and NAT subsystems are more general and more powerful than the stunted versions within ipchains and ipfwadm.
This split allowed iptables, in turn, to use the information which the connection tracking layer had determined about a packet: this information was previously tied to NAT. This makes "iptables" superior to "ipchains" because it has the ability to monitor the state of a connection and redirect, modify or stop data packets based on the state of the connection, not just on the source, destination or data content of the packet.
A firewall using iptables this way is said to be a stateful firewall versus ipchains, which can only create a stateless firewall (except in very limited cases). It can be said that ipchains is not aware of the full context from which a data packet arises, whereas iptables is, and therefore iptables can make better decisions on the fate of packets and connections.
Excerpt from 13

14. Stateful Firewall Local PC ip1 External Host ip2 14 TCP SYN
establishes “state” (ip1,ip2,tcp, 33489,80)
TCP SYN-ACK or RESET or relatedICMP
established “state” (ip1,ip2,tcp, 33489,80)
TCP ACK’s
established “state” (ip1,ip2,tcp, 33489,80)
TCP or UDP or ICMP
Not part of an established “state” 14

15. # iptables -L
Chain INPUT (policy DROP)
target prot opt source destination
DROP tcp -- anywhere anywhere tcp flags:SYN,RST/SYN,RST
DROP tcp -- anywhere anywhere tcp flags:FIN,SYN/FIN,SYN
ACCEPT udp -- anywhere anywhere udp spt:domain
ACCEPT tcp -- localhost anywhere tcp spt:smtp
ACCEPT tcp -- anywhere anywhere tcp spt:smtp state ESTABL ACCEPT udp -- anywhere anywhere udp spt:ntp
ACCEPT icmp lancope.com anywhere
DROP all / anywhere
DROP all -- anywhere /8
DROP icmp -- anywhere anywhere state NEW
DROP all -- anywhere anywhere
Chain FORWARD (policy DROP)
Chain OUTPUT (policy DROP)
ACCEPT icmp -- anywhere /24
ACCEPT icmp -- anywhere anywhere state RELATED,ESTABLISHED
Warning- the “-l” listing does not indicate the restriction on network port (lo, eth0, eth1, ...).
For example, the first line may appear to accept all tcp packets, but not show this applies only to tcp packets from the “lo” port. 15

16. Router Setup with Network Address Translation (NAT)
Addresses 10.x.y.z and x.y reserved for private networks. 16

17. 17

18. 18

19. Router 24.88.48.47 with NAT that Masquerades
FTP Client
Internet
To :21
from :x
To :x
from :21 1 4
Router with NAT that Masquerades
could be a “dual-homed bastion host”
To :x
from :21
To :21
from :x 3 2
Host
Host
Host
Host
Web Server
port 80
FTP Server
port 21
External FTP client accessing an internal FTP server
Note: x is a high port number, usually
,535 19

20. Router 24.88.48.47 with NAT that Masquerades
Web Server
Internet
To :x
from :80
To :80
from :x 3 2
Router with NAT that Masquerades 4
To :x
from :80
To :80
from :x 1
Host
Web Client
Host
Host
Web Server
port 80
FTP Server
port 21
Local Web client accessing an external Web server 20

21. Combined Firewalls and IDS
(see also: ISS Proventia) 21

22. 22 Protocol Anomaly Detection
WatchGuard Transparent Application layer proxies examine entire connection data streams, identifying protocol anomalies and discarding harmful or questionable information.
In addition, WatchGuard firewalls perform:
* Packet Handling - prevents packets from entering the network until they are reassembled and examined.
* Packet Reassembly - reassembles packet fragments to prevent fragment overlap attacks such as Teardrop and other Layer 3 protocol anomaly based attacks.
Signature Element Analysis
Rather than using signatures that precisely identify specific attacks, WatchGuard systems look at what any attack of a certain type (e.g., ) must do to succeed (e.g., auto-execute an attachment). With rule sets, you can choose to allow or deny traffic, or even deny all traffic from a source for a specific period.
In addition to rigorous rule sets, the firewall processes policy-based configurations, and management subsystems perform state and content analysis. These processes protect against entire known and unknown attack classes, and can narrow the vulnerability window without having to make you wait for updated attack-specific signatures.
Behavior-Based Analysis
Although behavior-based intrusion detection is a relatively new technology, WatchGuard has mechanisms in place within the firewall to identify known attack behaviors, such as:
* Port scans and probes
* Spoofing
* SYN flood attacks
* DoS and DDoS attacks
* The misuse of IP options such as source routing 22