Choosing an ISP Managing and Maintaining Your VPN
10.2 Solving VPN Problems
There are numerous points of failure with VPNs. This makes tracking down the cause of a problem more difficult than it might be for a normal WAN or remote access connection. Among the possible problems are connectivity problems, authentication errors, and routing problems.10.2.1 Connectivity Problems
Anyone familiar with maintaining or dialing into remote access servers—or into an ISP for that matter—is also familiar with the frustration of trying to pinpoint the problem of a bad connection. The main difficulty with connectivity problems is that they have so many causes. Here are a few possibilities: • Telco problems o Bad lines o Busy switch • ISP problems o Busy signals probably from a user-to-modem ratio thats too high o Bad modem or router • End-user problems o Bad modem or router o A modem or router thats incompatible with the ISPs o Configuration problem Besides these general communication problems, you may discover problems with port usage on firewalls. As youve seen, several VPN packages use specific TCP or UDP ports in order to communicate for example, PPTP uses TCP port 1723. If these ports arent open, you may not be able to make a VPN connection or transport data across the VPN. Its possible that these ports may be blocked at your ISP or on your own routers.10.2.2 Authentication Errors
Authentication problems are common in the realm of dial-up connections, even when VPNs arent involved. Here are the two most common authentication problems: • A mismatched username or password, which occurs when either the connecting machine or the far end thinks that the username or password is something other than what it is. This is sometimes caused by a simple typographical error. Likewise, there could be mismatched keys in a public key system. • The connecting system and the destination are using different authentication methods. For instance, the connecting machine might be attempting PAP authentication, while the destination system is expecting CHAP. There is a third level of authentication problems involving public key infrastructures. Its important to use the same key exchange protocol. For example, some IPSec products allow 161 for a number of key exchange options: Manual, SKIP, or IPSec. In a public key infrastructure there could also be problems with certificate authorities and certificates.10.2.3 Routing Problems
Routing problems occur when youre able to connect successfully to your ISP, but have trouble getting to certain hosts over the Internet, or getting out to the Internet at all. These problems are commonly due to configuration errors. Either the IP address, netmask, or gateway on your system is set incorrectly, or your ISP doesnt have a route for you. Chances are that any one of the problems discussed in this chapter—authentication, connectivity, or routing—is caused by a configuration mismatch on your equipment, the ISPs equipment, or the equipment on the far end of the connection. The routing problem, however, could also be due to any one of the numerous connection points on the Internet backbone between you and the destination. You, and your ISP, will have little control over these problems, but its nice to know where the problem is so that you can report it to the proper people. In Chapter 5 , in Section 5.6 , we mentioned two useful utilities for testing routes: ping and traceroute. Both of these tools can be used to troubleshoot problems on other VPNs as well. ping is a utility found on Unix, Windows 9598, and Windows NT systems. It sends packets to a given destination and awaits a return. It doesnt tell you what route the packets take, but it does tell you if they get there at all and if theres any packet loss. traceroute is a program on Unix systems. The Windows 9598NT equivalent is TRACERT. traceroute will actually show you the path packets take to their destination. This information can be useful to pinpoint exactly where a problem is occurring. traceroute information can sometimes be confusing. Be sure to read up on what the various latencies mean, as well as asterisks, exclamation marks, and other symbols. A good TCPIP networking book will explain how to read traceroutes output, as will its manual page on a Unix system. Also keep in mind that an ISP or company may be blocking traceroutes UDP packets at their firewall for security reasons, so you may want to contact them and find out if this is the case. If the problem appears to be with a backbone provider, the best thing to do is still to contact your ISP. They can then contact the backbone provider and see what the problem is.10.2.4 Dealing with an ISP
Working with an ISP to solve a VPN problem may prove difficult, especially if the ISP doesnt support VPNs. As a network administrator, therefore, youll want to know your VPN product inside and out. The most important thing to remember when troubleshooting a problem with an ISP is to give them as much information as possible. At a minimum, give them this information: • What VPN product youre using. • What the IP address of your system should be. • What the IP address of the destination VPN server or router should be e.g., the address of your PPTP server. • The TCP or UDP ports that your VPN product uses, in case your ISP has those ports blocked at a firewall.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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