The encapsulation process Dissecting a PPTP Packet
4.2.5 PPTP Security
Like most security systems, PPTP has two components: authentication to prevent improper connections, and encryption for data sent once the connection is made.4.2.5.1 RAS authentication methods
PPTP uses Windows NT RAS authentication. The choices for the different authentication types the RAS server can accept are located in the RAS properties under Encryption Settings. This setting lets you specify the level of authentication that the RAS server will perform against the clients login attempt. This section discusses the options you have: standard encrypted authentication, Microsoft- enhanced encrypted authentication, and allowing any type of authentication. Your choice will determine how secure your VPN will be. 664.2.5.1.1 Accept encrypted authentication
Encrypted authentication in RAS is actually the Internet authentication standard known as CHAP Challenge Handshake Authentication Protocol. CHAP is described in RFC 1994 as an extension to PPP in which clear-text passwords are not passed between the client and server. Instead, both the client and server have an agreed-upon password, called a secret, that is never sent over the link unencrypted. Heres how CHAP authentication occurs: 1. The server challenges the client to identify itself when the client tries to connect. 2. The client sends the secret through a one-way hashing algorithm, RSAs MD5. The algorithm uses mathematical formulas and random factors to come up with a hash value. One-way means that the hash value cannot be reversed into the original elements, and the use of random elements means that someone sniffing the connection will be less likely to see the same value twice. The hash value is sent across the connection to the server. 3. The server compares the value the server sent to its own calculation of the hash value. If the two values match, the connection is authenticated. If not, the connection is terminated. Another benefit of CHAP is that this authentication process can take place several times during the course of a connection. This limits the probability of being bumped off and having an impostor hijack your connection. In the case of PPTP, the secret is the password the user uses to log into the NT domain, which is also known by the RAS server either directly or through NT domain services.4.2.5.1.2 Accept Microsoft encrypted authentication
Microsoft encrypted authentication is also known as MS-CHAP. MS-CHAP performs RSAs MD4 hash, as well as the DES hashing technique. Windows 9598 and Windows NT RAS clients use the MD4 hash, which doesnt require clear-text passwords on the client or server. DES allows for backward compatibility with older RAS clients such as Windows for Workgroups 3.11 and RAS 1.1a. Otherwise, MS-CHAP operates the same way as CHAP. The main drawback of MS-CHAP is that not every platform has a PPP client that supports it. If your remote users are all on Windows systems, however, its the best protocol to use. In addition, you must use it to get the added benefit of data stream encryption over PPTP. Well explain why in the section on data encryption.4.2.5.1.3 Accept any authentication, including clear text
Accepting any authentication, including clear text, means that the RAS server will accept MS- CHAP, CHAP, or the Password Authentication Protocol PAP. PAP has long been a common way to authenticate a PPP connection. In fact, most ISPs use PAP authentication for their PPP dial-up connections. Its main drawback is that it sends the password over the connection in clear text, meaning that someone monitoring the connection between the client and server may be able to see the login exchange, then log in later as that person. PAP is an unsuitable authentication method for a VPN, since secure authentication over a public network is a VPNs primary goal. Its therefore suggested that you require CHAP or MS- CHAP authentication on your PPTP server. If your remote users are on varied platforms, you may find that not every client on every platform supports CHAP or MS-CHAP authentication.Parts
» Virtual Private Networks 2nd 1999
» How VPNs relate to Intranets
» What Are We Protecting with Our VPN?
» Firewalls How VPNs Solve Internet Security Issues
» Authentication How VPNs Solve Internet Security Issues
» Encryption How VPNs Solve Internet Security Issues
» Tunneling How VPNs Solve Internet Security Issues
» A Note on IP Address and Domain Name Conventions Used in This Book
» Packet restriction or packet filtering routers
» Bastion host What Types of Firewalls Are There?
» DMZ or perimeter zone network
» Proxy servers What Types of Firewalls Are There?
» A Brief History of Cryptography
» Cryptography: How to Keep a Secret
» Cryptography in Network Communications
» Hash algorithms Cryptographic Algorithms
» Secret key systems Cryptographic Algorithms
» Public key cryptosystems Cryptographic Algorithms
» Use of Cryptosystems and Authentication in a VPN
» ESP Encapsulating Security Payload
» AH Authentication Header VPN Protocols
» Internet Key Exchange, ISAMKPOakley
» ISO X.509 v.3 Digital Certificates
» LDAP Lightweight Directory Access Protocol Radius
» PPTP Point-to-Point Tunneling Protocol
» Basic Firewalling Methodologies for Compromising VPNs
» Ciphertext-only attack Cryptographic Assaults
» Known plaintext attack Cryptographic Assaults
» Chosen plaintext attack Cryptographic Assaults
» Chosen ciphertext attack Cryptographic Assaults
» Brute force attacks Cryptographic Assaults
» Password guessers and dictionary attacks
» Social engineering Cryptographic Assaults
» Address spoofing Network Compromises and Attacks
» Session hijacking Network Compromises and Attacks
» Man-in-the-middle attack Network Compromises and Attacks
» Replay attack Network Compromises and Attacks
» Detection and cleanup Network Compromises and Attacks
» Patents and Legal Ramifications
» General WAN, RAS, and VPN Concepts
» Telco Small to Medium Solutions
» Security, scalability, and stability
» Hardwaresoftware Small to Medium Solutions
» Administration Small to Medium Solutions
» Hardwaresoftware Administration Security, scalability, and stability
» Differences Between PPTP, L2F, and L2TP
» Dialing into an ISP That Supports PPTP
» Dialing into an ISP That Doesnt Support PPTP
» Where PPTP Fits into Our Scenario
» The encapsulation process Dissecting a PPTP Packet
» Accept encrypted authentication RAS authentication methods
» Accept Microsoft encrypted authentication
» Accept any authentication, including clear text
» Data encryption PPTP Security
» Availability Features of PPTP
» Easy Implementation Features of PPTP
» Multiprotocol Tunneling Features of PPTP
» Ability to Use Corporate and UnregisteredIP Addresses
» Choosing the protocols to tunnel
» Choosing your authentication method
» IP address negotiation using DHCP
» Outbound authentication using PPTP filtering
» Filtering caveats PPTP Filtering
» Installing PPTP Filtering by IP Address
» Configuring Users for Dial-up Access
» Configuring PPTP for Dial-up Networking on a Windows NT Client
» Configuring PPTP for Dial-up Networking on a Windows 95 or 98 Client
» Setting up global PPTP parameters Setting up a port for PPTP
» Configuring PPTP on an Ascend MAX 4004
» Making the Calls Configuring and Testing Layer 2 Connections
» The Event Viewer Login problems
» The Dial-Up Networking Monitor
» ping and traceroute Connectivity Testing
» Fixed IP addresses How to Allow PPTP Through Firewalls
» How PPTP Can Bypass a Proxy Server
» Three-part encryption technique Security
» Support for an emerging security standard
» Support for Security Dynamics SecureID
» Accessibility Flexibility Advantages of the AltaVista Tunnel System
» Platform Limitations AltaVista Tunnel Limitations
» Extranet server System Considerations
» Telecommuter client System Considerations
» Planning How the AltaVista Tunnel Works
» AltaVista Tunnel Extranet server
» Security procedures The Guts
» AltaVista Tunnel Telecommuter Client
» Sample configuration Implementing a LAN-to-LAN Tunnel
» Tunnel server configuration Implementing a LAN-to-LAN Tunnel
» Firewall configuration Host configuration
» Sample configuration Implementing Single Connections-to-LAN Tunnels
» Tunnel server configuration Implementing Single Connections-to-LAN Tunnels
» Firewall configuration Implementing Single Connections-to-LAN Tunnels
» Local host configuration Implementing Single Connections-to-LAN Tunnels
» Remote PC configuration Implementing Single Connections-to-LAN Tunnels
» Sample configuration Implementing PC-to-WAN Tunnels
» Tunnel server configuration Implementing PC-to-WAN Tunnels
» Tracing the packets Implementing PC-to-WAN Tunnels
» Preparing to Install Installing the AltaVista Tunnel
» Windows NT 4.0 Installing the AltaVista Tunnel Extranet Serverfor Windows NT
» Installing the AltaVista Tunnel Telecommuter Client for Windows
» Installing the AltaVista Tunnel Telecommuter Client for MacOS
» Initial configuration Adding Routes and Dynamic Addresses
» Managing routes and dynamic IPs
» Group configuration Adding Tunnel Groups
» Tunnel client information Adding Tunnel Groups
» Tools for Tunnel Management Changing Port Settings
» Rekey Interval and Minimum Encryption Settings
» Configuring Unix-to-Windows NT Tunnel Connections
» Getting Busy Configuring the AltaVista Telecommuter Client
» Tunnel Server and Client Configuration Checks
» Local Network and Internet Gateway Configuration Checks
» Encryption Capabilities The SSH Software
» Useful sshd parameters for our purposes
» Understanding SSH authentication ssh
» Useful ssh parameters for our purposes
» The VPN Components Creating a VPN with PPP and SSH
» Setting up the master and slave Linux systems
» Creating a user account on the slave
» Setting up SSH authentication
» Configuring sudo on the slave
» Putting pty-redir on the master
» Setting up the slaves scripts
» Testing the Connection Creating a VPN with PPP and SSH
» A Performance Evaluation Creating a VPN with the Unix Secure Shell
» ISP Assigned Addresses Global Pool
» Hardware solution Advantages of the PIX Firewall
» Superior to Unix and other router firewalls
» Single point of controlfailure
» Dynamic address translation Advantages of the PIX Firewall
» PIX acts like a proxy server
» Ease of configuration and maintenance
» High-speed access Advantages of the PIX Firewall
» Links Advantages of the PIX Firewall
» Hardware solution Limitations of the PIX Firewall
» Dynamic address use Limitations of the PIX Firewall
» Budgetary considerations Limitations of the PIX Firewall
» Maintenance Limitations of the PIX Firewall
» A Sample Configuration Configuring the PIX as a Gateway
» Firewall Configuration on the PIX
» debug xlate Testing, Tracing, and Debugging
» arp Testing, Tracing, and Debugging
» show interface Testing, Tracing, and Debugging
» Offering Services to the Internet Through Conduits and the static Command
» Tunneling with the link Directive
» Choosing an ISP Managing and Maintaining Your VPN
» Connectivity Problems Solving VPN Problems
» Authentication Errors Solving VPN Problems
» Routing Problems Dealing with an ISP
» Compatibility with Other Products
» Delivering Quality of Service
» Restrict What VPN Users Can Get To
» Avoid Public DNS Information for VPN Servers and Routers
» Keeping Yourself Up-to-Date Managing and Maintaining Your VPN
» Network Connections Hardware and Operating System VPN Package
» Connection Hardware and Operating System VPN Package
» Connection Hardware and Operating System
» VPN Package Remote Access Users
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