PPTP VPN passthrough stopped working on 18.04

Question

I've just upgraded to 18.04, and I have a problem that my wife cannot connect to her work PPTP-based VPN through our home network (Linux box operates as router). If I bypass the linux box, the VPN connection connects fine.

I had trouble setting this up back in the day, until I found I needed load module nf_nat_pptp. After that, it worked fine until the upgrade. I've tried disabling UFW, but it still won't work.

I can't seem to get much debug information on this. (Any debug tips?) The only thread I've got on the problem is a RasSstp error in Windows with message, "A certificate chain processed, but terminated in a root certificate which is not trusted by the trust provider." Would the Linux box (as router) need to be aware of the certification authority to allow the route?! Or perhaps it's inserting its own certificate in the chain, and that isn't a trusted CA at the Windows server end?

I just don't know enough about the protocol to unwind this---any help would be greatly appreciated.

Solution

Ubuntu 18.04.1 server
PPTP PassThrough started to work after adding

net.netfilter.nf_conntrack_helper = 1

and adding the module with:

modprobe ip_nat_pptp

This should fix the problem.

Comments

[MuntingInsekto]

What do you mean by sudo systemctl edit dnsmasq?

[Mark Johnson]

That's the command-line to bring up editor for that particular configuration file.

[Mark Johnson]

BTW, I ended up going back to ifupdown due to dnsmasq race issue.

[anthls]

Looking at the answer from @MarkJohnson, I believe the file to add the line net.netfilter.nf_conntrack_helper = 1 is /etc/sysctl.conf but @sciensys perhaps you could clarify? Thanks.