This document discusses key concepts related to network defense in depth. It defines common terms like firewalls, DMZs, IDS, and VPNs. It also covers techniques for packet filtering, application inspection, network address translation, and virtual private networks. The goal of defense in depth is to implement multiple layers of security and not rely on any single mechanism.

1. COEN 350
Network Defense in Depth
Firewalls

2. Terms of the 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.

3. Terms of the 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.

4. COEN 351 E-Commerce Securit
 E-Commerce Security Course Homepage
 Lecture Notes

5. Terms of the 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.

6. Defense in Depth
 Rule 1: Multitude of security measures.
 Do not relay on one security mechanism.
 Rule 2: Do not make security so
expensive / burdensome that you give
legitimate users an incentive to
circumvent security.

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

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

9. 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.

10. 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.

11. Filtering
 Signature Triggers
 Pattern 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

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

13. Packet Filtering
 Static Packet Filtering
 Allow or deny access to packets based on
internal characteristics.
access list 111 deny ip host 205.205.205.205.1 any
access list 111 permit tcp host 205.205.205.205.1 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

14. 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

15. 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.

16. 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 port comment
block * * faucet * We don’t trust
these people.
allow OUR-
GW
25 * * Connection to our
SMTP server

17. 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 port comment
allow * * * 25 Connection to
their SMTP port

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

19. 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.

20. 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.

21. 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.

22. 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.

23. 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

24. 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).

25. 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.

26. 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.

27. 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.

28. Application Inspection
 Dynamic Firewalls allow selective
inspection of applications:
 http
 ftp
 dns
 icmp
 …

29. 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.

30. 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, …

31. Network Address Translation
 Originally designed to address the IPv4
address shortage:
 Use internal IP addresses

192.168.x.x

172.16.x.x, 172.32.x.x

10.x.x.x
 NAT box is dual hosted:

One connection to interior network

Other connection to exterior network
 with “overloaded” or “public” address

32. Network Address Translation
 Internal host initiates TCP connection to the
outside.
 NAT box takes TCP package, replaces
source IP with its public IP, port with a port
chosen for that connection
 When reply return to NAT box, forwards
package to internal host.
 NAT uses stored connection data to
determine the interior address

33. Network Address Translation
 NAT increases security:
 NAT hides host identities
 NAT hides weak sequence number generating
hosts by randomizing sequence numbers.
 NAT disables inbound TCP connections
 NAT breaks most UDP protocols
 NAT allow load balancing and transparent
failover

34. Network Address Translation
 Full Cone NAT (one to one NAT)
 all requests from same internal IP address
and port are mapped to the same external
IP address and port.
 Allows external host to send package to
the host by using the mapped external
address.

35. Network Address Translation
 Restricted Cone NAT
 All requests from same internal IP address
and port are mapped to the same external
IP address and port.
 External host can only send package to
internal host if the connection has already
been established

36. Network Address Translation
 Port Restricted Cone NAT
 Like restricted cone NAT,

but only for certain port numbers
 Symmetric NAT
 All request from the same internal IP address and port to a
specific destination IP and port are mapped to a unique
external source IP address and port.
 If the same hosts sends to the same port, but another IP
address, then a different mapping is used.
 External host needs to receive a packet before sending a
UDP packet back to the internal host.
 In practice:
 combinations between these behaviors.

37. Virtual Private Networks

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

39. Virtual Private Networks

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

41. 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)

42. COEN 351 E-Commerce Securit
 E-Commerce Security Course Homepage
 Lecture Notes

43. 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.