Workings of a Virtual Private Network in Linux—Part 2

I found the HOWTO's treatment of routing less than comprehensive, perhaps beyond its intended scope. It goes as far as informing each VPN server about the population of workstations on the opposite network, but it doesn't teach those workstations the reverse, and it doesn't provide for end-to-end awareness by workstations on one network about the workstations on the other.

The provided routing from the main script

route add -net 193.6.37.0 gw 192.168.0.2

puts an entry in the local VPN server's routing table that says “If you handle any packets whose address starts with 193.6.37, the clearinghouse for them is 192.168.0.2 (the remote VPN server), send them to him. He'll know what to do with them”, since that's the address of the network to which he belongs.

When I tried to ping a 193.6.37 from the local VPN server, it failed. While I had a route to those machines, the problem was their ignorance of a reverse route back to me. They got my ping packets all right, but they couldn't answer back with a reply packet because their routing table had never heard of me—dead-letter office. So, I never saw the evidence of my half-success.

There are two solutions. One, an entry in each of their routing tables that points the way, unfortunately requires separate adjustments on these multiple machines. The other requires a single change at the remote VPN server, using the proxyarp option of pppd.

ARP (address resolution protocol) is for use in an Ethernet environment. It's insufficient for workstations that talk over Ethernet to know one another's IP address. The only way they can trade TCP/IP packets is inside of Ethernet packets. Those go nowhere unless the destination Ethernet (not IP) address is known.

Machines maintain a directory for looking up a machine's Ethernet address, given its IP address. (The Linux arp command prints it.) If a desired target IP isn't in your list, you automatically emit an ARP broadcast to the network. If any other machine can come up with the matching Ethernet address, it will send it to you.

The pppd proxyarp command makes a white-lie entry in the remote VPN server's directory that equates the local VPN server's TCP/IP address with the remote VPN server's Ethernet address. This fixed my ping problem. If the remote workstations can get replies back to the remote VPN server, they can certainly get them the rest of the way back to me at the local VPN server. (See section 9 of “Introduction to the Internet Protocols” for more about ARP.)

The other routing problem is that local and remote workstations don't have routes to each other. The fix is an entry for each local workstation specifying that either the local or remote VPN server offers the route to the remote network. For each remote workstation, either the remote or the local server offers the route.

For local workstations, the command is

route add -net 193.6.37.0 gw 192.168.0.1
or
route add -net 193.6.37.0 gw 192.168.0.2

For remote workstations,

route add -net 193.6.35.0 gw 192.168.0.2
or
route add -net 193.6.35.0 gw 192.168.0.1

For local workstations,

route add -net 193.6.37.0 gw 192.168.0.1
route add -net 193.6.35.0 gw 192.168.0.2
or
route add -net 193.6.37.0 gw 192.168.0.2
route add -net 193.6.35.0 gw 192.168.0.1

(The second one works only if you invoked proxyarp when you ran pppd.) For Microsoft machines on either net, give that net's VPN server as the Gateway under TCP/IP properties.

Now you are a VPN expert, so you also know the VPN is worthless unless the computer where it runs is secured in other ways. For instance, rsh and rlogin had better be disabled. ssh was designed to replace them. If they're still hanging around, you have one door double-bolted and another wide-open. Other services like telnet and ftp should be turned off, too. You can do that in the inetd.conf file. And, firewall rules should be deliberately applied. Though beyond the scope of this article, these important considerations require mention.

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