I got home today and was greeted by a pile of emails from logcheck, informing that pptpd was upset. Here is a snippet:
Jun 26 20:02:37 lazarus pptpd[3060]: MGR: initial packet length 4930 outside (0 - 220) Jun 26 20:02:43 lazarus pptpd[3060]: MGR: initial packet length 4930 outside (0 - 220) Jun 26 20:03:52 lazarus pptpd[3060]: MGR: initial packet length 4930 outside (0 - 220) Jun 26 20:04:04 lazarus pptpd[3060]: MGR: initial packet length 22415 outside (0 - 220)
It seems to have been happening about twice a minute for the last couple of hours.
Any clues what it might be?
I've started a tcpdump, and with any luck that will turn something up...