Re: PPTP problem with Linux Client

On Tue, 10 Aug 2004, Alex Bihlmaier wrote:

> I have a Debian Unstable Client with pptpconfig (as well as MPPE Support 
> in the kernel).
> This client can happily connect to another PPTP Server and can get the 
> traffic routed.
> 
> But with the M0n0wall as PPTP Server I am getting the following log 
> output after a sucessfull login into the pptp service:
> 
> Aug 10 23:28:22 erwin mpd: [pptpc0] LCP: rec'd Protocol Reject #17 link 
> 0 (Opened)
> Aug 10 23:28:22 erwin mpd: [pptpc0] LCP: protocol 0x2145 was rejected

It looks like there's a difference of opinion over whether Protocol Field
Compression is in use.  PPP protocol types are nominally 16-bit values,
but they're coded in a manner that permits the common ones to be sent as
single bytes if both parties agree to it.  0x0021, a.k.a. 0x21, is the PPP
protocol type for IPv4, and 0x45 is the first byte of an optionless IPv4
header.  So it seems that MPD is using PFC while the Debian client isn't
recognizing it, and is instead trying to interpret 0x2145 as the PPP
protocol.

There's no way to tell which party is at fault without seeing the LCP
negotiations.  If PFC was negotiated on, then Debian is to blame for not
handling it correctly.  If it was negotiated off (or not negotiated at
all, since off is the default), then MPD or the FreeBSD kernel is to blame
for using it without permission.  If there's an option to disable PFC, you
might try it.

If you send me a packet trace I'll take a look.  You should be able to do
that on the Debian box with something like:

        tcpdump -w pptp.cap -s 1536 -i <interface> ip proto 47

                                        Fred Wright