[pptp-server] how to detect GRE filtering?

[Neale Banks]

On Wed, 30 May 2001, kukulkan wrote:

> Hi there again,
>         I am trying to check that whether my internet connection(our network and
> ISP) is implementing some form of filtering. Help me to make a decision.  I have
> downloaded the patch for GRE traceroute , compile and run successfully. Again,
> please, please  correct me if I am wrong.
> 
> I also have tried to use the traceroute with -G option:
> 
> test  # 1 - traceroute -G 192.168.216.98
> 
> and it gave me several  error just before the pptp server, the router  in front
> of  my PPTP server(192.168.216.98)
> 
> test # 2 - traceroute -G -p 1723 192.168.216.98
> 
> and both  gave me the same error.
> 
> 1  e0.kch8.xxx.my (xxx.142.221.9)  116.096 ms  117.333 ms  120.209 ms
>  2  fe0-0-0.kch.xxx.my (xxx.142.221.1)  119.418 ms  129.945 ms  119.755 ms
>  3  fe0.kch15.xxx.my (xxx.142.221.16)  119.861 ms fe1.kch15.xxx.my
> (xxx.142.0.74)  119.767 ms fe0.kch15.xxx.my (xxx.142.221.16)  119.798 ms
>  4  xxx.142.33.210 (xxx.142.33.210)  339.815 ms !X  321.584 ms !X *

Acording to "man traceroute" on my Debian system, that "X" means
"communication   administratively  prohibited".

I suspect that you've just identified the point that's blocking GRE ;-)

> My question is,
> 
> i) is there any  difference with the traceroute with -G(use GRE packets instead
> of UDP packets) test # 1.
> ii)or  the -G with port specification  1723?
> 
> in other words, is it *neccessary*  for me to include the port specification -
> port 1723 to check whether my GRE is being filtered?

No, GRE does not use port numbers.

PPTP uses two separate channels:
 - a contol/setup channel over TCP which uses TCP port 1723.
 - a data channel for the PPP which uses GRE (no port# applicable).

HTH,
Neale.