Download Local Area Networks

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 email, now for Web access, e-commerce, ... .
Makes the world accessible, but now the world also has access to you.
1
Connectivity Provided by the
Georgia Backbone Network
• Schools
• Libraries
• Kiosks
WWW
• Citizens
• Contractors
• City & County
Governments
State WWW Gateway
State Internet
Agency Gateway &
Web Server
Other Agencies
Agency Virtual
Private Network
LANs at Agency
Offices across Georgia
Agency
Server
Private Virtual
Connection
Non-Agency
State Server
2
Agency Firewall -- Protects
Protects Agency
Agency Subnets
Subnets
from Unwanted Connections
Subnet 1
Subnet 2
Gateway
WAN
Gateway
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
• Email messages assembled and inspected, then passed to
internal email server machine.
Prevent Cyber Loafing - Exploring the Internet for fun.
3
Browser
Web Server
Application
Layer
(HTTP)
Port 80
Transport
Layer
(TCP,UDP)
Segment No.
Network
Layer (IP)
IP Address
130.207.22.5
E'net Data
Link Layer
Ethernet
Phys. Layer
Router-Firewall
can drop packets
based on
source or destination,
ip address and/or port
Network
Layer
Network
Layer
Token Ring
E'net Data
Link Layer Data Link Layer
E'net Phys.
Layer
Token Ring
Phys. Layer
Application
Layer
(HTTP)
Port 31337
Transport
Layer
(TCP,UDP)
Segment No.
Network
Layer (IP)
IP Address
24.88.15.22
Token Ring
Data-Link Layer
Token Ring
Phys. Layer
4
Process
Application
Layer (HTTP,
FTP, TELNET,
SMTP)
Transport or
App.-Layer
Gateway, or Proxy
Transport
Transport
Layer
Layer
(TCP, UDP)
(TCP, UDP)
Network
Network
Layer (IP)
Layer (IP)
E'net Data
TR Data
E'net Data
Link Layer
Link
Link
Layer
Layer
E'net Phys.
Layer
E'net Phys.
TR Phys.
Layer
Layer
Transport
Layer
(TCP, UDP)
Network
Layer (IP)
Process
Application
Layer
(HTTP(HTTP,
FTP, TELNET,
SMTP)
Transport
Layer
(TCP,UDP)
Network
Layer (IP)
TR Data
Link Layer
TR Phys.
Layer
5
Policy
No outside Web access.
Firewall Setting
Drop all outgoing packets to any IP, Port 80
Outside connections to Public Web
Server Only.
Drop all incoming TCP SYN packets to any IP
except 130:207:244.203, port 80
Prevent Web-Radios from eating up
the available bandwidth.
Drop all incoming UDP packets - except DNS
and Router Broadcasts.
Prevent your network from being
used for a Smuft DoS attack.
Drop all ICMP packets going to a “broadcast”
address (130.207.255.255 or 130.207.0.0).
Prevent your network from being
tracerouted or scanned.
Drop all incoming ICMP, UDP, or TCP echorequest packets, drop all packets with TTL < 5.
6
Firewall Attacks
Firewall Defense
IP Internal-Address Spoofing.
Drop all incoming packets with local address.
Source Routing (External Spoof).
Drop all IP packets with Source-Routing Option.
Tiny Fragment Attacks.
Drop all incoming packets with small offset.
2nd-Fragment Probes.
Assemble IP fragments (hard work).
SYN-ACK Probes.
Be “Stateful” -keep track of TCP outgoing SYN
packets (start of all TCP connections) (hard work).
7
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 from
Engineering.
Solution 2 - implement “IP Chains” to limit access to individual
computers at the lowest protocol level possible, to specific hosts
and subnets.
8
IP Chains
/etc/hosts.deny
ALL:ALL
/etc/hosts.allow
in.telnetd: 199.77.146 24.88.154.17
in.ftpd: 199.77.146.19 199.77.146.102
UNIX and Linux computers allow network contact to be limited
to individual hosts or subnets (199.77.146 means 199.77.146.any).
Above, telnet connection is available to all on the 199.77.146.0
subnet, and a single off-subnet host, 24.88.154.17 FTP service is
available to only to two local hosts, .19 and .102.
The format for each line is “daemon:host-list”
9
Router Setup with Network Address Translation (NAT)
Addresses 10.0.0.0 and 192.168.0.0 reserved for private networks.
FTP Client
130.27.8.35
Internet
To 24.88.48.47:23
from 130.27.8.35:x
To 130.27.8.35:x
from 24.88.48.47:23
Router 24.88.48.47 with NAT that Masquerades
could be a “dual-homed bastion host”
To 192.168.0.40:23
from 130.27.8.35:x
Host
192.168.0.10
Web Server
port 80
Host
192.168.0.20
To 130.27.8.35:x
from 192.168.0.40:23
Host
192.168.0.30
Host
192.168.0.40
FTP Server
port 23
Note: x is a high port number, 1024-65,535
11
Web Host
130.27.8.35
Internet
To 24.88.48.47:x
from 130.27.8.35:80
To 130.27.8.35:80
from 24.88.48.47:x
Router 24.88.48.47 with NAT that Masquerades
To 192.168.0.20:x
from 130.27.8.35:80
Host
192.168.0.10
Web Server
port 80
Web Client
192.168.0.20
To 130.27.8.35:80
from 192.168.0.20:x
Host
192.168.0.30
Host
192.168.0.40
FTP Server
port 23
12