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Network Security Attacks

There are several types of Network Security Attacks as described below:

  1. SYN Flood

    Here the attacker sends a SYN request from a spoofed source address. When the server responds with SYN-ACK, the source never replies back leaving the server handing with a half-open connection (Typically the client sends a SYN-ACK-ACK to complete the three way handshake). Half open connections consume resources eventually degrading the performance of the server.

    CISCO routers employ ‘TCP Intercept’ and ‘CAR – Committed Access Rate’ features to combat SYN-FLOOD. You can also change the default setting for the ‘maximum number of half-open TCP connections’

  2. UDP Flood

    Here the server is flooded with UDP requests, degrading the performance of the Server

  3. ICMP Flood

    Here the Server is flooded with ‘echo’ requests (which is an ICMP request), degrading the performance of the Server.

    It is best to drop ICMP packets at the router or Firewall.

  4. Smurf

    Here the attacker sends ICMP request packets to the broadcast address of the target network using a spoofed source address. The target responds with a echo request to all the hosts in the network, eventually overwhelming the network.

  5. Fraggle

    A flavor of Smurf attack which uses UDP Echo packets (UDP Port 7) instead of ICMP packets. Cisco routers can be configured to disable the TCP and UDP services (TCP and UDP small servers) to defend against Fraggle

  6. Bluejacking and bluesnarfing

    Here the Bluetooth enabled devices are attached. In Bluejacking, unsolicited messages are sent. In Bluesnarfing, personal information such as pictures and contacts, and cell phone information such as serial numbers are stolen.

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Protecting Wireless Networks using WEP,WPA and WPA2

Wired Equivalent Privacy:

The intention of WEP (Wired Equivalent Privacy) was to provide the same level of security as in Wired Networks. But it fell short greatly.

WEP uses 128bit key (with 24 bit initialization vector) which is very easy to crack. It uses RC4 (Rivest Cipher 4) stream cipher.

Two modes:

Open Systems Authentication:

No need of credentials from the client. After the initial association with AP (Access Point), WEP encrypts the whole conversation.

Shared Key Authentication:

Requires Client to present credentials to connect to AP before the encryption beings.

WEP can be enhanced by using ssh or tunneling.

WiFi Protected Access (WPA and WPA2):

WPA uses TKIP(Temporal Key Integrity Protocol), a sequence counter to prevent replay attacks and a 64 bit message integrity check. It combines a secret root key with initialization vector.

WPA2 uses AES with Cipher Block chaining message Authentication code Protocol (CCMP).

Both WPA and WPA supports several EAP extensions such as EAP-TLS, EAP-TTLS (Tunneled Transport Layer Security) and Protected EPA (PEAPv0,v1)

VPN (Virtual Private Network) Security Protocols

Commonly used VPN security technologies are:

  1. Point to Point Tunneling Protocol (PPTP)
  2. Layer2 Forwarding Protocol (L2F)
  3. Layer2 Tunneling Protocol (L2TP)
  4. IPSec
  5. SSL

Point to Point Tunneling Protocol (PPTP):

  1. Uses PAP, CHAP, EAP
  2. Typically used in dial-up connections in Windows platform
  3. Operates at Data Link Layer

Layer 2 forwarding Protocol (L2F):

  1. Developed by CISCO
  2. Similar to PPTP
  3. Operates at Data Link Layer

Layer 2 tunneling Protocol (L2TP)

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Remote access security technologies

There are 5 major remote access security technologies

  1. RAS (Remote Access Service)
  2. Radius
  3. Diameter

RAS (Remote Access Service):
Uses PPP (Point to Point Protocol) to secure dial-in, ISDN and serial links. Uses the following authentication mechanisms.

PAP (Password Authentication Protocol):

  1. Two way hand shake
  2. Sends passwords in clear text
  3. No protection against replay or brute force attacks

CHAP (Challenge Handshake Protocol):

  1. Uses three way hand shake
  2. Both server and client need to have a shared secret preconfigured
  3. Shared secret is stored in clear text. MS-CHAP allows the shared secret to be stored in encrypted form

EAP (Extensible Authentication Protocol):

  1. Used primarily in Wireless networks
  2. Supports various authentication mechanisms like MD5-Challenge, S/Key, generic token card and digital Certificates


RADIUS (Remote Authentication Dial-In Service)
  1. Open-Source UDP based.
  2. Provides authentication and accountability
  3. Use provides username/password to a RADIUS client using PAP or CHAP. Radius client encrypts password and sends to RADIUS Server for authentication


  1. Improved version of RADIUS
  2. Uses TCP. Supports IPSsec, TLS


TACACS (Terminal Access Controller Access Control System):
  1. Uses UDP. Provides Authentication, Authorization and Accountability
  2. XTACACS is an improved version but no longer used
  3. TACACS+ is the current version. Supports several authentication mechanisms – PAP,CHAP,MS-CHAP,EAP,KERBEROS,Token Cards

Firewall Classifications and Architectures

Classifications of Firewalls:

  1. Packet Filtering
  2. Circuit Level Gateway
  3. Application Level Gateway

Architectures of Firewall:

  1. Screening Router
  2. Dual Homed Gateway
  3. Screened-Host Gateway
  4. Screened Subnet






Packet Filtering

Basic. Operates at Network or Transport Layers. Examines TCP,IP,ICMP,UDP headers from the packet and routes based on a firewall ACL

  1. In expensive and Fast
  2. Easy to setup
  3. Transparent to users
  1. No Context level routing
  2. Can be hit by Spoofing
  3. Limited Logging
  4. No strong user authentication

Circuit Level Gateway

Operates at Session Layer. Uses state information about the established connections. Once the virtual circuit is formed, no packet analysis is done.

  1. Fast
  2. Low maintenance
  1. Limited Logging.
  2. Once connection is established, no further analysis is done

Application Level Gateway

Operates at Application Layer. Implemented as a Proxy Server.

  1. Supports Strong user authentication
  2. Data is not directly sent to the destination.
  1. Low performance because packet needs to be brought all the way up to Application layer for analysis
  2. High maintenance.






Screening Router

Basic Packet Filtering Firewall

  1. Cheap
  2. Transparent to users
  1. Makes internal Network structure complex
  2. No user authentication
  3. Single point of failure

Dual homed Gateway

It is bastion host with two network interface cards. It may be connected to an external screening router

  1. Fail safe mode. If it fails, nothing is allowed access
  2. Internal network structure is masked
  1. Additional auth required for users
  2. May slow down performance
  3. May not be available for all services.

Screened Host Gateway

External Screening router and internal Bastion Host.

  1. Transparent outbound access and restricted inbound access
  1. Screening router can by-pass the Bastion host
  2. Masking internal network is difficult

Screened subnet

Most secure. Forms a DMZ network between external and internal firewall

  1. Transparent, flexible
  2. Internal Network is masked
  1. Difficult to maintain
  2. Expensive


Layer 5,6,7 protocols (higher level protocols)

Here are the protocols commonly used in higher levels (5,6,7 of the OSI model)

Layer 5 (Session):

  1. NetBIOS
  2. NFS
  3. RPC
  4. SSH
  5. SIP

Layer 6: (Presentation):

  1. ASCII
  3. MPEG
  4. JPG
  5. GIF

Layer 7 (Application):

  2. SNMP
  3. SMTP
  6. POP3,IMAP
  7. PEM
  9. S-RPC

IP address classes

IP (Internet protocol) is a Network Layer protocol (Layer 3) that considered ‘routed’ protocol. It addresses the Network Packets so that routing protocols like OSPF,BGP and RIP can correctly route the packet.

IP defines the IP addresses. IP address is a 32 bit number (4 octets). It comprises of Network and Host numbers. The higher order bits define the Network number as shown below.

There are 5 classes of IP addresses:




Leading bits

Size of network
Number bit field

Size of rest bit field

Number of Networks

Addresses Per Network

Start address

End address





128 (27)

16,777,216 (224)





16,384 (214)

65,536 (216)





2,097,152 (221)

256 (28)



Class D is defined as Multicast. Address Range: 224 – 239

Class E is experimental. Address Range: 240 – 254 to is defined as loop back address range.


Also, a range of IP addresses are reserved for Private use (i.e not routable in internet). They are


Class A –

Class B –

Class C –


IPV6 uses 128 bit addresses and primarily introduced to address the depleting IPV4 addresses.


Open Shortest Path First (OSPF) Protocol:

It is a Link-State dynamic routing protocol used primarily in large networks. It routes packets within an AS (Autonomous System) – Interior gateway protocol (IGP). OSPF networks as assigned an Area identifier (32 bit length). The area identifier can be same as the IP address. OSPF can handle duplicate ip addresses without any conflict.

OSPF does not use UDP or TCP but rather directly encapsulated into IP datagrams.

OSPF areas include Backbone area (area 0), Stub area, not so stubby area (NSSA).

Border Gateway Protocol (BGP):

It is a path-vector based dynamic protocol that is widely used in ISP. It is an exterior gateway protocol (EGP)

WAN (Wide Area Network) types

There are 5 major WAN technologies available

  1. Point to Point Link
  2. Circuit Switched Network
  3. Packet Switched Network
  4. High Level Data Link control (HDLC)
  5. Synchronous Data Link control (SDLC)

Point to Point Link

Uses a public career for establishing WAN connectivity

Technology Details
SLIP (Serial Line IP)
  1. Enables serial devices such as modems to connect to remote network
  2. Asynchronous
  3. Slow speed
  4. Little or no security
Point to Point Protocol (PPP)
  1. Successor of SLIP
  2. Asynchronous and Synchronous operation
  3. More security features than SLIP
Point to Point tunneling protocol
  1. Relies on PAP,CHAP or EAP to provide encryption
  2. Developed by Microsoft
  3. Used in VPNs
Layer 2 Forwarding protocol
  1. Used in VPN with PPP
  2. Little or no security
  3. Developed by CISCO
Layer 2 Tunneling protocol
  1. Used in VPN
  2. Uses IPSec for encryption
  3. Uses UDP port 1701

Circuit Switched Network

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WAN (Wide Area Network) Circuits




64 Kbits/Sec


1.544 Mbits/Sec or 2.048 Mbits/Sec


44.736 Mbits/Sec


1.544 Mbits/Sec


44.736 Mbits/Sec

E1 (Used in Europe)

2.048 Mbits/Sec

E3 (Used in Europe)

34.368 Mbits/Sec

OC-1 (Synchronous Optical WAN)

51.84 Mbits/Sec

OC-3 (Synchronous Optical WAN)

155.52 Mbits/Sec

OC-12 (Synchronous Optical WAN)

622.08 MbitsSec

OC-48 (Synchronous Optical WAN)

2.488 Gbits/Sec

OC-192 (Synchronous Optical WAN)

9.9 Gbits/Sec