MTU f**k you

So a few months ago I replaced my cable model internet connection with an ADSL connection from my company.

It meant I had to use the company’s ADSL route on the front-end and my wireless router acting as a wireless bridge and switch.

Then I noticed my PPTP VPN’s were broken. One would disconnect in under 5 seconds, the other would last a few minutes then the connection would be dropped (i.e. I could not brown sites, fetch mail or IM) but the VPN would still be “connected”.

Initially I thought the problem was the two routers. But a quick check directly from the primary router discounted that. I hd the same behaviour via wireless or ethernet on the primary router.

My console log (I am using OSX Lion) showed some weird things in pptp.log, for example hundreds of entries after the connection dropped saying :

        Protocol-Reject for unsupported protocol 0x9264

After checking that Tunnelblick was not causing problem, stopping virtual interfaces, stopping the bonjour service and stopping ESET anti-virus, I stumbled upon the solution:

Change the MTU size on the device.

My new ADSL router could not handle a PPTP connection with an MTU of 1500. Setting MTU to 1450 on both the wireless and ethernet adapters 1450 fixed it. On OSX you find the MTU settings under System Preferences –> Network –> Select adapter (e.g. wireless or ethernet) –> Advanced — Hardware.

Putting this out there in case it helps someone else.

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