Question

1. Discuss in detail on IPSEC tunneling and transport mode mechanisms.
2. Discuss the various AAA based hardening techniques in routers.
3. Discuss basic router hardening techniques.
4. What are
   1. Application layer firewalls
   2. Circuit based
   3. Packet filtering
   4. Stateful firewall

Answer 1

# IPSEC TUNNEL MODE
IPSec tunnel mode is the default mode. With tunnel mode, the entire original IP packet is protected by IPSec. This means IPSec wraps the original packet, encrypts it, adds a new IP header and sends it to the other side of the VPN tunnel (IPSec peer).

Tunnel mode is most commonly used between gateways (Cisco routers or ASA firewalls), or at an end-station to a gateway, the gateway acting as a proxy for the hosts behind it.
Tunnel mode is used to encrypt traffic between secure IPSec Gateways, for example two Cisco routers connected over the Internet via IPSec VPN. Configuration and setup of this topology is extensively covered in our Site-to-Site IPSec VPN article. In this example, each router acts as an IPSec Gateway for their LAN, providing secure connectivity to the remote network:

Another example of tunnel mode is an IPSec tunnel between a Cisco VPN Client and an IPSec Gateway (e.g ASA5510 or PIX Firewall). The client connects to the IPSec Gateway. Traffic from the client is encrypted, encapsulated inside a new IP packet and sent to the other end. Once decrypted by the firewall appliance, the client’s original IP packet is sent to the local network.
In tunnel mode, an IPSec header (AH or ESP header) is inserted between the IP header and the upper layer protocol. Between AH and ESP, ESP is most commonly used in IPSec VPN Tunnel configuration.
The packet diagram below illustrates IPSec Tunnel mode with ESP header:

Original IP Packet ESP Header Header UDP ESP Auth Trailer ESP New IP Header IP eader HeadePDATA ESPESP Aut Encrypted with ESP Header Signed by ESP Auth Trailer

ESP is identified in the New IP header with an IP protocol ID of 50.

The packet diagram below illustrates IPSec Tunnel mode with AH header:

The AH can be applied alone or together with the ESP, when IPSec is in tunnel mode. AH’s job is to protect the entire packet. The AH does not protect all of the fields in the New IP Header because some change in transit, and the sender cannot predict how they might change. The AH protects everything that does not change in transit. AH is identified in the New IP header with an IP protocol ID of 51.


IPSEC TRANSPORT MODE
IPSec Transport mode is used for end-to-end communications, for example, for communication between a client and a server or between a workstation and a gateway (if the gateway is being treated as a host). A good example would be an encrypted Telnet or Remote Desktop session from a workstation to a server.

Transport mode provides the protection of our data, also known as IP Payload, and consists of TCP/UDP header + Data, through an AH or ESP header. The payload is encapsulated by the IPSec headers and trailers. The original IP headers remain intact, except that the IP protocol field is changed to ESP (50) or AH (51), and the original protocol value is saved in the IPsec trailer to be restored when the packet is decrypted.
IPSec transport mode is usually used when another tunneling protocol (like GRE) is used to first encapsulate the IP data packet, then IPSec is used to protect the GRE tunnel packets. IPSec protects the GRE tunnel traffic in transport mode.

The packet diagram below illustrates IPSec Transport mode with ESP header:

Notice that the original IP Header is moved to the front. Placing the sender’s IP header at the front (with minor changes to the protocol ID), proves that transport mode does not provide protection or encryption to the original IP header and ESP is identified in the New IP headerwith an IP protocol ID of 50.
The packet diagram below illustrates IPSec Transport mode with AH header:

The AH can be applied alone or together with the ESP when IPSec is in transport mode. AH’s job is to protect the entire packet, however, IPSec in transport mode does not create a new IP header in front of the packet but places a copy of the original with some minor changes to the protocol ID therefore not providing essential protection to the details contained in the IP header (Source IP, destination IP etc). AH is identified in the New IP header with an IP protocol ID of 51.
In both ESP and AH cases with IPSec Transport mode, the IP header is exposed.

2) Router hardening techniques
Disable unused router interfaces—that’s right, find ANY interface that is not in use and make sure you issue the shutdown command
Disable unused services—these typically include:

• BOOTP
• CDP
• Configuration autoloading
• FTP
• TFTP
• PAD
• TCP and UDP minor services
• DEC MOP
• Disable management protocols that you are not using—these typically include:
• SNMP
• HTTP or HTTPS
• DNS
• Disable features that are techniques for re-directing your traffic:
• ICMP Redirects
• IP Source routing
• Disable features that are techniques for probes and scans in reconnaissance attacks:
• Finger
• ICMP unreachables
• ICMP mask reply
• Ensure security of terminal connections:
• IP identification service
• TCP keepalives
• Disable gratuitous ARP and proxy ARP
• Disable IP-directed broadcasts

You also should inspect the network management protocols in use in your network infrastructure. Remember the following:
SNMP version 1 and 2c transfer passwords (called community strings) in clear text—if security is required—consider SNMP version 3.
HTTP authentication also sends clear text passwords—when needed, use HTTPS instead.
Attackers can respond to broadcast DNS lookups—disable DNS when not in use or configure securely.
Telnet is a clear text protocol—do not use it; instead, use SSH.
Router Hardening with the Cisco Router and Security and Device Manager (SDM)
Now one of the reasons that we love Cisco is that they are always trying to make it easy on us. We see this in the area of router hardening. In the Graphical User Interface for managing your perimeter routers, Cisco provides a Security Audit feature. This feature provides two “modes” of operation. The first is the Security Audit Wizard and the second is the One-step Lockdown mode.
The Security Audit Wizard examines your router and then lets you choose which potential security flaws you want to correct. The One-step Lockdown mode automatically makes the router hardening configurations that Cisco would recommend.
Here are the steps of the Security Audit Wizard:
Within the Cisco SDM, choose

Configure then Security Audit.
Click the Perform Security Audit button.
Click Next and the Security Audit Interface Configuration page appears —here you are presented with your router interfaces. It is up to you to select which of the interfaces connect to your internal networks, and which of the interfaces connect to your external networks. After you select these options, choose Next.
The security audit now runs. When it is complete, the SDM presents you with a report of potential security vulnerabilities on your device. You can click Save Report in order to save it. If you select Close, the Security Audit Wizard continues to the next phase.
In the final phase of the Security Audit Wizard, you can check or uncheck the vulnerabilities that you want the wizard to automatically repair. Notice there is even an option for Fix All.
As you might guess, using the One-step Lockdown mode is even easier!
Within the Cisco SDM, choose Configure then Security Audit.
Click the One-step Lockdown button.
Click the Yes button that you want to continue and the SDM goes about its business of locking down the device for you.
What exactly does the One-step lockdown do on your device? Well, it is very busy indeed. Here is a list of the changes made by this GUI:

• Disable Finger Service
• Disable PAD Service
• Disable TCP Small Servers Service
• Disable UDP Small Servers Service
• Disable IP BOOTP Server Service
• Disable IP Identification Service
• Disable CDP
• Disable IP Source Route
• Enable Password Encryption Service
• Enable TCP Keepalives for Inbound Telnet Sessions
• Enable TCP Keepalives for Outbound Telnet Sessions
• Enable Sequence Numbers and Time Stamps on Debugs
• Enable IP CEF
• Disable IP Gratuitous ARPs
• Set Minimum Password Length to Less Than 6 Characters
• Set Authentication Failure Rate to Less Than 3 Retries
• Set TCP Synwait Time
• Set Banner
• Enable Logging
• Set Enable Secret Password
• Disable SNMP
• Set Scheduler Interval
• Set Scheduler Allocate
• Set Users
• Enable Telnet Settings
• Enable NetFlow Switching
• Disable IP Redirects
• Disable IP Proxy ARP
• Disable IP Directed Broadcast
• Disable MOP Service
• Disable IP Unreachables
• Disable IP Mask Reply
• Disable IP Unreachables on NULL Interface
• Enable Unicast RPF on Outside Interfaces
• Enable Firewall on All of the Outside Interfaces
• Set Access Class on HTTP Server Service
• Set Access Class on VTY Lines
• Enable SSH for Access to the Router
• Router Hardening with the Cisco’s AutoSecure

Cisco also provides a One-step lockdown-like feature at the command line! This feature is called AutoSecure. It uses the command shown below:
auto secure [management | forwarding] [no-interact | full] [ntp | login | ssh | firewall | tcp-intercept]

Notice that this command can run fully automated like the One-step Lockdown mode of the Security Audit feature in SDM. You would issue the command auto secure no-interact. You should notice also that you can run “subsets” of the command’s full capabilities. For example, you could run auto secure managementin order to just harden the network management capabilities of the router.
You might not be surprised to learn that the Command Line AutoSecure feature is capable of doing a bit more than the graphical user interface counterpart. Specifically, Cisco SDM does not implement these Cisco AutoSecure features:

• Disabling NTP
• Configuring AAA
• Setting SPD values
• Enabling TCP intercepts
• Configuring antispoofing ACLs on outside interfaces

The Cisco SDM also implements some of the Cisco AutoSecure features differently. For example:
The SDM disables SNMP but does not configure SNMPv3 (on some routers).
The SDM enables and configures SSH on crypto Cisco IOS images, but does not enable SCP or disable other access and file transfer services, such as FTP for example.
3) a.Application layer firewalls
An application firewall is an enhanced firewall that limits access by applications to the operating system (OS) of a computer. Conventional firewalls merely control the flow of data to and from the central processing unit (CPU), examining each packet and determining whether or not to forward it toward a particular destination. An application firewall offers additional protection by controlling the execution of files or the handling of data by specific applications.
For best performance, a conventional firewall must be configured by the user. The user must know which ports unwanted data is likely to enter or leave through. An application firewall prevents the execution of programs or DLL (dynamic link library) files which have been tampered with. Thus, even though an intruder might get past a conventional firewall and gain entry to a computer, server, or network, destructive activity can be forestalled because the application firewall does not allow any suspected malicious code to execute.

c.Packet filtering
In a software firewall, packet filtering is done by a program called a packet filter. The packet filter examines the header of each packet based on a specific set of rules, and on that basis, decides to prevent it from passing (called DROP) or allow it to pass (called ACCEPT).
There are three ways in which a packet filter can be configured, once the set of filtering rules has been defined. In the first method, the filter accepts only those packets that it is certain are safe, dropping all others. This is the most secure mode, but it can cause inconvenience if legitimate packets are inadvertently dropped. In the second method, the filter drops only the packets that it is certain are unsafe, accepting all others. This mode is the least secure, but is causes less inconvenience, particularly in casual Web browsing. In the third method, if the filter encounters a packet for which its rules do not provide instructions, that packet can be quarantined, or the user can be specifically queried concerning what should be done with it. This can be inconvenient if it causes numerous dialog boxes to appear, for example, during Web browsing.

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