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6. Addenda

6.1 Pitfalls

Here are just a few of the snags that I've run into while using this system. I put them here so that you can hopefully avoid them. If you run into any new ones, please so that I can keep track, and help others avoid them.

read: I/O error

This error apparently comes from pppd. It's associated with mis-matched versions off pppd. If you get it, try upgrading both ends of the connection to the latest version of pppd. I've found that pppd version 2.2 has this problem, and use version 2.3.7 or 2.3.8 instead.

SIOCADDRT: Network is unreachable

This error is generated by route. I've seen it happen when the sleep time between ssh and pppd is not long enough. If you get this error, run ifconfig, you may see that there is no pppX interface. This means that ssh was not done authenticating before pppd was launched, and therefore pppd did not make the connection. just increase the delay, and your problems will be solved.

I wonder however if there might be some pppd option that will fix this problem.

IPv4 Forwarding and 2.2 kernels

In the new 2.2 kernel, you must specifically enable IP forwarding in the kernel at boot up. This with the following command:

# echo 1 > /proc/sys/net/ipv4/ip_forward

Without this, the kernel will not forward any packets, and hence the server will not work, nor will any of the gatewaying clients.


it should go without saying, but be careful when you are routing real numbers that you don't route traffic destined for the VPN server's external address through the tunnel. It won't make it. (yes, this is from personal experience.)

6.2 Hardware and Software Requirements

Minimum Hardware Requirements

Believe it or not, this system has been run on a 486SX33 with 8 megabytes of RAM. It didn't run very well though, it had trouble handling heavy traffic.

It doesn't take much more to make it work though. This system does work very well on a Pentium 75 with 16 megs of RAM, using an LRP distribution running off of a floppy, with a 6 meg ramdisk, and 10 megs of main memory. I've tested this setup by running a 700kbit RealVideo stream through it for over an hour.

I now typically run it on Pentium 90's, as their PCI clocking plays nicer with cheap 100Mbit Ethernet cards.

Software Requirements

This system works with both the 2.0 and 2.2 kernels. The script to keep the tunnel up requires a reasonably modern bash. I have however noticed that certain distribution's versions of bash don't play too well with the script.

Also, if someone could help me refine my scripts (or even write an executable?) that would helps things a lot. I'm not sure why, but even my own bash doesn't follow the rules and doesn't seem to interpret signals correctly. If you do make any improvements, please email me at

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