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This is beyond weird. One of my remote VPN clients decided today to upgrade his 11.10 laptop to 12.10. Everything was working prior to that.

Now, his OpenVPN client connects (as usual) and I can ping his machine without issues. However, as soon there is any traffic over the tunnel (e.g. I ssh to his machine and type ls -al, I get two-three lines and the link is dead). Restarting the service repeats the process.

I've checked and re-checked his client.conf files and there is nothing in the logs that differs one iota from my setup (also 12.10, no issues) and there are no issues in the server logs. There is no way he can tell me what he clicked on during the upgrade (I'm suspecting it's one of those "do you want to overwrite conf files or keep the old ones?"). There is a 12.10 machine behind the same router he's on and its VPN works flawlessly.

Any idea where and what to look for?

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it looks like a MTU issue, but ifconfig tun0 mtu 1200 didn't solve things, as soon as there's any traffic, the tunnel hangs. I've resolved the issue by creating a separate openvpn server instance bound to a different port and using udp (the former uses tcp for various reasons) and configuring it with mssfix and fragment 1400. I'd still like to know how the upgrade messed things up on the client's end. –  sostacked Jan 10 '13 at 2:00
I'm having a very similar problem, but the mssfix and fragment 1400 directives don't seem to help. One possibly-useful datapoint: I have duplicated the problem successfully on a number of Ubuntu machines (12.04 through 13.04). I have another machine that runs CentOS 6 with the same OpenVPN client version. I don't see any problems when connecting to the VPN from the CentOS machine. I suspect that it may be an issue related to the newer kernel version that Ubuntu uses. –  Jason R Jun 19 '13 at 14:06
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