SSCP – System Security Certified Practitioner (SSCP)

Reference / correct answer:

The Address Resolution Protocol (ARP) is used to match an IP address to an Ethernet address so the packet can be sent to the appropriate node.

Shon Harris in her book says:

MAC and IP addresses must be properly mapped so they can be correctly resolved. This happens through the Address Resolution Protocol (ARP). When the data link layer receives a frame, the network layer has already attached the destination IP address to it, but the data link layer cannot understand the IP address and thus invokes ARP for help.

ARP broadcasts a frame requesting the MAC address that corresponds with the destination IP address. Each computer on the subnet receives this broadcast frame, and all but the computer that has the requested IP address ignore it.

The computer that has the destination IP address responds with its MAC address. Now ARP knows what hardware address corresponds with that specific IP address. The data link layer takes the frame, adds the hardware address to it, and passes it on to the physical layer, which enables the frame to hit the wire and go to the destination computer.

ARP maps the hardware address and associated IP address and stores this mapping in its table for a predefined amount of time. This caching is done so that when another frame destined for the same IP address needs to hit the wire, ARP does not need to broadcast its request again. It just looks in its table for this information.

Man-In-The-Middle attack

Because ARP does not require authentication, an attacker could place bogus entries into the ARP cache of a remote host (gratuitous ARP replies) to carry out attacks, such as a man-in-themiddle attacks. This attack is called ARP poisoning.

The following answers were incorrect:

RARP is used to match an Ethernet address to an IP address.

ICMP is a management protocol whose function is to send message between network devices.

Routing tables are used by routers to choose the appropriate interface to route packets.

Reference(s) used for this question:

Harris, Shon (2012-10-25). CISSP All-in-One uide, 6th Edition, Chapter 6 Telecommunications and Network Security, Pages 580-581 or on the Kindle edition look around Locations 12298-12306. McGraw-Hill. Kindle Edition.

and

Schneiter, Andrew (2013-04-15). Official (ISC)2 Guide to the CISSP CBK , Third Edition: Telecommunications and Network Security, Page 342.

Reference / correct answer:

IPSec can be run in either tunnel mode or transport mode. Each of these modes has its own particular uses and care should be taken to ensure that the correct one is selected for the solution:

Tunnel mode is most commonly used between gateways, or at an end-station to a gateway, the gateway acting as a proxy for the hosts behind it.

Transport mode is used between end-stations or between an end-station and a gateway, if the gateway is being treated as a host—for example, an encrypted Telnet session from a workstation to a router, in which the router is the actual destination.

As Figure 1 shows, basically transport mode should be used for end-to-end sessions and tunnel mode should be used for everything else. (Refer to the figure for the following discussion.) Figure 1 Tunnel and transport modes in IPSec.

Figure 1 displays some examples of when to use tunnel versus transport mode:

Tunnel mode is most commonly used to encrypt traffic between secure IPSec gateways, such as between the Cisco router and PIX Firewall (as shown in example A in Figure 1). The IPSec gateways proxy IPSec for the devices behind them, such as Alice's PC and the HR servers in Figure 1. In example A, Alice connects to the HR servers securely through the IPSec tunnel set up between the gateways.

Tunnel mode is also used to connect an end-station running IPSec software, such as the Cisco Secure VPN Client, to an IPSec gateway, as shown in example B.

In example C, tunnel mode is used to set up an IPSec tunnel between the Cisco router and a server running IPSec software. Note that Cisco IOS software and the PIX Firewall sets tunnel mode as the default IPSec mode.

Transport mode is used between end-stations supporting IPSec, or between an end-station and a gateway, if the gateway is being treated as a host. In example D, transport mode is used to set up an encrypted Telnet session from Alice's PC running Cisco Secure VPN Client software to terminate at the PIX Firewall, enabling Alice to remotely configure the PIX Firewall securely.

AH Tunnel Versus Transport Mode

Figure 2 shows the differences that the IPSec mode makes to AH. In transport mode, AH services protect the external IP header along with the data payload. AH services protect all the fields in the header that don't change in transport. The header goes after the IP header and before the ESP header, if present, and other higher-layer protocols.

In tunnel mode, the entire original header is authenticated, a new IP header is built, and the new IP header is protected in the same way as the IP header in transport mode.

Figure 2 AH tunnel versus transport mode.

AH is incompatible with Network Address Translation (NAT) because NAT changes the source IP address, which breaks the AH header and causes the packets to be rejected by the IPSec peer.

ESP Tunnel Versus Transport Mode

Figure 3 shows the differences that the IPSec mode makes to ESP. In transport mode, the IP payload is encrypted and the original headers are left intact. The ESP header is inserted after the IP header and before the upper-layer protocol header. The upper-layer protocols are encrypted and authenticated along with the ESP header. ESP doesn't authenticate the IP header itself.

NOTE Higher-layer information is not available because it's part of the encrypted payload.

When ESP is used in tunnel mode, the original IP header is well protected because the entire original IP datagram is encrypted. With an ESP authentication mechanism, the original IP datagram and the ESP header are included; however, the new IP header is not included in the authentication.

When both authentication and encryption are selected, encryption is performed first, before authentication. One reason for this order of processing is that it facilitates rapid detection and rejection of replayed or bogus packets by the receiving node. Prior to decrypting the packet, the receiver can detect the problem and potentially reduce the impact of denial-of-service attacks.

Figure 3 ESP tunnel versus transport mode.

ESP can also provide packet authentication with an optional field for authentication. Cisco IOS software and the PIX Firewall refer to this service as ESP hashed message authentication code (HMAC). Authentication is calculated after the encryption is done. The current IPSec standard specifies SHA-1 and MD5 as the mandatory HMAC algorithms.

The main difference between the authentication provided by ESP and AH is the extent of the coverage. Specifically, ESP doesn't protect any IP header fields unless those fields are encapsulated by ESP (tunnel mode). Figure 4 illustrates the fields protected by ESP HMAC.

Figure 4 ESP encryption with a keyed HMAC.

IPSec Transforms

An IPSec transform specifies a single IPSec security protocol (either AH or ESP) with its corresponding security algorithms and mode. Example transforms include the following:

The AH protocol with the HMAC with MD5 authentication algorithm in tunnel mode is used for authentication.

The ESP protocol with the triple DES (3DES) encryption algorithm in transport mode is used for confidentiality of data.

The ESP protocol with the 56-bit DES encryption algorithm and the HMAC with SHA-1 authentication algorithm in tunnel mode is used for authentication and confidentiality.

Transform Sets

A transform set is a combination of individual IPSec transforms designed to enact a specific security policy for traffic. During the ISAKMP IPSec security association negotiation that occurs in IKE phase 2 quick mode, the peers agree to use a particular transform set for protecting a particular data flow. Transform sets combine the following IPSec factors: Mechanism for payload authentication—AH transform

Mechanism for payload encryption—ESP transform IPSec mode (transport versus tunnel)

Transform sets equal a combination of an AH transform, plus an ESP transform, plus the IPSec mode (either tunnel or transport mode).

This brings us to the end of the second part of this five-part series of articles covering IPSec. Be sure to catch the next installment.

Cisco Press at: http://www.ciscopress.com/articles/printerfriendly.asp?p=25477 and

Source: TIPTON, Harold

F. & KRAUSE, MICKI, Information Security Management Handbook, 4th Edition, Volume 2, 2001, CRC Press, NY, Pages 166-167.

Reference / correct answer:

Although a auditing is a part of corporate security, it in no way supercedes the requirments for a disaster recovery plan. All others can be blamed for a plan going out of date. Source: HARRIS, Shon, All-In-One CISSP Certification uide, McGraw-Hill/Osborne, 2002, chapter 9: Disaster Recovery and Business continuity (page 609).