ping and traceroute Connectivity Testing

87 modem, you might be able to check the carrier detect to see if youre actually connected. • Your PPP link is incorrectly configured. Double-check the PPP and TCPIP information given to you by your ISP against what you have in your TCPIP properties. Unless you have a fixed IP address, your IP address and gateway should be assigned by the ISPs PPP server. Also, make sure you have your ISPs correct DNS servers listed. • Congestion or backbone routing problems on the greater Internet are preventing you from reaching the corporate network a common occurrence. A good test would be to attempt to ping your corporate Internet gateway routers IP address. If you can reach it across the Internet, then you can probably assume the RAS server is configured incorrectly or not connected. Another good tool for testing connectivity is Traceroute. Traceroute is familiar to Unix users as the traceroute program, which traces the path of packets sent from the starting host to the destination host, listing the hops other gateway routers along the way. On Windows 9598 and Windows NT systems, Traceroute is named TRACERT. The only problem you might have with using Traceroute is that it sends UDP packets on an invalid and high port, and some ISPs or companies block incoming UDP packets for security reasons. If Traceroute doesnt trace past a certain ISPs router and onto their internal network, it doesnt necessarily mean that the network or machine is down.

5.7 Using PPTP with Other Security Measures

What weve covered so far are the basic steps for setting up a VPN using PPTP. The viability of VPNs is directly affected by security measures implemented on the destination LAN. PPTP is a protocol like any other, and must be allowed to pass through or bypass a firewall or proxy server in order to work successfully.

5.7.1 How to Allow PPTP Through Firewalls

Like most IP-based tunneling protocols, PPTP operates on a specific IP port—in this case, TCP port 1723. On your firewall or filter, youll want to allow IP access to and from that port for your RAS server. If your firewall also filters by protocol, youll need to allow GRE IP protocol 47 to pass through. Its a good idea to block every other port off on your RAS server, especially the nefarious NetBIOS name service, datagram, and session ports of 137, 138, and 139. These ports can be used to browse the NetBIOS names and shares of the machines on your network.

5.7.1.1 Fixed IP addresses

Since remote PPTP users will be dialing in through ISPs, they may not always have the same IP address. This eliminates the possibility of host-based filtering and means that a PPTP VPN will rely strictly on its user-based authentication system. A fixed IP address, where a user will be assigned the same IP address every time they dial in, is a way around this problem. Some ISPs offer a fixed IP address as an account add-on for a nominal monthly fee. If available, this is a great way to enhance security by allowing only hosts with one of those IP addresses to pass through port 1723 of the firewall and connect to the RAS server. 88

5.7.2 How PPTP Can Bypass a Proxy Server

A proxy server acts as a go-between for your internal hosts and hosts youll need access to on the Internet. Typically, a proxy server will be the only machine on a network, outside of an Internet server such as SMTP mail, thats allowed to pass through a firewall. An administrator can control who has access to what Internet services outside of the firewall. Although they seem like a perfect match, PPTP will not work with some proxy servers one exception to this is Microsoft Proxy Server 2.0. At this time, PPTP clients and servers dont know how to interoperate with the sockets services on proxy servers. This means that you cant have your PPTP server sitting behind a proxy server on your network. The clients wont be able to get through. If youre running a proxy server, youll still need to keep your RAS server open to the firewall. In such a case, both the RAS server and the proxy server may need to be multi- homed between your router and your local network. Multi-homed servers are connected via two network cards to two separate LAN segments. For these situations, youll want to turn on IP Forwarding routing in the TCPIP Properties for each network card.