Virtual Private Networks in Objective-C

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then the packet is dropped since the signature in it is either a fake or the packet was tampered with between the source and destination
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Authentication: Peer Validation Another important component of any VPN solution, including IPSec, is validating the peer s identity through some form of authentication IPSec supports two forms of authentication: device and user (commonly called extended authentication or XAUTH for short) L2L sessions support only device authentication while remote access supports both Device authentication supports two methods of validating a peer:
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Pre-shared keys (PSKs)
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PSKs require that a pre-shared symmetric key be configured on each VPN peer This key is then used, along with identity information from the peer, to generate a signature The remote end can then validate the signature using the same PSK RSA signatures use asymmetric keys for authentication Hashes of signatures (created with a private key) are placed on digital certificates generated by a central certificate authority (CA) The signatures can then be validated with the associated public key
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Of the two, PSK is the easier to implement; however, certificates using RSA signatures are a much more scalable solution
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IPSec is actually a group of standards, protocols, and technologies that work together to build a secure session, commonly called a tunnel, to a remote peer An IPSec tunnel comprises three connections: one management connection and two unidirectional data connections The tunnel is built across two phases The management connection is built during Phase 1 and is used to share IPSec-related information between the two peers The two data connections are built during Phase 2 and are used to transmit user traffic All three connections are protected Here is a brief description of these protocols used to build a tunnel:
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The Internet Security Association and Key Management Protocol is used to build and maintain the tunnel; it defines the format of the management payload, the mechanics of a key exchange protocol for the encryption algorithms and HMAC functions, negotiates how the tunnel will be built between the two devices, and authenticates the remote device
25: WAN Introduction
IKE
The Internet Key Exchange Protocol is responsible for generating and managing keys used for encryption algorithms and HMAC functions Actually, it is a combination of ISAKMP and IKE working together that secures the tunnel between two devices: they use UDP as a transport and connect on port 500 The Diffie-Hellman process is used to securely exchange the encryption and HMAC keys that will be used to secure the management and data connections The Authentication Header protocol is used only to validate the origination and validity of data packets (on the data connections) received from a peer; it accomplishes this by using HMAC functions, where the signature created is based on almost the entire IP packet Its two main disadvantages are that it breaks if it goes through any type of address translation device and it does not support encryption The Encapsulation Security Payload protocol is used to provide packet confidentiality and authentication It provides confidentiality through encryption and packet authentication through an HMAC function Because it supports encryption, it is the protocol companies use to protect the data connections; however, its downside is that its signature process does not protect the outer IP header and thus cannot detect packet tampering in the header, whereas AH can ESP s other main advantage is that it can work through address translation devices doing NAT without any changes, but it requires an encapsulation in a UDP packet to work through a PAT or firewall device This part of the IPSec standard is called NAT Transparency or Traversal, or NAT-T for short
DH
AH
ESP
Remember the IPSec protocols (ISAKMP, IKE, DH, AH, and
ESP) and their descriptions in the above bullets
Virtual Private Networks
INSIDE THE EXAM
Wide Area Networking Overview
Remember that WANs operate at the physical and data link layers You should be familiar with what a DCE and DTE are, what their main functions are, and examples of these kinds of devices WAN services that use VCs, such as Frame Relay and ATM, are used to connect multiple locations together using a single WAN interface Be able to compare and contrast different data link layer encapsulation types, such as HDLC and PPP, as shown in Table 25-2 Serial interfaces are used for synchronous connections
PPP should be used in a mixed-vendor environment Know the services PPP provides: authentication via CHAP and PAP, support for multiple encapsulated protocols, compression, multilink, error detection/ correction, and support for synchronous and asynchronous circuits Be able to explain what LCP and NCP do within PPP and the differences between PAP and CHAP authentication As with HDLC, know how to troubleshoot PPP connections by examining the status of the LCP state with the show interfaces command, and be able to identify problems with misconfigured PAP or CHAP authentication by examining a router s configuration with the show running-config command
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