PPTP connection error: GRE: Bad checksum from pppd

If you check the various forums and mailing lists you will find notes about the MPPE option (PPP Encryption,) and MS Chap v2. If you haven't already tried tinkering with those settings I suggest you google for information about them - they might help in your case.

They didn't help in my case.

It turns out that there are packets, called GRE packets, that might be blocked in your configuration. Most of the instructions that I found on the net don't mention this - but it's important.

Here's a page that does mention Protocol 47:

http://forums.whirlpool.net.au/archive/1957524

Bottom line: If there's something between, for example, your cable modem (ie: your public IP) and your VPN server: the problem may simply be that the GRE packets are not getting through. So,

• If you are using one of the many router boxes that are commonly available: Browse to the admin pages for the box and check the advanced settings. You will want to find and enable a "PPTP passthrough" feature that knows how to handle the GRE packets. (Remember that many ISP's provide you with a cable modem that has a built-in router.)

• If you are using Arno's IPTables Firewall: Enable the plugin for PPTP (it's in the plugins folder - follow the instructions.)

Unfortunately your configuration may require another solution - something to handle NAT'ing the GRE packets between your VPN server and your (cable modem / public IP.) I'm guessing that the routers will be the issue for many people so I hope this note helps in your case, too.