I run Poptop 1.1.2 on four Slackware Linux 8.0/8.1 boxes with kernels
ranging from v2.4.17 to v2.4.19. I occasionally get complaints that
connections from Windows clients fail with "Loopback detected".
Actually I see this happening from time to time too in the logs of a
FreeBSD server I use to monitor the availability of the servers (it uses
pptp-client to simulate connections).
Encryption is not used at all.
PPP debug logs actually show that LCP packets seem to be echoed back
to the client verbatim.
I can not relate this to anything, it's pretty much random. Except in
one occasion: when I upgraded the Linux kernel of one server from
2.4.17 to 2.4.19 and Poptop from 1.0.? to 1.1.2, it began happening
for all connections until I also upgraded pppd to 2.4.2-cvs (2.4.2
wasn't released at that time - haven't checked if it is now).
Anyone can shed light on what could cause these "Loopback detected"
failures? Obviously the explanation "your modem wires are
short-circuited" hardly applies here ;-)
Sr. System Administrator
CS Communications Co. Ltd. - Thailand